Origin of the volcanic rocks in Dianzhong Formation, central Lhasa Terrane, Tibet: implication for the genesis of syn-collisional magmatism and Neo-Tethyan slab roll-back

The origin and formation of the continental collision-related magmas remain elusive. The volcanic rocks erupted during India-Asia continental collision offer an ideal opportunity to explore their genesis and geodynamic process. Here, we report new zircon U-Pb dating results and Hf isotope, whole-rock element and Sr-Nd isotope data of volcanic rocks in Linzhou Basin, central Lhasa Terrane, southern Tibet. These volcanic rocks are mainly comprised of andesites and belong to Dianzhong Formation. The timing of their formation is ca. 63–66 Ma, coeval with the India-Asia continental initial collision in the central part of southern Tibet. All these rocks show an arc-like geochemical affinity and they have more depleted Sr-Nd-Hf isotopic compositions (⁸⁷Sr/⁸⁶Sr_i = 0.705006–0.705963, ε_Nd(t) = −1.78 to 3.52, zircon ε_Hf(t) = 1.2–7.0) than the Cretaceous pre-collisional andesites and Eocene ancient lithospheric mantle-derived melts beneath the central Lhasa Terrane. The correlation between Mg# and CaO, TiO₂, Al₂O₃, Sr/Y, ⁸⁷Sr/⁸⁶Sr, ε_Nd(t) suggests that they were likely hybrid production between an isotopically depleted end-member and the enriched lithospheric mantle (ELM). The oceanic crust-derived melts would be the best candidate for the former end-member according to their depleted Sr-Nd isotopic compositions. The andesitic rocks of Dianzhong Formation in the central Lhasa Terrane were most likely stemmed from partial melts of altered Neo-Tethyan crust and then mixed with the ELM-derived melts. Given the Cretaceous-Tertiary upper crustal shortening, back-arc extension, and voluminous volcanism in the Lhasa Terrane, along with abrupt increasing in the magma temperature and the convergence rate between India and Asia during Palaeocene, the occurrence of the Dianzhong Formation volcanic rocks can be well explained by the Neo-Tethyan slab roll-back. The mixing between the oceanic crust- and the continental lithospheric mantle-derived melts induced by the oceanic slab roll-back would be a complimentary scenario for the formation of the syn-collisional magmatism in collisional orogeny belts.

与大陆碰撞相关的岩浆起源与形成机制至今仍悬而未决。印亚大陆碰撞期喷发的火山岩，为探究其成因与地球动力学过程提供了理想契机。本文报道了西藏南部拉萨地体（Lhasa Terrane）中部林周盆地（Linzhou Basin）火山岩的最新锆石U-Pb定年、铪（Hf）同位素、全岩主微量元素以及锶-钕（Sr-Nd）同位素数据。这些火山岩以安山岩为主，隶属于典中组（Dianzhong Formation），形成时代约为63~66 Ma，与西藏南部中部的印亚大陆初始碰撞时限同期。所有岩石均表现出类弧地球化学亲和性，且相较于拉萨地体中部白垩纪碰撞前安山岩以及始新世古老岩石圈地幔成因熔体，其Sr-Nd-Hf同位素组成更为亏损（⁸⁷Sr/⁸⁶Sr_i = 0.705006~0.705963，ε_Nd(t) = -1.78~3.52，锆石ε_Hf(t) = 1.2~7.0）。Mg#与CaO、TiO₂、Al₂O₃、Sr/Y、⁸⁷Sr/⁸⁶Sr、ε_Nd(t)之间的相关性表明，这些岩石可能是同位素亏损端元与富集型岩石圈地幔（ELM）熔体的混合产物。结合其亏损的Sr-Nd同位素组成，洋壳成因熔体最有可能是该亏损端元的候选物。拉萨地体中部典中组安山岩最有可能源自蚀变新特提斯洋壳的部分熔融，并随后与富集型岩石圈地幔熔体发生混合。考虑到拉萨地体在白垩纪-古近纪经历的上地壳缩短、弧后伸展以及大规模火山作用，再结合古新世时期岩浆温度与印亚板块汇聚速率的陡然升高，典中组火山岩的产出可通过新特提斯洋板片回撤得到合理解释。由洋板片回撤诱发的洋壳与大陆岩石圈地幔成因熔体的混合作用，可作为碰撞造山带同碰撞岩浆作用的一种补充成因机制。