The Research Of The Paleozoic Ophiolites And Volcanic Rocks And The Tectonic Evolution In The Beishan Area(Northwest China)

Beishan is located in the South margin of CAOB(Central Asia Orogenic Belt), the research of Paleozoic Tectonic evolution has become the hot issues in this area. The study of ophiolite and volcanic rocks are very important part of tectonic unit division and tectonic evolution, however, there is a significant dispute about the chronology and tectonic setting. The method of chronology and petrogeochemisry was used for the research of structural attribute, Formation age, tectonic setting of the several ophiolites belt and tectonic setting of volcanic rocks in this article. In addition, on the basis of existing research, the Early Paleozoic ocean-land transition and tectonic evolution history of Beishan was retrieved and reconstructed by the present author.There are four ophiolite belts(From north to south: Hongshishan-Baiheshan-Pengboshan, Jijitaizi-Xiaohuangshan, Hongliuhe-Niujuanzi-Xichangjing and Huitongshan-Zhangfangshan) which are nearly in E-W direction in Beishan region. The author conducted the chronological study of zircons from gabbros for the several ophiolite belt by means of LA-ICP-MS U-Pb, the weighted mean ages were obtained as following 446.1±3.0 Ma(Huitongshan ophiolite), 362.6±4.0 Ma(Zhangfangshan ophiolite), 446.5±4.0 Ma(Niujuanzi ophiolite), 321.2±5.7 Ma(Jijitaizi ophiolite) and 346.6±2.8 Ma(Hongshishan ophiolite); Analyses of volcanic rocks geochemistry show that the Huitongshan-Huaniushan, Hongliuhe-Niujuanzi-Xichangjing and Hongshishan-Baiheshan-Pengboshan ophiolite belts belong to the type of MORB. The limited ocean basin which represented by Hongliuhe-Niujuanzi-Xichangjing ophiolite belt was formed in the Early Paleozoic without the significance of subdivision of plates. The other ophiolite belts(Zhangfangshan, Jijitaizi-Xiaohuangshan and Hongshishan-Baiheshan-Pengboshan) are the product of the rift extention in Late Paleozoic, which are the geological record of the embryonic ocean crust.There are a lot of the Paleozoic volcanic-sedimentary rocks outcropping in Beishan area, Ordovician volcanic rocks are outcropped in Huaniushan and Xichangjing in southern Beishan, Yihatuoli and Xirehada in northern Beishan; Silurian volcanic rocks are most widely exposed in Gongpoquan and Dongqiyishan area; the Devonian volcanic rocks that is the key period of ocean-land transition are mainly exposed in Dundunshan area. The followings are the main achievement by means of petrogeochemistry:(1) the author consider that volcanic rocks in this area mainly belong to tholeiite series, the samples collected from Huaniushan and Xichangjing are the part of the ophiolite, the samples from Yihatuoli and Xirehada are enriched in LREE, flat in HREE and without obvious Eu anomaly in the chondrite-normalized REE pattern, and slightly depleted in HFSE(Nb, Ta, Ti), which are similar with those of the island arc volcanic rocks. However, they have ratio values of(Th/Nb)N>>1 and Nb/La﹤1, as well as Zr/Nb similar to the N-MORB, and no depletion in HFSE(Nb, Ta and Ti), which suggested that the volcanic rocks were contaminated by the crust. Combined with the research of tectonic evolution in this area, the author concluded that Ordovician volcanic rocks in Yihatuoli and Xirehada formed at a extensional rifting setting with the plate;(2) the samples collected from Silurian Gongpoquan Group mainly belong to tholeiite series, except Dongqiyishan samples which belong to calc-alkaline series. The volcanic rocks are chatacterized by enrichment in LREE and LILE(K,Rb and Ba),and have negative anomaly in Nb, Ta, Ti(especially Yihatuoli), which suggest a characteristic of volcanic rocks related to subduction. PositiveεNd(t)and lower(87Sr/86Sr)t values(0.70340~0.70519)suggest that both fluid-sediments and silicate melts derived from subducted oceanic slab are involved in the genesis of basalt in Gongpoquan Group.(3) the samples collected from Devonian Sangejing Formation mainly belong to calc-alkaline series, and Devonian Dundunshan Group belong to tholeiite and calc-alkaline series. The volcanic rocks of Sangejing Formation are characterized by enrichment in LREE, depletion in HREE and negative anomaly in Eu slightly, Dundunshan Group are characterized by stronger enrichment in LREE and more obvious negative anomaly in Eu than Sangejing Formation. The volcanic rocks of Dundunshan Group were formed at 367 Ma±10 Ma, corresponding to Late Devonian. In addition, the volcanic rocks from Devonian Sangejing Formation have a positive εNd(t)which suggest a depleted mantle-related source, but the volcanic rocks from Devonian Dundunshan Group have negative εNd(t)which sugggest a crust or enriched mantle source. Combined with the tectonic setting discrimination diagram with trace element, sedimentary characteristics of Sangejing Formation and Dundunshan Group, and the research of ophiolite, it’s suggested that Sangejing Formation formed in an island arc setting owing to the southward subduction of Beishan limited ocean, whereas the Dundunshan Group were produced by rift extension and entered into a new phase, i.e.d Late Paleozoic intraplate extention.Based on the existing research and this study, the Paleozoic tectonic evolution history of Beishan was retrieved and reconstructed in this paper. In other word, Beishan experienced a history of Paleozoic tectonic evolution from supercontinent breakup and ocean-continent transition, to intraplate extention and rift basin development. The real stage of intracontinental evolution started at Early Permian.