EVIDENCE FOR K-FELDSPAR METASOMATISM IN LOWER CRUSTAL MAFIC XENOLITHS A. Montanini (1), D.Harlov (2)...
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0 10 20 30 40 50 60 70 80 mol% An Ab Or EVIDENCE FOR K-FELDSPAR METASOMATISM IN LOWER CRUSTAL MAFIC XENOLITHS EVIDENCE FOR K-FELDSPAR METASOMATISM IN LOWER CRUSTAL MAFIC XENOLITHS A. Montanini (1) , D.Harlov (2) (1) Dipartimento di Scienze della Terra, Università di Parma, [email protected] (2) GeoForschungsZentrum, D-14473 Potsdam, FR Germany, [email protected] 1 mm Ortopyroxene porphyroclast with clinopyroxene exsolution lamellae OPX 6. Concluding remarks 6. Concluding remarks • K-feldspar textures, e.g. microveins and randomly scattered antiperthitic plagioclase grains, suggest a likely metasomatic origin due to solid-state infiltration of K-rich fluids after the granulite facies recrystallization •The occurrence of complex sodic-calcic haloes around Type-3 kfs indicates is consistent with a replacement origin and variation of K /(Na+K) fluid (Griffin (1969) Lithos 2, 171-186). The wide range of albite content of K- feldspar is also consistent with a variable K /(Na+K) fluid •The formation of K-feldspar in the mafic xenoliths reflects metasomatic processes, which operated in the lower crust during in situ high-T recrystallization of relatively dry rocks characterized by refractory Ti- rich biotite. An external K-rich fluid source is therefore required • Alkaline fluids/melts related to the Plio-Quaternary volcanism may be ruled out on the basis of isotopic data. • We may speculate that the agent responsible for the K-feldspar metasomatism was a KCl-rich brine. The fluid, although primarily KCl, possibly contained NaCl and/or CaCl2 brine component, as suggested by the absence of monazite inclusions in apatite (Harlov and Forster, Amer. Rb ppm Sr ppm 87 Sr/ 86 Sr Sm (ppm ) Nd (ppm ) 143 Nd/ 144 Nd NB 324 W R (xenolith) 14.13 284.8 0.706482 3.124 13.795 0.512455 NB324 K-feldspar 57.27 390.8 0.706556 0.026 0.252 0.512483 NB8 W R (xenolith) 19.06 250 0.706455 3.033 13.531 0.512448 NB8 K-feldspar 84.64 384.7 0.706597 0.02 0.211 - Hostlava (basanite) 83.7 1224.2 0.704377 9.09 55.6 0.512578 4. Hydrous and halogen-bearing minerals 4. Hydrous and halogen-bearing minerals Biotite Small amounts of Ti-rich biotite (TiO 2 = 4.7-6.5 wt.%) characterized by low to moderate F (0.24- 1.19 wt.%) and low Cl (0.04-0.20 wt. %) may be present in both K-feldspar bearing and K- feldspar-free rocks. It is in textural equilibrium with the granulite facies assemblage, therefore mainly pre-dating the K-feldspar formation. Amphibole Ti-pargasite (Ti = 0.37, Na A = 0.46, K = 0.26 apfu) almost completely replaced by very fine- grained semi-opaque products (= late-stage heating by the host magma) is a rare mineral in the xenoliths 5. Isotopic constraints 5. Isotopic constraints Sr-Nd isotopic ratios of K-feldspar measured in two samples (Montanini et al., in prep.) are distinct from those of the host lava and very close to the whole-rock ratios. 0.5 mm 0.25 mm 1. Introduction The relatively common occurrence of metasomatic K-feldspar micro-veining in felsic gneisses, and in granulite-facies metabasites, has been interpreted as evidence of migrating alkali-rich fluids (e.g. KCl-NaCl brines) in high-grade terrains which may be responsible for the melt-absent dehydration of high-grade rocks from amphibolite to granulite facies (Harlov and Förster (2002) J Petrol 43, 769). Here we present new data on the occurrence of metasomatic K-feldspar in mafic granulite xenoliths which represent samples from the lower crust of Sardinia brought up to surface by recent (3. 9 Ma) eruptions of alkaline magmas. 