METAMORPHIC LOADING AND EXHUMATION RATES OF THE NARRAGANSETT BASIN, RHODE ISLAND

Preliminary microprobe analysis of garnet bearing assemblages in the Narragansett basin of Rhode Island indicates a range in core temperatures and pressures from 500⁰C to 675⁰C and 7kb to 9.5kb. Thermobarometric estimates were calculated using garnet-biotite Mg exchange, garnet-plagioclase Ca exchange reactions, and the winTWQ 2.3 software developed by R.G. Berman (2007). The four samples analyzed in detail are from Prudence Island (PI), southeast of Davisville (Da), Bonnet Shores (BS), and Stook Hill (SH). Samples PI, Da, and SH, are dominated by quartz, feldspar, and large subhedral biotite porphyroblasts. The BS sample is dominated by large subhedral biotites with lesser amounts of quartz, feldspar, and muscovite.

PI garnets, at the garnet isograd, show core to rim zoning of X(prp) 2- 4% and X(grs) 21-12%. These compositions yield core-rim temperatures and pressures of 500⁰C - 7kb and ~500⁰C - 7.25 kb. Da garnets are euhedral with core-rim compositions of X(prp) 8-2% and X(grs) 12- 30%. Core compositions yield temperatures and pressures of 625⁰C and 8.25 kb while rims show 500⁰C and 5.5 kb. BS contains euhedral garnets with core-rim compositions of X(prp) 3.5-5% and X(grs) 14-8%. These yield core-rim conditions of 575⁰C - 9kb and 545⁰C - 9kb. Our SH samples contain abundant 0.1-0.2 mm euhedral to subhedral garnets with a core-rim composition of X(prp) 9-8% and X(grs) 5-6%. These yield conditions of 675⁰C and 9.5 kb in the core falling to 550⁰C and 5kb in the rim.

Core pressure and temperature estimates create a smooth trend from lower garnet grade rocks (PI) to kyanite/sillimanite grade (SH). Given a depositional age of ~305 Ma (Lyons, 1978) and peak T at ~280 Ma we estimate a typical loading rate of ~1-2 km/Ma. ⁴⁰Ar/³⁹Ar cooling ages (Dallmeyer, 1982) indicate that the high grade assemblages passed through hornblende and biotite closure temperatures of ~500⁰C and ~300⁰C at 260 Ma and 245 Ma respectively. Using these ages, and the above pressures, we estimate an initial exhumation rate of ~0.35 mm/yr from peak conditions through the hornblende cooling temperature and an increased rate of ~0.5 mm/yr between the hornblende cooling temperature and the biotite temperature.