The Himalayan passive margin from Precambrian to Cretaceous times

The Himalayan region has a record of several phases of igneous activity which indicate a continued N—S orientated tensional regime since at least late Precambrian time. Though the earlier volcanic eruptions were restricted both in time and space, the Late Carboniferous-Triassic activity was widespread in terms of both the time interval and area covered. The Abor Volcanic activity in the Eastern Syntaxial Bend of the Himalayan fold belt is coeval with the initiation of Late Carboniferous—Triassic Panjal Trap volcanism of the Western Syntaxial Bend and eruption of the Sikkim lavas. Chemical characters of the Abor Volcanics indicate (1) a within-plate rift tectonic setting of the lava eruptions, (2) a transitional nature between tholeiitic and alkalic compositions, (3) relative enrichment in incompatible elements, and (4) high Fe-Ti values similar to many such basalts erupted in the regions of active lithospheric rifting. The olivine-free nature, a linear trend for MgO and low Co contents of the studied rocks are suggestive of high pressure olivine fractionation for the generation of the bulk chemistry. However, large variations in the incompatible element abundances and ratios of rocks at a similar fractionation stage discount the evolution of the lava chemistry exclusively by simple fractionation or partial melting processes. Instead, these characters together with the low Zr/Nb ratio and within-plate rift tectonic setting suggests the modification of an upper mantle source region, possibly by fluid phase CO 2 immediately before the partial melting and lithospheric rifting. Combining geology of the region with the geochemistry and inferred tectonic setting of these rocks, the lava eruptions are suggested to have been associated with the 9rift valley stage9 of the Tethys Ocean formation in the present Himalayan region.

Abor Volcanics: further evidence for the birth of the Tethys Ocean in the Himalayan segment

The Kumbalgarh Group of the south Delhi fold belt are the main bedrock series exposed in the axial region of the Aravalli craton. Quartzites and greywackes, the chief clastic constituents of this group, are well exposed. Petrographic and bulk-rock analyses of these rocks permit determination of their provenance, tectonic setting of the basin, and the Archaean to Proterozoic crustal evolution. Greywackes comprise quartz, plagioclase, amphiboles, K-feldspar, and rock fragments. Based on mineralogy, we divided the quartzites into three categories: QTZ1 is chiefly composed of quartz with a silty matrix and a minor quantity of feldspars and QTZ2 contains significant mafic minerals as well as quartz and feldspars, whereas QTZ3 is more feldspathic than the other groups. All the lithounits have SiO2/Al2O3 ratios <∼10 suggesting textural immaturity consistent with their sedimentary petrography. Greywackes display the least fractionated rare earth elements (REEs) (La/Yb N : avg. 2.55) with positive Eu anomalies (av...

Petrographic and geochemical characteristics of Mesoproterozoic Kumbalgarh clastic rocks, NW Indian shield: implications for provenance, tectonic setting, and crustal evolution

Bayana Basin, sited along the eastern margin of the north Delhi fold belt of the Aravalli Craton, contains an ∼3000 m-thick sequence comprising one volcanic and seven sedimentary formations of the Delhi Supergroup. The sedimentary units are the Nithar, Jogipura, Badalgarh, Bayana, Damdama, Kushalgarh, and Weir formations in order of decreasing age. Petrographic study of the sandstones as well as major and trace elements (including rare earth elements) and bulk-rock analyses of the shales and sandstones allow the determination of their provenance, source-rock weathering, and basinal tectonic setting. The sandstones are quartz rich and were derived mainly from exhumed granitoids typical of a craton interior. Geochemical patterns of the sandstones and shales are similar. However, trace element abundances are low in sandstones, probably due to quartz dilution. The coarser clastic Damdama and Weir sandstones, which occur at higher stratigraphic levels, have strikingly low trace element concentrations compared ...

Geochemistry and detrital modes of Proterozoic sedimentary rocks, Bayana Basin, north Delhi fold belt: implications for provenance and source-area weathering

Ideas on the genesis and magmatectonic setting of three prominent Proterozoic, Grenvillian mafic dyke swarms in southern and central Sweden are presented. Two of the linear dyke swarms are associated with the Protogine Zone, a zone currently interpreted as a taphrogenic complex. The third dyke swarm represents very extensive mafic magmatism in central Sweden and forms large sheets and feeder dykes. The first two consist of the c. 1180 Ma old alkaline Protogine Zone Dolerite Group (PZDG) and the early Late Proterozoic, Late Grenvillian c. 930 Ma old Blekinge Dalarna Dolerite Group (BDDG), composed of olivine and quartz tholeiites. The third consists of the c. 1220 Ma old weakly alkaline to transitional Central Scandinavian Dolerite Group (CSDG). Petrogenetic modelling of the REE distribution patterns of the dyke swarms indicates partial batch melting followed by fractional crystallization. It is shown that the dyke swarms have been generated by low to moderate degrees of partial melting of a garne...

M. Raza

Papers

Geochemical clues to elucidate the tectonic environment of the Chamoli Volcanics, Lesser Himalayas, Uttar Pradesh, India