THE ROLE OF FLUIDS IN MOBILIZING THE CRUSTAL COLUMN BENEATH THE RUBY MOUNTAINS-EAST HUMBOLDT RANGE METAMORPHIC CORE COMPLEX

Monazite, zircon, and feldspar textures in Cretaceous pegmatitic and equigranular granitic gneisses from the Ruby Mountains-East Humboldt Range metamorphic core complex were texturally characterized using optical- and SEM-CL, BSE imaging, and element mapping.

Monazites from equigranular granitic gneisses have oscillatory zoned cores surrounded by mantles and rims with lobate, curvilinear domains and sharp compositional boundaries. In situ U-Pb and trace element analysis identified three complex age populations attributable to monazite growth or reactivity that may be assigned to 84-90 Ma, 65-80 Ma, and 30-40 Ma. The oscillatory zoning in monazite cores, coupled with zircon geochronology, dates emplacement and crystallization of these gneisses to ~90 Ma. Metamorphism during Sevier orogenesis is recorded in monazite as a series of textural domains attributed to dissolution-reprecipitation accompanying fluid infiltration. The 65-80 Ma dates for monazite overlap with emplacement of a voluminous leucocratic pegmatitic granite emplaced as a network of small-volume dikes, sills and sheets. The pegmatites were emplaced over a > 10 m.y. (U-Pb, zircon) during crustal thickening in the Sevier Orogeny, which is compatible with the duration of monazite reactivity in the equigranular gneisses.

Monazite domains in equigranular gneisses and zircon populations in pegmatite gneisses document metamorphism during 30-40 Ma core complex formation. CL investigation shows the development of fluid-driven alteration of feldspar, which contain inclusions of zircon with mottled-patchy CL zoning. Localized partial melting resulted in recrystallized feldspar and crystallization of zircon with oscillatory zoning. Dates for zircon, as well as monazite domains in the equigranular gneiss, span an 11 m.y. period, which is interpreted to be the minimum duration of core complex formation. The prevalence of alteration and partial melting textures is attributed to pervasive infiltration of fluids. The implication may be that hydrothermal fluids induced partial melting, which increased buoyancy and plasticity of the crustal column, contributing to mid-crustal flow, uplift and doming, and eventual collapse of the core-complex by low-angle extension along the shear zone.