| As an important part of the continental crust, granite is one of the symbols which is different from the ocean crust. Its formation is closely related to the growth of the continental crust. As the most active location of the continental crust, the activity of the orogenic belt is violent. It is also a good area of granite development. As the segmentation unit, the Eastern Kunlun orogenic belt is the part of central orogenic system. It has a long complicated geologic evolution history. Multiphase tectonic magmatism make it to constitute a huge tectonic-magmatic belt. The Caledonian and hercynian are main periods of magmatism. It has rich mantle magma underplating and migmatization. The Eastern Kunlun magma belt is a good study place for crustal mantle action and crustal growth.The study area is located in Heihai region, the southwest of Geermu. It belongs to the southern subduction complex wedge. Through field geological survey, we found abundant peraluminous granites in this area. Many methods will be carried out to finish the comprehensive research, such as petrography, major elements and trace elements analysis, electron microprobe analysis(EPMA) and zircons LA-ICP-MS U-Pb analysis. With the regional geological data, This paper will discuss the early Paleozoic formation and evolution of the southern subduction complex wedge and provide some related geological basis to study the early Paleozoic crust growth and Tectonic evolution.(1)The Caledonian peraluminous granites is mainly composed of biotite tonalite, biotite granodiorite and biotite granite, two-mica granite and muscovite granite, intruded into the Silurian Saishiteng group. Outcrop characteristics showed white muscovite granite is the latest intrusions. The Silurian Saishiteng group contain metasandstone, cordierite Sandstone and cordierite schist. Peraluminous granites mineral composition are simple, mainly composed of plagioclase, potassium feldspar, mica and quartz. The mica types and accessory mineral species are different between them. The micas of Biotitetonalite are biotite. Mainly, the micas of Biotite granodiorite are biotite.They also have needle rutile. Muscovites are small and often occur in edge of biotite. Biotite granite’s mica are composed of biotite and muscovite, the content of biotite is slightly higher than muscovite. Biotite’s crystal shape is good than muscovite. In two mica granites, the contents of biotite and muscovite are similar, with similar size and shape. Muscovite granites have muscovite. In addition, muscovites have rich aluminum garnet. According to the degree of particle size and the shape, they can be divided into two classes. One has good shape and small particles, another has poor shape, crack and bigger grain.(2)On the composition of major elements, tonalite and granodiorite have relatively low Si, high Ti,Mg,Fe,Mn, high Al, high Ca,P, low K,Na Biotite granite, two mica granite and muscovite granite have relatively rich silicon, aluminum, low iron, manganese, low sodium and high potassium phosphate. All rocks belong to calc-alkaline-high potassium calc-alkalineseries. M ost A/CN K are greater than 1.1. Cal cul ati on standard mi neral corundum content isgreater than 1%. They are strong peraluminous granites, belongs to S type granite. Muscovite granite is leucogranite. The main elements characteristics suggest that source magma is aluminum. On the composition of rare earth elements, from tonalite to white mica granite, ∑ REE gradually reduce. They have similar chondrite standardized distribution curves, relative enrichment in LREE and loss of HREE. Heavy rare earth fractionation is not obvious Generally, they have moderate Eu negative abnormality. On trace element compositions, Standardization of the primitive mantle curves are right deviation, with relative enrichment of Cs, Rb, Th, K and relative loss of Ba, Sr. Nb and Ti losses significantly, Rb/Sr ratios show increased trend, low Sr/Y ratio are similar to leucogranite, Nb/Ta ratios are close to the shell source rocks.(3)Electron microprobe analysis results show that:Plagioclases have Ab=67.74%-97.27%, An=2.46%-31.96%, Or=0.16%-1.56%, the main species are albite-oligodase. They are acidic plagioclase. Among them, An of biotite granites and some two-mica granites plagioclase are slightly higher than that of muscovite granites and tonalites. Potassium feldspars have Or =91.901%-97.916%, Ab=2.37%-7.829% and extremely low An. The perthites of biotite granites are orthoclase-perthites. Biotites have high iron. The trend of Al2O3 and TiO2 contents from granodiorites to two-mica granites reflects diagenetic temperature decreases. The characteristics of TiO2/MnO ratios and Mg/(Mg+Fe) ratios indicate that biotites are magmatogene. Muscovites have high Al, K, Fe and low Mg, Ti, Na garnets are almandine (Alm)+spessartine (Spess), They all have Compositional zonation. From core to edge, One has decreased almandine (Alm) and increased spessartine, Another has oscillatory zone. The characteristics of rich Mn and high Al in host rocks showed that garnet may crystallize from peraluminous magma, under the low pressure.(4)The magmatic zircon LA-icp-ms U-Pb age of Muscovite granite is 420.4±3.5 Ma (MSWD=0.025), mica granite magmatic zircon LA-icp-ms U-Pb age was 424.0±2.6 Ma (MSWD=2.0),421.5±4.4 Ma (MSWD=2.6). Dating results show that peraluminous granites in Heihai region formed in the late Caledonian. The age of Muscovite granite (420.4±3.5 Ma) represents the upper limit of the magmatism event.(5)The characteristics of Hf isotope of zircon indicate that It had occurred mantle upplating during Late Silurian Epoch. Not only mantle had supplied the heat source, but also it had participated in the formation of juvenile crust.(6)The TZr of Biotite tonalite and biotite granodiorite are 827-861℃ and 836℃, respectively, slightly higher than the biotite granite’s TZr (772-826℃) and two-mica granite’s TZr(755-835℃), muscovite granite’sTZr(647-724℃) are the lowest. The temperatures gradually reduce, which may be associated with degree of source heat, From tonal ite to muscovite granite, source area melting heat supply gradually decreased, suggesting that tectonic-magmatic activity intensity gradually weakened. Two feldspar and quartz eutectic pressure(0.1-0.2 Gpa) is low. The characteristics of trace element show that the formation pressure is medium-low. It represents the thicker-normal crust became thinning. Source is likely a mixed source with graywacke and argillaceous rock. Partial melting of the source area are mainly induced by muscovite dehydration, under the condition of no water.(7)Discriminant analysis show that the peraluminous granites are similar with volcanic arc granites and collision granites, probably formed in the active continental margin environment. Combination with regional geological evolution data, this paper conclude the possible mechanism:in the environment of the southern oceanic plate retreat, with the mantle underplating, the lower crust produced different degrees of partial melting. With the decreased temperature, partial melting of the crust formed the biotite tonal ite-white mica granite series. Biotite tonalite and biotite granodiorite might formed in the initial stage. Two-mica granite and biotite granite may represent the the maximum extension. As the latest intrusion, Muscovite granite represents that the block retreat might stop, and the next oceanic crust subduction might start. In conclusion, during the late Silurian, the study area might be in the stage of the tectonic switch setting which changed from compression to extension. This study may provide new geological basisesabout crustal growth and tectonic evolution. |
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