2. The samples The xenoliths are quartz-free metagabbronorites characterised by orthopyroxene + clinopyroxene + plagioclase porphyroclasts (An 50-66 ) in a recrystallized matrix composed of pyroxenes + plagioclase (An 56-72 ) + Fe-Ti oxides ± Ti-biotite ± Ti-pargasite ± apatite. Olivine rimmed by spinel-pyroxene symplectitic intergrowths at the olivine-plagioclase interface rarely occurs. Whole-rock compositions show slight LREE enrichment, flat HREE patterns and HFSE depletion. Sm-Nd data on mineral separates are close to the WR present-day ratios, in agreement both with a very young age of the protoliths and equilibration at the high (lower crustal) temperatures recorded by two-pyroxene and opx- biotite thermometry. Calculated V P on the basis of modal composition (7.1 km s -1 ) are in good agreement with those reported for the lower crust of central-western Sardinia by seismic refraction surveys. The xenoliths are meta-igneous rocks originated from underplating of subduction-related mafic liquids which underwent cooling and recrystallisation in the deep crust (T ~ 850-900°C, P ~ 8-10 kbar, Montanini et al., in prep.). 1 mm Recrystallized xenolith with granoblastic polygonal texture CPX OPX OPX PL BT 0.5 mm 0.5 mm Kfs 0.5 mm Plagiocla se 0.5 mm Kfs 0.25 mm 0.5 mm Kfs 3. The K-feldspar Small amounts of K-feldspar (up to 2-3 vol. %) have been found in about 50 % of the examined xenoliths. Texturally, the K- feldspar occurs in a variety of different modes that may be commonly observed within the same sample Type 1. Micro-veins along plagioclase- plagioclase and plagioclase-pyroxene grain rims. Plagioclase close to microveins is usually (but not always) slightly depleted in Na Type 2. Intergrowths with Ca-rich plagioclase along plagioclase- plagioclase grain boundaries. Plagioclase intergrown with K-feldspar is relatively Na-rich, whereas increasing anorthite content in the plagioclase towards the contact with the K-feldspar myrmekite-like intergrowths is observed. Type 3. Rods, blebs, and irregular patches in a random scattering of plagioclase grains. Pl close to K- feldspar rods and blebs may show Na depletion or more complex variations, i.e an inner sodic halo and an outer calcic halo Type 4. Discrete elongate grains showing irregular shape and sometimes partial recrystallization. They are usually characterized by tiny albite exsolutions. No significant compositional variations have been observed in the adjacent plagioclase. Type 5. Rims or interstial patches associated with local occurrence of fine-grained coronas around orthopyroxene. They are likely related to infiltration of the host magma reacting with opx (e.g.: opx + silica-undersaturated alkali- rich melt = olivine + cpx + K- feldspar). It is distinct from Type 1-4 for considerably higher albite, anorthite and BaO contents (up to 2.4 wt%). It does not show any textural relationship with the former types. Slight Na enrichment occurs in the plagioclase approaching the K-feldspar. Apatite Grey, yellow, or brownish yellow fluorapatite characterized by relatively high Cl amounts (0.4-0.8 wt %) is a common accessory mineral in the xenolith. Apatite crystals do not show monazite or type of mineral inclusion. Ab Or An Ab Or An Plagioclase composition of xenoliths K-feldspar composition Type 1 Type 2 Type 3 Type 4 Type 5 Distance () 0 120 30 60 90 Plagioclas e Plagioclas e Plagioclas e Plagioclas e Kfs Kfs Kfs Kfs Kfs Cpx Kfs Kfs opx opx cpx opx opx Type 1- BSE images Plagioclase 1 Plagioclase 2 Kfs Kfs Plagioclas e 2 Plagioclas e 1 Plagioclas e A Plagioclas e B Kfs Kfs Opx Kfs Plagioclas e Plagioclas e Plagioclas e Plagioclas e Plagioclas e Kfs Kfs Kfs Kfs Kfs Kfs Kfs A Plagioclas e Pl Kfs Opx Plagioclas e Cpx Kfs Opx Plagioclas e Kfs Opx Olivin e Cpx Kfs Type 2- BSE images Linescan1 Linescan 1 Linescan 1 Linescan 2 Linescan 2 apatite opx cpx Plagioclas e apati te opx cpx Plagioclas e Bioti te Type 3- BSE images Type 5- BSE images Type 4- BSE image K -FE LD S P A R TYPE C aO (w t% ) N a2O (w t% ) K 2O (w t% ) B aO (w t%) 1. M icroveins 0.15-0.53 2.5-3.9 13.8-11.6 0.2-0.8 2. Intergrow ths w ith plagioclase 0.13-0.80 1.2-3.6 14.5-11.8 0.4-1.0 3. B lebs and rods into plagioclase 0.21-0.43 2.7-3.8 13.2-11.5 0.5-0.9 4. Large discrete crystals 0.16-0.34 2.3-2.6 14.3-13.9 0.3-0.5 5. C oronas around opx 0.13-1.58 5.4-5.9 8.8-6.6 0.1-2.4 An 53 Ab 43 Or 4 An 51 Ab 45 Or 4 An 70 Ab 27 Or 3 (An 60 Ab 35 Or 5 ) An 59 Ab 37 Or 4 An 54 Ab 39 Or 4 An 1 Ab 17 Or 82 A 400
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Transcript of EVIDENCE FOR K-FELDSPAR METASOMATISM IN LOWER CRUSTAL MAFIC XENOLITHS A. Montanini (1), D.Harlov (2)...
