DECIPHERING THE CENOZOIC DEVELOPMENT OF THE RUBY-EAST HUMBOLDT METAMORPHIC CORE COMPLEX USING MONAZITE, TITANITE AND ALLANITE PETROCHRONOLOGY

Metamorphic core complexes record exhumation of the middle to lower crust through a combination of tectonic denudation (isostacy) and buoyancy-driven flow. Evaluating the mechanisms of buoyancy-driven flow requires detailed characterization of high-temperature processes, such as partial melting and metasomatism, that facilitate enhanced buoyancy and rheological weakening. The Ruby-East Humboldt metamorphic core complex, Nevada, is a unique setting to study such high-temperature processes, as ductile attenuation of the metamorphic core preceded tectonic denudation by ca. 10 Ma suggesting isostatic-forces were minimal during its development. Here we present a study of the mechanisms driving high-temperature metamorphism and deformation within the core complex using new monazite, titanite and allanite split-stream LA-ICP-MS petrochronology. We studied seven quartzofeldspathic and metapelitic schist samples from within and below the mylonite zone across the northern Ruby Mountains and southern East Humboldt Range. Monazite in rocks from the lower mylonite zone and below yield 100-80 Ma ²⁰⁸Pb-²³²Th dates that record monazite core growth during prograde Late Cretaceous metamorphism. At all structural levels, 80-60 Ma monazite cores show elevated Y-concentrations, enriched HREE and increased Eu-anomalies interpreted to record initial garnet resorption and melt crystallization (in deepest samples) following peak metamorphism. Titanite grains in rocks from the upper and middle mylonite zone yield arrays of 100-40 Ma ²⁰⁷Pb-corrected ²⁰⁶Pb/²³⁸U dates representing titanite growth during prograde to retrograde conditions, and partial resetting at 40 Ma. In all monazite, titanite and allanite samples studied, a prominent population of 40-30 Ma dates record enhanced retrograde metamorphism driven by an influx of mantle-derived dioritic intrusions. Mottled zoning textures, high Th/U ratios and increased common-Pb contents in monazite indicate enhanced fluid flow and metasomatism between 40-30 Ma. These data support a model where mafic magmas heated the middle crust, remobilized Late Cretaceous leucogranites and enhanced fluid flow leading to increased buoyancy and rheological weakening, thus allowing buoyant ascent of the middle crust and ductile attenuation of the mylonitic carapace.