THE ALLEGHANIAN TECTONIC SETTING OF THE ACADIAN HIGHLANDS OF WESTERN CONNECTICUT REVEALED BY PERMIAN INTRUSIVE FELSITES

A 15 km long NW trend of felsic intrusions in the Acadian highlands of western Connecticut provides new insights into the Alleghanian orogeny. This trend includes the Pinewood Adamellite stock (PA), dikes of rhyolite and dacite porphyries (Crowley, 1968), and the high-temperature hydrothermal fluoride mineralization at the Old Mine Park (OMP) area, Trumbull. The ~4 km² PA is generally a 1-2mm equigranular, corundum normative granite with porphyritic, aplitic, and pegmatitic phases. Trace element compositions (Rb, Y, and Nb) corroborate its present tectonic setting with predominantly syn-collisional and lesser intraplate affinities. Perthitic K-feldspar phenocrysts show grid twinning, plagioclase shows albite twinning, and its muscovite has interlobate grain boundaries with quartz and feldspars. NW-trending quartz ± fluorite veins commonly with muscovite-mantled margins cut the adamellite. Small outcrops of rhyolite and dacite porphyries 3 km south and 4 km north, respectively, consist of subhedral feldspar phenocrysts in a fine-grained matrix. Contacts are not exposed, but outcrop shapes suggest they are dikes.

The occurrence of accessory fluorite in these rocks suggests they are part of a single, synchronous, NW-trending magmatic/hydrothermal “fluoride” zone. A new ICP-MS U-Pb concordia age of zircon of 289 ± 1 Ma from the PA documents its crystallization during the Alleghanian orogeny. The fine-grained porphyritic textures of the intrusive dikes suggest the country rocks were cool at the time of intrusion. Additionally, regional biotite ⁴⁰Ar/³⁹Ar cooling ages (~270 Ma) and a new K-feldspar ⁴⁰Ar/³⁹Ar upper end age of 245 ± 3 Ma from the country rock confirm that high-grade Acadian metamorphic conditions had dropped to lower greenschist facies at the time of intrusion. Thus, these rocks were cool and strong enough to fracture during the Alleghanian orogeny. The NW trend defined by both mapped quartz-fluorite veins and the “fluoride” zone is parallel to Alleghanian shortening in eastern New England. We conclude that the rocks of western Connecticut acted as a buttress and deformed by NW trending extensional fractures during the NW-directed shortening and thrusting well documented in eastern New England.