Yong-Feng Zhu talks with
ScienceWatch.com and answers a few questions about
this month's Emerging Research Front in the field of
Geosciences.
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.."
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.