Metamorphic patterns in orogenic systems and the geological record

Abstract

AbstractRegional metamorphism occurs in plate boundary zones. Accretionary orogenic systems form at subduction boundaries in the absence of continent collision, whereas collisional orogenic systems form where ocean basins close and subduction steps back and flips (arc collisions), simply steps back and continues with the same polarity (block and terrane collisions) or ultimately ceases (continental collisions). As a result, collisional orogenic systems may be superimposed on accretionary orogenic systems. Metamorphism associated with orogenesis provides a mineral record that may be inverted to yield apparent thermal gradients for different metamorphic belts, which in turn may be used to infer tectonic setting. Potentially, peak mineral assemblages are robust recorders of metamorphic P and T, particularly at high P–T conditions, because prograde dehydration and melting with melt loss produce nominally anhydrous mineral assemblages that are difficult to retrogress or overprint without fluid influx. Currently on Earth, lower thermal gradients are associated with subduction (and early stages of collision) whereas higher thermal gradients are characteristic of back-arcs and orogenic hinterlands. This duality of thermal regimes is the hallmark of asymmetric or one-sided subduction and plate tectonics on modern Earth, and a duality of metamorphic belts will be the characteristic imprint of asymmetric or one-sided subduction in the geological record. Accretionary orogenic systems may exhibit retreating trench–advancing trench cycles, associated with high (>750 °C GPa−1) thermal gradient type of metamorphism, or advancing trench–retreating trench cycles, associated with low (<350 °C GPa−1) to intermediate (350–750 °C GPa−1) thermal gradient types of metamorphism. Whether the subducting boundary advances or retreats determines the mode of evolution. Accretionary orogenic systems may involve accretion of allochthonous and/or para-autochthonous elements to continental margins at subduction boundaries. Paired metamorphic belts, sensu Miyashiro, comprising a low thermal gradient metamorphic belt outboard and a high thermal gradient metamorphic belt inboard, are characteristic and may record orogen-parallel terrane migration and juxtaposition by accretion of contemporary belts of contrasting type. A wider definition of ‘paired’ metamorphism is proposed to incorporate all types of dual metamorphic belts. An additional feature is ridge subduction, which may be reflected in the pattern of high dT/dP metamorphism and associated magmatism. Apparent thermal gradients derived from inversion of age-constrained metamorphic P–T data are used to identify tectonic settings of ancient metamorphism, to evaluate the age distribution of metamorphism in the rock record from the Neoarchaean Era to the Cenozoic Era, and to consider how this relates to the supercontinent cycle and the process of terrane export and accretion. In addition, I speculate about metamorphism and tectonics before the Mesoarchaean Era.