Early Neoproterozoic granitic gneisses in the Amdo micro-continent, Tibet: petrogenesis and geodynamic implications

ABSTRACT

The palaeogeographic locations of ancient continental blocks in the Tibetan Plateau, especially of micro-continental blocks during the Neoproterozoic supercontinent cycle, remain enigmatic. To clarify the petrogenesis of Neoproterozoic granitic gneisses and the early evolution of the Amdo micro-continent, we conducted an integrated study of the petrography, geochronology, and geochemistry of biotite monzonitic gneisses from the Precambrian metamorphic basement. The biotite monzonitic gneiss sample yielded a weighted mean zircon U–Pb age of 820 ± 11 Ma, indicating an early Neoproterozoic magmatic event. All gneiss samples show enrichment in light rare-earth elements and depletion in high-field-strength elements in N-MORB-normalized spider diagrams, which suggests that their precursor magma was generated in a subduction-related active continental margin. In addition, these rocks have mostly positive ε_Hf(t) values (−1.0 to 7.0) and corresponding crustal model ages of 1.3 − 1.8 Ga. Zircon Hf isotopes and whole-rock geochemistry indicate that the gneisses were derived from partial melting of juvenile crust material. Recently published age spectra of detrital zircons from different blocks suggest that the Amdo micro-continent had a closer affinity to the Lhasa terrane than to the Qiangtang terrane prior to and during the break-up of the Rodinian supercontinent. However, the gneisses are geochemically distinct from back-arc magmatic rocks in the Lhasa terrane and similar to coeval Andean-type magmatic rocks in blocks that constituted the northwestern margin of Rodinia (including Madagascar, Seychelles, and Malani). This study therefore provides new constraints on the Neoproterozoic palaeogeographic location and evolution of the Amdo micro-continent.

KEYWORDS:

• Tibetan Plateau
• Amdo micro-continent
• early Neoproterozoic
• granitic gneiss
• zircon U–Pb age

Research highlights

• Early Neoproterozoic (~820 Ma) gneisses occur in the Amdo micro-continent, Tibet.

• Their geochemical data suggest a subduction-related setting.

• Amdo micro-continent had a close affinity to the Lhasa terrane prior to and during the break-up of the Rodinian supercontinent.

• Amdo micro-continent should be part of an Andean-type orogeny along the NW margin of the Rodinia.

Acknowledgments

We thank editor and reviewers for constructive and helpful comments, Professor Li Su and Hongyu Zhang at China University of Geosciences (Beijing, China) for their help during analyses. This research was funded by National Natural Science Foundation of China (Grant numbers 41872231, 41402190), Key Laboratory of Mineral Resources Evaluation in Northeast Asia, Ministry of Land and Resources of China (DBY–ZZ–18–06) and Graduate Innovation Fund of Jilin University (Grant numbers 419100200483).

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary material

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Additional information

Funding

This work was supported by the National Natural Science Foundation of China [41402190, 41872231]; Key Laboratory of Mineral Resources Evaluation in Northeast Asia, Ministry of Land and Resources of China [DBY–ZZ–18–06]; Graduate Innovation Fund of Jlilin University [419100200483].