GEOCHEMISTRY GEOBAROMETERY AND TECTONIC SIGNIFICANCE OF A LATE OTTAWAN GRANITE BODY: NORTHWEST ADIRONDACK MOUNTAINS, NEW YORK STATE

The Grenville Province exposed in the Adirondack uplift is a high-grade metamorphic terrane (P=6-8kbar T=600-750^oC) that is crosscut by numerous shear zones. Grenville-aged rocks in the Adirondack uplift have been subdivided into smaller terrains separated by shear zones. The Carthage Colton Shear Zone (CCSZ) separates the Central Metasedimentary Belt Trerrane (CMB) and Central Granulite Terrain (CGT). This work reports on the results of field, geochemical, and geobarometric studies carried out over the past 3 years. Detailed mapping across the CCSZ focused on field relationships of a weakly-deformed hornblende + biotite bearing granite body in the CGT. Field data demonstrate that the pluton truncates to the west against a prominent southeast-dipping lineament. This lineament crosscuts the CCSZ indicating the lineament is younger than the CCSZ. This observation offers two possibilities with respect to the timing of granite intrusion: 1. the granite is cut by and hence older than the lineament or 2. the granite exploited the lineament and postdates its formation The mineralogy and lack of recrystallization of these hornblende granites allowed for the use of the total Al in hornblende geobarometer. Crystallization pressures of 3.7-4.2 +/- 1 kbar were recovered and show that the granites crystallized at shallow crustal depths. Geochemical data show that the granite is an A-I hybrid type. REE and spider plot data are consistent with a continental arc type intrusive environment for this body, however, the granite samples plot in the WPG (Within Plate Granite) field on discrimination diagrams suggesting a rifting environment. These data, coupled with the presence of magmatic hornblende, and low crystallization pressure suggest that this granite body intruded at the end of a rapid tectonically driven exhumation event. This study identified igneous activity in the CGT at pressures well-below those determined for peak Ottawan metamorphism, and is believed to be driven by rapid post Ottawan exhumation of the CGT. Hydration of this granitic magma is most likely due to the shallow depth of crystallization.