Triassic-Jurassic granitoids and pegmatites from western Kunlun-Pamir Syntax; implications for the Paleo-Tethys evolution at the northern margin of the Tibetan Plateau

The Western Kunlun-Pamir-Karakorum (WKPK) at the northwestern Tibetan Plateau underwent long-term terrane accretion from the Paleozoic to the Cenozoic. Within this time span, four phases of magmatism occurred in WKPK during the Early Paleozoic, Triassic-Jurassic, Early Cretaceous, and Cenozoic. These voluminous magmatic rocks contain critical information on the evolution of the Tethys Oceans. In this contribution, we provide field observations, petrography, ages, whole-rock elemental and Sr-Nd isotopic compositions, and zircon in situ Lu-Hf isotopes of the Triassic-Jurassic granitoids and pegmatites from the Dahongliutan in Western Kunlun and Turuke area at the Pamir Plateau, in an attempt to constrain their petrogenesis and to decipher a more detailed Paleo-Tethys evolution process. The Dahongliutan pluton is composed of diorites (ca. 210 Ma) and monzogranite (ca. 200 Ma). The diorites have moderate SiO2 (56.77-62.22 wt.%), variable Mg# (46-49), and low Cr (34.4-50.6 ppm) and Ni contents (7.0-14.5 ppm). They show LREE-enriched patterns (LaN/YbN=4.3-17), with variable negative Eu anomalies (0.63-0.91) and variable ratios of Nb/La (0.27-0.97). Isotopically, the diorites display enriched whole-rock εNd(t) (-5.43 to -7.67) and negative to positive zircon εHf(t) values (-6.6 to 0.4). They were most likely generated by melting of a subduction-modified mantle source with subsequent assimilation and fractional crystallization. The Turuke monzogranites (ca. 202-197 Ma) have S-type granite characteristics and are characterized by high SiO2 (70.36-76.12 wt.%) and A/CNK values (1.19-1.36), variable LREE-enriched patterns (LaN/YbN=8.87-14.40), negative Eu anomaly (0.07-0.56), relatively uniform whole-rock εNd(t) (-10.49 to -11.22), and variable negative zircon εHf(t) values (-10.7 to -1.3). They were probably generated by muscovite-dehydration melting of dominantly metapelitic sources. The widespread pegmatites (ca. 195 Ma) at the Dahongliutan area record an extensional setting after the collision of Karakorum with the South Kunlun-Tianshuihai terrane. Combining our new data with the previous studies, we propose a divergent double-sided subduction of the Paleo-Tethys Ocean (243-208 Ma) and a gradual closure of the Paleo-Tethys Ocean from east (ca. 200 Ma) to west (ca. 180 Ma) to explain the Triassic-Jurassic tectono-magmatism in the WKPK.