Southward continuation and slab rollback of the Neotethyan arc-back-arc system: Insights from Eocene mafic intrusions from North Sumatra, SE Asia

The Neotethyan subduction and collision with the Asian, Indian, and Australian plates is evidenced by a giant Andean-type arc system and a remarkable Neotethyan igneous belt along the southwestern Eurasian margin. However, the southward extent, and cause, of Neotethyan subduction is poorly understood. Here, a zircon U-Pb dating and whole-rock geochemical study of two groups of Eocene (50-48 Ma) mafic intrusions from Tangse, North Sumatra, helps to unravel the Neotethyan subduction story. Group 1 consists of diabase samples, which have mid-oceanic-ridge- and arc-like geochemical affinities, with low chondrite-normalized La/Yb-(cn) (similar to 0.6) ratios and a high measured epsilon(Nd)(t) value of +8.7. Such geochemical signatures indicate that the magmatic rocks were derived from a refractory depleted mantle source that was metasomatized by slab melts. Group 2 consists of gabbro-diorites, which are enriched in the large-ion lithophile elements and depleted in some high field strength elements. These samples have high La/Yb-(cn) (2.9-6.6), low Nb/La-(cn) (0.2-0.3), epsilon(Nd)(t) values of +1.1 to +5.0, and epsilon(Hf)(t) values of +8.8 to +15.3. Group 2 samples were produced by the melting of a depleted mantle wedge metasomatized by slab fluids. The magmatism provides evidence for the southern continuation of the Neotethyan igneous belt and tectonic system, whereby slab rollback triggered melting and upwelling in an arc-back-arc geodynamic system from South Tibet to SE Asia.