Origin And Tectonic Setting Of Late Paleozoic-Early Mesozoic Granites In Daqingshan District, Inner Mongolia

The Late Paleozoic - Early Mesozoic is an extremely important time for global geological evolution. A supercontinent, Pangea, is commonly recognized, that it was formed in this period along with the splicing of Laurasia in the north and Gondwana in the south. The occurrence of Mid-Carboniferous collision of Laurania and Gondwana in Hercynian is the mark for this period. The collision led to the formation of the supercontinent in Late Triassic. To Laurasia, the splicing procedure of Siberian Plate North China Plate attracts the extensive attention to geologists for a long time. The most reliable evidence for the splicing of the two plates has been especially provided by the discovery of Radiolaria from deep sea in Guapulu epoch of Permian near Xilamulun River tectonic belt. Nevertheless, the researches, which are conducted by the majority of scholars, on the splicing procedure of the two plates focus mainly on the aspects of stratigraphy, palaeoecology, petrotectonics, and tectonics of Xilamulun River and Erlian-Hegenshan tectonic belts as well as their surrounding regions. There are few studies on the geological events, which took place within the North China Plate or in the splicing belt of the two plates, and specially lack of systematically researching achievements.The author chose the mid-northern edge of the North China Plate as the study area and Late Paleozoic-Early Mesozoic granites in Daqingshan area as the research objects. Under the guidance of the tutor, the author conducted a field geological survey and systematically collected lithogeochemical and isotopic dating samples based on the corresponding previous geological data. By a comprehensive study, the Late Paleozoic-Early Mesozoic granites in Daqingshan district were classified into the following six patterns: Early Permian Aguigou magmatic body, Mid-Permian Laoyinhada magmatic body, Late Permian Halaheshao magmatic body, Late Triassic Taolegai magmatic body, Late Triassic Gechoushan magmatic body, and Late Triassic Shadagai magmatic body. The formation ages of the different types of the magmatic bodies were determined by applying azurite SHRIMP or TIMS dating methods. In accordance with the lithogeochemical data, the author analyzed the genesis of the different types of the magmatic bodies and the tectonic setting of the North China Plate and the Siberian plate during their splicing procedure. The characteristics for the different patterns of the magmatic bodies are as follows:Aguigou magmatic body consists of gabbro, diorite, quartz diorite, and granodiorite. Its feature is that the body is rich in mafic inclusions. All the rocks of the body contain amphiboles. The formation age, determined by azurite SHRIMP, is 284.5±2.9Ma or 283.7±3.7Ma for the quartz diorite, and 281.1±3.4Ma for granodiorite. Thus, it is Early Permian for the body age. The lithogeochemical feature analysis indicates that the body obviously has a trend of calc-alkalic evolution, which is time-continuous, characterized by that SiO2 is negatively related to FeOT, MgO, CaO, Al2O3, TiO2, and MnO, while positively related to Na2O, and K2O, and that the rock is rich in CaO. The body is characterized by moderate lanthanon concentration, LREE enrichment, HREE depletion, and with unobvious europium anomaly. Of microelements, LILE, such as Rb, Ba, and HSFE, including Nb, Ta, Zr, and Hf, are positively related to SiO2 and K2O in content, while transitional elements including Co, Ni, V, and Sc are negatively related to SiO2 and K2O. It is clearly deficient in Nb, and Ti, while rich in Ba. The genesis of the body belongs to I-type granite according to I-, S-, M-, and A-type granite classification system. Its basic features are similar to that of ACG proposed by Barbarin (1996, 1999). The tectonic setting is CAG, which is proposed by Maniar and Piccoli (1989).Laoyinhada magmatic body comprises fine biotite monzonitic granite and porphyritic biotite monzonitic granite. The azurite SHRIMP age is 272±4Ma for the fine biotite monzonitic granite. Thus its formation age is Early Permian. The lithogeochemical feature investigation indicates that the body is rich in silicon, potassium, and sodium while deficient in iron, magnesium, calcium, titanium, and manganese. It has a trend of calc-alkalic evolution, characterized by that SiO2 is negatively related to FeOT, MgO, CaO, Al2O3, TiO2, and MnO, and while positively related to Na2O and K2O. The body is rich in well fractionally distilled LREE while deficit in low differentiated HREE, and with relatively weak negative europium anomaly. Microelement analysis shows that the body is obviously deficit in Nb, Ti, and P while rich in Ba, La, and Zr. The genesis of the body is I-type granite according to I-, S-, M-, and A-type granite classification system. Its basic features are similar to that of KCG proposed by Barbarin (1996, 1999). The tectonic setting is CAG, which is proposed by Maniar and Piccoli (1989). Halaheshao magmatic body is a group of medium-coarse