Jurassic plutonism and crustal evolution in the central Mojave Desert,California |
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| Authors: | Jonathan S Miller Allen F Glazner |
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| Institution: | (1) Department of Geology CB⋕3315, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3315, USA, US |
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| Abstract: | Middle to Late Jurassic plutonic rocks in the central Mojave Desert represent the continuation of the Sierran arc south of
the Garlock fault. Rock types range from calc-alkaline gabbro to quartz monzonite. Chemical and isotopic data indicate that
petrologic diversity is attributable to mixing of crustal components with mantle melts. Evidence for magma 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 the gabbros do not represent pure liquids but are, at least partly, cumulates. The cumulate nature of the gabbros coupled
with field evidence for open-system contamination means that trace element contents of gabbros cannot be used to fingerprint
the Jurassic mantle source, nor can isotopic data be unequivocally interpreted to reflect the isotopic composition of the
mantle. Correlation of Sr and Nd isotropic composition with bulk composition allows some constraints to be placed on the mantle
isotopic signature. Gabbros and mafic inclusions from localities north of Barstow, CA have the most depleted mantle-like isotopic
signatures (Sr
(
i
)≈0.705 and ɛNd
(t)=≈0 to +1). However, these rocks have likely seen some contamination as well, so the mantle source probably has an even more
depleted character. Gabbros with the lowest Sr(
i
) and highest ɛNd
(t) are also characterized by the highest 207Pb/204Pb and 206Pb/204Pb in the entire data set. This may be a feature of the mantle component in the Jurassic arc indicative of minor source contamination
with subducted sediment as has been observed in modern continental arcs. Locally exposed Precambrian basement and metasedimentary
rocks have appropriate Sr, Nd and Pb isotopic signatures for the crustal end members and are possible 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 gabbros contrasts with the enriched subcontinental mantle component in Jurassic arc
plutons further to the east and suggests there may have been a major mantle lithosphere boundary between the two areas as
far back as 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 composition with Sr and Nd isotopic composition
and because values may differ depending on the age of the rocks sampled within a given area. Data from plutons intruded into
rocks known or inferred to be Precambrian are, however, shifted dramatically (highest Sr(
i
) and lowest ɛNd(t)) toward Precambrian values. The least isotopically evolved rocks (lowest Sr(
i
) and highest ɛNd(t)) occur within the eugeoclinal belt of the Mojave Desert. This zone has been previously identified as a Precambrian rift zone
but more likely represents a zone where mantle magmas have been intruded into isotopically similar crustal rocks of the eugeocline
with minor input from old Precambrian crust.
Received: 12 August 1993/Accepted: 8 July 1994 |
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