Partial melting of high-P-T metapelites from the Tshenukutish terrane (Grenville Province): Petrography and U-Pb geochronology

Kyanite-bearing metapelites from the Tshenukutish terrane (Manicouoagan Imb ricate zone, Grenville Province) display evidence of anatexis by means of d ehydration melting of micas. These rocks were metamorphosed during a Grenvi llian high-P-T crustal thickening event with monazite ages ranging from 104 0 to 1017 Ma. Samples that best preserve textural differences between forme r melt and restitic domains provide evidence for dehydration melting of whi te mica at similar to 1400-1600 MPa followed by extensive to complete dehyd ration melting of biotite up to temperatures in excess of 850 degreesC, and subsequent crystallization of the melt at lower pressures (similar to 1100 MPa) during cooling to similar to 750 degreesC. Dehydration melting of bio tite was accompanied by growth of garnet with distinctive Ca, Y and Cr patt erns, locally around subsolidus garnet. In addition, garnet in one sample d isplays evidence of partial consumption before the latest stage of growth. This is consistent with dehydration melting of phengite instead of muscovit e, according to a theoretical ly defined reaction: Phe + Grt + Qtz = Bt + K y + Kfs + L. In all samples, melt crystallization was accompanied by growth of retrograde biotite and was completed at temperatures above the stabilit y 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.