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EVIDENCE FOR K-FELDSPAR METASOMATISM IN LOWER CRUSTAL MAFIC EVIDENCE FOR K-FELDSPAR METASOMATISM IN LOWER CRUSTAL MAFIC XENOLITHSXENOLITHS
A. Montanini (1) , D.Harlov (2)
(1) Dipartimento di Scienze della Terra, Università di Parma, [email protected](2) GeoForschungsZentrum, D-14473 Potsdam, FR Germany, [email protected]
1 mm
Ortopyroxene porphyroclast with clinopyroxene exsolution lamellae
OPX
6. Concluding remarks6. Concluding remarks
• K-feldspar textures, e.g. microveins and randomly scattered antiperthitic plagioclase grains, suggest a likely metasomatic origin due to solid-state infiltration of K-rich fluids after the granulite facies recrystallization
•The occurrence of complex sodic-calcic haloes around Type-3 kfs indicates is consistent with a replacement origin and variation of K /(Na+K)fluid (Griffin (1969) Lithos 2, 171-186). The wide range of albite content of K-feldspar is also consistent with a variable K /(Na+K)fluid
•The formation of K-feldspar in the mafic xenoliths reflects metasomatic processes, which operated in the lower crust during in situ high-T recrystallization of relatively dry rocks characterized by refractory Ti-rich biotite. An external K-rich fluid source is therefore required
• Alkaline fluids/melts related to the Plio-Quaternary volcanism may be ruled out on the basis of isotopic data.
• We may speculate that the agent responsible for the K-feldspar metasomatism was a KCl-rich brine. The fluid, although primarily KCl, possibly contained NaCl and/or CaCl2 brine component, as suggested by the absence of monazite inclusions in apatite (Harlov and Forster, Amer. Mineral. in press)
Rb ppm Sr ppm 87Sr/86Sr Sm (ppm) Nd (ppm) 143Nd/144Nd
NB 324 WR (xenolith) 14.13 284.8 0.706482 3.124 13.795 0.512455NB324 K-feldspar 57.27 390.8 0.706556 0.026 0.252 0.512483
NB8 WR (xenolith) 19.06 250 0.706455 3.033 13.531 0.512448NB8 K-feldspar 84.64 384.7 0.706597 0.02 0.211 -
Host lava (basanite) 83.7 1224.2 0.704377 9.09 55.6 0.5125784. Hydrous and halogen-bearing minerals4. Hydrous and halogen-bearing minerals
Biotite
Small amounts of Ti-rich biotite (TiO2 = 4.7-6.5 wt.%) characterized by low to moderate F (0.24-1.19 wt.%) and low Cl (0.04-0.20 wt. %) may be present in both K-feldspar bearing and K-feldspar-free rocks. It is in textural equilibrium with the granulite facies assemblage, therefore mainly pre-dating the K-feldspar formation.
Amphibole
Ti-pargasite (Ti = 0.37, NaA = 0.46, K = 0.26 apfu) almost completely replaced by very fine-grained semi-opaque products (= late-stage heating by the host magma) is a rare mineral in the xenoliths
5. Isotopic constraints5. Isotopic constraints
Sr-Nd isotopic ratios of K-feldspar measured in two samples (Montanini et al., in prep.) are distinct from those of the host lava and very close to the whole-rock ratios.
0.5 mm
0.25 mm
1. IntroductionThe relatively common occurrence of metasomatic K-feldspar micro-veining in felsic gneisses, and in granulite-facies metabasites, has been interpreted as evidence of migrating alkali-rich fluids (e.g. KCl-NaCl brines) in high-grade terrains which may be responsible for the melt-absent dehydration of high-grade rocks from amphibolite to granulite facies (Harlov and Förster (2002) J Petrol 43, 769). Here we present new data on the occurrence of metasomatic K-feldspar in mafic granulite xenoliths which represent samples from the lower crust of Sardinia brought up to surface by recent (3. 9 Ma) eruptions of alkaline magmas.
2. The samplesThe xenoliths are quartz-free metagabbronorites characterised by orthopyroxene + clinopyroxene + plagioclase porphyroclasts (An50-66) in a recrystallized matrix composed of pyroxenes + plagioclase (An56-72) + Fe-Ti oxides ± Ti-biotite ± Ti-pargasite ± apatite. Olivine rimmed by spinel-pyroxene symplectitic intergrowths at the olivine-plagioclase interface rarely occurs. Whole-rock compositions show slight LREE enrichment, flat HREE patterns and HFSE depletion. Sm-Nd data on mineral separates are close to the WR present-day ratios, in agreement both with a very young age of the protoliths and equilibration at the high (lower crustal) temperatures recorded by two-pyroxene and opx-biotite thermometry. Calculated VP on the basis of modal composition (7.1 km s-1) are in good agreement with those reported for the lower crust of central-western Sardinia by seismic refraction surveys. The xenoliths are meta-igneous rocks originated from underplating of subduction-related mafic liquids which underwent cooling and recrystallisation in the deep crust (T ~ 850-900°C, P ~ 8-10 kbar, Montanini et al., in prep.).
