IGCP 510 – A-TYPE GRANITES AND RELATED ROCKS THROUGH TIME
	GLOBAL CORRELATION OF A-TYPE GRANITES AND RELATED ROCKS, THEIR MINERALIZATION, AND SIGNIFICANCE IN LITHOSPHERIC EVOLUTION

The project will gather together an active, enthusiastic worldwide group of experts to carry out and correlate research in key areas involving all continents and the entire age spectrum of A-type granites.

The overall aim of the project is to correlate the petrology, geochemistry, and metallogeny of A-type granites in various tectonic settings through geologic time. Specific topics for detailed investigation will include, but not be limited to:

1. Petrogenesis of the A-type granites

Since their definition in the late 1970’s, it has become evident that the A-type granites span a wide range of petrotectonic associations. Granites and other felsic igneous rocks belonging to this category are found in both continental and oceanic settings and comprise plutons (and minor hypabyssal and supracrustal rocks) that sharply transect the surrounding cratonic or oceanic crust. The oldest (~2.73 and 2.75 Ga) A-type granites have been described, respectively, from southern Botswana where they are associated with contemporaneous mafic (mostly anorthositic) rocks, and Carajás, Brazil. The end of the Paleoproterozoic and Mesoproterozoic times (~1.9 to 1.0 Ga) were characterized by the emplacement of voluminous A-type granites (often rapakivi-textured) and associated mafic rocks (gabbro-anorthosites, diabase dikes, rare volcanic rocks) that constitute continent-wide belts, e.g., in northern Europe and North America, and Amazonia. Near the end of the Brasiliano–Pan-African cycles, Upper Proterozoic to Cambrian, post-tectonic A-type granites are common in Atlantic provinces of Brazil and also in Africa (e.g., Algeria). Phanerozoic examples of A-type granites include the Permian Oslo Graben, the Devonian in southeastern Australia and New England, the Tertiary intrusions of the British Isles, the Younger Granites of Nigeria, and the Permo-Triassic Western Mediterranean province. The Cenozoic topaz rhyolites and synextensional granites of the western United States, the rhyolites of the Yellowstone hotspot track, as well as alkali granites of oceanic islands (e.g., Kerguelen Archipelago) are prime examples of young A-type granite magmatic activity. Petrogenetic models applicable to these occurrences are obviously many, including extreme fractionation of mantle-derived mafic melts (a particularly likely scenario for the oceanic A-type granites), partial melting of a (juvenile) mafic underplate (consistent with the strongly reduced character of many A-type granites), and anatexis of a source in pre-existing (refractory or fertile) continental crust. It is probable that no single mechanism can account for the wide range of petrotectonic associations recorded by the A-type granites, and one prime aim of the project is to scrutinize the possible petrogenetic scenarios, and whether or not they show secular variation with geologic time.

2. Metallogenic significance of the A-type granites

A-type granite associations have a significant role as carriers of several metallic commodities and their global correlation and resultant enhancement of prospecting and exploration will be the principal applied result of the project. Proterozoic A-type (rapakivi) granites are hosts of significant tin-polymetallic deposits particularly in South America (Brazil) and such occurrences are also found in Missouri, Finland, the Ukraine, and India. They are also associated with Fe-Cu deposits in southeastern Missouri, and the huge Olympic Dam Cu-U-Au-Ag deposit of South Australia is also hosted by an A-type granite. Moreover, one of the major rare earth element resources currently in production, the Mountain Pass deposit of southern California, is associated with a ~1.4 Ga shonkinite that is presumably related to the mid-Proterozoic A-type granites of the southwestern United States. The project arranges field trips to mineralized A-type granite terrains. One goal of the project is to examine how mineralization types vary with time, tectonic setting, and A-type granite characteristics, and the new results may well lead to establishment of further occurrences of valuable resources related to A-type granites.

3. Granite typology

The geological literature includes a myriad different schemes for the classification of granitic rocks and no generally accepted practice exists for their typology. The “alphabet classification” involving A-, I-, S-, M-, and C-type granites that is largely based on compositional criteria reflecting the ultimate source of the granite magmas, has been relatively widely applied but has also been criticized. The alphabet categories often show some overlap and, within the individual groups themselves, the need for a more detailed subdivision has emerged. The University of Wyoming group has recently proposed a classification scheme based solely on the major element composition of granites, as an alternative to the other schemes that often build on trace element contents that can be more vulnerable for redistribution in the post-magmatic stage and thus cause problems in classification. The data collected during the project will be utilized in the evaluation of the proposed classification and in pursuit of a comprehensive granite classification scheme.

4. Relation to lithospheric evolution

Overall, the tectonic setting of the A-type granites is non-compressional, including such regimes as oceanic islands near spreading ridges, apparently stable cratons, and intraplate extensional continental rifts which, in many cases, imply a principal role of mantle processes in the initiation of the magmatism. However, post-collisional or post-orogenic settings have also been invoked and some of the “anorogenic” A-type complexes have been associated with concurrent subduction processes in distant craton margins. The project gathers a global database of the tectonic settings of the A-type granites and assess the variation in their tectonic regimes through space and time. We will also try to locate the oldest A-type granite assemblages to see how much earlier than 2.75 b.y. ago processes creating these rocks have been in operation.