PROSPECTS FOR IDENTIFYING HIGHLY SILICIC GRANITE SOURCES IN THE DETRITAL RECORD

The detrital record is an important supplement to the crystalline rock record due to the continual recycling of Earth’s crust, and is the only record of Earth’s pre-4 Ga crust. As highly silicic granites (HSG) derive from either high degrees of magmatic differentiation or from melting of previously differentiated materials, they are expected in evolved continental igneous settings. Unfortunately, they are lower volume than other portions of the magmatic record and readily lost to the crystalline record. It would therefore be helpful to determine markers for HSGs in the detrital record to trace their presence in Earth’s crust over time, including potentially in Earth’s earliest crust. This study focused on zircon as a potential marker for HSGs, given its hardiness and well-preserved U-Pb system. We examined trace element and mineral inclusion contents in HSG and other granitoid zircon suites. Samples come from the Cretaceous California continental arc (Peninsular Ranges and San Bernardino Mountains), Miocene Spirit Mountain Batholith (southern Nevada), and Paleozoic Lachlan Fold Belt (southeastern Australia). Most HSG zircon suites overlap the field of other granitoids in trace element measures of differentiation, but several suites are more likely to have Hf > 14,000ppm, Eu/Eu* <0.1, and U >1000ppm. HSG zircon suites are also more likely than other granitoid suites to contain >5% inclusion phases derived from highly incompatible elements (HIC; e.g., monazite, U-Th minerals). The case for a detrital zircon suite being derived from highly silicic sources should be made statistically based on the proportion of HIC inclusions and/or trace element distributions. Detrital zircons in sediments draining the HSGs in our study show lower % HIC (<7%) due to dilution by other zircon sources; these results bear comparison with other detrital zircon suites to determine whether this dilution will limit HSG source identification. Additionally, differences in zircon suite HSG markers with varying SiO₂ and whole rock trace element markers for differentiation may help further distinguish if any given subdivision of HSGs is most recognizable in the detrital zircon record. This will have broad implications for understanding how to use mineral inclusions to connect detrital minerals to their host rocks, including searching for HSG origins among early Archean and Hadean zircon.