Giant country rock blocks within Searchlight pluton, southern Nevada
Perrault, Daniel Scott
Searchlight pluton, a steeply tilted, 10 km thick Miocene intrusion in southern Nevada, exposes a zone with abundant, 5-400 m long blocks of Proterozoic gneiss. Blocks are present within a pair of subparallel horizons that make up a 2 km thick zone and extend 5 km laterally away from the pluton’s N-margin, slightly oblique to the initially subhorizontal boundary between the pluton's middle granite and lower quartz monzonite units. Blocks are commonly polylithologic and well foliated, with long and intermediate dimensions parallel to their own foliation, the foliation of the host granitoid, and the orientation of flattened magmatic enclaves. Blocks have a sub-angular tabular geometry with an average aspect ratio of ~3.7. We interpret the blocks as stoped and emplaced after repeated wall collapse events onto a crystal-rich mush and transported laterally primarily by matrix-supported debris flows. SHRIMP ages of granitoid zircons constrain block emplacement between ~16.1-16.3 Ma, a small fraction of the lifetime of the pluton (15.8-17.7 Ma). Though blocks are locally abundant (~20% exposure over ~0.3 km2), they constitute <1% of the entire exposure, suggesting that stoping played a minor role in the emplacement process. LA-ICPMS analysis of zircons from host granitoids suggest minor host contamination by block disaggregation - a sample adjacent (15 cm) to a block contained ~8 % inherited Proterozoic zircons, while samples from elsewhere in the pluton yielded <<1%. Scaled settling experiments suggest tabular blocks deposit with their long and intermediate axes parallel to surfaces; blocks with a geometry controlled by foliation would on average possess a subparallel alignment of both their geometric shape and foliation. Experiments also show that fluid sublayers may become trapped beneath blocks and allow lateral transport by hydroplaning.