ISOTOPIC DISEQUILIBRIUM AMONG COMMINGLED HYBRID MAGMAS - EVIDENCE FORA 2-STAGE MAGMA MIXING-COMMINGLING PROCESS IN THE MT PERKINS PLUTON, ARIZONA

The syn-extensional Miocene Mt. Perkins pluton, northwestern Arizona, cooled rapidly due to its small size (6 km(2)) and shallow emplacement (7.5 km) and allows examination of commingled rocks that experienced little isotopic exchange. Within the pluton, quartz dioritic to granod ioritic host rocks (58-68 wt % SiO2) enclose dioritic enclaves (50-55 wt % SiO2) and a portion contains enclave-free granodiorite (70-74 wt % SiO2). Fine-grained, crenulate enclave margins and a lack of advance d mixing structures (e.g., schlieren, flow fabrics, etc.) indicate an incipient stage of commingling. Isotopic variation between enclaves an d enclosing host rocks is large (6.8 to 10.6 epsilon(Nd) Units; 0.0036 to 0.0046 Sr-87/Sr-86 units), suggesting isotopic disequilibrium. Com parison of an enclave core and rim suggests that isotopic exchange wit h the host magma was limited to the enclave rim. Enclaves and hosts co llectively form a calc-alkaline suite exhibiting a large range of epsi lon(Nd) (+1.2 to -12.5) and initial Sr-87/Sr-86 (0.705 to 0.71267) wit h a correlation among epsilon(Nd), initial Sr-87/Sr-86, and major and trace element compositions. Modeling sugests that the suite formed by magma hybridization involving magma mixing accompanied by fractional c rystallization. The magma mixing must have predated commingling at the present exposure level and indicates a larger mixing chamber at depth . Isotopic and trace element data suggests mixing end-members were ast henospheric mantle-derived mafic and crustal-derived felsic magmas. Fr actional crystallization facilitated mixing by reducing the theologica l contrasts between the mafic and felsic mixing end-members.