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The Tethys Himalaya tectonic belt has long been a passive continental margin north of Gondwana, commonly known as the Tethys Himalaya, which Indo-Asian collision orogeny developed until the Cenozoic era. It is of great scientific significance to determine its geo-tectonic evolution over geological history. Therefore, to reveal the geodynamic background of the Late Cretaceous Tethys Himalaya and its internal connection with the subduction of the Neo-Tethys ocean, in this study, we analysed the petrology, and whole-rock geochemistryof the Zongzhuo Formation basalt, which is widely distributed in the Tethys Himalaya Rongbu area. Further, we investigated the basalt types, magma source, tectonic setting and causes that gave rise to the Zongzhuo Formation to further research on the evolution of the Tethys Himalaya tectonic domain. The Zongzhuo Formation features a basalt set of compatible elements and is characterised by low Cr (average: 86.838 ppm) and enrichment of high-field-strength elements (e.g. Nb, Ta and Hf). The characteristics of the major elements, i.e. high TFeO (average: 10.43 wt %), TiO2 (average: 3.44 wt %) and P2O5 (average: 0.47 wt %), indicate an oceanic–island–basalt (OIB) geochemical signature with obvious light REE and heavy REE fractionation. The characteristics of the trace elements confirm the alkalinity of the Zongzhuo Formation basalts. Based on these results, the Zongzhuo Formation basalt was assigned to within–plate OIB. The magma might be the metasomatism products of the oceanic lithosphere mantle with low crystal fractionation and no crustal contamination, its tectonic background includes a passive–to–active transformation in the continental margin. By analysing the Tethys Himalaya belt in the south of Tibet, we concluded that the Zongzhuo Formation basalt strata in the Rongbu area were likely formed owing to Mesozoic volcanic activity and the extension and thinning of the Tethys Himalaya in the area. [ABSTRACT FROM AUTHOR]

The Lhunze region of southeast Tibet uniquely juxtaposes Cenozoic granitic plutons with subvolcanic rocks in the Tethyan Himalayan Sequence. In this study, the potential petrogenetic connection between two-mica granite and subvolcanic rhyolite porphyry from the Lhunze area is investigated using petrology, geochronology, and geochemistry. Monazite Th Pb ages imply the two-mica granite crystallized during the mid-Eocene at c. 43.5–43.1 Ma, consistent with previous zircon U Pb isotope data. Monazite in the porphyry also records Th Pb ages of c. 44.5–42.9 Ma , suggesting contemporaneous emplacement with the two-mica granite. Porphyritic structure, feldspar aggregates and comb alignment demonstrate that the two-mica granite resulted from transient melt flow associated with melt extraction. Some plagioclase phenocrysts from the porphyry have consistent An contents with the two-mica granite, indicating that both the two-mica granite and the porphyry probably originated from a similar melt. However, the porphyry has a higher SiO 2 concentration (72.2–75.9 wt%) than the two-mica granite (69.0–73.0 wt%). In addition, the porphyry is characterized by lower TiO 2 , MgO, CaO, Ba, Sr, and Zr contents as well as Zr/Hf, Sr/Y, Nb/Ta ratios than the two-mica granite. Moreover, strontium and REE contents of monazite in both the two-mica granite and porphyry also imply that crystal fractionation dominated melt evolution. Whole-rock Sr Nd and in situ monazite Nd isotope compositions indicate that the porphyry is slightly more radiogenic than the two-mica granite, implying that the porphyry likely assimilated more ancient country rocks during its evolution. We interpret the two-mica granite and porphyry as comagmatic intrusives linked by fractional crystallization of a cognate magma mush, in which the crystals accumulated to form the two-mica granite, and the complementary high silicic melt segregated to form the porphyry. Plagioclase accumulation is responsible for the generation of the high Sr /Y adakitic geochemical feather within two-mica granite. The scenario of complementary melt extraction and crystal fractionation might also have been an important process in forming the Himalayan leucogranites. Regionally, Neo-Tethyan slab breakoff during the Eocene could have increased mantle heat flow and triggered the generation and extraction of large volumes of silicic melt. • Two-mica granite and subvolcanic rhyolite porphyry have a coeval crystallization age at 44–43 Ma. • Two-mica granite represents cumulate and the porphyry as its complementary highly fractionated melt. • Magma differentiation via melt extraction from crystal mush is an effective mechanism to produce Himalayan leucogranites. [ABSTRACT FROM AUTHOR]