BACKARC ORIGIN FOR ULTRAHIGH TEMPERATURE METAMORPHISM, ECLOGITIZATION AND OROGENIC ROOT GROWTH

Contraction of continental crust during orogeny results in mountainous topography at the surface and a root at depth. Thermomechanical models suggest root growth is enhanced by thickening of thermally softened thin lithosphere. A >400 km² region of Archean gneiss, referred to as the Upper Deck domain, contains abundant mafic sills and dikes with MORB-like chemistry. Heat from the sills and dikes facilitated melting of supracrustal host rocks along a prograde P-T path culminating at T >950°C and P>1.4 GPa in the Neoarchean. A basalt sill, converted to eclogite near the base of the domain, exhibits positive Eu-anomalies consistent with plagioclase accumulation at shallow crustal levels prior to burial. Eclogite facies metamorphism previously dated at 1.90 Ga is here revised to 2.54 Ga based on existing zircon dates from the sill and new monazite dates from the paragneiss that hosts the sill. The results support a scenario where upper crustal materials were thermally softened in a backarc setting prior to being thrust to lower crustal levels during ultrahigh temperature metamorphism and orogenic root growth.