Underplating-related adakites in Xinjiang Tianshan, China

Intermediate to acidic porphyries and dacites in the Awulale and Sanchakou area of the Xinjiang Tianshan region, China, conform to the definition of high-SiO2 adakite (HSA). These volcanic or subvolcanic rocks are characterized by high Na2O (Na2O > K2O), high Sr/Y (51–327), and strong depletions in HREE and HFSE (Nb, Ta, and Ti). Despite these characteristics, they are distinct from a suite of adakites in the same area that are inferred to be derived from partial melting of subducted slab. Differences between the two suites are evident mainly in their rock association, isotopic ages and compositions of Sr and Nd isotopes. The subducted slab-related adakites are associated with common island arc rocks, niobium-enriched basalt (NEB) and high-Mg andesite (HMA), formed in the late Carboniferous (320–334 Ma) and are characterized by relative high ɛNd(t) (+ 3.4 − + 11.6) and low (87Sr/86Sr)i ratios(0.7032–0.7063). In contrast, the Awulale and Sanchakou adakites were generated during the middle to late Permian (260–278 Ma), possess a wide range of low ɛNd(t) (+ 0.75 − + 5.69), a greater range of (87Sr/86Sr)i (0.7039–0.7054) and have young Nd model ages (T2DM 472–699 Ma). In addition, their Mg# (35–56) and compatible element contents (MgO = 0.93–2.22 wt.%; Cr = 4.95–16.41 ppm; Ni = 2.9–25.8 ppm) are relative low compared to the subducted slab-related adakites(Mg# = 55–71; MgO = 1.22–6.78 wt.%; Cr = 24–132 ppm; Ni = 2.28–45.61 ppm). Based on these characteristics, the regional association of igneous rocks, and evidence from a global geoscience transect (GGT), the Awulale and Sanchakou adakites represent a genetically distinct suite in the Xinjiang region. We proposed that the source of these adakites was basaltic rock underplated to the base of the lower crust, that partial melted at the rutile-bearing amphibole-eclogite facies (> 650 °C and 1.5 Gpa minimum pressure (≥ 50 km)).