Geochemistry of the 2.51 Ga old Rampur group pelites, western Himalayas: implications for their provenance and weathering

Abstract

The late Archean volcano-sedimentary succession of the Rampur group (RG) represents the oldest superacrustal rocks exposed in the Himalayas. The sedimentary component is pelite dominated in the lower part and siliciclastic (with associated mafic volcanics) dominated in the upper part, indicating shallowing of the basin over time. The bulk chemistry of the RG pelites shows illite–chlorite control that gets diluted with increase in quartz content. They have a general similarity with the post-Archean shales but a slightly higher average SiO 2 and lower Al 2O 3 contents and severe depletion in CaO, Na 2O, Sr and Ba relative to the North American shale composite. REE patterns are highly fractionated with (La/Yb) N ratio ranging from 8.8 to 18.6; HREE show slight fractionation [(average=Gd/Yb) N <2], and all the samples have large negative Eu anomalies (average, 0.61). (La/Yb) N decrease with increase in Al 2O 3/Na 2O, but lack any relationship with K 2O, thereby discounting clay mineral control. HREE have a positive correlation with TiO 2, Th, Y and Nb but not with P 2O 5 and Zr. The increase in HREE seems likely due to the increased mobilization of HREE-enriched phase(s) during intense weathering. High Zr and Th contents and Th/Ni, La/Sc and (La/Yb) N ratios as well as Sc–Th/Sc relations favor a dominantly felsic source with only a minor contribution from mafic rocks. The most likely source lithology, compatible with the southerly paleocurrent direction and first cycle-sediment textural features of the RG sediments, appears to be the banded gneissic complex (BGC) of the northern Indian shield. The same conclusion is reached when RG data are plotted in Al 2O 3–CaO+Na 2O–K 2O (A–CN–K) compositional space, the data define a trend which indicates unweathered plagioclase-K-feldspar source composition similar to that of the BGC. Pre-metasomatized high chemical index of weathering as well as the high plagioclase index of weathering for a majority of the samples conforms to the world-wide warm, moist climate prevalent during the time of their deposition.