Geochemical response to varying tectonic settings: An example from southern Sulawesi (Indonesia)

The South arm of Sulawesi was an active continental margin from approximate ly 60 to 10 Ma, when it collided with the microcontinental fragment of Buto n. pre-collisional samples analyzed for this study are characterized by a g eochemical signature typical of are volcanics: high LILE/HFSE ratios; Sr-87 /Sr-86 slightly higher than MORB;Nd-143/Nd-144 ratios similar to MORE. Syn- collisional samples have more enriched isotopic signatures, and are relativ ely potassium rich. This is interpreted to reflect a larger contribution fr om subducted sediments, added to the mantle wedge as a silicic melt. Meltin g of subducted sediments is interpreted to result from a decrease in subduc tion rate and an increase of temperature in the slab. Magmatism that postda tes the collisional event by 10 Ma is characterized by higher Nb/Y ratios t han the pre- or syn-collisional samples, and Sr and Nd isotopic signatures intermediate between these two groups. This is likely to reflect melting of a subduction-modified mantle, with a significant contribution from the sub continental lithospheric mantle. Comparison with post-collisional magmatism from other areas of the world suggests that trace element signatures are s imilar, but isotopic characteristics are variable. The latter are likely to reflect both the age of the sub-continental lithospheric mantle and the ri me lag between cessation of subduction and formation of the post-collisiona l magmas. Copyright (C) 1994 Elsevier Science Ltd.