Compositional and tectonomagmatic evolution of Early Cretaceous magmatism in the central Lhasa suberrane, Tibet: Implications from the Zenong Group volcanic rocks

The Lhasa Terrane in southern Tibet is widely recognized as having separated from the northern margin of Gondwana with a Precambrian basement and undergoing a protracted and intricate evolution. Abundant Early Cretaceous volcanic rocks are present in the central Lhasa subterrane, Tibet, playing an essential role in models aimed at comprehending the tectonic-magmatic evolution and mantle-crust interaction of this terrane. In this study, we present a well-preserved section of Zenong Group volcano-sedimentary sequence in Eyang, Xainza area within the central Lhasa subterrane. Our new data combined with existing literature data indicate that there was an extensive period of magmatism (approximately 140 Ma to 102 Ma) throughout the Early Cretaceous in the central Lhasa subterrane, reaching its peak around 113 Ma with remarkable compositional diversity.

However, the composition of Early Cretaceous volcanic rocks in the central Lhasa subterrane underwent a temporal transition from high-silica rhyolites to dacites and andesites, exhibiting a reverse cyclicity. Moreover, the intermediate rocks from the upper section display elevated whole rock εNd(t) and zircon εHf(t) values, as well as decreased ⁸⁷Sr/⁸⁶Sr ratios compared to the high-silica rocks from the lower section. These observations collectively suggest: (a) involvement of open-system processes encompassing mantle-derived magmas and ancient crustal-derived materials; (b) an increasing contribution of mantle sources in the magma genesis; (c) variable magma origins with distinct petrogenetic histories rather than a uniform source involving assimilation-fractional crystallization processes.

The high-silica rhyolites from the bottom of the Eyang section display characteristics of fractional crystallization and exhibit varying zircon ε_Hf(t) values (−16.7 to −7.8), negative ε_Nd(t) values (−13.7 to −13.1), highly variable initial Sr isotopic compositions, and radiogenic Pb isotopic signatures, indicating that a combined process of magma mixing (involving crustal-derived felsic melts and mantle-derived mafic melts) followed by fractional crystallization was primarily responsible for their formation. The dacitics from the upper part of the Eyang section show higher ε_Hf(t) values (−9.9 ~ +0.5) and ε_Nd(t) values (−10.6 to −9.5) than the high-silica rhyolites, suggesting that these dacitic rocks were also largely derived from anatexis of ancient crustal material with more involvement of mantle-derived magmas. The andesites exhibit more enriched Sr-Nd-Hf isotopic compositions compared to the contemporaneous dacitics, as well as less radiogenic Pb isotopic compositions, suggesting their likely derivation from partial melting of an enriched mantle wedge previously metasomatized by melts derived from subducted sediments.

We propose that the high-silica rhyolites in the lower section of the Xainza area (≥ ca. 120 Ma) are associated with slab roll-back, while the dacites and andesites in the upper section (≤ca. 120 Ma) are linked to slab break-off during southward subduction of Bangong-Nujiang Ocean lithosphere. Furthermore, it is evident that the ancient basement of the central Lhasa subterrane underwent localized reworking by mantle-derived melts.