1 mm
Recrystallized xenolith with granoblastic polygonal texture
CPXOPX
OPX
PL
BT
0.5 mm
0.5 mm
Kfs
0.5 mm
Plagioclase
0.5 mm
Kfs
0.25 mm
0.5 mm
Kfs
3. The K-feldspar Small amounts of K-feldspar (up to 2-3 vol. %) have been found in about 50 % of the examined xenoliths. Texturally, the K-feldspar occurs in a variety of different modes that may be commonly observed within the same sample
Type 1. Micro-veins along plagioclase-plagioclase and plagioclase-pyroxene grain rims. Plagioclase close to microveins is usually (but not always) slightly depleted in Na
Type 2. Intergrowths with Ca-rich plagioclase along plagioclase-plagioclase grain boundaries. Plagioclase intergrown with K-feldspar is relatively Na-rich, whereas increasing anorthite content in the plagioclase towards the contact with the K-feldspar myrmekite-like intergrowths is observed.
Type 3. Rods, blebs, and irregular patches in a random scattering of plagioclase grains. Pl close to K-feldspar rods and blebs may show Na depletion or more complex variations, i.e an inner sodic halo and an outer calcic halo
Type 4. Discrete elongate grains showing irregular shape and sometimes partial recrystallization. They are usually characterized by tiny albite exsolutions. No significant compositional variations have been observed in the adjacent plagioclase.
Type 5. Rims or interstial patches associated with local occurrence of fine-grained coronas around orthopyroxene. They are likely related to infiltration of the host magma reacting with opx (e.g.: opx + silica-undersaturated alkali-rich melt = olivine + cpx + K-feldspar). It is distinct from Type 1-4 for considerably higher albite, anorthite and BaO contents (up to 2.4 wt%). It does not show any textural relationship with the former types. Slight Na enrichment occurs in the plagioclase approaching the K-feldspar.
Apatite
Grey, yellow, or brownish yellow fluorapatite characterized by relatively high Cl amounts (0.4-0.8 wt%) is a common accessory mineral in the xenolith. Apatite crystals do not show monazite or type of mineral inclusion.
Ab Or
An
Ab Or
An
Plagioclase composition of
xenoliths
K-feldspar composition
Type 1
Type 2Type 3Type 4Type 5
Distance ()
0 12030 60 90
PlagioclasePlagioclase
Plagioclase
Plagioclase
Kfs
Kfs
Kfs
Kfs
Kfs
Cpx
Kfs
Kfs
opxopx
cpxopx
opx
Type 1- BSE images
Plagioclase 1
Plagioclase 2
Kfs
Kfs Plagioclase 2
Plagioclase 1
Plagioclase A
Plagioclase B
Kfs
Kfs
Opx
KfsPlagioclase
Plagioclase Plagioclase
Plagioclase
Plagioclase
Kfs
Kfs
Kfs
Kfs Kfs
Kfs
Kfs
A
Plagioclase
PlKfs
Opx
Plagioclase
Cpx
Kfs
Opx
Plagioclase Kfs
Opx
Olivine
Cpx
Kfs
Type 2- BSE images
Linescan1
Linescan 1 Linescan 1Linescan 2
Linescan 2
apatite
opxcpx
Plagioclaseapatite
opxcpx
Plagioclase
Biotite
Type 3- BSE images
Type 5- BSE images Type 4- BSE image
K-FELDSPAR TYPE CaO (wt%) Na2O (wt%) K2O (wt%) BaO (wt%)
1. Microveins 0.15-0.53 2.5-3.9 13.8-11.6 0.2-0.8
2. Intergrowths with plagioclase 0.13-0.80 1.2-3.6 14.5-11.8 0.4-1.0
3. Blebs and rods into plagioclase 0.21-0.43 2.7-3.8 13.2-11.5 0.5-0.9
4. Large discrete crystals 0.16-0.34 2.3-2.6 14.3-13.9 0.3-0.5
5. Coronas around opx 0.13-1.58 5.4-5.9 8.8-6.6 0.1-2.4
An53Ab43Or4
An51Ab45Or4
An70Ab27Or3
(An60Ab35Or5)
An59Ab37Or4
An54Ab39Or4
An1Ab17Or82
A
400
