Post-collisional Magmatism In Sawu'er Region, West Junggar, Xinjiang, China

Post-collision can be defined as: Continuous plate convergences will result in intracontinental thrusting, wrench structure, the block transverse compression or extention, or the inner-plate tectonic environment of significant block lateral movement, magmatism and the horizontal accretion, and vertical accretion of continental crust along horizontal direction following gigantic shear zone after major oceans shut down. Post-collision is a plate interaction which has taken place in a period between after the major orogenic movement and before the post orogeny (the tectonic environment of inner-plate). Post-collision is an important stage of the whole orogenic cycle in plate tectonics, while post-collisional magmatic activities (including intrusive and extrusive magmatism) are important tectonic events. The study on post-collisional magmatic activities has very important meanings for both clarifying the integrity of plate tectonic activities and establishing relationships between magmatic rocks and plate tectonics.Sawu'er region is located in West Junggar, Jimunai County of Altay District and Hefeng County of Tacheng District, Xinjiang of China. It is along the north margin of Kazakstan-Junggar plate. Based on the field study, this work has carried out macroelements, microelements, rare earth elements, isotope geochemical analysis and isotope precise dating of the magmatic rocks in Sawu'er region, as well as studies on the rules of Permian volcanic-intrusive rocks in Sawu'er Region, including the geological characteristics and spatial and temporal distributions of magmatic rocks, petrology, petrochemistry, geochemical characteristics and geodynamic backgrounds etc. The forming tectonic environment of both the late Paleozoic intrusive rocks and the Permian volcanic rocks and their implications to the evolution of the Central Asian Orogenic Belt are discussed so as to furnish forceful supports for the reseach of Palaeozoic two-dimensional crustal growth in the Central Asian Orogenic Belt and the continental dynamic background.The post-collisional magmatic rocks of late Paleozoic is widespread in Sawu'er. The intrusions formed from 328.2 Ma to 290.7 Ma (from late Carboniferous to early Permian). The geochemical evolvement characteristics of the intrusions from early to late (Sentasi intrusion, Wokensala intrusion→Tasite intrusion→Ka'erjia intrusion→Kuoyitasi intrusion, Qiaqihai intrusion) are obviously. The volcanic rocks in Haerjiawu Group and Kalagang Group formed from 296.7Ma to 280Ma (early Permian). The volcanic rocks in Ha'erjiawu Group have weak bimodal characteristics and those in Kalagang Group have typical bimodal characteristics, indicating that they formed in an extensional setting. Based on the time frame, geochemical evolvement characteristics and tectonic setting of the late Paleozoic magmatic rocks, we think that the Sawu'er region was in a post-collisional setting from late Carboniferous to early Permian, that evolved from an compression-extension translational stage to an extensional stage gradually.The intrusions in the region consist of I granite and A ones, in which Qiaqihai and Kuoyitasi intrusions are important in the region with the characteristics of A-type granite, they belong to A2-type granite that were formed in an extensional tectonic setting of post-collision. The REE chondrite-normalized patterns of the intrusions show LREE enrichment and theδEu values are lower. The Nd, Sr, Pb isotope compositions of the intrusions indicate a mantle source, while the lowδ¹⁸O values resulted from the isotope exchange with meteoric water. According to SHRIMP U-Pb age dating results, the crystallization age of Qiaqihai intrusion is 290.7±9.3 Ma(1σ),and that of Kuoyitasi intrusion is 297.9±4.6 Ma(1σ),corresponding to the beginning of early Permian.The A₂-type granites indicate that the region was in the extensional period of the post-collisional stage at the beginning of early Permian in the Sawu'er region. The A-type granites in the Sawu'er region in west Junggar discovered from this work is analogous to the A-type granites found in east Junggar of the Ulungur alkali granites belt. The confirmatiom of post-collisional A-type granites of early Permian in the Sawu'er region provides new evidence for the regional vertical continental crust growth in early Permian. The former proposed Ulungur alkali granites belt may extend from the east Ulungur through the west Ulungur and to the Zhaisang in Kazakstan westwards.The I-type granites consist of Sentasi, Wokensala and Tasite intrusions. According to the SHRIMP U-Pb age analysis results, the crystallization age of Sentasi intrusion is 328.2±5.7 Ma(1σ) and that of Wokensala