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The emplacement age of the Songshugou ultramafic massif: LA-ICP-MS U–Pb zircon dating
A628
Goldschmidt Conference Abstracts 2006
The emplacement age of the Songshugou ultramafic massif: LA-ICP-MS U–Pb zircon dating W.D. SUN2, Y. SUN1, B. ZHOU1, J.F. LIU1 1
The Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, China ([email protected]) 2 Guangzhou Institute of Geochemistry, Chinese Academy of Science, 510640, China ([email protected]) The Songshugou massif is the largest Alpine-type ultramafic body in China, covering an area of 50 km2, which consists mainly of dunite (80%) with minor harzburgite, dunite–pyroxenite, pyroxenite, etc. It is located to the north of the Shang-Dan fault, intruding into the Proterozoic Qinling Group in the eastern Qinling Mountains, central China. There is a garnet amphibolite ‘‘coating’’ of 2–10 m thick, surrounding the ultramafic massif formed during thermal contact metamorphism (Sun et al., 1997). The emplacement age of the Songshugou ultramafic massif is very important for understanding the evolution of the Qinling orogenic belt (Sun et al., 2002). Published results however, range from 500 Ma to 1000 Ma (Li et al., 1991; Su et al., 2004; Liu and Sun, 2005), which led to different models for the early history of the Qinling orogenic belt and the interaction between the North and South China blocks, e.g., the effects of Jinning movement on the Qinling orogenic belt. Here we show LA-ICP-MS U-Pb dating results for zircon from the garnet amphibolite, which yield a concordant age of 506 ± 7 Ma. The studied zircon grains are small in size (30– 100 lm), anhedral, with typical metamorphic characteristics of low Th/U values (<0.1). The U, Th contents range from 260 ppm to less than 10 ppm. Cathodoluminescence images show two ‘‘generations’’ of zircon, with lighter cores and darker rims. The ages of the cores and the rims are identical to each other. Petrological observations suggest that the studied zircon grains were most likely formed during the amphibolite metamorphism induced by the emplacement of the ultramafic massif. Therefore the Songshugou ultramafic massif emplaced at 500 Ma, instead of 1000 Ma. This provides an important constraint on the paleozoic evolution of the Qinling orogenic belt.
Shahewan pluton: Implication on the evolution of the Qinling orogenic belt W.D. SUN2, Y. SUN1, J.F. LIU1, B. ZHOU1 1
Department of Geology, Northwest University, Xi’an 710069, PR China ([email protected]) 2 Gangzhou Institute of Geochemistry, Chinese Academy of Sciences, 510640, PR China ([email protected]) The Shahewan pluton is located near Shangzhou in central China. It was regarded as orogenic rapakivi, forming a Triassic rapakivi belt along with several other granite found in the Qinling orogen. The age of these granites was taken as the end time of the main orogeny and beginning of the post-collision of Qinling orogen belt (Zhang et al., 1999). Rapakivi is usually formed 150 Ma after collision. Therefore, a rapakivi belt in the Qinling orogen implies that the main collision between the South and North China blocks occurred much earlier than generally accepted (Triassic). This contradicts with many observations, e.g., sediments of the South Qinling are successive marine deposit from the Simian to middle Triassic (Yu and Cui, 2003), syn-collisional granites were formed in Triassic (206–220 Ma) (Sun et al., 2002), which are the same as that of the Shahewan pluton (214 Ma) (Zhang et al., 1999), etc. In this study, we analyzed the major and trace element compositions of samples from Shahewan and nearby syn-collisional granites. Our results show that Shahewan batholith has the same geochemical characteristics as syn-collisional granites nearby and all along the South Qinling granite belt (Sun et al., 2002), which are obviously different from typical rapakivi (figure).
References Li, S.G., Chen, Y., Zhang, G., Zhang, Z., 1991. Geol. Rev. 37, 235–242. Liu, J.F., Sun, Y., 2005. Geol. Rev. 51, 189–192. Su, L., Song, S.G., Song, B., Zhou, D.W., Hao, J.R., 2004. Chin. Sci. Bull. 49, 1209–1211. Sun, W.D., Li, S.G., Sun, Y., Zhang, G.W., Li, Q.L., 2002. JAES 21, 69–76. Sun, W.D., Zheng, Y., Li, S.G., Sun, Y., Zhang, G., 1997. Acta Petrol. Sin. 13, 162–167.
