Sandstone provenance of north-western sectors of the intracontinental Cenozoic Qaidam basin, western China: Tectonic vs. climatic control

摘要

Sandstone samples from all Cenozoic formations of the nonmarine, internally drained Qaidam basin have been collected for provenance analysis to monitor possible changes in sandstone composition driven either by tectonics or climate. Provided an appropriate hinterland, biotite contents may be a reasonable indicator for aridity. Moderately sorted sandstones with subangular to subrounded grains from the northwestern part of the Qaidam basin are immature indicated by a high proportion of feldspars and lithic fragments. Nevertheless, they have relatively high quartz contents of 29-65%, averaging between 40% and 45%. The average framework compositigns of the formations show only a slight variation over time, limited to the oldest and youngest Cenozoic formations. Contents of white mica and biotite are significantly higher in upper portions of the section. White mica contents increases from 1.0% to 3.7% and biotite from 0.1% to 1.6%, respectively. This change can be explained by increasing aridity during Neogene and surface uplift of Altyn Mountains and Qimantagh, which led to higher relief. When plotted on basic variation diagrams (total quartz-feldspar-lithic fragments) there is a slight shift from continental block field to orogenic field, which might reflect the motion of the Altyn Tagh fault since Oligocene times. Although climate changed during the Cenozoic from arid over semiarid back to arid, we could not find any distinct changes in the framework mineral compositions over time that could be related to climatic events. Few selected mudstones from Eocene to Lower Miocene formations have been geochemically analysed by ICP-MS. All samples display uniform high enrichment in light REEs in chondrite-normalized diagrams. With one exception, the REE concentrations are decreasing in a stepwise manner, although insignificantly, from formation to formation with time due to a diluting effect by carbonate.