THE METAMORPHIC AND STRUCTURAL HISTORY OF CLOVER HILL, NEVADA, PART OF THE RUBY-EAST HUMBOLDT CORE COMPLEX

Clover Hill, south-southwest of Wells, Nevada, is a domical topographic feature bounded on its east by a Quaternary normal fault. The structural architecture is broadly divisible into a hanging-wall and footwall separated by a brittle, low-angle normal-sense fault. Slices of metamorphosed to non-metamorphosed miogeoclinal sedimentary rocks occur along this tectonic boundary, whereas a tilted stratigraphic sequence of Middle Eocene volcanic/volcaniclastic rocks in turn disconformably overlain by Middle to Upper Miocene sedimentary and volcanic rocks forms the adjacent hanging-wall block. The footwall consists of mylonitic Cambrian and Neoproterozoic metasedimentary rocks, chiefly flaggy, micaceous-feldspathic quartzite and subordinate pelitic schist (equivalent to the Prospect Mountain Quartzite and McCoy Creek Group). Structurally deeper, orthogneiss and paragneiss comprise the core of Clover Hill, and are equivalent to Paleoproterozoic–Late Archean rocks exposed at Angel Lake in the northern East Humboldt Range. The pelitic schists are characterized by a GRAIL metamorphic mineral assemblage: muscovite + biotite + plagioclase + garnet + kyanite + sillimanite + rutile + ilmenite. The metamorphic textures in kyanite-bearing pelitic schists support at least two overprints: (1) fibrolitic sillimanite that grew as a late, minor mineral phase on large kyanite porphyroblasts, and (2) kinked kyanite porphyroblasts, probably mechanically deformed during Tertiary mylonitization. New thermobarometric data using GASP assemblages and the TWQ program indicate P–T conditions of ~7.2 kb and ~720–760º C—consistent with the GRAIL assemblage. These data, in additional to prior regional studies, indicate that significant crustal thickening (early Late Cretaceous?) was subsequently overprinted by high-temperature mylonitization during the Late Eocene–Early Oligocene and subsequently by brittle, low-angle to high-angle normal faulting in Middle Miocene to Quaternary time.