Latest Carboniferous closure of the Junggar Ocean constrained by geochemical and zircon U-Pb-Hf isotopic data of granitic gneisses from the Central Tianshan block, NW China

Xiaoran Zhang, Guochun Zhao*, Paul R. Eizenhöfer, Min Sun, Yigui Han, Wenzhu Hou, Dongxing Liu, Bo Wang, Qian Liu, Bing Xu

*Corresponding author for this work

Research output: Contribution to journalArticle

42 Citations (Scopus)

Abstract

Once situated between the Central Tianshan and Junggar terranes during Paleozoic time, the Junggar Ocean was a major southern branch of the Paleo-Asian Ocean. Thus, when and how it was closed are essential in understanding the final assembly of the Central Asian Orogenic Belt. However, the exact closure time of the Junggar Ocean remains unresolved due to the lack of reliable timing of collision-related regional metamorphism. This paper reports whole-rock geochemical and zircon U-Pb and Lu-Hf isotopic data for granitic gneisses from the northern margin of the Central Tianshan block, which can provide crucial constraints on the final closure of the Junggar Ocean. Mineral assemblages and geochemical suggest that the protoliths of the Central Tianshan gneisses are weakly peraluminous high-K calc-alkaline I-type granites, possessing typical subduction-related features such as strong enrichment in LREE and LILE and depletion in HFSE. Negative Eu anomalies (δEu=0.46-0.81) and highly variable zircon Hf isotope compositions indicate various amounts of residual plagioclase in the source and crustal contamination during magma formation. LA-ICPMS U-Pb dating on magmatic-type zircons (72%), characterized by euhedral to subhedral shapes, concentric oscillatory zoning, high Th/U ratios (0.30-2.05) and large ranges of εHf(t) values (-3.4 to +8.7; up to 6 epsilon units in each sample), yields consistent weighted mean 206Pb/238U ages of ca. 325-320Ma, interpreted as the crystallization ages of the granitic protoliths. Geochemical signatures and dominantly positive zircon εHf(t) values reveal that the protoliths were emplaced in a continental arc setting, pinpointing the development of a late Early to early Late Carboniferous continental arc system on the northern margin of the Central Tianshan block, probably related to the southward subduction of the Junggar oceanic plate. Meanwhile, younger ages at ca. 303-301Ma were obtained on recrystallized zircon-rims and unzoned/homogeneous zircons (25%) with narrow ranges of εHf(t) values (-1.8 to +7.2; <3 epsilon units in each sample), considered as the timing of a later tectonothermal event, which was associated with strong deformation that formed the gneissosity of the granitic gneisses. Combined with previous studies, we suggest that this event most likely resulted from the arc-continent collision/accretion between the island arcs (e.g. Bogda, Dananhu) in the Junggar Ocean and the Central Tianshan block during the final closure of the Junggar Ocean, which led to the final assembly of the Eastern Tianshan during the latest Carboniferous time.

Original languageEnglish
Pages (from-to)26-36
Number of pages11
JournalLithos
Volume238
DOIs
Publication statusPublished - 1 Dec 2015
Externally publishedYes

Keywords

  • Central Tianshan block
  • Geochemistry
  • Granitic gneiss
  • Hf isotope
  • Junggar Ocean
  • U-Pb dating

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