PETROLOGY OF UPPER AMPHIBOLITE FACIES MARBLES FROM THE EAST HUMBOLDT RANGE, NEVADA, USA - EVIDENCE FOR HIGH-TEMPERATURE, RETROGRADE, HYDROUS VOLATILE FLUXES AT MIDCRUSTAL LEVELS

Petrographic and petrologic investigations of upper amphibolite facies metacarbonates from the East Humboldt Range core complex, Nevada, provide important constraints on P-T-X(CO2) conditions and fluid flow during metamorphism. Three marble assemblages are observed [(1) dol + cc + bt + cpx + q+ ru +/- scap +/- ksp +/- amph; (2) cc + bt + cpx + plag + q + sph +/- scap +/- ksp; (3) cc + cpx + plag + q + sph +/- scap +/- ksp], all of which equilibrated with relatively CO2-rich fluid compositions, at P-T conditions of approximately 6 kbar and 600-750-degrees-C. The most recent equilibration event is recorded in some calcsilicate gneisses where retrograde amphibole and epidote + garnet replace clinopyroxene and plagioclase, respectively. This is attributed to infiltration of H2O-rich fluids at and/or after peak metamorphic temperatures, which continued as the rocks were cooled and rapidly uplifted after a Tertiary extension-related heating event. Likely sources for the retrograde fluids are the abundant pegmatitic leucogranites in the area. Volumetric fluid-rock ratios of 0.02-0.4 are required to generate the retrograde assemblage, and observed leucogranite proportions are more than adequate to provide the required volume of fluid. Estimates of retrograde fluid fluxes range from 0-25 to 5 x 10(2) cm3/cm2 for a transient temperature gradient of 5-degrees-C/m, to 3 x 10(3) to 7 x 10(4) cm3/cm2 for a temperature gradient of 35-degrees-C/km. These gradients are characteristic of a skarn-type contact metamorphic environment and a regional crustal geotherm, respectively. They imply different time-scales and length-scales for the retrograde fluid flow system, with the former more akin to a contact metamorphic setting with local, meter-scale retrograde fluid flow, and the latter to a regional metamorphic setting with regionally high mid- and lower-crustal temperatures and fluid flow throughout a significant thickness of the middle crust. Higher gradients are considered more likely given the proximity of leucogranites to retrogressed calc-silicate gneisses, and the resultant relatively small fluxes are consistent with a magmatic source. The length-scale of reaction within the retrograde fluid flow system was of the order of meters to hundreds of meters and involved both pervasive and (later) fracture-controlled down-temperature flow. Retrograde fluid flow in this terrane, as well as others dominated by magmatic volatiles, is in the form of multiple discrete bursts of fluid released in a discontinuous manner potentially over long periods of time (1-10 Ma) with locally variable thermal gradients along the flow path.