ANISOTROPY OF MAGNETIC SUSCEPTIBLY AND GEOCHEMISTRY THE HYDE SCHOOL GNEISS AND ROCKPORT GRANITE, NORTHWEST ADIRONDACK LOWLANDS

In the Northwest Adirondack Lowlands massive to foliated granitoid rocks such as the Hyde School and Rockport Gneisses are interlayered with and/or intrude a variety of metamorphosed sedimentary and other rocks. ~60 samples were collected in the Lowlands of the U.S. and a few kilometers into Canada for chemical and magnetic susceptibility analysis.

ICP-MS and XRF analysis of 20 leucogranite samples yield a stunning diversity compositions among rocks that look almost identical in the field. Most rocks are typical LREE-enriched, high-K granites with flat or saddle-shaped MREE-HREE patterns and prominent negative Nb-Ta anomalies indicative of derivation from melting of arc-type continental crust. Four samples from the North Hammond body have FLAT REE patterns and NO Nb-Ta anomalies, suggestive of derivation from MORB-like crustal sources. These four samples are unique in the Lowlands, based on the 200 samples in our data set. One sample from the Fish Creek body has strongly depleted HREE suggestive of considerable garnet involvement, like some Antwerp Gneiss in the Lowlands (Carl et. al, 1990) southeast of our sampling area. There is no consistency of results from several tectonic setting discriminant diagrams, other than the dominance of melting sources with arc involvement at some point in their history. This suggests that the source regions for these granites across the Lowlands was highly inhomogeneous. Ongoing work to understand the geographic variability of Lowlands granitoid compositions may yield insights into the deeper source regions, with possible tectonic implications.

Anisotropy of magnetic susceptibly (AMS) was tested on 48 cored samples of the Hyde School and Rockport Gneisses from 20 locations. The purpose was to compare to field evidence for strain across the Black Lake Shear Zone (BLSZ), a possible buried docking boundary between the Adirondack Lowlands/Highlands and Canadian Frontenac region, northwest of the Carthage-Colton Shear Zone. AMS strain magnitude (k1/k2), lineation, and foliation data are consistent with a NE-SW-oriented shear zone with a clear progression of increased strain into the BLSZ.