Petrogenesis and Geodynamic Implications of Late Triassic Mogetong Adakitic Pluton in East Kunlun Orogen, Northern Tibet: Constraints from Zircon U–Pb–Hf Isotopes and Whole-Rock Geochemistry

Abstract

Adakites or adakitic rocks usually show special geochemical signatures and are petrological probes to reveal the tectono–magmatic evolutionary history of paleo–orogenic belts. Here, we present a comprehensive study on the zircon U–Pb geochronology, whole-rock geochemistry, and zircon Lu-Hf isotopes of Mogetong adakitic pluton in East Kunlun orogen, Northern Tibetan Plateau, to constrain its petrogenesis and tectonic setting, and thus to reveal its implications for the Paleo–Tethyan orogeny. The studied pluton comprises of quartz monzonite porphyry with zircon U–Pb crystallization age of ca. 215 Ma, which is coeval to their diorite enclaves (ca. 212 Ma). The quartz monzonite porphyries have intermediate SiO2 (63.31–65.74 wt%), relatively high Al2O3 (15.52–16.02 wt%), K2O (2.83–3.34 wt%), and Sr (462–729 ppm), but low Y (9.14–15.7 ppm) and Yb (0.73–1.39 ppm) with high Mg# (47–55), Sr/Y (30–57) and La/Yb ratios resembling typical high–K calc-alkaline and high Mg# adakitic rocks. Zircon Lu–Hf isotopes show that the studied samples have weakly juvenile zircon Lu-Hf isotopes (εHf(t) = 1.80–4.03) with older model age (1.00–1.14 Ga). The relative low content of Cr (14–59 ppm) and Ni (8–30 ppm), as well as the petrological, geochemical, and Lu-Hf isotopic data, indicates that the Mogetong adakitic rocks were generated by partial melting of thickened lower crust with a certain contribution of the underplated mantle-derived magma in slab break-off setting. This study shows that the Late Triassic adakitic magmatism in East Kunlun orogen may be the response of tectonic transition from oceanic subduction to post–subduction extension, and the reworking of ancient continental crust with subsequent variable crust-mantle magma mixing is the major mechanism of continental crust evolution in the Paleo–Tethyan orogenic belt.

Petrogenesis and Geodynamic Significance of the Early Triassic Nanpo Adakitic Pluton of the Luang Prabang-Loei Tectonic Belt (Northwestern Laos) in the East Tethys Domain: Constraints from Zircon U-Pb-Hf Isotope Analyses and Whole-Rock Geochemistry

“…The Nanpo adakitic diorite rock is not likely to have formed via basaltic slab melting since the examined adakitic rocks exhibit weak juvenile zircon Hf isotopes (Hf(t) = 1.80-4.03), which are substantially lower than the Paleo-Tethyan MORB (Hf(t) = 15-20) [36]. Additionally, experimental studies and phase equilibrium modeling show that the melting of either pristine or changed oceanic basaltic slab typically results in the production of high-SiO2 and strongly sodic adakitic melts [27]. The examined Nanpo adakitic rocks, however, have low levels of Al2O3 (15.97%-18.97% wt%) and moderate contents of SiO2 (56.26%-65.60% wt%) and MgO (1.28-2.63 wt%), in contrast to the melts, which are typically rich in SiO2 (>68 wt%) and Al2O3 (>18 wt%) but poor in MgO (<0.2 wt%).…”

“…Therefore, the model of mid-ocean ridge subduction and plate refolding has been excluded [3,11]. In addition, the associated magma formed by lithospheric disintegration would have interacted with mantle wedge peridotite to form adakitic magma with high Ni and Cr contents [2,27], inconsistent with the low Cr and Ni contents of the Nanpo adakites. The Nanpo adakites show a relative enrichment in LREE and large ion lithophile elements (Rb, Ba, K) and are deficient in high-field-strength elements (Nb, Ta, Ti).…”

“…In recent years, different models for the generation of continental crust during the subduction, ablation, and collision of the Paleo-Tethys Ocean have been proposed. Some researchers have demonstrated that one key mechanism for the growth and evolution of the continental crust in the orogenic belt is the remelting of the ancient continental crust that was predominately composed of lithospheric mantle material and the addition of a small amount of mantle material [27,28]. Another key reason for the genesis of the orogenic belt, according to some researchers who have researched the early Mesozoic granites in the Central Asian orogenic belt, is the partial melting of the Paleo-Tethys oceanic crust or continental deposits during subduction [43].…”

“…Adakites or adakitic rocks are igneous rocks that form in volcanic arc environments by the melting of young (≤25 Ma) subducted oceanic crust [26]. In recent years, the understanding of adakites has made major progress [27,28]. Based on the geochemical characteristics of plutonic adakitic rocks and the inferred crystallization conditions, it is recognized that their formation is related to the growth of the continental crust [28].…”