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EMERGING RESEARCH FRONTS

Yong-Feng Zhu talks with ScienceWatch.com and answers a few questions about this month's Emerging Research Front in the field of Geosciences.
Zhu Field: Geosciences
Article: The zircon SHRIMP chronology and trace element geochemistry of the Carboniferous volcanic rocks in western Tianshan Mountains
Authors: Zhu, YF;Zhang, LF;Gu, LB;Guo, X;Zhou, J
Journal: CHIN SCI BULL, 50 (19): 2201-2212 OCT 2005
Peking Univ, Key Lab Orogen Belts & Crustal Evolut, Sch Earth & Space Sci, Beijing 100871, Peoples R China.
Peking Univ, Key Lab Orogen Belts & Crustal Evolut, Sch Earth & Space Sci, Beijing 100871, Peoples R China.
(addresses may have been truncated)
 

Does it describe a new discovery, methodology, or synthesis of knowledge?

This paper reports on the first zircon SHRIMP chronology study in the western Tianshan Mountains (Xinjiang, NW China), and presented high-quality evidence on zircon SHRIMP ages as well as interpretations of well-established geochemistry data. Based on this new data, we suggested that the volcanic rocks largely distributed across this region represent a moved Late Paleozoic continental arc rather than the previously so-called "mantle plume" or "rift."

We started working in this region in 2004, and have also published several related papers in various other scientific journals as follows:

  • Yongfeng Zhu and Yoshihide Ogasawara, "Clinopyroxene phenocrysts (with green salite cores) in trachybasalts: implications for two magma chambers under the Kokchetav UHP massif, North Kazakhstan," Journal of Asian Earth Sciences, 22 [5]: 517-527, 2004

  • Yongfeng Zhu, Yishan Zenga and Libing Gua, "Geochemistry of the rare metal-bearing pegmatite No. 3 vein and related granites in the Keketuohai region, Altay Mountains, northwest China," Journal of Asian Earth Sciences, 27 [1]: 61-77, 2006

  • Zhu Yongfeng and Xin Xu, "The discovery of Early Ordovician ophiolite melange in Taerbahatai Mts., Xinjiang, NW China," Acta Petrologica Sinica 22 [12]: 2833-2842, 2006

  • Yongfeng Zhu, Xuan Guo, Lifei Zhang and Biao Song, "Geochemistry and zircon SHRIMP dating on the Late Paleozoic volcanic rocks in west Tianshan Mountains (Central Asia, Xinjiang)" Geochimica et Cosmochimica Acta, 70 [18]: Supplement 1, Page A755, 2006

  • Zhu Yongfeng and Xin Xu, "Exsolution texture of two-pyroxenes in Iherzolite from Baijiangtan ophiolitic melange, western Junggar, China," Acta Petrologica Sinica, 23 [5]: 1075-1086, 2007

  • Yongfeng Zhu, Jing Zhoua and Yishan Zenga, "The Tianger (Bingdaban) shear zone hosted gold deposit, west Tianshan, NW China: Petrographic and geochemical characteristics," Ore Geology Reviews, 32 [1-2]: 337-365, 2007

Would you summarize the significance of your paper in layman’s terms?

The genesis of the Carboniferous volcanic rocks in the Dahalajunshan group, mainly consisting of trachytes and trachy-andesites and distributing widely in the western Tianshan Mountain range, remains controversial. It has been proposed to be attributable to "rift" or "plume." Detailed petrology and geochemical data presented in this paper show that these volcanic rocks represent typical continental arc magmatism. The volcanic rocks are mainly trachy-andesitic, and the magma source is enriched in LILE, Th and Pb, and depleted in HFSE and Ce. Trace element geochemical study suggests that the basalts could be modeled by 7-11% partial melt of garnet Iherzolite. The volcanic rocks in the Dahalajunshan group are neither the products of "rift" nor so-called "plume" but represent the continental island arc of the Paleo-Southern Tianshan Ocean.

The mantle wedge had been modified by the melt generated in the subduction zone during the long evolution history of this island arc. The continental crust materials (i.e., mainly sediment on the ocean floor) had been added into island-arc through melt in subduction zones. Volcanic rocks occurring in different regions might represent magma eruption during different time periods. The zircon SHRIMP dating indicates that the ages of the basalt varies between 334.0 Ma and 394.9 Ma. The 13 analyses give an average age of 353.7 +/- 4.5 Ma (MSWD = 1.7). The apparent ages of zircons in trachy-andesite vary between 293.0 Ma and 465.4 Ma.

All analyses fall on the U-Pb concordant line and are divided into two groups. Eight analyses produce an average age of 312.8 +/- 4.2 Ma (MSWD = 1.7), which represents the crystallizing age of zircon rims in trachy-andesite. These two ages (i.e. 354 and 313 Ma) belong to the Early Carboniferous and Late Carboniferous epochs, respectively. Thus, in our opinion, the Dahalajunshan group would be separated into several groups with the accumulation of high-quality age-dating and data of trace elements and isotopic geochemistry. The following works obtained more zircon SHRIMP data which support this conclusion.

  • Yongfeng Zhu, Xuan Guo, Lifei Zhang and Biao Song, "Geochemistry and zircon SHRIMP dating on the Late Paleozoic volcanic rocks in west Tianshan Mountains (Central Asia, Xinjiang)" Geochimica et Cosmochimica Acta, 70 [18], Supplement 1, Page A755, 2006

  • Zhu Yongfeng, Zhou Jing, Song Biao, Zhang Lifei, and Guo Xuan, "Age of the "Dahalajunshan" Formation in Xinjiang and its disintegration" Geology in China, 33 [3]: 487-497, 2006

Where do you see your research leading in the future?

I believe our research related to these articles suggests that isotopic chronology has to be based on serious geology and petrology studies, otherwise, the study of zircon SHRIMP will lead in the wrong direction for modern geology.

Yong-Feng Zhu
Professor of Petrology & Geochemistry
School of Earth and Space Sciences
Peking University
Beijing, PR CHINA

RELATED> Also see the Special Topic of Zircon Dating.



2008 : February 2008 : Yong-Feng Zhu