Isotopic Composition of Oligocene Mafic Volcanic Rocks in...

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 96, NO. B8, PAGES 13,593-13,608, JULY 30, 1991

Isotopic Composition of Oligocene Mafic Volcanic Rocks in the Northern Rio Grande Rift: Evidence for Contributions of Ancient Intraplate

and Subduction Magmatism to Evolution of the Lithosphere CLARK M. JOHNSON

Department of Geology and Geophysics, University of Wisconsin, Madison

REN A. THOMPSON

U.S. Geological Survey, Denver, Colorado

Mafic lavas erupted during initiation of regional extension at 26 Ma in the northern Rio Grande rift were derived from at least two isotopically distinct mantle sources. One is characterized by 87Sr/86Sr = 0.70495, •md -- -4, 2ø6pb/2ø4pb = 18.2, and the other by 87Sr/86Sr = 0.7044, •Nd -- 0, 2ø6pb/2ø4pb = 18.2. The 1OW-Smd value source (MANTLE 1) is interpreted to largely reflect the isotopic compositions of the lithospheric mantle in the region. Isotopic compositions of Cenozoic mafic lavas and Proterozoic rocks are used to constrain models for evolution of the lithosphere. The low 8Nd values of the MANTLE 1 component were probably produced by evolution of a light rare earth element-enriched upper mantle or input of low-s material during development of the lithosphere in the Early Proterozoic. The 8Nd values Nd . and 2ø6pb/2ø4pb ratios •n evolved rocks are as low as -7 and 17.3, respectively, indicating interaction with lower crust that had •Na -< -12 and 2ø6pb/2ø4pb _< 17.0. Initial 87Sr/86Sr ratios both increase and decrease slightly in evolved rocks, indicating interaction with lower crust that had Sr isotope ratios that were generally similar to those of the mantle. The 8Nd value of the modern lithospheric mantle beneath the northern Rio Grande rift is >18 units lower than the projected modern values of the asthenospheric mantle from which the Proterozoic crust was originally derived. The higher-ema value source (MANTLE 2) may reflect mixing of asthenosphere and lithosphere components. The mantle source for most late Cenozoic mafic lavas in the northern Rio Grande rift region lies at 2ø7pb/2ø4pb ratios that are significantly higher than the 2ø6pb/2ø4pb - 2ø7pb/2ø4pb array defined by Proterozoic crust in the region. Although the crustal array is also displaced to higher 2øTpb/2ø4pb ratios as compared to the northern hemisphere oceanic mantle a•ray, indicating incorporation of Archean Pb during crust formation, the still higher 2øTpb/2ø4pb ratios in the Rio Grande rift mantle source is interpreted to reflect continued input of Archean Pb into the developing Early Proterozoic lithospheric mantle after crust formation. These relations are strong evidence that the lithosphere became stabilized shortly after the major crust formation events in the Early Proterozoic.

INTRODUCTION

Rift-related volcanism in the northern Rio Grande rift is

largely basaltic in composition, in contrast to prerift magmatism in the region, which was dominated by intermediate- to silicic-composition ash flow tuffs and related lavas [Steven, 1975; Lipman and Mehnert, 1975]. Most studies of rift-related volcanic rocks in the northern Rio

Grande rift have concentrated on relatively young volcanic fields that formed significantly after initiation of extension. These include the Taos Plateau, Rayton-Clayton, and Ocate fields (Figure 1) [Stormer, 1972a, 1972b; Lipman and Mehnert, 1979; O'Neill and Mehnert, 1988], as well as lavas exposed at Los Mogotes volcano, in the Tusas Mountains, and near Amalia (Figure 1)[Lipman and Mehnert, 1975; Lipman et al., 1986]. Early rift lavas are exposed only on intrarift horsts and on the flanks of the rift in the San Juan and Latir volcanic fields (Figure 1) [Lipman and Mehnert, 1975, 1979; Thompson et al., 1986; Lipman et al., 1986]. Late Cenozoic basaltic lavas in northern Colorado are temporally correlative with rift-related volcanic rocks but were erupted significantly outside the Rio Grande depression [Larson et al., 1975]. San Luis Hills

Copyright 1991 by the American Geophysical Union.

Paper number 91JB00342. 0148-0227/91/91 JB-00342505.00

(Figure 1) preserves the largest volume of early rift (26 Ma) volcanic rocks in the northern Rio Grande rift. These lavas

provide an important and previously poorly known view of volcanism that occurred during inception of the Rio Grande rift.

We report Sr, Nd, and Pb isotope data for early rift mafic lavas of the Hinsdale Formation that are exposed at San Luis Hills. These data bear on mantle sources of volcanism

associated with inception of rifting and models for development of the lithospheric mantle. Compositional variations and isotopic compositions indicate that many San Luis Hills magmas interacted with lower crust, as has been proposed for other lavas in the region. Detailed consideration of chemical and petrologic characteristics of the lavas, however, allows identification of primitive, uncontami- nated compositions that can be used to determine isotopic compositions of the mantle. The data define two mantle source regions, at least one of which is interpreted to reflect the lithospheric mantle that had been enriched in light rare earth elements (LREE) during stabilization and growth of the lithosphere in the Proterozoic.

GEOLOGY AND PETROLOGY OF SAN LUIS HILLS LAVAS

San Luis Hills are the surface expression of a major intrarift horst within the northern Rio Grande rift that is

largely buried beneath late Cenozoic sedimentary and vol-

13,593

13,594 JOHNSON AND THOMPSON: ISOTOPIC COMPOSITION OF MAFIC VOLCANIC ROCKS

COLORADO

NEW MEXICO

SAN LUIS HILLS

PROTEROZOIC BASEMENT

MID-CENOZOIC CALDERAS

,SAN LUIS HILLS

• I I I I I o 5o IOOKM

Fig. 1. Generalized map of the northern Rio Grande rift region showing major Cenozoic volcanic areas, including San Luis Hills, the subject of this report. SJVF, San Juan volcanic field; TPVF, Taos Plateau volcanic field; TBM, Timber Mountain and Brushy Moun- tain; AL, Amalia lavas; LVF, Latir volcanic field; R-CVF, Rayton- Clayton volcanic field; OVF, Ocate volcanic field; LM, Los Mogotes volcano.

canic rocks [Kleinkopf et al., 1970]. Other surface expo- sures include Timber Mountain and Brushy Mountain (Fig- ure 1) [Thompson et al., 1986]. Early rift lavas (26.4-25.7 Ma) of the lower Hinsdale Formation, the subject of this report, overlie intermediate-composition lavas at San Luis Hills that are temporally correlative with precaldera lavas of the Latir volcanic field [Thompson et al., this issue]. Silicic volcanic rocks are conspicuously missing from San Luis Hills, in contrast to volcanic sections exposed on the flanks of the rift in the San Juan and Latir volcanic fields.

Particularly notable is the absence of the Amalia Tuff, which erupted from the Questa caldera of the Latir volcanic field contemporaneous with initiation of regional extension at 26 Ma [Lipman et al., 1986; Hagstrum and Lipman, 1986]. These relations, in addition to the general lack of Los Pinos Formation age sedimentary rocks at San Luis Hills [Thompson and Machett, 1989], suggest that the horst was a topographic high during early evolution of the northem Rio Grande rift.

