Exhumation of Granulites Within a Transpressive Regime: an Example from Southern Madagascar

Abstract

Granitic magmas extracted from crustal sources can form over a wide variety of P, T and aH2O conditions. Both fluid-present and effectively fluid-absent conditions can yield granitic magmas, though the majority are formed through high-T, fluid-absent reactions because, in the deep crust, most available H2O within rock bodies is contained within minerals rather than in free fluids. Fluid-present partial melting generally results in the formation of migmatites (and sometimes diatexites) under upper amphibolite-facies conditions. By definition, aqueous fluid-present melting begins at temperatures very close to the relevant ‘wet’ solidus. Consequently, studying mid-crustal migmatites, and the poorly mobile and mainly small intrusions that may result from such melting, provides no guide to the temperatures and fluid conditions that are involved in the genesis of highly mobile granitic magmas that facilitate crustal differentiation. Although zircon-saturation temperatures are commonly used to infer magma temperatures, and even melting temperatures, they seldom resemble actual magmatic temperatures. Recent notions about the persistence of granite-forming melts at conditions well below the experimentally determined H2O-saturated (‘wet’) solidi for granites (i.e., generally <650 °C) are commonly based on the results of TitaniQ (titanium-in-quartz) thermometry on silicic plutonic rocks. These results are problematic and inconsonant with TitaniQ thermometry on rhyolites, including those thought to represent erupted near-solidus crystal mushes. Thus, the hypothesis that low-T, aqueous fluid-fluxed granitic magmas have significance for the recycling and large-scale differentiation of the continental crust is poorly founded.