Paper No. 4
Presentation Time: 1:50 PM


METCALF, Rodney V.1, MILLER, Calvin F.2, DANIELSON, Lisa3, RHODES, Gregory2 and WOODEN, J.L.4, (1)Department of Geoscience, University of Nevada, Las Vegas, 4505 S. Maryland Parkway, Las Vegas, NV 89154-4010, (2)Earth & Environmental Sciences, Vanderbilt University, Nashville, TN 37235, (3)Astromaterials Research and Exploration Science, NASA JSC, Jacobs Engineering, Mail Code JE23, Houston, TX 77058, (4)Stanford University, Stanford, CA 94305,

The Miocene Mt. Perkins Pluton is a small composite intrusive body emplaced in the shallow crust as four separate phases during the earliest stages of crustal extension. Phase 1 (oldest) consists of isotropic hornblende gabbro and a layered cumulate sequence. Phase 2 consists of quartz monzonite to quartz monzodiorite hosting mafic microgranitoid enclaves. Phase 3 is composed of quartz monzonite and is subdivided into mafic enclave-rich zones and enclave-free zones. Phase 4 consists of aphanitic dikes of mafic, intermediate and felsic compositions hosting mafic enclaves.

Phases 2-4 enclaves record significant isotopic disequilibrium with surrounding granitoid host rocks, but collectively enclaves and host rocks form a cogenetic suite exhibiting systematic variations in Nd-Sr-Pb isotopes that correlate with major and trace elements. Phases 2-4 record multiple episodes of magma mingling among cogenetic hybrid magmas, the latter formed via magma mixing and fractional crystallization at deeper levels. The mafic end-member was alkali basalt similar to nearby 6-4 Ma basalt with enriched OIB-like trace elements and Nd-Sr-Pb isotopes. The felsic end-member was a subalkaline crustal-derived magma.

Phase 1 isotropic gabbro exhibits elemental and isotopic compositional variations at relatively constant SiO2, suggesting generation of isotropic gabbro by an open-system process involving two mafic end-members. One end-member is similar in composition to the OIB-like mafic end-member for phases 2-4; the second is similar to nearby 11-8 Ma tholeiite basalt exhibiting low Nd, and depleted incompatible trace elements. Phase 1 cumulates record in situ fractional crystallization of OIB-like mafic magma with isotopic evidence of crustal contamination by partial melts sourced in adjacent Proterozoic gneiss.

The Mt Perkins pluton records a complex history of a lithospheric scale magma system involving two distinct mantle-derived mafic magmas and felsic magma sourced in the crust. Mixing and fractional crystallization of these magmas at various levels in the crust generated a suite of intermediate composition magmas. U-Pb zircon SHRIMP ages of phase 1 (15.7 ± 0.2 Ma), phase 3 (15.8 ± 0.2 Ma) and phase 4 (15.4 ± 0.3 Ma) document a 100-300k year timescale for the petrogenetic processes recorded in the Mt Perkins magma system.