HIGH SPATIAL-RESOLUTION ISOTOPE GEOCHEMISTRY OF MONAZITE (U–PB & SM–ND) AND ZIRCON (U–PB & LU–HF) IN THE OLD WOMAN PIUTE RANGE BATHOLITH, MOJAVE DESERT, CALIFORNIA
Zircon and monazite crystals were analysed for U–Pb & Hf and U–Pb & Sm–Nd isotopes respectively, by in situ LA-ICP-MS and LA-MC-ICPMS using the recently developed Laser Ablation Split Stream (LASS) approach. This high spatial-resolution approach allows a detailed assessment of geochemical changes in the magmatic system at a fine spatial and temporal scale. Additional zircon U–Pb data were collected by Q-LA-ICP-MS.
The U–Pb data show widespread inheritance in zircon cores, yielding ages varying between ~1400 and ~1800 Ma, consistent with regional Proterozoic crustal building events. Only four monazite grains yield inherited cores ~1700 Ma, all of which are found in the Sweetwater Wash two-mica granites. Zircon and monazite rims give crystallisation ages between 70–75 Ma, with monazite rims typically slightly younger than the zircon rims from the same samples, and ages between samples and between plutons are within uncertainty of each other. The zircon and monazite crystals also preserve the Hf and Nd isotopic ratios of their crustal sources. Correct age determinations are crucial for this interpretation for monazite grains, illustrating the utility of the LASS technique in integrating two isotopic systems from the same ablation volume. The OWPB shows a large range in εHfi (young = -8.2– -19.2, old = -0.9– 8.4) and εNdi (young = -12.6 – -21.8, old = -0.8– -4.1) with the North Piute pluton being the most isotopically homogenous. Isotopic data is consistent with derivation of the OWPB Batholith from the ancient crust into which it intrudes, which is spatially coincident with the inferred edge of Precambrian North America.