Pleistocene Underplating and Metasomatism of the Lower

JOURNAL OF PETROLOGY VOLUME 41 NUMBER 3 PAGES 331–356 2000

Pleistocene Underplating and Metasomatismof the Lower Continental Crust: a XenolithStudy

PETER M. SACHS∗ AND THOR H. HANSTEENGEOMAR FORSCHUNGSZENTRUM FUR MARINE GEOWISSENSCHAFTEN AN DER CHRISTIAN-ALBRECHTS-

UNIVERSITAT KIEL, ABTEILUNG VULKANOLOGIE UND PETROLOGIE, WISCHHOFSTR. 1–3, D-24148 KIEL, GERMANY

RECEIVED OCTOBER 1, 1998; REVISED TYPESCRIPT ACCEPTED SEPTEMBER 2, 1999

Xenoliths from Engeln–Kempenich in the East Eifel volcanic field lower-crustal mafic granulites. The brittle–ductile transition appearsto be a preferred level of magma stagnation.(Germany) comprise gabbroic to ultramafic cumulates, and meta-

igneous and meta-sedimentary granulite- to amphibolite-facieslithologies. They provide evidence for Pleistocene heating andmetasomatism of the lower continental crust by mafic magmas. The

KEY WORDS: continental lower crust; fluids; magma chamber; meta-metamorphic xenoliths were divided into three types: (1) primitivesomatism; xenolithstype P, which are little affected by metasomatic replacement structures;

(2) enriched type E1 defined by metasomatic replacement of primarypyroxene and garnet by pargasitic amphibole and biotite; (3) enrichedtype E2 defined by breakdown of hydrous phases. Type E rocks are

INTRODUCTIONgeochemically related to type P and cumulate xenoliths by com-positional trends. During modal metasomatism, type E rocks were Basaltic magmas contribute to the formation of loweroxidized. Type E1 rocks were typically enriched in Rb, Th, U, continental crust by ‘underplating’ through intrusion andNb, K, light rare earth elements (LREE) and Zr, and E2 enriched crystallization (e.g. Cox, 1983; Furlong & Fountain, 1986;in Rb, Th, U, Nb, K, REE, Zr, Ti and Y, relative to type P Rudnick & Fountain, 1995). These basaltic intrusionsrocks. Formation of the hydrous, chlorine-bearing phases amphibole interact with the lower crust by transfer of heat andand scapolite containing glass and fluid inclusions in the E1 rocks matter, especially by metasomatism, which may be in-provides evidence for a water and Cl-bearing fluid phase coexisting duced by fluid phases released from the intruded magmaswith silicate melt. Accordingly, we calculated 10 mol % H2O (e.g. Newton et al., 1980; Touret, 1986; Katz, 1987). Theback into the CO2-dominated fluid inclusions, in agreement with components added to and/or removed from the lowerexperimental data on the composition of a fluid phase coexisting crust, the nature of the transporting fluids and the relativewith mafic alkaline melts at elevated pressure. Primary CO2- importance of metasomatism during the evolution of thedominated fluid inclusions coexisting with glass inclusions in lower crust, are poorly constrained.metamorphic corona phases and neoblasts, and in cumulate xenoliths, Xenoliths are fragments of the wall-rocks of the magmahave overlapping densities. Fluid inclusion barometry using the plumbing system and can provide important evidencecorrected densities indicates that both cumulates and metamorphic for metasomatic interaction of the magma with the lowerxenoliths originated from the same depth at 22–25 km (650 ± crust. The clearest evidence for metasomatism of lower-50 MPa). This is interpreted as being a main magma reservoir crustal rocks is the appearance of new phases in thelevel within the upper part of the lower crust close to the Conrad xenoliths. Metasomatism involving development of newdiscontinuity, where the xenoliths represent wall-rocks. The Conrad phases is referred to as ‘modal’ (Harte, 1983). Here wediscontinuity separates an upper-crustal layer, consisting of pref- present a study of xenoliths with different degrees of

modal metasomatism, sampled from a Pleistoceneerentially ductile granodioritic and tonalitic gneisses, and more brittle

∗Corresponding author. e-mail: [email protected] Oxford University Press 2000

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Fig. 1. Location of the Engeln–Kempenich locality and the Pleistocene East Eifel volcanic field. After Worner et al. (1985).

(<450 ka; Schmincke et al., 1990) phreatomagmatic ne- studied petrographically mafic garnet granulite xenoliths,phelinitic tephra deposit of probably one single eruption which are considered to be part of the lower crust. Stoschof the East Eifel volcanic field (Engeln–Kempenich, Ger- & Lugmair (1984) and Stosch et al. (1986) found Sm–Ndmany) (Fig. 1). We suggest a model in which meta- isotopic evidence for metasomatic overprinting of thesesomatism of the lower crust is due to the formation of lower-crustal xenoliths. They suggested that the additionmafic magma chambers. Barometry of fluid inclusions of a light rare earth element (LREE-)enriched me-and mineral thermometry of cumulate and metamorphic tasomatic component had occurred more recently thanxenoliths was used to locate the depth of a magma Jurassic times. The source of the metasomatism wasreservoir and to estimate the conditions of formation of attributed to fluids from the upper mantle.the metasomatic phases.

GEOLOGICAL SETTING ANDPHASE ASSOCIATIONS

PREVIOUS STUDIES OF THE EIFEL The studied xenoliths from Engeln–Kempenich compriseCRUSTAL XENOLITHS (1) gabbroic and clinopyroxenitic cumulates, whichThe East Eifel volcanic field is part of the Rhenish Massif, are probably comagmatic with the host magma;which has been undergoing uplift since the late Tertiary (2) rare granulite-facies meta-sedimentary quartz-(Meyer et al., 1983), probably as a result of upwelling bearing garnet–sillimanite gneisses;asthenosphere. The Pleistocene volcanism in the Eifel (3) mafic to ultramafic meta-igneous granulites, meta-has been summarized by Schmincke et al. (1990). Xenolith clinopyroxenites and meta-hornblendites;studies of the upper and middle crust beneath the East (4) rare peridotites (highly recrystallized spinel harz-Eifel volcanic field have been published by Worner et al. burgites and amphibole-bearing spinel harzburgites of(1982), Voll (1983), Worner & Fricke (1984) and Mengel the upper mantle), which are not considered here.et al. (1991). The uppermost Eifel crust is characterized

[See Table 1; abbreviations of mineral names are fromby folded Devonian sediments. The Devonian strata areBucher & Frey (1994).] All studied xenoliths from Engeln–underlain by greenschist-facies rocks and by medium- toKempenich are well rounded. The crustal xenoliths havehigh-grade amphibolite-facies rocks (e.g. staurolite schists,diameters up to 20 cm. Peridotite xenoliths are generallygranitic to tonalitic gneisses). Okrusch et al. (1979), Voll

(1983), Loock et al. (1990) and Mengel et al. (1991) have smaller, having diameters Ζ3cm.

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Table 1: Phase associations of granulite-facies xenoliths, mineral temperatures (only for underlined samples; mineral rims,

clinopyroxene–orthopyroxene solvus, garnet–biotite, appearance of early-textured glass inclusions and interstitial glasses)

Sample nos T Wells T BK Phases

K5/48, K5/61, K5/74, 835 816 Rt ±Ttn Di Opx Prg, Hbl Pl ±Ap

K5/76, K9/30

K5/55, K5/65, K9/28, k9/32 781 664 Ilm Di Prg, Hbl Pl

K5/57, K5/71 868 773 ±Ti-Mag Rt Di Prg, Hbl Pl ±liq1

K5/51, K5/59, K5/68 Grt ±Ilm Rt Opx Prg, Hbl Pl ±Ap

K5/31 Grt Ti-Mag Di Prg Pl

K5/45, K5/56, K5/70, k5/72, 827, 846, 797, 690, 741, 650, Grt ±Ti-Mag ±Ilm Di Opx Prg, Hbl ±Bt Pl ±Ap

K5/73, K5/75, K5/77, K9/24, 844, 867, 805 734, 762, 695

K9/35, K9/70

K5/52, K5/54, K9/23b 800, 827 668, 761 Grt Hc-Mag ±Ti-Mag Di Opx Prg, Hbl Pl ±Scp1 ±Ap

K5/64, K9/23a, K9/27 827, 1230 761, 1082 Grt Hc-Mag Di Opx Prg, Hbl ±liq1

K5/47, K5/67 >s >s Hc-Mag ±Ti-Mag Di Opx Prg Pl ±Scp1 Ap

K9/29, K9/69 >s, >s >s, >s ±Hc-Mag ±Ti-Mag Di Prg Pl ±Scp1 Ap liq1

K5/58, K9/21, K9/22, K9/31 >s, >s, >s >s, >s, >s ±Hc-Mag ±Ti-Mag Di Prg ±Scp1±Scp2 Ap liq1

K9/32 Ti-Mag Ol Di Opx Prg

K9/33 >s >s Ti-Mag Ol Di Prg liq1

Sample Bt–Grt Phase associations in Grt cores Corona Matrix Replacement of Grt + Bt by

no. thermometry around

Mineral rims Grt

K4/10 792 (IM-A) Grt Hc1 Sil Qtz Pl1 Rt liq1 Bt Pl2 Qtz Rt Ilm Zrn liq3 Opx Hc2 Pl3 liq2

759 (IM-B)

747 (PL)

684 (B)

The mafic and ultramafic phase assemblages contain additional pyrrhotite. Sample K9/23 is a spinel- and garnet-bearing, composite xenolith (the spinel is ahercynite–magnetite solid solution), consisting of a garnet–spinel websterite dyke (K9/23a) in a garnet–spinel pyriclasite (K9/23b). Temperatures derived frommineral core compositions are >20–80°C below rim temperatures. T Wells, clinopyroxene–orthopyroxene thermometry after Wells (1977); T BK, orthopyroxene–clinopyroxene thermometry of Brey & Kohler (1990). Garnet–biotite thermometry (mineral rims): IM-A, IM-B, Indares & Martignole (1985), models A and B; PL,Perchuk & Lavrent’eva (1983); B, Bhattacharya et al. (1992).

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All studied xenoliths show no or low degrees of al- (1) tephrite–basanite to basalt, and (2) phonotephrite andtephriphonolite [using the classification of Le Maitre etteration. Glasses (<1 vol. %) occurring along grain bound-

aries and in interstitial pockets are fresh or only partly al. (1989)]. An analysis of the glass inclusion in K9/34with the highest S, Cl and F concentrations is given inaltered into sheet silicates or palagonite. Minerals are

completely fresh, except olivine, which is partly or com- Table 2. The glass inclusions are associated with high-density CO2-fluid inclusions indicating a cogenetic origin.pletely iddingsitized (sample K9/33).

