The Geological And Geochemical Characteristics And Its Petrogenesis For Caledonian Granites In The Central-Southern JiangXi Province

Caledonian (Early Paleozoic) granitoids outcropped in the South central of Jiangxi province are one of important parts of granites in South China, which has attracted many geological workers'attentions. Due to the lack of accurate geochronological and geochemical data, the classification, petrogenesis and tectonic settings of granitic rocks are still controversy. In order to answer these questions, this study selected five representative plutons, including Fufang, Le'an, Longhui, Shangyou and Ninggang. Based on the investigation of the field geology and the advanced measurment methods, the thesis studied the petrology, isotopic geology, geochemistry and geochronology of five granitic plutons and discussed the genesis of the granites and migmatites, the relationship between the granites and the tectonics. Finally, combining with the previous works, the thesis suggested the new geological settings of the Caledonian granites in South China.The geochronological data showed that Fufang pluton is a complex pluton, composed of Fufang pluton and Sangfengshan pluton. The crystallization age of the Fufang granodiorite and Fufang monzogranite were 443.9±3.5Ma and 443.1±4.6Ma respectively, belong to the first-stage intrusions. The second-stage intrusion is Sanfengshan monzogranite with the age of 433±3.9Ma. These ages are consistent with field geological characteristics. Three zircon U-Pb ages obtained from Le'an monogranite were 401.1±3.8Ma,405.1±3.3Ma and 403.9±2.9Ma respectively. The ages of Longhui pluton were 401.1±3.7Ma and 441.3±3.3 Ma. Four zircon U-Pb ages of Shangyiu complex pluton were 440.1±4.7Ma,443.9±3.2Ma and 408.2±4.7Ma respectively. The fine-grained porphyritic biotite-granite (440.0±4.7 Ma) and the fine-medium grained porphyritic biotite-monzogranite (443.9±3.2 Ma) from the complex pluton were the products of early-stage magmatism. The medium-grained porphyritic biotite-monzogranite possibly crystallized from the different facies of the homologous source. Doushui pluton (408.2±4.7 Ma) formed by the later stage magmatic activity. The zircon U-Pb age of Ninggang pluton was 433.8±2.2Ma. Above mentioned data showed that all studied granites formed from 444 Ma to 400 Ma, belong to the late Caledonian period. Combining with the field investigation, we considered that Caledonian granites have the characteristics of multi-stage intrusions.The petrology of the Caledonian granites showed that they contained abundant muscovite and a few hornblende only occurred in granodiorites. The zoned plagioclase and rapakivi K-feldspar phenocryst typically in magma mixing phenomena is absent.The micrograined enclaves are not found in Caledonian granites. It is suggested that the magma mixing did not occur in Caledonian granites.Caledonian granites studied here have similar geochemical compositions, high contents of SiO2, K2O+Na2O, Al2O3, A/CNK value with the average of 71.31%,7.37%,14.05% and 1.11, respectively, and low content of CaO, P2O5 as well as mafic components. They enriched in large ion trace elements such as Cs, Rb, Th, Zr, U and rare earth elements in La, Ce, Nd, depleted relatively in Ba, Sr, Nb, P, Ti and had higher Rb/Sr ratio (the average of 1.54) and Rb/Nb ratio (the average of 13.76). These granites have significant LREE enrichment with relatively flat HREE patterns and moderate depletion in Eu (the average 8Eu of 0.49).The values of (87Sr/86Sr) i of granites changed from 0.70353 to 0.71937, in agreement with that of S-type granites. According to Nd isotopes, these granites were divided into two types, one type with the lowerεNd(t) value(-13.32～-8.64) and higher Nd model ages(2252～1838Ma) and the other with higherεNd(t) value(-7.00～-4.06) and lower Nd model ages(1735～1491 Ma). Their isotopic geochemistry reflected the different composition of the distinctive basements. Granites from Le'an, Longhui and Shangyiu pluton with the characteristics of higherεNd(t) value and lower Nd model ages possibly indicated that their source was related to the Mid-to Neo-proterozoic basement containing the basic volcanic rocks.The petrology, geochemistry isotopic geology of the Caledonian granites showed that they could be classified into S-type granite or crust-derived granites. They were originated from partial melting of meta-greywacke, meta-greywacke-pelite or meta-pelite in low mature Proterozoic crust. Doushui pluton was possibly originated from remelting of garnet-rich metamorphic rocks residue after the formation of Shangyou pluton at the early stage, because unstable garnet released more HREEs in the condition of decompression. Moreover, the fractionation of plagioclase and accessory minerals such as zircon, apatite, monazite and allanite occured. Fufang pluton was originated from partial melting of the meta-greywacke.Migmatites well developed in Wuyi Mountain and distributed in HP amphibolite facies region. The field geological characteristics suggested that migmatites formed by partial melting and represented the initial stage of crustal remelting with the syn-tectonic deformations during partial melting. The superimposed fold leaded the crust to be thicken, so the heat supplied by the thick crust is hot enough to form HP amphibolite facies metamorphism, deeply re-melting and finally form the migmatites. However because of dissimilar thickness of the crust or crust thickening in different degrees, HP amphibolite facies in some areas have not been happened to produce the migmatites. Also this is the reason that leaded the discontinuous distribution of the migmatites.Caledonian granites have similar geochemical characteristics of post-orogenic granite. Tectonically, the rocks distributed along large fault belt, which suggested that the intrusions were controlled by the fault. Temporally, Caledonian granitic magmatism formed after the fold-orogenesis and in the post-orogenic tectonic settings.It is showed that during Early Paleozoic the superimposed fold developed with the basement involved, the strong deformation of the basement and broad regional greenschist phase metamorphism also happened. There were absent of obvious volcanism, ophiolite suites and immingling ophiolite rocks or super lithosphere fault in studied area have not been found. The sedimentary environments of Early Paleozoic stratum are shallow to half-deep sea. As a whole, the Caledonian granites in this area were classified into S-type granite or crust-derived granite, dispersively distributing. The petrology, isotopic geology, geochemistry and geochronology are not same with that in the granites of the epicontinental orogenic belt. The Caledonian granites formed in the intracontinental orogenic setting, possibly related to the amalgamation and collision between Cathaysian block and the Yantze block, and the amalgamation amongst many Cathaysian remnant blocks.