Oligocene Basaltic Volcanism of the Northern Rio Grande ...geoscience.wisc.edu/icp-tims/wp-content/uploads/... · THOMPSON ET AL.: OLIGOCENE BASALTIC VOLCANISM, RIO GRANDE RIFT 13,579

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 96, NO. B8, PAGES 13,577-13,592, JULY 30, 1991

Oligocene Basaltic Volcanism of the Northern Rio Grande Riff' San Luis Hills, Colorado

REN A. THOMPSON

U.S. Geological Survey, Denver, Colorado

CLARK M. JOHNSON

Department of Geology and Geophysics, University of Wisconsin, Madison

HARALD H. MEHNERT

U.S. Geological Survey, Denver, Colorado

The inception of the Rio Grande rift in northern New Mexico and southern Colorado was accompanied by voluminous mafic volcanism preserved in part as erosional remnants on an intrarift horst within the current axial rift graben of the San Luis Valley. Oligocene (--•26 Ma) volcanic rocks of the Hinsdale Formation at San Luis Hills range from 49 to 57 wt % SiO2 and include nepheline and hypersthene normative lavas. A mildly alkalic series consisting of trachybasalt, basaltic trachyandesite, and trachyandesite is volumetrically dominant, olivine tholeiites are subordinate, and xenocrystic trachyandesites containing abundant quartz and plagioclase xenocrysts occur only locally. Relative to the San Luis Hills olivine tholeiites which have La/Smn --• 2, the more alkaline series are enriched in light rare earth elements (LREE) and have La/Sm ratios that increase in the trachybasalt-basaltic trachyandesite suite (La/Smn --• 3) to xenocrystic trachyandesites that are the most LREE enriched (La/Smn --• 4). Chondrite-normalized, trace element patterns for the lavas in the San Luis Hills are similar in shape within the mildly alkaline to transitional series; they have characteristic Nb and Ta depletions and high K and Th relative to Ta, Nb, and LREE. Major and trace element constraints support a petrogenetic model of fractionation plus lower crustal assimilation for petrologic suites within the San Luis Hills rocks, although the model cannot relate lavas for the entire series to a common parent. Most mafic lavas of the San Luis Hills were evolved (Mg # <60) and contaminated by LREE-enriched silicic partial melts of granulitic lower crust depleted in Rb, Th, and U. Pb isotopes are the most sensitive indicators of crustal contamination, whereas shifts in Nd and Sr isotope ratios are associated with large amounts of assimilation. However, relatively noncontaminated lavas can be identified and indicate at least two mantle source regions were involved.

INTRODUCTION

Propagation of rifling into northern New Mexico and southern Colorado in the late Oligocene was accompanied by widespread basaltic volcanism extending north- northwestward from northern New Mexico to the Wyoming state line (Figure 1). Within this region, Oligocene and Miocene basaltic rocks are preserved as erosional remnants that are interbedded with basin fill sedimentary rocks (Figure 2) that have been subjected to late Tertiary regional uplift, erosion, and extensional faulting associated with the inception of the Rio Grande rift. Pliocene and Pleistocene volca-

nism of the northern Rio Grande rift is well documented, especially the Taos Plateau volcanic field [Dungan et al., 1986; McMillan and Dungan, 1986; Dungan, 1987; McMillan and Dungan, 1988] and the central Rio Grande rift [Bald- ridge, 1979; Baldridge et al., 1982], although the extensive basaltic volcanism that is associated with the onset of rifting [Lipman and Mehnert, 1975; Lipman et al., 1986; Leat et al., 1988; Thompson and Machette, 1989] is less well studied. We present new field, geochronologic, and geochemical data for mafic volcanic rocks of the Hinsdale Formation exposed

Copyright 1991 by the American Geophysical Union.

Paper number 91JB00068. 0148-0227/91/91 JB-00068505.00

at San Luis Hills, which preserve the most compositionally diverse and largest volumes of early rift basaltic rocks of southern Colorado and northern New Mexico (Figure 1). The spatially and temporally restricted nature of volcanism in the San Luis Hills provides an exceptional opportunity to examine the compositional diversity of early rift magmatism and the role of crustal contamination in their evolution.

These data are in turn examined in light of current models for Pliocene rift volcanism and predictive models of volcanism associated with lithospheric extension and rifting [e.g., Perry et al., 1987; Glazner and Ussler, 1989].

GEOLOGIC SETTING

The late Cenozoic Rio Grande rift developed in Protero- zoic age crust along earlier structures related to the ancestral Rocky Mountain (late Paleozoic) and Laramide (Late Cre- taceous-early Tertiary) orogenies. The current physio- graphic expression of the rift is a series of north trending en echelon basins extending from Chihuahua, Mexico northward to central Colorado [Tweto, 1979]. In contrast to the southern and central parts of the rift, which are bounded on the west by the Basin and Range province, the northern rift basins are confined to a narrow axial zone. Bordered on the

east by the Great Plains and the west by the Colorado Plateau (Figure 1), the northern rift region currently stands

13,577

13,578 THOMPSON ET AL.' OLIGOCENE BASALTIC VOLCANISM, RIO GRANDE RIFT

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Fig. 1. Regional map showing locations of early rift basaltic volcanic rocks of northern New Mexico and Colorado and their relation to major uplifts of the southern Rocky Mountains. Solid areas represent Oligocene to Pliocene basaltic volcanic fields, principally erosional remnants of an aerially more extensive field during the late Tertiary. Heavy stippled areas are Pliocene and younger basalt fields of the northern Rio Grande rift and Jemez zone of volcanism (indicated by arrows). Modified from Tweto [1979].

THOMPSON ET AL.: OLIGOCENE BASALTIC VOLCANISM, RIO GRANDE RIFT 13,579

Basaltic Volcanism 0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

Sedimentation

Fig. 2. Stratigraphic summary of Oligocene to Miocene basaltic volcanism and rifting in northern New Mexico and Colorado. Vertical lines represent the range of published ages. The sources of geochronologic data are as follows: Flat Tops [Larson et al., 1975]; Grande Mesa [Marvin et al., 1966]; Glenwood Springs [Larson et al., 1975]; Elkhead Mountains [Luedke and Smith, 1978]; Hinsdale Formation lavas of the San Juan Mountains [Hon and Mehnert, 1983]; Jemez Mountains [Gardner et al., 1986; Goffet al., 1989]; San Luis Hills (this study); Amalia [Lipman et al., 1986].

at high elevation. The crest of the Sangre de Cristo Range (4000 m) rises 1700 m above the San Luis Valley floor. However, during the Oligocene and Miocene, basaltic volcanism was associated with shallow basin sedimentation that

marked the early stages of rift development (Figure 2). Erosional remnants of these aerially extensive basaltic volcanic fields trend northwestward from northern New Mexico

to northern Colorado (Figure 1). The volcanic stratigraphy exposed in the southern Colo-

rado segment of the northern Rio Grande rift is similar to that preserved in north central Colorado (Figures 1 and 2), where mesa-capping erosional remnants of a formerly more extensive basalt field occur in the vicinity of Flat Tops. Mildly alkaline to transitional 24-20 Ma basalts up to 200 m thick [Larson et al., 1975] interfinger with basin fill sedimentary rocks of the Browns Park Formation, a temporal equivalent of the Santa Fe Group in southern Colorado. Younger basaltic andesites that erupted between 14 and 10 Ma have accumulated to thicknesses of 200 m and are

interbedded with tuffaceous sedimentary rocks [Larson et al., 1975]. Broadly contemporaneous basaltic volcanism to the southwest in the Grand Mesa area (9.7 Ma) produced a series of lava flows that have an aggregate thickness of-•250 m [Marvin et al., 1966] and in the Glenwood Springs area 8-11 Ma basalt flows cap erosional mesas and unconformably overlie Pennsylvanian sedimentary rocks.

