Trace element constraints on the parental melt of gabbroic cumulates from the Naga Ophiolite Complex, North-East India

The best exposures of mafic cumulate (gabbros and gabbronorites) rocks of Naga Ophiolite Complex (NOC) occur north-east of Moki village, east of Wazeho in the Indian State of Nagaland. Gabbro comprises predominantly of clinopyroxene (40–50 vol%) and plagioclase (40–50 vol%), while gabbronorite mainly consist of plagioclase (40–50 vol%), clinopyroxene (30–40 vol%) and orthopyroxene (~ 10 vol%) with minor iron oxides and apatite. Compositionally clinopyroxenes are augite, plagioclases are bytownite and orthopyroxenes are enstatitic. In primitive mantle and chondrite normalized diagrams, clinopyroxenes in gabbronorite show greater enrichment in trace elements and REEs than in gabbro. However, whole rock chemistry of both rock types show similar trends except for negative Ti, Sr and Eu anomalies in gabbronorite. Whole rock geochemical results suggest gabbronorite and gabbro as cogenetic but unrelated to the ultramafic rocks of NOC. P–T estimates using clinopyroxene-only thermobarometers suggest crystallization at 1204–1273 °C and 4.4–9.4 kbar for gabbros and 1193–1230 °C and 1.3–3.6 kbar for gabbronorites. Isobaric equilibrium crystallization models indicate clinopyroxene as the first crystallizing phase in gabbro with a liquidus temperature of 1289 °C, whereas it is plagioclase in gabbronorite with liquidus temperature of 1248 °C. Liquidus temperatures are comparable to the maximum crystallization temperatures obtained for gabbro and gabbronorite. Parental melt calculated using equilibrium distribution method (EDM) corresponds to the melt compositions at trapped melt fractions = 5% and 10% for gabbro and gabbronorite, respectively. Parental melt calculated using the concentration ratio method corroborates EDM calculations.