Formation and evolution of granite magmas from migmatites: An example from the Ashuanipi subprovince in the Superior Province, Quebec

The Ashuanipi subprovince and contiguous metasedimentary subprovinces in the Superior province contains reworked Archaean crust that displays a continuous section from mid-crustal anatectic rocks to shallow level granite plutons. In other words, the superior province represents a crustal section which is continuous from partially melted source to upper crustal magma sink and the Ashuanipi portion of it is an example of a source region for granulitic magma. There, the transition from palaesome to granite was an open system process through intermediate stages of metatexite and diatexite.; Field, petrographic and geochemical methods have been used to estimate how much granitic melt was formed and extracted from granulite facies terrane, and to determine what the grain and outcrop-scale melt-flow paths were during the melt segregation process. More than 85% of the metasediments in the Ashuanipi subprovince are of metagreywacke composition that was metamorphosed at mid-crustal conditions (820--900°C and 6--7 kbar). Decrease in modal biotite and quartz as orthopyroxene and plagioclase contents increase, together with preserved former melt microstructures indicate that anatexis was by the biotite dehydration reaction: biotite + quartz + plagioclase = melt + orthopyroxene + oxides. Using melt/orthopyroxene ratios for this reaction derived from experimental studies, the modal orthopyroxene contents present in the Ashuanipi rocks indicate that the metagreywacke rocks underwent an average of 31 vol% partial melting.; The metagreywackes are enriched in MgO, CaO and FeO and depleted in SiO 2, K2O, Rb, Cs and U, have lower Rb/SR, higher Rb/Cs and Th/U ratios and positive Eu anomalies compared to their likely protolith. These compositions are modelled by the extraction of between 20 and 40 wt % granitic melt from typical Achaean low-grade metagreywackes.; The distribution of relict melt at thin section and outcrop scales indicates that in layers without leucosomes melt extraction occurred by a pervasive grain boundary (porous) flow from the site of melting, across the layers and into bedding planes between adjacent layers. In other rocks, pervasive grain boundary flow of melt occurred along the layers for few to tens centimetres followed by channelled flow of melt in a network of short interconnected conduits. Very little (<5% biotite + orthopyroxene) residual material remained in the leucosomes, indicating that the melt fraction was well separated from the residuum left in situ as melt-depleted granulite. Only 1--3 vol percentage melt remained in the melt-depleted granulites, hence, the extraction of melt generated by biotite dehydration melting in these granulites, was virtually complete under the conditions of P, T and deformation that prevailed during partial melting.; Chapter IV and Chapter V focus on the morphological, petrological and geochemical continuity from diatexite migmatites to granitic rocks in the Ashuanipi subprovince. The extracted anatectic melt was pooled and redistributed into large low pressure sites and become contaminated with some of the residuum it passed through. These bodies of contaminated anatectic melt form diatexite migmatites. The solid and melt fractions in the diatexite magma became progressively separated during flow of the diatexite magma, and thus generated both crystal-enriched and melt-enriched diatexite migmatites. Plagioclase-rich diatexite is a contaminated cumulate rock composed principally of plagioclase that crystallised from the anatectic melt, whereas the K-feldspar rich diatexite is consistent with the putative liquid lost from the plagioclase-rich diatexite rocks. Plagioclase + K-feldspar-rich diatexites approximate to the contaminated anatectic melt composition. Most of the diatexite magmas in the Ashuanipi subprovince are modified by the effects of fractional crystallization and very few actually preserve primary melt compositions.; The diatexite magma is parental to the monzogranit...