Neoproterozoic Magmatism and Tectonic Evolution in South China

Abstract

AbstractNeoproterozoic magmatic rocks are widespread in South China, which can be divided into three major groups based on their formation ages and their genetic relationships with regional tectonics, metamorphism and basin evolution: ① Group 1 formed during the syn-orogenic process (1.0–0.9 Ga); ② Group 2 formed in the early rift phase (0.85–0.80 Ga); and ③ Group 3 formed in the major rift phase (0.79–0.75 Ga). Despite numerous studies during the past two decades, the tectonic settings responsible for the Neoproterozoic magmatism, in particular the intensive granitoid plutonism in South China is still an issue of hot debate. It is noteworthy that the geochemical characteristics of granitoid rocks are reflective of their source compositions as well as the melting and crystallization histories of the melts, thus could not be simplistically used to assess the tectonic regime under which the granitoids were formed. On the other hand, geochemistry of basaltic rocks (including basalts and basaltic dikes) reflects the compositions and thermal structures of their mantle sources that are relevant to different petrotectonic associations. In this chapter we compile the published high-precision isotopic ages and geochemical and isotopic data for the Neoproterozoic basaltic rocks and the pre-Neoproterozoic crystalline basement rocks from South China. These datasets, combined with other aspects of geological records, are used to investigate the petrogenesis of the basaltic rocks, as well as their mantle source compositions and potential temperatures, aiming to shed new lights on the Neoproterozoic petrotectonic evolution in South China. The Group 1 syn-orogenic basaltic rocks occur sporadically along the Yangtze Craton margins, including the ca. 0.9 Ga Yanbian basalts and the ca. 1.0 Ga Huili mafic dykes in western margin, the ca. 0.95–0.89 Ga Xixiang basalts in northwestern margin and the ca. 0.96 Ga Pingshui spilites in southeastern margin. They are closely associated with the calc-alkalinic intermediate-acidic volcanic/granitic rocks; all of the rocks are clearly deformed and metamorphoses to varying degrees. The Xixiang basalts and Pingshui basalts are calc-alkaline in compositions with geochemical characteristics similar to those of island arc basalts (IAB). In Yanbian area, both tholeiitic and calc-alkaline basalts are exposed, and their geochemical features are similar to those of the back-arc basin basalts (BABB). Overall, the Group 1 basaltic rocks surrounding the Yangtze Craton are likely formed in active continental margins. The Group 2 basaltic rocks formed during the early rift phase consist mainly of the tholeiitic and alkali basalts as well as dolerites around the Yangtze Craton margins, including the ca. 0.85 Ga Zhengzhushan bimodal volcanic rocks and the Shenwu dolerites in southeastern margin, ca. 0.83–0.82 Ga Yiyang komatiitic basalts, the Guangfeng alkaline basalts and the Yingyangguan spilites in southern margin of the Yangtze Craton, and ca. 0.82–0.80 Ga Suxiong alkaline basalts, Bikou and Tiechuanshan tholeiitic basalts along the western to northwestern margin of the Yangtze Craton. In the Cathaysia Block, the Mamianshan bimodal volcanic rocks (alkaline basalts and rhyolites) in northwestern Fujian are also dated at ca. 0.82 Ga. The majority of the ca. 0.83–0.82 Ga bimodal volcanic rocks occurred at the bottom of the volcano-sedimentary sequences in a number of mid-Neoproterozoic rift basins. The geochemical compositions of the Group 2 basaltic rocks are either similar to those of typical oceanic island basalts (OIB), or transitional between OIB and IAB due to various degrees of contamination of the lithospheric mantle and/or continental crust materials. The Group 3 basaltic rocks formed in the major rift phase include the ca. 0.79 Ga Shangsu basalts and Daolinshan dolerites in southeastern margin of the Yangtze Craton, the ca. 0.77 Ga spilites in northern Guangxi and dolerites in western Hunan and the numerous 0.78–0.75 Ga mafic dykes occurring in the Kangdian rift in western margin of the Yangtze Craton. All these basaltic rocks belong to tholeiitic series and alkaline series, and their geochemical features are similar to those of OIB. The calculated mantle potential temperatures (Tp) of the Group 1 basaltic rocks and the ca. 0.85 Ga Shenwu dolerites (Group 2) range from 1355 to 1420 ℃, which are consistent with those of the Neoproterozoic mid-oceanic-ridge basalt (MORB)-source mantle (ca. 1350–1450 ℃). On the contrary, the ca. 0.82 Ga Yiyang komatiitic basalts were generated by melting of an anomalously hot mantle source with Tp up to ca. 1618 ℃, ca. 260 ℃ higher than the MORB-source mantle, suggesting derivation from an abnormally hot mantle plume. The Tp for the other Group 2 and 3 basaltic rocks are ca. 25–140 ℃ higher than that of the MORB-sourced mantle, indicating various degrees of contribution from hot mantle plumes. The changes of rock types, geochemical compositions and the mantle potential temperatures are attributed to the regional tectonic transformation of the South China Block (SCB) from ca. 1.0–0.9 Ga Sibao Orogeny to ca. 0.85–0.75 Ga intracontinental rifting. Mantle plume/superplume activities play important roles in the mid-Neoproterozoic rift magmatism. Our interpretations of the geochemical results of basaltic rocks are consistent with other aspects of geological records. The Yangtze Cratob and Cathaysia block eventually amalgamated at ca. 0.9 Ga, forming a united South China continent that links Australia–East Antarctica and Laurentia in the Rodinia supercontinent. Termination of the Sibao Orogeny indicates that the Rodinia might have eventually assembled at ca. 0.9 Ga. The mid-Neoprterozoic (0.83–0.75 Ga) mantle plume/superplume activities resulted in the formation of widespread anorogenic magmatism and rift basins in South China.KeywordsSouth ChinaNeoproterozoic magmatismBasaltic rocksOrogenyRift baisnRodinia supercontinent