Initiation of Rifting

Early estimates of the ages of rift inception were based on age determinations of volcanic rocks that are interbedded with coarse clastic rocks of the basal Sante Fe Group [Lipman and Mehnert, 1975]. Recent estimates, which have

generally shown that rifting began earlier than originally thought, have focused on ages of dikes that also demonstrate the orientation of least principal stress at the time of intru- sion [Dasch et al., 1969; Aldrich et al., 1986]. Magmatism is associated with crustal extension at 31 Ma in the Trans-

Pecos area of southwest Texas and northeastern Chihuahua

[Price and Henry, 1984; Price et al., 1986] and at 32 Ma in southern New Mexico. The oldest ages of rift-related basalts are generally younger to the north; by 26-24 Ma, dominantly basaltic volcanism was well established throughout west Texas, New Mexico, and Colorado. Magmatism and faulting, associated with northward propagation of the rift, ex- tended into northern New Mexico and southern Colorado at

26 Ma [Lipman and Mehnert, 1975], immediately following the waning stages of major caldera-related magmatism in the San Juan volcanic field (29-26 Ma) [Lipman et al., 1970] and coincident with the eruption of Amalia Tuff from the Questa caldera in the Latir volcanic field [Lipman et al., 1986]. Extensional structures in the Questa caldera tilted calc- alkaline to peralkaline extrusives and intrusives at about 26 Ma [Lipman, 1981; Hagstrum and Lipman, 1986; Lipman et al., 1986].

Early extension-related faults and dikes tend to have northwest trends [Aldrich et al., 1986; Henry and Price, 1986; Lipman, 1981]. Pliocene to Quaternary extension in the Rio Grande rift has produced north-south normal faulting within the axis of the rift (sigma 3 is east-west), in accord with estimates of regional stress orientations [Zoback et al., 1981]. The younger phase of rifting, which followed a period of relative tectonic and magmatic quiescence in the late Miocene (20-12 Ma), is marked by narrowing of rift basins. Early basin fill sedimentary rocks of the Sante Fe Group commonly crop out on uplifted blocks that are adjacent to current rift basins [e.g., Butler, 1971; Chapin and Seager, 1975; Tweto, 1979; Lipman, 1981]. Rejuvenation of rift tectonism was accompanied by widespread, dominantly basaltic volcanism, active regional uplift, and large-scale slip on basin-bounding faults that produced deep, asymmetric basins. Rift basins in northern New Mexico and Colorado

are arrayed in en echelon patterns and are offset along commonly northeast trending oblique structures or accom- modation zones [Rosendahl, 1987; Chapin, 1988].

San Luis Hills

The San Luis Hills consist of a series of flat-topped mesas and irregular hills that trend north to northeasterly for approximately 45 km from the Colorado-New Mexico bor- der (Figure 1). They are the northernmost surface expression of a major intrarift horst within the central depression of the San Luis Valley part of the northern Rio Grande rift (Figures 3 and 4). For most of its length, the horst is confined to the subsurface [Kleinkopf et al., 1970; Brister, 1990]; two additional surface exposures are at Brushy Mountain and Timber Mountain [Thompson et al., 1986]. Exposures at all three localities are entirely middle Tertiary volcanic and intrusive rocks.

Two Tertiary igneous sequences are exposed at San Luis Hills, an older intermediate-composition volcanic-intrusive assemblage, and unconformably overlying basaltic rocks of the Hinsdale Formation. The older assemblage was originally thought to be correlative with the intermediate- composition Oligocene lavas and breccias of the 33-29 Ma

13,580 THOMPSON ET AL.: OLIGOCENE BASALTIC VOLCANISM, RIO GRANDE RIFT

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I • I i• HJrmclale Basalt 0 10 20Kin '•;..• Bemin Fill 8edtm• Rocks • Tertiary Inlmeions

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Fig. 3. Diagrammatic cross section of the southern San Luis Valley at approximately the location of A-A' on Figure 1. Vertical exageration is approximately 5.5X. Modified from Lipman and Mehnert [1975].

Conejos Formation [Lipman et al., 1970] in the San Juan Mountains and Tusas Mountains to the west and southwest

respectively [Burroughs, 1971; Thompson and Machette, 1989]. A recent 4øAr/39Ar age determination on biotite from a late stage dike of the older igneous sequence of the San Luis Hills yielded an age of 27.7 Ma (unpublished age determination by D. Lux and M. A. Dungan, (written communication, 1989)). The older sequence therefore post- dates major caldera-forming volcanism of the southeastern San Juan volcanic field (29-28.2 Ma) and is temporally related to precaldera, intermediate-composition volcanic rocks of the Latir volcanic field to the southeast [Lipman et al., 1986]. Basement rocks are not exposed in the San Luis Hills, but Precambrian crystalline rocks have been found at a depth of 1675 m [Tweto, 1979] in a subsurface section of the horst 50 km north of the San Luis Hills and at depths of 3200 m 55 km to the northwest [Brister, 1990]. Precambrian rocks are exposed in the San Juan Mountains and Sangre de Cristo Mountains.

Older sequence mafic and intermediate-composition lavas are regnant at San Luis Hills, in contrast to volcanic rocks exposed on the flanks of the rift, which include intermediate- composition lavas that are overlain by major silicic ash flow eruptions. Andesitic to dacitic eruptions appear to have been followed closely in time at San Luis Hills by intrusions of subvolcanic, cogenetic plutons. Emplacement of quartz monzonite plutons and diorite stocks was accompanied by dike swarm intrusions that are offset from the main quartz monzonitic intrusions, which crop out 8 km to the northeast. Rapid unroofing of the subvolcanic complex prior to eruption of the Hinsdale Basalt indicates positive local topo- graphic relief within an area characterized by broad shallow basins during early rifting. The paucity of Los Pinos Forma- tion basin fill sedimentary rocks in the San Luis Hills section and the absence of the Oligocene Amalia Tuff, which crops out on the west side of the Taos Plateau farther south, suggest that the horst may have been a structural high as well during the Oligocene. Brister [1990] has determined through correlation of stratigraphic units in well logs and interpretation of seismic data that much of the structural relief observed

in the subsurface of the northern San Luis Valley, including definition of the axial horst, predates early rift extension and is probably associated with Laramide deformation.

Whole rock K-Ar age determinations (Table 1) on four samples that represent the compositional and stratigraphic extent of Hinsdale basalt suites in the San Luis Hills average

26 Ma, suggesting nearly contemporaneous eruption of all suites. These ages are similar to the oldest ages reported for basalts of the Hinsdale Formation in the San Juan Mountains

[Lipman and Mehnert, 1975]. Trachybasalts to basaltic trachyandesites are the dominant mafic compositions erupted during the Oligocene and Miocene in the San Luis Hills and in the centers of the San Juan and Tusas Moun-

tains; the tholeiitic volcanism was subordinate. In contrast, Pliocene rift magmatism of the San Luis Valley was dominated by olivine tholeiite lavas and related calc-alkaline rocks of the Taos Plateau volcanic field; eruption of transitional to alkaline magmas was minor. Scattered basalts erupted during Miocene rifting are also preserved along the northern parts of the Latir volcanic field [Lipman et al., 1986; Lipman and Reed, 1989], and erosional remnants crop out near the crest of the Sangre de Cristo Mountains north of the Latir volcanic field and are probably equivalent to Hinsdale Formation rocks.

