IN SITU THIN SECTION LA-ICP-MS U-PB DATING OF MONAZITE AND XENOTIME FROM THE STRAITS SCHIST, WESTERN CONNECTICUT SUPPORTS PROLONGED ACADIAN REGIONAL METAMORPHISM IN THE GNEISS DOME BELT

Amphibolite facies rocks in western Connecticut shown on the Bedrock Geological Map of Connecticut (Rodgers, 1985) as the Hartland and Gneiss Dome Belts were accreted to the Laurentian margin beginning in the Late Ordovician and both belts record Acadian regional metamorphism. In situ thin section LA-ICP-MS U-Pb dating of monazite and xenotime from The Straits Schist in the Gneiss Dome Belt has produced ages that record prolonged Acadian metamorphism. The Straits underwent amphibolite facies metamorphism at ~600-750 °C and ~0.65-0.9 GPa. Nappes outlined by The Straits were juxtaposed with rocks of the Waterbury dome (WD) prior to 378 Ma. Monazite included in garnet rims from a sample of The Straits located south of the WD has concordant ages between 387 ± 10 and 358 ± 7 Ma (n = 4), and a KDE plot of 66 concordant analyses of matrix monazite in this sample has peaks at 363 and 377 Ma. In another sample south of the WD, two monazite inclusions in staurolite have concordant ages of 390 ± 7 and 384 ± 5 Ma, and a KDE plot of 63 concordant analyses of matrix monazite defines a broad peak between ~365 and 395 Ma. In this same sample, five matrix xenotime grains with three to four analytical spots per grain have concordant ages of 374 ± 6 Ma, 368 ± 6, 367 ± 5 Ma, and 367 ± 5 Ma. From a sample of The Straits located along the western flank of the Bristol dome, a KDE plot of 57 concordant analyses of matrix monazite yields a prominent peak at ~377 Ma and two minor peaks at ~370 and ~360 Ma. Two matrix xenotime grains from this sample have the same concordant age of 377 ± 6 Ma. Inclusions of monazite in garnet and staurolite suggest that monazite growth began at ~390 Ma and monazite last grew at ~360 Ma. The spread in matrix monazite ages is interpreted to reflect a combination of episodic growth during this 30 m.y. interval and partial resetting of the U-Pb isotopic system. Keeping rocks deep and hot for 10s of millions of years implies a nexus of tectonics, climate, and surface processes that limited denudation. A modern analog might be the Transhimalayan and Ladakh ranges in northern India which have been tectonically quiescent since the Early Miocene (Kirstein et al., 2009, J. Geol. Soc. Lond. 166 667), where precipitation is only 400-500 mm/a (TRMM, data; NASA, 2009), and where bedrock erosion rates are extremely slow, less than 2 m/m.y. (Dietsch et al., 2015, ESPL 40 389).