UNRAVELING MULTIPLE METAMORPHIC/TECTONIC EVENTS IN NEW ENGLAND: A PERSISTENT CHALLENGE

Detailed geologic mapping has provided the foundation of our understanding of the geology of New England for many years. However, we have come to understand that mapping provides only one basis for that understanding, and that at its core a map is an interpretation that is not above re-interpretation as new data become available. Regional mapping has provided evidence for the Taconic, Acadian, and Alleghanian orogenies that in places overprint one another. Geochronology has revealed complicated age patterns, and the Salinic, Neo-Acadian, and Fundian orogenies have been proposed to account for these ages. Our next challenge is to establish the plate tectonic context of the many orogenic events: are they punctuated or progressive or continuous? Thermochronology, geochronology, metamorphic petrology, and structural geology, together with geodynamic modeling will be at the heart of many new breakthroughs. Thermochronology is increasingly successful at identifying (1) the times and rates of cooling and (2) overprinting metamorphic histories in metamorphic field gradients. One challenge of geochronology is that we want to date events, but we can date only minerals. Where dissolution/precipitation is the deformation mechanism, and where datable minerals participate in defining fabrics, we may be able to date the fabric directly. Sphene at amphibolite facies and muscovite at greenschist facies where these minerals can crystallize below their respective closure temperatures are promising systems, and have demonstrated that Alleghanian deformation is more widespread than previously thought. Monazite is widely explored but complex structures and overgrowths make the multiple crystallization events difficult to unambiguously assign to specific events on a PTt path. P(max) is rarely preserved in mineral assemblages and geodynamic modeling is our best hope for estimating maximum pressures on CW PTt paths. Forward modeling to predict PTt paths is becoming mature, but inverse modeling that can identify geodynamically plausible depths for PTt paths lags behind. Progress in crustal studies will require thorough integration of these fields of study – difficult in a single investigator. Close collaboration among specialists will be necessary for the next increment of progress in unraveling the geologic history of polymetamorphic rocks.