USING DETRITAL SANIDINES TO DECIPHER MAJOR TECTONIC EVENTS: MIDDLE MIOCENE(?) UPLIFT AND EXPOSURE OF THE RUBY MOUNTAINS-EAST HUMBOLDT METAMORPHIC CORE COMPLEX

The pre-/syn-kinematic Clover Creek basin adjacent to the Ruby Mountains-East Humboldt Range (REHR) metamorphic core complex, northeast Nevada, is poorly dated. Megabreccia in lower basin strata reflects initial normal-fault exhumation of the REHR; precise dating of the deposits is crucial to resolve the timing of regional extension that contributed to exhumation. Other constraints reveal that normal-sense mylonitic shearing in the REHR footwall occurred in the Oligocene (possibly older), whereas regional extension initiated post-17 Ma. We applied high-precision ⁴⁰Ar/³⁹Ar dating of detrital sanidine grains from 8 samples in and overlying megabreccia. We obtained 796 dates that were ≤42 Ma, matching the regional igneous record. The maximum depositional age (MDA) of the youngest dates requires deposition ≤20.1 Ma. All deposits, including overlying ≤16 Ma sediments, lack ~20-17 Ma detrital sanidines because the nearest felsic volcanic sources of that age were ≥400 km to the south. Precise sanidine dates and K/Ca allow some sanidine age populations to be tied to unique sources in the 37-19 Ma ignimbrite flareup across Nevada. Locations of igneous sources and outflow tuffs indicate the sanidine grains are reworked from pyroclastic fall deposited near the REHR, not from source calderas or outflow deposits. Although the MDA allows megabreccia to be ~20 Ma, we interpret that it was deposited beginning ~17 Ma in a basin bounded by a high-angle normal fault, now rotated to subhorizontal, based on equal tilting of 40-15 Ma rocks, analysis of changing source regions, the reworked character of deposits, and the lack of ~20-17 Ma erupted felsic rocks to provide such sanidine grains. A ≤17 Ma age for megabreccia would be synchronous with other records of regional extension and range exhumation, whereas an ~20 Ma age does not correlate with any known major event. Our study showcases the utility of detrital sanidine dating. (1) Although sanidine is not as resistant to physical/chemical destruction as zircon, the data signal is not swamped by reworked pre-Cenozoic grains. (2) The combination of precise age and K/Ca to identify unique sources can be used to evaluate source-to-sink conditions. Complex depositional and detrital sanidine population changes in a thick sedimentary sequence show the need for multi-sample detrital dating.