The meta-igneous granulites are composed of clino- Most glass inclusions show little or no evidence of partialcrystallization but frequently contain fluid vesicles (0–pyroxene, pargasite or pargasitic hornblende, ± garnet,

± spinel (hercynite–spinel–magnetite solid solutions),± 10% of inclusion volume).The latest phase is interstitial, clear light brownishorthopyroxene,± biotite,± plagioclase,± scapolite,±

glass,± magnetite,± ilmenite,± rutile, and composite vesicular phonotephrite glass (Table 2, under K9/34).Vesicles in the glass are partly filled with sheet-silicates,granulite xenoliths with websterite dykelets. Meta-clino-

pyroxenites and meta-hornblendites are dominated by variably enriched by Cu (up to 1 wt %). Fluorine isenriched in the glass adjacent to vesicles and cracks.clinopyroxene and pargasitic amphibole, respectively,

and have variable contents of olivine (two samples only), The second representative rock, ultramafic cumulate(K9/25), has a nephelinitic bulk composition (see Tablehercynitic spinel, scapolite, plagioclase, apatite, Ti-mag-

netite and glass. All studied xenoliths contain high-density 4, below). It consists of early crystallized cumulus clino-pyroxene and minor late-stage phlogopite (Phl80Ann20),CO2-dominated fluid inclusions. The spinel-bearing

meta-igneous phase associations and the garnet– and vesicular foiditic interstitial glass (Table 2). Clino-pyroxenes are zoned, with pleochroic light brownish–sillimanite gneisses have not been described previously.

Metasomatism is a texturally late-stage process and greenish cores (En38Fs11Wo51), and brownish outer rims.Cumulus phases are medium grained (average grainincludes hydration and dehydration reactions. From their

petrographic appearance, the xenoliths can be subdivided size 1–3 mm) and sub- to euhedral, defining a euhedralgranular texture. Clinopyroxene has a prismatic, andinto enriched (metasomatized) and primitive types as

follows: phlogopite a tabular morphology, without preferred ori-entation.(1) an enriched type E, with modal metasomatic over-

printing, which can be subdivided into (1a) subtype E1 Glass inclusions show strong conspicuous crys-tallization. The glass inclusions are apparently cogeneticwith replacement structures of pyroxene and garnet by

hydrous phases amphibole and biotite, and (1b) subtype with high-density CO2-fluid inclusions.E2 defined by breakdown of hydrous phases;

(2) a primitive type P generally lacking type E re-placement textures. Meta-igneous xenoliths

The granulite xenoliths are dominated by meta-igneous,mafic to ultramafic phase associations and represent

Cumulate xenoliths products of phase transitions of meta-gabbroic rockswithin the granulite facies above >500 MPa. Table 1The cumulate xenoliths comprise gabbros and phlogopite

clinopyroxenites. We have chosen two representative shows the phase associations roughly in the order ofincreasing equilibration pressure and temperature. Therocks for detailed descriptions.

The first is a gabbroic heteradcumulate (K9/34), which petrography of some of the mafic xenolith types (garnetgranulites) has been described earlier (Okrusch ethas the bulk composition of a basalt (see Table 4, below),

using the total alkalis vs silica diagram of Le Maitre et al., 1979; Voll, 1983; Loock et al., 1990; Mengel et al.,1991).al. (1989). Cumulus phases crystallized in the order: Ti-

Mag + Ap + Cpx, Ttn, Am (compositions are given Mafic granulites have granoblastic inequigranular toequigranular microstructures [for microstructural termsin Table 2). They are medium grained (1–2 mm) and

scattered without common orientation in coarse-grained and definitions we follow Passchier & Trouw (1996)].Plagioclase in mafic, equigranular granulite xenoliths(5 mm) potassic oligoclase (An31Ab56Or13), defining an

ophitic texture. Clinopyroxene is zoned from pleochroic shows undulose extinction, kinking, tapering deformationtwins, bent twins and deformation bands. The rockslight green cores (En29Fs22Wo49) to darker greenish rims.

Except for sub- to anhedral amphibole and plagioclase, have suffered different degrees of recrystallization ofplagioclase, scapolite, pyroxene and amphibole. Mantlesminerals have a subhedral to euhedral morphology, with

prismatic clinopyroxene and apatite, octahedral Ti-mag- of plagioclase neoblasts are common, with a sharp bound-ary around cores of old grains, whereby typical core-netite and sphenoidal titanite.

Clinopyroxene, titanite and Ti-magnetite contain prim- and-mantle structures have developed, characterized bya well-defined bimodal grain size distribution and by aary glass inclusions of dark to light brown colour (Fig.

2a). The glass inclusions define two compositional groups: large difference in grain size.

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SACHS AND HANSTEEN PLEISTOCENE UNDERPLATING AND METASOMATISM

Table 2: Mineral analyses (EMP, electron-microprobe; SYXRF, synchrotron-XRF microprobe)

Sample: K9/34 K9/25

Phase: GI in Interstitial Cpx Am Pl core Ti–Mag Tit core Ap core Interstitial Cpx core Bt core

Cpx glass core core core glass

wt % EMP

SiO2 53·71 55·13±0·27 45·41 38·71 62·56 29·10 38·91±0·22 44·66 36·47

TiO2 2·52 1·59±0·03 2·25 4·50 0·05 9·52 34·74 2·88±0·09 2·65 5·40

Al2O3 18·28 15·94±0·15 6·59 12·09 22·71 3·19 1·77 15·18±0·13 9·34 16·87

CaO 3·74 6·46±0·11 21·60 11·45 5·22 0·00 27·85 55·30 11·88±0·61 23·71 0·15

FeO∗ 5·98 6·30±0·16 12·20 15·35 0·40 79·11 1·42 0·24 10·58±0·48 6·32 8·27

MgO 0·89 1·75±0·04 9·32 10·14 0·04 2·40 0·00 0·14 3·71±0·04 12·60 18·78

MnO 0·32 0·35±0·05 0·51 0·58 0·00 1·27 0·12 0·01 0·31±0·01 0·07 0·04

Na2O 7·07 6·60±0·04 1·51 2·50 5·26 0·00 5·52±0·08 0·40 0·44

K2O 3·38 3·57±0·04 0·00 1·92 1·80 0·00 6·32±0·34 0·00 9·22

P2O5 0·33 0·29±0·02 40·00 1·32±0·02 0·04

F 0·47 0·22±0·02 0·04 3·00 0·52±0·22 0·50

Cl 0·25 0·21±0·02 0·05 0·21 0·38±0·01 0·04

SO2∗ 0·40 0·31±0·03 0·93 0·44±0·02 0·00

Sum 96·93 98·73 99·39 97·33 98·02 95·49 95·00 99·83 97·95 99·77 96·22

Cu lg/g EMP 840

Zn 150 2050

Ni 460 660

Ba 5320

En29 XMg = An31 En38 Phl80

Fs22 0·54 Ab56 Fs11 Ann20

Wo49 Or13 Wo51

Modal abundances of volatile-bearing phases are evidence for significant amounts of HSO4− [in-

terpretation of the spectra following Swayze & Clarkhighly variable; amphibole ranges from 5 to 95 vol. %,biotite between 0 and 5 vol. % and scapolite from 0 to (1990)]. The scapolite2 porphyroclasts have a remarkably

constant major element, sulphur (Table 2, sample K5/30 vol. %. Amphibole occurs as texturally apparentlyprimary grains, in apparent equilibrium with similar- 58) and fluorine composition. Chlorine concentrations

decrease about 40% from the core to the rim, e.g. fromsized clinopyroxene, as coronas around pyroxene andgarnet, and as texturally early grains partially replaced 0·47 to 0·28 wt %, and bromine shows a decrease of

about 80%, e.g. from 30 to 6 lg/g (sample K5/58).by clinopyroxene or orthopyroxene. Biotite occurs as-sociated with pargasitic hornblende as coronas around Meta-pyroxenites and meta-hornblendites are por-

phyroclastically recrystallized or show mosaic structures.garnet. Scapolite occurs (1) as an apparently texturallyequilibrated early phase (scapolite1), forming equi- Clinopyroxene and pargasitic amphibole neoblasts fre-

quently contain glass inclusions and are in contact withgranular clusters, with numerous inclusions of pyrrhotiteor haematite and surrounded by coronas of Pl + Hem, vesicular glass occurring in pockets, indicating re-

crystallization under hypersolidus conditions. Idio-and (2) as a porphyroclastically recrystallized phase (sca-polite2) with centimetre-sized porphyroclasts and no co- morphic clinopyroxene and pargasitic hornblende in

contact with glass provide evidence for continued growthrona formation. The porphyroclasts show highly unduloseextinction and occur in two varieties: as an optically from the melt after incorporation of the xenoliths by the

magma.homogeneous phase, and with ubiquitous pyrrhotite in-clusions. Scapolite is the most important CO3

2−, SO42−, Some meta-clinopyroxenitic and meta-hornblenditic

xenoliths have compositions similar to the cumulateCl and Br phase. Fourier transform IR (FTIR) analysisof homogeneous scapolite2 in sample K5/58 provides xenoliths but are distinguished from them by the ap-

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Table 2: continued

Sample: K9/29 K5/71

Phase: Sp Sp Cpx Am Opx Opx Cpx Rt Am

Phase association: Sp + Cpx + Am + Core, Rim to Rim to Rim, late-

vesicular glass replaces Cpx Opx stage corona

Am around Opx

wt % EMP

SiO2 0·0 0·0 50·55 39·33 53·47 54·11 52·90 0·0 46·03

TiO2 0·14 0·03 0·33 1·80 0·10 0·05 0·24 98·36 0·02

Al2O3 56·41 53·51 5·12 14·54 2·19 1·37 2·09 0·17 10·44

CaO 0·18 0·11 20·77 10·46 0·35 0·67 23·02 0·61 11·59

FeO∗ 27·15 32·02 10·70 16·88 15·68 15·10 5·04 0·16 8·52

MgO 14·61 13·07 10·08 9·04 27·74 28·18 15·98 16·62

MnO 0·36 0·36 0·26 0·27 0·38 0·38 0·12 0·14

Na2O 1·68 2·93 0·0 0·02 0·42 1·84

K2O 0·0 1·29 0·0 0·0 0·0 0·39

F <0·01 <0·005

Cl 0·05 0·057

Sum 98·85 99·10 99·49 96·59 99·91 99·88 99·81 99·30 95·647

Method EMP EMP EMP, EMP, EMP, EMP, EMP EMP,

SYXRF SYXRF SYXRF SYXRF SYXRF

Cu lg/g EMP 99

Zn 800 800 630 <50 <50 <50 <50 396

Ni 100 600 1040 85 <50 <50 <50 693

Cr 1400 700 140 453 295 495

En32Fs19 XMg = En75Fs24 En76Fs23 En45Fs8 XMg = 0·78

Wo48 0·49 Wo1 Wo1 Wo47

pearance of scapolite and by deformation structures such & Trouw (1996)]. Biotite (Phl59–60Ann41–40) forms coronasas porphyroclastic recrystallization, deformation twinning around garnet. Garnet and biotite are separated by aof plagioclase, and undulose extinction and kinking of younger phase assemblage of vesicular glass + fine-olivine, clinopyroxene, pargasitic amphibole, plagioclase grained (0·5 mm) Hc + Opx + Pl. Rutile (1 vol. %)and scapolite. occurs as small inclusions in garnet, and in the matrix

as single millimetre-sized prismatic porphyroclasts andas clusters of neoblasts, frequently associated and epi-tactically intergrown with zircon. Matrix rutile in contactGarnet–sillimanite gneisseswith plagioclase neoblasts and glass is surrounded by

The garnet–sillimanite gneiss xenolith K4/10 is macro- coronas of ilmenite. In the garnet cores, inclusions ofscopically characterized by alternating layers of garnet sillimanite, quartz and green spinel (hercynite–([1 cm; 50 vol. %) and plagioclase ([0·5 cm), and minor spinel–magnetite solid solutions) and inclusions of pris-amounts (Ζ10 vol. %) of sillimanite, quartz, green spinel matic sillimanite with spherical inclusions of dark green(Spl–Hc–Mag solid solution), biotite, rutile, ilmenite and spinel (hercynite–spinel–magnetite solid solution, Fig. 2b)glass. Quartz is not in contact with spinel. Garnet provide evidence that spinel has been consumed by the(rim Prp34Alm59Grs5Adr0Spess2; core Prp32Alm62Grs4- processAdr0Spess2), plagioclase (An36–39Ab61–57Or3), rutile andquartz are highly porphyroclastically recrystallized, and Hc+ Qtz→ Grt+ Silthe microstructure is inequigranular polygonal to in-equigranular interlobate [after Moore (1970), in Passchier either by isobaric cooling and/or by a pressure increase.

