Metamorphic terrane in East Kunlun

The metamorphic terrane of East Kunlun is located between the suture zones in south-central Kunming, and mainly consists of Mesoproterozoic Kuhai Rock Group, Neoproterozoic Granite Gneiss, Neoproterozoic Wanbaogou Group and Nachitai Group. On the north side of metamorphic terrane, the suture zone in the middle of East Kunlun (hereinafter referred to as the Kunzhong suture zone) starts from the north bank of Ayakumu Lake in Xinjiang, passes through the north of Tengtag in kajakka, the steep south of Kaimuqi, Dagangou, Wutuo and other places eastward, and reaches Qingshuiquan and its Jirimai area. The length of the discontinuity is about 1000km and the width is about 2 ~ 5 km. The two ends are cut off by the Wahongshan fault and strike-slip fault on the southern margin of Altun Mountain, and generally spread in NW-NW direction. The suture zone at the southern foot of the East Kunlun Mountains on the south side of the metamorphic terrane refers to the ophiolite melange belt composed of Buqingshan and Anima Qing extending in the northwest and the Kuhai-Saishitang ophiolite melange belt extending in the northeast, which was formed later than the suture zone in the middle of Kunming, and belongs to the Late Paleozoic to the Early Mesozoic. The Kunzhong suture zone has been introduced in detail in the second chapter, and now it is appropriate to supplement the Kunnan suture zone.

1. Composition and characteristics of Kunnan suture zone

Kunnan suture zone can be divided into three branches, the west branch is Buqingshan ophiolite melange zone, the east branch is Anima ophiolite melange zone, and the north branch is Kuhai-Saishitang ophiolite melange zone. Buqingshan ophiolite melange belt is located in the west branch of Kunnan suture zone, extending in the direction of-SEE, and is divided into three secondary structural belts from north to south, namely, the island arc volcanic belt in the north, the ophiolite melange belt in the middle and the flysch belt in the south. Ophiolite melange is embedded in slate in the form of ophiolite blocks of different sizes. Ophiolite consists of strong serpentine peridotite, harzburgite, picrite, pyroxenite, gabbro, diabase wall, pillow basalt and massive basalt.

Anima ophiolite melange belt also extends along NWW—SEE direction, connecting Buqingshan ophiolite melange belt in the west and Wenxian, Mianxian and Lueyang ophiolite melange belts in the east. The suture zone consists of three secondary structural zones from north to south: island arc volcanic belt, oceanic ridge ophiolite belt and flysch belt. The isotopic age of volcanic rocks in Wu Xia island arc in northern Qinghai, Anima is (260 10)Ma.

The Kuhai-Saishitang ophiolite melange belt is distributed in the eastern part of East Kunlun Mountain and the eastern edge of Qaidam Block, extending in the NE-SW direction, reaching the Mawengenshan area of * * and * * in the north, and forming a three-dimensional structural belt together with Buqingshan, Anima Qing-Wenxian-Mianxian-Lueyang ophiolite melange belt. Sun Tinggui and others believe that the Kuhai-Saishitang ophiolite melange belt constitutes the northern branch of the Qinling-Kunlun three-way structural belt, and different rock units in the melange are mixed in Carboniferous-Early Permian argillaceous slate and tectonic rocks with medium-high pressure and low greenschist facies metamorphism. The rocks mainly include strong serpentine dunite, plagioclase peridotite, gabbro, diabase, basalt and siliceous slate, among which basalt belongs to intraplate basalt and E-MORB type, indicating the structural environment of the transition from mature rift valley to initial ocean basin. Zircon U-Pb dating is (263.9 2.1) Ma, which is consistent with the age of volcanic rocks in the Animaqing island arc.

Sun Tinggui and others proposed that the evolution model of the suture zone in southern Kunming is as follows: first, GGG tee (G = graben) was formed in the background of the initial rift in Devonian, commonly known as the triple rift; Further extension makes the western branch and the eastern branch form Buqingshan and Anima Qingfinite ocean basins, but the northern branch dies out, forming * * * and aulacogen, which is called RRA-type combination (ridge-aulacogen). The late Permian developed into SSS type three-arc volcanic belt (S = subduction, that is, subduction type), and the subduction lasted until the end of late Paleozoic. This evolution model involves whether there is a proto-Paleotethys ocean, or whether it evolved from a triple rift to a limited ocean basin on a unified continent, so it is still debatable. However, the Buqingshan-Anima Qinghe Kuhai-Saishitang ophiolite melange, as the southern structural boundary of the metamorphic terrane in East Kunlun, provides abundant geological evidence for the post-Devonian evolution.

