AFTER THEIR ACCRETION TO THE LAURENTIAN MARGIN, ROCKS IN WESTERN CONNECTICUT STAYED DEEP AND HOT FOR 60 MILLION YEARS

Amphibolite- and upper amphibolite facies rocks in western Connecticut that compose the Hartland and Gneiss Dome Belts shown on the Bedrock Geological Map of Connecticut (Rodgers, 1985) were accreted to the Laurentian margin beginning in the Late Ordovician. Both belts have been interpreted to record overprinting New England Appalachian Taconic and Acadian regional metamorphism, supported, for example, by the seminal work of Hames, Tracy, and others (AJS 291 887). U-Pb zircon crystallization ages of plutonic rocks and U-Pb ages of sphene and monazite growth, combined with the compilation and synthesis of previously published argon mineral ages (Hillenbrand et al., 2019 GSA Abstracts with Prog. 51 doi: 10.1130/abs/2019NE-328393 and Hillenbrand and Williams, 2019, GSA Abstracts with Prog. 51 doi: 10.1130/abs/2019NE-328385) show that these multiply-deformed rocks remained deep and hot from the onset of Taconic metamorphism until the onset of slow regional exhumation beginning at ca. 380 Ma. In current tectonic models (summarized by Jacobi and Mitchell, 2018, GSA Spec. Pap. 540 617), Taconic thrusting began at ca. 455 Ma. Staurolite-grade metamorphism in northwestern Connecticut was attained at 445 Ma (Hames, Tracy, et al.). Intrusion of qtz-diorite and tonalite, and migmatization in dome cores at 436 Ma (Dietsch et al. 2010, GSA Mem. 206 141) and intrusion of ms-bt leucogranite into higher structural levels at 430 Ma signal maximum crustal thickening. Across both belts, hornblende argon ages between 383-378 Ma signal cooling and denudation following Acadian orogenesis; the rocks cooled to argon closure in muscovite between 360-355 Ma (Dietsch et al., 2006, GSA Abstracts with Prog. 38 20 and 2010). 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 erosion rates are extremely slow, less than 2 m/m.y. (Dietsch et al., 2015, ESPL 40 389).