Early rift lavas at San Luis Hills are divided into four suites based on petrographic characteristics, chemical compositions, and stratigraphic position. Suites 1-4 represent a general sequence from oldest to youngest. Suite 1 lavas occur at the base of the section and represent N60 vol % of the lavas exposed at San Luis Hills. They erupted from multiple centers and are characterized by up to 20 vol % phenocrysts of olivine and clinopyroxene [Thompson et al., this issue]. These lavas are transitional between tholeiite and alkali basalts, as determined by Na20+K20-SiO 2 variations, and are similar to other Hinsdale Formation lavas exposed at Los Mogotes volcano and in the eastern parts of

400

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JOHNSON AND THOMPSON: ISOTOPIC COMPOSITION OF MAFIC VOLCANIC ROCKS 13,$95

TABLE 1. Sr, Nd, and Pb Isotope Data for San Luis Hills Lavas

87Sr/86Sr Rb, Sr, 87Sr/86Sr Measured ppm ppm Initial

143Nd/144Nd Sm, Nd, Measured ppm ppm t•Nd(T ) 206pb / 204pb 207pb / 204pb 208pb / 204pb

Suite 1 T84-12

72 74 88

98 99

152

Suite 2 T84-38

126

163 204

Suite 3 T84-46

89 125

150 205

0.704890+ 9 46 706 0.704820+ 13 46 971

0.704882+ 7 46 780 0.704819+ 10 0.705568+21 46 665 0.705494+ 25 0.704980+ 8 52 710 0.704902+ 12 0.704853+ 8 45 901 0.704800+ 11 0.705015+10 40 647 0.704949+ 13

(0.705001+ 7) (0.704935+ 10)

0.704761ñ 7 62 1179

(0.704769ñ 8) 0.704730ñ 8 48 954

(0.704716ñ10) 0.704731ñ13 57 1187 0.705549ñ10 109 1119

0.704363ñ 8 17 489 0.704475ñ10 20 395 0.704911ñ 8 33 971 0.704609ñ 8 48 1014 0.704344ñ 7 9 360

0.512404ñ 7 6.65 31.5 -3.97ñ0.15 0.512448ñ 6 8.13 37.7 -3.12ñ0.13 0.512408ñ 7 7.29 35.5 -3.88ñ0.15 0.512380ñ11 6.36 28.1 -4.46ñ0.23 0.512414ñ 7 6.98 33.5 -3.76ñ0.15 0.512392ñ 7 9.10 45.3 -4.18ñ0.15 0.512409ñ 7 7.48 36.0 -3.86ñ0.15

0.704705ñ 10 0.512476ñ10 10.4

(0.704713ñ 11) 0.704676ñ 11 0.512415ñ 6 8.20

(0.704662ñ 13) 0.704680ñ 16 0.512536ñ 7 8.92 0.705445ñ 15 0.512267ñ 8 9.60

17.626 15.468 37.195 17.605 15.474 37.351 17.677 15.458 37.222 17.988 15.535 37.585 17.808 15.492 37.371 17.580 15.447 37.127 18.133 15.523 37.569

56.9 -1.91ñ0.21' 17.959

42.4 -3.13ñ0.13' 17.697

15.556 37.800

15.549 37.504

47.7 -1.34ñ0.15 17.963 15.526 37.682 49.7 -6.60ñ0.17 17.698 15.491 37.920

0.704326ñ 10 0.512560ñ 9 4.98 21.2 -0.97ñ0.19 17.862 15.511 37.470 0.704421ñ 13 0.512593ñ 6 4.35 17.4 -0.36ñ0.14 17.784 15.493 37.353 0.704875ñ 10 0.512309ñ 8 8.10 41.4 -5.79ñ0.17 17.457 15.458 37.116 0.704558ñ 11 0.512371ñ 7 6.53 33.3 -4.58ñ0.15 17.308 15.455 36.986 0.704317ñ 8 0.512596ñ10 4.60 18.8 +0.30ñ0.21' 18.234 15.562 37.761

Suite 4 T84-119 0.704541ñ 8 54 706 0.704459ñ 12 0.512442ñ 8 8.22 48.1 -3.14ñ0.17 17.787 15.523 37.330

140 0.704478ñ 8 39 1135 0.704441ñ 10 0.512560ñ 6 8.13 41.7 -0.89ñ0.13 17.698 15.484 37.375 140 0.704461ñ10 39 1219 0.704427ñ 12 0.512460ñ 8 8.94 47.5 -2.24ñ0.17' 17.518 15.485 37.248

Duplicate analyses (separate dissolutions) are noted in parentheses. *Nd isotope analyses performed after repair of amplifier housing; measured i43Nd/144Nd not corrected for shift, but /•md (r) values corrected by +0.59 units. Analytical procedures noted in text.

contain more abundant olivine and clinopyroxene phenocrysts. Suite 3 lavas have Sr and Zr contents and chon- drite-normalized La/Yb ratios ([La/Yb]m) that are substan- tially lower than those of the other San Luis Hills suites (Figure 2 and Table 1) [Thompson et al., this issue]. Suite 3 lavas are compositionally similar to the Servilleta Basalt and andesite of the Taos Plateau volcanic field [Dungan et al., 1986]. Suite 4 lavas are the youngest Hinsdale Forma- tion rocks exposed at San Luis Hills and are characterized by large xenocrysts of partially resorbed quartz, plagio- clase, and rare clinopyroxene crystals in otherwise apha- nitic or sparsely porphyritic lavas. This suite contains the highest alkali contents, at a given SiO 2 content, of the San Luis Hills suite and are trachybasalts and trachyandesites. They are compositionally similar to the most alkaline lavas of the Hinsdale Formation that are exposed in the western San Juan volcanic field [Lipman and Mehnert, 1975]. This suite also contains the highest Zr contents of the rocks exposed at San Luis Hills, consistent with its alkaline nature (Figure 2) [Thompson et al., this issue].

Several samples of suites 2 and 3 contain MgO and Cr contents that are higher than those predicted for their SiO 2 contents if they evolved largely by crystal fractionation (Figure 2) [Thompson et al., this issue], suggesting that they represent mixtures of fractionated and primitive magmas. These relations are similar to those observed for other

mafic- to intermediate-composition lavas in the region, most notably in the Latir and Taos Plateau volcanic fields [Dungan et al., 1986; Thompson et al., 1986; Johnson and Lipman, 1988]. Suite 1 lavas contain the highest Cr con-

tents of the four San Luis Hills suites, and the high-Cr samples are interpreted as primitive, mantle-derived lavas.

ANALYTICAL METHODS

Whole rock powders analyzed for Sr, Nd, and Pb isotope ratios are the same as those used for chemical analyses reported by Thompson et al. [this issue]. Strontium and Nd were separated using 2.5M HC1 for Sr separation, followed by group separation of the rare earth elements (REE) using 6M HC1 and separation of Nd using 0.150M and 0.225M 2- methyllactic acid. Lead was separated using 0.6M HBr and 6M HC1 on an anion exchange column. Total procedural blanks were 200-400 pg for Sr, 40-80 pg for Nd, and 0.6- 1.5 ng for Pb, which are negligible.

Strontium was mass analyzed on a VG Instruments Sec- tor 54 6-collector mass spectrometer at-•3x10 -•1A 88Sr (10 TM ohm resistors) using single Ta filaments and H3PO4 and a three collector triple-jump mode (dynamic multicollection) that removes all collector biases and beam instability factors. Measured ratios were exponentially corrected for mass fractionation using 86Sr/88Sr=0.1194. Within-run errors noted (Table 1) are +2-sigma standard error (2SE) using n=100 (number of ratios calculated from three jumps). Long-term drift (-1 year) in the 87Sr/a6Sr ratio of NBS-987 is negligible using three collector dynamic analysis. The a?Sr/a6Sr ratio measured for NBS-987 during this study was 0.710215 +5 2SE (23 analyses). The a7Sr/86Sr ratio measured for BCR-1 during this study was 0.704990 +7 2SE (11 analyses). Rubidium and Sr contents were


determined by energy-dispersive X-ray fluorescence [Thompson et al., this issue] and are precise to +5%. Errors in initial 87Sr/a6Sr ratios (hereafter referred to as Isr ratios) are calculated using a squared-sum partial derivative expression of the decay equation which propagates errors for the measured 8*Sr/86Sr ratio, Rb and Sr contents, and sample age (taken as 26 +1 Ma).

Neodymium was mass analyzed as Nd + (~lx10 -• A •44Nd) using double Re filaments and a five collector triple- jump mode (dynamic multicollection) with exponential mass fractionation correction of •46Nd/•44Nd = 0.7219. Within-run errors noted (Table 1) are +2SE using n=100 (number of ratios calculated from three jumps). Prior to work on the amplifier housing in July 1989, the following Nd isotope ratios on standards were determined: BCR-1, •43Nd/•44Nd=0.512619 +8 2SE (eight analyses); internal laboratory normal standard (Ames National Laboratory ultrapure metal), 143Nd/144Nd=0.512143 +4 2SE (16 analyses); La Jolla Nd, •43Nd/rand=0.511859 _+4 2SE (10 analyses). We take the 143Nd/144Nd ratio of BCR-1 as equivalent to the present-day ratio for CHUR [e.g., Wasserburg et al., 1981], which produces an •md value of our internal Ames Nd standard of-9.29 _+0.08. For comparison, 143Nd/144Nd measured for BCR-1 and our internal Ames Nd standard at

the U.S. Geological Survey, Menlo Park, using a Finnigan- MAT 261 single-collector mass spectrometer was 0.512618 _+6 2SE (11 analyses) and 0.512139 _+11 2SE (seven analyses), respectively, in 1985-1987 [Johnson et al., 1990]. The pooled 145Nd/144Nd ratio of 34 standard analyses at University of Wisconsin was 0.348413 _+4 2SE. This ratio is identical within error to the pooled 145Nd/144Nd ratio measured on 15 San Luis Hills samples prior to work on the amplifier housing, which is 0.348410 _+5 2SE.

