Abstract

There is a major belt of Proterozoic, so-called anorogenic, igneous rocks that extends from the Baltic Shield through southern Greenland and eastern Canada to southern USA. All rocks are situated within a zone of 1.9-1.6 Ga arc-accretionary orogens that consist of juvenile mantle-derived material especially accretionary prism sediments that provided the fertile source for abundant granitic crustal melts. There were two main periods of formation of the anorogenic rocks. 1.76-1.55 Ga rocks are mainly rapakivi granites and rhyolites that formed about 60-200 Ma after the last compressional events in the Svecofennian, Ketilidian and Penokean orogens, the time-lag being caused by the slow thermal relaxation of thickened lower crust that had commonly been depleted by the extraction of earlier granitic crustal melts. 1.45-1.4 Ga anorogenic rocks are mainly rhyolites and peraluminous granites in central/southern USA and anorthosites in eastern Canada that formed in the continental margin of the Grenvillian ocean, the subduction of which led to the Grenville orogen. These anorogenic igneous rocks formed in extensional regimes during the formation and breakup of a 1.5 Ga supercontinent.

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