Provenance of the Upper Permian Gequ Formation in the Southern Margin of the East Kunlun Mountains: Constraints from Detrital Composition and Geochemistry

Key words: 
• Upper Permian Gequ Formation /
• East Kunlun Mountains /
• provenance /
• tectonic evolution

Abstract: The Upper Permian (Lopingian) Gequ Formation in the southern margin of the East Kunlun Mountains is composed of conglomerates, sandstones and calcareous mudstones of fan delta-sea carbonate plateau. It recorded the tectonic evolution of the East Kunlun orogenic belt during Late Hercynian-Early Indosinian. This paper studies the detrital modes and the compositions of major, trace and rare earth elements for sandstones of the Gequ Formation in Tandikejie area, north of Huashixia, and investigates the tectonic attributes and provenance types. Sandstone detrital modes show that the Gequ Formation consists of arkose and lithic arkose. The QtFL averages of sandstones are Qt 50.9%, F 36.8%, L 12.4%, feldspars are mainly K-feldspar, the composition of gravel and lithic is complex. Geochemistry analysis shows that average concentrations of SiO₂, Al₂O₃, TiO₂, TFe₂O₃, MgO, K₂O and Na₂O are 62.06%, 13.26%, 0.44%, 4.53%, 3.02%, 2.42% and 3.18%, respectively. Trace elements ratios such as Th/Sc, Th/U have high averages of 1.06, 7.71. The Gequ Formation sandstones have REE gross of 120.43×10^-6. In the sandstones, LREE is relatively rich, the LREE/HREE ratio is 9.34, the (La/Yb)_N ratio is 12, δEu is 0.96 and the anomaly of δEu is not obvious. According to the REE features, the Gequ Formation show similarities to the greywackes from continental island arc and affinity to the Caledonian subduction-collision arc type granites of the East Kunlun terrane. Based on the features and discriminant diagrams of clastic composition, major elements and trace elements, the provenance of the Gequ Formation sandstones came from the south microblock of East Kunlun, which was rifted from the East Kunlun terrane during Early Hercynian. The source rocks consisted of mainly granites and little sedimentary rocks, metamorphic rocks and acid-basic volcanic rocks, and the granites belong to the Caledonian continental island arc. Combined with regional geological background and previous basin studies, the Gequ Formation formed in back-bulge of foreland basin system.