Petrogenesis of Early Carboniferous adakitic dikes, Sawur region, northern West Junggar, NW China: Implications for geodynamic evolution

Abstract

Adakitic dikes are widespread in the Sawur region, northern West Junggar, northwestern China. Zircon U–Pb analyses of the dikes have yielded consistent ages of ca. 334 Ma. The dikes are characterized by relatively high SiO 2 (55.5–61.8 wt.%), Al 2 O 3 (14.5–16.3 wt.%), and Sr (468–1005 ppm), and low Y (10.1–14.1 ppm) and Yb (0.93–1.39 ppm) contents, with high Sr/Y (34–74) ratios and slight Eu anomalies, which are analogous to those of slab-derived adakites. In addition, the dikes are relatively MgO-rich (1.75–3.57 wt.%; Mg # = 44–56), with high Th/Yb ratios and positive εNd (334 Ma) (+6.2 to +6.5) and εHf (334 Ma) (+11.3 to +15.8) values. The data suggest that the dikes were generated by partial melting of subducted oceanic crust and overlying sedi- ments. However, these Early Carboniferous adakitic rocks and associated I-type granites in the northern part of West Junggar are petrochemically distinct from the Late Carboniferous adakitic plutons from the southern part ofWestJunggar.Thelatterareassociatedwithcharnockites,A-typegranites,tholeiites,andNb-enriched,alkaline basalts, as well as magnesian dikes, which were generated by ridge subduction during the Late Carboniferous. The distinct lithological association of the Early Carboniferous adakites is consistent with flat slab subduction, which wascharacterizedbyanomalousheating oftheslaband consequentialpartialmeltingtogenerateadakitic magma. Aftergeneration of theEarlyCarboniferous adakites, the subducting slabcooled, revertedto a morenor- mal subduction angle in the late Early Carboniferous, and was associated with more typical I-type granite arc magmatism.