Early Wang Yuzhen (1983) and Li Songling (1985) defined ophiolite as "Neoproterozoic", and the Rb-Sr age of amphibole vein in ultramafic complex was 860Ma. Pan Yusheng et al. (1990, 1994), Wang Dongan et al. (1989), Yang Shufeng et al. (1999) and Deng Wanming (1995) think that it was formed in the early Paleozoic, representing Zhou Hui et al. (1998, 1999) found early Paleozoic radiolaria in some Kegou and determined the ductile shear zone in the late early Paleozoic, which greatly supported the above understanding. Ding Daogui et al. (1996) obtained the whole rock-mineral Sm-Nd isochron age of 65 1Ma, suggesting that ophiolite was formed in Sinian-Early Paleozoic. The Rb-Sr isochron age of Di Ku basalt obtained by Jiang Chunfa et al. (2000) is 359Ma, indicating that it belongs to Devonian (? )—Carboniferous ophiolite belongs to the back-arc small ocean basin environment, which is supported by the Carboniferous radiolarian fossils discovered by Fang Aiming et al. (2000) in some Kegou. Xiao Wenjiao et al. (2000) and Xiao et al. (2002) believe that ophiolite belongs to arc or back-arc basin ophiolite superimposed on subduction zone in early Paleozoic by studying the structural characteristics and geochemistry of different units in the area. Table 2- 1 summarizes the previous understanding of ophiolite age and its tectonic setting in Di Ku for comparative analysis. In addition, we also listed the relevant dating data in Di Ku area for comprehensive analysis (Table 2-2).
I. Regional geological analysis
The "Di Ku ophiolite" defined by predecessors consists of three parts, including ultramafic rocks in Di Ku, part of Kegou basalt in northern Di Ku and flysch deposits on basalt (Figure 2- 1, Figure 2-2). The ultramafic rocks in Di Ku are exposed in Buzwandaban, Di Ku, and are mainly composed of pure peridotite, pyroxenite, pyroxenite and lenticular chromite (ore). Shen Buming et al. (1996) made a thorough and detailed study on peridotite. Gabbro veins are developed in many places in the rock mass, and the width of the veins varies greatly, generally from ten centimeters to tens of centimeters. Some tiny veins are only 1 ~ 2 centimeters, forming staggered network veins. Between gabbro vein and peridotite, there is a condensed edge of 2 ~ 5 mm, from the condensed edge to the center of gabbro vein under the thin slice, it changes from glassy and fine-grained microcrystal to pure, and a small number of wide vein centers form pegmatite gabbro. The long axis of feldspar single crystal in pegmatite gabbro can reach tens of centimeters, and the largest pyroxene single crystal can reach 80 centimeters. The above structural characteristics show that gabbro intruded into pure olivine in the later stage, and the intrusion depth was relatively deep, which gave it sufficient time to crystallize and form megacrysts. Another important component of ophiolite in Di Ku is Di Ku North Basalt, some of which are the most complete outcrops in Kegou. On this profile, the lower part is massive basalt, and the upper part is pillow basalt, with an exposed thickness of more than 200m m m. ; Yuan Chao et al., 1999), and the southern and 128 rocks (471ma; ; Yuan Chao et al., 1999) are in fault contact (fig. 2- 1, fig. 2-2). There is a set of flysch assemblage on basalt, including conglomerate, miscellaneous sandstone, siltstone, tuff and siliceous rock. (Figure 2-2), and a large number of sedimentary structures with flysch characteristics, such as flame-like structures and Ma Bao sequences (mainly composed of A-E and A-D-E). Develops in rock formations.
Table 2- 1 Preliminary understanding of ophiolite age and tectonic setting in Di Ku
Table 2-2 Age Characteristics of Related Rocks in Di Ku Area of West Kunlun Mountain
Second, age information.
1. bimodal intrusive complex in southern Di Ku.
In the gneiss exposed in the reservoir, we decomposed some gneiss intrusive rocks (Figure 2-3). The intrusive complex consists of two parts, namely granite and gabbro. The two types of rocks are interwoven on outcrops (equivalent to mixed structures), indicating that they belong to bimodal intrusive complex. Zircon SHRIMP U-Pb age of bimodal intrusive complex granite is (78 1 10) Ma. Geochemical research shows that the gneissic bimodal intrusive complex belongs to alkaline series and was formed in the extensional continental cracking background (Zhang et al., 2006).
The gneiss-like bimodal intrusion complex in this area intrudes into gneiss, indicating that gneiss in Di Ku area belongs to Precambrian, while zircon U-Pb dating of the Cambrian gneiss in front of Kunlun main ridge shows that this set of deep metamorphic rocks was deposited in late Neoproterozoic-early Paleozoic, early Paleozoic and Indosinian metamorphism. Therefore, the gneiss in Di Ku and the gneiss in the metamorphic terrane in the south of West Kunlun (Sangzhutag Group and Bulunkuole Group) are completely different sets of metamorphic rocks, and this part of gneiss in Di Ku suture zone is similar to the "Beidahe Group" in North Qilian suture zone and should belong to Precambrian detritus. Geochemical study of the bimodal intrusive complex in Di Ku shows that the granite in it came from the remelting of the Mesoproterozoic basaltic lower crust, while the gabbro magma came from the depleted mantle and was contaminated by the mature continental crust. In northern Di Ku, our research on Mesoproterozoic volcanic rocks in Tiekelike uplift belt shows that (Zhang Chuanlin et al., 2003b；; Zhang et al., 2003), in which the basalt has exactly the same nd isotopic composition as the A-type granite, so it can be judged that the gneiss in the region originally belonged to the Tarim basement and was involved in the orogenic belt during the formation of the suture zone.
