Formation and stabilization of the Dunhuang Block, NW China: Constraints from the Late Paleoproterozoic A-type granites of the Dunhuang Complex

The geochemical composition and temporal evolution of A-type granites may hold key information regarding critical geodynamic processes that occurred during the evolution of the continental crust. Moreover, these aspects may provide an indication of when orogenic processes ended and craton formation/stabilization began in Early Precambrian blocks. In this study, we report the results of zircon U-Pb ages, Hf isotope data and wholerock geochemistry for the Proterozoic Huoyanshan potassic granites in the Dunhuang Block, northwest China. Zircon U-Pb dating reveals that the Huoyanshan granites were emplaced at ca. 1791-1786 Ma, representing a Late Paleoproterozoic magmatism event. The granites have high SiO2 (70.71-72.20 wt%), total alkali contents (K2O + Na2O = 8.81-8.96 wt%; K2O/Na2O = 1.57-1.66), TiO2/MgO ratios (2.19-2.57), low CaO (0.77-1.27 wt%), MgO contents (0.12-0.16 wt%), and Mg-# values (7.72-8.67), with A/CNK ratios of 0.96-1.04, pointing to alkaline and metaluminous to weakly peraluminous affinities. These granites are characterized by high total rare-earth element concentrations (Sigma REE = 419-755 ppm) and variable (La/Yb)(N), values (8-24) with significantly negative Eu anomalies (Eu/Eu* = 0.31-0.42). The primitive mantle-normalized trace element patterns suggest that the Huoyanshan plutons are depleted in Ba, Nb, Ta, Sr, P, and Ti, but enriched in Rb, Th, and Pb. Additionally, the granites have a high zirconium saturation temperature of 855-897 degrees C, with an average of 878 degrees C. These geochemical characteristics indicate that Huoyanshan potassic granites are an aluminous A-type granite, and further classified as A(2)-type. Together with the zircon Hf isotope data (epsilon(Hf)(t) = - 11.21 to - 2.76), indicates that these granites were generated from a crustal source. The heterogeneous Hf isotope compositions in zircons are principally caused by the disequilibrium melting of Zr-bearing sources. In addition, two-stage Hf model ages of zircons from the granites are ca. 2.6-3.1 Ga and plot on the average crustal evolution trend of Archean tonalite-trondhjemite-granodiorite (TTG) gneiss (similar to 2.5 to 3.0 Ga) in the Dunhuang Block. Thus, we propose that A-type granites in the study area were generated from the disequilibrium partial melting of Archean tonalitic to granodioritic rocks under high-temperature and low-pressure conditions according to their low Al2O3 + CaO + Na2O + K2O (molar) values and high 10000 x Ga/Al values ( > 2.9). Subsequently, the magma experienced prominent fractional crystallization of K-feldspar, plagioclase, orthopyroxene, and/or magnetite. Our results, with available data, suggest that the Huoyanshan granites formed in a tectonic transform regime within a collisional to extensional setting, and serve as a precursor for the formation and stabilization of the Dunhuang Block during the Late Paleoproterozoic. Furthermore, it is probable that the post-collisional magmatism event at ca. 1.79 Ga in the Dunhuang Block was closely related to the assembly of the Columbia supercontinent.