Following work on the amplifier housing in July 1989, the 143Nd/144Nd and •45Nd/•44Nd ratios measured for our internal Ames Nd standard decreased to 0.512113 6 _+2SE

and 0.348402 3 _+2SE (28 analyses), respectively. This is equivalent to a 0.0059 _+0.0014% and 0.0032 _+0.0014% decrease, respectively, and these factors have remained constant through the end of 1990. The 143Nd/144Nd ratio measured for BCR-1 during this time interval was 0.512611 4 _+2SE (four analyses); this shift is not as great as that measured for our internal Ames Nd standard, although only four analyses of BCR-1 were made. No shift in 87Sr/a6Sr ratios within the average external reproducibility of +0.000010 was observed during this time for NBS-987 and BCR-1, consistent with the repeated analysis of sample 152 (Table 1), which was done in February 1990. Four San Luis Hills samples (38, 126, 141 and 204; Table 1) were analyzed for Nd isotope ratios following work on the amplifier housing, and the •md values for these have been increased by 0.59 units to maintain a common reference (Table 1), although the 143Nd/144Nd ratio is reported as measured (Table 1). The 145Nd/144Nd ratio of these four samples is 0.348406 +7 2SE.

Neodymium and Sm contents were determined by instru- mental neutron activation analysis (INAA) [Thompson et al., this issue], and are precise to _+5%. Present-day 147Sm/144Nd ratio for CHUR taken as 0.1967 [Jacobsen and Wasserburg, 1980]. Errors in •md values are calculated in a manner similar to that used for calculating errors in Isr ratios.

Lead isotope ratios were determined using single Re filaments and a mixture of dissolved silica gel and H3PO 4 and four collector static (non peak jumping) multicollection. Long-term (~ 1 year) drift in collector biases, as determined by a reference voltage comparison, or static multicollection analysis of Sr and Nd, is a factor of 20 less than the estimated precision to which Pb isotope ratios may be determined (_+0.10 %). Lead isotope ratios were corrected for mass fractionation by +0.10% per mass (pooled error, _+0.02 2SE n=13), as determined by 2ø7pb/2ø6pb and 2ø8pb/2ø6pb ratios measured on NBS-981 and NBS-982 standards, respectively. Standards were run at temperatures similar to those of the samples (1100ø-1400øC). Sig- nificant excursions from this value are found in our labora-

tory only for rare analyses where large changes in filament temperature are required to maintain a ~2x10 '11 A 2ø8pb signal; mass fractionation did not correlate with absolute temperature within the range 1000ø-1500øC for analyses that maintained relatively constant temperatures. Mea- sured Pb isotope ratios for NBS-981 during this study were 2ø6pb/2ø4pb=16.904 _+8, 2ø7pb/2ø4pb=15.449 _+10, 2ø8pb/2ø4pb =36.568 _+10, and 2ø7pb/2ø6pb=0.91395 _+22 2SE (four analyses). Measured Pb isotope ratios for NBS-982 during this study were 2ø6pb/ 2ø4pb=36.642 _+20, 2ø7pb/2ø4pb=17.094 _+13, 2ø8pb/2ø4pb=36.563 _+40, and 2ø8pb/2ø6pb=0.99784 _+57, 2SE (nine analyses). Errors for samples are approximately 1-2 times larger than those of standards, based on random replicate analyses using different filament loads. Measured Pb isotope ratios are taken as initial ratios, because the U contents of the San Luis Hills lavas are very low (R. Thompson, unpublished data, 1987).

RESULTS

Strontium, Nd, and Pb isotope analyses were performed on samples that span the compositional range of the four suites. Both primitive lavas and those that have petrographic characteristics and chemical compositions which indicate contamination with continental crust were ana-

lyzed.

Sr Isotope Data

Initial 87Sr/86Sr (Isr) ratios for most San Luis Hills rocks are between 0.7042 and 0.7050, similar to younger, rift- related lavas exposed at Los Mogotes volcano and the Taos Plateau volcanic field (Figure 3a). In contrast, Isr ratios for preextension mafic- and intermediate-composition rocks at the San Juan and Latir volcanic fields are significantly higher than those at San Luis Hills (Figure 3a) [Lipman et al., 1978; Colucci et al., this issue]. Isr-SiO 2 relations for suites 1 and 2 are scattered, although Isr ratios modestly increase with increasing SiO 2 contents for suite 3 lavas. Despite clear petrographic evidence for crustal assimilation in suite 4 lavas, Isr ratios for these rocks are relatively constant with increasing S iO 2 contents.

Nd Isotope Data

The •md values for suites 2, 3, and 4 decrease with increasing SiO 2 contents (and decreasing Cr and Mg con-

JOHNSON AND THOMPSON: ISOTOPIC COMPOSITION OF MAFIC VOLCANIC ROCKS 13,597

0.706

•o 0.705

• 0.704 z

0.703

2

"• AMALIA LATIR A

'"' / '•© ' © ß ..... 1' -4,',';, ', ," ............. == ...... " .....

ß ,._ ......... , ..................... / --,

• /•' AMALIA

LOS MOGOTES • SUITE 1 ß SUITE 3 •) SUITE 2 ß SUITE 4

- /

19.0

1•.5 J AMALIA i ß :., .......

18.0 ......

.... e -.

+' .... , ..... ................... 17.o • -"'"'•

48 52 56 60 64

wt• SiO 2

Fig. 3. Variations in (a) initial 87Sr/86Sr (ISr) ratio, (b)/•Nd value, and (c) 2ø6pb/2ø4pb ratio with SiO 2 contents of San Luis Hills lavas (symbols) and other lavas in the region (outlines). Data from Dungan et al. [1986], Johnson et al. [1990], M. Dungan and S. Moorbath (unpublished data, 1990), and M. Dungan and J. Davidson (unpublished data, 1988). Asterisk, estimated SiO 2 content for suite 3 sample 205.

tents) but are relatively constant for most suite 1 lavas (Figure 3b). The high Cr and Mg contents of the mafic suite 1 lavas suggest that they are relatively primitive, and we interpret the average end value of-4 as indicative of a mantle value. The San Luis Hills lavas have Nd isotope ratios that overlap those of lavas at Amalia and the Taos Plateau volcanic field (Figure 3b). Primitive lavas at Los Mogotes volcano have slightly higher end values (•- +1 to +2) than the highest found at San Luis Hills (Figure 3), as do Quaternary lavas in the Tusas Mountains (Brazos basalt [Williams, 1984]) and mafic feldspathoidal lavas in the Rayton-Clayton field [Phelps et al., 1983]. The lowest end values at San Luis Hills overlap those of intermediate- composition rocks at the San Juan and Latir volcanic fields [Johnson et al., 1990; Colucci et al., this issue]. Mafic lavas of the northern Rio Grande rift region have end values

that are higher than those of late-Cenozoic mafic lavas in northwestern Colorado, which vary from-4 to -10 [Leat et al., 1988, 1989, 1990; Thompson et al., 1989], but are generally lower than Miocene and younger mafic lavas in the central and southern Rio Grande rift, which vary from 0 to +8 [Menzies et al., 1983; Perry et al., 1987; Roden et al., 1988].

Pb Isotope Data

The 2ø6pb/2ø4pb ratios for suites 2 and 3 decrease with increasing SiO 2 contents, similar to rocks at the Taos Pla- teau volcanic field (Figure 3c). 2ø6pb/2ø4pb-SiO2 relations for suite 1 are scattered. Evolved rocks from suite 4 that contain crustal xenocrysts have 2ø6pb/2ø4pb ratios that are both lower and higher than one relatively mafic sample. The highest 2ø6pb/2ø4pb ratios for rocks at San Luis Hills (•-18.2) are similar to those of primitive lavas that crop out near Amalia, Los Mogotes volcano, and at the Taos Plateau volcanic field (Figure 3c). The 2ø6pb/2ø4pb ratios of San Luis Hills rocks include ratios that are as low (-17.3) as those found in the San Juan and Latir volcanic fields [Lipman et al., 1978; Johnson et al., 1990; Colucci et al., this issue], although they are not as low as those found in some evolved rocks of the Taos Plateau volcanic field [Dungan et al., 1986].

