Tertiary and quaternary magmatism in Central Sulawesi: Chronological and petrological constraints

Abstract

40K- 40Ar ages (27 new ones) and geochemical data (major and trace elements) allow the recognition of four main magmatic events in Central Sulawesi (South of Palu and in the Toraja area). The oldes magmatic activity took place during Late Eocene and Oligocene, and led to the emplacement of island arc tholeiites and calc-alkaline magmas as intrusions cutting the Tinombo Formation near Donggala (34.5 My) and the Kambuno granitic body in the Toraja area (29.9 My). The Lamasi Volcanics followed, but their KAr ages, which range from 33 to 15 My, cannot be safely interpreted as magmatic ages. They have a MORB-type affinity, flat to slightly depleted incompatible trace-element patterns and Nb and Ta negative anomalies indicative of their BABB affinity. Their origin and mode of emplacement are still enigmatic. Then an important magmatic event, with a strong shoshonitic affinity, occurred in a short time interval (10.1−11.9 My), leading to the emplacement of intrusions cross-cutting the Mamasa and Kambuno granitic bodies and to the eruption of lava flows (Sekata, Talaya and Sesean Volcanics). This shoshonitic episode is probably equivalent to the post-subduction magmatic associations emplaced in South Sulawesi during the late Miocene and might derive from the melting of mantle material after the mantle was metasomatised during a former episode of subduction. The last magmatic event occurred between 6.5 and 0.6 Ma. The corresponding products are granitic rocks (Kulawi, Kamarora, Palopo), rhyolites (Gimpu, 1.9 My) and widely distributed rhyolitic pyroclastics known as the Barupu tuff (0.6 Ma). All these magmatic rocks are characterised by their K-rich calc-alkaline composition, by the lack of basaltic and intermediate magmas (SiO 2 > 60%) and by high enrichments in incompatible elements a radiogenic Sr isotopic signature. This is consistent with the hypothesis of a strong crustal imprint by melting of an underthrusted continental crust in a collisional context.