RECOVERING EARLY-FORMED P-T CONDITIONS OF POLYPHASE, HIGH-GRADE ROCKS IN WESTERN CONNECTICUT USING RAMAN SPECTROSCOPY OF INCLUSIONS IN GARNET

In western CT across the Hartland Belt east of Cameron’s Line and along the belt of domes west of the Mesozoic Hartford Basin, the imprint of Acadian deformation and regional metamorphism is strong. Nappes were emplaced along a décollement over rocks in the core of the Waterbury dome (WD) and in the nappes, early-formed west-vergent isoclinal F1n (n=nappe) synclines cored by The Straits Schist (TSS) of Silurian-Devonian age are recumbently refolded by east-vergent F2n folds with a strongly developed axial planar S2n, the dominant regional foliation. More upright F3n folds deform S2n into an S3n regional crenulation cleavage that also deforms core rocks of the WD. Based on hornblende Ar ages, crenulation cleavage and dome formation was over by 378 Ma. TSS in the saddle between the Waterbury and Bristol domes yields peak conditions of 9±1 kb at 700±50°C and broad zones of ky+st and sil extend across the Hartland Belt. But Acadian nappes were emplaced on already-deformed and metamorphosed rocks: syn-migmatitic orthogneiss in the core of the WD crystallized at ca. 435 Ma and mesoscopic field relations reveal even older pre-migmatitic foliation. In an effort to document early-formed P-T conditions in the WD, we used Raman spectroscopy (with a Horiba T64000 using 457.9 nm excitation from an Ar+ laser) to analyze minute inclusions preserved in garnet porphyroblasts deformed and aligned in migmatitic foliation. The inclusions we analyzed range in size from a few to tens of microns across. In migmatitic pelitic schist with ky+kfs+bt+grt+ms+qtz+pl, we analyzed inclusions in 5 garnets and discovered the following sets: qtz-ms-bt-ap; qtz-ky-ru-zrn; qtz-ru; qtz-ky; and qtz-?pyroxene. The inclusion “assemblage” of qtz+ky+ru without ilm or st indicates minimum T and P of 650°C and 11-12 kb (depth of about 40 km). These results imply that WD rocks were tectonically buried during Taconic thrusting and post-Taconic, pre-Acadian exhumation was significant.