Tectonic evolution of the northwestern Yangtze block in central China during the Early- Middle Devonian: constraints from petrology, zircon U- Pb dating and trace element composition of the Tapo Formation in the Lüeyang area
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    Abstract:

    The provenance of the Early- Middle Devonian Tapo Formation in the northern margin of Bikou Terrane has long been considered to have derived from the Meso- Neoproterozoic Bikou Group, but lack of support from the detrital zircon provenance data. In this paper, systematic petrology, zircon types, zircon LA- ICP- MS U- Pb dating and trace element compositions of the clastic rocks from three different lithologic sections of the Tapo Formation in the Lueyang area, northern margin of Yangtze were studied. The comparative analysis of zircon crystals shows that there are two types of zircons: detrital zircons with metamorphic accretion rims (upper lithologic section) and detrital zircons of typical magmatic genesis (lower and middle lithologic sections). The dating results show that the former has two age peaks (~2. 0 Ga main peak and ~2. 5 Ga sub- peak), upper and lower interception ages of 2473±24 Ma and 359±84 Ma, respectively; the latter has a ~2. 5 Ga main peak, a ~2. 0 Ga sub- peak and a ~1. 39 Ga micro- peak. The concordant ages of three samples are older 1. 3 Ga, which can not define the age of the Tapo Formation, and does not support the previous understanding that the provenance of the Tapo Formation was mainly derived from the Bikou Group. Geochemical discrimant diagram of zircon compositions shows that source rocks are granitoid (granodiorite and quartze diorite), mafic rock and lamproite formed mainly in a magmatic arc/orogenic environment. Based on the type of rock fragments, regional geology, zircon ages correlation and paleocurrent data, it is considered that the Tapo Formation originated from the Archean- Paleoproterozoic basement (Yudongzi and Kongling complexs) located in the northern margin of Yangtze and the Yudongzi complex was widely exposed/uplifted in the late deposition of the Tapo Formation. At the same time, this study indicates that the Bikou Terrane may have been amalgamated with the Yangtze plate at least as early as the Early- Middle Devonian.

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WANG Xunlian, ZHOU Hongrui, WANG Zhentao, SHEN Yang, YU Zidong, YANG Zhihua.2019. Tectonic evolution of the northwestern Yangtze block in central China during the Early- Middle Devonian: constraints from petrology, zircon U- Pb dating and trace element composition of the Tapo Formation in the Lüeyang area[J]. Acta Geologica Sinica,93(12):2997-3019

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History
  • Received:October 22,2019
  • Revised:November 17,2019
  • Adopted:November 29,2019
  • Online: December 03,2019
  • Published: