SCALE AND SIGNIFICANCE OF TRANSPORT OF SEDIMENT IN EAST GONDWANALAND DURING THE CAMBRIAN

A remarkable distance of sediment transport and degree of sediment mixing across the vast area of East Gondwanland during the Cambrian is demonstrated by detrital zircon age spectra. These spectra derive from samples taken both across and along the strike of the Himalayan orogen, from northwest India to Bhutan. Grenville- and Pan-African age grains dominate the age spectra, although distinctive Mesoproterozoic, Paleoproterozoic and Archean peaks also exist. The presence of 1.4 Ga. zircons in 7 of 9 samples (ranging from 3-13% of the total populations) indicates sediment transport from one of two possible sources. First, these zircons may have a provenance in an A-type granite belt conjectured to exist in the East Antarctic shield, which represents a continuation of a magmatic province extending from southwest U.S. into Antarctica within Rodinia (SWEAT hypothesis). Beginning in the latest Early Cambrian, an extensive wedge of syn- and post-orogenic sediment was shed outward from the Ross orogen and these deposits contain abundant 1.4 Ga zircons. Alternatively, zircons of this age may have been derived from African sources. The Mesoproterozoic Nzilo Group of the Kibaran Belt contain a population of ~1330 to ~1500 Ma detrital zircons, including abundant early 1.40–1.45 Ga grains. These grains were presumably derived from the Mitwabi Orthogneiss within Kibaran Belt, situated in south-central Africa at the juncture between the Congo Craton and the Tanzania–Bangweulu Block. Late Neoproterozoic sedimentary rocks of central Madagascar, which are placed in Rodinian reconstructions to the northeast of the Tanzania Craton, also contain 1.4 Ga detrital zircons. The presence of 1.4 Ga grains in Middle and Upper Cambrian Himalayan samples suggests that syn- and post-orogenic sediment extended remarkably long distances either from the Ross Orogen or the Kibaran Belt to the northern Indian continental margin. This sediment combined with sediment sourced from Pan-African and Arabian-Nubian Shield regions, and was dispersed across the northern Indian margin. The great transport distances and degree of homogenization of detrital zircon populations is possibly unmatched in geologic history. The degree of long-distance sediment mixing may have resulted from a combination of extreme relief associated with Gondwana assembly and sediment transport across a non-vegetated landscape.