MELT DISTRIBUTION AND MOVEMENT IN A MELT-DEPLETED GRANULITE TERRANE

Criteria now exist for recognising former melt at grain and outcrop scales in anatectic rocks, thus, melt distribution can be mapped. Melt formed in shallow (few 100 bars), short-lived, contact aureoles appears to remain in situ, but melt in deeper, longer duration aureoles moved into structural sinks. Relict melt textures occur in regional metamorphic rocks too, and enable their melt extraction processes to be studied. Metagreywackes in the Ashuanipi subprovince (Quebec) melted by the reaction Bt+Pl+Q=Opx+melt+oxides to produce up to 42% melt (average 25%). Mapping former melt textures shows a uniform distribution of melt; mainly on grain boundaries parallel to the foliation. After melt extraction 1 to 3% melt remained on grain boundaries and about 5% in the outcrops as leucosomes. Since sedimentary structures are preserved the melt-loss process was not destructive; melt moved pervasively along grain boundaries for many centimetres before entering larger channels (leucosomes). A simple mass balance; 0.25 (average partial melting) x 0.85 (fraction of metagreywacke) x 90000 km² (area) x 30 km (0.75 x crustal thickness), yields 573750 km³ of granite melt generated, and lost from the metagreywackes. The largest reservoir of anatectic melt in the Ashuanipi is diatexite migmatite. Most diatexite is a cumulate formed of the early crystallized products and residual material from the melt extracted from the metagreywackes. Most of the fractionated part of the anatectic melt is not in the Ashuanipi, it moved to higher crustal levels represented by the Opinaca and Nemiscau subprovinces which contain monzogranites with strongly fractionated compositions; 5 to 6 % K₂O and Rb/Sr upto 3, high total REE contents and large negative Eu anomalies.