JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 96, NO. B8, PAGES 13,593-13,608,JULY 30, 1991 IsotopicComposition of OligoceneMafic VolcanicRocks in the Northern Rio GrandeRift: Evidencefor Contributionsof AncientIntraplate andSubduction Magmatism to Evolutionof the Lithosphere CLARK M. JOHNSON Departmentof Geologyand Geophysics,University of Wisconsin,Madison REN A. THOMPSON U.S. Geological Survey,Denver, Colorado Mafic lavas eruptedduring initiation of regional extensionat 26 Ma in the northernRio Granderift werederived from at leasttwo isotopicallydistinct mantle sources. One is characterizedby 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-Smdvalue source (MANTLE 1) is interpretedto largelyreflect the isotopiccompositions of the lithosphericmantle in the region. Isotopiccompositions of Cenozoicmafic lavasand Proterozoicrocks areused to constrainmodels for evolutionof thelithosphere. The low 8Ndvalues of theMANTLE 1 componentwere probablyproduced by evolution of a light rare earth element-enrichedupper mantle or inputof low-sNd material. during development of thelithosphere in theEarly Proterozoic. The 8Nd values and2ø6pb/2ø4pb ratios •n evolvedrocks are as low as-7 and 17.3,respectively, indicating interaction with lowercrust that had •Na -< -12 and 2ø6pb/2ø4pb _<17.0. Initial87Sr/86Sr ratios both increase and decrease slightly in evolved rocks, indicatinginteraction with lower crust that had Sr isotoperatios that were generallysimilar to thoseof themantle. The 8Nd value of themodern lithospheric mantle beneath the northernRio Grande rift is >18 units lower than the projectedmodern values of the asthenospheric mantlefrom which the Proterozoic crust was originally derived. The higher-ema value source (MANTLE 2) may reflect mixing of asthenosphereand lithospherecomponents. The mantle sourcefor most late Cenozoicmafic lavasin the northernRio Granderift regionlies at 2ø7pb/2ø4pbratios that are signifi- cantlyhigher than the 2ø6pb/2ø4pb- 2ø7pb/2ø4pb array defined by Proterozoiccrust in the region. Althoughthe crustalarray is alsodisplaced to higher2øTpb/2ø4pb ratios as comparedto the northern hemisphereoceanic mantle a•ray, indicating incorporationof Archean Pb during crust formation, the stillhigher 2øTpb/2ø4pb ratios in theRio Granderift mantlesource is interpretedto reflectcontinued input of ArcheanPb into the developingEarly Proterozoiclithospheric mantle after crust formation. These relations are strong evidence that the lithosphere became stabilized shortly after the major crust formation eventsin the Early Proterozoic. INTRODUCTION (Figure 1) preservesthe largestvolume of early rift (26 Ma) Rift-related volcanism in the northern Rio Grande rift is volcanic rocks in the northern Rio Grande rift. These lavas largely basaltic in composition, in contrast to prerift provide an importantand previouslypoorly known view magmatismin the region, which was dominatedby inter- of volcanism that occurredduring inception of the Rio mediate- to silicic-compositionash flow tuffs and related Grande rift. lavas [Steven, 1975; Lipman and Mehnert, 1975]. Most We reportSr, Nd, andPb isotopedata for earlyrift mafic studies of rift-related volcanic rocks in the northern Rio lavas of the Hinsdale Formationthat are exposedat San Granderift have concentratedon relatively youngvolcanic Luis Hills. These data bear on mantle sources of volcanism fields that formed significantlyafter initiationof extension. associatedwith inceptionof rifting andmodels for develop- Theseinclude the TaosPlateau, Rayton-Clayton, and Ocate ment of the lithosphericmantle. Compositionalvariations fields (Figure 1) [Stormer, 1972a, 1972b; Lipman and and isotopic compositionsindicate that many San Luis Mehnert, 1979; O'Neill and Mehnert, 1988], as well as Hills magmasinteracted with lower crust,as hasbeen pro- lavas exposed at Los Mogotes volcano, in the Tusas posedfor otherlavas in the region. Detailedconsideration Mountains, and near Amalia (Figure 1)[Lipman and of chemical and petrologic characteristicsof the lavas, Mehnert, 1975; Lipman et al., 1986]. Early rift lavas are however, allows identification of primitive, uncontami- exposedonly on intrarift horstsand on the flanks of the rift natedcompositions that can be usedto determineisotopic in the SanJuan and Latir volcanicfields (Figure 1) [Lipman compositionsof the mantle. The data define two mantle and Mehnert, 1975, 1979; Thompsonet al., 1986; Lipman sourceregions, at leastone of which is interpretedto reflect et al., 1986]. Late Cenozoic basaltic lavas in northern the lithosphericmantle that had beenenriched in light rare Coloradoare temporallycorrelative with rift-relatedvolca- earth elements(LREE) during stabilizationand growth of nic rocksbut were eruptedsignificantly outside the Rio the lithospherein the Proterozoic. Grande depression[Larson et al., 1975]. San Luis Hills GEOLOGY AND PETROLOGY OF SAN LUIS HILLS LAVAS Copyright1991 by the AmericanGeophysical Union. San Luis Hills are the surface expressionof a major Papernumber 91JB00342. intrarift horst within the northern Rio Grande rift that is 0148-0227/91/91 JB-00342505.00 largelyburied beneath late Cenozoicsedimentary and vol- 13,593 13,594 JOHNSONAND THOMPSON:ISOTOPIC COMPOSITION OF MAFIC VOLCANICROCKS 400 ] I•'SUITE 1i A ' i ß SUITE ,.3 ß ß I- m [ I ,,,,..."', ' , ..................... • LATIR m ',,, .