A Late Carboniferous–Early Permian slab window in the West Junggar of NW China: Geochronological and geochemical evidence from mafic to intermediate dikes

Abstract

The Late Carboniferous to Early Permian is a critical period for the formation of particular high-temperature magmatic associations in West Junggar, NW China and the geodynamic processes of ridge subduction and related slab windows in the Central Asian Orogenic Belt. Mafic to intermediate dikes, exposed in the central and southeastern West Junggar extensively, mainly consist of Nb-enriched and magnesian suites. The Nb-enriched dikes (305Ma) are characterized by high Nb contents (5.63–9.08ppm) and large variations of major element contents, exhibiting high-K to medium-K calc-alkaline characteristics with high Na2O/K2O (1.33–3.85) and low Sr/Y (9–32) ratios. The dikes have moderate Mg# (44.6–52.9), LREE-rich and sub-horizontal HREE patterns ((La/Yb)N=3.05–7.18; (Gd/Yb)N=1.43–1.78) with weak Eu anomalies (Eu⁎/Eu=0.81–1.10) and positive εNd (t) values (+5.2 to +8.0), and likely originate from partial melting of mantle peridotite that was metasomatized by subducted sediment-derived melts and slab fluids. The medium-K magnesian dikes (305Ma) are characterized by high MgO (3.05–7.28wt.%), Cr (22.3–311ppm) and Ni (32.6–199ppm) contents and all plot in the field of high-Mg andesite. Some have very high Mg numbers (Mg#>60), suggesting a primitive melt composition. These samples display positive Ba, Sr, K and negative Nb–Ta–Ti anomalies as well as strongly-depleted HREE patterns with weak positive Eu anomalies. They have high Sr/Y (27–126) and relatively high εNd (t) values (+6.0 to +8.3), which suggest a similar petrogenesis as the Nb-enriched dikes, but were metasomatized by adakitic melts. The high-K magnesian dikes (284Ma) are high-K calc-alkaline and have lower MgO (3.63–5.57wt.%) compared with the medium-K suite. They have high Sr (422–819ppm), Sr/Y (26–62) ratios and εNd (t) values (+7.0 to +7.1) with relatively flat REE patterns ((La/Yb)N=2.4–5.5), probably originating from partial melting of K-enriched mantle wedge peridotite metasomatized by adakitic slab melts. Based on these petrogenetic features, we suggest that a ridge subduction regime likely prevailed in West Junggar in the Late Carboniferous. The opening of related slab windows and upwelling asthenosphere have possibly triggered the formation of Late Carboniferous–Early Permian magmatic associations in West Junggar.