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Sample: K9/27

Mineral: Grt core Grt rim Hc Pl Opx Cpx Am

Corona around Grt Rim to Px+Hc

wt % EMP

SiO2 40·03 40·32 46·49 49·64 51·25 41·49

TiO2 0·0 0·09 0·18 0·0 0·0 0·09 0·22

Al2O3 22·50 22·81 57·58 34·11 7·27 4·14 16·04

CaO 4·12 3·86 0·07 17·45 1·73 13·58 10·68

FeO∗ 19·32 18·70 27·97 0·30 17·40 11·61 10·07

MgO 13·12 13·74 12·52 0·02 22·45 17·84 14·52

MnO 0·90 0·88 0·57 0·01 0·89 0·59 0·15

Na2O 0·0 0·0 1·45 0·0 0·16 2·58

K2O 0·0 0·0 0·02 0·29

Sum 99·99 100·40 98·91 99·85 99·38 99·26 96·04

Zn (lg/g EMP) 1200 700 300 600

Ni 500 100

Cr 400 100 1000

Prp47·8 Prp50·0 Ab13An87 En67Fs29 En52Fs19 XMg = 0·72

Alm39·5 Alm38·1 Or0 Wo4 Wo29

Grs10·6 Grs9·9

Sps1·9 Sps1·8

Adr0·2 And0·0

Spherical zircon occurs as inclusions in garnet and in scapolite, (b) in corona phases: amphibole and hercyniticspinel replacing garnet; clinopyroxene or orthopyroxenethe matrix, indicating an earlier event of zircon corrosion.replacing amphibole.

In the garnet–sillimanite gneiss K4/10, CO2-fluid in-Fluid and glass inclusions clusions are cogenetic with early-textured glass inclusionsMode of occurrence (liq1) in garnet containing OH−, H2O, CO2 and CO3

2−

(analyses by FTIR). Similarly, CO2 inclusions in am-Several generations of fluid and glass inclusions occurphibole and scapolite in clinopyroxenites and hornblen-abundantly in the xenoliths. The fluid inclusions aredites are paragenetic with melt inclusions (e.g. Fig. 2h).CO2 dominated and are common in clinopyroxene,Primary inclusions in the gabbroic cumulate K9/34 occurorthopyroxene, garnet, plagioclase, hercynitic spinel, sca-in the cores of clinopyroxene and titanite idioblasts andpolite (scapolite1 and -2) and amphibole, and in garnetare associated with glass inclusions.and plagioclase in the garnet–sillimanite gneiss K4/10.

Many fluid and glass inclusions show textural evidenceInclusions can be divided into three groups: (1) texturallyof partial decrepitation (e.g. Hansteen et al., 1991). Sizesearly inclusions in porphyroclasts, occurring singly orrange from <2 to 40 lm; adjacent texturally early in-in groups, well removed from host grain boundaries,clusions can have widely varying sizes. Many late fluidespecially common in garnet and scapolite (fluid andinclusions also contain brown to colourless glass in vari-glass inclusions, liq1 in Table 1), and to a lesser extentable proportions.in amphibole, clinopyroxene and orthopyroxene; (2) tex-

turally late fluid and glass inclusion trails in porphyroclastsComposition of fluid inclusions(liq2), reaching or cross-cutting grain boundaries and also

other inclusion trails; (3) primary inclusions (fluid and Samples chosen for fluid inclusion analysis are cumulatesand metamorphic xenoliths showing different degrees ofglass inclusions, liq1) occurring singly or in groups (a) in

neoblasts of clinopyroxene, amphibole, plagioclase and modal metasomatism and deformation (Table 3).

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inclusions show final CO2 melting at−57·2 to−57·8°C,Table 2: continuedpreceded by melting intervals varying between 0·2 and0·8°C, suggestive of significant but small amounts ofSample: K5/58components additional to CO2 (Van den Kerkhof, 1990).The additional minor components could again be N2,Phase: Scp 2 core, porphyroclastCH4, CO or noble gases in amounts less than>2–4 mol % (Burke & Lustenhouwer, 1987; Van denMethod: EMP, SYXRFKerkhof, 1990). Such minor components are, however,CO2 bynot significant for our thermobarometric calculations,differenceand will be discussed elsewhere.

wt % lg/g

SiO2 48·83 Cu <1

TiO2 0·07 Zn 22·2 GEOTHERMOBAROMETRYAl2O3 26·86 Ni <1

Mineral thermobarometryCaO 15·86 Cr <1

P–T conditions related to ancient granulite metamorphismFeO∗ 0·26 Ga 15·8The association of green spinel and quartz within theMgO 0·0 Br 28·6garnet cores of the garnet–sillimanite gneiss (sample K4/MnO 0·00 Rb 32·610) was probably paragenetic at an earlier stage and thusNa2O 4·02 Sr 8275·7provides evidence for a high-temperature event with TK2O 0·27 Y 67·5>800–1100°C. We derived a lower stability limit atCl 0·421 Zr 104950 MPa of the paragenesis Grt + Qtz + Sil (molar

SO2∗ 0·594 Nb 19bulk XMg= 0·35; Aranovich & Podlesskii, 1983) inserting

CO2∗ 2·815 Sn <2temperatures from garnet–biotite thermometry (see

Sum 100 Ba 264below). The spinel-consuming reaction Spl + Qtz →

La 26Grt + Sil, preserved in the phase associations, indicates

Ce 52 that the estimated pressure post-dates the high-tem-Ta <5 perature stage.Pb 3

Mei68Ma32

P–T conditions related to late-stage heating and metasomatism

Coexisting mineral pairs were analysed for major ele-ments, to estimate late-stage temperatures. Results ob-tained from mineral rim compositions are shown in TableAll of the fluid inclusions froze to aggregates of solid1. Temperature uncertainties associated with deviationsCO2 and vapour when cooled to −65 to −100°C.from equilibrium may be large, because PCO2 q PH2O,Further cooling to about −190°C produced no visiblebut are impossible to quantify at present.phase changes, implying that CH4 or N2 can be present

Garnet–biotite thermometry (Perchuk & Lavrent’eva,only in minor quantities, if at all (e.g. Thiery et al., 1994).1983; Indares & Martignole, 1985; Bhattacharya et al.,Heating of the inclusions from about −190°C caused1992) applied to garnet–sillimanite gneiss K4/10 (biotitethe following three phase transitions: (1) initial meltingcoronas not in contact with garnet) indicates a late-stage(Ti ) of CO2 crystals in the temperature interval −57·8temperature increase from garnet cores to rims betweento −56·4°C (CO2 triple point at −56·6°C), in many>20 and 70°C. This temperature increase may be par-cases coinciding with (2) final melting of CO2 (Tm) attially reflected in coronas of vesicular glass + Hc +−57·2 to −56·4°C, and (3) final homogenization ofOpx + Pl, separating garnet and biotite. Temperaturesliquid+ vapour (L+ V) into liquid or vapour (Thl andabove the solidus are recorded by texturally early andThv, respectively) at <31·1°C (the inclusions are of thelate glass inclusions in garnet.microthermometric type H3; Van den Kerkhof, 1990).

We also applied the orthopyroxene–clinopyroxeneMost inclusions show a well-defined triple point meltingsolvus thermometers of Wells (1977) and Brey & Kohlerof CO2 at −56·6 ± 0·2°C and no melting interval,(1990). Mineral rims of clinopyroxene- and or-indicating essentially pure CO2, which has also beenthopyroxene-bearing granulites yield temperatures of ap-confirmed in some inclusions by FTIR analysis. Aboutparent last equilibration between >670 and 900°C25% of the inclusions, melting between −56·8 and[thermometer of Brey & Kohler (1990)] or 779 and−57·2°C, show a melting interval covering 0·1 or 0·2°C,950°C [thermometer of Wells (1977)] (Fig. 3). Mineralpossibly indicating minor amounts of additional com-

ponents such as N2, CH4, CO or noble gases. Several rim temperatures of mafic granulites are up to 70°C

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Table 2: continued

Sample: K 4/10

Phase: Grt porphyro- Grt porphyro- Bt corona Rt porphyro- Ilm corona Pl neoblast

clast centre clast rim to Bt around Gt clast centre around Rt in matrix

porphyroclast

wt %

SiO2 38·68 38·77 36·22 0·0 0·0 59·36

TiO2 0·0 0·0 5·1 98·07 49·94 0·0

Al2O3 21·63 21·78 16·47 0·26 0·53 26·02

V2O3 0·84

FeO∗ 27·85 27·91 15·35 0·57 43·91 0·19

MnO 0·81 0·92 0·1 0·03 0·86 0·0

MgO 8·27 9·11 13·01 0·0 2·77 0·0

CaO 2·71 2·01 0·03 0·02 0·01 7·98

Na2O 0·0 0·0 0·05 0·0 0·0 6·43

K2O 0·0 0·0 9·64 0·0 0·0 0·54

Sum 100·07 100·65 95·97 99·88 98·03 100·51

lg/g

Ni 37 43 <1 <1

Cu <1 <0·1 <1 55

Zn 110 122 297 65

Ga <1 <0·1 29 <0·1

Br <0·1 <0·1 <0·1 <0·1

Rb 1 6 199 <0·1

Sr 2 4 23 <0·1

Y 316 607 <1 <1

Zr 14 35 14 18

Nb <1 <1 2 35

Prp34·4 Prp34·1 Phl60 An39

Alm59·4 Alm59·5 Ann40 Ab57

Grs7·2 Grs5·2 Or3

Sps1·7 Sps2·0

Adr0·1 Adr0·2

Ti-Grt0·0 Ti–Grt0·0

higher than the core temperatures. Further, temperatures Type E2 xenoliths are defined by dehydration reactionsabove the solidus are recorded by glass-bearing xenoliths. (Fig. 2). On average, type E2 rocks record higher tem-Glass occurs in two varieties: as pockets of vesicular peratures of last equilibration than the other xenoliths.interstitial glass in contact with Cpx ± Spl (Hc–Mag An important type of amphibole-consuming and clino-solid solution) ± Hbl (pargasitic hornblende) ± Pl; pyroxene-forming reaction is found in samples K9/31and as texturally early glass inclusions in scapolite2, and K9/22:clinopyroxene and plagioclase (neoblasts and por-

Hbl + Cpx1 → Cpx2 +phyroclasts) where it is associated with early CO2-fluidSpl(Hc-Mag solid solution) + liq(vesicular glass)inclusions. The phase association Spl + Cpx + Prg +

Pl + liq1 provides evidence for temperatures between (Hbl is pargasitic hornblende) and can be attributed to900°C and 1030°C [XCO2= CO2/(CO2+ H2O)= 0·5] a temperature rise above 1050°C. The glass occurs asaccording to melting experiments of Springer (1992) on glass inclusions and pockets of interstitial glass in contact

with clinopyroxene2 and spinel.the Kempenich granulite suite.

