P-T Conditions, Mechanism And Timing Of The Localized Melting Of Metapelites From The Petronella Shear Zone And Relationships With Granite Intrusions In The Southern Marginal Zone Of The Limpopo Belt, South Africa

Evidence is presented for the localized melting of cordierite-orthopyroxene-biotite metapelites within the Petronella shear zone in the Southern Marginal Zone of the Limpopo Belt of Southern Africa. This process is expressed by the formation of centimetre-scale, K-feldspar-rich, garnet-orthopyroxene-bearing leucosome patches. Structural data indicate that the leucosomes formed after regional peak metamorphic and deformational events, but were syn-to late tectonic with respect to the shear fabric of the host metapelites. Phase equilibria modelling using the PERPLE_X software shows that the leucosomes were produced via fluid-deficient partial melting of the metapelites at temperatures above 900 degrees C and pressures of 6.0-6.5 kbar. Melting reactions involving mainly biotite, plagioclase, quartz and accessory phases (including sulfides) produced peritectic Cr-enriched orthopyroxene, as well as Sc, Y, Cr, P-enriched garnet along with the potassium-rich melt. The melt was segregated around the peritectic minerals into the leucosomes. During near-isobaric cooling, the segregated melts crystallized to produce abundant K-feldspar and Sc, Y, Cr and P-poorer garnet inside the leucosome patches. Partial melt loss from the patches on cooling assisted with the preservation of anhydrous assemblages inside the patches and with re-hydration of the surrounding melanosome, with extensive formation of biotite + quartz +/- sillimanite assemblages after orthopyroxene and cordierite, as well as late Sc, Y, Cr, and P-free garnet (+sillimanite + quartz) at temperatures down to < 600 degrees C. Field and P-T data suggest a link between the localized melting of metapelites and trondhjemite intrusions that were syn-to late-tectonically injected into the Petronella shear-zone at about 2.67 Ga after the peak metamorphic and deformational event in the Southern Marginal Zone. This link is proven by U-Pb ages for monazites (a weighted average Pb-207/U-235 age of 2666 +/- 4 Ma) from the leucosome patches. The trondhjemites provided additional heat and, probably, also a small amount of fluids, for the leucosome formation.