Suarez, Kaitlyn
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Suarez
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Kaitlyn
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Publication MIGMATITE FORMATION, GEOCHRONOMETER PETROGENEISIS, AND RARE EARTH ELEMENT MINERALIZATION IN THE ADIRONDACK MOUNTAINS, NY(2023-09) Suarez, KaitlynThe Adirondack Mountains in upstate New York contain exposures of complex partially melted rocks, in addition to iron oxide-apatite (IOA) deposits with variable rare earth element (REE) concentrations. Previous workers have suggested that melting occurred during the ca. 1150 Ma Shawinigan and the ca. 1050 Ma Ottawan orogenies. However, there are challenges in determining the timing of melting and the number of partial melt events. Further, tectonic models must be developed to describe the petrogenesis of IOA and REE mineralization. Migmatites are present along Rt. 4/22 near Whitehall, NY. In chapter two, all layers of a single migmatitic rock were analyzed to describe the evolution of gneissic layering and the texture and timing of monazite and zircon developed during biotite dehydration melting. The gneissic layers are interpreted to reflect the differential preservation of reactants and products formed during the forward and reverse progress of the melting reaction. The heterogeneous layered gray gneiss provides robust constraints on the timing of melting (ca. 1050 Ma Ottawan orogenesis), melt crystallization, post-melting retrogression, and information about earlier metamorphic events. The IOA deposits are typically hosted in the Lyon Mountain Granite Gneiss (LMG) and are spatially correlated with extensive Na metasomatism. The results in chapter three demonstrate the utility of hand-held instruments (i.e., gamma ray and portable X-ray fluorescence spectrometers) for identifying cryptic alteration gradients in the LMG. Further, elevated Th and U values in the LMG proved to distinguish the high-REE deposits. The abundance of REEs is variable within the IOA deposits. The mineralogy, composition, and texture of high, medium, and low-REE deposits are compared in chapter four. High-REE deposits are characterized by clinopyroxene, apatite (14 wt% REEs), and magnetite (~4,000 ppm Ti) with accessory quartz (~12 ppm Ti). Medium-REE deposits are dominated by apatite (9 wt% REEs) and magnetite (~5,000 ppm Ti) with accessory quartz (~10 ppm Ti). The low-REE deposits consist of abundant quartz (~6 ppm Ti), magnetite (~60 ppm Ti), and chlorite. The three deposit types are interpreted to reflect varying temperatures of the immiscible liquids generated from a gabbroic magma and subsequently emplaced into dilatant zones coeval with the LMG.Publication In-situ Zircon and Monazite Geochronology from Compositionally Distinct Layers in a Single Migmatitic Paragneiss Sample Located in the Eastern Adirondack Mountains, NY(2019-05) Suarez, KaitlynMigmatites are a common rock type in the Adirondack Mountains, NY. We analyzed a single sample of biotite-garnet-sillimanite paragneiss with foliation parallel leucosome along Route 22 south of Whitehall, NY in order to determine the timing of melting using both in-situ monazite and zircon U/Pb geochronology from the restite and leucosome layers of the same rock. Monazite was analyzed via in-situ EMPA on the Ultrachron microprobe at the University of Massachusetts. Zircon was analyzed via LA-ICP-MS (in-situ and mounted mineral separates) at the LaserChron Center. Monazite analyses from the restite yielded six compositionally distinct populations with dates of 1178 ± 16, 1139 ± 4, 1064 ± 6, 1049 ± 4, 1030 ± 5, and 1004 ± 10 Ma. Yttrium and heavy REEs decrease in monazite in two steps: one dramatic drop from ca. 1150 to 1065 Ma and another between ca. 1065 and 1050, interpreted to reflect two periods of garnet growth and melting. Analyses from the restite zircon separate yielded a significant single peak near 1050 Ma. These zircon grains exhibit fir-tree sector zoning texture which is interpreted to indicate crystallization from melt. Monazite from leucosome yielded a unimodal population at ca. 1050 Ma, however, backscatter images document alteration of monazite to apatite on the edges of the grains, and abundant uranothorite inclusions. Leucosome zircon analyses yielded a ca. 1150 Ma population from cores and a 1050 Ma population from rims. Cathodoluminescence imaging reveals that the zircon rims have textures indicative of fluid alteration. The data are consistent with these rocks undergoing two periods of melting. The first event at ca. 1150 Ma may have involved a non-garnet producing melting reaction, such as muscovite dehydration-melting. The second event at 1065 Ma involved significant garnet growth, interpreted to represent biotite dehydration-melting. Subsequently, the rocks underwent hydrothermal alteration at 1050 Ma. Monazite grains with dates at 1030 ± 5 and 1004 ± 10 Ma have higher yttrium concentrations suggesting garnet breakdown and monazite growth during decompression and retrograde metamorphism. A combination of monazite and zircon dating techniques from each compositional layer is necessary to constrain leucosome-restite relationships and to accurately interpret the timing of melting from migmatites that have experienced multiple phases of melting.