ULTRAHIGH-TEMPERATURE METAMORPHISM IN THE CALEDONIAN, VARISCAN, AND APPALACHIAN OROGENS: RARE, UNEXPOSED, OR JUST UNRECOGNIZED?

Ultrahigh-temperature (UHT; >900 ^oC) regional metamorphic rocks form in the most extreme thermal environments known in the crust. Attainment of UHT conditions has a profound impact on crustal rheology, melt generation and, thus, orogen-scale geodynamics. The recent summary by Kelsey and Hand (2015) tabulates more than 50 localities for UHTM worldwide. Most are Cambrian in age or older. Despite the growing list of UHTM discoveries, reported localities from the Caledonian-Appalachian-Variscan orogen remain rare. These include: (1) xenoliths in gabbro, Cortlandt Complex, New York, USA (Caporuscio and Morse, 1978; Dorfler et al., 2015); (2) Central Maine Terrane, New England, USA (Ague et al., 2013); and (3) Gruf Complex, Central Alps (Galli et al., 2011). Of these, the xenoliths in (1) were heated by the gabbroic magma and (3) is associated with post-Variscan rifting. Broad episodes of UHTM likely correlate with supercontinent formation in the Precambrian and lowermost Paleozoic (Brown, 2006, 2007), including significant UHTM during Gondwana assembly. It is thus puzzling why the mountain building events of the Caledonian, Appalachian, and Variscan orogenies including Pangea formation would produce comparatively little UHTM. The increasing abundance of UHT localities with age could reflect a general decrease in orogenic geothermal gradients with time, but strong decreases from Gondwana to Pangea would be necessary to explain temporal trends in the Phanerozoic. Perhaps UHT rocks formed but are not exhumed very well. This seems unlikely given the deep exhumation including ultrahigh-pressure rocks represented in the orogen. Another possibility is that UHT rocks are exposed but unrecognized. Classic UHT mineral assemblages such as sapphirine + quartz are favored in Mg-rich metapelites. However, in rocks with more “normal” Mg/Fe ratios, UHT assemblages can be dominated by quartz, garnet, sillimanite, plagioclase, K-feldspar, and melt ± cordierite ± biotite ± spinel. These assemblages are common in the orogen (e.g., Central Maine Terrane) and may reflect unrecognized UHTM. If UHTM is more widespread than thought, geodynamic settings of orogenesis in the mountain belt, whether ultimately related to continental collision, extension, or crustal recycling at subduction zones, must be thoroughly reexamined.