The four petrologic suites of lower Hinsdale Formation lavas reported in this paper, defined on the basis of miner- alogy and whole rock chemistry, correspond with minor exception to mappable lithologic units reported in Thompson and Machette [1990] and simplified in Figure 4. The earliest eruptions of Hinsdale lavas in the San Luis Hills (suite 1 trachybasalts to trachandesites) constitute approximately 60 vol % of the eruptive products and include lavas, associated breccia, and near-vent pyroclastic deposits. Vent areas are characterized by steeply dipping accumulations of red- dishbrown cinders and spatter agglutinate that are commonly deeply eroded and overlain by lavas flows. Massive flows range in thickness from 3 to > 10 m and form a thick accumulation of lavas (300 m) on the southwestern side of Flat Top mesa (Figure 4), Intersuite sedimentary breaks of predominantly aeolian sediments form distinct marker beds only locally and probably represent brief time intervals. These lavas are overlain by trachybasalts of suite 2 (Thp, Hinsdale Formation, Figure 4) which form the upper third of the section at Pinon Hills and occur as minor erosional

remnants in the South Pinon Hills. Suite 3 (Tht, Hinsdale Formation, Figure 4) consists of a veneer (50 m) of thin pahoehoe lavas of diktytaxitic olivine tholeiite that contain distinctive vesicular segregations that form vertical pipes and horizontal sheets. Suite 3 lavas are restricted to the top of the section at South Pinon Hills. Subordinate sparsely vesicular andesites of suite 3 usually occur as isolated flows (not shown at scale of Figure 4) and are often separated from

THOMPSON ET AL.' OLIGOCENE BASALTIC VOLCANISM, RIO GRANDE RIFT 13,581

EXPLANATION

F• Qa F• Tha ß :-i• QIs '..'i• Tid ',[• Tx ','• Tiq ;'• Toa F• Tcu ,• Ts F] Tc13 n Thx El n Tht F• Tc11 n Thp F] Tca n Thb

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Fig. 4. Generalized geologic map of the San Luis Hills region modified from Thompson and Machette [1989]: Qa, Quaternary alluvium; Qls, Quaternary landslide deposits; Tx, Pliocene xenocrystic basaltic andesitc; Toa, Pliocene olivine andesitc; Ts, Servilleta Basalt. Hinsdale Formation: Thx, xenocrystic andesitc (suite 4); Tht, tholeiitic basalt (suite 3); Thp, pyroxene trachybasalt (suite 2); Thb, olivine trachybasalt to basaltic trachyandesite (suite 1); Tha, andesitc. Intrusive rocks: Tid, porphyritic dacite stocks and dikes; Tiq, quartz monzonite stocks. Pre-Hinsdale intermediate composition volcanic rocks: Tcu, upper andesitc; Tell, Tel2, Tel3, porphyritic dacite; Tca, pyroxene andesitc.

overlying flows by minor aeolian deposits. Xenocrystic trachyandesites (suite 4) containing abundant quartz and plagioclase xenocrysts are found in local monogenetic centers east of the Rio Grande (Figure 4). These erosional

remnants of isolated volcanic centers are probably the youngest of the mafic flows, although their stratigraphic relationship to the other San Luis Hills lavas is uncon- strained.

TABLE 1. Whole Rock K-Ar Age Determinations for San Luis Hills Rocks

Location 40 Ar*, Latitude Longitude K20, 10 -]ø 4øAr*, Age --- 2tr

Sample DKA Field North West % mol/g % Ma

1 4771 T84150 37012.38 ' 105049.20 ' 2.84 1.075 77.0 26.1 +__ 1.2 2 4770 T84098 37ø11.50' 105047.90 ' 2.78 0.657 80.9 26.4 + 1.2 3 4769 T84163 37008.37 ' 105049.40 ' 2.32 0.867 88.5 26.0 + 1.1 4 4768 T84089 37ø02.15 ' 105ø49.18 ' 0.60 0.226 45.3 25.7 + 1.8

Field sample numbers same as in Table 2; locations shown by Thompson and Machette [1989]. Decay constants, 4øKit• = 0.581 x 10 -]ø yr-1; At• = 4.962 x 10 -lø yr-]; 4øK/K = 1.167 x 10 -4 [Steiger and Jager, 1977].

13,582 THOMPSON ET AL..' OLIGOCENE BASALTIC VOLCANISM, RIO GRANDE RIFT

PETROGRAPHY AND MINERAL CHEMISTRY

Suite 1

Trachybasalts of suite 1 (Thb, Hinsdale Formation, Figure 4) contain olivine and clinopyroxene phenocrysts and Fe-Ti oxide microphenocrysts in a fine- to medium-grained tra- chytic groundmass composed of p!agioclase, clinopyroxene, and Fe-Ti oxides. Total phenocryst content by volume is typically 8-10 % (78% ol, 20% cpx, 2%, Fe-Ti oxides) but can be as high as 20% with an accompanying increase in modal proportions of clinopyroxene and Fe-Ti oxides (45% ol, 35% cpx, 20% Fe-Ti oxides). Euhedral to subhedral olivine phenocrysts are partially altered to iddingsite but have unaltered core compositions of Fo85_80 and rims as iron rich as Fo77. Low Mg samples (e.g., T84088) contain olivine phenocrysts with core compositions as low as Fo77. These values are consistent with the partitioning relations of iron and magnesium between olivine and basaltic liquid determined by Roeder and Emslie [1970]. Consequently, the olivine compositions of phenocrysts in suite 1 are thought to represent equilibrium liquidus phases in magmas of the host compositions. The thin low-Fo phenocryst rims are attributed to efficient equilibration between olivine and evolving residual liquids, as they are ubiquitous throughout the suite. Clinopyroxene phenocrysts are subhedral and have normal core to rim zoning (En50_45 Fs•4_•0 Wo43_39). Glomerocrysts of olivine, clinopyroxe, and Fe-Ti oxides occur sporadically in the rocks and compositionally overlap with phenocryst compositions.

Suite 2

Trachybasalts of suite 2 (Thp, Hinsdale Formation, Figure 4) are distinguished by relatively abundant phenocrysts (10-15%), a greater proportion and size of clinopyroxene phenocrysts (typically >2 mm and constituting up to 30% of phenocrysts) and abundant euhedral plagioclase (up to 50 modal %) as compared to suite 1 lavas. Olivine phenocrysts (5%) and Fe-Ti oxide microphenocrysts (15%) are subordinate in a fine-grained groundmass of plagioclase, clinopyroxene, and Fe-Ti oxides. Trachyandesites contain the same mineral assemblage but with an increased proportion of plagioclase (up to 60 modal %). Small olivine phenocrysts are altered to iddingsite but locally have unaltered cores of Fo72_76. Euhedral clinopyroxene phenocrysts (typically > 1 mm) have Fe-Ti oxide inclusions, are normal zoned (En55_ 48Fs 11-8Wo37-40), and commonly form glomerocrystic aggregates with plagioclase and Fe-Ti oxides.