Lead isotope compositions of the San Luis Hills lavas generally form a linear array for all suites on 2ø7pb/2ø4pb- 2ø6pb/2ø4pb and 2øSpb/2ø4pb-2ø6pb/2ø4pb diagrams, similar to other mafic lavas in the region (Figure 4). A Pb-Pb (2ø7pb*/2ø6pb*) isochron age of 2006 +540 Ma is calculated for the San Luis Hills lavas (excluding sample 126 of suite 2, which has an anomalously high 2ø7pb/2ø4pb ratio), similar to those that may be calculated for other mafic lava suites in the northern Rio Grande rift: precaldera Latir, 1387 340 Ma [Johnson et al., 1990]; Amalia, 1439 +1200 Ma [Johnson et al., 1990]; Taos Plateau, 1760 +240 Ma [Dungan et al., 1986]; extension-related (


38.0

37.8

37.6 37.4

37.2

37.0

15.7

15.6

15.5

LOS 0 Mo MOGOTES

,• • • • • I•IORB/HA•'AII 15.4

17.0 17.5 18.0 18.5 19.0

206 Pb/ 204 Pb

Fig. 4. (a) 2ø6pb/2ø4pb - 2ø8pb/2ø4pb and (b) 2ø6pb/2ø4pb - 2ø?pb/2ø4pb variations for San Luis Hills lavas and other lavas in the region. Data sources cited in Figure 3. "1,2" and "3" next to large open squares indicate MANTLE 1 and 2 and MANTLE 3 compositions, respectively, discussed in text. Stacey-Kramers average crustal Pb evolution curve shown in heavy line [Stacey and Kramers, 1975]. Proterozoic crustal array regression line (1710 60 Ma) shown by stippled bar. Low 2ø8pb/2ø4pb ratios of the crust indicate a Th/U ratio of-2 (J. Wooden et al., unpublished data, 1987). Data for MORB and Hawaiian lavas from Tatsumoto [1978], Cohen et al. [1980], Duprg and All•gre [1980], Cohen and O'Nions [1982], Stille et al. [1983], Staudigel et al. [1984], and Hegner et al. [1986].

those of the Taos Plateau volcanic field, suggesting that evolution of the two magmatic systems involved similar endmember components. An important exception is the occurrence of petrologically primitive lavas at San Luis Hills that have low/•Nd values of ~ -4.

Crustal Interaction

Decreasing /•Nd values with increasing SiO 2 contents for suites 2, 3, and 4 is interpreted to reflect coupled assimilation/fractional crystallization (AFC [Taylor, 1980; DePaolo, 1981a]) involving crust that had low/•Nd values, such as Proterozoic crust in the region [DePaolo, 1981b; Nelson and DePaolo, 1984, 1985]. The presence of crustal components in suites 2, 3, and 4 is also indicated by decreasing 2ø6pb/2ø4pb ratios with increasing SiO 2 contents for suites 2 and 3, in addition to the common occurrence of crustal xenocrysts in suite 4. Although isotopic variations relative to SiO 2 contents for suite 1 are scattered, Sr and Pb isotope ratio and trace element variations indicate some interaction with crustal rocks (Figure 5). Except for one sample (88) that has an anomalously high lsr ratio, 2ø6pb/2ø4pb and lsr ratios decrease with increasing differen- tiation, as indicated by increasing Sr and Zr contents. Inas- much as SiO 2 contents of mafic magmas can increase or

19.0

18.5 13_

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• 18.o-

o

17.5

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17.0 ß 0.703

0.70475 0.70500

' ' ' ' 18.2 I SUITE 1 B ;'21 A

I MAIN TREND E4Z .157 t I I-

............... LO-; ........ "I . 7eo..15e / ..... 0--- i, • 706...155 ....... MOG TE$ ":l m œ0.1 .1',•' , 17.6 .-'"" . ...... ' .... i ! .z 1

............ 12' - -/._._ I

AMALIA i 14, ..... ß / I-ATIR

_ _,:• 4` '"',,,,,,,

ß I ß ß ß 0.704 0.705 0.706

INITIAL 87 Sr/86 Sr

Fig. 5. Initial 87Sr/86Sr (/Sr) - 2ø6pb/2ø4pb variations for San Luis Hills and other lavas in the region. Inset for the main trend of suite 1 also shown with expanded scale. Size of boxes in inset note 2SE analytical error. "A" refers to Sr isotope analysis of sample 152 prior to work on amplifier housing, and "B" refers to repeated analysis (dotted line) following work on amplifier housing. Decreasing 87Sr/86Sr - 2ø6pb/2ø4pb trend for most suite 1 lavas is interpreted as reflecting assimilation of lower crust that had 87Sr/86Sr ratios lower than that of the mantle. Numbers in italics in inset note Sr, Zr contents. "1", "2", and "3" next to open boxes in main figure note MANTLE 1, 2, and 3 compositions, respectively. Data sources cited in Figure 3.

decrease slightly during early fractionation depending upon the phases that are fractionated, trace elements such as Sr and Zr are better indicators of fractionation in the olivine-

and clinopyroxene-phyric lavas of suite 1. Variations in 2ø6pb/2ø4pb ratios with SiO 2 or trace element

contents for the majority of San Luis Hills rocks indicate interaction with crust that had 2ø6pb/2ø4pb ratios less than 17.3. A possible exception is one evolved sample from suite 4 (119), which has a relatively high 2ø6pb/2ø4pb ratio of 17.8 (Table 1). The generally small variation in lsr ratios with SiO 2 contents for suites 2, 3, and 4 suggests interaction with crust that had 87Sr/g6Sr ratios that were similar to those of the parental magmas. One sample from suite 2 that has a low /•md value and 2ø6pb/2ø4pb ratio (204), suggestive of crustal interaction, has a relatively high lsr ratio, indicating interaction with crust that had an 87Sr/86Sr ratio > 0.706. Most samples from suite 1 indicate interaction with crust that had an 87Sr/g6Sr ratio < 0.7045.

Although some variability in isotopic compositions of the crustal components in the San Luis Hills lavas is indicated by the scatter in the data, the general isotopic compositions of these components are remarkably similar to those proposed as crustal contaminants in several other volcanic sequences in the northern Rio Grande rift region. Contami- nation by crust that has 87Sr/86Sr=0.7045 to 0.7070, /•Nd


TABLE 2. Assimilation/Fractional Crystallization Models Suite 1 Suite 2 Suite 3 Suite 4

Parental MANTLE 1 MANTLE 2 MANTLE 2 composition (sample 152) (extrapolated from (sample 205)

sample 163) a?Sr/a6Sr 0.70495 0.7044* 0.7044

ENd -4 0 0 2ø6pb/2ø4pb 18.2 18.2 18.2

Most contaminated Main trend (sample 204) (samples 125 composition (sample 99) and 150)

a?Sr/a6Sr 0.70480 0.7055 0.7049

ENd -4 -7 -6 2ø6pb/2ø4pb 17.6 17.7 17.3

Crustal 8?Sr/86Sr 0.7030-0.7040 0.7060-0.7080 0.7060-0.7080 R 0.15 Ma/Mo = 0.10-0.20

MANTLE 2

(extrapolated from sample 140)

0.7044

0

18.2

(sample 119)

0.7045

-3

17.5

negligible assimilation

required -12 to -15

R~l/4 50-70% AFC

Ma/Mo = 0.17-0.25

16.0-17.0

R = 1/10 to 1/4 50-70% AFC

Ma/Mo = 0.06-0.20

R is mass ratio of assimilated crust to crystallized minerals [DePaolo, 1981a]. Ma/Mo is mass ratio of assimilated crust to initial mass of magma [Farmer and DePaolo, 1983]. AFC refers to coupled assimilation/fractional crystallization. * Choice of 87Sr/86Sr = 0.7047 (ratio for sample 163) for initial composition decreases calculated amount of assimilation based on Sr isotope variations by ~10%.

