The Umarizal Igneous Association, Borborema Province, NE Brazil: Implications for the Genesis of A-Type Granites

The Umarizal igneous association (NE Brazil) consists of three A-type plutons, intruded over an interval of about 15 Ma, during the post-collisional phase of the Brasiliano orogeny. All the plutons have Sr isotopic compositions which indicate important or dominant crustal contributions. Crystallization of the Umarizal sill, formed of fayalite quartz syenite and syenogranite, and of the Lagoa stocks, formed by mangerite and granite, commenced under water-undersaturated conditions at pressures around 700-800 MPa and temperatures around 900°C, and continued during magma rise with the crystallization of hornblende at about 480-570 MPa. Crystallization of the Açمo stock, which contains a rapakivi-like facies, commenced at similar temperatures, and hornblende appeared at slightly less than 800°C and around 500 MPa. Different fO2 conditions controlled the compositions of the ferromagnesian phases and the nature of the oxide mineral assemblage. Simple fractional crystallization models from homogeneous parent magmas are insufficient to explain the chemical variation of the rocks suites.