Suite 3

Olivine tholeiites of suite 3 (Tht, Hinsdale Formation, Figure 4) contain small olivine phenocrysts and microphenocrysts that have core to rim zonation of Fo72_63. The lavas have an ophitic-diktytaxitic texture of groundmass olivine, plagioclase clinopyroxene, and titanomagnetite. Plagioclase is both coarse and abundant in the diktytaxitic interiors of flows, but distinction of phenocrysts from groundmass plagioclase is equivocal in most cases. Andesites typically contain olivine and Fe-Ti oxide microphenocrysts and sparse clinopyroxene microphenocrysts in a fine-grained pilotaxitic groundmass. Rare skeletal olivine phenocrysts up to 3 mm in diameter typically have Fo79 core compositions

that are considerably more Fo-rich than the microphenocrysts. McMillan and Dungan [1988] have interpreted similar features in andesites of the Taos Plateau volcanic field as

evidence for preeruptive recharge and mixing of evolved magmas with more mafic tholeiites. Although the evidence presented here cannot substantiate the existence of long- lived magma chambers where recharge was a significant factor, the presence of large volumes of mafic lavas erupted over short time periods (--•1 Ma) and in restricted areal extent is suggestive of recharge mechanisms.

Suite 4

Trachyandesites and basaltic trachyandesites of suite 4 contain phenocrysts of euhedral to subhedral plagioclase, minor olivine (Fo60) and Fe-Ti oxide microphenocrysts in a fine-grained intersertal groundmass. Suite 4 lavas are notable by the occurrence of large (up to 5 mm) xenocrysts of embayed quartz and plagioclase. Plagioclase xenocrysts are resorbed with seived textures and embayed rims. Xenocryst populations are variable and tend to be characteristic of individual centers. Similar relations are observed in the

Hinsdale lavas of the southeastern San Juan Mountains

[Lipman, 1975; Lipman and Mehnert, 1975], in the Taos Plateau volcanic field in late stage xenocrystic trachyandesite centers, and in some precaldera lavas of the Latir volcanic field [Johnson and Lipman, 1988]. Little detailed study of widespread xenocrystic trachyandesites of the Hinsdale Formation has been done, although they appear to be representative of late stage eruptions from isolated centers. Doe et al. [1969] demonstrated that feldspar separates from Hinsdale basalts rich in quartz and sodic plagioclase xenocrysts had Pb isotope ratios that were less radiogenic than non xenocrystic lavas (2ø6pb/2ø4pb --• 17.9 and 18.2) and attributed this difference to assimilation of lower crustal

granitic rocks. Johnson and Thompson [this issue] have shown that Pb isotope ratios for suite 4 xenocrystic lavas are consistent with assimilation of up to 20% crust. Kinsel [ 1986] documented the complex association of xenocrysts and glomerophyric aggregates in similar Pliocene lavas of the Taos Plateau volcanic field and attributed much of the

mineralogic and chemical diversity to variable contamination by crust.

MAJOR AND TRACE ELEMENT CHEMISTRY

Twenty seven samples of suites 1-4 were analyzed for major and trace element compositions, and an additional 27 were analyzed by energy-dispersive X ray fluorescence spectrometry for Rb, Sr, Y, Zr, and Nb. Nineteen representative analyses are presented in Table 2. Pb, Sr, and Nd isotope analyses for the same suite of rocks are presented by Johnson and Thompson [this issue]. Whole rock major element concentrations were determined by wavelength- dispersive X ray fluorescence analysis at the U.S. Geological Survey (USGS) by the methods outlined by Taggart et al. [1987]. Rare earth elements (REE), Cr, Sc, Hf, Ta, W, Th, and U were determined by instrumental neutron activation analysis (INAA) at the Imperial College Reactor Center, using a high purity germanium, low-energy planar spectrom- eter [cf. Henderson and Williams, 1981] and at the Johnson Space Center in Houston by the method of Jacobs et al. [1977]. Rb, Sr, Y, Zr, and Nb were determined by energy-


o

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0

48

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• +

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wt% SiO2 Fig. 5. Alkali-silica diagram for Oligocene volcanic rocks from

the San Luis Hills area, southern Colorado. Solid triangles, suite 1 lavas; solid diamonds, suite 2 lavas; solid circles, suite 3 lavas; crosses, suite 4 lavas. Divisions based on IUGS classification [Le Bas et al., 1986]. Fields: basalt (B); trachybasalt (TB); basaltic tranchyandesite (TBA); trachyandesite (TA); andesite (A); basaltic andesite (BA).

dispersive X ray fluorescence spectrometry by methods similar to those outlined by Johnson and King [1987].

Compositions of San Luis Hills mafic volcanic rocks range from 49 to 59 wt % SiO2 (Figures 5 and 6) and include mildly alkaline to transitional trachybasalt, basaltic trachyandesite, and trachyandesite (suites 1, 2, and 4) as well as subalkaline tholeiite to andesite (suite 3) based on alkali-silica variations. The transitional nature of the mildly alkaline lavas are characteristic of the Hinsdale Formation, although the mafic rocks of the San Luis Hills are volumetrically more extensive and compositionally more diverse than their temporal equivalents in the southeastern San Juan Mountains to the northwest or the Tusas Mountains to the southwest (Figure 6). The tholeiitic basalt-andesite series of the San Luis Hills overlaps compositionally with lavas of the extensive Pliocene Taos Plateau volcanic field (Figure 6) [Lipman and Mehnert, 1975; Kinsel, 1986; Dungan et al., 1989], although Pliocene rift volcanism was dominated by subalkaline tholeiites and their differentiates. The silicic alkalic basalts and

xenocrystic basaltic andesites of Lipman and Mehnert [1975] and Dungan et al. [1989] are compositionally equivalent to members of suites 1 and 4 of the San Luis Hills, respectively, and probably reflect derivation from similar mantle sources, followed by interaction with similar crustal lithologies. Andesites of the series are volumetrically minor, occurring only as isolated flows typically separated from major eruptive sequences by minor sedimentary interbeds. Conspicu- ously absent from the early rift volcanic sequences are the 62-68% SiO2 dacites which comprise a significant proportion of the Pliocene Taos Plateau lavas.

Suite 1 lavas include the most primitive compositions at San Luis Hills. Mg # (Mg # = 100 x (Mg/(Mg + 0.9 Fe)) are up to 62 (49% SiO2, 8.1% MgO, 338 ppm Cr, Table 2), although more evolved rocks have Mg # down to 52 (53% SiO2, 5.5% MgO, <200 ppm Cr). The high-Cr contents and Mg # of some basaltic lavas suggest that suite 1 trachybasalts represent some of the least contaminated or evolved early rift magmas of the region and consequently provide important constraints on isotopic compositions of mantle sources during early stages of rifting [Johnson and Thomp-

son, this issue]. Trachybasalts and trachyandesites are hypersthene normative and plot midway between the alkaline- subalkaline fields of Le Bas et al. [1986] on alkali-silica variation diagrams (Figure 5). Highly incompatible elements (Rb, Ba, REE, Zr, Nb, Hf, and alkalies) are poorly corre- lated with Mg # or SiO2 contents, precluding simple models of petrogenesis. Sr concentrations are highly variable in the suite, ranging from 500 to 1000 ppm; most contain 700 to 900 ppm Sr. Suite 1 lavas are moderately enriched in light rare earth elements (LREE) (La/Smn = 2.3 to 3.3) and are characteristically depleted in Th, Nb, and Ta (La/Ta n = 1.4-2.2) on chondrite-normalized trace element variation diagrams (Figure 7). The four San Luis Hills suites are readily distinguished on La-Zr and Th-Zr variation plots (Figure 8), where suite 1 lava compositions plot midway between tholeiites of suite 3 and the divergent trends of suites 2 and 4 at higher Zr concentrations (La/Zr = 0.17- 0.25; Th/Zr = 0.017-0.03).

