Miocene adakitic volcanism in eastern Lhasa Terrane, Tibet: perspective from zircon U–Pb geochronology, whole-rock geochemistry and Sr–Nd–Hf isotopes

ABSTRACT

The Riduo intermediate-felsic volcanic rocks are located in the eastern Lhasa terrane in southern Tibet. Zircon U–Pb dating, geochemical and Sr–Nd–Hf isotope analyses were carried out to constrain the age, petrogenesis and magma source of the Riduo magma suite, including andesite, dacitic tuff, and quartz diorite. Zircon U–Pb dating yielded Early Miocene ages of 19.5 ± 0.5 Ma (andesite), 17.6 ± 0.3 Ma (dacitic tuff), and 15.6 ± 0.5 Ma (quartz diorite). Geochemically, this early Miocene suite is adakitic with high SiO₂ (55.76–69.46 wt.%), Al₂O₃ (15.21–16.61 wt.%), Na₂O (3.82–4.83 wt.%), Sr (427–1033 ppm), and Sr/Y (24.5–145), but low Y (< 17.8 ppm) and Yb (< 1.82 ppm). The rocks also have low to high K₂O (0.44–3.82 wt.%). They are enriched in light rare earth elements (LREEs), large ion lithophile elements (LILEs, e.g. K, Rb, Th and U), but depleted in high-field strength elements (e.g. Nb, Ta) and heavy rare earth elements (HREEs), and have weakly negative to positive Eu anomalies. Isotope analyses reveal that most samples have high whole-rock initial ⁸⁷Sr/⁸⁶Sr ratios (0.705611–0.708807), low ε_Nd(t) (−2.25 to −1.15), and positive zircon ε_Hf (t) (+1.29 to +6.49). We suggest that the early Miocene magmas in the eastern Lhasa terrane were likely sourced from post-collisional partial melting of the juvenile lower crust with minor mantle input. The magmas may have been crustal-contaminated during their ascent to the upper crust along the N-S-trending Cona-Riduo rift.

KEYWORDS:

• Adakite
• Miocene
• juvenile lower crust
• Riduo
• eastern Lhasa terrane

Highlights

• The Miocene Riduo igneous suite in the Gangdese Copper Belt is investigated.

• Intermediate-felsic volcanism was U-Pb dated to be Miocene (ca. 19.5–15.6 Ma).

• The volcanic rocks have adakitic geochemical affinities.

• Riduo adakitic magma likely derived from partial melting of juvenile lower crust.

Acknowledgments

We thank the editor and reviewer who have contributed to improve this work with their insightful comments and suggestions. We thank Fang-lin Han, Xin-lin Li, and Lian-dang Zhao for their field assistance, and Zhi Zhang for the insightful discussion.

Disclosure statement

No potential conflict of interest was reported by the author(s).

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Additional information

Funding

This research was financially supported by the second Tibetan Plateau Scientific Expedition and Research (STEP) [Grant No. 2019QZKK0802] and China Geological Survey [Project DD20160015].