New U-Pb zircon ages of plagiogranites from the Nagaland-Manipur Ophiolites, Indo-Myanmar Orogenic Belt, NE India.

We report new U-Pb zircon ages and whole-rock chemistry of plagiogranites from the Nagaland-Manipur Ophiolites, exposed in the Indo-Myanmar Orogenic Belt, NE India, which represents the southeastern extension of the Indus- Yarlung-Tsangpo Suture of Neo-Tethyan origin. The plagiogranites (SiO2 = 57 and 76 wt%) are characterized by high sodium (Na2O = 5.9 - 7.2 wt%) and low potassium (K2O = 0.1 - 0.6 wt%), coupled with low Rb (1.8 - 9.5 ppm), low Rb/Sr (<0.1) and A/CNK < 1 (molar). Geochemically they are oceanic plagiogranites with trondhjemitic compositions and metaluminous affinities. Their rare earth element (REE) patterns exhibit depletion in light REE (LaN/SmN = 0.50 - 1.13) relative to heavy REE (SmN/YbN = 0.59 - 0.88), with low ΣREE and incompatible trace element contents that indicate derivation from a depleted mantle source. High ratios of large ion lithophile elements to high field strength elements with pronounced Nb, Ta and Ti depletions furthermore suggest the presence of the subduction component in the source region. Zircon U-Pb geochronology yielded mean 206Pb/238U ages between 116.4 ± 2.2 and 118.8 ± 1.2 Ma that record crustal formation in the Early Cretaceous. Collectively the data indicate that the Nagaland-Manipur Ophiolites are broadly coeval and geochemically comparable with Neo-Tethyan ophiolites elsewhere in the Indus-Yarlung-Tsangpo Suture. [ABSTRACT FROM AUTHOR]