The early rift tholeiites of suite 3 are compositionally similar to tholeiites from the Servilleta Basalt (Figure 6) for major elements (50-53 wt % SiO2, --7 wt % MgO, Mg # 60-53), although the most primitive compositions in the Servilleta Basalt (Mg # > 60) are not represented in the Hinsdale Formation. Increases in SiO2 from basalt to andesite (50-58 wt %) are accompanied by nearly linear decreases in MgO, total Fe as Fe203, CaO, and by increases in P205 and total alkalies. Titanium initially increases and subse- quently decreases with the onset of Fe-Ti oxides as fraction- ating phases in the andesites. Suite 3 lavas are lower in total alkalies, P205, and TiO2, total REE contents and are less enriched in LREEs than the more alkaline suites 1, 2, and 4. La/Sm n and La/Yb n chondrite-normalized ratios increase from 1.8 to 2.7 and 5 to 8 in the basalts to 3.1 to 3.5 and 15

to 19 in the andesites (Figure 7). Increases in LREEs are accompanied by a threefold increase in Ba, Rb, Th, and Zr (Figures 7 and 8). The increase in LREE/HREE ratios of the suite can be explained by assimilation/fractional crystallization (AFC) models involving LREE-enriched crustal melts. San Luis Hills tholeiites are more LREE enriched than

average Servilleta tholeiites (Figure 7) but are more similar to "San Cristobal type" Servilleta Basalt of Dungan et al. [1986], which contain slightly higher SiO2, K, P, LREEs, and the highly incompatible elements Ba, Rb, Th, Zr, Nb, and Hf, as compared to the dominant Servilleta tholeiite. The slightly more alkaline nature of the San Luis Hills tholeiites and the San Cristobal lavas may reflect an integra- tion of lithospheric mantle sources during melting or subsequent melt extraction to produce the enriched lavas.

The mafic rocks of suites 2 and 4 are the least understood

and chemically most variable volcanic rocks of the San Luis Hills. The clear petrographic distinctions among these two suites, respectively dominantly augite-phyric and xenocrystic, are reflected in major element and several trace element compositions. Suite 4 rocks are hypersthene normative and have lower CaO and MgO and higher TiO2 and Na20, at a given SiO 2 content, than any of the other suites. Suite 4 rocks are also uniformly high in m1203 . In suite 4 samples, the elevated Na and A1 contents can be attributed to the

abundant plagioclase xenocrysts. In contrast, suite 2 lavas contain up to 2.4% normative nepheline and share major element characteristics of both suites 1 and 4 (Figure 6). Suite 2 lavas are strongly depleted in MgO (factor of 2) with respect to suite i lavas at the same SiO2 content but are


10

13

11

8

6

4

2

2

1.5

1

I i i i i

/• 4oy, FC /

.;

+

/ 65% ß o

ß +

18

16

14

12

-I-

TL_ .5 I I I I I I ' ' ' ' ' 0

48 50 52 54 56 58 60 48 50 52 54 56 58 60

Wt% SiO 2 wt% SiO 2 Fig. 6. Variation diagrams for major elements in upper Oligocene volcanic rocks from the San Luis Hills area and

ranges of compositions for other regional lavas of the Hinsdale Formation (dotted lines) and the Taos Plateau volcanic field (dashed lines). Regional data from Lipman [1975]; Lipman and Mehnert [1975]; Dungan et at. [1986], and McMittan and Dungan [1988]. Oxides as wt %, total FeO as Fe203. Symbols as in Figure 5. Dashed arrows illustrate the effect of fractional crystallization on liquid lines of descent for an average "type A" Servilleta tholeiite [Dungan et at., 1986]. Percent fractionation is indicated adjacent to the arrowhead. Dotted arrow shows the same for suite 1 Hinsdale basalt (T84099).

markedly higher in A1203, Na20, and P205. Relative to the suite 1 trachybasalts and suite 3 tholeiites, the more alkaline series of suites 2 and 4 are more LREE enriched; La/Sm ratios increase through the trachybasalt-basaltic trachyandesite suite (La/Smn = 3.3 to 4; chondrite normalized values), and xenocrystic trachyandesites have the highest LREE enrichment (La/Smn = 3.7 to 5.5). Chondrite- normalized trace element diagrams (Figure 7) are similar in shape within the series, exhibiting characteristic Nb and Ta depletions (La/T% - 2.0 to 3.4). Prominent troughs at Sr and Ti, especially for the xenocrystic trachyandesites can be attributed to plagioclase and Fe-Ti oxide fractionation which occur as abundant phenocryst phases. Suites 2 and 4 can also be readily distinguished on plots of La versus Zr and Th versus Zr (Figure 8) where suite 2 compositions diverge from those of suite 4 along trends of higher La and Th concentrations at a given Zr content.

ISOTOPIC DATA

Johnson and Thompson [this issue] report initial 87Sr/86Sr (Isr) and i43Nd/144Nd ratios, and Pb isotope ratios for 19 samples from rock suites 1-4 that encompass the extent of major element compositions observed in the San Luis Hills. Ranges of Isr, SNa, and 2ø6pb/2ø4pb of suites 1-4 are respectively: suite 1 (0.70480 to 0.70549, -3.1 to -4.5, 17.58 to 18.13); suite 2 (0.70466 to 0.70545, -1.3 to -6.6, 17.70 to 17.96); suite 3 (0.70432 to 0.70488, +0.3 to -5.8, 17.31 to 18.23); suite 4 (0.70443 to 0.70446, -0.9 to -3.1, 17.52 to 17.79).

The 2ø6pb/2ø4pb ratios generally decrease with increasing SiO2, Zr, and Sr (Figure 9) indicating assimilation of U-depleted lower crust. This trend is most pronounced in the suite 3 rocks. Andesites have markedly lower 2ø6pb/2ø4pb ratios than the olivine tholeiites (2ø6pb/2ø4pb = 18.2 at 49 wt


1000

o x

o

• 10

I I I I I I I I I I I I I I I I

Ba Rb Th K Nb Ta La Ce Sr P Sm Zr Hf Ti Tb Y Yb

0.35 120 0.02 0.865 6.84 620 2.0 ........ 0.22

Ba Rb Th K Nb Ta La Ce Sr P Sm Zr Hf Ti Tb Y Yb

Fig. 7. Chondrite-normalized, trace element abundance diagrams (spidergrams) for representative San Luis Hills basaltic volcanic rocks and Servilleta basalts of the Taos Plateau volcanic field. (a) Solid triangles, suite 1 trachybasalts (T84012, T84098). Solid diamonds, suite 2 trachybasalt and basaltic trachyandesite (T84038, T84163). Open triangle, suite 4 xenocrystic trachyandesite (T84119). (b) Open diamonds, Pliocene tholeiites of the Taos Plateau volcanic field including an average of 10 "type A" Servilleta basalts and one representative sample of "San Cristobal type" Servilleta Basalt. All data from Dungan et al. [1986]. Solid triangles, suite 3 tholeiites (T84046, T84089). Solid squares, suite 3 andesites (T84125, T84150). Normalization factors are given on the diagrams of and discussed by Thompson [1982].

