The lower Lesser Himalayan sequence: A Paleoproterozoic arc on the northern margin of the Indian plate
The lower Lesser Himalayan sequence marks the northern extremity of the exposed Indian plate, and is generally interpreted as a passive margin. Five lines of evidence, however, collectively suggest a continental arc setting: (1) igneous intrusions and volcanic rocks occur at this stratigraphic level across the length of the Himalaya, (2) ages of intrusive and metavolcanic (?) rocks cluster at 1780–1880 Ma but also indicate a long-lived igneous process, (3) detrital zircon ages in clastic rocks cluster at 1800–1900 Ma, with a unimodal age distribution in some rocks, (4) the mineralogy and chemistry of metasedimentary rocks differ from typical shales and suggest a volcanogenic source, (5) trace-element chemistries of orthogneisses and metabasalts are more consistent with either an arc or a collisional setting. Intercalation of volcanic rocks with clastic sediments and a general absence of Proterozoic metamorphic ages do not support a collisional origin. An arc model further underscores the profound unconformity separating lower-upper Lesser Himalayan rocks, indicating that a Paleoproterozoic arc may have formed the stratigraphic base of the northern Indian margin. This, in turn, may indicate disposition of the Indian plate adjacent to North America in the ca. 1800 Ma supercontinent Columbia. Felsic orthogneisses (“Ulleri”) likely represent shallow intrusions, not Indian basement.