THE “LOG-JAM” SCHIST OF WESTERN CONNECTICUT: ITS FABRICS AND ORIGIN

The colloquially named "Log-Jam" schist (LJS) is as a single set of exposures in a 500x300 m sub-elliptical map area within the pelitic Rowe schist. The LJS is known for its spectacularly large kyanite (Ky) crystals, some >50 cm long, that has landed samples in museums and on book covers. The moniker infers Ky is without preferred orientation, but on inspection Ky defines a foliation at minimum. Less appreciated is that Ky is largely (not exclusively) within 5-10 cm-thick layers spaced 5 cm to 1 m within host medium- to coarse-grained quartzo-feldspathic gneiss that dominates the exposures. The Ky-rich layers have 15-20% Ky in a matrix of 1-3 cm-size Bt (+Chl), and fine-grained Pl, Qtz and Rt, collectively indicating very high alumina content. This has led some to speculate that the LJS composition resulted from pre-metamorphic Al-enrichment in pelitic sediment by a hydrothermal vent system, further supported by the isolated elliptical map pattern and the bulk composition of the Rowe schist.

Structural and petrographic analyses suggest a simpler interpretation. At outcrop, Ky-rich layers are moderately W-dipping, sub-parallel to the host gneiss foliation, and consistent with regional structure. All minerals are aligned preferentially. From 11 stations, 475 total Ky c-axis attitudes were measured revealing a distinct stereogram girdle with best fit great circle 004/29W, and less than 15% of data as outliers defining a Ky foliation sub-parallel to layers and matrix foliation. A subset of 349 crystals show full length and plots of this subset show similar contoured stereograms, and that all >12 cm-long crystals plunge generally W. [100] planes were determined using 295 b-axis attitudes. Poles to [100] planes are subparallel to poles to matrix and regional structures. Therefore, we interpret that these data record Appalachian strains.

The LJS origin is found in the mineral assemblage of the Ky-rich layers in context of the host quartzo-feldspathic gneiss. Aluminosilicate-producing metasomatic reactions are consistent where Ky and mica are produced at the expense of feldspars. Given the dominant host quartzo-feldspathic gneiss, and the outcrop pattern similar to metaplutonic granitic rocks in the area we favor the LJS to be a metamorphosed granitic pluton where LJS layers are instead metasomatic coplanar veins.