Petrogenesis and geodynamic implications of Late Carboniferous sanukitic dikes from the Bieluagaxi area of West Junggar, NW China

Abstract

Sanukitic dikes, mainly consisting of pyroxene diorite porphyrite, diorite porphyry, quartz diorite porphyrite and quartz diorite, were widely distributed in the Bieluagaxi area, southern West Junggar. In this contribution, we present new LA-ICP-MS zircon U-Pb ages, major and trace elements, and Sr-Nd-Hf isotopes for these dioritic dikes. These rocks formed in the early Late Carboniferous (ca. 319–315 Ma), coeval with abundant magmatic rocks in the region, and exhibit high MgO, Cr and Ni concentrations and K/Na ratios, and also show strong enrichment in large ion lithophile elements relative to high field strength elements, with pronounced negative Nb, Ta, and Ti anomalies, analogues to those of Cenozoic sanukitoids of the Setouchi Volcanic Belt. Compositionally, the sanukitic dikes are divided into two groups. Group 1 have MgO contents greater than 6 wt% (6.18–9.34 wt%), with Mg# values ranging from 58.2 to 73.7. They also have relatively low SiO2 (51.7–55.7 wt%) and Sr (356 ppm in average) contents, and low Sr/Y (15.1–25.4) ratios, but high concentrations of Cr (6.2–490 ppm) and Ni (12.8–163 ppm). Compared to the Group 1, the Group 2 samples have slightly lower MgO (2.12–5.58 wt%; Mg# = 45.8–68.5), Cr (10.1–329 ppm) and Ni (2.84–136 ppm), but relatively higher SiO2 (58.6–65.7 wt%) and Sr (304–521 ppm) concentrations and Sr/Y (19.2–41.0) ratios, making them akin to adakitic rocks elsewhere. These dikes also exhibit depleted Sr-Nd-Hf isotopic signatures, with (87Sr/86Sr)i = 0.70353–0.70480, εNd(t) = +5.03 to +7.61, εHf(t) = +10.9 to +15.3. Geochemical and Sr-Nd-Hf isotopic data suggest that the Group 1 rocks are similar to the typical sanukitoids of the Setouchi Volcanic Belt, and were likely derived from partial melting of a depleted mantle source with subducted sediment melts. While the Group 2 rocks show a remarkable geochemical affinity with the Northwestern Karamay sanukitoids, and were derived from partial melting of a depleted mantle source metasomatized by slab-derived adakitic melts and fluids. Combined with the identified and reported subduction-related magmatism, i.e., adakites, sanukitoids, Nb-enriched basalts, lagged arc volcanic rocks, and I-type granitoids in the Western Junggar region and taking into consideration the previous achievements, therefore, we consider a multi-stage subduction-accretionary orogeny model may account for the difference of subduction duration between the northern segment and southern segment of Western Junggar.