Geology and Geochronology of the Spirit Mountain Batholith, Southern Nevada: Implications for Timescales and Physical Processes of Batholith Construction

Abstract

The Spirit Mountain batholith (SMB) is a ~250 km2 composite silicic intrusion located within the Colorado River Extensional Corridor (CREC) in southernmost Nevada. Westward tilting of 40-50º has exposed a cross section of the SMB. Piecemeal construction is indicated by zircon geochronology, field relations, and elemental geochemistry. U/Pb data (zircon SHRIMP) demonstrate a ~2 million year (17.4-15.3 Ma) history for the SMB. Individual samples contain zircons with ages that span the lifetime of the batholith, suggesting recycling of extant zircon into new magma pulses. Field relations reveal distinct magmatic episodes and suggest a common injection geometry of stacked horizontal sheeting. The largest unit of the SMB is a gradational section (from roof to depth) of high-silica leucogranite through coarse granite into foliated quartz monzonite and has a ~million year history. The 25 km2 x 2 km2 leucogranite zone was emplaced incrementally as subhorizontal sheets throughout the history of this section, suggesting repeated fractional crystallization and melt segregation events. Age data from this gradational unit show multiple zircon populations within individual samples. Subsequent distinct intrusions that cut this large unit preserve a sheeted, sill-on-sill geometry. We envision the SMB to have been a patchwork of melt-rich, melt-poor, and entirely solid zones throughout its active life. Preservation of intrusion geometries and contacts depended on the consistency of the host rock. Zircons recycled into new pulses of magma document remobilization of previously emplaced crystal mush, suggesting the mechanisms by which evidence for initial construction of the batholith became blurred.