Late Carboniferous – Early Permian mafic dikes and granitoids in the heart of the Western Tianshan Orogen, NW China: Implications for a tectonic transition from a syn- to post-collisional setting

Abstract

Syn- to post-collisional magmatism in the Chinese western Tianshan Orogen is recorded in the Late Carboniferous to Early Permian magmatic rocks of the Awulale Mountains. However, the orogen's tectono-magmatic evolution remains controversial. In particular, two contrasting genetic models have been proposed, i.e., post-collisional extension vs. mantle plume. Here, we report zircon U-Pb ages, whole-rock geochemical data and Nd-Hf isotopic data from the mafic dikes and granitoids of the western Awulale Mountains. These data better constrain the rock's petrogenesis and geodynamic processes following the amalgamation of the Tianshan Orogen. In combination with previous data, we have identified two magmatic episodes in the Awulale Mountains. The Late Carboniferous (ca. 319–307 Ma) episode emplaced the mafic gabbro dikes and intermediate-acidic granitoids with I-type and adakitic affinity. The latest Carboniferous-Permian (ca. 302–286 Ma) episode emplaced bimodal intrusions containing mafic diabase dikes, acidic adakite and I- and A-type granites. Mafic dikes and granitoids of both episodes show depleted whole-rock Nd and zircon Hf isotopic compositions (εNd(t): +2.21 ~ +7.54, εHf(t): +6.48 ~ +15.97), indicating a juvenile magmatic source. However, the mafic dikes have variable geochemical features that clearly suggest their derivation from two distinct mantle sources during the two magmatic episodes. The earlier gabbro dikes have lower Th/Yb (0.08–0.12) and Zr/Yb ratios (15.32–25.45) that resemble a normal mid-ocean ridge basalt (N-MORB) from a depleted mantle source. In contrast, the later diabase dikes show enrichment in the light rare earth elements (LREE) with relatively higher Th/Yb (0.22–0.86) and Zr/Yb ratios (38.05–68.97) that are similar to an enriched mid-ocean ridge basalt (E-MORB) mantle source. Also, the Nb/Ta ratios from the earlier adakitic granitoids (21.26–23.28) differ markedly from the later acidic adakites (0.41–19.50), which implies the transition from a thicker to a thinner crust. Integrating our results with the previous geological records, leads us to propose a two-stage tectonic model for the Chinese western Tianshan Orogen's transition from syn- to post-collision. The Late Carboniferous stage of magmatism was triggered by the break-off of the North Tianshan Oceanic slab as the Yili Block and Junggar Terrane collided. The subsequent latest Carboniferous-Permian stage followed the switch to a post-collisional setting with delamination of the thickened lower crust and the intrusion of voluminous lithospheric mantle-induced bimodal magmatism.