Abstract

The Qinling orogenic belt is one of world's great collisional orogenic belts, which extends from the Pamirs–West Kunlun to the Korean peninsula and occupies a key position in understanding of the evolution and assembly of Asia. This belt separates the North China and South China blocks. Extensive, thick Devonian clastic rocks, sandwiched between the Shangdan and Mianlue suture zones, are mainly infills of the Xicheng Basin and provide the principal sedimentary record of tectonism in the Qinling orogen. Devonian infills were subdivided into three subunits: the Upper Devonian Dacaotan Group, the Middle–Upper Devonian Shujiaba Group, and the Lower–Upper Devonian Xihanshui Group. Their provenance and tectonic setting are critical to understanding not only the tectonic evolution of Asia but also the global aspects of collisional regions. Compared with Devonian strata in the eastern part of the Qinling orogen, the Xicheng Basin was not seriously modified by Mesozoic tectonism.

Petrologic studies demonstrate that the Dacaotan Group consists mainly of feldspathic litharenite with abundant volcanic fragments. In contrast, the Shujiaba and Xihanshui Groups contain more metamorphic and sedimentary fragments and fewer magmatics than other lithic fragments; these relations demonstrate that magmatic plutons were not extensively exposed at the time of deposition of these groups. On Q-F-L plots sandstones fall into the field of transitional arcs, but on Qm-P-K and Lm-Lv-Ls diagrams they plot in the fields of continental-arc and continental-suture rocks. The presence of volcanic lithic grains in the sandstones suggests they were derived mainly from a continental arc.

Major-element geochemistry of siltstones and mudstones demonstrate that most were derived from a continental arc, but some from an oceanic island arc. Trace-element geochemistry favors a continental-arc provenance mostly from felsic rocks. A high Cr content of siltstones and mudstones demonstrates that some mafic or ultramafic rocks were exposed in the source area and provided some detritus. Rare-earth-element patterns are very similar to those of sandstones in modern continental arcs. Devonian paleocurrent indictors demonstrate that this continental arc was located at the northern edge of the basin. This deduction demonstrates that the Qinling arc in the southern margin of the North China block was the major source of clastic debris in the Devonian Xicheng Basin; these relationships suggest that the sediments were most probably deposited in a forearc basin.

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