Neoproterozoic Crustal Accretion of the Northern Margin of Yangtze Plate:Constrains from Geochemical Characteristics, LA-ICP-MS Zircon U-Pb Chronology and Hf Isotopic Compositions of Trondhjemite from Zushidian Area, Hannan Region
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    Abstract:

    A large area of ultrabasic—basic—acid rocks which are Called "Hanna Complex" outcropped in the northern margin of Yangtze Plate. The acid plutons mainly exposed in Erliba,Wudumen, and Zushidian areas. The Zushidian pluton is a suit of trondhjemite characterized by high contents of Si (SiO2=73.18%~77.18%) and Na (Na2O=4.55%~5.49%), low contents of K (K2O=2.01%~3.01%) and Mg (MgO=0.14%~0.25%), belonging to slight peralurninous and medium K calc alkaline series. The chondrite normalized REE patterns are charaeterized by smoothly REE right deviation and slightly positive Eu anomaly. The primitive mantle normalized spidergrams are also charaeterized by enrichment of LILE (Rb,Ba and Th) and depletion of HFSE (Ni,Ta and Ti), with Sr/Y and (La/Yb) ratios of 28.16~50.05 and 7.04~14.21, respectively. Geochemical data shows that the pluton has the feature of arc granite. The Zushidian pluton is not adakite compared to the Erliba and Wudumen plutons. The characteristics of high Sr and low Y indicate that the formation pressure for the Zushidian pluton is relatively high, with residues of garnet and rutile, and without plagioclase. By Using saturated zircon thermometer and zircon titanium thermometer, the crystallization temperature for the Zushidian pluton can be constrained at 680~694℃, resulting from low partial melting of basaltic rocks. LA ICP MS zircon U Pb dating yield the formation age of 728±3 Ma for the Zushidian pluton and the captured zircon age of 786±4 Ma, and the edge of captured zircons also recorded the ca.728 Ma magmatic event . The initial ,n(176Hf)/n(177Hf) values of zircons range from 0.282470~0.282683, corresponding to εHf(t) =+5.0~+12.2. Zircon two stage Hf model ages (TDM2) range from 871 Ma to 1325 Ma, with a peak age of ca. 1100 Ma, which represents a crustal accretion event in the northern margin of Yangtze Plate at Neoproterozoic. In this paper, we speculate that the rocks possibly formed in the oceanic crust subduction setting,and resulted from the partial melting of juvenile oceanic crust with water added. Combining with the available data, we propose that the time—space relationship of granitic rocks in the Hannan area reflect the evolution and migration processes of island arcs in the north margin of Yangtze Plate during Neoproterozoic, and the Zushidian trondhjemite was formed at the island arc transformational stage that the immature arc to the mature marginal island arc.

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.2014. Neoproterozoic Crustal Accretion of the Northern Margin of Yangtze Plate:Constrains from Geochemical Characteristics, LA-ICP-MS Zircon U-Pb Chronology and Hf Isotopic Compositions of Trondhjemite from Zushidian Area, Hannan Region[J]. Geological Review,60(6):1393-1408.

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History
  • Received:December 09,2013
  • Revised:August 22,2014
  • Adopted:
  • Online: November 20,2014
  • Published: