GENERATION OF LEUCOGRANITES BY SHEAR-HEATING DURING THRUSTING

Leucogranites are commonly found in deformed metapelitic/metagraywacke sequences that have been thrusted over basements during crustal thickening associated with continental collisions. Several examples include the leucogranites of the High Himalayas, the Avalon terrane of Maine, and the Black Hills, South Dakota. Often, pegmatites are associated with the granites. The granite and pegmatite melts generally ascended as dikes along preferred foliation directions of the host rocks, along faults, and along shear zones. Almost invariably, isotopic and trace element data indicate that the host metasedimentary rocks in a given terrane are the source of the included granite suite. Generation of the granites has been found to be temporally proximal to deformation and metamorphism of the host rocks. The source of heat that drives leucogranite generation has been debated. Simple crustal thickening does not result in sufficient increase in temperatures to cause melting in most of the crust and unusually high radiogenic heat production is generally not supported by relevant data. Input of heat into the crust by underplating or intrusion of mafic magma is usually not evident. Melting of deep-seated rocks during rapid decompression may be a viable mechanism, but does not seem to be supported by the generally low- to mid-pressures recorded by mineralogy of the host metamorphic rocks. The lack of evidence for high-pressure of melt generation and the other noted geologic settings are consistent with melt production during heating associated with shearing of metasedimentary rocks. We successfully numerically modeled the production of Black Hills leucogranites by shear heating using published P-T-t constraints for the terrane during the Trans-Hudson continental collision and data for rheology of metapelites at high temperatures.