Proterozoic ferroan feldspathic magmatism

Abstract

The Proterozoic eon is characterized by an unusual abundance of ferroan feldspathic rocks that range in composition from granite to quartz syenites to feldspathoid-bearing syenites. Three associations of ferroan rocks may be recognized on the basis of major element compositions: (1) metaluminous alkalic to alkali-calcic syenites and granites associated with anorthosites and iron-rich basaltic rocks; (2) alkaline intrusions in continental rifts; and (3) calc-alkalic metaluminous to peraluminous granitoid plutons that are less strongly ferroan than the other two groups. The first two groups form by differentiation of tholeiitic to alkali basaltic parental magmas; the third by partial melting of quartzo-feldspathic crust. Although all three associations may form in intraplate settings, only the alkaline intrusions, which typically contain high incompatible trace element abundances, plot entirely within the “within plate granite” fields on trace element discrimination diagrams. Processes of crustal melting and magma mixing have had a greater effect on the trace element compositions of ferroan granitoids from the other two associations, causing some rocks to plot outside this field.Of these three associations, the granitoids associated with anorthosites are the most voluminous and are essentially restricted to the Proterozoic. Special conditions, perhaps related to the presence of a long-lived supercontinent under which widespread mantle upwelling took place, appear required to explain the abundance of these rocks. Ferroan alkaline rocks, although rarely preserved in Archean crust, are present in Proterozoic and Phanerozoic rifts. The earliest alkaline intrusions thus may mark the establishment of rigid continents capable of rifting. The formation of ferroan calc-alkalic granites does not appear to be time-dependent: these granites have been generated throughout Earth history in response to crustal melting.