A-type granite

A general term for granitoids typically occurring in rift zones and the interiors of stable continental plates. The prefix A stands for anorogenic. Usually mildly alkaline and K-rich.

Since the original definition of A-type granites by Loiselle and Wones as alkaline, anhydrous, and anorogenic rocks distinct from M-, S-, and I-type granites, their diagnostic criteria have been progressively refined. Contemporary classification emphasizes the characteristic high contents of SiO2, FeOT, Na2O + K2O, and Zr + Nb + Ce + Y (>350 ppm), as well as distinctive 10,000 Ga/Al ratios (>2.6). These granites are typically generated under high temperatures and low-pressure conditions in extensional settings, and can be aluminous. A-type granites can be subdivided into A1 and A2 subtypes based on their characteristic geodynamic environments. The A1 subtype (Y/Nb < 1.2) is derived from oceanic inland basalts in continental rift or intraplate settings, whereas the A2 subtype (Y/Nb > 1.2) originates from continental or underplated crust in post-collisional extensional regimes or during slab rollback events. A2-type granites are widespread in the subducted slab rollback and post-collisional settings, such as those found in the Central Asian Orogenic Belt, NW China, Gan-Hang Belt, SE China, Sakarya Zone, and NE Turkey.