Petrogenesis of the Early Paleozoic S-type granites in southern Alxa, Northwest China and its implications on fluid flourishing in the middle and lower crust

Abstract

Fluids play a key role in the partial melting of crustal materials which leads to the formation of the andesitic continental crust. However, the origin of the aqueous fluid in the lower and middle crust remains unclear. Here, we present three early Paleozoic S-type granites of the Yabulai Mountain in southern Alxa in an attempt to clarify the origin of the fluid and its implications for the geodynamic processes of the Late Silurian to Early Devonian. The coarse-grained two-mica granite and fine-grained two-mica granite formed during 423–417 Ma, whereas the muscovite granite was relatively younger (408 Ma). The fine-grained two-mica granites contain high SiO2 (72.56–75.77 wt%) and are metaluminous to weakly peraluminous (A/CNK = 0.97–1.07), while the coarse-grained two-mica granites contain comparable SiO2 (72.18–74.56 wt%) and are strongly peraluminous (A/CNK = 1.10–1.16). The muscovite granites (SiO2 = 72.50–76.04 wt%) are different from the two-mica granites in having higher Na2O (3.38–4.58 wt%) and lower K2O (3.70–4.90 wt%) with higher Na2O/K2O ratios (0.71–1.24). In addition, they have higher Ba (1071–2152 ppm) and Sr (640–801 ppm) contents and lower Rb (66.7–107 ppm) and U (0.23–0.74 ppm) contents and lower Rb/Sr (0.09–0.17) ratios and higher Eu/Eu* (0.95–3.92) ratios than those of the two-mica granites. These geochemical features imply that the two-mica granites were derived from fluid-absent partial melting and the muscovite granites were formed by fluid-present partial melting and their magma sources were plagioclase-rich metapsammite. The whole-rock Sr–Nd and zircon Hf isotopic compositions suggest that the metapsammite sources originated from parts of the Mesoproterozoic basement in the Alxa block. It is proposed that a tectonic shift from southward advancing to northward retreating of the Paleo-Asian slab subduction took place during 423 to 397 Ma and this caused a progressive back-arc extension in southern Alxa. Such a tectonic transformation may promote fluids release from the mantle wedge and upward infiltration through the crustal fractures resulting in intense fluid-present melting in the back-arc setting.