Partial Melting of High-P–T Metapelites from the Tshenukutish Terrane (Grenville Province): Petrography and U–Pb Geochronology

Abstract

Kyanite-bearing metapelites from the Tshenukutish terrane (Manicouagan Imbricate zone, Grenville Province) display evidence of anatexis by means of dehydration melting of micas. These rocks were metamorphosed during a Grenvillian high-P–T crustal thickening event with monazite ages ranging from 1040 to 1017 Ma. Samples that best preserve textural differences between former melt and restitic domains provide evidence for dehydration melting of white mica at ∼1400–1600 MPa, followed by extensive to complete dehydration melting of biotite up to temperatures in excess of 850°C, and subsequent crystallization of the melt at lower pressures (∼1100 MPa) during cooling to ∼750°C. Dehydration melting of biotite was accompanied by growth of garnet with distinctive Ca, Y and Cr patterns, locally around subsolidus garnet. In addition, garnet in one sample displays evidence of partial consumption before the latest stage of growth. This is consistent with dehydration melting of phengite instead of muscovite, according to a theoretically defined reaction: Phe + Grt + Qtz = Bt + Ky + Kfs + L. In all samples, melt crystallization was accompanied by growth of retrograde biotite and was completed at temperatures above the stability field of white mica. In samples that achieved textural equilibrium during or after melt crystallization only the composition of garnet provides some hints about the partial melting history.