No abstract

40 Ar/ 39 Ar dating of detrital white mica from Cenozoic sediments from the nonmarine Qaidam Basin in western China yields several age groups. These include (1) ca. 350-450 Ma, (2) 220-280 Ma, subordinate clusters with (3) 122-140 Ma, and (4) a minor group ca. 500 Ma. Ages between 350 and 450 Ma are the most common and dominating in the analyzed samples. Detrital mica in the western Qaidam Basin can be of a northern (Altyn Mountains) and/ or a southern (Qimantagh-Kunlun Mountains) provenance. The main age group of 350-450 Ma is well known from intrusions in the Altyn Mountains, whereas the young ages (120-126 Ma) have been found within shear zones along the Central Kunlun and Altyn Tagh faults. However, hinterland sectors beyond the Altyn Tagh and Central Kunlun faults must be considered as potential source areas, because the imbalance between the amount of basin fi ll to eroded basement material requires additional sources. Early Paleozoic ages (ca. 500 Ma) have been documented so far only in the Hongliugou suture zone north of the Altyn Tagh fault. This suggests late or slow continuous uplift of the southern Altyn Range, and because of sinistral slip along the Altyn Tagh fault, further units of the same age are now exposed farther westward. The age group of 220-280 Ma has been documented only from the northern basin margin. This age group may indicate recycling from Triassic-Jurassic cover sequences of the Altyn Mountains or provenance from Permian-Triassic granite bodies that possibly originated in the Qilian Mountains.

Carbon and oxygen stable isotopic composition of Cenozoic lacustrine carbonates from the intramontane Qaidam Basin yields cycles of variable length and shows several distinct events driven by tectonics and climate changes. From Eocene to Oligocene, the over-all trend in the d 13 C composition of lacustrine carbonates shows a shift toward higher values, possibly related to higher proportions of dissolved inorganic carbon transported to the lake or lower input of soil derived CO 2 . At the same time, the d 18 O composition of lacustrine carbonates is decreasing in accordance with the global cooling trend and northwards drifting of the whole region. During the Miocene, distinct isotopic events can be recognized, although their interpretation and linkage to a certain tectonic event remains difficult. These events may be related to uplift in the Himalayas, to the strongest phase of uplift in the Altyn Mountains, to pronounced subsidence of the Qaidam Basin or to the expansion of C4 plants on land. Generally cold, highly evaporative conditions can be deduced from enrichment of d 18 O isotopic compositions during Pliocene and Quaternary times.

SamplesEight sandstone samples containing abundant detrital white mica (200-ABSTRACT New single-grain 40 Ar/ 39 Ar detrital white-mica ages from the Lulehe section at the eastern Qaidam Basin yield uniform Permian ages between 250 ± 3 and 279 ± 3 Ma throughout the whole Cenozoic sequence. This is inconsistent with the present hinterland, which is composed of early Palaeozoic metamorphic units with subordinate early Palaeozoic and few Permian granites. The new data indicate that Permian tectonic units are likely more widespread at the north-eastern margin of the Tibetan plateau as known at present, particularly within the Qilian Mountains. The preferred explanation is that the Qaidam block represents a rigid indenter, which indented during late Tertiary times into early Palaeozoic orogenic units. This is consistent with recent findings of a NW-trending sinistral Permian ductile shear zone and a dextral, NW-trending Tertiary fault system close to the north-eastern margin of the Qaidam Basin.

When classic petrographic analysis of the modal composition of sandstones yields no distinction between different source regions, 40 Ar/ 39 Ar dating of detrital white mica can provide vital information on the age of a source area and thus link the sediments to a specific provenance in the hinterland. This approach is exemplified by a case study of the intramontane Qaidam Basin (western China). While the geology of the surrounding mountains of the Qaidam Basin shows considerable lithological variation and the basin's palaeoclimate changed from semi-arid to arid, modal analysis of sandstones from two sections in the northwestern basin, as well as a section on the eastern margin, yielded no significant spatial or temporal differences. All sandstones, most of them classified as lithic wackes with matrix/cement contents between 14 and 39%, plot mainly in the recycled orogenic field of Dickinson's ternary discrimination diagrams for a tectonic environment. The sandstones are quartz dominated, with quartz contents of 33-65% and relative high contents of feldspar and lithic grains. On the other hand, 40 Ar/ 39 Ar total-fusion age data obtained from detrital white mica of between 123 and 546 Ma yielded three age clusters (120-180, 220-280, 350-450 Ma) that could be assigned to certain provenance areas within the early Palaeozoic and Permian basement in the Altyn and Qimantagh mountains. This contrasts with the Lulehe section in the east of the basin, where exclusively Permian ages between 250 and 279 Ma were found. This significant difference in age distribution, and thus provenance, could not be deduced from sandstone composition. The results of this study show how 40 Ar/ 39 Ar thermochronology can complement classic point-count analysis.