Crystallographically-oriented Lamellae Phases in Garnet and Their Potential Use as Petrogenetic Indicators, Central Maine Terrane, Connecticut

Yale University, New Haven CT

Investigators

Abstract

Tectonic plates are consumed at subduction zones, vast undersea regions which stretch thousands of miles across the face of our planet. During subduction, one plate sinks beneath another to be recycled into Earth's mantle. Subduction zones are the sites of active volcanism, the largest known earthquakes, and a wide array of mineral deposit types. Tracing the history of subduction as recorded in mountain belts is essential for reconstructing plate movements, paleogeography, paleobiology, and paleoclimate throughout geologic time. The Appalachian-Caledonide mountain chain is one of the largest in the world, stretching from the southern and eastern United States up through Canada and Greenland and across to Europe. Nonetheless, the record of subduction in the northeastern United States is surprisingly cryptic in terms of the rock record it left behind, particularly in terms of the metamorphic rocks comprising the deep roots of the mountain belt. New discoveries, however, are providing fresh perspectives, including the recognition of "ultrahigh-temperature" metamorphic rocks in northeastern Connecticut (CT) formed in excess of 900 °C (about 1650 °F). Of major significance is that this CT locality also preserves tantalizing evidence for subduction of crust to great depths-perhaps in excess of 60-100 miles. The research project will focus on these CT rocks as they may be the first "ultrahigh-pressure" (UHP) metamorphic rocks definitively recognized in the United States; their presence would catalyze a rethinking of how, where, and why rocks were subducted during Appalachian mountain building. More broadly, a deeper understanding of the tectonic architecture of the region may help resolve longstanding questions involving Paleozoic plate movement and associated implications for the location and geologic formation of natural resources in the mountain belt (which extends into the Arctic and potentially as far west as Alaska). The project will support and train Yale Ph.D. and undergraduate students, and integrate research results into public science outreach programs at the Yale Peabody Museum of Natural History. Gneisses in the field area host garnets that contain spectacular examples of crystallographically-oriented lamellae minerals (rutile, ilmenite, apatite, amphibole, pyroxene, quartz, and other phases). Comparisons with other localities worldwide indicate that such lamellae correlate broadly with pressure-temperature conditions and, thus, they hold considerable but as yet mostly untapped potential as petrogenetic indicators. For example, lamellae assemblages including rutile, ilmenite, quartz, amphibole, and apatite are documented only from UHP settings. In order to be used to diagnose metamorphic regimes, however, lamellae origins must be understood. Consequently, this project will undertake mineralogical and petrological study of the lamellae in the CT garnets involving: (1) Characterization of lamellae assemblages and chemical compositions via electron imaging and microprobe techniques together with Raman spectroscopy. (2) Electron-backscatter diffraction and transmission electron microscopy studies to determine crystallographic relationships between garnet and lamellae. (3) Chemical mapping using the electron microprobe to image closed and open-system garnet behavior. (4) Field work to identify other lamellae-bearing garnet localities in the region. The results of the above research will be used to test whether or not the lamellae are the products of exsolution from garnet, or if they formed by some other mechanism such as coupled infiltration and precipitation. If the former, then the Si-, Ti-, and P-rich garnet compositions point strongly toward majoritic garnet precursors formed at UHP conditions ~5 GPa or higher during subduction. Regardless of hypothesis test outcomes, lamellae formation processes must be better understood to help guide petrotectonic interpretations in New England and other orogenic belts across the globe.