DISCOVERY OF A NEOPROTEROZOIC ARC IN THE NW HIMALAYA – TECTONIC IMPLICATIONS

The Himalayan nappe sequence encompasses a series of tectonic units whose magmatic components are usually separated into Paleoproterozoic (c. 1800 Ma) units of the Lesser Himalayan Sequence (LHS) and early Paleozoic (c. 500 Ma) units of the Greater Himalayan Sequence (GHS). Here we report new chronologic data demonstrating that a third major magmatic component exists in the Shimla (Himachal) region of NW Himalaya that is Neoproterozoic (800-1000 Ma) in age and sandwiched structurally between GHS and LHS thrusts. Zircon grains from 14 orthogneisses of the GHS, LHS, and structurally intervening Munsiari (or Jutogh) Group were analyzed for U-Pb ages and trace element geochemistry to determine if the Munsiari unit correlates chronologically with GHS or LHS units, and to explore its magmatic origins. Orthogneiss ages regionally separate into three distinct age groups, including expected ~500 Ma GHS rocks (8 samples) and ~1800 Ma LHS rocks (2 samples). However, four samples from the Munsiari Group yield ²⁰⁷Pb/²⁰⁶Pb ages of 823±8, 823±9, 826±8, and 976±9 Ma. These ages correspond both with prior reports for the Chor granite and a Neoproterozoic detrital zircon age peak in the study area. Zircon trace element compositions indicate a continental arc affinity. These ages correspond with the timing of the Rodinian supercontinent, and correlate to the south and west with magmatic rocks of the Aravalli Delhi Belt, Seychelles Island (Indian Ocean), and eastern Madagascar; they may also extend northward to connect with the (structurally repositioned) Yangtze Block of the South China craton. Evidently, a Neoproterozoic arc bounded western Rodinia and is now represented in the NW Himalaya by the Munsiari Group. If so, the nappe sequence in the NW Himalaya should be classified differently than elsewhere in the Himalaya, recognizing the Munsiari rocks as a separate subsection. Structurally, the Munsiari thrust sheet occupies a similar level as the Ramgarh thrust sheet in Nepal. However, unlike the Ramgarh thrust sheet, whose magmatic rocks are Paleoproterozoic in age, the magmatic rocks of the Munsiari Group and thrust sheet represent a wholly different Neoproterozoic origin. Neoproterozoic detrital zircon ages are prominent in the NW Himalaya, but die out along strike into Nepal, only to reappear in the eastern Himalaya, likely pointing to a separate Neoproterozoic arc there.