Early Palaeozoic arc-related gabbro-diorite suite in East Junggar, southern Central Asian Orogenic Belt: petrogenesis and tectonic implications

ABSTRACT

The Armantai and Kalamaili oceans were both important branches of the Palaeo-Asian Ocean between the Altai microcontinent and the Junggar juvenile crust; however, their geological evolution is not completely clear. This paper reports detailed petrological, zircon geochronological, whole-rock elemental and Sr-Nd-Pb isotopic data for a newly discovered gabbro-diorite suite (Akputo pluton) in East Junggar to constrain this issue. Quartz diorite and hornblende gabbro yield intrusion ²⁰⁶Pb/²³⁸U ages of ca. 444–440 Ma, which indicates that mafic-intermediate magmatism occurred in the earliest Silurian. The gabbroic-dioritic rocks have tholeiite to calc-alkaline affinities and are characterised by moderately fractionated rare earth element patterns with flat heavy rare earth elements, enrichment in Rb, Ba, K, Sr, and depletion in Nb, Ta, Ti. Low initial Sr and Pb isotopic compositions and positive ε_Nd(t) indicate a derivation from the isotopically depleted mantle sources. The geochemical compositions and melting calculations suggest that the gabbro-diorite suite originated from partial melting of spinel-bearing mantle sources that had been metasomatised by subduction-derived fluids and sediments. The Akputo intrusions formed in an arc setting related to the southward subduction of the Armantai Ocean. Taking the regional angular unconformity between the Ordovician sediments and Silurian volcanics into account, we further suggest that the Early to Middle Silurian was a transition period of the closure of Armantai Ocean and the initial spreading of Kalamaili Ocean.

KEYWORDS:

• Diorite
• petrogenesis
• subduction
• East Junggar
• CAOB

Article Highlights

• Detailed data on the geochronology and geochemistry of a Silurian gabbro-diorite suite from East Junggar are presented;

• The distribution of early Palaeozoic magmatism around East Junggar is presented and the geochemical characteristics are described;

• Intrusions were derived from spinel-bearing mantle sources that had been metasomatised from subduction-derived fluids and sediments;

• These rocks were formed in a Late Ordovician to earliest Silurian arc setting;

• A new early Palaeozoic tectonic evolution model for the southern Central Asian Orogenic Belt is presented.

Acknowledgments

We are sincerely grateful to editor Robert J. Stern, three anonymous reviewers and Ernst Hegner for their detailed and constructive comments, which substantially improved our manuscript. We thank Liangyu Wang, Chaobin Hu, Ting Luo, Jinming Tian, Pan Zhang and Hongfei Liu for their field assistance. Thank Xueming Yang for his valuable suggestions. This study was jointly supported by the National Natural Science Foundation of China (Grant No. 41672295) and China Geological Survey Projects (Grants No. DD20160060 and No. 12120114042801).

Disclosure statement

No potential conflict of interest was reported by the authors.

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Additional information

Funding

This work was supported by the National Natural Science Foundation of China [41672295]; China Geological Survey Projects [12120114042801,DD20160060].