Geochemistry, geochronology, and petrogenesis of mid-Cretaceous Talabuco volcanic rocks, central Tibet: implications for the evolution of the Bangong Meso-Tethys

The widespread mid-Cretaceous igneous rocks in the northern margin of the Lhasa Block play an important role in understanding deep geologic processes, matter exchange at depth, and tectonic evolution of the Tibetan Plateau. In this paper, we report new zircon U-Pb ages, whole-rock major and trace element data, and Sr-Nd-Pb-Hf isotope data from the Talabuco andesites and basalts, which were dated at ~111 Ma. These rocks belong to the high-K calc-alkaline and shoshonite series, and show enrichment in terms of large-ion-lithophile elements (LILEs, e.g. Rb, U, and Th) and light rare earth elements (LREEs), but depletion of high-field-strength elements (HFSEs, e.g. Nb and Ti). The (87Sr/86Sr)i ratios of the Talabuco andesites range from 0.7043 to 0.7048, and the εNd(t) contents range from 0.68 to 4.33. The ratios of 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb are 18.6064–18.8993, 15.6233–15.6707, and 38.8634–39.1720, respectively. The 176Lu/177Hf and 176Hf/177Hf ratios of one sample range from 0.00081 to 0.00206 and 0.28280 to 0.28296, respectively. The εHf(t) values for this sample range from 3.4 to 9.1, and the two-stage model Hf age (TDM2) is 0.59–0.95 Ga. Combined with previous studies, the geochemical and isotopic data reveal that the parental magma of the Talabuco andesites was probably derived by partial melting of EM II-type sub-continental lithospheric mantle (SCLM). The Talabuco andesites are most likely generated by fractionation of mafic magma contaminated by subducted oceanic sediment and represent product of arc magmatism due to northward subduction of the Yarlung Zangbo Neo-Tethyan slab or southward subduction of the Bangong Meso-Tethyan slab.

拉萨地块北缘广泛发育的白垩纪中期火成岩，对于解析青藏高原深部地质过程、深部物质交换及构造演化具有重要科学意义。本文报道了塔拉布科（Talabuco）安山岩与玄武岩的最新测试数据，包括锆石U-Pb年代学数据、全岩主量与微量元素数据以及Sr-Nd-Pb-Hf同位素数据，其锆石U-Pb定年结果约为111 Ma。该套岩石属于高钾钙碱性系列与钾玄岩系列，表现出大离子亲石元素（large-ion-lithophile elements, LILEs，如Rb、U、Th）和轻稀土元素（light rare earth elements, LREEs）富集，而高场强元素（high-field-strength elements, HFSEs，如Nb、Ti）亏损的地球化学特征。塔拉布科安山岩的初始87Sr/86Sr比值介于0.7043~0.7048之间，εNd(t)值介于0.68~4.33之间；其206Pb/204Pb、207Pb/204Pb、208Pb/204Pb比值分别为18.6064~18.8993、15.6233~15.6707与38.8634~39.1720。对其中一件样品的测试结果显示，其176Lu/177Hf比值介于0.00081~0.00206之间，176Hf/177Hf比值介于0.28280~0.28296之间；该样品的εHf(t)值介于3.4~9.1之间，两阶段Hf模式年龄（TDM2）为0.59~0.95 Ga。结合前人研究成果，地球化学与同位素数据表明，塔拉布科安山岩的母岩浆可能起源于EM II型陆下岩石圈地幔（SCLM）的部分熔融。塔拉布科安山岩最可能是受俯冲洋壳沉积物混染的镁铁质岩浆经分异结晶形成的产物，代表了雅鲁藏布江新特提斯洋板片北向俯冲，抑或是班公湖中生特提斯洋板片南向俯冲引发的弧岩浆作用产物。