Late Paleozoic underplating in North Xinjiang: Evidence from shoshonites and adakites

Shoshonitic series volcanic rocks (SSVR) and adakites are widely distributed in the Permian terrestrial volcanic strata of the Yishijilike–Awulale range of west Tianshan, north Xinjiang, China. Isotopic dating yields Permian ages of 280–250 Ma. The SSVR include absarokite, shoshonite and banakite which are characterized by enrichment of alkalis, particularly in K, combined with lower Ti, higher Al (A/NKC = 0.70–0.99, metaluminous) and Fe2O3 > FeO. The SSVR that are rich in LILE with high REE contents and Eu/Eu⁎ range from 0.59 to 1.30. They are rich in LREE ((La/Yb)N 2.15–11.97) and depleted in Nb, Ta and Ti (TNT negative anomalies). The adakites are metaluminous to weakly peraluminous (A/NKC = 0.85-1.16) and belong to the high-SiO2 type of adakite (HSA, SiO2 = 62%–71%). They are characterized by lower ΣREE with strong LREE enrichment ((La/Yb)N 13–35). Pronounced positive Eu anomalies (Eu/Eu⁎ = 1.02–1.27), very low Yb contents and distinct TNT-negative anomalies are evident. The SSVR have eNd(t) (+ 1.28 to + 4.92) and (87Sr/86Sr)i (0.7041–0.7057) that are similar to adakites in the regions which are characterized by eNd(t) = 0.95 to + 5.69 and (87Sr/86Sr)i = 0.7050–0.7053. Trace element, REE and Sr/Nd isotopic compositions suggest that both SSVR and adakites possess similar source regions associated with underplated mantle-derived basaltic materials. Lithosphere extension driven by magmatic underplating was responsible for the generation of both the SSVR and adakites. This magmatism serves as a petrological indicator of underplating during the Permian. Obviously thickened crust (62–52 km), a complex Moho discontinuity, high heat flow (~ 100 mw·m− 2), widespread contemporary alkali-rich granites, basic dike swarms (K–Ar ages of 187–271 Ma, Ar–Ar ages of 174–270 Ma and Rb–Sr ages of 255 ± 28 Ma; eNd(t) + 1.84 to + 10.1; (87Sr/86Sr)i 0.7035 and 0.7065), and basic granulites (SHRIMP U–Pb age of 268–279 ± 5.6 Ma) provide additional evidences for the underplating event in this area during Permian.