biotite-bearing monzonitic granites and large porphyritic-bearing monzonitic granite. The azurite TIMS age is 260±0.5Ma for the biotite-bearing monzonitic granite. The formation age is Late Permian. Its geochemical characteristics show that silicon and alkali are well concentrated, while aluminium, iron, magnesium, and calcium are less concentrated. The contents of SiO2, Al2O3, and K2O+Na2O vary over a range as wide as from 73.61% to 76.81%, 12.63% to 13.7%, and near or more than 8.5% (the lowest content is 8.44%), respectively. These features reflect that the granite belongs to the weak peraluminous calc-alkali series. The body is rich in well fractionally distilled LREE while deficit in not obviously differentiated HREE, and with negative europium anomaly. It is rich in Rb, U, Th, Nb, and Ta while deficit in Sr, Ba, Co, Cr, Ni, V, and Sc. The body belongs to A2 sub-class, proposed by Eby (1992), which is formed at the tectonic environment of post-collision, or is non-orogenic rapakiwi granite, or PA-type granite classified by Hong (1995). The tectonic environment belongs to post-orogenic granites.Taolegai magmatic body consists of medium-fine granite, medium-coarse granite, porphyritic-bearing granite, and fine granite. The combination of the host minerals is potash feldspar+plagioclase+quartz+biotite±muscovite±garnet. The zircon Shrimp age is 224±3Ma for medium-coarse granite. Thus the formation age is Late Triassic. Silica and aluminum are well concentrated in the rocks. Index A/CNK is between 1.01 and 2.11. Therefore, the rocks belong to peraluminous rocks, and the most of them are strong peraluminous rocks. However, the geochemical contents are not evenly distributed in the magmatic body. The rocks are commonly low concentrated in iron, magnesium, and calcium, but they can be still classified into the two classes according to the relative contents of FeOT and MgO. One class is well concentrated in iron and magnesium, and the other is low concentrated in iron and magnesium. The class one is marked by higher contents of iron, magnesium, REE, Ba, and Sr, while relatively lower contents of Rb; and the class two is marked by lower contents of Fe, Mg, REE, Ba, and Sr, while relatively higher content of Rb. Its genesis is light color granite co-occurred with muscovite peraluminous granites (MPGs). The tectonic environment belongs to post-orogenic granites (POG).Gechoushan magmatic body is medium-fine monzonitic granite, a kind of typical muscovite granites. Its formation era is Late Triassic. Main rock-forming oxides are characterized by well concentrated silica, aluminum, and alkali while low concentrated Fe, Mg, Ca, and Ti. REE features show that both LREE and HREE are badly differentiated. LREE do not fractional distillate at all while HREE fractional distillate to some extent. There is a very clear negative Eu anomaly.δEu is extremely low. The value is from 0.006 to 0.03. The trace elements are characterized by high content of Nb and Rb but low content of Ba and Sr. Its genesis belongs to S-type granite according to the I-, S-, M-, and A-type granite classification system. According to Barbarin's classification strategy (1996, 1999), it is a muscovite-bearing peraluminous granite (MPG). The tectonic environment belongs to post-orogenic granite, that is, belongs to Sylvester's (1998) post-collision strong peraluminous granite.Shadegai magmatic body is mainly composed of biotite granites. The zircon TIMS age is 211.2±0.7Ma for medium-coarse biotite granite. Therefore, its formation age is Late Triassic. Being rich in silicon, aluminium, and alkali while deficit in iron and magnesium is the highlighted lithogeochemical feature of the body. The contents of SiO2, Al2O3, and K2O+Na2O range from 68.8% to 73.45%, 13.52% to 14.96%, and 7.95% to 9.88% with K2O>Na2O, respectively. It belongs to potassium-bearing calc-alkali rock series. It is rich in well fractionally distilled LREE, deficit in indistinctively differentiated HREE, and with negative europium anomaly. Trace elements Nb, Sr, P, and Ti are badly concentrated but Rb, Th, Hf, etc. are well concentrated in the rocks. The tectonic setting belongs to post-orogenic granites.From the above, a conclusion can be drawn that the Late Palaeozoic-Early Mesozoic granites in the study area basically reveal all the magmatic events occurred during the whole procedure from Late Palaeozoic orogeny, to post-orogeny, and to intracontinental orogeny in the north edge of the north China plate. Early Permian Aguigou magmatic body consists of magmatic arc granite, formed in the continental edge in the early period of the middle Asia ocean plate subduction. Mid-Permian Laoyinhada magmatic body belongs to magmatic arc granite, formed in the continental edge in the late period of the plate subduction. Late Permian Halaheshao magmatic body is composed of granite formed in the colluvial period of a post-orogeny after two tectonic plates have collided and spliced. Late Triassic Taolegai, Gechoushan, and Shadegai magmatic bodies belong to granites formed by intracontinental nappes after Lausasia land is generated.