Composition and characteristics of pre-Devonian metamorphic terrane in East Kunlun

The metamorphic terrane in East Kunlun consists of Wanbaogou Group and Nachitai Group, but a set of "Kuhai Group" is exposed near the Kunzhong fault zone in the east of East Kunlun, and its main body is Neoproterozoic granite gneiss. In the southwest of Qingshuiquan in Xiangride Town, dulan county City, Qinghai Province, a set of medium-deep metamorphic supracrustal rocks and ancient plutonic intrusive rocks, with an area of nearly 65,438 000 km2, are exposed to the south of Kunzhong Fault, which was formerly called Kuhai Group. The supracrustal rocks of Kuhai Group are mainly composed of amphibole, mica adamellite, mica adamellite and amphibole adamellite. These metamorphic supracrustal rocks occur in Neoproterozoic granite gneiss as xenoliths of different scales. The zircon U-Pb ages of granite gneiss are (875±25)Ma and (86710) Ma, respectively (Lu Songnian et al., 2002). There are many small diorite bodies in the exposed area of Kuhai rock group. The graphic discrimination of trace elements shows that its magma belongs to calc-alkaline series and was formed in the tectonic environment related to island arc. The U-Pb age of zircons from diorite TIMS method is (446.5 9.1) Ma, which is equivalent to the data of (446.2±2.2)Ma and (447.9±4.4)Ma of island arc-type intermediate-acid volcanic rocks in Nachitai Islands in the north (Chen Nengsong et al., 1999).

A set of shallow metamorphic slate and marble is widely developed from Wanbaogou to Halabayigou in the middle part of East Kunlun Mountain, and the stratigraphic name and age attribution have changed (Ji Qiang, 1993). In this book, Jiang Chunfa (1992) and Li Guangcen (1982) are used to divide this shallow metamorphic rock series into Wanbaogou Group in the lower part and Nachitai Group in the upper part, which are from late Neoproterozoic to Cambrian and Ordovician to Silurian respectively. Wanbaogou Group consists of lower clastic rock formation, volcanic rock formation, marble formation and upper clastic rock formation from bottom to top. Nachitai Group is divided into Cement Plant Formation, Shihuigou Formation and Halabayigou Formation from bottom to top. 1993, Ji Qiang et al. separated the CAMBRIAN Meishucun small-shell animal fossils from the marble formation and upper clastic rocks of Wanbaogou Group, and the age of Wanbaogou Group extended to early Paleozoic.

Kang et al. (1992, 1995) proposed that these two groups of basalts belong to "ocean island basalt" by studying their geochemical characteristics. Guo et al. (2004) thought that the two groups of sedimentary strata were sedimentary-structural mixed reservoir-forming systems, which were divided into two parts: matrix system and foreign system. The element 19 in the matrix system is obviously different from that in foreign systems, especially the silicon isotope characteristics of the basic system reflect the sedimentary environment of siliceous rocks. The average δ30Si of siliceous rocks deposited in deep-sea environment is -0.6 ‰ ~ 0.8 ‰, that of siliceous rocks deposited in semi-deep-sea environment is 0. 1 ‰ ~ 0.6 ‰, and that of siliceous rocks deposited in shallow-sea environment is -0.35 ‰ ~ 3.4 ‰. The δ30Si values measured by Guo et al. are between-0.7 ‰ and 0.1‰, and the δ30Si values of Nachitai Group are -0.6‰, all of which reflect very low δ30Si values, which are consistent with the δ 30Si values of siliceous rocks deposited in deep-sea environment, and represent the deep-water sedimentary environment far from the continental source area. Based on the previous research results, we tend to think that the main bodies of Wanbaogou Group and Nachitai Group belong to the "seamount" volcanic sedimentary sequence. When the ocean basin between the southern margin of Qaidam block and the metamorphic terrane of East Kunlun disappears, the seamounts that constitute the main part of the metamorphic terrane are not dragged deep into the crust, but collaged in the southern margin of Qaidam block. Therefore, the metamorphic terrane in East Kunlun belongs to accretionary collage in the southern margin of Qaidam block.