Triassic Shoshonite Association-A-type Granites Belt In South China And Its Geological Implications

South China, consisting of Yangtze Block and Cathaysia Block, is one of the most famous granite provinces in China and even in the world. Large scale granitic magma activities and related metal mineralization in Mesozoic developed in the area, forming many world-class mineral deposits. The relationships between magmatism and metal mineralization have been studied as early as the last century and still attract much attention now. Most Triassic magma rocks in the area were granitic rocks while mafic rocks were rarely reported except the mafic xenoliths in Daoxian, and volcanics were not reported. There were some studies summed up the spatial distribution regularities of the Triassic magma rocks, and with more and more high precision age data recently, many people considered that Indosinian granite were developed all over the Indosinian period. The exiting researches were mainly focused on the area to the west of the Mesozoic volcanics line of South China, while the research of the east of the line is still relatively weak.The thesis selected the Triassic shoshonite series and A-type granite located in several areas on the east of the Mesozoic volcanics line as study objects, which including the quartz monzonite-(quartz) syenite in the middle of Zhejiang province, quartz monzonite-syenogranite-monzogranite in the southwest of Zhejiang province, syenites-syenogranite in the west of Fujian province and quartz monzonite-monzogranite in the east of Jiangxi province. Detailed studies on field geology and petrography were taken in the first period, and the comprehensive studies on the zircon U-Pb age, major elements, trace elements, Sr-Nd isotope and zircon Hf isotope were taken later. Combined with the previous study achievements on Triassic geology in south China, the thesis discusses the petrogenesis, spatial distribution and tectonic setting of the Triassic shoshonite series and A-type granite in south China This paper has some understandings as follows:(1) The Triassic plutons in central Zhejiang province include Shiban intrusion, Dashuang intrusion and Zhouzhuang intrusion. The lithology of Shiban intrusion is quartz-syenite, and Dashuang intrusion consists of quartz-syenite and porphyritic quartz monzonite, while the Zhouzhuang intrusion consists of porphyritic quartz monzonite. LA-ICP-MS zircon U-Pb dating for the different lithologies of the three intrusions gives magma crystallization ages of238±2Ma,240±3Ma,236±3Ma and237±2Ma, respectively. These ages rang from236Ma to240Ma, indicating magmatism occurred at middle Triassic. The Shiban and Dashuang syenites belong to peraluminous granites (A/CNK>1.02), Dashuang quartz monzonites are metaluminous granites (A/CNK=0.90-0.98), while the Zhouzhuang quartz monzonites distribute in both metaluminous and peraluminous areas, with most of the samples are peraluminous granites (A/CNK=0.93-1.05). All the four type samples are shoshonite granites. Syenites in this area show enrichment of Th, K, depletion of Ba, Sr and high field strength elements (Ta, Ti). The chondrite-normalized REE patterns show that all syenites are enriched in light rare earth elements (LREE) with (La/Yb)N of106-175, and have pronounced negative Eu anomalies with Eu/Eu*values of0.280～0.40. The quartz monzonite samples are enriched in Ba and Th, and are depleted in Sr, P, Ta and Ti. The chondrite-normalized REE patterns show that all quartz monzonite samples are enriched in LREE ((La/Yb)N=30.12-43.79), and have pronounced negative Eu anomalies with Eu/Eu’ values of0.60～0.70. Total bulk Sr-Nd isotope results show that Shiban syenite, Dashuang syenite, Dashuang quartz monzonite and Zhouzhuang quartz monzonite have εNd(t) values of-8.60～-8.71,-8.35～-8.46,-8.26～-10.92and-9.24～-10.13, respectively, and the corresponding TDM2of1.70-1.71Ga,1.68-1.71Ga,1.68-1.89Ga and1.77-1.83Ga. The zircon Hf isotope results show that Shiban syenite, Dashuang syenite and Dashuang quartz monzonite have εHf(t) values of-6.6±0.33,-6.91±0.33and-11.21±0.72, respectively, and the corresponding TDM2of1.62-1.75Ga,1.64-1.79Ga and1.8-2.07Ga.