References doi:10.1016/j.gca.2006.06.1165
Sun, W.D., Li, S.G., Chen, Y.D., Li, Y.J., 2002. J. Geol. 110, 457–468. Yu, Z.P., Cui, H.F., 2003. J. Northwest Univ. 33, 65–69. Zhang, Z.Q., Zhang, G.W., Tang, S.H., Lu, X.X., 1999. Chin. Sci. Bull. 44, 2001–2004. doi:10.1016/j.gca.2006.06.1166
Goldschmidt Conference Abstracts 2006
The emplacement age of the Songshugou ultramafic massif: LA-ICP-MS U–Pb zircon dating W.D. SUN2, Y. SUN1, B. ZHOU1, J.F. LIU1 1
The Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, China ([email protected]) 2 Guangzhou Institute of Geochemistry, Chinese Academy of Science, 510640, China ([email protected]) The Songshugou massif is the largest Alpine-type ultramafic body in China, covering an area of 50 km2, which consists mainly of dunite (80%) with minor harzburgite, dunite–pyroxenite, pyroxenite, etc. It is located to the north of the Shang-Dan fault, intruding into the Proterozoic Qinling Group in the eastern Qinling Mountains, central China. There is a garnet amphibolite ‘‘coating’’ of 2–10 m thick, surrounding the ultramafic massif formed during thermal contact metamorphism (Sun et al., 1997). The emplacement age of the Songshugou ultramafic massif is very important for understanding the evolution of the Qinling orogenic belt (Sun et al., 2002). Published results however, range from 500 Ma to 1000 Ma (Li et al., 1991; Su et al., 2004; Liu and Sun, 2005), which led to different models for the early history of the Qinling orogenic belt and the interaction between the North and South China blocks, e.g., the effects of Jinning movement on the Qinling orogenic belt. Here we show LA-ICP-MS U-Pb dating results for zircon from the garnet amphibolite, which yield a concordant age of 506 ± 7 Ma. The studied zircon grains are small in size (30– 100 lm), anhedral, with typical metamorphic characteristics of low Th/U values (<0.1). The U, Th contents range from 260 ppm to less than 10 ppm. Cathodoluminescence images show two ‘‘generations’’ of zircon, with lighter cores and darker rims. The ages of the cores and the rims are identical to each other. Petrological observations suggest that the studied zircon grains were most likely formed during the amphibolite metamorphism induced by the emplacement of the ultramafic massif. Therefore the Songshugou ultramafic massif emplaced at 500 Ma, instead of 1000 Ma. This provides an important constraint on the paleozoic evolution of the Qinling orogenic belt.
Shahewan pluton: Implication on the evolution of the Qinling orogenic belt W.D. SUN2, Y. SUN1, J.F. LIU1, B. ZHOU1 1
Department of Geology, Northwest University, Xi’an 710069, PR China ([email protected]) 2 Gangzhou Institute of Geochemistry, Chinese Academy of Sciences, 510640, PR China ([email protected]) The Shahewan pluton is located near Shangzhou in central China. It was regarded as orogenic rapakivi, forming a Triassic rapakivi belt along with several other granite found in the Qinling orogen. The age of these granites was taken as the end time of the main orogeny and beginning of the post-collision of Qinling orogen belt (Zhang et al., 1999). Rapakivi is usually formed 150 Ma after collision. Therefore, a rapakivi belt in the Qinling orogen implies that the main collision between the South and North China blocks occurred much earlier than generally accepted (Triassic). This contradicts with many observations, e.g., sediments of the South Qinling are successive marine deposit from the Simian to middle Triassic (Yu and Cui, 2003), syn-collisional granites were formed in Triassic (206–220 Ma) (Sun et al., 2002), which are the same as that of the Shahewan pluton (214 Ma) (Zhang et al., 1999), etc. In this study, we analyzed the major and trace element compositions of samples from Shahewan and nearby syn-collisional granites. Our results show that Shahewan batholith has the same geochemical characteristics as syn-collisional granites nearby and all along the South Qinling granite belt (Sun et al., 2002), which are obviously different from typical rapakivi (figure).
References Li, S.G., Chen, Y., Zhang, G., Zhang, Z., 1991. Geol. Rev. 37, 235–242. Liu, J.F., Sun, Y., 2005. Geol. Rev. 51, 189–192. Su, L., Song, S.G., Song, B., Zhou, D.W., Hao, J.R., 2004. Chin. Sci. Bull. 49, 1209–1211. Sun, W.D., Li, S.G., Sun, Y., Zhang, G.W., Li, Q.L., 2002. JAES 21, 69–76. Sun, W.D., Zheng, Y., Li, S.G., Sun, Y., Zhang, G., 1997. Acta Petrol. Sin. 13, 162–167.
References doi:10.1016/j.gca.2006.06.1165
Sun, W.D., Li, S.G., Chen, Y.D., Li, Y.J., 2002. J. Geol. 110, 457–468. Yu, Z.P., Cui, H.F., 2003. J. Northwest Univ. 33, 65–69. Zhang, Z.Q., Zhang, G.W., Tang, S.H., Lu, X.X., 1999. Chin. Sci. Bull. 44, 2001–2004. doi:10.1016/j.gca.2006.06.1166