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Table 3: Samples chosen for fluid inclusion barometry

Sample Type Degree of deformation Phase association

K9/34 Glass + Pl + Cpx + Am + Ap + Tit + Ti-Mag

Cumulate

K5/71 P No recrystallization, deformation

twinning of Plg Opx + Cpx + Am + Pl + Rt

K5/54 P No recrystallization, deformation

twinning of Pl Grt + Cpx + Opx + Pl + Am + Scp1 + Mag + Ilm

K5/47 E1 Strong Pl recrystallization, core

and mantle structures Cpx + Opx + Am + Scp1 + Pl + Mag + Ilm + Tit + Po + glass

K5/31 P porphyroclastic recrystallization Grt + Opx + Bt + Pl + Mag + Ilm + Zirc

K4/10 E1 porphyroclastic recrystallization Grt + Sil + Qtz + Pl + Rt + Hc + Pl + Opx + Bt + Ilm + glass

K5/58 E1 porphyroclastic recrystallization Hc-Mag + Cpx + Am + Scp2 + Ap + glass

K9/27 E1 mosaic textured Grt + Hc + Opx + Cpx + Am + Pl + Po + glass

Further pyroxene-forming reactions in meta-horn- spinel, pyroxene?) and probably indicate short-time syn-blendites (e.g. sample K9/27, Table 2; Fig. 2h) are also eruptive heating and decompressive melting after in-due to a temperature increase: corporation of the xenoliths by the host magma.

Grt ± Cpx1 → Cpx2 + Hc + PlGrt → Opx2 + Hc + Pl.

Fluid inclusion thermobarometryThe reactions proceeded in the presence of a CO2-Composition of the magmatic fluidsdominated fluid, as indicated by high-density CO2 in-In addition to the occurrence of CO2-fluid inclusions,clusions in hercynitic spinel.the presence of a CO2-dominated fluid phase is evidencedSuitable mineral barometers are not available for theby FTIR analyses of carbonate-rich homogeneous sca-mafic granulites, and can give only maximum pressurepolite2 porphyroclasts [Sachs & Hansteen (1996), un-values; this is >950 MPa for the garnet pyriclasitespublished data]. We have no direct evidence for the(Loock et al., 1990).presence of H2O as a major component in the fluidThe temperatures obtained from mineral thermometry,inclusions. In the E1-type rocks, however, the activity ofhowever, apparently correlate positively with the degreeH2O during overprinting with silicate melt was obviouslyof recrystallization of plagioclase, and provide evidencehigh enough to stabilize the hydrous phases amphibolethat the recorded temperatures reflect the conditions ofand biotite. But even texturally early inclusions in am-plagioclase recrystallization.phibole contain no detectable water, at variance withLate-stage coronas around garnet in some meta-ig-

neous xenoliths consist of glass+ crystals (sizesΖ1 lm; experimental results showing that diffusive hydrogen loss

Fig. 2. Photomicrographs. (a) Gabbroic cumulate K9/34, showing clinopyroxene in phonotephritic interstitial glass. The clinopyroxene containstephritic glass inclusions (GI). (b) Garnet–sillimanite gneiss K4/10, polarized light, showing two sillimanite inclusions in the core of a garnetporphyroclast. One sillimanite contains a spherical inclusion of hercynitic spinel (black). The garnet core also contains inclusions of quartz. (c)Garnet pyriclasite K5/75, a type E1 xenolith. Orthopyroxene and garnet are partially replaced by a corona of pargasite. The xenoliths havehighly variable corona sizes. The blackish corona around garnet is probably due to heating after incorporation of the xenolith by the hostmagma. (d) Meta-pyroxene hornblendite K9/29, showing a type E1 reaction: pargasite replaces older clinopyroxene. Close to the grain boundary,high-density CO2 fluid, glass and amphibole inclusions coexist in clinopyroxene. (e) Same sample as in (d). Pure CO2 inclusions (FI, partlydecrepitated; FTIR, no water detected) and cogenetic glass inclusions (GI), consisting of brownish, partially altered, OH-bearing glass and alarge vesicle filled with CO2. (f ) Sample K5/58, showing three primary, vesicular glass inclusions in a scapolite porphyroclast (Mei68Ma32) andreplacement of clinopyroxene by pargasite. The vesicles in the glass inclusions are filled with high-density liquid CO2. The occurrence of primaryglass inclusions indicates hypersolidus conditions during scapolite formation. (g) Sample K5/58, showing porphyroclastic recrystallization andreplacement structures. Clinopyroxene (neoblasts) is replaced by pargasite (neoblasts), followed by late-stage formation of scapolite (scapolite2,homogeneous variety; porphyroclasts and neoblasts). (h) Sample K9/27, showing hercynitic spinel and plagioclase + orthopyroxene (light),which have completely replaced garnet. The matrix consists of pargasitic hornblende and minor clinopyroxene and plagioclase.

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scapolite2, NaCl/(NaCl + H2O) of the coexisting fluiddecreased during the growth of the porphyroclasts. Thisis accompanied by a similar but more pronounced de-crease of bromine, which can be expected to have ageochemical behaviour similar to chlorine.

In summary, the CO2-dominated fluids in questionmust have contained some H2O and additionally sig-nificant but unknown amounts of chloride. The activityof chloride decreased during the metasomatic stage.

Fluid inclusion densities

To compensate for the expected water loss from the fluidinclusions, we calculated 10 mol % water back into theCO2-fluid inclusions using the assumption that the vol-umes of the inclusion cavities did not change after in-clusion formation:

DCorr= DMeas+MwH2O/9V ′CO2

where DCorr and DMeas are the corrected and measuredinclusion densities, V ′CO2 is the measured molar volumeof CO2 in the inclusion and MwH2O is the molecularweight of H2O.

The corrected densities are 4·5% higher than thosefor the corresponding pure CO2 inclusions. We believeFig. 3. Mineral thermometry. Temperature distributions for clino-

pyroxene–orthopyroxene and garnet–biotite pairs. (See Table 1 for that the corrected compositions and densities provide theabbreviations.) best first-order estimate of the fluids originally trapped

during inclusion formation. We use such corrected in-clusion densities for the further thermobarometric con-from fluid inclusions at high temperatures is probablesiderations.(e.g. Bakker & Jansen, 1991). CO2+H2O fluid inclusions

The homogenization temperatures and correspondingcan thus selectively release considerable amounts of theirdensities of fluid inclusions are shown in Fig. 4a and b.H2O content. For example, growth imperfections alongOur fluid inclusion data comprise both texturally earlyhealed cracks and lattice defects formed during por-and late inclusions in each sample. The histograms canphyroclastic recrystallization provide possible routes forbe subdivided into three groups, as follows.fluid transport. Bakker & Jansen (1991) demonstrated

Group A (samples K9/34, K5/54, K5/71) is char-that secondary CO2+ H2O fluid inclusions in quartzacterized by a sharp density maximum correspondingoriginally containing 20 mol % CO2 increased their CO2within the analytical error to the broad density maximumcontents up to 54 mol % during 1 month when the ex-for the cumulate sample K9/34 at 0·82–0·89 g/cm3.ternal pressure was decreased from 200 to 100 MPa atThis density maximum for K9/34 includes both primarya temperature of 850 K. Similarly, water is expected as anand secondary inclusions in clinopyroxene, where theoriginal component in the fluids that caused metasomaticsecondary fluid inclusions coexisting with glass inclusionsoverprinting of the xenoliths. Experiments on the de-have the higher densities of 0·85–0·89 g/cm3. K5/71gassing of CO2 and H2O from basanitic and nepheliniticadditionally contains fluid inclusions with densities in themelts, i.e. of compositions similar to the host melts of therange 0·90–1·05 g/cm3. Primary group A inclusionsxenoliths, show that the fluid phase at the expectedoccur in plagioclase, clinopyroxene, scapolite (Scp1) andpressure of about 650 MPa should have an XH2O=H2O/in corona amphibole around garnet.(CO2 + H2O) close to 0·1 (Dixon, 1997). Provided that

Group B (samples K5/47, K5/31, K5/58) shows athe fluid inclusions in the xenoliths originated fromsimilar maximum of secondary inclusions as group A,mafic alkaline magmas, this line of evidence also stronglybut at slightly higher densities of 0·91–0·97 g/cm3.indicates that the fluid inclusions have lost significantK5/47 additionally contains texturally early fluid in-amounts of H2O after entrapment.clusions with densities of 0·98–1·05 g/cm3. In sampleScapolite is a sensor of the activity of NaCl in the fluidK5/58 [Cpx + Am + Scp2 + Ap + Spl (Mag–Hc)phase (e.g. Ellis, 1978; Mora & Valley, 1989). Derived+ Po + liq (fluid + silicate melt)], the main densityfrom the nearly constant eqivalent anorthite and S con-

tents, and on the decrease of Cl from cores to rims of maximum of secondary inclusions is practically identical

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Fig. 4.

to densities of primary inclusions in amphibole neoblasts. (corresponding to 1·07 g/cm3 for the pure CO2 fluid)(Fig. 4b).The porphyroclast cores of K5/58 additionally contain

In some samples, a small additional maximum ofinclusions with densities slightly higher than the inclusionspartially decrepitated inclusions occurs at lower densitiesin the neoblasts.of between 0·55 and 0·75 g/cm3.Group C consists of the garnet–sillimanite gneiss

K4/10 and the spinel–garnet–pyroxene amphiboliteFluid inclusion pressures and depths of originK9/27. Compared with the densities of secondary in-

clusions in the cumulate sample K9/34, the density Isochores for fluid inclusions containing 90 mol % CO2

maximum of secondary and primary inclusions in am- and 10 mol % H2O were calculated using the modifiedphibole and plagioclase show a strong shift towards higher Redlich–Kwong equation of state of Kerrick & Jacobsdensities of 0·99–1·02 g/cm3. Early inclusions in garnet (1981). The isochores calculated using such corrected

densities yield pressures 14–19% higher for a givencores of sample K4/10 have densities up to 1·12 g/cm3

