Proterozoic Anorthosite Massifs

Abstract

Major anorthositic massifs intruded stable cratonic crust in the North Atlantic region about 1.4 to 1.7 Ga ago. Similar (meta-) anorthosite massifs (0.9–1.6 Ga) in Grenville and Sveconorwegian terranes exhibit varying degrees of deformational and metamorphic overprint and consequently have ambiguous tectonic settings. Close correspondence of rock and mineral associations in both types points to common origins. Mineral assemblages, chemical, and isotopic data for pristine and meta-anorthosite massifs strongly imply varying degrees of interaction between mantle-generated magmas and deep continental crust occurred prior to final emplacement. Olivine-bearing massifs consistently show less evidence of crustal assimilation than olivine-free massifs. Parent magmas of anorthosite massifs were LREE-enriched with moderately high Fe/Mg, consistent with fractionated basic magmas or their derivatives. Recently reported Nd and Sr isotopic relationships imply that the parent magmas were not simply partial melts of undepleted subcontinental mantle. Appropriate dry, plagioclase-saturated or -supersaturated magmas can be generated on a large scale only within a depth interval ~35–50 km, embracing the lowermost continental crust and upper mantle. Fractionation of pyroxenes ±olivine was an important factor in determining the chemistry of parent magmas and in decreasing their densities prior to intrusion into the continental crust. The high level intrusions of Fennoscandia and the East European platform are not obviously related to crustal rifting. Widespread non-orogenic granitic suites, including rapakivi types, span a similar time interval and provide supporting evidence for local heat sources perhaps related to subcratonic mafic magma ponding. This unique period of geological history may mark a transition to Phanerozoic tectonic regimes in which large scale magmatism became increasingly confined to plate margins and intracontinental rift zones.