% SiO2 versus 2ø6pb/2ø4pb = 17.3 at 57 wt % SiO2). Similar relations are observed in the suite 1 rocks although there is scatter in the mafic rocks (2ø6pb/2ø4pb = 18.1 to 17.6 at 49 to 50 wt % SiO2). Suite 3 and suite 4 lavas have 2ø6pb/2ø4pb ratios that are less radiogenic than observed for the more primitive lavas of the San Luis Hills although relations are not internally consistent for each suite. For example, the xenocrystic andesite T84119 has the highest 2ø6pb/2ø4pb ratio (17.8) and the highest Si content (wt % SiO2 = 58.7) of suite 4. This sample locality is noted for the ubiquitous quartz xenocrysts in the lavas which may reflect contributions from more radiogenic crust than observed for most of the samples. Doe et al. [1969] concluded that primitive basalts of southern Colorado have 2ø6pb/2ø4pb ratios of

-18.2 based on determinations from younger Hinsdale lavas. The most radiogenic lavas of the San Luis Hills are characterized by this signature although we interpret most, including mafic lavas of suite 1 (Mg # = 60 to 62), to have experienced some contamination by U-depleted lower crust based on the Pb isotope compositions.

The sensitivity to crustal contamination apparent in the Pb isotopes is not reflected to the same degree in the Sr and Nd isotope data due to the higher Nd and Sr contents of the parental basalts. Sr and N d isotope ratios are generally shifted only with relatively large amounts of crustal assimilation as compared to the Pb isotope ratios [Johnson and Thompson, this issue]. This is readily apparent for suite 1 lavas where the moderate variation in 2ø6pb/2ø4pb ratios is


8O

2O

I I i ß

+

+

i ß ß 0 100 200 400 0 100 200 300 400

ppm Zr ppm Zr

Fig. 8. Incompatible trace element variation diagrams of (a) La and (b) Th versus Zr for the San Luis Hills volcanic suites. Arrow indicates AFC trend of suite 3 subalkaline rocks assuming and average "type A" Servilleta Basalt parent (see text). Tick marks indicate "F" increments of 5% using the equations of DePaolo [1981] where where F is mass of magma/original mass of magma, r is mass of assimilated material/mass of crystallized material equal to 0.5. Symbols as in Figure 5.

accompanied by only slight variation in ENd and Isr ratios. The most radiogenic sample (T84152) has ENd -- -3.9 and Isr = 0.70495. The least radiogenic sample (T84099) has ENd -- -4.18 and Isr = 0.70480. Only in suite 3 lavas, which have the largest variation in 2ø6pb/2ø4pb ratios, is covariation of Nd and Sr isotopes observed. Decreases in ENd values and 2ø6pb/2ø4pb ratios are accompanied by increases in Isr. The isotopically most primitive sample (T84205) has ENd = +0.30, 2ø6pb/2ø4 = Pb = 18.2, and Isr = 0.70432. The most evolved andesite of the suite 3 (T84125, SiO2 = 57.7) has ENd -- -5.79, 2ø6pb/2ø4pb = 17.4, and Isr = 0.70488. Isotopic variations at San Luis Hills are similar to those of olivine tholeiites and andesites of the Taos Plateau volcanic field

[Dungan et al., 1986; M. A. Dungan, written communication, 1989]. Dungan et al. [1986] concluded that basaltic and andesitic lavas of the Taos Plateau are the products of variable fractionation, followed by mixing with dacitic magmas that contained a high proportion of continental crust. The isotopic data discussed above for the suite 3 lavas are supportive of an AFC model involving contamination of olivine tholeiite magmas by U-depleted crustal melts similar to those proposed by Dungan et al. [ 1986] and McMillan and Dungan [1988] for contamination of the Pliocene Taos Pla- teau lavas.

Relatively noncontaminated lavas characterized by 2ø6pb/ 2ø4pb ratios ---18.2 are recognized in both suite 1 and suite 3 lavas. Associated Nd isotope ratios are distinctly different for each suite, ENd "- --4 and ENd "- 0 to + 1 for suites 1 and 3, respectively, and indicate at least two mantle source regions were involved in the genesis of early-rift mafic lavas. Johnson and Thompson [this issue] have proposed that suite 1 ENd values reflect a discrete, previously unrecognized mantle source in the northern Rio Grande rift resulting from progressive enrichment of lithospheric mantle by LREE- enriched magmatism or mantle contamination by low 143Nd/ 144Nd components during Proterozoic subduction.

PETRoGENESIS

Thompson et al. [1986]; Johnson and Lipman [1988], and Johnson et al. [1990] have demonstrated that intermediate- composition lavas of the Latir volcanic field were derived from "Hinsdale-like" mafic magmas via fractional crystallization accompanied by assimilation of continental crust (AFC), similar to models for the rift-related Taos Plateau

lavas. The results of petrogenetic studies of San Luis Hills lavas constrain models of magma-crust interaction during the inception of rifting and provide an additional basis to evaluate models of regional crustal contamination.

The effects of fractional crystallization and AFC on differentiation paths for select major elements using the program TRACE.FOR [Nielsen, 1988] are illustrated in Figure 6. For isobaric conditions at one atmosphere, TRACE.FOR en- ables the determination of temperature dependent compositions of mafic magmas undergoing fractional crystallization or AFC processes. The one atmosphere restriction limits its applicability in some of the San Luis Hills basaltic volcanic rocks. On normative plots of liquidus phase relations for basaltic melts projected from plagioclase onto the diopside- olivine-silica surface of the simplified basalt phase system from Stolper [1980] and Walker et al. [1979], suite 3 rocks plot nearest the 1-atm olivine-clinopyroxene cotectic. The transitional suite 1 and ne-normative suite 2 are substantially shifted toward the olivine apex and lie along the experimen- tal olivine-clinopyroxene cotectic at 100 mPa corresponding to lower crustal conditions in the northern Rio Grande rift

[Baldridge et al., 1984, and references therein]. The expan- sion of the clinopyroxene stability field at higher pressures may be responsible for the abundance of large clinopyroxene phenocrysts in suite 2 rocks and the inability of TRACE.FOR to model the observed phase compositions in this suite. TRACE.FOR models with reasonable certainty mineral phases and proportions observed for the suite 3 tholeiites and suite 1 trachybasalts and can be used to estimate major element lines of descent during phenocryst fractionation.

