ISOTOPIC CONSTRAINTS ON THE PETROGENESIS OF JURASSIC PLUTONS, SOUTHEASTERN CALIFORNIA

The 165 Ma Eagle Mountain intrusion is a heterogeneous, enclave-bearin g, metaluminous remnant of the Cordilleran Jurassic are that cuts regi onally metamorphosed pre-Mesozoic rocks in the southeastern Mojave Des ert of California. The main phase of the intrusion consists of granodi orite to tonalite host facies, diorite mixed facies, and homogeneous m onzogranite facies. The host facies contains microdiorite enclaves int erpreted as intermingled masses of mafic magma. Late-phase leucogranit e stocks cut the main phase. Mineral equilibria indicate emplacement a t similar to 6.5 km depth, with solidus temperatures ranging from 760 degrees C for diorite to 700 degrees C for felsic granodiorite. Althou gh uniform radiogenic-isotope compositions (Sr-i = 0.7085, epsilon Nd- i = -9.4) suggest derivation from a single source, no known source has the composition required. A hybrid source is proposed, consisting of various proportions of juvenile mantle and recycled lower crust. Calcu lations indicate that the source of the Eagle Mountain intrusion compr ised >60% juvenile mantle and <40% recycled crust. On the basis of the ir isotopic compositions, other mafic Jurassic plutons in the region w ere derived from sources containing different proportions of mantle an d crustal components.