THE PINEWOOD ADAMELLITE: A RECORD OF ALLEGHANIAN TECTONICS IN THE ACADIAN METAMORPHIC HIGH OF WESTERN CONNECTICUT

The Pinewood Adamellite is unique in western Connecticut as the only Alleghanian intrusion (monazite age of 291 ± 4 Ma; Sevigney and Hanson, 1993) into the high-grade Acadian rocks of western Connecticut. Its internal textures and structures are surprisingly varied, perhaps reflecting its relatively shallow level of intrusion. Textures are dominated by hypidiomorphic-granular granites with subordinate aphanitic and porphyritic phases. Pegmatite dikes ranging from 0.5-50 cm wide cut the Adamellite, and become more abundant toward the margins of the body. K-feldspar dominates the pegmatites with lesser quartz, minor plagioclase, and rare muscovite. Pegmatites may be zoned with quartz or aplite cores. Pegmatite dikes show no preferred trend within the adamellite, but the measured bearings may suggest a radiating pattern perpendicular to the contact of the intrusion with the host Straits Schist. Aplite occurs in dikes alone or with pegmatite margins 1-5 cm in width, or in gently dipping bands up to 40 cm thick with gradational contacts with the granite. The Adamellite is also cut by steeply dipping quartz veins from 1mm-50cm wide. Quartz always dominates, but the veins may contain minor feldspar. Many quartz veins are mantled by muscovite, with flakes showing a very strong preferred orientation perpendicular to the walls of the vein. In contrast to the random bearing of pegmatites, quartz veins have a strong alignment at 315° with a subordinate set toward 010°.

The Adamellite contains accessory fluorite, apatite, and magnetite, and is one of three unfoliated intrusions within Long Hill & Bridgeport quadrangles that define a trend of 315°. This mineralogy is similar to the topaz and fluorite veins in Old Mine Park (OMP), Trumbull. This trend of 315° is close to the trend of the OMP veins (325°, Moritz et. al, 2019) and to the quartz veins in the Pinewood Adamellite. This coincidence of structure and geochemistry suggests that NW-directed Alleghanian shortening was responsible for the extension producing these parallel structures, and that Alleghanian metamorphism caused partial melting below the already metamorphosed Acadian crust.