STRUCTURE, PETROGRAPHY AND GEOCHEMISTRY OF THE LOG JAM SCHIST, WESTERN CONNECTICUT

The curious "Log-Jam" schist (LJS) is a set of NNW-SSE-trending outrcops in a 500x300m area within the Rowe meta-turbidite in W Connecticut. The LJS is known for its common and large Ky porphyroblasts, some >50 cm long, distinct from local, Ky-absent, pelitic-semipelitic Rowe-Ratlum Mtn formation amphibolite facies mica schist. LJS rocks are layered compositionally sub-parallel to local structure (moderately W-dipping), and the W-dipping Ky foliation (shape- and lattice-preferred orientation; see Solar et al., 2016, NEGSA). Ky is mostly <15 cm long within very coarse-grained 5-10 cm-thick Ky-phyllosilicate (KP) layers spaced 5-40 cm with 10-20 cm-thick, Ky-bearing coarse-grained quartzo-feldspathic gneiss (QFG). Unlike QFG layers, KP layers are dominated by coarse Chl.

We compare LJS structures, petrography, and whole-rock geochemistry. Compared to local rocks all LJS rocks are coarser grained, and Ky porphyroblasts are unique. Ky growth and alignment was syntectonic. Late strain in Ky is recorded by deformation bands and local gentle c-axis folds. Ky is 15-20% of KP layers, and <10% of QFG layers. Ky in KP layers has low inclusion density within a poorly-ordered matrix of 1-2 cm Chl, and fine Bt, Pl, Qtz, and Rt. In contrast, Ky in QFG layers is within coarse continuous Qtz + feldspar foliation with Bt + Chl. We separate QFG rocks into %Ky categories of lower (<5%) and higher (5-8%). Ky is poikiloblastic in higher-%Ky, but skeletal in lower-%Ky consistent with fabric overprinting. Relative to local rocks, the LJS specimens have similar Na₂O, and (FeO_t+MgO) concentrations and REE patterns, but higher MgO and lower FeO_t, CaO, K₂O, and some trace elements (e.g., Rb, Sr, Pb). SiO₂ is higher in lower-%Ky QFG, similar in higher-%Ky QFG, and lower in KP rocks that also have higher Al₂O₃ and TiO₂ whereas in all QFG rocks these oxides are similar to local rocks.

Together, petrography, Ky fabrics, meta-turbidite structure, and geochemistry preclude a previous interpretation of a granitic pluton protolith and supports a common metasedimentary rock protolith and peak metamorphic grade of the LJS and the local rocks. The very different mineral contents are from metasomatism of the LJS protolith. We speculate deeper-level magmatism caused local dehydration and fluid flow through what are now the LJS rocks. It is weird schist.