Stein and Crock, 1990; Johnson and Fridrich, 1990]. Crust of this nature apparently comprised the major crustal component in Cenozoic magmatism in the northern Rio Grande rift region that spanned more than 40 m.y. and covered over 100,000 km2.

AFC models summarized in Table 2 are constrained to

realistic percentages of crystallization (_


2 • :"-.."•- LOS IdOGOTES .........

o. • i"'• ...... I.J"-¾,. I • I •,.. • '"•"•-,.

ß / V •.•../ b, • •", /..:' © \",,

.......... 2 '"'",,, "o } AMALIA Z AIdALIA

t8 ß .' ß ß ß ß ß ß ß 0.703 '0.•04 '0.•05 '0.706

INITIAL 87 St/ 86 Sr

Fig. 7. Initial 87Sr/86Sr (/Sr) - •Nd variations for San Luis Hills and other lavas in the region. "1 .... 2", and "3" next to open boxes note MANTLE 1,2, and 3 compositions, respectively. Data sources cited in Figure 3.

Based on detailed study of primitive and cont•aminated Hinsdale Formation lavas exposed in the San Juan and Jemez volcanic fields, Doe et al. [1969] conclude that the mantle source region for the basalts had a 2ø6pb/2ø4pb ratio of-* 18.2. This ratio is similar to that measured for the suite 1 sample that has the highest Cr and lowest Sr and Zr contents (152) and the suite 3 sample that has the highest ENd value and lowest SiO 2 content (205). The SiO 2- 2ø6pb/2ø4pb and eNd-2ø6pb/2ø4pb variations for suite 2 lavas define similar trends as compared to other lavas in the region and at San Luis Hills, suggesting that the mantle source regions for suite 2 magmas had a 2ø6pb/2ø4pb ratio of 18.0-18.2 (Figures 3 and 6). Although the highest 2ø6pb/2ø4pb ratio for suite 4 lavas is 17.8, the contaminated nature of these lavas preclude estimation of a mantle Pb isotope composition. The mantle source regions for primitive lavas exposed near Amalia and at the Taos Plateau volcanic field also had a 2ø6pb/2ø4pb ratio of-•l 8.2 [Dungan et al., 1986; Johnson et al., 1990]. This ratio lies within the range measured for mantle xenoliths from California, Ari- zona, and New Mexico [Zartman and Tera, 1973; Galer and O'Nions, 1989; Meijer et al., 1990]. An important characteristic of the mantle source for northern Rio Grande

rift mafic lavas, with the exception of lavas at Los Mogotes volcano, is its markedly higher 2ø7pb/2ø4pb ratio, at 2ø6pb/2ø4pb-18.2, as compared to northern hemisphere oceanic basalts ("MANTLE 1, 2"; Figure 4). Moreover, the mantle source for the majority of nodhern rift lavas ("MANTLE 1, 2"; Figure 4) lies at a 2ø7pb/2ø4pb ratio that plots above the 2ø7pb/2ø4pb-2ø6pb/2ø4pb array defined for Early Proterozoic crust in the region (Figure 4). In contrast, primitive lavas at Los Mogotes volcano have the lowest 2ø7pb/2ø4pb ratios of primitive lavas in the northern rift, and this source has Pb isotope compositions that are similar to those of mid-ocean ridge basalts (MORB) in the northern hemisphere and Hawaiian lavas ('ZMANTLE '3; Figure 4).

Distinction of the two mantle sources for San Luis Hills

lavas lies in their Sr and Nd isotope compositions (Figures 5-7). Extrapolation to primitive compositions indicate one source that has 87Sr/S6Sr -• 0.70495 and end "• -4 ("MANTLE 1; Figures 5-7), and a second source that has 875r/a6Sr 0.7044 and end '•' 0 ("MANTLE 2"; Figures 5-7). The MANTLE 1 source appears to have been tapped only dur-

ing early volcanism at San Luis Hills (suite 1) affd is not found in any younger rift-related volcanic rocks. The MANTLE 2 component was the source for suites 2, 3 and probably 4 at San Luis Hills, as well as primitive lavas at the Taos Plateau volcanic field and alkali-olivine basalts

near Amalia (Figures 5-7). A third mantle source is indicated for Los Mogotes volcano, which has 87Sr/86Sr-• 0.7038 and end'* +2 ("MANTLE 3"; Figures 5-7); this is similar to the source postulated for the Brazos basalts in the Tusas Mountains and mafic feldspathoidal lavas at the Raton-Clayton volcanic field [Phelps et al., 1983; Wil- liams, 1984]. A basanite near Amalia has an anomalously high Isr ratio (0.706), which may reflect an additional source (Figures 5 and 7).

The MANTLE 1 component has Sr and Nd isotope ratios that are similar to those of the EMI reservoir proposed by Zindler and Hart [1986], Hart et al. [1986], and Hart [1988], although ihe 2ø6pb/2ø4pb ratio of MANTLE 1 is somewhat higher than that of EMI. The end values of lavas largely derived from the oceanic asthenosphere (MORB), as well as virtually all ocean islands, are significantly higher than the end value of MANTLE 1, leading us to ascribe MANTLE 1 as a lithospheric composition. The higher end values of MANTLE 2 and 3 components that are the sources for younger lavas in the northern rift may reflect a mixture of asthenosphere and lithosphere sources that occurred during continued extension, as has been interpreted for rift-related lavas in the central and southern parts of the Rio Grande rift [Perry et al., 1987, 1988]. That the 2ø7pb/2ø4pb and 2ø6pb/2ø4pb ratios of MANTLE 1 and 2 do not lie in the field for oceanic lavas in the northern hemi-

sphere further support a lithospheric component for these sources.

COMPOSITION OF THE LOWER CRUST

The isotopic compositions of contaminated lavas in the northern Rio Grande rift region, including those at San Luis Hills, bear on current debates regarding the isotopic compositions of the lower crust. As noted above, most lavas in the northern rift region have Pb isotope compositions which indicate a crustal component that has 2ø6pb/2ø4pb ratios that are less than 17.0. Recent studies of lower

crustal xenoliths, however, have suggested that the lower crust has relatively high 2ø6pb/2ø4pb ratios that lie to the rioht c•ftho ooc•chrcm [o o •'•n•,rrtnYt •,t rtl 1 QRR'

and Goldstein, 1990; Kempton et al., 1990]. Many lower crustal xenolith suites have been interpreted to represent recent magmatic underplating [Rudnick et al., 1986; Rudnick and Taylor, 1987; Rudnick and Williams, 1987; Kempton et al., 1990; K.L. Cameron et al., Granite-facies xenoliths from north central Mexico' Evidence for a major pulse of mid-Cenozoic crustal growth, submitted to Jour- nal of Geophysical Research, 1991 ], which may indicate that lower crust which has high 2ø6pb/2ø4pb ratios is more commdn in regions of recent tectonic or magmatic activity [e.g., Rudnick and Goldstein, 1990]. Some lower crustal xenolith suites that have high 2ø6pb/2ø4pb ratios are interpreted to represent lower crust of Proterozoic age, such as at Camp Creek, Arizona [Esperanca et al., 1988], although a Proterozoic age for these xenoliths has been debated [Esperanca et al., 1990; Johnson, 1990].

The necessity of a nonradiogenic crustal component in


evolved igneous rocks of the northern Rio Grande rift region highlights the importance of Precambrian crust that has low 2ø6pb/2ø4pb ratios. Other studies of evolved, young igneous rocks that were emplaced in Precambrian crust support the presence of low 2ø6pb/2ø4pb lower or middle crust [e.g. Doe et al., 1968; Peterman et al., 1970; Dickin, 1981; Doe et al., 1982]. Many studies have shown that U depletion is characteristic of granulite-grade rocks, including high-grade crustal xenoliths [e.g., Dostal and Capedri, 1978; Fowler, 1986; Rudnick and Taylor, 1987; Reid et al., 1989; Whitehouse, 1989; Kempton et al., 1990], and we endorse the view that the lower crust should have generally low U/Pb ratios [Zartman and Doe, 1981; Zartman and Haines, 1988]. The importance of Precambrian crust that has low 2ø6pb/2ø4pb ratios is demonstrated by the fact that such crust is found in high-grade terranes on every continent [e.g., Moorbath et al., 1969; Gray and Oversby, 1972; Moorbath et al., 1975; Sobotovich et al., 1973; Leeman, 1979; Griffin et al., 1980; Tilton and Barreiro, 1980; DePaolo et al., 1982; Bernard-Griffiths et al., 1984; Cohen et al., 1984; Ovchinnikova et al., 1987; van Calsteren et al., 1988].