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Fig. 4. Fluid inclusion data. Microthermometric measurements of CO2-dominated inclusions in various mineral phases. (a) Homogenizationtemperatures. The data represent homogenization temperatures of the phase transition liquid–gas to liquid [Th(l)]. (b) Densities calculated fromhomogenization temperatures assuming that the inclusions originally contained 10 mol % H2O. Stippled bars represent examples of formationtemperatures of fluid inclusions in the metamorphic xenoliths, assuming they were trapped at a pressure of 650 MPa. A, B and C indicate groupA, B and C inclusions.

temperature in the range of interest as compared with is 0·85–0·89 g/cm3. If we assume a temperature of1200°C for the host magma, the secondary inclusionsisochores calculated for the pure CO2 fluids. Addition of

up to 6 molal NaCl solutions to the CO2-dominated fluids would have been trapped at a pressure of 650± 50 MPa,which we interpret to represent the pressure of a magmainstead of pure water would lead to slightly increased

isochore pressures of between 0 and 1% relative (Brown chamber at 22–25 km depth (Fig. 5). The most importantfeature of group A secondary fluid inclusions is that the& Lamb, 1989; Joyce & Holloway, 1993), which is

insignificant for the thermobarometric results. density maxima overlap completely with the densities ofinclusions in clinopyroxene of the cumulate K9/34. WeThe density maximum for secondary fluid inclusions

coexisting with glass inclusions in the cumulate K9/34 therefore assume that all group A fluids had nearly the

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distribution of fluid inclusions in the metamorphic xeno-liths can best be explained in terms of wall-rock reactionsclose to a magma chamber, i.e. by isobaric heating.

Using the assumption that the pressure was fixed at650 MPa, the highest density of the texturally earlyinclusions within the samples K5/71, K5/47, K5/58and K9/27 translates into isochore temperatures between660 and 760°C (Fig. 4b). This is very similar to thetemperature of 650°C obtained from Grt–Bt ther-mometry of mineral cores. The density maxima for groupB and C fluid inclusions would in the isobaric heatingcase correspond to temperatures about 250–400°C lower,respectively, than the magma temperature. Similarly,fluid inclusions having densities even higher than thedensity maxima in samples K5/71, K5/47 and K5/58can most simply be explained by assuming that thetexturally late inclusions were formed at temperaturesabout 200–500°C higher than the early inclusions. Heat-ing during neoblast formation is also reflected by mineral

Fig. 5. Pressure–temperature–time (P, T, t) paths derived from thermo- thermometry.barometry. The shaded bars indicate P, T conditions inferred for the The small maximum of partially decrepitated inclusionsmagma reservoir and the xenoliths. Relative positions and thickness of

in most samples at densities of between 0·55 and 0·75the bars correspond to the frequency maxima of the homogenizationtemperatures, Th(l), in Fig. 4. E, early; L, late; P, primary; S, secondary g/cm3 can at best be explained as due to the syneruptiveinclusions. Isochores are for inclusions containing 90 mol % CO2 and pressure release of the xenoliths.10 mol % H2O. TM (shaded area), estimated host magma temperature.EMD, range of most dense, early inclusions of all meta-igneous xeno-liths. The EMD is considered to represent starting temperatures of the

Trapping mechanisms for the late fluid inclusionsgranulites before heating. The starting temperatures are similar to thoseobtained from mineral rim thermometry (light shaded vertical bar, Tx). The assumption that Group A fluids had nearly hostK5/58 indicates homogenization temperatures of primary inclusions magma temperature during the entrapment process doesin neoblasts and of secondary inclusions in porphyroclasts. Partial

not necessarily implicate a similar temperature for thedecrepitation of many fluid inclusions occurred at 250–450 MPa.host crystals of the inclusions. In our case, the preservationof the primary density of the fluid is possible only if thetime to isolate the fluid from its environment,same composition and temperature during entrapment.Dt(isolation), is considerably shorter than that necessarySuch high temperatures are supported by the observationto cool the fluid to the ambient temperature,that the fluid inclusions are usually associated with,Dt (equilibration), i.e. Dt(isolation) p Dt(equilibration).apparently cogenetic, glass inclusions. The occurrence ofSuch a process is probably possible only within a smallprimary group A fluid inclusions in late phases in thedistance from the host magma, and if silicate melt isxenoliths (amphibole and hercynitic spinel replacing gar-available in addition to fluid, to support the healingnet and pyroxene; clinopyroxene and orthopyroxeneprocess by sealing as defined by Brenan (1991).replacing pargasite; scapolite) provides first-order evi-

In this scenario, the temperature of the fluid decreasesdence that the phases originated by fluid and silicatequickly after fluid migration into mineral cracks. Thus themetasomatism from the magma chamber.densities of the inclusions depend on the rate of the crack-The coexistence of glass and fluid inclusions providessealing process. When the sealing rate is fast, the fluid in-strong evidence for a common origin from a CO2-jected from the magma into the wall-rock will cool relativelydominated silicate-rich fluid penetrating cracks that werelittle until complete isolation of the inclusions occurs. Theopened towards the host magma. We suggest that theresulting inclusions would thus have a lower density thancracks were formed as a result of the intrusive processthose formed if the sealing rate was slow, at which the in-of the host magma, driven by the temperature differencejected fluid would cool further before complete isolation ofand by viscous forces acting between the magma andan inclusion from its environment. A final possibility is thatthe wall rock.the fluids can equilibrate thermally with their host crystalsbefore sealing: Dt(isolation)[ Dt(equilibration).

Isobaric heating This model does not require that the total rock was attemperatures close to that of the host magma during in-We use a combination of mineral-pair thermometry

and fluid inclusion barometry to show that the density clusion formation. Such high temperatures would lead to

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high degrees of melting for the mafic granulites, i.e. a melt reflect the granulite suite before the onset of modalmetasomatism. Type P meta-igneous rocks show a trendfraction of more than>30 wt % (Springer, 1992), in strong

contrast to the actually observed melt fraction of <1 vol. %. in CIPW normative compositions from plagioclase–olivine websterite toward tholeiitic and alkaline gabbrosThe best illustration of this model is probably the

ultramafic sample K5/58, where the lowest fluid densities and anorthosites. MgO correlates positively with Cr, Ni,Co, FeO, heavy rare earth elements (HREE), Y and Zn,overlap with those in the cumulate K9/34, and the

displacement of the inclusion densities towards higher and correlates negatively with Ga and Al (Fig. 6). Weinterpret the correlations as a protolithic cumulate trendvalues can be explained as due to a temperature decrease

of the fluids by about 280°C from 1200°C to >920°C, involving olivine, spinel, clinopyroxene and plagioclasefractionation. The molar fractions of Mg, XMg = Mg/until isolation of the inclusions (Fig. 4b). Because the prim-

ary inclusions in amphibole neoblasts and the secondary (Fe2+ +Mg), scatter from 0·46 to 0·81, with a frequencymaximum at 0·75. The meta-igneous P-type rocks involveinclusions in amphibole porphyroclasts in K5/58 indicate

an identical entrapment temperature, we infer that the the following compositional features (Figs 6, 7 and 8):(1) samples with low (sample K5/31: MgO = 1·9 wtfluids in the cracks thermally equilibrated with their host

crystals before sealing. In sample K5/58, porphyroclastic %) to intermediate MgO content (up to 13 wt %), andanorthositic and olivine tholeiitic to quartz tholeiiticrecrystallization was apparently contemporaneous with

the formation of secondary inclusions. compositions; e.g. sample K5/52 with MgO = 12·61wt %, XMg = 0·72, SiO2 = 48·7 wt %, K2O = 0·13The only possible exceptions to our model involving

CO2–H2O-fluids are the early inclusions in Gt cores in wt %, TiO2= 0·50 wt %, CaO/Al2O3= 0·85, chondrite-normalized ratios (La/Sm)n = 1·7 and (Gd/Yb)n = 1·5sample K4/10. In this case, the assumption that the fluid

had the same composition as the fluids of the host magma (Fig. 7).(2) MgO-rich members, e.g. the pyroxenitic sampleleads to unrealistically low temperatures of <600°C.

Using the assumption of a pure CO2-fluid at the time K9/30 with XMg= 0·82, MgO= 22·41 wt % and (La/Sm)n = 0·6, (Gd/Yb)n = 1 (Fig. 7). Major elementof formation of the garnet cores under granulite-facies

conditions, a temperature of >650°C is obtained. Both parameters are SiO2 Ζ 53·54 wt %, K2O <0·9 wt %,TiO2 <0·9 wt %, CaO/Al2O3 >1.temperatures contradict our conclusion of a temperature

T >800–1100°C derived from the possible earlier para-The proposed cumulate trends of the protoliths overlapgenesis of Spl+ Qtz associated with the inclusions. Thewith picritic and komatiitic compositions (e.g. Arndt &early inclusions in garnet are therefore possibly relics ofNisbet, 1982) (Fig. 6).an ancient high-pressure stage of the rock.

Compared with N-type MORB (McCulloch &Hornblenditic veinlets (thickness >100 lm to 1 mm)Gamble, 1991; Rollinson, 1995), meta-igneous type Ppreserved in type E1 samples (K9/23, K5/63, K5/65,xenoliths have trace element patterns characterized byK5/71) indicate that a considerable amount of am-higher Th, U and Pb concentrations, and by lower K,phibole-forming matter has been transported alongREE, Sr, P, Zr, Ti, Y and Sc.microfractures. The formation of the veinlets is possibly

related to fragmentation of the wall-rocks, and thus in awider sense to xenolith formation. Clinopyroxenes ad-jacent to the veinlets are partially replaced by pargasitic Enriched (type E) xenolithshornblende and contain numerous inclusions of pargasitic The XMg of type E1 and E2 xenoliths have a frequencyamphibole, which we interpret to have been formed maximum at >0·7, which is similar to the type Psimultaneously with the secondary glass inclusions, i.e. rocks. The xenoliths are characterized by a well-definedthrough healing of microcracks. The relationship between correlation of Fe2O3/FeO with Fe2O3, and a weak cor-the formation of of amphibole inclusions and secondary relation of Fe2O3/FeO with total Fe (Fig. 9). Fe2O3 doesglass and fluid inclusions is demonstrated by fluid and not correlate with MgO. The P-type samples are theOH− and H2O-bearing glass inclusions coexisting with least oxidized. The gabbroic cumulate K9/34 can beamphibole inclusions in clinopyroxene, e.g. in sample interpreted as a hypothetical endmember at high Fe2O3/K9/40 (Fig. 2d and e). FeO. Fe2O3 correlates strongly with V and Ti, providing

evidence that Fe–Ti oxides are the most important Fe3+

phases and that alteration plays no significant role for theoxidation state of the xenoliths, which is also confirmed byMAJOR AND TRACE ELEMENToptical microscopy and by electron microprobe analyses.

