| Granitoid is the chief component of continental crust,and its formation is closely related to the tectonic setting and geodynamic background.South China region is widely distributed in different ages of granite,is one of the world's rare granite province.The Early Paleozoic granites are closely related to the Wuyi-Yunkai orogenic movement,and the study on the tectonic-magmatic activity of Early Paleozoic in South China is still relatively weak.The Maixie pluton is located in the Xingan County,NW Jiangxi Province,and the tectonic position is located in the east of the Jiangshao fault zone and in close proximity to the center of Wuyi-Yunkai orogenic belt.In this paper,the petrography,mineralogy,zircon U-Pb chronology,main and trace element geochemistry and Sr-Nd isotopic geochemistry of the Maixie pluton biotite granodiorite are systematically studied.Systematically discussed its petrogenesis and tectonic setting.And provide new evidence for further study on the Early Paleozoic tectonic-magmatic evolution history of South China.Maixie pluton is predominantly composed of fine-to medium-grained and equigranular biotite granodiorite with light gray colour,granite texture and massive structure.Rock-forming minerals are quartz,feldspar,plagioclase,biotite and a small amount of fine granular amphibole,the accessory minerals are zircon,apatite,sphene and Fe Ti oxides etc.The biotite in biotite granodiorite is the origin of the primary magma,which is mainly composed of magnesio-biotite and ferri-biotite.The FeOT content of biotite ranged from 15.61 to 21.27%,with an average of 20%;and the content of MgO ranged from 7.74% to 10.88%,with an average of 9.79%;Fe2+/?Fe2++Mg?=0.380.54,with an average of 0.49.According to the calculation formula of the crystallization pressure of biotite Al,the crystallization pressure P=233381MPa,with an average of 293 MPa,corresponding to solidification depth H=8.814.4km,with an average of 11.1km.Biotite Ti thermometer show the crystallization temperature of biotite is 494671?.According to the composition of biotite,the oxygen fugacity in the process of diagenesis is low,ranging from-25.6 to-18.2,with an average of-20.6.LA-ICP-MS Zircon U-Pb dating yields weighted mean 206Pb/238 U ages of 433.4±7.1Ma?MSWD=0.82?and 431.8±6.0 Ma?MSWD=0.43?for the Maixie pluton,indicating that it was emplaced in the early Paleozoic,in the peak of the early Paleozoic granites crystallization age in Southern China,lagging behind the peak period of metamorphism of Wuyi-Yunkai orogeny?before 445Ma?.The results show that it may not be formed under the syn-collision tectonic setting.The Maixie granodiorites are characterized by high silicon?SiO2=62.0%71.43%?,high potassium?K2O/Na2O=0.882.42?,alkali-rich?K2O+Na2O?=5.39%8.12%),low Fe2O3?2.61%5.70%?,MgO?0.96%2.90%?,CaO?1.39%5.11%?,Ti O2?0.27%0.67%?,P2O5?0.06%0.10%?content,weakly peraluminium?A/CNK=0.981.25,with average value of 1.08?.The oxidation coefficient of biotite in the samples was between 0.118 and 0.469,with an average of 0.157;M=0.450.61,with an average of 0.50,MF=0.911.23,with an average of 1.0,similar with I type granite.And the content of P2O5 decreased with the increase of SiO2 content,suggesting that Maixie granodiorites belong to high-K calc-alkaline I-type granite.The zircon saturation thermometer is 736?825?.The granodiorites are enriched in Rb,Th,LREE and depleted in Nb,Ta,Ti,P,Ba,Sr and other elements,with distinct LREE and HREE fractionation??La/Yb?N= 5.00 40.42,with a mean of 16.97?and weak to medium Eu negative anomalies??Eu=0.631.00,with an average of 0.77?.The 87Rb/86 Sr ratio varied from 2.12882.2306,and the corresponding?87Sr/86Sr?i=0.7084440.708962;147Sm/144Nd= 0.0974570.117699,143Nd/144Nd=0.5119760.512046,?Nd?t?=-7.48-7.22,TDM2=1.751.87 Ga,with an average of 1.81 Ga.Their relatively higher?87Sr/86Sr?i,lower ?Nd?t?values and older Nd mode ages suggest that it originated from the partial melting of the Early-Middle Proterozoic crust.Based on the experimental petrology and the geochemical characteristics of our studied samples,it can be inferred that the Maixie biotite granodiorites were formed in the water-unsaturated partial melting of the intermediate-basic rocks?e.g.,amphibolite?in the lower-to middle-crust,with a certain content of mantle-derived materials at the initial stage of magmas,and underwent extensive fractional crystallization of amphibole,biotite,ilmenite,titanite,plagioclase and apatite.The mantle-derived materials come from the partial melting of the Paleoproterozoic subduction-modified lithospheric mantle.According to the regional geological background and our new data,together with published data,it is suggested that Maixie pluton were formed in post-collisional intro-continental environment.The early Paleozoic intracontinental magmatic rocks in South China are linked to lower-to middle-crustal anatexis,triggered by underplating of mantle-derived magmas. |
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