PROVENANCE OF THE PERMIAN-LOWER TRIASSIC SANDSTONES IN BOGDA MOUNTAINS, NW CHINA: IMPLICATIONS FOR UNROOFING HISTORY OF NORTH TIANSHAN AND ALTAY SUTURE ZONES

The provenances for the lower Permian-Lower Triassic sandstones in Bogda Mountains, greater Turpan-Junggar basin, NW China, are interpreted from petrographic data of 191 sandstones and paleocurrent directions. Samples are from Tarlong-Taodonggou (TR-TDG) and Zhaobishan (ZBS) areas in the southern, and Dalongkou (DLK) section in the northern foothills, which are ~100 km apart to each other. Framework grain composition is used to construct QFL diagrams to interpret provenance tectonic settings (Dickinson, 1985).

116 sandstones in TR-TDG have mean compositions of Q3F11L86 (Sakmarian), Q5F21L74 (Capitanian), Q12F7L81 (Lopingian), Q21F4L74 (Induan) and Q5F0L95 (Olenekian). 91% of samples are from undissected arcs; the rest in Lopingian and Induan are from transitional arc and recycled orogen. 41 sandstones in ZBS have mean compositions of Q4F8L88 (Sakmarian-Artinskian), Q42F32L26 (Lopingian), Q32F13L55 (Induan) and Q34F16L50 (Olenekian), indicating an undissected arc source in lower Permian and recycled orogen in the upper Permian-Lower Triassic. 34 sandstones in DLK have mean compositions of Q43F30L27 (Capitanian), Q7F18L75 (Lopingian), and Q27F11L62 (Induan), indicating the source changed from a recycled orogen in Capitanian to a new undissected arc in late Permian, which was denudated in Induan. The provenance for the lower-middle Permian sandstones in all three areas is probably the northern Tianshan suture.

This interpretation is supported by the overall northward paleocurrent direction measured in the areas. The same provenance provided sediments for the upper Permian-Lower Triassic sandstones in the southern foothills. However, the upper Permian-Lower Triassic sediments in DLK were probably derived from a new volcanic arc to the north, likely in the Altay suture zone(s). In all three areas, sedimentary lithics are present, indicating a minor contribution from local horsts. The northern Tianshan suture probably formed before early Permian, but suturing between Junggar plate and Siberian craton along the Altay probably occurred in late Permian.