MANTLE UPWELLING FROM ~ 410 KM TRANSITION ZONE BELOW THE NEO - TETHYAN INDUS OPHIOLITE IN NIDAR, NW HIMALAYA. BASU,

The Nidar & Sumdo area display the 10 km thick, well preserved Indus ophiolite in NW Himalaya, India. The suite is part of the Neo Tethyan ophiolite belt which represents Indus Suture Zone (ISZ) – the geological boundary between India and Asia. The lower ultramafic part of the ophiolite is ~ 7 km thick and consists of harzburgites, lherzolite tectonites and crosscutting dunite channels with layered and podiform chromitites. With the aid of petrography, Laser Raman spectroscopy and Electron Micro Probe Analyses , we have discovered systematic high pressure--low pressure mineral phase transitions in two Nidar peridotites, from coesite → quartz, high-pressure C2/c clinoenstatite → orthoenstatite and β – Mg₂SiO₄ → Cr spinel exsolution needles in olivine (Das et al., 2015). Coesite with typical palisade texture and C2/c clinoenstatite are found within enstatite porphyroclasts. C2/c clinoenstatite are also observed as lamellar inclusions in orthoenstatite porphyroclasts. The retrogressed β - Mg₂SiO₄ are included within Cr spinels. In places, the matrix olivines show numerous Cr spinel exsolution needles indicating β → α Mg₂SiO₄ transition. The phase stabilities of these UHP phases require derivation from the depth of mantle transition zone (~ 410 km) in continous,focused convective flow beneath the Neo-tethyan spreading center. The ~3km thick mafic crust above these peridotites have typical , depleted N-MORB trace element and isotopic signature. The crystallization age of the Indus Ophiolite over a slow-spreading ridge is much earlier than its emplacement on the leading edge of the Indian plate. Our comprehensive study of the Indus Ophiolite will have implications for current models of supra subduction zone ophiolite formation .