Symplectite growth in the presence of alkaline fluids: evidence from high-aluminous metasediments of the Highland Complex, Sri Lanka

Abstract

High-Al garnet-sillimanite-graphite gneisses (khondalites) from the Matale area in central Sri Lanka show evidence for open system formation of symplectites. Three types of khondalite (type A, B, and C) were collected from two localities (Lo1 and Lo2) in this area. In Lo1, khondalite type A shows a stable mineral assemblage with garnet (Grt), ribbon quartz (Qtz), prismatic sillimanite (Sil), alkali feldspar (Akfs) and minor graphite. The abundant leucosome is interpreted as (modified) crystallised melt (Liq). Qtz is invariably rimmed by Akfs double corona. In type B, Grt and Sil are partially rimmed by spinel (Spl)-Akfs symplectites. Type C samples show the extensive development of Spl-Akfs symplectites, locally with Bt, as well as corundum (Crn)-Akfs symplectites after Sil in domains relatively remote from Grt. Mineral inclusions in Grt suggest advanced biotite dehydration melting producing Grt ± Spl + Liq, similar to other Sri Lankan metapelitic granulites. Peak P-T conditions are ~8–9 kbar and ~ 850 °C. The symplectites developed on the retrograde path, with Ti in Bt thermometry indicating 700–720 °C. The chemical potential calculations indicated that the chemical potential gradient from garnet to sillimanite could well have produced the AKfs+Spl symplectite. Hence the most critical point is the initiation of Grt breakdown reactions which can facilitate the formation of Spl + Akfs and Crn + Akfs symplectites after sillimanite via the chemical potential gradient. Textural observations coupled with mass balance of symplectitic reaction textures indicate that addition of external Na and K to reaction sites is needed to trigger the symplectite formation. Excess Fe and Mg from Grt breakdown probably diffused away from garnet along a chemical potential gradient. As a result, Sil in the vicinity of Grt has broken down to Spl-Akfs symplectites, whereas Sil further away from Grt produced Crn-Akfs symplectites. Spl-Akfs and Crn-Akfs symplectites after Grt and Sil in khondalites are restricted to the boundary zone between the Wanni and Highland Complexes. The formation of symplectites is inferred to have taken place during or just after a decompression stage with minor cooling, under the influence of an alkaline fluid derived from nearby pegmatites, when the assemblage Grt + Sil was already metastable. The combination of fluids and symplectites indicates a very fast process rate.