(2) The Triassic magmatic rocks in Southwest Zhejiang province include Meitian intrusion and Jingju intrusion. The lithologies of Meitian intrusion include porphyritic monzonite granite and porphyritic quartz monzonite, while the Jingju intrusion consists of syenogranite and quartz monzonite. LA-ICP-MS zircon U-Pb dating for Meitian monzogranite, quartz monzonite and quartz monzonitic dyke show crystallization ages of238±2Ma,236±2Ma and228±2Ma, respectively, indicating these intrusions formed in middle Triassic. The Meitian granites belong to peraluminous granites (A/CNK=1.00-1.23), and the Meitain quartz monzonite (A/CNK=0.96-1.09) and Jingju syenogranite (A/CNK=0.88-1.12) show wide rang between metaluminous and peraluminous areas, while the Jingju quartz monzonites mainly belong to metaluminous granites (A/CNK=0.83-1.04). All the Triassic plutons in Southwest Zhejiang province are shoshonitic granites except few samples in Meitian monzogranite which are high-K calc-alkaline granites. On the Harker diagrams for the quartz monzonites, the TiO2, Fe2O3T, CaO and P2O5show linear negative correlations with SiO2, while others show nearly no correlations with SiO2, For the monzogranites and syenogranites, the MgO, Al2O3, CaO, TiO2, Fe2O3T and P2O5show linear negative correlations with SiO2, while the MnO show negative correlations with SiO2first and then become positive. All the Harker diagrams suggest that the magma might have evolved by fractional crystallization of feldspars, iron-magnesium minerals, Ti-Fe oxides and apatites. Quartz monzonite in this area show enrichment of Th, Rb, and depletion of Ba, Sr and high field strength elements (Ta, Ti). The chondrite-normalized REE patterns show that quartz monzonite are enriched in LREE ((La/Yb)N=15.53-34.05), and show pronounced negative Eu anomalies with Eu/Eu values of0.58-1.04. The monzogranites in Meitian are enriched in Rb, Th, and depleted in Ba, P, Ta and Ti. The chondrite-normalized REE patterns show that monzogranites are enriched in LREE ((La/Yb)N=24.61～47.30), and show pronounced negative Eu anomalies with Eu/Eu values of0.85～2.32. Syenogranite are enriched in large ion lithophile elements (K, Rb, Th), and depleted in U, Ba, P, Ta and Ti. Samples in syenogranite have high rare earth content (933-1772ppm), and are enriched in LREE (La/Yb)N=50.23～255.45), and show pronounced negative Eu anomalies with Eu/Eu*values of0.38-0.77. Total bulk Sr-Nd isotope results show that Meitian quartz monzonite and monzonite granite have εNd(t) values and TDM2of-13.72～-14.25and-14.57, and2.12-2.16Ga and2.19Ga, respectively. The zircon Hf isotope results show that Meitian monzonite granite, quartz monzonite and quartz monzonitic dyke and Jingju syenogranite have εHf(t) values of-17.51±0.43,-19.01±0.46,-15.19±0.48and-10.90±0.67, respectively, with the corresponding TDM2of2.37～2.54Ga,2.29-2.45Ga,2.14-2.3Ga and1.84～2.08Ga.(3) The Triassic magmatic rocks from the Western Fujian (Mingxi-Qingxi) area are composed of Pinghu biotite-pyroxene syenites, Pinghu syenites, Baiheng syenogranites and Julin syeno granites. LA-1CP-MS zircon U-Pb analyses suggest the formation ages are232Ma±2Ma,235.0±1.3Ma,236±2Ma and225±2Ma, respectively. The samples from Pinghu biotite-pyroxene syenites have low A/CNK ratios ranging from0.42to0.45, and show metaluminous-peralkaline and shoshonitic compostions. The Harker diagrams show negative correlations between CaO, Fe2O3T, MnO and SiO2but positive correlations between Al2O3, Na2O and SiO2, indicating the fractionation of mafic minerals, feldspar and Fe-Ti oxides. All the samples are characterized by enrichment of LREE relative to HREE ((La/Yb)N=53.75±63.76), slight negative Eu anomalies, enrichment of Rb and Ba, depletion of high field strength elements (HFSE, such as Nb, Ta and Ti) and enriched εHf(t)(-11.28±0.48) with old TDM2(1.88-2.05Ga). The Pinghu syenites have A/CNK ratios ranging from0.50to0.80, plotted in the metaluminous and peralkaline areas on the A/NK-A/CNK diagrams. All samples exhibit egative correlations between MgO, Fe2O3T, MnO, P2O5and SiO2but positive correlations between A12O3and SiO2on the Harker diagrams, suggesting the fractionation of mafic minerals, feldspar, Fe-Ti oxides and apatites. They are characterized by enrichment of LREE relative to HREE ((La/Yb)N=41.41-87.63), negative Eu anomalies (Eu/Eu*=0.74～0.84), depletion of Sr, P, Nb, Ta and Ti, and enriched εHf(t)(-9.88±0.36) with old TDM2(1.8-1.96Ga).The Baiheng syenogranites have A/CNK ratios ranging from0.80to1.06, plotted in the metaluminous, peralkaline and peraluminous areas on the A/NK-A/CNK diagrams. All samples exhibit negative correlations between MgO, A12O3, Fe2O3r, MnO, P2O5, CaO, K2O and SiO2on the Harker diagrams, suggesting the fractionation of mafic minerals, feldspar, Fe-Ti oxides and apatites. They are characterized by enrichment of LREE relative to HREE ((La/Yb)N=7.22～19.23), negative Eu anomalies (Eu/Eu*=0.35～0.71),enrichment of Th and U, depletion of Ba, P, Ta and Ti, and enriched εHF(T)(-6.54～-5.29) with old TDM2(1.60-1.68Ga).The Julin syenogranites have A/CNK ratios ranging from1.06to1.10, plotted in the metaluminous and peralkaline areas on the A/NK-A/CNK diagrams. All samples exhibit negative correlations between MgO, Al2O3, Fe2O3T, P2O5, CaO, TiO2and SiO2on the Harker diagrams, suggesting the fractionation of mafic minerals, feldspar, Fe-Ti oxides and apatites. They are characterized by strong enrichment of LREE relative to HREE ((La/Yb)N=9.99～22.86), negative Eu anomalies (Eu/Eu*=0.14-0.23), enrichment of Th and U, depletion of Ba, P, Sr, Ta and Ti, and enriched εHf(t)(-8.86±0.27) with old TDM2(1.79～1.88Ga).