•. LOS ß -....... ,ooJ ,,,,,,, . mTAOS '•"•', -,,, ,.•' -" xl.•.AMALIA ............. © '-... © ,SAN LUIS o.... •*.,,,, ....•...... :•": ...... :..... :...... :..... :-*'--: .....:...... :..... •...... HILLS " B COLORADO .340• TAOS- ALKALIC NEW MEXICO ..•_.. 280 ......... ........-,i;,' HILLSSANLUIS •, 220 e•" ,...... © .................................. • // ............. PROTEROZOIC BASEMENT 160ß1 -,- .•- - •/•'.•.•.;-",a,, ,,.•"•/ / AMALIA•F ............... ............ MID-CENOZOIC ! '• ..'_.•.' ? ./ . ............. CALDERAS • I I I I I 100 1_-:,'-..-".•; ß i ...... •!" ............... ß ................... o 5o IOOKM :1',, ' ............•-X' •os - •.o.[,,•,• Fig. 1. Generalized map of the northern Rio Grande rift region 4.0i-- " ß.......... ß ß ß --ß LOS ß MOGOTE$ß ß ß ß ß . ß ß ß 48 52 56 60 64 showing major Cenozoic volcanic areas, including San Luis Hills, the subjectof this report. SJVF, San Juanvolcanic field; TPVF, Taos wf%SiO 2 Plateau volcanic field; TBM, Timber Mountain and Brushy Moun- 400 • C tain; AL, Amalia lavas; LVF, Latir volcanic field; R-CVF, Rayton- = Clayton volcanic field; OVF, Ocate volcanic field; LM, Los Mogotes LOSMOGOTES • volcano. ,.300i ß ' ''' AMALIA canic rocks [Kleinkopf et al., 1970]. Other surfaceexpo- (• ...........-=/•'-...- -• i .... suresinclude Timber Mountain and BrushyMountain (Fig- ure 1) [Thompsonet al., 1986]. Early rift lavas (26.4-25.7 E200 Ma) of the lower Hinsdale Formation, the subject of this '-.•L.C... report, overlie intermediate-compositionlavas at San Luis 1O0 '"'•<.::... '- TAOS Hills that are temporally correlativewith precalderalavas of the Latir volcanic field [Thompson et al., this issue]. Silicic volcanic rocks are conspicuouslymissing from San 0 ß ß ß ß ß , ß , ß ß Luis Hills, in contrastto volcanic sectionsexposed on the 80 120 160 200 240 2•0 320 flanks of the rift in the San Juan and Latir volcanic fields. pprn Zr Particularly notable is the absence of the Amalia Tuff, Fig. 2. Chemicalvariation diagrams (a) Cr-SiO2, (b) Zr-SiO2, and which eruptedfrom the Questacaldera of the Latir volcanic (c) Cr-Zr. Symbols shown for lower Hinsdale Formation lavas field contemporaneouswith initiation of regional extension exposedat San Luis Hills, the subjectof this report;outlines for other lavasin the northernrift region also shown. The samesymbols and at 26 Ma [Lipman et al., 1986; Hagstrum and Lipman, outlinesused for Figures3, 4, 5, 6, 7, and 10. Data from Dungan et 1986]. These relations, in addition to the general lack of al. [1986],Johnsonand Lipman [1988], Thompsonet al. [this issue], Los Pinos Formation age sedimentary rocks at San Luis andM. Dungan(unpublished data, 1988). Asterisk,estimated SiO 2 Hills [Thompsonand Machett, 1989], suggestthat the horst contentfor suite 3 sample205. was a topographichigh during early evolutionof the north- em Rio Grande rift. the San Juan volcanic field [Lipman and Mehnert, 1975]. Early rift lavas at San Luis Hills are divided into four Suite 2 lavas contain 10-15 vol % phenocrystsof plagio- suitesbased on petrographiccharacteristics, chemical com- clase and subordinate clinopyroxene and minor olivine, positions,and stratigraphicposition. Suites 1-4 representa have slightly higher alkali contentsthan suite 1 rocks, at a general sequencefrom oldest to youngest. Suite 1 lavas given SiO2 content, and are trachybasaltsand trachyande- occur at the base of the sectionand representN60 vol % of sites. Suite 2 lavas are notable for their very high Rb and the lavas exposed at San Luis Hills. They erupted from Th contents,up to 120 and 14 ppm, respectively,as com- multiple centers and are characterizedby up to 20 vol % pared to most Hinsdale Formation lavas in the region [Lip- phenocrystsof olivine and clinopyroxene[Thompson et al., man et al., 1973]. The high Rb and Th contents,however, this issue]. These lavas are transitional between tholeiite are similar to those of precalderalavas of the nearby Latir and alkali basalts,as determinedby Na20+K20-SiO 2 varia- volcanic field [Johnsonand Lipman, 1988]. Suite 3 lavas tions, and are similar to other Hinsdale Formation lavas are tholeiitic basalts and andesites.