The relatively low ENd values for volcanic rocks at San Luis Hills that contain a large crustal component (Table 2), in addition to lavas in the northern rift region that are similarly contaminated [Johnson et al., 1990; Riciputi and Johnson, 1990; Colucci et al., this issue], suggest interaction with crust that had present-day/•md values that were less than -12. If the low 2ø6pb/2ø4pb ratios of many of these rocks is indicative of interaction with lower crust, then the present-day /•md values of the upper and lower crust in northern New Mexico and southern Colorado may be similar [DePaolo, 1981b; Nelson and DePaolo, 1985]. If these conclusions are generally applicable to the crust in western North America, they stand in contrast to recent Nd isotope studies of lower crustal xenoliths in western North

America. Several studies of xenoliths have suggested that the Proterozoic lower crust may have /•md values that are significantly higher than those of exposed Proterozoic crust [e.g., Esperanca et al., 1988; Ruiz et al., 1988]. Critical to this interpretation, however, is the evidence that the xenoliths represent lower crust that is of Proterozoic age, and this debate continues [Cameron and Robinson, 1990; Esperanca et al., 1990; Johnson, 1990; Ruiz et al., 1990].

EVOLUTION AND STABILIZATION

OF THE LITHOSPHERIC MANTLE

A large number of studies of mafic lavas in the western United States have interpreted low 143Nd/144Nd ratios as diagnostic of an ancient lithosphere mantle source, similar to our interpretation of the MANTLE 1 component at San Luis Hills [e.g., Menzies et al., 1983; Vollmer et al., 1984; Fraser et al., 1985; Dudas et al., 1987; Carlson and Hart, 1987; Fitton et al., 1988; Ormerod et al., !988; Farmer et al., 1989; Thompson et al., 1989; Kempton et al., this issue]. Although the mechanisms for generating low •43Nd/•44Nd ratios in the lithospheric mantle often are not clear, metasomatic and subduction processes are some of the more common mechanisms that have been proposed [e.g., Ormerod et al., 1988; Fitton et al., 1988; Menzies, 1989; Kempton et al., this issue]. Neodymium and Pb

isotope compositions of mafic lavas in the northern Rio Grande rift region are discussed below in the context of models for evolution and stabilization of the lithospheric mantle. Moreover, the exceptional data base of Nd and Pb isotope ratios and REE concentrations that is available for Proterozoic rocks in Colorado and New Mexico provides an important temporal framework for interpretation of the isotopic compositions of Cenozoic lavas.

Nd Isotope Constraints

Tholeiite lavas that are LREE depleted are common in lower Proterozoic sections exposed in New Mexico and Colorado. These include the Tijeras, Pecos, and Dubois greenstone sequences [Condie and Budding, 1979; Condie, 1980; Condie and Nuter, 1981; Nelson and DePaolo, 1984; Knoper and Condie, 1988; Robertson and Condie, 1989]. If these rocks are products of large degrees of partial melting of mantle that did not contain garnet as a residual phase, then the high measured mSm/•44Nd ratios would be approximately the same as those of the mantle source regions. Assuming that this mahtle was incorporated into the lithosphere and evolved as a closed system, the average present-day /•md value would be ~ +7 (Figure 8). If the LREE-depleted lavas were generated by relatively small degrees of partial melting of mantle that contained residual garnet, the mSm/144Nd ratio of the source would be sub- stantially higher than that measured for the lavas, and this would require a very high average present-day/•md value for the mantle (> +12; Figure 8).

Many Early •Proterozoic mafic lavas have relatively high LREE contents, however, and melting calculations predict that these were derived from mantle source regions that had relatively low •47Sm/•44Nd ratios (Figure 8). For example, assuming that the calc-alkaline Pecos lavas were derived by 30% partial melting with garnet as a residual phase, the average 147Sm/144Nd ratio of the mantle source region would be 0.15-0.16, which would produce a present-day /•md value of-7 during 1750 m.y. of evolution (Figure 8). Gabbroic parts of the Pikes Peak batholith in Colorado appear to have been derived from mantle source regions that had low mSm/144Nd ratios [Barker et al., 1976], which would produce low present-day/•md values (Figure 8).

Although the earliest Proterozoic mafic lavas in Colo- rado and New Mexico were derived from a depleted mantle, as indicated by the high 147Sm/•44Nd ratios measured for some lavas, and the uniformly high /•md values calculated for all Proterozoic lavas (Figure 9a) [Nelson and DePaolo, 1984], we propose that continued mafic magmatism became more LREE enriched with time. Geo-

graphically extensive 1700-1600 Ma plutonic rocks in New Mexico and Colorado [e.g., Condie and Budding, 1979] probably reflect significant magmatic thickening of the lithosphere, and LREE-enriched mafic magmas tend to be most common in regions that are underlain by relatively thick lithosphere [e.g., Pearce, 1982, 1983). LREE enrichment of the lithospheric mantle may have occurred by metasomatism associated with Early and Middle Protero- zoic magmatism, in addition to crystallization of LREE- rich basaltic magmas in the uppermost mantle. It seems likely that the abundant LREE-enriched crustal magmatism that occurred shortly after initial crust formation in the region was associated with an increase in LREE contents


throughout the entire lithospheric column as it became isolated from the convecting asthenospheric mantle (Fig- ure 9b).

Input of low end value crustal material through subduction in the Proterozoic would also generate an average evolution of the lithospheric mantle toward low present- day end values (Figure 9c). Evolution of the lithospheric mantle to low end values may also occur if Proterozoic mantle-derived magmas stalled at the crust-mantle bound- ary and assimilated lower crust. Crustal recycling may occur by this mechanism through return of cumulate miner-

EVOLVED FROM DEPLETED MANTLE AT 1750 Mo

(o) -15 -5 5 15 25

I I I I I I I I I

1000 Mo PIKES PEAK BATHOLITH

4 • MEASURED SLL1½1½ PART

2 • MEASURED MAFIC PART

0 --! m ß ß ß m m m m ß m m m m m m m m mm m m

lO B 8

6 PECOS

m • • CALC-ALKALINE 4 BASALT 2 • MEASURED 0 ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß m

15 m OTHER

J • MAFIC LAVAS 5

0 ............. ;";"•"•, .... 12

10 m DUBOlS D 8 I GREENSTONE 4

2

0 ß ß ß ß ........... ß ....

PECOS E THOLElITE > 0.3 lO

LAVAS

• MEASURED

........... ß ß ß ß ß ß ß ß

15

10

5

0

F

lO TIJERAS GREENSTONE

5 • MEASURED

ß ß

O. 11 0 1• 0.21 0.26 0.31 147 Srn/144 Nd

MIINTL E SOURCE

MELTING m 30 % MELTING

als to the upper mantle [Arndt and Goldstein, 1989; John- son et al., 1990]. We note, however, the importance of carefully assessing the effects of crustal contamination in the young lavas. Several studies have suggested that decreasing end values with increasing La/Nb ratios may reflect the presence of an ancient subduction component [e.g., Fitton et al., 1988; Kempton et al., this issue], although this same trend is produced in the San Luis Hills suite by crustal contamination (Figure 10).

The low SNd values for late Cenozoic mafic lavas in northwest Colorado (-4 to -10) [Leat et al., 1988, 1989, 1990; Thompson et al., 1989] may reflect lithospheric mantle that has remained isolated from interaction with the

asthenosphere since the Proterozoic. The lowest end value for mafic lavas from the northern Rio Grande rift that

represent mantle values is -4 (MANTLE 1 component, this study), and this is interpreted to largely reflect a lithospheric mantle source, although it may also represent a mixture of Nd derived from lithospheric (low end value) and asthenospheric (high end value) mantle.

