DETAILED THREE-DIMENSIONAL MAPPING OF VARIATIONS IN MINERAL CONTENT AND TEXTURE IN METAGABBRO, HOOPER GARNET MINE, SOUTHEASTERN ADIRONDACKS, NEW YORK

The metagabbroic rocks found in and around the abandoned Hooper garnet mine in the Adirondacks have recorded significant partitioned strain during deformation and metamorphism associated with the Grenville orogeny. Exposure is nearly 100% in the 150 x 100m quarry, with 30m walls permitting 3-D examination. Rocks range from dominantly garnet amphibolite to subordinate banded metagabbroic gneiss composed of amphibole+plagioclase (~ 1:1 proportion). All rocks contain mineral aggregates that define fabrics of variable alignment and intensity. Foliation is variable in intensity and dip, but the strikes of foliation and trends of lineations are consistently oriented across the entirety of the quarry to 110°. This trend is consistent with structures across the southern Adirondacks. Garnet porphyroblasts are found throughout, and are typically draped with matrix minerals to form tails of various symmetry, and elongate along the 110° trend.

To determine whether mineral content affected textural development in these rocks, we collected compositional, geometrical and orientation mineral data from the millimeter- to the quarry-scales. A fabric and compositional map was produced of the mine that allowed us to identify priority locations for finer-scale studies at the meter-scale, and for specimen collection for laboratory analyses. In order to determine correlated 3-D variations in composition, fabric intensity, shapes of grain aggregates, and the area percentage of garnet present, the specific areas chosen were documented down to grain-by-grain analyses. Examination of specimens, which were cut according to fabric orientations and thin sections (cut from those surfaces) augmented field data. Results show that highly-concentrated clusters of small (~5mm) garnet crystals define distinct bands in dominant amphibolite, whereas relatively isolated large (up to 10cm) garnet crystals with distinct matrix mineral tails tend to occur within the more leucocratic metagabbroic gneiss. Independent of rock composition, field and laboratory data show a dominant L>S tectonite fabric in areas that can be traced across the quarry. It is apparent from our data that fabrics in the rocks are the result of localization of strain controlled by compositional variation, and by the concentration and size of garnet porphyroblasts.