1. Ordovician
During this period, volcanic rocks were distributed in the area of Polokolo Mountain and exposed in Nerengeledaban Group of Middle Ordovician. In the early stage of volcanic eruption, basalt was dominant, and the eruption cycle was obvious. From the profile of the field Guozigou, there are at least four eruption cycles (see figure 1-4). A set of acid-base volcanic rocks and pyroclastic rocks containing limestone are exposed in Xikeguqin Mountain. Volcanic rocks are characterized by low alkali bimodal combination. According to the graphic analysis of quantitative classification of volcanic rocks by Ding Ganjun et al. (1990), it is divided into calc-alkaline basalt and alkaline basalt areas, which shows that alkaline series and tholeiite series coexist. All the above volcanic rocks are associated with deep-sea clastic rocks on the continental margin, indicating that the Ili Ocean opened in Ordovician and entered the stage of magmatic passive continental margin volcanic activity. The Ordovician magmatic passive continental margin can be explained by the continental lithospheric extension model proposed by who (1994) (Figure 2- 1). According to the time and environment when the continental crust started magmatism in the extension stage, magmatism can start from the basic rock assemblage of alkaline series, or from the intermediate basic rock assemblage of calc-alkaline or calc-alkaline series, and evolve to bimodal assemblage in the middle period. Due to the expansion of the ocean, a detachment zone is formed in the depth of the ocean floor, and the magmatic area of the underlying asthenosphere uplift can shift to the continental margin, forming a passive continental margin of magma in this area. The characteristics of Ordovician volcanic-sedimentary assemblage and its tectonic setting in this area show that the Ordovician volcanic belt in Polokolo has typical characteristics of magmatic passive continental margin.
Figure 2- 1 Schematic diagram of passive continental margin structure model of Ordovician continental lithosphere extension in this area.
2. Silurian
Silurian volcanic rocks are distributed in Polokolo area and Nalati area. Along the east Tianshan-Beishan structural belt in Gansu Province, Silurian volcanic activity is relatively strong. The tectonic background of volcanic rocks is also varied, including continental volcanic arc type, island arc type, ophiolite (ocean) type and back-arc basin type. According to the structural pattern analysis of Polokolo and Nalati in this area, the former is of continental margin volcanic arc type and the latter is of volcanic island arc type.
(1) Polokolo shelf volcanic arc: The volcanic rocks in this area are distributed in a beaded shape along the continental margin of the northern margin of the Ili ocean basin, which is the result of the steep subduction of the oceanic crust plate to the northern margin. Therefore, there is no back-arc basin on the north side of the volcanic chain, but a shelf shallow sea. Early Silurian volcanic rocks were mainly intermediate-acid dacite pyroclastic rocks, which turned into basalt and andesite eruption in the middle period and ended in the late andesite lava mixed with pyroclastic rocks. The whole eruption is in the order of intermediate acidity → basic → neutral eruption. It reflects the volcanic suite formed by mantle-derived and crust-derived mixed magma on the continental crust basement, in which pyroclastic rocks are developed and have the characteristics of calcium-alkaline combination. The distribution curve of rare earth elements is right-leaning light rare earth enrichment type, which is consistent with the curve of high-potassium andesite in orogenic belt (Ding Ganjun, 1990). According to the characteristics of petrochemistry and structural framework in this area, SILURIAN volcanic rocks are defined as continental volcanic arcs.
(2) Nalati Island Arc Volcanic Rock Belt: Only Late Silurian volcanic-sedimentary rock series were found in this belt. The volcanic sequence is tuff and tuffaceous sandstone → andesite → andesite volcanic breccia → andesite porphyrite. Volcanic rock series contains multi-layer shallow sea limestone containing coral. The above volcanic rocks were formed in the shallow island arc environment with the continental crust as the basement when the Ili oceanic crust plate subducted southward.
(3) Bayinbuluke island arc volcanic belt: The island arc volcanic belt is exposed on the southern slope of Daban, Huladudun. The age is Late Silurian, with layered andesite tuff in the early stage and andesite basalt and breccia basalt in the late stage. According to the research of Wang Zhiliang (2004), the volcanic rock series belongs to calc-alkaline series and island arc volcanic rock series. Island arc volcanic rocks were formed in the back-arc micro-ocean basin of South Tianshan. When the volcanic arc on the oceanic crust of the southern Tianshan micro-ocean basin dived northward, it was spliced into the Wusun-Aura microplate suture zone. Volcanic rock series experienced late hydrothermal process, with obvious copper mineralization and many small copper deposits with high grade.
(4) The micro-ocean basin in the Huolashan-Kule Rift: located in the marginal sea of the southern Tianshan Mountains, the continental crust cracked due to local stretching in the Middle Silurian, and explosive volcanic breccia appeared in the early stage (Ⅲ-5 plate), mainly pillow basalt (Ⅲ-6 plate) in the middle and late stage, with purple radiolarian siliceous rocks and radiolarian fossils above the pillow lava. The representative species are golden dragon with five claws (D3) and common golden dragon with five claws (C 1) (Jun Gao et al., 1996), etc. (Figure 2-2). Long et al. (2006) measured the age of gabbro by zircon SHRIMP U-Pb method, belonging to Middle Silurian. According to Feng Xinchang's (2005)1∶ 250,000 regional survey and mapping, it is found that the Yeyungou Formation of the Lower Carboniferous was not integrated on ophiolite, so the ophiolite age was determined as Middle Silurian to Devonian.
Fig. 2-2 Schematic diagram of pillow lava and siliceous rock interlayer in S2-D ophiolite in Kule Lake area.