Petrology of Upper Amphibolite Facies Marbles from the East Humboldt Range, Nevada, USA; Evidence for High-Temperature, Retrograde, Hydrous Volatile Fluxes at Mid-Crustal Levels

Abstract

Petrographic and petrologic investigations of upper amphibolite facies metacarbonates from the East Humboldt Range core complex, Nevada, provide important constraints on P-T-XCO2 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 ∼6 kbar and 600–750°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 5×102 cm3/cm2 for a transient temperature gradient of 5°C/m, to 3×103 to 7×104 cm3/cm2 for a temperature gradient of 35°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 terrance, 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.