Abstract The Yardoi dome is located in the eastern end of the north Himalayan domes (NHD), distributed in northwest‐southeast direction. Previous mapping and analytical studies established the distribution of three lithologic–tectonic units, positions of the upper detachment fault and the lower detachment fault. The lower detachment fault is a strongly deformed top‐NW shear zone with ∼800 m thick, termed as the Yardoi detachment fault (YDF). LA‐ICP‐MS zircon U–Pb dating yielded a crystallization age of 19.57 ± 0.23 Ma ∼15.5 ± 0.11 Ma for the leucogranite dyke swarm, which indicates that the ductile motion along the YDF began at ca.20 Ma. Muscovite from the three studied samples yielded 40 Ar/ 39 Ar ages between 14.05 ± 0.2 Ma and 13.2 ± 0.2 Ma. The biotite from the garnet‐bearing two‐mica gneiss have 40 Ar‐ 39 Ar age of 13.15 ± 0.2 Ma. These results suggest that the exhumation led to cooling through the 370°C Ar closure temperature in muscovite at ∼14 Ma to the 335°C Ar closure temperature in bioitte at ∼13 Ma. Thermochronological data indicates that ductitle motion along the YDF continued until at least 13 Ma. We correlate the YDF to the South Tibet detachment system (STDS) on the basis of lithological, metamorphic, structural, and chronological criteria and consider the YDF, a top‐to‐northwest shear zone, may represent the northern extension of the STDS. Comparing the Yardoi dome to the other domes in the NHD, the cooling ages show a clear diachronism and they are progressively younger from the west to east Himalaya.
The Kangxiwar ductile strike-slip shear zone marks the southern boundary of the West Kunlun terrane, a large, nearly E-W trending metamorphic terrane in the western Qinghai Tibet Plateau region. This ductile shear zone is ∼7 km wide, and consists of mylonitized khondalites. Protoliths of the khondalites were alumina-rich pelitic sedimentary and subordinate volcanic rocks. The pelitic khondalites have pronounced positive Th anomalies and subdued positive Ce and Zr anomalies, whereas the metavolcanic rocks have positive Nb and Zr anomalies. Both types of khondalite are LREE enriched, and show weak HREE depletions and moderate negative Eu anomalies. P-T conditions for the formation of the khondalites are estimated to be 6.8 kbar and 700°C. The khondalites formed in the Caledonian orogeny (428-445 Ma) and underwent strong shear deformation during the Indosinian (250-210 Ma). SHRIMP dating of detrital zircons in the khondalites suggests that they were derived from an older metamorphic basement, probably older than 644 Ma. The Kangxiwar khondalites are similar in their protoliths, trace- element and rare-earth element geochemistry, P-T conditions, and age of formation to those of the South Altyn Tagh khondalite series. This lateral correlation suggests that the West Kunlun and Altyn Tagh terranes were once contiguous, and provides evidence for the existence of a Caledonian orogenic belt in this region.
Abstract The Gangdese magmatic belt is located in the southern margin of the Lhasa terrane, south Tibet. Here zircon U–Pb ages and Hf isotopic data, as well as whole‐rock geochemistry and Sr–Nd isotopes on andesites from the Bima Formation with a view to evaluating the history of the Gangdese magmatism and the evolution of the Neotethys Ocean. Zircon U–Pb dating yields an age of ca 170 Ma from six samples, representing the eruptive time of these volcanic rocks. Zircon Hf isotopes show highly positive ε H f (t) values of +13 to +16 with a mean of +15.2. Whole‐rock geochemical and Sr–Nd isotopic results suggest that the magma source of these andesites was controlled by partial melting of a depleted mantle source with addition of continental‐derived sediments, similar to those in the southern arcs of the Lesser Antilles arc belt. In combination with published data, the volcanic rocks of the Bima Formation are proposed to have been generated in an intra‐oceanic arc system, closely associated with northward subduction of the Neotethyan oceanic lithosphere.
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