Pb Isotope Constraints

The 2ø?pb*/2ø6pb* isochron "ages" of mafic lava suites in the western United States have been used to infer the age of the underlying lithosphere [e.g., Doe et al., 1982; Dudas et al., 1987]. Because rocks from the northern Rio Grande rift that have 2ø6pb/2ø4pb < -• 18.2 are interpreted to have assimilated crust that had non-radiogenic Pb isotope ratios, the 2ø?pb*/2ø6pb* isochron "ages" calculated for these rocks place no constraints on the age of the lithospheric mantle. That the 2ø?Pb*/2ø6pb* isochron "ages" calculated for most northern rift lavas are similar (within large errors) to the age of the Proterozoic basement in the region simply reflects the combination of a large difference in 2ø6pb/2ø4pb ratios of the crust and mantle components and the fact that the mantle source has Pb isotope ratios that plot near the crustal array (Figure 4). These features produce contamination trends that are sub-parallel to the crustal array. In contrast, a steep trend on a 2ø?pb/2ø4pb-2ø6pb/2ø4pb diagram (i.e., very old 2ø?pb*/2ø6pb* "age") may be produced by mixing between a mantle component that has Pb isotope ratios that lie off the crustal array and crust that has

Fig. 8. Histograms of 147Sm/•44Nd ratios measured for Early Prot- erozoic mafic rocks exposed in New Mexico and Colorado (coarse diagonal pattern) and those calculated for their mantle source regions assuming 5% melting (fine diagonal pattern) and 30 % melting (solid pattern). Silicic rocks of Pikes Peak batholith also shown (no pattern). Predicted present-day ENd values of mantle sources shown at top, assuming derivation from depleted mantle at 1750 Ma [DePaolo, 1981b]. Melting calculations assume modal equilibrium melting with 10% garnet, 30% clinopyroxene, and 60% olivine. Mineral/ melt distribution coefficients from Grutzeck et al. [1974], Shimizu and Kushiro [1975], Irving and Frey [1978, 1984], Fujimaki et al. [1984], and Green and Pearson [1985]. Nonmodal melting involving garnet or melting without garnet will shift calculated source compositions toward lower 147S1TIf •44Nd ratios and would produce lower present-day ENd values. Data for Proterozoic rocks from Barker et al. [1976], Condie and Budding [1979], Condie [1980], Condie and Nuter [1981], Condie and McCrink [1982], Nelson and DePaolo [ 1984], Boardman and Condie [ 1986], Knoper and Condie [ 1988], and Robertson and Condie [ 1989].

JOHNSON AND THOMPSON' ISOTOPIC COMPOSITION OF MAFIC VOLCANIC ROCKS 13,603

-8

-12

-16

z

z

^•E (•o) 0 500 1000 1500 2000

........ - PLUTONIC AND I

......... /- • I- OT...oc.s I

EVOLUTION BY / • B

EVOLUTION BY SUBDUCTION -

Fig. 9. eNa evolution diagrams showing (a) net evolution of continental crust and lithospheric mantle, (b) interpretive evolution by episodic intrusion into the lithospheric mantle of LREE-enriched magmas, and (c) interpretive evolution by episodic contamination of lithospheric mantle by ancient subduction. High end values for Early Proterozoic mafic lavas indicate that the early crust was originally underlain by depleted mantle, which would have evolved to very high present-day end values had this mantle remained isolated beneath the crust. Cenozoic lavas exposed in northwest Colorado and the lowest end lavas in the northern Rio Grande rift region may largely represent a lithospheric mantle composition, whereas higher end lavas in the northern rift may represent mixtures of asthenospheric and lithospheric mantle. Solid boxes in Figure 9a at end = -4, 0, and +2 represent MANTLE 1, MANTLE 2, and MANTLE 3 reservoirs for the northern Rio Grande rift, respectively, as discussed in text and previous figures. Data for Proterozoic rocks from DePaolo [1981b] and Nelson and DePaolo [1984, 1985].

2ø6pb/2ø4pb ratios that are close to those of the mantle. This may be the best explanation for the Pb isotope variations at Los Mogotes volcano (Figure 4).

Assuming that the Proterozoic crust and lithospheric mantle in northern New Mexico and Colorado ("MANTLE 1 and 2"; Figures 4 and 11) were originally derived from the asthenospheric mantle, such as that represented by oceanic lavas in the northern hemisphere, their relatively high 2ø7pb/2ø4pb ratios require explanation. We propose that Pb which had a high 2ø7pb/2ø4pb ratio, similar to that of the Stacey-Kramers Pb evolution curve at 1750 Ma, was trans- ported into the mantle during arc-related magmatism that occurred during the Early Proterozoic in Colorado and New Mexico (Figures 11 and 12) [e.g., Condie, 1982]. This is similar to models proposed for Pb isotope evolution of Precambrian crust in other areas of the western United

States [Wooden and Mueller, 1988; Wooden et al., 1988]. The sources of this Pb could be subducted oceanic sedi-

ments or crustal material from the adjacent Archean craton (Figure 12), similar to sources proposed to explain shifts

-2

-4

-6

0.5

•.------' .... •' SUITE 4 ß •'•-•

\ l• "-'•., .• '•.• x \

_•.

•.•.•__.•'•

'•'•.•.•.•.• ' ' 2;5 ' s55 ' 455 ' 5.5

La/Nb

Fig. 10. end - La/Nb relations for San Luis Hills lavas (fields added for emphasis). San Luis Hills data define similar trends on a 2ø6pb/2ø4pb-La/Nb diagram; these variations indicate that increases in La/Nb ratios are largely due to crustal contamination. The crustal component probably had high La/Nb ratios, suggesting that it was LREE rich. This is consistent with the low end values that are inferred for the crustal contaminant. eNd-La/Nb variations at San Luis Hills overlap those of other mafic lavas in the western United States [e.g., Fitton et al., 1988].

toward high 2ø7pb/2ø4pb ratios of Cenozoic arc-related and behind-arc lavas in the western United States [Church, 1976; Carlson, 1984; Carlson and Hart, 1987]. The steep trend defined by Early Proterozoic ore leads from Colorado and New Mexico cannot represent an isochron age, but instead probably reflects mixing between subducted Pb of --1750 Ma Stacey-Kramers average crust composition and asthenospheric mantle (Figure 11).

That the MANTLE 1 and 2 components have Pb isotope compositions which plot above the crustal array (Figures 4 and 11) place important constraints on models for development of the Proterozoic lithosphere. Subduction that continued after initial crust formation may have resulted in greater contamination of the developing lithospheric mantle with 2ø7pb as compared to the crust. Extensive contamination of the lithospheric mantle could produce a lithospheric mantle composition that lies above the Prot- erozoic crustal array on a 2ø7pb/2ø4pb-2ø6pb/2ø4pb diagram (Figure 11). If the major flux of crustal Pb occurred at 1750 Ma, the contaminated lithospheric mantle would have evolved with a 238U/2ø4pb ratio of 8.1 ("MANTLE 1 and 2"; Figure 11). This is slightly lower than the ratio of 9.7 calculated for the mantle source for Los Mogotes volcano ("MANTLE 3"; Figure 11), which may represent asthenospheric mantle.

SUMMARY AND CONCLUSIONS

Early rift volcanism in the axis of the Rio Grande depression in northern New Mexico and Colorado was more mafic

and contained significantly smaller crustal components than volcanic rocks that were erupted on the flanks of the rift immediately prior to extension. Restriction of mafic and isotopically primitive early rift lavas to the axial part of the rift zone suggests that the modem rift depression was the primary locus of mantle upwelling at 26 Ma. Primitive lavas at San Luis Hills indicate that at least two isotopically distinct mantle sources were involved in early rift magmatism. One source, which is characterized by 87Sr/86Sr=0.70495, end = -4, and 2ø6pb/2ø4pb=18.2, has


15.7

15.6

15.5

15.4

15.3

15.2

1750 Ma

S-K COUP

MANTLE 1 2

MANTLE 3 HAWAII

I MIXING IN $UBDUCTION ZONE AT 1750 Me 1750 Me ASTHENOSPHERE

15.1 15.0 16.0 17.0 18.0 19.0

206 Pb/204 Pb

a 1750 Ma

LOW 143Nd •t [ LOW 143Nd HIGH 207pb HIGH 207pb f •.

