FLUID-ABSENT MELTING BEHAVIOR OF AN F-RICH TONALITIC GNEISS AT MID-CRUSTAL PRESSURES: IMPLICATIONS FOR THE GENERATION OF ANOROGENIC GRANITES

Abstract

Fluid-absent melting experiments on a biotite (20 wt.%) and hornblende (2 wt.%) bearing tonalitic gneiss were conducted at 6 kbar (900–975°C), 10 kbar (875–1075°C), and 14 kbar (950–975°C) to study melt productivity from weakly peraluminous quartzofeldspathic metamorphic rocks. At 6 kbar, biotite dehydration–melting is completed at 975°C via incongruent melting reactions that produce orthopyroxene, two oxides, and {small tilde}25 wt.% granitic melt. At 6 kbar, hornblende disappears at 900°C, probably in reaction with biotite. At 10 kbar, biotite dehydration–melting produces <10 wt.% melt up to 950?c via incongruent melting reactions that produce orthopyroxene, garnet, and granitic melt. hornblende disappears in the satne temperature interval either by resorption or reaction with biotite. widespread biotite dehydration␓melting occurs between 950 975?c produces two oxides, {small tilde}20 fluorine-rich (up 0?31 wt.%) at 14 kbar only a trace of is present 950?c, amounts are virtually same as starting material. 975?c, gone tilde}10 produced both hornblende. our results show hornblende-bearing assemblages cannot go through on their own (although they can combination biotite) if ca content source rock too low stabilize clinopyroxene. such rocks, will resorb under fluid-absent conditions, intrusion hot, mantle-derived magmas into lower crust necessary initiate rocks compositions similar those discussed here. we argue high thermal stability material caused mainly incorporation fluorine. relatively f (0?47 suggests has experienced dehydroxylation its past. enrichment previous partial event excluded because lack phases orthopyroxene garnet which would have been produced. experiments f-enriched f-rich liquids, within range a-types granites, leaves behind granulitic residue dominated quartz, plagioclase. this study therefore supports notion a-type granites be generated h2O-undersaturated melting of rocks of tonalitic composition (Creaser et al., 1991), but does not require that these source rocks should be residual after a previous melting event.