Provenance And Tectonic Implications Of The Gequ Formation At Area North Of Huashixia,East Kunlun Region

The Upper Permian(Lopingian) Gequ Formation in Tandikejie area, north of Huashixia and the southern margin of the East Kunlun Mountains is composed of conglomerates, sandstones and calcareous mudstones of fan delta-shallow sea. It recorded the tectonic evolution of the East Kunlun orogenic belt during late Hercynian-early Indosinian. The determinations of the provenance and tectonic setting of the Gequ Formation have great significance for studying the tectonic history of the study area from Neopaleozoic to Early Mesozoic. This paper studies the sedimentary features, detrital modes geochemistry and detrital zircons dating for sandstones of the Gequ Formation in Tandikejie area. In combination with previous studies, this paper comprehensive research the material characteristics of Gequ Formation, the nature of the provenance and the basin characteristic. On this basis, rebuilt the tectonic evolution process of south area of the East Kunlun mountain during Hercynian- Indosinian. The main viewpoints and conclusions in this thesis are as follow:1. 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%, the average of Qt/(F+L) is 1.03, quartz is mainly monocrystalline quartz, feldspar is mainly K-feldspar. Lithic contents are low, the composition of gravel and lithic is complex, including sedimentary, metamorphic, volcanic and granite rock fragment. According to features and discriminant diagrams of detrital modes, source area of the Gequ Formation shows characteristics of ―uplift basement‖. Geochemistry analysis shows that average concentrations of SiO2, Al2O3, TiO2, TFe2O3, MgO, K2 O and Na2 O 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 features and discriminant diagrams of major elements and trace elements, the source terranes are mainly felsic magmatic rocks. 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.2. The Gequ Formation sandstone sample has ages of 412 Ma to 2448 Ma, which can be mainly divided into four age ranges: 1869~1602Ma, 1396Ma~1270Ma, 1197~877Ma and 572~412Ma. The age ranges indicate the source regions have experienced complex multi-phase geological events, which corresponding to the Early Paleoproterozoic tectono-magmatic event, Late Paleoproterozoic assembly event, Mesoproterozoic rifting event, Late Mesoproterozoic-Early Neoproterozoic tectonomagmatic event and Late Neoproterozoic-Early Paleozoic rifting-extension-assembly event. Magmatic rocks formed during Late Neoproterozoic-Early Paleozoic are the mainly provenance of the Gequ Formation.3. Based on the comprehensive analysis of detrital zircons dating, detrital modes, gravel composition and geochemistry, 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. Basin of the Gequ Formation was a ―cold basin‖ similar to foreland basin.4. Comparing with the Upper Permian, there are many more ages >2000Ma in the Low-mid Permian, and the dated zircons yielded Hercynian-Indosinian ages were first found in the Lower Triassic. Combined with regional geological background and previous studies, the two significant changes were the sedimentary response to the two tectonic events which were the causes of the unconformities between Upper Permian and Low-mid Permian, Lower Triassic and Upper Permian, respectively, and they existed a good tectono-depositional coupling relationship. In which, the close of central ocean of the East Kunlun and the initial subduction of the A’nimaqing Ocean coexisted in Late Permian. And the back-bulge basin was mainly controlled by the collisional orogenesis of central ocean of the East Kunlun in the latest Middle Permian. The A’nimaqing Ocean was in subduction-collision stage during the Early to Middle Triassic. The unconformity under the bottom of the Lower Triassic and the foreland basin were mainly controlled by the anabatic subduction of the A’nimaqing Ocean.