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Xenoliths in ultrapotassic volcanic rocks in the Lhasa block: direct evidence for crust–mantle mixing and metamorphism in the deep crust

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Abstract

Felsic granulite xenoliths entrained in Miocene (~13 Ma) isotopically evolved, mantle-derived ultrapotassic volcanic (UPV) dykes in southern Tibet are refractory meta-granitoids with garnet and rutile in a near-anhydrous quartzo-feldspathic assemblage. High F–Ti (~4 wt.% TiO2 and ~3 wt.% F) phlogopite occurs as small inclusions in garnet, except for one sample where it occurs as flakes in a quartz-plagioclase-rich rock. High Si (~3.45) phengite is found as flakes in another xenolith sample. The refractory mineralogy suggests that the xenoliths underwent high-T and high-P metamorphism (800–850 °C, >15 kbar). Zircons show four main age groupings: 1.0–0.5 Ga, 50–45, 35–20, and 16–13 Ma. The oldest group is similar to common inherited zircons in the Gangdese belt, whereas the 50–45 Ma zircons match the crystallization age and juvenile character (εHf i +0.5 to +6.5) of Eocene Gangdese arc magmas. Together these two age groups indicate that a component of the xenolith was sourced from Gangdese arc rocks. The 35–20 Ma Miocene ages are derived from zircons with similar Hf–O isotopic composition as the Eocene Gangdese magmatic zircons. They also have similar steep REE curves, suggesting they grew in the absence of garnet. These zircons mark a period of early Miocene remelting of the Eocene Gangdese arc. By contrast, the youngest zircons (13.0 ± 4.9 Ma, MSWD = 1.3) are not zoned, have much lower HREE contents than the previous group, and flat HREE patterns. They also have distinctive high Th/U ratios, high zircon δ18O (+8.73–8.97 ‰) values, and extremely low εHf i (−12.7 to −9.4) values. Such evolved Hf–O isotopic compositions are similar to values of zircons from the UPV lavas that host the xenolith, and the flat REE pattern suggests that the 13 Ma zircons formed in equilibrium with garnet. Garnets from a strongly peraluminous meta-tonalite xenolith are weakly zoned or unzoned and fall into four groups, three of which are almandine-pyrope solid solutions and have low δ18O (+6 to 7.5 ‰), intermediate (δ18O +8.5 to 9.0 ‰), and high δ18O (+11.0 to 12.0 ‰). The fourth is almost pure andradite with δ18O 10–12 ‰. Both the low and intermediate δ18O groups show significant variation in Fe content, whereas the two high δ18O groups are compositionally homogeneous. We interpret these features to indicate that the low and intermediate δ18O group garnets grew in separate fractionating magmas that were brought together through magma mixing, whereas the high δ18O groups formed under high-grade metamorphic conditions accompanied by metasomatic exchange. The garnets record complex, open-system magmatic and metamorphic processes in a single rock. Based on these features, we consider that ultrapotassic magmas interacted with juvenile 35–20 Ma crust after they intruded in the deep crust (>50 km) at ~13 Ma to form hybridized Miocene granitoid magmas, leaving a refractory residue. The ~13 Ma zircons retain the original, evolved isotopic character of the ultrapotassic magmas, and the garnets record successive stages of the melting and mixing process, along with subsequent high-grade metamorphism followed by low-temperature alteration and brecciation during entrainment and ascent in a late UPV dyke. This is an excellent example of in situ crust–mantle hybridization in the deep Tibetan crust.

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Acknowledgments

This study was funded by CSIRO OCE postdoc three-year (2014–2017) project to Rui Wang, Australian Research Council DP110102543 to Roberto F. Weinberg and DP120104004 to William J. Collins, a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada to Jeremy Richards, the China National Funds for Distinguished Scientists (No. 41302054) to Li-min Zhou, and IGCP/SIDA-600. Anna Oh is thanked for SIMS zircon sample preparation and dating, and Robert Creaser is thanked for whole-rock Sr–Nd isotopic analysis.

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Authors and Affiliations

  1. CSIRO Mineral Resources Flagship, Kensington, Perth, WA, 6151, Australia

    Rui Wang

  2. NSW Institute for Frontiers Geoscience, University of Newcastle, Newcastle, NSW, 2308, Australia

    William J. Collins

  3. School of Earth, Atmosphere and Environment, Monash University, Clayton, VIC, 3800, Australia

    Roberto F. Weinberg

  4. Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, 100085, People’s Republic of China

    Jin-xiang Li

  5. Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037, People’s Republic of China

    Qiu-yun Li & Zengqian Hou

  6. School of Earth and Space Sciences, Peking University, Beijing, 100871, People’s Republic of China

    Qiu-yun Li

  7. State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences and Resources, China University of Geoscience, Beijing, 100083, People’s Republic of China

    Wen-yan He

  8. Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, T6G 2E3, Canada

    Jeremy P. Richards & Richard A. Stern

  9. National Research Centre for Geoanalysis, Chinese Academy of Geological Sciences, Beijing, 100037, People’s Republic of China

    Li-min Zhou

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  1. Rui Wang
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Correspondence to Rui Wang.

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Wang, R., Collins, W.J., Weinberg, R.F. et al. Xenoliths in ultrapotassic volcanic rocks in the Lhasa block: direct evidence for crust–mantle mixing and metamorphism in the deep crust. Contrib Mineral Petrol 171, 62 (2016). https://doi.org/10.1007/s00410-016-1272-6

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