Boron isotopic signature of obducted mantle rocks at Indian-Eurasian convergent plate margin

The Indus Suture Zone (ISZ) in north western Himalayas is major tectonic suture dividing the Indian plate from Kohistan Island Arc (KIA) and Eurasian plates. It coincides with the subduction zone where the Tethys oceanic lithosphere subducted beneath KIA and Eurasia causing the obduction of several bodies of Tethyan upper mantle along ISZ, two prominent among them are Jijal Complex and Sapat Complex. Mantle peridotites of these complexes individually lie at a distance of ~ 80 Km and consist of harzburgites, pyroxenites, dunites and serpentinites.

In order to understand the effects of subduction and fluid rock interaction we measured the boron isotopic signatures of Jijal and Sapat mantle rocks through LA-ICPMS at FIERCE. Whole rock δ¹¹B was analyzed on nano-powder pressed pellets and mineral δ¹¹B of olivine and pyroxene were obtained in-situ on polished thin sections. The whole rock δ¹¹B of Jijal ultramafics ranges from -4.32 to 6.43 ‰ with average δ¹¹B of 0.36 ‰. On the contrary, the Sapat ultramafics show higher average δ¹¹B of 2.80 ‰ and range of -0.13 ‰ to 7.66 ‰. The results from both mantle sections are significantly higher than the depleted mantle δ¹¹B value of -7.1 ‰. The difference between boron isotopic signature of Jijal and Sapat complex suggests forearc position of the later and potentially different magma compositions. The overall higher values of both mantle bodies indicate towards a more enriched mantle component or the influence of slab derived fluids from subducted serpentinites.