Metamorphic Conditions in Orogenic Belts: A Record of Secular Change

Abstract

In general, Archean rocks exhibit rather ordinary moderate-P-high-T facies series metamorphism; neither blueschists nor any record of deep continental subduction and return are documented. However, the abundance and scale of ultrahigh-temperature (UHT) metamorphic belts from the Neoarchean to the Cambrian imply a significant change in geodynamics during the Neoarchean Era, after which transient sites of high heat flow were available at intervals throughout this period of Earth evolution. Many Neoproterozoic-Cambrian UHT metamorphic belts appear to have developed in settings analogous to modern backarcs that were closed and inverted during crustal aggregation and formation of the Gondwana supercontinent. If backarcs were the general setting for UHT metamorphism, then on a hotter Earth the cyclic formation of supercratons (in the Neoarchean Era) and supercontinents (in the Proterozoic Eon) required the destruction of oceans floored by thinner lithosphere that may have generated hotter backarcs than those associated with the current destruction of the Pacific Ocean on the modern Earth. The inherent weakness of the lithosphere in a hotter thermal regime inevitably localized magmatism and deformation at these sites contemporaneously with UHT metamorphism. Medium-temperature eclogites of crustal derivation and associated highpressure granulites are also first recognized in the Neoarchean, for example within the Belomorian Mobile Belt, and they occur at intervals throughout the Proterozoic, for example in the Orosirian Usagaran Orogen and in the Grenvillian belts of the Proto-Atlantic region, and Paleozoic, for example in the circum-North Atlantic Caledonides and European Variscides. Eclogite-high-pressure granulite (E-HPG) metamorphism is predominantly a Proterozoic-Paleozoic phenomenon—complementary to but sparser than UHT metamorphism to begin with, but extending further into the Paleozoic than does UHT metamorphism—that is inferred to record subduction-to-collision orogenesis. Blueschists appear in the Neoproterozoic Era, becoming common through the Phanerozoic Eon; they record the low thermal gradients associated with modern subduction. Lawsonite-bearing blueschists and eclogites, and ultrahigh-pressure (UHP) metamorphism characterized by coesite or diamond are predominantly Phanerozoic phenomena related to deep subduction within subductionto-collision orogens. In general, UHP metamorphic belts in subduction-to-collision orogens are not associated with a contemporary magmatic arc in the hanging wall; this suggests that deep subduction of continental crust may inhibit the generation of calc-alkaline magmas, a feature that may have enabled preservation during exhumation of the mineralogical evidence for extreme pressures. However, an enigma concerning UHP metamorphism is the first evidence of deep subduction of continental crust in the rock record. At issue is the recycling implied by the geochemistry of Archean and Proterozoic diamonds (since entrained from mantle to upper crust in younger magmatic events), which requires a supracrustal component. Are these diamonds evidence of deep subduction of continental crust and an early record of UHP metamorphism, or was some other mechanism (e.g., delamination of underthrust lithosphere, some form of slab break-off) responsible for taking a supracrustal component deep into the mantle source? The Archean and Proterozoic eons were characterized by higher but decreasing mean mantle temperatures and radioactive heat production (RHP), and a thinner thermal boundary layer (TBL) with a shorter residence time than modern Earth. Modeling the effect of increased RHP on the thermal evolution of crust instantaneously doubled in thickness predicts that metamorphic rocks in Archean collisional orogens should have experienced maximum temperatures several hundreds of degrees Celsius higher than those recorded by metamorphic rocks in modern collisional orogens. However, there is no evidence of this—the Archean record is dominated by rather ordinary P-T conditions and crustal melting at relatively low temperatures probably fluxed by water. I argue that a duality of metamorphic belts—reflecting a duality of thermal environments—is the characteristic metamorphic imprint of plate tectonics in the rock record, and it appears only since the Neoarchean Era. Based on the occurrence of both UHT and E-HPG metamorphic belts since the Archean-to-Proterozoic transition, I suggest this transition records the onset of a "Proterozoic plate tectonics regime," although the total ridge length and the number of plates undoubtedly were larger in the beginning than on modern Earth and the style of tectonics likely involved some differences. The Neoproterozoic transition to the "modern plate tectonics regime" registers a change to subduction of continental crust deeper into the mantle and its (partial) return from depths of up to 300 km, a change perhaps related to whole mantle convection as oceanic lithosphere became thicker with decreased thermal gradients. Although there was overlap in time and space between E-HPG and UHP metamorphism during the Paleozoic, since the Carboniferous UHP metamorphism has dominated, and extreme thermal metamorphism is rare.