From melting to emplacement: the importance of fractional crystallization

Abstract:

The composition of anatectic melt extracted from the deep crust evolves during transport due to processes associated with melt–residuum separation and fractional crystallization. In the Cretaceous Fosdick migmatite–granite complex, P–T phase equilibria modeling of migmatitic paragneisses and orthogneisses, and the occurrence of leucosome-bearing normal-sense shear zones are consistent with suprasolidus conditions in the complex extending into the early stages of doming and exhumation. Sub-horizontal sheeted granites at shallower structural levels and variably oriented granites in networks at deeper structural levels within the complex commonly have coarse blocky plagioclase and/or K-feldspar grains within interstitial quartz, consistent with early crystallization of feldspar. The granites yield U–Pb zircon crystallization ages from 115 to 100 Ma, with a dominant grouping at 107–100 Ma, which corresponds to the timing of dome formation during regional oblique extension that facilitated exhumation of the complex. Whole rock Sr and Nd and zircon Hf and O isotope compositions are consistent with derivation from regionally-associated source materials. Although the major and trace element chemistry of these granites is highly variable, they typically have large positive Eu anomalies and correlated Rb/Sr/Ba covariation, features consistent with the early accumulation of feldspar and quartz and subsequent drainage of fractionated melt. The granites in networks are interpreted to represent clogging of magma transport channels through the middle crust by crystal accumulation as drainage of fractionating magma slowed during doming and exhumation. By contrast, the sheeted granites record collapse of sub-horizontal partially-crystallized layers of magma by filter pressing and drainage of melt during vertical shortening associated with doming. Processes that separate cumulates from evolved melt are likely to have been important in the evolution of granites in other migmatitic gneiss domes. Furthermore, since the evolved melt is expected to have higher concentrations of heat-producing elements, this study has significant implications for the tectonic history of large hot orogens, the cooling of UHT metamorphic terrains and the long-term stability of the continental crust.