ARCHEAN MELT INCLUSIONS IN BARBERTON AMPHIBOLITES (SOUTH AFRICA)

Archean amphibolites derived from tholeiithes were exhumed as a tectonic melange in the Inyoni shear zone near Barberton (South Africa) at 3.2 Ga. They are composed of abundant garnet crystals surrounded by a finer grained matrix made of amphibole, plagioclase and quartz. Symplectitic retrograde coronae of epidote and quartz are developed around the garnet prophyroblasts. These rocks underwent partial melting at 720–800 °C and 1.1–1.2 GPa. Melt inclusions (MI) are present within large (c. 5 mm in diameter) garnet crystals. They are isolated or occur in clusters indicating a primary entrapment (i.e., they were trapped when host garnet was growing). MI have variable shapes, from isometric (negative crystal shape) to tubular or irregular, and variable size (5-60 μm). Former melt crystallized to Qtz+Pl+Bt+Ep+Kfs±Ms during cooling at depth. Biotite has a low TiO₂ content (0.5-0.6 wt%) consistent with crystallization from a felsic melt. Amphibole, clinopyroxene, sphene and apatite may be present as trapped phases (i.e., phases which have favored the entrapment of melt). A porosity is sometimes observed. Fragments of MI-bearing garnets were heated at 750, 800, 850 °C, 15 kbar for 24 h using a piston cylinder. Re-melted MI have been observed in 800 and 850 °C runs. The abundance of disequilibrium microstructures in the MI (i.e., irregular MI walls characterized by embayments, recrystallized Grt domains at the Grt-MI boundary, bubbles) decreases from 850 to 800 °C, and the experiment at 800 °C shows a closer approach to equilibrium. MI remelted at 800 °C contain a granodioritic melt with K/Na=0.8-1.3, ASI=0.90-1.10 and H₂O=11-18 wt%. The primary nature of MI and the peritectic nature of the host garnet indicate that MI were formed during prograde melting of the host amphibolite. The granodioritic composition of the trapped melt is somewhat unexpected, but consistent with the presence of K-bearing phases within crystallized MI. The K-rich character along with the high H₂O content may suggest that MI reflect the earliest melting increments. Trace element contents will permit to better define the melting reaction.