Figure 2- 1 Geological Schematic Diagram of Di Ku Area
Figure 2-2 Geological Profile of Kegou Yishak Group in Di Ku
1- granite; 2- massive basalt; 3- pillow basalt; 4- almond basalt; 5- pillow basalt containing almonds; 6- Volcanic agglomerate; 7- almond andesite; 8-crystal tuff; 9- Joint venture; 10-gravelly sandstone; 11-sandstone; 12-siltstone; 13-argillaceous siltstone; 14-mudstone; 15-bedding occurrence; 16- layered code
2. Chronology and geochemistry of ophiolite in Di Ku.
The debate about "ophiolite" in Di Ku is mainly due to the lack of reliable isotopic age data. In ultramafic complexes, the zircon SHRIMP U-Pb age of pegmatite intrusive peridotite and pyroxene is 525Ma, which is consistent with the age value obtained by Xiao et al. (2005), and it can be confirmed that the age of ultramafic rocks is no later than 525Ma. For some Kegou basalts, early Paleozoic radiolarians (late Ordovician-early Silurian; Zhou Hui et al., 1999), and the late Paleozoic radiolarian assemblage was found in the upper flysch assemblage, so the "Isaac Group" in some Kegou essentially has two rock assemblages. In addition, our SHRIMP U-Pb dating of zircon from the lower basalt shows that the age of basalt formation is between 420 and 430 Ma (Zhang Chuanlin et al., 2004), which is basically consistent with the age defined by radiolarians. Therefore, it is reasonable to speculate that Kegou basalt in Di Ku is the product of Early Paleozoic, which is obviously different from ultramafic rocks and cannot be put together to form ophiolite suite.
Fig. 2-3 Geological schematic diagram of bimodal intrusive complex in southern Di Ku.
A large number of geochemical studies have been carried out on pillow basalt and massive basalt in the lower part of some Kegou sections in Di Ku (Fang Aimin et al., 2003; Yang Shufeng et al.,1999; Yuan Chao et al., 2002; Pan Yusheng et al., 2000), different researchers have different interpretations of geochemistry, but there are two main understandings: one is that he was formed in the mid-ocean ridge and belongs to oceanic ridge basalt (Pan Yusheng et al., 2000; Yang Shufeng et al.,1999); Secondly, it is considered that it was formed in the island arc environment (including the ocean inner arc, see Yuan Chao et al., 2002; Zhou Hui et al., 2003; Xiao et al., 2002b). Because Yuan Chao et al. found high-magnesium andesite in some Kegou, and some basalts are rich in large ion elements (such as rubidium, barium, strontium, thorium, etc. ) and the relatively low initial value of nd, it is unlikely that some Kegou basalts were formed in the mid-ocean ridge. According to the sedimentary characteristics of flysch in the upper part of the profile, it is basically consistent with the flysch assemblage in island arc zones (such as pre-arc or inter-arc basins).
Three. discuss
Based on the above analysis, we draw the following conclusions: ① Di Ku suture zone is composed of ultramafic rocks in Di Ku, basalt in Issac Group, turbidite and Precambrian gneiss; ② The ultramafic rocks in Di Ku are distributed in the suture zone in the form of fragments, and their formation age is not later than 525Ma, which may represent a part of dismembered ophiolite (accumulation body); ③ The geochemical characteristics and age of some Kegou basalts and flysch in their upper sedimentary strata (about1000 ma later than ultramafic rocks) do not support them to form a complete ophiolite assemblage with ultramafic rocks * * *, the Issac Group represents an arc volcanic-sedimentary rock assemblage, and the ultramafic rocks in Di Ku belong to the product of the mid-ocean ridge; ④ Gneiss in suture zone was invaded by nearly 800Ma rock mass, and the protolith may have formed in Mesoproterozoic (early). Ar-Ar plateau ages of amphibole and biotite in gneiss confirm the existence of early Paleozoic thermal events in Di Ku area (Zhou Hui et al., 2003), indicating that they are Precambrian tectonic stratigraphic detritus involved in the early Paleozoic orogenic belt; ⑤ The existence of Di Ku suture zone indicates that there is an ocean basin between the metamorphic terrane of West Kunlun and the southern margin of Tarim Craton. When the western Kunlun metamorphic terrane was combined with the Tarim Craton, that is, after the ocean basin was closed, ophiolite, arc volcanic sedimentary rocks and their Precambrian structural fragments remained during the closing of the ocean basin.