Silurian alkaline magmatism in the Saur area, northern West Junggar: Evidence for the Middle Palaeozoic amalgamation of the Kazakhstan Block at the south‐west of the Central Asian Orogenic Belt

Abstract

Although it is generally accepted that the Kazakhstan Block is an assemblage of Precambrian microcontinents and Early Palaeozoic island arcs, the details and timing constraint of the collage process need to be precisely studied in local regions. Thus, we conducted combined studies of geochemistry, zircon U–Pb geochronology and Sr–Nd–Hf isotopes upon the Awusiqi alkali‐feldspar granite and volcanic rocks from the Saur area in northern West Junggar. The Awusiqi volcanic rocks are mainly composed of andesites, rhyolites, and pyroclastic rocks. The alkali‐feldspar granite directly intrudes into the surrounding volcanic strata. Zircon U–Pb ages of the Awusiqi andesites, rhyolites, and alkali‐feldspar granite are 431 ± 3 Ma, 428 ± 5 Ma, and 426 ± 6 Ma, respectively. These new age data imply for the existence of Silurian magmatism in the Saur area. The alkali‐feldspar granite and rhyolites with high silica and alkali contents belong to metaluminous to weakly peraluminous and alkaline series and show seagull‐type REE patterns with strongly negative Eu anomalies. They are relatively enriched in the light rare earth elements (LREEs), K, Rb, Ba, Cs and other large‐ionic lithophile elements (LILEs), and are poor in P, Sr, and Eu, geochemically similar to those of A2‐type granites. The alkali‐feldspar granites have positive zircon εHf(t) values varying from +11.03 to +16.87 and young Hf model ages (TDM2 = 686 – 425 Ma). These features suggest that the Awusiqi alkali‐feldspar granite and rhyolites (except sample 18XJSF22C) were generated by partial melting of the juvenile lower crust with fractional crystallization, whereas sample 18XJSF22C shows mantle‐derived features. Awusiqi andesites show typical sanukitic high‐Mg andesites characteristics(Mg# = 48.08 – 65.52). The andesites are enriched in LREEs and LILEs, and are depleted in Nb, Ta, and Ti. They also have relatively high initial 87Sr/86Sr ratios (0.70543 – 0.70547), positive εNd(t) values (+2.5 – +5.7), and ancient Nd model ages (TDM1 = 1,093 – 815 Ma). These features suggest the andesites were likely derived from partial melting of a depleted mantle source previously metasomatized by slab‐derived fluids and melts. The discovery of Silurian magmatism in the Saur area has further confirmed the existence of Early Palaeozoic Tarbagtay arc basement and widespread Middle Palaeozoic A2‐type granitoids. We favor a post‐collisional setting in the Silurian in northern West Junggar, which also is the local embodiment of the Middle Palaeozoic amalgamation of the Kazakhstan Block.