intrusion is 323.8±6.2 Ma(1σ). According to ⁴⁰Ar/³⁹Ar age analysis result, the crystallization age of Tasite intrusion in the Sawuer region is 313.6±3.2 Ma(1σ), which is corresponding to the productions of late Carboniferous. The intrusions are the partial alkali rock, whose REE distribution patterns are of the LREE rich type andδEu values in Sentasi, Wokensala intrusions are lower, whileδEu values are middle. The Nd, Sr, Pb isotope compositions of three intrusions indicate a mantle source, while theδ¹⁸O values are normal. These intrusions belong to post-collisional granite, indicating that the Sawu'er region indeed as well as the west Junggar was in the post-collisional setting in late Carboniferous. They might form in the extension period or from compressional to extensional period during the post-collisional stage.The Kaejiao intrusion in the Sawu'er region is mainly albite granite porphyry, potassium feldspar granite porphyry, it belongs to the transitional type of granite. The intrusion formed at the telophase of late carboniferous as its SHRIMP U-Pb zircon age is 302.6±7.6Ma(1σ) .The intrusion is the partial alkali, whose REE distribution patterns are of the LREE enrichment type,δEu value is low and Nd,Sr,Pb isotope reflects a mantle source characteristic. The O isotope of intrusion is low for the isotope exchange with meteoric water. Diagenetic distinguishing shows it formed in compressing structuring setting of the late orogenic period (post-collisional phase).The characteristics of A-type and I-type granites in the Sawu'er region are obviously different, because the A-type and I-type granites formed at different stages of the post-collisional period. The Sentasi intrusion and Wokensala intrusion are the I type granites that formed at the late of the post-collisional period, and there was compressional stress. The characteristics of rare earth elements show that the fractional crystallization of the I-type granites is indistinct, and the sources of the I-type granites come from the mantle. The Kuoyitasi intrusion and Qiaqihai intrusion are the A-type granites, they were formed at the end of the post-collisional period in tensional setting. The characteristics of rare earth elements show that the fractional crystallization of the A-type granites is distinct. The sources of the A-type granites came from the mantle but contaminated by the crust.The Nd,Sr,Pb isotope characteristics of the I-type granites are similar to those of A-type granites. The I-type and A-type granites came from the mantle magma. They were the results of different evolusional phases of the same magma source. The O isotope and the REE characteristics of the I-type and the A-type granites were distinctly different. The A-type granites in the west of Zhungaer Sawu'er region should belong to the Ulungur alkali granites belt in East Zhungaer.The volcanic activities occurred in Sawu'er region during the middle Devonian to early Permian. Volcanic rocks are in the Haerjiawu Group and Kalagang Group of early Permian. The Haerjiawu Group consists of a series of basic to neutral continental volcanic rocks and pyroclastic rocks, and the Kalagang Group consists of a series of basic to acidic volcanic rocks and pyroclastic rocks. The volcanic rocks include olivine basalt, basalt, dolerite, andesite, trachyandesite, rhyolite, pyroclastic rock and so on. According to the characteristics of section, lithology and lithfacies, the Permian volcanic activity in Sawu'er Region can be grouped to five phases from early to late, neutral volcanic activity phase, intermadiate basic volcanic activity phase, acidic volcanic activity phase, small-scale intermadiate neutral volcanic activity phase and basic volcanic activity phase. The Permian volcanic rocks belong to continental volcanic rocks and have the characteristics of typical bimodal volcanic rock characteristics. This study indicates that they formed in an extensional tectonic setting.Based on the analysis of the REE's geochemical characteristics of the volcanic rocks in the Sawu'er region, the correlativity of REE and major elements and diagrams of covariant relation of REE, and the research results of the regional geological setting, petrology and petrochemistry characteristics, it is inferred that the mechanism for formation of Premian basic and intermediate volcanic rocks is mainly the equilibrium partial melting, and the magmatic fractional crystallization is not significant, while the rock-foming processes of intermediate-acidic volcanic rocks were influenced by both the partial melting and the fractional crystallization. The REE geochemical research has provided the important evidence for the mechanism of magmatism of the Premian post-collision stage and the geodynamical evolution process.