Liquid lines of descent illustrating the effects of olivine plus plagioclase fractionation from an average Servilleta tholeiite are illustrated in Figure 6 for the major oxides MgO, CaO, Fe20 3, TiO 2, and K20. The average Servilleta Basalt composition was used instead of the Hinsdale tholeiites in favor of the slightly more primitive nature of the former. Removal of the olivine rapidly depletes tholeiitic magmas in MgO and cannot account for the elevated concentrations observed in the evolved tholeiites and andesites. Addition-

ally, fractional crystallization of phenocryst phases does not produce the strong Fe enrichment trends modeled by TRACE.FOR. The same relations hold for olivine tholeiites and andesites of the Taos Plateau volcanic field. In contrast,


19

18.8

18.6

18.4

18.2

18

17.8

17.6

17.4

17.2

17

19

18.8

18.6 -

18.4 -

18.2 -

18 -

17.8 -

17.6 -

17.4 -

17.2 -

17

19

18.8

18.6 -

18.4 -

18.2 -

18 -

17.8 -

17.6 -

17.4 -

17.2 -

17

Taos Plateau

Oilvine Tholeiites/Andesites

wt% si02

' 1206pb/204pb I

I I I

150

ppm Zr

! I

&

&

ß +

I I I ; ; ; ; I ,

5OO 8O0 1100

ppm Sr

35O

1400

Fig. 9. Variation diagrams of (a) SiO2, (b) Zr, and (c) Sr versus 206 204

Pb/ Pb for the volcanic suites of the San Luis Hills. Symbols as in Figure 5. Taos Plateau data from Dungan et al. [1986] and M. A. Dungan (written communication, 1989). Outlined field indicates range of compositions observed for Taos Plateau olivine tholeiites and andesites.

tions of actual lower crustal assimilants, the trends produced are in accord with those observed for both San Luis Hills

and Servilleta olivine tholeiites. Low 2ø6pb/2ø4pb ratios required of the assimilant [Johnson and Thompson, this issue] indicate a crust at granulite grade, as exemplified by the Lewissian Complex.

McMillan and Dungan [1988] modeled the trace element patterns of andesites and dacites using r = 0.5 and an assimilant composition based on a trondjemitic gneiss from the Lewissian Complex [Weaver and Tarney, 1980] using equation 6a of DePaolo [1981]. The same models were applied to the San Luis Hills tholeiites and andesites of suite 3 (Figures 8 and 10). This model approximates the observed data reasonably well at 30% crystallization for LREEs, Ba, K, Nb, and Zr (Figure 10a) but fails to account for the elevated Rb and Th contents that are characteristic of the

San Luis Hills tholeiites. The same models using the Rb and Th enriched "San Cristobal type" Servilleta parent can accommodate the higher Rb and Th values but overestimates other incompatible elements.

The results of isotopic models using lower crustal assimilant values of 2ø6pb/2ø4pb • = 17, end = --12, and Isr = 0.7055 for the northern Rio Grande rift region [Johnson and Thompson, this issue] indicate that 35-40% fractionation at r = 0.5 can accommodate the range of compositions observed in the suite 3 rocks and also provides a reasonable fit for the Taos Plateau tholeiite/andesite suite. These results

are consistent with those presented by McMillan and Dun- gan [1988] and provide a reasonable model for crustal contamination of both Oligocene and Pliocene tholeiite to andesite suites.

The lack of consistent variation of moderately or strongly incompatible elements of suite 1 trachybasalts and basaltic trachyandesites precludes simple models of petrogenesis such as those presented above. However the similarity in shapes of chondrite-normalized trace element patterns (Fig- ure 7), their discrimination on La-Zr and La-Th variation diagrams (Figure 8), and similar petrographic and isotope characteristics, especially for Pb (Figure 9), suggest similar petrogenetic histories. Most of the observed chemical variation result from minor mantle source heterogeneities and subsequent modification along independent fractionation paths, modified to a minor extent by assimilation of Rb-, Th-, and U-depleted lower crust [Johnson and Thompson, this issue]. Additionally, the LREE/HREE ratio of the lower crust is typical of crustal averages since the Nd isotope data require an assimilant having end = --12. Hence the lower crust must be LREE enriched, similar to that proposed for the models of suite 3 rocks.

DISCUSSION

AFC models involving assimilation of partial melts of granulitic crust will effectively moderate the Mg and Fe concentrations at increasing SiO2 values. For example, using a 68 wt % SiO2 melt having 1.4 wt % MgO and 2.5 wt % Fe203) as an assimilant, 46% crystallization of parental tholeiite coupled with high rates of assimilation (r is mass of assimilated material/mass of crystallized material equal to 0.5) can generate a 57 wt % SiO2 andesite having 4.3 wt % MgO and 11.6 wt % Fe20 3. Whereas a precise match of model and observed data is not possible due to the uncertain composi-

The transition from intermediate to basaltic composition volcanism in the northern Rio Grande rift region was marked by the eruption of 26 Ma Hinsdale basaltic volcanic rocks on rapidly eroding volcanic highlands that ultimately became the axial rift depression known as the San Luis Valley. Whereas the appearance of basaltic volcanism at 26 Ma was abrupt in southern Colorado, as well as to the north in the Flat Tops area, the cessation of intermediate to silicic volcanism was protracted by ---3 m.y. Intermediate to silicic volcanism culminated in the eruption of the Amalia Tuff from the Questa caldera on the eastern margins of the rift


• O.Ol .Ol O.Ol

I I

Ba Rb Th

0.7 0.8 0.9

0.01 0.01 0.01 0.02 0.01 .E• 0•. • 0.06 0.01 0.01 0.02 0.05 0.•)1 0 0.06 I I I I I I I I I I I •\1 I K Nb Ta La Ce Sr P Sm Zr Hf Ti Tb • Yb

K' _ .,•,.,,

.• -•::_z•:•.. "r" ........................................... ,'-.'.,. 0.7

..• 10 "' .... 0.9

I

B. T. T. L. Ce Sm .f Fig. 10. Chondrite-normalized trace element abundance diagrams illustrating the affect of contamination of suite 3

tholeiitic rocks by Rb-, Th-depleted granulite facies crust similar to trondhjemitic gneiss of the Lewisian Complex in northwest Scotland. (a) Open squares, granulite facies lower crust, sample 7H of Weaver and Tarhey [1980]. Open diamonds, average Servilleta Basalt from Figure 6. Solid squares, suite 3 andesite (T84150). (b) Open diamonds, suite 3 tholeiite (T84046). Other symbols as in Figure 10a. Dashed lines represent results of AFC modeling using the equations of DePaolo [1981] where r is mass of assimilated material/mass of crystallized material equal to 0.5.

basin at 26 Ma, followed by postcaldera volcanism to 23 Ma [Lipman et al., 1986]. At approximately the same time (23 Ma), eruption of postcollapse lavas in the Summitville caldera continued in the southeastern San Juan Mountains

[Lipman, 1975]. That mafic magma played a substantial role in the long-

lived "prerift" volcanism of the region is demonstrated in the Latir volcanic field by petrogenetic models which require fractional crystallization of substantial volumes of mantle- derived basaltic magma of near-chondritic Nd isotope ratios, accompanied by assimilation of crust [Johnson and Lipman, 1988; Johnson et al., 1990]. In the San Juan volcanic field, sustained mid-Tertiary volcanism was prolonged by input of mantle-derived mafic magmas, up to 300,000 km 3, which regionally preheated the crust, facilitated anatexis, and increased its mean crustal density through hybridization of existing crust and addition of new mafic crust [Riciputi and

Johnson, 1990; Johnson, this issue]. Glazner and Ussler [1989] proposed that increased crustal densities could favor buoyant rise of basaltic magmas. Once extension was initi- ated, eruption to the surface could be facilitated or enhanced by crustal fracturing or asthenospheric upwelling [Hildreth, 1981], and surface eruption was restricted to areas where the upper crust was not currently occupied by silicic magma chambers. This may account in part for the aerial restriction of early rift basalts to the margins of active intermediate to silicic composition volcanic fields.