(4) The Triassic magmatic rocks from the Eastern Jiangxi (Ningdou-Xingguo) consist of porphyritic monzonitic granites and Quartz monzonites. LA-ICP-MS zircon U-Pb analyses suggest that the formation age of monzonitic granites are228±2Ma. On the A/NK-A/CNK diagrams, most samples from the two rock types plot in the metaluminous area (A/CNK ratios are0.93～1.07and0.88～0.99, respectively), but some from the monzonitic granites plot in the peraluminous and weak peraluminous. Most samples exhibit shoshonitic compostions. However, some samples from monzonitic granites show High K calc-alkali compositions. On the Harker diagrams, all the samples from the two rock types exhibit no correlation with SiO2. The samples from Quartz monzonites display enrichment of LREE ((La/Yb)N=12.22～46.03), Th, U, Rb and K, depletion of Ba, P, Sr, Nb, Ta and Ti, and obvious negative Eu anomalies (Eu/Eu*=0.35～0.65) The samples from monzonitic granites are characterized by strong enrichment of LREE relative to HREE ((La/Yb)N=10.76～28.81), negative Eu anomalies (Eu/Eu*=0.36～0.75), enrichment of Th,U,Rb and K, depletion of Ba, P, Sr, Nb, Ta and Ti. They have relatively high HREE contents, indicating the presence of heavy minerals. The samples from monzonitic granites have enrichedd(t)(-9.6～9.99) and εHft)(-7.68±0.48), with old T2DM(1.79～1.82Ga) and TDM2(1.67～1.85Ga).(5) In summary, the Triassic quartz monzonites from the central Zhejiang area were derived from partial melting of old deep crustal materials with the participation of mantle materials and crystal fractionation of plagioclases, hornblendes, biotites, apatites and Fe-Ti oxides. However, the syenites were derived partial melting of relatively young crustal materials and crystal fractionation of alkali-feldspars, hornblendes, biotites and apatites. The Triassic monzonitic granites from Southwestern Zhejiang were generated by partial melting of old crustal materials and crystal fractionation of alkali-feldspars, biotites, apatites and Fe-Ti oxides, while the quartz monzonites were generated by partial melting of crustal materials with the participation of some mantle materials and crystal fractionation of plagioclases, hornblendes, biotites, apatites and Fe-Ti oxides. And the late monzonitic granites were mainly generated by partial melting of relatively young crustal materials. The syenites were derived partial melting of an enriched lithospheric mantle, while the Baiheng syenogranites were generated by partial melting of crustal materials and crystal fractionation of alkali-feldspars, hornblendes, biotites, apatites and Fe-Ti oxides. The origin of Julin syenogranites is similar to the syenogranites from the southwest Zhejiang area. The Baohuashan granites from Eastern Jiangxi area have the same compositions as the syenogranites from the western Fujian area, indicating they have similar genesis.(6)This paper compared the geology of South China to that of North Vietnam for understanding the tectonic settings of the Triassic magmatisms in South China. They have different times, strikes and kinematic directions of tectonothemal events. The tecotonothemal events developed in the North Vietnam concentrated in Early to Middle Triassic, striked Northwest (NW) and had dextral ductile strike-slip shearings. However, the tecotonothemal events developed in the South China (Guangxi and Zhejiang) concentrated in Middle to Late Triassic, striked NE or NNE. Most areas had sinistral ductile strike-slip shearings, but Southwestern areas have dextral ductile strike-slip shearings. Therefore, the Indosinian orogeny developed in the North Vietnam had less impact on the South China, and was not used to interpret the generation of Triassic NE trending shoshonitic series-A-type granitic belt in the South China. Late Permian to Early Triassic Arc magmatisms had been reported in the Korean Peninsula, Janpan and Philippines in the past two years, indicating the subduction of Paleo-Pacific plate. Take together, we propose that the Triassic magmatisms in South China are mainly related to the subduction of Paleo-Pacific plate.