Igneous and Metamorphic Petrology and Geochemistry
earth and environmental sciences
PETROLOGICAL AND GEOCHEMICAL STUDIES AS A GUIDE TO TERRANE ANALYSIS
Currently, much of the research in igneous and metamorphic petrology and geochemistry carried out by our students is aimed at understanding the origin and evolution of selected igneous rocks as a guide to terrane analysis. These studies are often integrated with those discussed in the previous section on regional geology. Major and trace element analyses, carefully used, combined with petrological and regional observations can be powerful discriminators of the environments in which these rocks formed. Thus, our studies strive to not only understand the origin and evolution of particular igneous rocks, but to use these rocks as guides to understanding the larger picture of the terrane formation and plate tectonic processes in the northern Appalachians. This approach has been successfully applied to igneous and metamorphic rocks from different terrane settings in southeastern New England, Massachusetts, Rhode Island, Vermont and Maine, and other students are currently applying them in areas outside of the northern Appalachians.
GEOCHEMISTRY AS A GUIDE TO TERRANE ANALYSIS IN SOUTHEASTERN NEW ENGLAND
Southeastern New England represents a collage of tectonic terranes formed on the eastern margin of the Iapetus Ocean that collided with the North American margin during the Acadian and Alleghanian Orogenies. Using major and trace element geochemistry , we are able to understand the formation and evolution of these enigmatic terranes. For instance, it is now clear that much of southeastern New England is underlain by a fragment of the composite Avalon terrane. Geochemistry indicates it was part of a continental margin volcanic arc in the late Proterozoic. At that time mafic, mantle derived magmas, mixed with crustal material to form at least some of the intrusions of batholithic proportions in Southeastern New England. Later, this terrane was intruded by an entirely different suite of alkalic magmas in a different tectonic regime during the early and middle Paleozoic. Inboard of the Avalon terrane of Southeastern New England is the Nashoba terrane, which geochemistry indicates was part of an Ordovician arc, likely ìpicked-up by the Avalonian margin as it closed with North America. There is yet much that needs to be learned about the terranes that make up the eastern margin of the Appalachians and their amalgamation. Geochemical studies are proving to be key to obtaining a better understanding of these terranes.
METAMORPHIC STUDIES IN THE NORTHERN APPALACHIANS
Studies of metamorphism help define the thermal evolution of an area and its relationship to deformational events. In addition, coexisting metamorphic minerals can be used to determine the pressures and temperatures of individual metamorphic rocks and constrain the pressure, and temperature time paths of a terrane. Recent metamorphic studies have included analyses of the metamorphism of the Nashoba terrane in southeastern New England.
PETROLOGY, GEOCHEMISTRY, STRUCTURE AND TECTONICS OF WESTERN NEW ENGLAND
In a manner similar to our studies in Southeastern New England we are integrating studies from a broad range of sub-specialties to better understand the tectonic history of portions of western New England in Vermont and Maine. Geochemical analyses have recently supplemented field and structural studies to provide new insights into the evolution of this region. Using analyses of the immobile trace elements (including the Rare Earth Elements) we were able to reach back through the intense metamorphism in the area and determine the original plate tectonic settings in which the igneous rocks formed. Recently, for instance, isotope and trace element geochemical analyses indicated the existence of a previously unknown early Ordivician volcanic arc, the Shelburne Falls Volcanic Arc, in Western Vermont and adjoining areas. It is likely that collision of this arc with Laurentia is responsible for the well known Taconic Orogeny, not the collision of the younger Bronson Hill Arc as previously thought. Thus, our studies have revealed a new tectonic picture for this area of the Appalachians. This and similar studies are ongoing as we try to unravel the complex history of terrane interaction that led to the formation of the Appalachians.