ASTHENOSPHERE

Fig. 11. 2ø6pb/2ø4pb-2ø7pb/2ø4pb evolution diagram illustrating contamination (mixing) of subcontinental mantle with Archean crustal Pb during Early Proterozoic subduction, noted by arrow in lower left. Composition of 1750 Ma asthenospheric (oceanic) mantle from Plumbotectonics model of Zartman and Doe [ 1981 ]. Composition of Pb contaminant is thought to be similar to that of 1750 Ma Stacey- Kramers (S-K) average crust [Stacey and Kramers, 1975]. Evolution of asthenospheric mantle to MANTLE 3 composition requires a 238U/2ø4pb ratio of 9.7, whereas evolution of contaminated lithospheric mantle that had Stacey-Kramers composition at 1750 Ma to present-day MANTLE 1 and 2 composition requires a 238U/2ø4pb ratio of 8.1. This later evolution is interpreted to represent that of the lithospheric mantle, following formation and stabilization in the Early Proterozoic, and its isochron and evolution path are shown in dashed lines. Array for Proterozoic crustal rocks (double heavy lines) lies between the lithospheric mantle isochron and the field for oceanic lavas (J. Wooden, et. al., unpublished data, 1987). Field of Early Proterozoic ore leads in Colorado and New Mexico from Stacey et al. [1977] and Stacey and Hedlund [1983].

]43Nd/144Nd ratios that are slightly higher than those measured for late Cenozoic lavas in northwest Colorado that

erupted outside the rift, and this source is thought to largely reflect a lithospheric mantle composition. The other source, which is characterized by 87Sr/a6Sr=0.7044, and 2ø6pb/2ø4pb=l 8.2, may reflect a mixture of lithospheric and asthenospheric mantle compositions.

Comparison of the isotopic compositions of Cenozoic lavas and those of Proterozoic crust in the region provide important constraints on evolution of the lithosphere. Neodymium isotope compositions of Early Proterozoic mafic lavas in New Mexico and Colorado indicate that

depleted mantle underlay the earliest crust in the region [Nelson and DePaolo, 1984], which presumably reflects the major mantle source for magmatism that existed prior to formation of a stable lithospheric mantle (Figure 12a). During the major period of Early Proterozoic subduction in the region, progressively larger volumes of LREE-enriched magmas were emplaced into the developing lithosphere [e.g., Condie and Budding, 1979, Condie, 1982, 1986; Condie and McCrink, 1982; Knoper and Condie, 1988; Robertson and Condie, 1989], producing a decrease in the average 147Sm/]44Nd ratio of the lithospheric mantle (Figure 12b). Melting calculations indicate that the average •47Sm/•44Nd ratio of the mantle source region for the LREE- enriched magmas may have been NO. 15-0.16, which would produce an average present-day /•md value for the lithospheric mantle of-5 to -7, similar to Cenozoic lavas in northwest Colorado and rift-related lavas of suite 1 at San

Luis Hills. Anorogenic, intraplate plutonism occurred be-

b 1600 Ma

LOW 143Nd

LOW 143Nd •t HIGH 207pb HIGH 207pb

HEAN

.- X, TM x• '• i•x x , ••"•,, ..._-'-

• }-- - -,_Y_.2, T,,'?'.'" ---- - ---M9 HO

wr/_///?2)!• '"'• Z3eu _. w•:•J• Lo L• =8'1

ASTHENOSPHERE

1000 TO 1500 Ma

238 U -I0

147Sm 144N• =O'IZ 238U X X X X 204p b- I X X 147Sm

144Nd =0-12 TO 0.16 !• 9 9 238 U

204p• =8'1 •1

" ARCHEAN

!!?:• •'.;;- CRATON i.u Lo Lu • W

? MOHO 0

147Sm - =0.24

144Nd

238 U = 9.7

204pb

L,'gr..: :;:.:! ASTHENOSPHERE Lo Lu

Fig. 12. Interpretive cross sections through the crust and upper mantle illustrating formation and stabilization of the Proterozoic lithospheric mantle in Colorado and New Mexico. (a) Period of initial crust formation in primitive arc setting, (b) period of intense orogenic plutonism in mature arc setting, and (c) anorogenic plutonism. Lithosphere formation and stabilization is thought to be accompanied by contamination of the subcontinental mantle with Archean crustal Pb, either from subducted sediments or material from the adjacent Archean craton (Figures 12a and 12b). A net increase in the LREE content of the lithospheric mantle is thought to have occurred during major episodes of LREE-enriched orogenic (---1600 Ma) and anorogenic (1000-1500 Ma) plutonism. Magma intrusion and generation shown in solid pattern, crustal melting in cross pattern, and plutonic bodies in hatchered pattern.


tween 1500 and 1000 Ma in the region [e.g., Condie and Budding, 1979] and may have also led to a decrease in the average 147Sm/144Nd ratio of the lithospheric mantle (Figure 12c), as indicated by the presence of LREE-enriched mafic rocks in at least the Pikes Peak batholith [Barker et al., 1976). The •Sd value of the modem lithospheric mantle in the region is >18 units lower than that of the projected modem value for the asthenospheric mantle from which the Proterozoic crest was originally derived. If the lithospheric mantle evolved toward low present-day •Sd values, the amount of crustal recycling proposed for the genesis of Middle Proterozoic plutons has been markedly overesti- mated [DePaolo, 1981b).

Based on isotopic variations in mafic lavas, a number of workers have proposed the existence of low 2ø6pb/2ø4pb mantle beneath the continents [e.g., Doe et al., 1982; Fraser et al., 1985; Peng et al., 1986; Dudt•s et al., 1987; Thompson et al., 1989; Hawkesworth et al., 1990a; Kempton et al., this issue]. Direct evidence for low 2ø6pb/2ø4pb lithospheric mantle is provided by xenolith and kimberlite samples, although most studies have been re- stricted to regions of Archean lithosphere [e.g., Kramers, 1977, 1979; Smith, 1983; Cohen et al., 1984; Menzies et al., 1987; Walker et al., 1989; Hawkesworth et al., 1990b]. Analyses of clinopyroxene mineral separates from mantle xenoliths from the western United States do not support a model for exceptionally low 2ø6pb/2ø4pb mantle in the region [Zartman and Tera, 1973; Galer and O'Nions, 1989; Meijer et al., 1990], although the presence of a sulfide phase may be an important low 238U/2ø4pb component in the mantle [Kramers, 1979; Meijer et al., 1990]. Many studies have highlighted the extreme sensitivity of Pb isotope compositions of mafic magmas to modification by crustal contamination [e.g., Doe et al., 1969; Dickin, 1981; Cortini a•id van Calsteren, 1985], and we urge caution in interpret- ing the very low 2ø6pb/2ø4pb ratios measured for some mafic lavas in the western United States as unambiguously repre- senting a nonradiogenic lithospheric mantle. Although many lavas at San Luis Hills have near-primitive Sr and Nd isotope compositions, only two have Pb isotope ratios that probably reflect their mantle source regions. While com- pelling arguments may be made for the insensitivity to crustal contamination of Sr and Nd isotope ratios in alkaline rocks, which may contain up to 10 times greater Sr and Nd concentrations as compared to average crust [e.g., Vollmer et al., 1984; Fraser et al., 1985; Dudt•s et al., 1987], few mafic lavas contain Pb concentrations that are as high as the crest, including strongly alkaline rocks, mak- ing Pb isotope ratios susceptible to modification by even small amounts of crustal contamination.

The only valid constraint on the age of stabilization of the lithospheric mantle in the northern Rio Grande rift region is the fact that the Pb isotope composition of most primitive lavas lies on a 2ø7pb/2ø4pb-2ø6pb/2ø4pb isochron that lies above the Proterozoic crustal array and intersects the highest 2ø7pb/2ø4pb ratios for Early Proterozoic ore leads. The high 2ø7pb/2ø4pb ratios are best explained through incorporation of Archean Pb in a Proterozoic subduction zone.

Lead-lead isochrons of the mafic lavas do not reflect "ages" in the sense of recording a time of U/Pb fractionation in their source regions, but instead are artifacts produced by assimilation of lower crust, a conclusion also reached by Dungan et al. [1986] in their study of the Taos Plateau

volcanic field. In addition, Nd model ages for the San Luis Hills lavas, which vary from 718 to 1240 Ma (calculated relative to depleted mantle [De?aolo, 1981b]), probably do not record meaningful age information, due to the effects of heterogeneous isotopic compositions of the mantle and fractionation in Sm/Nd ratios during melt generation.

Volumetrically subordinate lavas in suites 2, 3, and 4 at San Luis Hills assimilated substantial amounts of crust and

suggest that the lower crust beneath the northern Rio Grande rift region has low 87Sr/86Sr and 2ø6pb/2ø4pb ratios of 0.703-0.704 and 16.0-17.0, respectively. The •Sd values of the lower crust that was assimilated must be _


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