Rapid Ascent of Granitoid Magmas from the Lower Crust

Abstract

Ascent rates and mechanisms are important in controlling the compositional modification of sihcic magma. When magmas undergo rapid ascent, contamination by country rock and fractional crystallization are minimal, and magma compositions are controlled by source characteristics, extent of melting, and melt/restite separation. During slow ascent, more modification can occur, and resultant magma compositions do not accurately reflect the nature of the source rock. Not only do magma ascent times constrain the possible geochemical histories, but they also delineate whether the method of migration is diapirism (~ 1 m/yr), or fracture propagation via dyke systems (>>1 m/yr, Clemens and Mawer, 1992; Petford et al., 1993). Recent studies based on numerical modeling advocate dyke transport of granitoid magma, and suggest that dykes can fill greater than 1000 km3-size silicic plutons in less than 1000 years (Clemens and Mawer, 1992). Here, we use direct petrologic constraints to evaluate the transport rates and mechanisms of granitoid magmas