GROWTH OF THE SPIRIT MOUNTAIN BATHOLITH (NEVADA, USA) AS REVEALED BY IN SITU HF ISOTOPES IN ZIRCON

The Spirit Mountain Batholith (SMB) is the largest of several Miocene intrusions lying with the Colorado River Extensional Corridor of southern Nevada (USA). West-tilting has exposed the SMB from its roof to a structural depth of ca. 15 km. Previous field, geochronologic and geochemical studies have shown that the SMB is zoned from a roof leucogranite downward into coarse-grained granite to foliated quartz-monzonite at the deepest exposures. Late fine-grained granite sheets and diorite masses (dikes and enclaves) intrude the granite and quartz monzonite. Zircon geochronologic data show that the SMB was assembled incrementally over 2 million years, and age and zircon trace element data suggest repeated magma input, remobilization of mush, and secular fractionation of melt to form the leucogranite.

New in situ Hf isotopic data from zircons are consistent with this view of the SMB but reveal additional information on the melt source, and the growth and evolution of the magma system. Hf isotopic variation (εHf) for all granitoid zircons ranges from -7 to -20, excluding two distinctly lower εHf zircons. A depth profile through one of these low εHf grains shows it is composite with a core εHf = -36 and a rim εHf= -14; otherwise isotopic zoning is either subtle or irresolvable. Intra-sample variability is typically only 3-4 εHf units and zircons from the granite-quartz monzonite have slightly higher εHf than roof leucogranite zircons. Diorite (53 wt.% SiO₂) zircons have εHf ranging from -5 to -8.

The Hf data indicate moderate melt source variability and appreciable (>50%) juvenile mass involved in granite melt production if the diorite zircons are indicative of local mantle Hf. The near absence of Precambrian cores agrees with the geochronology and suggests that zircons are magmatic, and that antecrysts were recycled and mixed with younger zircons in the magma system during repeated magma input. However, the limited intra-sample variability indicates mixing was domainal. The modest Hf zoning of the pluton is compatible with either minor assimilation of Precambrian roof rock or ‘basification’ of the deeper levels of the SMB.