Reduced rapakivi-type granites; the tholeiite connection

Reduced rapakivi-type granites are the most iron enriched and reduced (i.e., least oxidized) of the "anorogenic" granite association. The low oxygen fugacity and chemical composition of these granites severely limit their sources. In this paper we argue that reduced rapakivi-type granites and their eruptive equivalents, high-potassium fayalite rhyolites, are derived from mafic sources, because tholeiitic magmas and their derivatives have the required low oxygen fugacity. Reduced, rapakivi-type granites are produced either by extreme differentiation of basaltic melts or by partial melting of underplated basalts and their differentiated equivalents. They form in extensional environments where the asthenosphere is present at shallow depths. We envision three stages in the origin of these rocks: (1) tholeiitic melts are emplaced at the base of the crust, (2) continued introduction of heat partially remelts these tholeiitic rocks, and (3) the hot, dry melts so produced migrate into the middle crust to produce rapakivi batholiths or erupt as rhyolites. Partial melting of felsic continental crust may accompany the intrusion of rapakivi-type magmas, thereby producing the other metaluminous and peraluminous granite compositions of the anorogenic suite.