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Fawna Korhonen, University of Maryland (United States)
Michael Brown, University of Maryland (United States)
Satoshi Saito, University of Maryland (United States)
Christine Siddoway, Colorado College (United States)
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An unresolved issue in granite genesis is the nature of the physical link between lower crustal sources and upper crustal sinks. Information to address this issue is preserved in migmatite domes. Outcrop-scale structures, rock chemistry, mineral assemblages and compositions, microstructures and accessory phase chronometers in migmatites and granites within domes yield information about the P-T evolution and processes and timing of melt segregation, extraction and ascent in orogens. The Fosdick Mountains host an elongate migmatite dome of migmatitic paragneiss and orthogneiss, with associated granites. New data reveal the Fosdick migmatite dome experienced a punctuated history of melting and magmatism that ultimately controlled its form, kinematics and emplacement. We derive constraints on the conditions and timing of metamorphism and crustal melting from mineral assemblages in gneiss layers, monazite and zircon chronology, and geochemical and Sr-Nd isotope data from granites and migmatites. Potential protoliths in the dome include a Lower Paleozoic metasedimentary succession (Swanson Formation) and a calc-alkaline plutonic suite (Ford Granodiorite); residual paragneiss occupies a central domain within the dome. Microstructural observations, in situ monazite chronology, and mineral equilibria modeling constrains metamorphic conditions to 800-860°C and 7-10 kbar at ca. 368-348 Ma. This event corresponds to a well-established period of arc magmatism along the active margin of East Gondwana. Carboniferous granites (LA-ICP-MS zircon ages) derived from a Ford Granodiorite suite source record a major melting event, although an extensive Carboniferous leucosome network has not been recognized. Two stages of Jurassic-Cretaceous metamorphism and melting of orthogneisses (Ford Granodiorite protolith) and, locally paragneisses (Swanson Formation protolith) are recognized. Mineral equilibria modeling and in situ monazite geochronology suggest conditions of 820-870°C and 6.5-7.5 kbar at ca. 150-140 Ma, contemporaneous with Cretaceous arc magmatism in contiguous New Zealand. Monazite rim analyses yield an age of ca. 115 Ma, which corresponds to the onset of transtension, granitic magmatism, voluminous anatexis and formation of the Fosdick migmatite dome. The Jurassic-Cretaceous melting events are best preserved E and W from the residual paragneiss, where the proportion of leucosome and the number of thick concordant leucogranite sills increases dramatically with respect to intervening gneiss layers, resulting in a sheeted granite complex. Leucosome compositions are generally cumulate-rich suggesting loss of a K-rich melt, consistent with the extensive leucosome network observed in outcrop. The leucosome-sill networks are interpreted as remnants of a Cretaceous melt transfer system that allowed magma flux through a zone of anatexis within the middle crust during the transition from convergence to extension along the proto-Pacific margin of East Gondwana.
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