61 Pegmatites, Plutons, and Pyroclastics of the Northeast

Wednesday, 25 March 2015
8:00 AM - 12:15 PM
Grand Ballroom North

FIELD EVIDENCE FOR THE SYNTECTONIC [OR SALINIC?] INTRUSION OF THE BEARDSLEY ORTHOGNEISS BY THE PUMPKIN GROUND ORTHOGNEISS, PERI-LAURENTIA, CONNECTICUT

Paper number: 61-4
Program: Theme Sessions
Time: 9:00 AM – 9:20 AM

Authors

Abstract

A large outcrop in Seymour, Connecticut provides critical field evidence of the relative ages of units in the Prospect orthogneisses. Here the foliation in the dioritic Beardsley orthogneiss (Obl) is truncated by a granodioritic porphyry, the Pumpkin Ground Orthogneiss (Spg). Foliation in the Obl is defined by biotite and hornblende; small plagioclase phenocrysts are anhedral and transposed into the foliation, indicating at least moderate shear strain. In contrast, foliation in the Spg is defined by biotite alone, and large (≤3 cm) K-feldspar phenocrysts are euhedral and randomly oriented, suggesting less strain than in the Obl. These relationships indicate that the Spg is younger than the Obl, and confirms the U-Pb TIMS analysis of zircons (Sevigny and Hanson, 1993) that yielded discordant data with lower intercept ages of ~446 and ~428 Ma, respectively. Geochemical analyses show an increase in silica content in the Pumpkin Ground relative to the Beardsley, suggesting evolving magma fractionation during the Silurian.

Foliation in the Obl is also truncated by the Spg, indicating that a metamorphic event occurred in the early Silurian. However, shear zones truncate both gneisses, and local garnets in each demonstrates that upper-amphibolite facies deformation affected both units. This suite was further truncated by a series of pegmatitic veins. Early veins are transposed and attenuated into the overprinting fabric, later veins are boudinaged, and the youngest pegmatites are undeformed.

These observations confirm the lithologic classification previously described for these two units. It is significant that the contact between these units are not strongly overprinted by the mylonite or blastomylonite as described by Sevigny and Hanson (1993). A stronger fabric in the Beardsley unit relative to the Pumpkin Ground suggests Silurian deformation. This along with possible magmatic fractionation constitute evidence for the Salinic orogeny having affected this part of eastern Laurentia. However, multiple stages of pegmatite intrusion show progressive deformation during their emplacement, reflecting a protracted Acadian overprint.