JURASSIC PLUTONISM AND CRUSTAL EVOLUTION IN THE CENTRAL MOJAVE DESERT, CALIFORNIA

Middle to Late Jurassic plutonic rocks in the central Mojave Desert re present the continuation of the Sierran are south of the Garlock fault . Rock types range from calc-alkaline gabbro to quartz monzonite. Chem ical and isotopic data indicate that petrologic diversity is attributa ble to mixing of crustal components with mantle melts. Evidence for ma gma mixing is scarce in most plutons, but emplacement and injection of plutons into preexisting wallrocks (e.g, pendants of metasedimentary rocks) suggests that assimilation may be locally important. Field and petrographic evidence and major and trace element data indicate that t he gabbros do not represent pure liquids but are, at least partly, cum ulates. The cumulate nature of the gabbros coupled with field evidence for open-system contamination means that trace element contents of ga bbros cannot be used to fingerprint the Jurassic mantle source, nor ca n isotopic data be unequivocally interpreted to reflect the isotopic c omposition of the mantle. Correlation of Sr and Nd isotropic compositi on with bulk composition allows some constraints to be placed on the m antle isotopic signature. Gabbros and mafic inclusions from localities north of Barstow, CA have the most depleted mantle-like isotopic sign atures (Sr((i)) approximate to 0.705 and epsilon Nd-(t) = approximate to 0 to +1). However, these rocks have likely seen some contamination as well, so the mantle source probably has an even more depleted chara cter. Gabbros with the lowest Sr-(i) and highest epsilon Nd(t) are als o characterized by the highest Pb-207/Pb-204 and Pb-206/Pb-204 in the entire data set. This may be a feature of the mantle component in the Jurassic are indicative of minor source contamination with subducted s ediment as has been observed in modern continental arcs. Locally expos ed Precambrian basement and metasedimentary rocks have appropriate Sr, Nd and Pb isotopic signatures for the crustal end members and are pos sible contaminants. Incorporation of these components through combined anatexis and assimilation can explain the observed spread in isotopic composition. Evidence for a depleted mantle component in these gabbro s contrasts with the enriched subcontinental mantle component in Juras sic are plutons further to the east and suggests there may have been a major mantle lithosphere boundary between the two areas as far back a s the Late Jurassic. Crustal boundaries and isotopic provinces defined on the basis of initial isotopic composition (Sr-(i) = 0.706 isopleth ) are difficult to delineate because of the correlation of bulk compos ition with Sr and Nd isotopic composition and because values may diffe r depending on the age of the rocks sampled within a given area. Data from plutons intruded into rocks known or inferred to be Precambrian a re, however, shifted dramatically (highest Sr-(i) and lowest epsilon N d-(t)) toward Precambrian values. The least isotopically evolved rocks (lowest Sr-(i), and highest epsilon Nd-(t)) occur within the eugeocli nal belt of the Mojave Desert. This zone has been previously identifie d as a Precambrian rift zone but more likely represents a zone where m antle magmas have been intruded into isotopically similar crustal rock s of the eugeocline with minor input from old Precambrian crust.