PARTIAL MELTING OF METAGREYWACKES .1. FLUID-ABSENT EXPERIMENTS AND PHASE-RELATIONSHIPS

Island arcs, active and passive margins are the best tectonic settings to generate fertile reservoirs likely to be involved in subsequent gr anitoid genesis. In such environments, greywackes are abundant crustal rock types and thus are good candidates to generate large quantities of granitoid magmas. We performed a series of experiments, between 100 and 2000 MPa, on the fluid-absent melting of a quartz-rich aluminous metagreywacke composed of 32 wt% plagioclase (Pl) (An(22)), 25 wt% bio tite (Bt) (X(Mg)45), and 41 wt% quartz (Qtz). Eighty experiments, aver aging 13 days each, were carried out using a powder of minerals (less than or equal to 5 mu m) and a glass of the same composition. The mult ivariant field of the complex reaction Bt + Pl + Qtz double left right arrow Grt/Crd/Spl + Opx + Kfs + melt limited by the Opx-in and Bt-out curves, is located between 810-860 degrees C at 100 MPa, 800-850 degr ees C at 200 MPa, 810-860 degrees C at 300 MPa, 820-880 degrees C at 5 00 MPa, 860-930 degrees C at 800 MPa, 890-990 degrees C at 1000 MPa, a nd at a temperature lower than 1000 degrees C at 1500 and 1700 MPa. Th e melting of biotite + plagioclase + quartz produced melt + orthopyrox ene(Opx) + cordierite (Crd) or spinel (Spl) at 100, 200 and 300 MPa, a nd melt + orthopyroxene + garnet (Grt) from 500 to 1700 MPa (+ Qtz, Pl , FeTi Oxide at all pressures). K-feldspar (Kfs) was found as a produc t of the reaction in some cases and we observed that the residual plag ioclase was always strongly enriched in orthoclase component. The P-T surface corresponding to the multivariant field of this reaction is ab out 50 to 100 degrees C wide. At temperatures below the appearance of orthopyroxene, biotite is progressively replaced by garnet with increa sing P. At 850 degrees C, we observed that (1) the modal proportion of garnet increases markedly with P; (2) the grossular content of the ga rnet increases regularly from about 4 mol% at 500 MPa to 15 mol% at 20 00 MPa. These changes can be ascribed to the reaction Bt + Pl + Qtz do uble left right arrow Grt + Kfs + melt with biotite + plagioclase + qu artz on the low-P side of the reaction. As a result, at 2000 MPa, we o bserved the progressive disappearance of biotite without production of orthopyroxene. These experiments emphasize the importance of this rea ction for the understanding of partial melting processes and evolution of the lower continental crust. Ca-poor Al-metagreywackes represent f ertile rocks at commonly attainable temperatures (i.e. 800-900 degrees C), below 700 MPa. There, 30 to 60 vol.% of melt can be produced. Abo ve this pressure, temperatures above 900 degrees C are required, makin g the production of granitoid magmas more difficult. Thin layers of gn eisses composed of orthopyroxene, garnet, plagioclase, and quartz(+/- biotite), interbedded within sillimanite-bearing paragneisses, are qui te common in granulite terrains. They may result from partial melting of metagreywackes and correspond to recrystallized mixtures of crystal s (+ trapped melt) left behind after removal of a major proportion of melt. Available experimental constraints indicate that extensive melti ng of pelites takes place at a significantly lower temperature(850 deg rees C +/- 20) than in Al-metagreywackes (950 degrees C +/- 30), at 10 00 MPa. The common observation that biotite is no longer stable in alu minous paragneisses while it still coexists commonly with orthopyroxen e, garnet, plagioclase and quartz, provides rather tight temperature c onstraints for granulitic metamorphism.