Cumulates from the Nidar ophiolite, Ladakh: a record of pulsed slab-fluid supply in supra-subduction zone.
Author : Arijit Debroy, Abul Amir Khan, Salim Akhtar
Abstract : The Nidar Ophiolite Complex, along the Indus–Yarlung Tsangpo Suture Zone in Ladakh, preserves one of the most complete supra-subduction zone (SSZ) sections in the Himalaya. Its lower plutonic unit contains two cumulate types: orthopyroxene-bearing gabbronorites and orthopyroxene-free gabbros. Such pairings are commonly interpreted as evidence of separate magmatic pulses or different mantle sources. We tested this assumption using petrography, EPMA mineral chemistry, whole-rock major and trace elements, clinopyroxene thermobarometry and PELE forward crystallisation modelling on fourteen samples collected along two transects. The two rock types are interleaved on a decimetre-to-metre scale within a single structural panel. They yield overlapping clinopyroxene temperatures of approximately 1135 and 1155 °C and pressures of 3.1–10.2 kbar, equivalent to about 14–36 km depth. Their slab-flux and melting indices (Ba/La, Sr/Nd, Nb/Yb and Th/ Yb) are statistically indistinguishable and indicate approximately 5–10% melting of a spinel lherzolite source modified by slab-derived fluids, with back-arc basin basalt affinities. They differ mainly in features governed by melt water content and oxidation state: orthopyroxene stability, plagioclase anorthite content and amphibole abundance. We interpret this contrast as evidence of pulsed slab derived fluid supply within one polybaric plumbing system rather than separate magmatic events or mantle sources. Together with ultra-depleted and boninitic rocks reported from Nidar, the cumulate suite records transition from subduction initiation to early back-arc extension during Early Cretaceous Neo-Tethys evolution.
Keywords : Nidar Ophiolite, Supra-subduction Zone, Gabbronorite, Gabbro, Slab-derived Fluids, Cumulates, Polybaric Plumbing System, Neo-Tethys.
Conference Name : International Conference on Mineralogy and Crystallography Applications (ICMCA - 26)
Conference Place : Coimbatore, India
Conference Date : 19th Sep 2026
