The Lohit Plutonic Complex (LPC) represents Trans-Himalayan magmatic rocks in the easternmost sector of the Himalayan Orogen. It is divided into leucogranite-granodiorite-diorite dominant suite in the NE and the gabbro-diorite-granodiorite-granite-trondhjemite suite in the SE by the Walong Thrust. U-Pb geochronological analyses of euhedral zircon grains from different magmatic phases of LPC reveal episodic magmatism. Zircon U-Pb age of 153.4 ± 1.4 Ma of a garnetiferous granite gneiss suggest that the crustal sourced magmatism was active in Arunachal Trans-Himalaya during the late Jurassic. U-Pb ages of subduction induced magmatic rocks of LPC range between 117 ± 0.6 Ma and 49.5 ± 0.3 Ma. A significant contribution in crustal recycling during the Oligocene-Miocene is evident from crustally derived leucogranites having U-Pb ages ranging from 35.6 ± 0.3 Ma to 30 ± 0.3 Ma. A comparison of the crystallization ages of the Western Trans-Himalayan units with those of eastern Trans-Himalayan units reveals an earlier onset of Tethyan arc magmatism in the eastern sector. The most prominent phase in the eastern sector is during the Late Cretaceous while western and central sector show major activity during the Palaeocene-Eocene.

DOI = 10.3126/hjs.v5i7.1295 Himalayan Journal of Sciences Vol.5(7) (Special Issue) 2008 p.103-105

The sedimentary sequence of Barail Group preserves the records of depositional history of Indo-Myanmar Range during Eocene-Oligocene. Field investigation, geochemistry and detrital geochronology are used to study its geochemical characteristics, sediment provenance and tectonic setting. Major elements based discrimination plots indicate sediments of this group are derived from quartzose sedimentary sources and deposited in a passive margin tectonic set-up.Barail sandstone display a gentle sloping LREE and more or less at HREE pattern, and show negative Eu anomaly and positive Sm anomaly. Detrital zircon U-

No abstract

The sedimentary sequence of Barail Group preserves the records of depositional history of Indo-Myanmar Range during Eocene-Oligocene. Field investigation, geochemistry and detrital geochronology are used to study its geochemical characteristics, sediment provenance and tectonic setting. Major elements based discrimination plots indicate sediments of this group are derived from quartzose sedimentary sources and deposited in a passive margin tectonic set-up. Barail sandstone display a gentle sloping LREE and more or less flat HREE pattern, and show negative Eu anomaly and positive Sm anomaly. Detrital zircon U-Pb reveal age ranging from 3225 Ma to 90 Ma, with clusters at222–90 Ma, 1215 − 394 Ma, 1849 − 1364 Ma, 2500–2094 Ma and 3225–2094 Ma. These results indicate majority of the sediments of the Barail Group were sourced from the Tethyan Sedimentary Sequence and Trans-Himalayan granitoids and deposited in shallow marine condition to deltaic environment.