A crustal model which proposes an increasing crustal density also predicts that younger basaltic magmas will reach the surface with decreasing amounts of crustal contamination [Glazner and Ussler, 1989]. This does not appear to be the case in the northern Rio Grande rift, as early rift basaltic volcanic rocks discussed in this study, as well as Pliocene lavas in the region [Dungan et al., 1986; McMillan


and Dungan, 1988] are contaminated to similar degrees. This may in part be a reflection of the small degree of extension in the northern Rio Grande rift relative to the Basin and Range province or may reflect the fact that rift axis magmatism did not occur in areas that had experienced earlier caldera- related magmatism, which is the setting envisioned for the most extensive crust hybridization [Johnson, this issue].

Perry et al. [1987] have presented a model for rift evolution in which asthenospheric mantle progressively replaces lithospheric mantle during upwelling as extension progresses from the onset of rifting. This model was in part based on regional variations in end for basaltic rocks; end "- +7 to + 8 is envisioned to reflect largely asthenospheric sources, whereas t•Nd "- 0 to +2 is thought to reflect in part a lithospheric source. Perry et al. [1987] suggest that all northern Rio Grande rift volcanism is the product of lithospheric mantle melting, largely based on interpretation of the Nd isotope ratios. We propose [Johnson and Thompson, this issue] that while volcanism associated with rifting in the northern Rio Grande rift can be generated in the lithosphere, significant isotopic variation is observed within lithospheric melts which must reflect heterogeneities in the source. These heterogeneities are of the order (->4 end units) of the range predicted by Perry et al. [1987] for the distinction between lithosphere and asthenosphere. Observed differences in vol- umetric proportions of effectively similar rock types separated in time (Oligocene versus Pliocene) must then be accounted for by variation in the intensity of the thermal anomaly responsible for melting a heterogeneous mantle source. Lower thermal input may result in selective partial melting of a "plum pudding" mantle [Perry et al., 1987] in which less refractory enriched enclaves are preferentially melted to produce predominantly isotopically enriched alkaline series magmas. Higher thermal input which may have accompanied a rejuvenation of volcanism and extensional faulting during the Pliocene would result in larger degrees of melting, having a dilution effect that results in characteristic tholeiitic volcanism that has more depleted Nd and Sr isotope signatures.

SUMMARY

1. Early rift basaltic volcanism in the San Luis Valley segment of the northern Rio Grande rift was concentrated in a narrow zone currently occupied by the axial depression of the modern rift. Within this region, the volumetrically most extensive and compositionally most diverse suite of lavas were erupted over a short time period (---1 m.y.) and coincided with the onset of regional extension at approximately 26 Ma. Similar isolated eruptive centers occurred throughout the southern Rocky Mountain region during this time period and reflect the regional transition to extensional tectonism. In the San Luis Valley segment of the northern Rio Grande rift, basaltic volcanism was omnipresent with minor exception during 25 m.y. of rift evolution; major episodes occurred during the onset of rifting and again in the Pliocene, constituting the most magmatic segment of the Rio Grande rift.

2. Eruptive products of the Hinsdale Formation in the San Luis Hills include four suites of chemically distinct magma types that are dominated by mildly alkaline trachybasalts, basaltic trachyandesites and trachyandesites, and subordinate olivine tholeiites and associated andesites. With

the exception of suite 2 pyroxene-rich basaltic trachyandesites, Hinsdale Formation lavas of the San Luis Hills are analogous to lavas of the Pliocene Taos Plateau volcanic field. The major difference between the two eruptive rift cycles lies in the predominance of alkaline volcanism during the late Oligocene as compared to largely tholeiitic volcanism in the Pliocene.

3. Most mafic lavas of the San Luis Hills are evolved

(Mg # <60) and have been contaminated to some degree by LREE-enriched silicic partial melts of Rb-, Th-, and U-depleted granulitic lower crust. Pb isotopes are the most sensitive indicators of crustal contamination, whereas shifts in Nd and Sr isotope ratios are associated with large amounts of assimilation. Models for contamination of tholeiitic ba-

salts are regional in scope and equally applicable to Oli- gocene and Pliocene rift volcanism. Relatively noncontaminated lavas can be identified on the basis of chemical and Pb

isotope compositions, and supporting Nd and Sr isotopic data [Johnson and Thompson, this issue] indicate that at least two mantle source regions were involved.

Acknowledgments. Study of the San Luis Hills area was initi- ated under the auspices of the National Research Council's post- doctoral associate program in conjunction with the U.S. Geological Survey (USGS). Support has been provided by the USGS National Geologic Mapping Program and the Geothermal Research Program. INAA data from Imperial College was provided by P. T. Leat, from NASA by P. D. Kempton and by J. Budahn, D. Knight, and D. McKown of the USGS-Denver. XRF analyses were done by J. Taggart and A. Bartel of the USGS Denver. Their contribution is gratefully acknowledged. P. W. Lipman and M. A. Dungan en- dowed the project with stimulating discussion and shared unpublished data on aspects of the geology and geochemistry of volcanic rocks in the region. Acknowledgment is made is made by Johnson to the Donors of the Petroleum Research Fund, administered by the American Chemical Society, and the National Science Foundation (EAR-8803892) for partial support of this work. S. Baldridge and J. Stormer provided helpful journal reviews, and we thank P. Lipman for additional critical review.

REFERENCES

Aldrich, M.J., Jr., C.E. Chapin, and A. W. Laughlin, Stress history and tectonic development of the Rio Grande rift, New Mexico, J. Geaphys. Res., 91, 6199-6211, 1986.

Baldridge, W. S., Petrology and petrogenesis of Plio-Pleistocene basaltic rocks from the central Rio Grande rift, New Mexico, and their relation to rift structure, in Rio Grande Rift--Tectonics and Magmatism, edited by R. W. Riecker, pp. 323-353, AGU, Wash- ington, D. C., 1979.

Baldridge, W. S., F. V. Perry, and E. S. Gladney, Petrology and geochemistry of the Cat Hills volcanic field, central Rio Grande rift, New Mexico, Geol. Sac. Am. Bull., 93, 635-643, 1982.

Baldridge, W. S., K. H. Olsen, and J. F. Callender, Rio Grande rift Problems and perspectives, in Rio Grande Rift--Northern New Mexico, edited by W. S. Baldridge, P. W. Dickerson, R. E. Riecker, and J. Zidek, Field Conf. Guideb. N.M. Geol. Sac., 35, 1-12, 1984.

Brister, B. S., Tertiary stratigraphy and tectonic development of the Alamosa Basin, Rio Grande rift, south-central Colorado, Ph.D. thesis, part 3, 74 pp., N.M. Tech., Socorro, 1990.

Burroughs, R. L., Geology of the San Luis Hills, south central Colorado, Field Conf. Guideb. N.M. Geol. Sac., 22, 277-287, 1971.

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C. M. Johnson, Department of Geology and Geophysics, Univer- sity of Wisconsin, Madison, WI 53706.

H. H. Mehnert and R. A. Thompson, U.S. Geological Survey, Box 25046, Denver Federal Center, Denver, CO 80225.

(Received July 3, 1990; revised December 19, 1990;

accepted December 20, 1990.)

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