Petrogenesis and tectonic setting of Early Cretaceous magmatism in the Jiwa area, central Lhasa Terrane, Tibet

New zircon LA-ICP-MS U–Pb ages, Sr-Nd isotopic data, and whole-rock major and trace element data from Early Cretaceous volcanic rocks are reported for the Jiwa area in the southern central Lhasa Terrane of Tibet. These mainly silicic volcanic rocks and subordinate intermediate-basic volcanic rocks have long been considered to be Pliocene (Wuyu Group) or Eocene (Pana Formation) in age. However, our new zircon U–Pb ages constrain the timing of eruption to the Early Cretaceous (124.6 ± 1.6–126.1 ± 1.1 Ma); thus, we have redefined these volcanic rocks as the Lower Cretaceous Zenong Group. The silicic volcanic rocks feature high-K calc-alkaline to shoshonitic compositions and are mostly strongly peraluminous, rich in Rb, Th, and light rare earth elements (LREEs), and depleted in Nb, Ta, P, and Ti. They are also characterized by negative whole-rock εNd(t) values (–9.1 to –13.1) and variable 87Sr/86Sr ratios. Thus, the geochemical and zircon U–Pb age data of the Jiwa volcanic rocks suggest that these rocks are associated with a continental arc and are mostly likely derived from anatexis of ancient continental crustal material and minor basalt-derived melts. The discovery of Early Cretaceous volcanic rocks in the southern central Lhasa Terrane extends the duration of magmatism triggered by southward subduction of the Bangong-Nujiang oceanic lithosphere from the Late Jurassic to the Early Cretaceous. The spatial distribution of magmatism is also extended 70–80 km to the south. The Lower Cretaceous volcanic rocks in the Jiwa area are proposed to be a result of bi-directional subduction, with southward subduction of the Bangong-Nujiang oceanic crust and northward subduction of the Yarlung Zangbo oceanic crust. The bi-directional subduction of the oceanic lithosphere and gravitational sinking led to slab retreat at ca. 125 Ma. The roll-back of the slab would have then led to back-arc extension and asthenospheric upwelling. The subduction-induced decompression melting of the mantle led to the generation of widespread rhyolitic volcanism with continental arc geochemical signatures.本文报道了西藏拉萨地体中南部吉瓦地区早白垩世火山岩的激光剥蚀电感耦合等离子体质谱（LA-ICP-MS）锆石U-Pb年代学数据、锶-钕同位素数据，以及全岩主量与微量元素数据。该套岩石以长英质火山岩为主，伴生少量中基性火山岩，长期以来被认为形成于上新统（吾玉组）或始新统（帕那组）。然而，本文新获得的锆石U-Pb年代学数据将其喷发时代约束为早白垩世（124.6±1.6~126.1±1.1 Ma），因此将这套火山岩重新厘定为下白垩统则弄群（Zenong Group）。该套长英质火山岩具有高钾钙碱性至钾玄质的地球化学特征，多数属于强过铝质岩石，富集铷（Rb）、钍（Th）与轻稀土元素（LREEs），亏损铌（Nb）、钽（Ta）、磷（P）与钛（Ti）；其全岩εNd(t)值为负值（-9.1~-13.1），87Sr/86Sr比值变化范围较大。综上，吉瓦地区火山岩的地球化学与锆石U-Pb年代学数据表明，这套岩石形成于大陆弧环境，最可能来源于古老大陆地壳物质的深熔作用与少量玄武质熔体的混入。拉萨地体中南部早白垩世火山岩的发现，将班公湖-怒江洋岩石圈向南俯冲引发的岩浆作用时限从晚侏罗世延伸至早白垩世，同时将岩浆作用的空间分布向南拓展了70~80 km。本文认为吉瓦地区的下白垩统火山岩是双向俯冲作用的产物：即班公湖-怒江洋壳向南俯冲，以及雅鲁藏布江洋壳向北俯冲。大洋岩石圈的双向俯冲与重力沉陷作用导致板片在约125 Ma时发生后撤，板片回滚进而引发弧后伸展与软流圈上涌。俯冲作用诱发的减压熔融使得地幔产生广泛的流纹质火山作用，且具有大陆弧的地球化学特征。