GEOCHEMISTRY In general, Fe–Ti oxides of E-type xenoliths are dom-Primitive (type P) granulites inated by Ti-magnetite, whereas P-type xenoliths pre-

dominantly contain rutile or ilmenite. Ilmenite is partlyType P xenoliths, by definition, show little or no evidenceof modal metasomatism. We thus consider them to surrounded by a corona of Ti-magnetite. This provides

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Fig. 6. Major and trace element variation of the xenoliths. The compositions partly overlap with picritic and komatiitic compositions. The P-type xenoliths with the lowest and highest MgO concentrations correspond to the most plagioclase- and pyroxene-rich granulites, K5/31 andK9/30, respectively.

further evidence that oxidation of Fe2+ and not the wt %, compared with 49–54 wt % in pyroxene; Table2 samples K5/71 and K9/29) is reflected by the bulkaddition of Fe3+-enriched matter controlled the oxidation

state of the rocks. Oxidation could have been controlled compositions of the E1 xenoliths partly having lowerSiO2 contents and higher K2O/Na2O ratios than theby a CO2-rich fluid.

Our petrographic observations indicate that type E1 type P rocks (Fig. 9), and by higher K2O concentrations(Fig. 6). The enrichment of scapolite implies enrichmentrocks are enriched in K, OH, F and Cl, and depleted in

Si relative to type P rocks. The replacement of pyroxene of Cl, Br, S and CO32−, and also Rb, Nb, Sr, REE and

Y as seen from synchrotron X-ray fluorescence (SYXRF)by secondary amphibole (typical SiO2 contents of 39–46

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Fig. 8. Multielement diagram of representative type E and cumulatexenoliths. The sample concentrations are normalized by the averageconcentrations of type P xenoliths with 0·6 < XMg <0·8.

P rocks, E2-type xenoliths are enriched in Rb, Th, U,Nb, K, REE, Zr, Ti, Y and Fe3+, and are depleted inSr.

The Grt–Sil gneiss K4/10 can be considered to be aFig. 7. Chondrite-normalized REE diagrams (Nakamura, 1974).

E1-type xenolith because of the biotite coronas partiallyreplacing Prp–Alm garnet. Chondrite-normalized REE

analyses (Table 2, sample K5/58). Apatite is strongly concentrations in this meta-sediment (Fig. 7) are char-enriched in REE. acterized by a marked concave-up pattern, indicating a

The replacement of amphibole by pyroxene defining relative enrichment of the LREE and the HREE + Ytype E2 rocks indicates a relative increase in Si, and a as compared with the middle rare earth elements (MREE)loss of K, halogens and OH. (Nd, Sm, Eu). The enrichment of Y and HREE probably

As Fig. 7 shows, type E rocks appear to be enriched reflects earlier melting events and restite formation. Thein REE as compared with P-type rocks. The REE com- enrichment of Zr, Y, Sr, Rb, V and Zn in rims relativepositions of type E2 rocks overlap with the host magma to cores of the garnet porphyroclasts (SYXRF analyses,cumulates. To better visualize the typical compositional Table 2, sample K4/10) in K4/10 is probably cogeneticdifferences between the P- and E-type rocks, we use a with E1 metasomatism and with the formation of biotitemultielement diagram in which the E-type rocks are coronas around garnet.normalized to the most typical composition of our P-type rocks (Table 4, Fig. 8), with mg-numbers 0·6 < XMg

<0·8.DISCUSSIONThe following distinct compositional trends can beChemical evolution of the xenolithsobserved. E1-type xenoliths show an enrichment in in-

compatible elements Rb, Th, U, Nb and K. Sr and P We want to test the hypothesis that the compositions ofE-type xenoliths are genetically related to the host magmaare relatively depleted. The more compatible elements

Y, Yb, Cr, Mn, Co, Ni and Zn show no or little increase chamber. This can be tested by comparing elementpairs having similar bulk distribution coefficients duringrelative to average P-type compositions. Relative to type

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because the cumulates contain abundant clinopyroxene,Fe–Ti oxides, amphibole, ± biotite and apatite, whichare also the main constituents of the metamorphic xeno-liths. Therefore melts and fluids released from the magmawould tend to equilibrate at similar bulk partition co-efficients with both the cumulate crystals and the maficwall-rock phases. Element concentrations in the cu-mulates can therefore be expected to be most similar tothose metamorphic xenoliths that had the most intenseand longest contact with the host magma, probably thetype E2 rocks, because their phase associations generallyrecord higher temperatures of last equilibration than theother xenolith types.

The nearly constant ratios Y/Zn vs Zn (Fig. 10) andY/Co vs Co of P-type and E-type xenoliths indicate thatthese ratios are independent of Zn and Co concentrations.Thus in the presence of the proposed metasomatic agent,the geochemical behaviour of Zn, Co and Y is verysimilar. The absence of a mixing line relating the Zn andCo contents of the cumulate xenoliths to the granulitesindicates that the Y/Zn and Y/Co ratios are controlledby processes that are not related to the host magma ofthe xenoliths. The meta-sedimentary xenolith K4/10defines a separate compositional group, which expressesthe different protolith history. The representation of theratio Y/Cu vs Cu, however (Fig. 10), shows a well-defined trend with a distinct negative slope and with anendmember composition coinciding with the cumulatexenoliths at high Cu contents. A similar trend wouldresult if HREE, Ni, Co or Zn were used as referenceelements. The interstitial glass of the cumulate xenolithK9/34 is characterized by very low Cu and Y contents(Fig. 10). Although some Cu may have been lost by

Fig. 9. Selected major-element ratio plots. (a) E-type xenoliths partly syneruptive degassing at low pressure, as indicated byhave higher K2O/Na2O ratios than the type P rocks, and higher K2O the precipitation of Cu-enriched (sample K9/34: 0·2–1concentrations. The gabbroic cumulate K9/34 is a possible endmember

wt % Cu) sheet silicates at the walls of the vesicles, thisat high K2O/Na2O. (b) The xenoliths are characterized by a well-defined correlation of Fe2O3/FeO with Fe2O3, and a weak correlation confirms our hypothesis that these elements are muchof Fe2O3/FeO with total Fe. The P-type samples are the least oxidized. more abundant in the crystalline phases than in theThe gabbroic cumulate K9/34 can be interpreted as a hypothetical

silicate melt, such that the observed trends represent inendmember at high Fe2O3/FeO.situ compositions of the xenoliths.

Similar to the pair Y/Cu, the representations of thepairs LREE/Pb vs Pb, K/Nb vs Nb, Ba/Th vs Th, Ba/protolith formation by partial melting (of peridotitic sys-U vs U, Ti/Zr vs Zr (Fig. 10) and Ba/Rb vs Rb revealtems) and subsequent crystal fractionation, but will bewell-defined trends with negative slopes, and endmemberaffected by the presence of a metasomatic componentcompositions coinciding with the cumulate xenoliths atoriginating from the host magma reservoir. Thus wehigh Rb, Th, U, Nb, LREE, Zr and Cu. The garnet–chose elements that are characterized by positive orsillimanite gneiss K4/10 falling into these chemical trendsnegative deviations from the average P-type compositionin spite of a different protolith history provides furtherand compared them with reference elements with similar evidence that the element mobilization was controlled

bulk compatibility under fluid-absent conditions. Element by the host magma.pairs in question are Th–Ba, U–Nb, K–Nb, LREE–Pb,Zr–Ti, Zn–Y, Co–Y and Cu–Y. If the metamorphic

The role of fluid phases duringxenoliths were affected by the host magma, we wouldmetasomatism of the wall-rocksexpect a mixing line in bivariate plots, where the host

magma cumulates (samples K9/25 and K9/34) should Our chemical data strongly indicate that E1 andE2 metasomatic processes are related to the compositionsdefine an endmember composition. This can be expected

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JOU

RN

AL

OF

PET

RO

LO

GY

VO

LU

ME

41N

UM

BE

R3

MA

RC

H2000

Table 4: Selected whole-rock analyses of major (wt %) and trace elements (lg/g)

Sample: K9/25 K9/34 K4/10 Average of K9/30 K5/31 K5/52 K5/71 K5/65 K5/64 K5/62 K9/27 K9/35 K5/70 K9/32 K9/23 K9/22 K9/2111 samples

Rock type: Cumulate Cumulate Grt–Sil- ‘Typical’ P P P P E1 E1 E1 E1 E1 E1 E1 E1 E2 E2gneiss, E1 P, SD

