PLUTONIC HARRISON GNEISS AND SISCOWIT GRANITE IN THE STAMFORD AND POUND RIDGE 7.5 MINUTE QUADRANGLES, SOUTHWESTERN CONNECTICUT, PRESERVE IGNEOUS FLOW FABRICS, INTRUDE ALREADY-FOLIATED COUNTRY ROCKS, AND WERE ISOCLINALLY FOLDED, ALL UNDER HIGH-T CONDITIONS

The bedrock geology of the Stamford and Pound Ridge 7.5 minute quadrangles, southwestern CT, is dominated by the Harrison Gneiss and Siscowit Granite. Where least deformed, the Harrison is typically a medium-grained diorite gneiss and the Siscowit has at least four major variations, but is commonly a whitish-gray, fine- to medium grained bt granite. Based on correlations, the Harrison is likely to have an igneous crystallization age of 450-455 Ma (Sevigny and Hanson, GSA Bull. 107 487). Continued bedrock mapping has led to new insights about the (1) development of high-grade fabrics in the metasedimentary country rocks (dominated by bt schist), (2) development of fabrics in the two plutonic units, and (3) the relative timing of intrusion and regional deformation.

Across both quadrangles, Siscowit thoroughly permeates its country rock. All gradations can be seen in it from m-thick screens of bt schist to mm-thick bt schileren, all parallel to a high-T foliation in the granite defined by recrystallized qtz + homogeneous Kfs + aligned, unstrained bt. Country rock intruded by veins of granite forms stromatic migmatite. Harrison can be traced in outcrop into strongly foliated qtz diorite (bt-hbl schist), with undeformed plagioclase and recrystallized bt & hbl forming an amphibolite facies assemblage. In these outcrops, layer-parallel veins of Siscowit occur, commonly stretched and forming boudins. Both plutons intrude already-foliated country rocks and are folded by younger isoclinal folds; locally, a well-developed stretching lineation parallel to the axes of these isoclines plunges 220º to 320º, the variation produced by late open folding with a spaced axial plane cleavage.

We interpret field relations, macro-, and microscopic fabrics in the Harrison and Siscowit to, first, record magmatic flow as these magmas were intruded under a stress regime into foliated metasediments. The now-gneissic igneous rocks and the metasediments were then isoclinally folded eastwards(?), the gneissic foliation becoming in places refolded and in other places developing a new, foliation axial planar to these folds. In more deformed zones, a strong lineation parallel to fold axes developed. All these events took place under high-T conditions which continued post-tectonically, as recorded by static mineral recrystallization.