NEW U-PB DETRITAL ZIRCON AGE CONSTRAINTS FROM THE LESSER HIMALAYAN SEQUENCES, HIMACHAL-UTTARANCHAL AREA, NORTHWESTERN INDIA

Between the Kali river of Uttaranchal and Satluj river of Himachal Pradesh, NW India, the Lesser Himalayan Sequence (LHS) exhibits the maximum thickness. The LHS is bounded by the Main Central Thrust (MCT) to the north and Main Boundary Thrust (MBT) to the south. The LHS is divided into inner/older Lesser Himalaya and outer/younger Lesser Himalaya. Lack of systematic age constraints and stratigraphic ambiguity arise difficulty for regional correlation of the various Lesser Himalayan units. Here we present preliminary data from geochronological and field-based structural study of LHS from Himachal-Uttaranchal area, northwestern India. U-Pb ages of detrital zircons of the Berinag Formation, in the footwall of Munsiari/Jutogh/Ramgarh Thrust, from three different localities (Rampur window, southeast of Rohru and Askot area) yielded different youngest age component. Sericitic quartzite (the Berinag Formation) from Rampur window and Askot area yielded no grains younger than ~1800 Ma, whereas, a quartzite sample, southeast of Rohru, yielded much younger ages ~650 Ma, with clusters at ca. 1600 Ma and ca. 2500 Ma. Presence of ~650 Ma zircons is inconsistent with the age of the Berinag Formation (ca. 1800 Ma). Hence, this unit should not be mapped as the Berinag Formation. However, it has been mapped by several workers as the Berinag Formation based on the presence of thick bedded sericitic quartzite unit but our new geochronological data refute this. Detrital zircons from low-grade metamorphic Chail Group (silty quartzite), southeast of Simla, in the footwall of Munsiari Thrust show a distinct age peak at ca. 800 Ma, with a two grain age peak of ca. 700 Ma. A quartzite sample from northwest of Simla, in the footwall of Simla Klippe, contains zircons not younger than ca. 1000 Ma. Hence, we conclude that the Chail Group of rocks is stratigraphically overlying Simla Group of rocks in Himachal area. A few (n=4) spot analysis of the thick-bedded Chakrata Sandstone, near Ramgarh area Uttaranchal, show an age peak at ca. 1800 Ma, however, it is difficult to assign a maximum depositional age of this unit due to the lack of ample detrital zircon grains.