GEOCHRONOLOGICAL AND ISOTOPIC EVIDENCE FOR TRIASSIC-JURASSIC EMPLACEMENT OF THE EUGEOCLINAL ALLOCHTHON IN THE MOJAVE DESERT REGION, CALIFORNIA

The geologic history of the outer continental margin (eugeoclinal) roc ks in the El Paso Mountains and northern Mojave Desert has long been i mportant in models for the development of the active continental margi n in the western Cordillera, Current interpretations call for either s trike-slip or thrust juxtaposition of eugeoclinal rocks against miogeo clinal/cratonal (platformal) rocks, or some combination of both strike -slip faulting and thrusting, Two broad and interrelated aspects of th e history of the eugeoclinal rocks are at issue: (1) How much primary displacement is necessary to account for the present position of the e ugeoclinal rocks' and (2) When were the eugeoclinal rocks thrust again st platformal rocks? This study primarily addresses the second issue. Lithologic correlation indicates that the outcrop belt of eugeoclinal rocks is bounded by platformal rocks to the east and south. Platformal rocks are also present to the west, but many of these exposures resto re to a position structurally beneath the eugeoclinal rocks when Terti ary extension is restored, implying stacking by a thrust, New U-Pb zir con geochronology and whole rock geochemistry and Sr, Nd, and Pb isoto pic data for plutons in the El Paso Mountains and northern Mojave Dese rt lend insight into the timing of this thrusting, The outcrop belt of eugeoclinal rocks coincides with the only known Permian and Triassic plutons in the El Paso Mountains and northern Mojave Desert, which are dated at 260 to 240 Ma by U-Pb zircon in this report. Initial Sr and ENd(,, isotopic values from these plutons are distinct (less than or e qual to 0.704 and greater than or equal to+2, respectively) from conti nental lithosphere isotopic signatures (greater than or equal to 0.705 and less than or equal to-2) of both Middle Jurassic plutons in the s ame area and Triassic plutons in the southern and eastern Mojave Deser t. Feldspar common lead data for the Permian and Triassic plutons with in the eugeoclinal outcrop belt also indicate limited crustal involvem ent and do not overlap previously reported values for common lead data from Mesozoic plutons in the eastern Mojave Desert region where Prote rozoic basement is widespread, The observations and data reported here indicate that Late Permian-Early Triassic plutons in the northern Moj ave Desert and El Paso Mountains were generated within or passed prima rily through oceanic lithosphere, but later Jurassic plutons were deri ved from and/or interacted extensively with continental lithosphere. W e hypothesize that the eugeoclinal rocks were deposited on oceanic cru st that was thrust eastward over Precambrian cratonal basement and ove rlying strata between approximately 240 Ma and 175 Ma, a time of littl e documented tectonic activity in the Mojave Desert region, Postulated Permian or late Middle Jurassic east-directed thrusting is incompatib le with our data and observations, The data also fill an important gap in palinspastic reconstructions of the early Mesozoic are and indicat e that this are was northwest-trending and oblique to Paleozoic facies trends in the Mojave Desert.