World's youngest blueschist belt from Leti Island in the non-volcanic Banda outer arc of Eastern Indonesia

The Timor-Tanimbar region constitutes part of the non-volcanic outer Banda Arc of Eastern Indonesia. Here, the world's youngest 'A'-type high-pressure metamorphic belt crops out with different stages of evolution. Whereas an advanced domal uplift stage is seen in Timor island, the high-pressure (HP) metamorphic belt is still in the first stage of tectonic extrusion on the eastern small islands of Kisar, Leti, Moa, Sermata and Laibobar. The metamorphic rocks on Leti are among the best exposed in the islands. They are tectonically juxtaposed against overlying ultramafic rocks and underlying unmetamorphosed continental shelf sediments, bound by normal and reverse faults, respectively. The Leti metapelites display four progressive metamorphic zones: chlorite-biotite, garnet, chloritoid-staurolite and kyanite zones, with increasing grade. Zonation in Leti metabasite unit is comparable, and progressively changes from blueschist-greenschist transition (BS/GS), through epidote-amphibolite (EA), to amphibolite (AM) facies, with increasing grade. The highest-grade metapelites and metabasites occupy structurally intermediate levels. Overprinting mineral relationships in BS/GS transition schists indicate that decompression occurred from 5.5-7 kbar to < 4 kbar within the temperature range 300-400 °C. P-T estimates of the EA and AM units are 6.3-7.6 kbar and 461-521 °C, and 9.7-10.3 kbar and 580-626 °C respectively. The protoliths of Leti metamorphic rocks are originally Permo-Triassic. The younger sediments and igneous rocks at the margin of the northward advancing Australian continent entered the subduction zone immediately prior to the commencement of the Banda Arc-Australia collision in the Pliocene. Burial reached a maximum depth of 30-35 km at the main stage of peak P-T conditions, and was related to the main stage of the collisional event. Slab break-off at depth in the collision zone facilitated rapid uplift by wedge extrusion and active erosion during the exhumation stage. The tectonic juxtaposition of the schist units at mid-crustal levels and hydration may correspond to the final stage of Barrovian overprinting which continued through transportation from depth. The Leti Island belongs to the eastern portion of the world's youngest blueshist belt which has not yet entered the second-stage mountain building, and is slightly younger than the western Timor which evolved to the extensive mountain-building stage by doming.