PROVENANCE OF PRE-CENOZOIC DETRITAL ZIRCONS FROM THE HUMBOLDT FORMATION ALONG THE NORTHERN MARGIN OF THE WOOD HILLS AND ITS IMPLICATIONS FOR PRE-16 MA PALEOGEOGRAPHY AND EXTENSION IN THE EAST HUMBOLDT-WOOD HILLS-PEQUOP METAMORPHIC CORE COMPLEX, NE NV

The northern margin of the East Humboldt-Wood Hills-Pequop metamorphic core complex, NE NV, contains remnants of an extensional fault and basin system that was responsible for early exhumation of the core complex. Detrital zircon ages from the basin-fill (lower Humboldt formation) exposed in the northern Wood Hills indicate that this early phase of extension and basin-filling began by 27 Ma and ended at or before 16 Ma. Sedimentary rocks of this age in the region are rare and consequently the provenance of the sediment can provide important information about the paleogeography and structural exposure-level of basement rocks during this early phase of extension. To this end, we investigated U-Pb ages of 312 pre-Cenozoic detrital zircons from two conglomerate samples in the Wood Hills area to assess their provenance. The presence of 350 – 850 Ma zircons in these samples indicate that some of the grains were derived from Triassic formations in NE NV. Significant relative age-probability peaks at 1.7 to 1.9, 2.1, and 2.7 Ga in the samples indicate derivation from the Ordovician Valmy and Vinini formations of the Roberts Mountain allochthon and associated Mississippian foreland basin deposits. Petrologically, the conglomerate samples are texturally and compositionally immature, and are dominantly composed of sedimentary carbonate clasts with minor clasts of dark chert resembling that in the Ordovician Valmy and Vinini formations in NE Nevada. Taken together, the detrital zircon age data and petrology of the conglomerate samples indicate that sediment was sourced from local highlands composed of Mesozoic-upper Paleozoic miogeoclinal strata and the Roberts Mountain Allochthon and Mississippian foreland basin strata. These source rocks structurally represent the upper part of the pre-Cenozoic bedrock of NE Nevada. The provenance of the samples indicate that structurally-deep lower-Paleozoic miogeoclinal rocks presently exposed in the Wood Hills were not exhumed during the early phase of extension between 27 and 16 Ma, but instead were exhumed in the later phase of extension postdating 16 Ma.