wt %SiO2 41·68 47·00 45·00 53·54 54·9 48·7 50·21 48·03 47·24 42·00 43·34 45·23 47·01 46·97 46·12 41·18 43·19TiO2 3·51 2·35 1·31 1·01±0·52 0·36 0·8 0·5 0·48 1 0·31 3·33 0·37 2·06 0·71 0·68 1·325 1·735 3·37Al2O3 11·63 14·53 22·7 6·01 22·83 12·75 10·87 10·73 16·91 16·31 17·59 10·5 13·31 6·59 14·18 10·43 9·49Fe2O3 2·77 6·22 1·55 2·11±1·38 0·75 0·6 1·22 0·0 1·76 0·75 6·42 1·54 2·41 0·98 2·51 1·13 6·77 4·48MgO 12·79 4·64 4·8 22·41 1·91 12·61 14·96 13·47 11·78 7·66 14·23 14·37 13·86 19·13 14·32 10·36 13·61FeO 5·43 4·24 13·08 9·53 4·02 8·95 7·14 6·74 6·4 9·29 7·43 11·16 7·86 8·95 8·77 7·17 9·71MnO 0·10 0·23 0·39 0·15±0·02 0·22 0·09 0·18 0·16 0·36 0·12 0·21 0·13 0·21 0·17 0·20 0·14 0·27 0·18CaO 15·70 14·22 4·28 5·64 7·69 10·84 13·33 14·19 13·65 10·89 10·92 10·10 13·38 12·71 10·62 15·94 10·80Na2O 0·74 3·35 2·69 0·95 4·82 1·62 1·12 1·82 1·13 1·93 2·36 1·60 1·18 0·94 1·84 2·01 2·20K2O 2·76 0·65 0·61 0·13±0·06 0·08 0·35 0·13 0·06 0·25 0·05 0·10 0·24 0·30 0·57 0·19 0·14 0·46 0·64P2O5 0·15 0·73 0·23 0·36±0·15 0·09 0·15 0·26 0·34 0·23 0·19 0·14 0·16 0·23 0·26 0·14 0·12 0·79 0·33Sum 97·26 98·16 96·64 99·58 98·16 97·75 98·67 98·58 98·53 98·28 98·31 98·17 99·29 99·01 98·71 97·11 98·00XMg 0·81 0·66 0·40 0·6–0·8 0·81 0·46 0·72 0·79 0·78 0·77 0·60 0·77 0·70 0·76 0·79 0·74 0·72 0·71lg/gS 309 697 52 200 40 326 56 115 133 291 34 57 87 60 62 164 89Ga 20 21 22 8 23 14 15 11 14 22 12 15 14 11 13 20 18As 3 7 33 4 11 6 7 6Sc 37·2 20 69·9 33±6·3 24 36·3 39·4 44·7 30·1 37 11·7 25·8 41·4 46·3 29·1 42·4 29·5V 317 344 229 254±96 116 90 208 156 223 100 629 78 293 180 184 237 402 342Cr 62 3 220 280±214 1841 599 709 703 532 50 309 1005 295 1612 924 185 709Co 43·7 29 12·5 42±6·5 62·2 12 47·6 45·8 38·1 47·9 42 62 54·2 55·9 68·2 55·9 51·7 62·8Ni 75 27 21 90±36·5 492 33 90 184 121 164 67 338 366 184 332 339 128 480Cu 130·5 73 6·4 32±10·5 43·7 13 34 23·2 18·2 23·9 145 3·1 9·1 5·2 20 55 78·6 18·2Zn 95·5 104 48·4 78·6±25·5 74·2 50 97·6 50·3 55·7 44·7 99 39·7 123·4 54·6 71·3 63·5 130·4 134·3Rb 5 7 1±0 1 1 3 1 1 1 4 2 1 2 4Sr 488 1810 410 442±176 123 839 355 661 242 385 393 135 185 86 154 135 384 111Y 22·6 20 178·7 15·1±5·3 5·2 13·8 10·5 16·7 6·4 12 5·8 25·9 13·7 16 17·5 22 49·4Zr 223 387 23 38·9±16·1 11 705 34 21 37 13 28 13 42 42 68 40 185 47Nb 31 197 16 2±0·7 1 3 2 3 3 2 16 5 3 4 114 55Ba 242 560 442 137±53 76 381 107 280 105 133 148 91 80 107 183 38 221 117La 34·4 107 33·9 7±0·7 3·38 45 7·63 7·36 7·58 4·39 3·65 13·5 6·17 10·8 2·37 30·3 23·3Ce 84·2 174 54·5 16·3±1·3 6·39 49 18·2 15·6 18·7 7·4 36 6·9 37·7 14·5 25·2 6·9 94·6 73·2Pr 12·2 44 5·61 0·97 2·81 2·18 3·05 1·02 0·93 6·15 2·07 3·55 1·35 12·71 11·45Nd 50·5 96 20·0 10·5±1·1 3·97 12·2 9·2 14·5 5·1 4·26 28·7 9·31 15·8 7·71 53·6 53·5Sm 9·21 5·96 1·01 2·87 2·14 3·68 1·27 1·03 6·8 2·46 3·46 2·53 9·98 12·6Eu 2·55 2·41 0·35 101 0·79 0·68 1·11 0·56 0·51 1·96 0·75 0·99 0·95 2·74 3·1Gd 7·4 13·2 1·02 2·9 2·19 3·63 1·35 1·16 6·42 2·7 3·47 3·08 7·98 11·6Tb 1·04 3·4 0·18 0·46 0·35 0·6 0·22 0·19 0·98 0·46 0·54 0·53 1·08 1·83Dy 5·36 28·3 1·02 2·79 2·06 3·57 1·36 1·19 5·58 2·77 3·27 3·39 5·31 10·6Ho 0·96 6·91 0·22 0·56 0·42 0·7 0·27 0·25 1·03 0·55 0·63 0·69 0·92 1·97Er 2·52 22·8 0·61 1·65 1·16 1·93 0·73 0·67 2·83 1·53 1·81 1·96 2·4 5·45Tm 0·33 3·47 0·1 0·23 0·16 0·27 0·1 0·1 0·38 0·21 0·25 0·27 0·3 0·75Yb 2·11 24·8 1·2±0·3 0·63 1·61 1·12 1·69 0·62 0·64 2·47 1·32 1·62 1·75 1·88 4·82Lu 0·69 3·72 0·1 0·23 0·16 0·24 0·08 0·1 0·34 0·18 0·23 0·25 0·27 0·66Th 2·63 1·9 0·07±0·03 0·1 0·07 0·12 0·2 0·39 0·28 0·35 0·54 0·49 0·08 1·87 0·8U 0·44 0·35 0·06±0·02 0·06 0·04 0·08 0·1 0·4 0·3 0·07 0·1 0·21 0·04 0·3 0·13Pb 1·75 11·3 2·1±0·3 1·86 2·37 1·85 2·98 1·12 2·51 1·07 1·24 2·95 0·5 1·2 1·71Mo 1 1 2 1 1 1 1

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Fig. 10. Bivariate ratio plots of trace element concentrations (lg/g). The nearly constant ratios Y/Zn for P-type and E-type xenoliths indicatethat these ratios are independent of Zn concentrations. The absence of a mixing line relating the Zn contents of the cumulate xenoliths to thegranulites indicates that the Y/Zn ratios are controlled by processes that are not related to the host magma of the xenoliths. All other plotsindicate compositional trends toward a hypothetical endmember coinciding with the compositions of cumulate xenoliths K9/34 and K9/25.The P-type xenoliths define the other endmember. Xenoliths with compositions most similar to the cumulates are of type E2. The metasomatismof the metamorphic rocks could thus have had the same source as the cumulates. In the K-representation sample K9/25 was omitted becauseK was significantly enriched as a result of crystallization of phlogopite. MG, phonotephritic matrix glass of the cumulate K9/34, analysed bySYXRF.

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of the cumulate xenoliths and thus to the host magmas pressure interval of 650 ± 50 MPa, corresponding to aformer magma chamber at a depth of 22–25 km. Theby mixing curves. We can distinghuish two types ofrarity and the small sizes of peridotite xenoliths at Kem-chemical plots where (1) The P-, E1- and E2-type xeno-penich can be interpreted as the result of settling and/liths are separated into distinct compositional fields, andor assimilation of mantle xenoliths in this or a deeper(2) The P-type compositions overlap completely with E-reservoir (e.g. Sachs & Stange, 1993).type compositions.

The pronounced density maxima of fluid inclusions,Group 1 is represented by Nb, Zr, LREE, Th and U.giving constant pressure and overlapping temperatureP-type xenoliths have the highest Ba/U, Ba/Th, K/Nb,ranges for the granulite xenoliths, indicate that they arePb/LREE and Ti/Zr ratios. E2-type xenoliths have thepieces of fragmented wall-rocks of the magma chamber.lowest ratios and have concentrations of U, Th, Nb,Apparent equilibration temperatures derived from clino-LREE and Zr similar to the host magma cumulates. Thepyroxene–orthopyroxene and garnet–biotite rim com-decrease of the ratios from P to E2 xenoliths indicatespositions coincide with temperatures inferred fromthat the addition of high field strength elements (HFSE)microthermometry of texturally early fluid inclusions andU, Th, Nb, LREE and Zr was more important than thethus probably reflect the temperature field around theaddition of large ion lithophile elements (LILE) andintrusion.Ti during the progression from P to E1 and

This is supported by the appearance of compositionalE2 metasomatism. Probably, elements were enriched intrends relating the granulites to the cumulate xenoliths,the granulites through interaction with a silicate melt.demonstrating that the granulites belong to a geneticallyGroup 2 is represented by Cu, where the P-typeuniform rock unit, which may be relatively small. Wecompositions widely overlap with E-type compositions.therefore suggest that similar trends in other mafic gran-This provides evidence for a Cu-enrichment process thatulite xenolith suites might be indicative of an origin fromis decoupled from silicate metasomatism. We thereforea limited depth range, and that such xenoliths representinfer that significant amounts of Cu were preferentiallywall-rocks of a magma stagnation zone. By analogy, otheradded to the P-type xenoliths through the action of alower-crustal xenolith suites also may represent wall-fluid phase.rocks from magma chambers, as suggested by HansteenThe observation that P-type xenoliths have the highestet al. (1998) for the Canary Islands and Iceland.and E2 xenoliths the lowest ratios Pb/LREE, K/Nb,

Neoblasts in porphyroclastically recrystallized ultra-Rb/Ba is compatible with the hypothesis that the amountmafic xenoliths, containing primary glass inclusions, pro-of melt added to the P-type rocks was minimal and thatvide evidence for ductile deformation under hypersolidusa chlorine-bearing fluid has added Cu to the P-typeconditions, i.e. during formation of fluid and melt in-rocks. Such a fluid will be able also to dissolve andclusions. We thus infer that recrystallization is con-preferentially mobilize Pb, K and Rb, more than Ce,temporaneous with the formation of the host magmaNb and Ba, if no silicate melt is present (Keppler &chamber, and possibly was caused by the stress fieldWyllie, 1991; Keppler, 1993, 1996; Kravchuk & Keppler,of the intruding magma, as suggested for lower-crustal1994). The presence of the Cl- and Br-rich phases sca-gabbro complexes in the Ivrea zone (Sinigoi et al., 1994).polite (Table 2, sample K5/58), amphibole (Table 2,During deformation, the growing neoblasts and crackssamples K5/71 and K9/29), apatite and biotite indicatesin the porphyroclasts took up fluids that originated fromthat halogens were important during metasomatism.the magma chamber.An overprinting of the wall-rocks of the host magma

Our model agrees with a model of lower-crustal magmachamber by a CO2- and halogen-bearing fluid is inreservoirs in the Eifel postulated by Schmincke (1977)agreement with the hypothesis of, for example, Frost &and Duda & Schmincke (1985), and is supported byFrost (1987), who, on the basis of experimental data,geophysical observations (Fig. 11). Beneath Kempenich,suggested that additionally to CO2, fluids exsolved fromthe seismic P-wave velocities between the depth ofa mafic magma in the lower crust will also be enriched>18 km and the Moho at>30 km are anomalously lowin alkali chloride.at 6·25 km/s (Mechie et al., 1983; Raikes & Bonjer, 1983),indicating the occurrence of either felsic rocks or maficrocks at high temperatures (Mengel et al., 1991).

Cenozoic evolution of the lower continental P-wave velocities north of Kempenich (6·1–6·4 km/s)crust between 10 and 22 km depth probably reflect the velocityOn the basis of barometry of CO2-dominated fluid in- structure of the Eifel crust before the volcanism. Theclusions, we can conclude that the cumulate xenoliths velocities probably indicate the presence of amphibolite-originate from the same depth as the metamorphic xeno- facies gneisses of granodioritic and tonalitic compositionliths. Assuming a magma temperature of >1200°C, (Mengel et al., 1991). The velocity increase to 6·7 km/s

at a depth of 22 km defines the Conrad discontinuity.the primocrysts of the cumulates crystallized within the

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Fig. 11. Model of the Pleistocene lithosphere beneath the Kempenich–Engeln area. Present-day seismic P-wave velocity structures beneath andnorth of the East Eifel volcanic field, based on refraction seismic survey data and teleseismic tomography, are compared with inferred crustalcompositions. The residuals give the time delay in percent of teleseismic signals relative to a reference model. The depth distribution of seismicenergy inferred from hypocentre distribution of earthquakes (from Langer, 1990) indicates the position of a ductile region immediately abovethe Conrad discontinuity. The Conrad discontinuity separates an upper-crustal layer, consisting of preferentially ductile amphibolite-faciesgranodioritic and tonalitic gneisses, and more brittle lower-crustal granulites. The brittle–ductile transition appears to be a preferred level ofmagma stagnation.

