Migmatites formed by water-fluxed partial melting of a leucogranodiorite protolith: Microstructures in the residual rocks and source of the fluid

Abstract

The Opatica Subprovince in the Canadian Shield is a late Archaean (2761–2702 Ma) plutonic arc formed above a north-dipping subduction zone. Anatexis (2690–2677 Ma) of leucogranodiorite and leucotonalite orthogneisses in the Opatica generated migmatites in an area of north-vergent back thrusts visible at the surface and in L ithoprobe seismic profile 48. Schollen diatexite migmatites occur in the thrusts and metatexite migmatites between them. The modal mineralogy, microstructure, and whole rock major, trace and oxygen isotope compositions of the protolith and migmatites were investigated to; 1) determine the melting reaction, 2) find microstructural criteria for identifying residual rocks in leucocratic systems where there is no melanosome, and 3) to determine the source of the fluid involved in anatexis. Partial melting of the protolith did not change the mineral assemblage, but the abundance of quartz and microcline both declined and plagioclase and biotite increased in the residual rocks. Quartz, plagioclase and microcline show evidence for dissolution and biotite does not. Thus, water-fluxed melting of quartz + plagioclase + microcline occurred. A mass balance indicates 25–30% partial melting. The melting reaction consumed the microcline and created essentially monomineralic domains of plagioclase. Extraction of 80–90% of the melt left a thin film of melt on the grain boundaries, and crystallization of these in the plagioclase domains created diagnostic microstructures. Microcline fills the last remaining pore space and forms high-aspect ratio crystals between plagioclases or triangular crystals at grain junctions. Quartz shows a range of morphologies, from high-aspect ratio films through the “string of beads” to isolated rounded grains, as the microstructure progressively equilibrated after crystallisation. Most accessory phases, including zircon, remained in the residuum. However, almost all the schollen migmatites have high contents of Th, U, Nb, Ta and REE relative to the protolith, due to contamination by accessory phases derived from mafic rocks. Disaggregation of the mafic rocks may have been facilitated by the high strain in the back thrusts where the schollen diatexites formed. Average whole rock δ 18O for the protolith and migmatites are similar (ca 8.2‰), and the small difference between melt-rich (8.6‰) and residuum-rich rocks (8.0‰) is consistent with fractionation. Thus, the fluid that caused melting was probably of metamorphic origin with δ 18O similar to the protolith. The seismic profile shows several reflectors extending to a present depth of 20 km (ca. 40 km in the late Archaean) under the migmatites; these are the paths along which the metamorphic fluid migrated and generated the migmatites now at the surface. A new type of neosome reported in this study may have formed along fractures that the fluids migrated along, however, these are peripheral pathways in the metatexites adjacent to the back thrusts and schollen diatexites.