EARLY CRETACEOUS VOLCANICLASTIC AND QUARTZOSE SANDSTONES FROM NORTH CENTRAL NEPAL - COMPOSITION, SEDIMENTOLOGY AND GEOTECTONIC SIGNIFICANCE

Early Cretaceous sandstones and shales from the Thakkhola region (Chuk h and Tangbe Formations) were deposited on the subsiding northern (Tet hyan) margin of Gondwana; current directions indicate a source area to the south. Whereas the Berriasian Chukh Formation is dominated by qua rtz arenites, volcaniclastic sandstones and minor conglomerates domina te the Tangbe Formation of Valanginian to Aptian age. Up-section, the composition of the volcaniclastic components changes from mainly basal tic to dominantly dacitic/rhyolitic. Magmatism was preceded and accomp anied by strong uplift in the source area of the Chukh and Tangbe clas tics, resulting in the erosion of probably more than several hundred m eters of pre-Cretaceous sedimentary, metamorphic and plutonic rocks. A geochemical whole rock analysis of basaltic pebble fragments shows th eir alkaline character and suggests a 'within-plate' geotectonic setti ng for the volcanic source. This volcanotectonic event is probably rel ated to rifting between Australia and Greater India, where seafloor sp reading began in late Valanginian/Hauterivian times.