Thus, the xenolith suite originates from the uppermost (1) fluid inclusion barometry using histogram densitymaxima indicates that the granulite xenoliths from Engelnlower crust immediately below the Conrad discontinuity.

Studies of hypocentre distributions of earthquakes north were incorporated into the host magma at similar levelsto those at which the cumulate xenoliths crystallized, andof the Kempenich area indicate that this region cor-

responds to an aseismic zone that extends between 16 thus represent wall-rocks of a magma reservoir withinthe Pleistocene crust at 22–25 km depth (650± 50 MPa).and 24 km depth, meaning that the zone comprises the

lowermost part of the amphibolite-facies gneiss layer. (2) A depth of 22 km corresponds to the positionof the Conrad discontinuity. The Conrad discontinuityThe aseismic zone underlies a zone of high earthquake

activity, which characterizes the brittle upper crust. At a separates an upper-crustal layer, consisting probably ofdepth of 24–28 km, a second but smaller activity max- preferentially ductile amphibolite-facies granodioritic andimum indicates the presence of a further brittle layer in tonalitic gneisses, and more brittle lower-crustal gran-the lower crust (Langer, 1990). The brittle material could ulites. The brittle–ductile transition appears to be apossibly be composed of mafic granulites similar to the preferred level of magma stagnation.granulite xenoliths. (3) Fluid inclusions with densities higher than those in

The aseismic zone provides evidence that the rocks the cumulate xenoliths correspond to lower temperaturesare preferentially plastically deformed, i.e. the shearing than the host magma. The occurrence of high-densitystrength is controlled by the dynamic viscosity. A relative fluid inclusions in several xenoliths thus provides evidencedecrease of the shear strength could be the reason why for in situ heating of between 150 and 400°C, possiblythe magmas intruded at this depth. The ductile–brittle as a result of local heating induced by percolating fluidstransition is probably the most efficient barrier against released from the magma chamber.magma ascent. (4) By combining thermobarometry of fluid inclusions

with geochemical and petrological investigations, we haveshown that cumulate xenoliths may originate from the

CONCLUSIONS same reservoir as the metasomatic fluids that have over-printed the source rocks of the granulite xenoliths. Meta-We derive the following conclusions from this study of

granulite xenoliths: somatism is a texturally late-stage process and includes

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Cox, K. G. (1993). The Karoo province of southern Africa: origin of‘amphibolite’-forming hydration reactions (enriched typetrace element enrichment patterns. In: Hawkesworth, C. J. & Norry,E1: formation of amphibole and biotite) and ‘granulite’-M. J. (eds) Continental Basalts and Mantle Xenoliths. Nantwich: Shiva,forming dehydration by breakdown of volatile-bearingpp. 139–157.phases, in particular of amphibole (enriched type E2). Dixon, J. E. (1997). Degassing of alkalic basalts. American Mineralogist

82, 368–378.Duda, A. & Schmincke, H.-U. (1985). Polybaric evolution of alkali

basalts from the West Eifel: evidence from green-core clinopyroxenes.ACKNOWLEDGEMENTS Contributions to Mineralogy and Petrology 91, 340–353.

Ellis, D. E. (1978). Stability and phase equilibria of chloride andThanks are due to H.-U. Schmincke for initiating thiscarbonate bearing scapolites at 750°C and 4000 bar. Geochimica etproject. Discussions with H.-U. Schmincke, K. Hoernle,Cosmochimica Acta 42, 1271–1281.

A. Gurenko and E. Harms are gratefully acknowledged. Frost, B. R. & Frost, C. D. (1987). CO2, melts and granulite meta-Reviews by H. Downes, E.-R. Neumann, H.-G. Stosch morphism. Nature 327, 503–506.and B. Upton helped improve the manuscript. ICP-MS Furlong, K. P. & Fountain, D. M. (1986). Continental crustal under-

plating: thermal considerations and seismic–petrological con-analyses were performed by C. D. Garbe-Schonberg,sequences. Journal of Geophysical Research 91(B8), 8285–8294.and F. Lechtenberg is thanked for his support during

Garbe-Schonberg, C. D. (1993). Simultaneous determination of thirty-SYXRF analyses. This research was funded by theseven trace elements in twenty-eight international rock standards byDeutsche Forschungsgemeinschaft through Grants SchmICP-MS. Geostandards Newsletter 17(1), 81–97.

250/41-1/2 and Schm 250/47, and the Volkswagen Hansteen, T. H., Andersen, T., Neumann, E.-R. & Jelsma, H. (1991).Foundation (Grant I/68 581). Fluid and silicate glass inclusions in ultramafic and mafic xenoliths

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Lechtenberg, F., Garbe, S., Bauch, J., Dingwell, D. B., Freitag, J.,Bucher, K. & Frey, M. (1994). Petrogenesis of Metamorphic Rocks, 6th edn.Haller, M., Hansteen, T. H., Knochel, A., Radtke, M., Romano,Berlin: Springer, 318 pp.C., Sachs, P. M., Schmincke, H.-U. & Ullrich, H. J. (1996). The X-Burke, E. A. J. & Lustenhouwer, W. J. (1987). The application of aray fluorescence measurement place at beamline L of HASYLAB.multichannel laser Raman microprobe (Microdil 28) to the analyses

of fluid inclusions. Chemical Geology 61, 11–17. Journal of Trace and Microprobe Techniques 14(3), 561–587.

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Le Maitre, R. W., Bateman, P., Dudek, A., Keller, J., Lameyre, C. W., Synchrotronstrahlungslabor HASYLAB am Deutschen Elek-tronensynchrotron DESY, pp. 985–986.Le Bas, M. J., Sabine, P.A., Schmid, R., Sørensen, H., Streckeisen, A.,

Woolley, A. R. & Zanettin, B. (1989). A Classification of Igneous Rocks Sachs, P. M. & Stange, S. (1993). Fast assimilation of xenoliths inmagmas. Journal of Geophysical Research 98, 19741–19754.and Glossary of Terms. Oxford: Blackwell.

Loock, G., Stosch, H.-G. & Seck, H. A. (1990). Granulite facies lower Schmincke, H.-U. (1977). Eifel-Vulkanismus ostlich des Gebietes Rie-crustal xenoliths from the Eifel, West Germany: petrological and den-Mayen. Fortschritte der Mineralogie 55, 1–31.geochemical aspects. Contributions to Mineralogy and Petrology 105, 25–41. Schmincke, H.-U., van den Bogaard, P. & Freundt, A. (1990). Quaternary

McCulloch, M. T. & Gamble, J. A. (1991). Geochemical and geo- Eifel Volcanism. IAVCEI International Volcanological Congress,dynamical constraints on subduction zone magmatism. Earth and Mainz, 1990. Witten: Pluto Press, 188 pp.Planetary Science Letters 102, 358–374. Sinigoi, S. S., Quick, J. E., Clemens-Knott, D., Mayer, A., Demarchi,

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Rollinson, H. (1995). Using Geochemical Data: Evaluation, Presentation, Mineralogie, Abhandlungen 144, 29–55.Interpretation. Harlow, UK: Longman, 352 pp. Worner, G., Staudigel, H. & Zindler, A. (1985) Isotopic constraints on

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Sachs, P. M. & Hansteen, T. (1996). The geotherm and metasomatismin the lower crust beneath the E-Eifel volcanic field/Germany: anapplication of fast kinetics fluid inclusion barometry on granulitexenoliths. Berichte der Deutschen Mineralogischen Gesellschaft. Beihefte zum APPENDIXEuropean Journal of Mineralogy 8, 235. Methods

Sachs, P. M. & Lechtenberg, F. (1997). Synchrotron X-ray fluorescenceThe samples were prepared as polished thin and doubly(SYXRF) analysis of the international standards SY-3, JB-2, JF-2,

NIM-G and NIM-S. HASYLAB Annual Report. Hamburg: Hamburger polished thick sections under petroleum without water

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contact for optical microscopy, scanning electron micro- titration. Precision was better than 15%. Total sulphurwas analysed by an IR photometer, Rosemount CSAscopy (SEM), electron microprobe analyses, ionprobe,5003. Analytical precision is better than 20%.microthermometry, FTIR and SYXRF microprobe.

Trace element compositions in the phases were meas-Mineral analyses were performed on a Cameca SX-ured by SYXRF microprobe at the DORIS positron50 electron microprobe at GEOMAR applying the built-storage facility at HASYLAB/DESY in Hamburg. Ex-in PAP correction procedure (Pouchou & Pichoir, 1984).perimental setup and quantification of the spectra haveAnalytical conditions included an acceleration voltage ofbeen described by Lechtenberg et al. (1996). Detection15 kV, a beam current of 8–20 nA, and counting timeslimits are 0·1–3 lg/g for atomic numbers 21 (Sc) to 26of between 20 and 60 s on peaks. A focused beam was(Co); 0·1–1 lg for 28 (Ni) to 60 (Nd); >5 lg/g for >60used for olivine, pyroxenes and oxides, and a rastered(Nd). Analytical accuracy was checked by analysis ofbeam of 1–40 lm2 for other phases. Natural and syntheticinternational standards (analytical signal Ka; Th and Pb:minerals were used as standards and monitors. AnalyticalLa; Sachs & Lechtenberg, 1997), and are better thanaccuracy is <0·3% for concentrations of >10 wt % and5% at concentrations <100 lg/g, and better than 15%<5% for 0·1–10 wt %.at <10 lg/g.Central pieces of the samples were cut and selected

Microthermometric measurements were performed onfor whole-rock analyses, avoiding cracks and rims affected inclusions in clinopyroxene, amphibole, plagioclase, sca-by melt. Chemical analyses of rocks showing petrographic polite and garnet using Linkam THM 600 and Fluidevidence of melt infiltration along grain boundaries were Inc. heating–cooling stages, which were calibrated usingnot considered. Samples for whole-rock analyses were SYNFLINC synthetic fluid inclusion temperature stand-crushed and powdered in agate ball-mills. Before analysis, ards. Accuracy and precision was estimated at ±0·2°Cthe samples were dried at 110°C. Major and trace near the triple point of CO2 (−56·6°C), and at betterelements were determined by XRF on fused beads using than ±0·4°C at other temperatures. Isochores for thean automated Philips PW1480 spectrometer. All analyses CO2-dominated inclusions were calculated using the com-were performed with a Rh tube; calibration was per- puter program FLINCOR (Brown, 1989) utilizing theformed using international geological reference samples. Kerrick & Jacobs (1981) equation of state for CO2.Trace elements were analysed by inductively coupled Densities of CO2 were derived from Angus et al. (1976).plasma-mass spectrometry (ICP-MS) on a VG Plasma- Compositions of fluid inclusions were qualitatively ana-quad PQ1 at the Geological Institute of the Christian- lysed by FTIR on a Bruker IFS 120 instrument atAlbrechts University in Kiel (Garbe-Schonberg, 1993). Bayerisches Geoinstitut in Bayreuth (H. Keppler). De-

tection limits are >100 lg/g.FeO has been repeatedly analysed by potentiometric

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Pleistocene Underplating and Metasomatism of the Lower

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