Sediment contribution in post-collisional high Ba-Sr magmatism: Evidence from the Xijing pluton in the Alxa block, NW China

Liqi Zhang, Hongfei Zhang*, Chris Hawkesworth, Biji Luo, He Yang, Wangchun Xu, Liang Guo, Fabin Pan, Zhong Gao, Lu Tao

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)

Abstract

High Ba-Sr granitoids occur in a number of tectonic settings, and variable petrogenetic models have been proposed. Those from post-collisional settings are rarely studied and are the focus of this study. Zircon U-Pb geochronology and whole-rock elemental and Sr-Nd-Hf isotopic data are presented for a suite of high Ba-Sr granitoids from the southern margin of the Alxa block, NW China. U-Pb zircon dating shows that the Xijing high Ba-Sr granitoids were emplaced in two periods at ~440 Ma and ~410 Ma, and they are characterized by elevated Ba and Sr contents. The older ~440 Ma high Ba-Sr granitoids range from monzodiorite to quartz monzonite, and the younger ~410 Ma high Ba-Sr granites tend to be more evolved. Both suites have similar enriched Sr-Nd-Hf isotopic ratios, low Ba/Th and Sr/Th ratios, and high Th contents and Th/Ce ratios indicative of a sediment contribution. This is potentially linked to previous subduction of the North Qilian ocean slab. Major and trace element calculations suggest that the older ~440 Ma granitoids experienced up to ~60% fractionation at 4–6 kbar with a crystallizing assemblage dominated by amphibole and plagioclase. The younger ~410 Ma granites could in turn have been formed by a further 80% fractional crystallization at lower pressures (1–2 kbar). Trace element and Sr-Nd isotopic modeling suggest incorporation of ~5% slab-derived sediment into mantle magma source of these high Ba-Sr granitoids. We note that post-collisional granitoids with high Ba and Sr characteristics may also reflect a sediment contribution in their source region, and this may be a key aspect of why such granites plot in the field of post-collisional granites in the Rb vs Y + Nb diagram. Some sanukitoids with high Ba and Sr contents in the late Archean also appear to reflect sediment subduction and they hence may represent early stages of crustal recycling.

Original languageEnglish
Pages (from-to)177-192
Number of pages16
JournalGondwana Research
Volume69
Early online date2 Feb 2019
DOIs
Publication statusPublished - May 2019

Keywords

  • High Ba-Sr granitoids
  • Petrogenesis
  • Post-collisional setting
  • The Alxa block
  • The North Qilian

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