Geochronology, geochemistry and petrogenesis of the late Palaeoproterozoic A-type granites from the Dunhuang block, SE Tarim Craton, China: implications for the break-up of the Columbia supercontinent

The discovery of c. 1.77 Ga A-type granite in the Tarim Craton (TC) provides the first evidence that supports an extensional event related to fragmentation of the Columbia supercontinent in the late Palaeoproterozoic. We present laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb ages, Lu-Hf isotopic data and the whole-rock geochemical and Nd isotopic data of A-type granites in the Dunhuang area in the SE Tarim Craton. Zircon U-Pb dating for three granite samples indicate that they were emplaced at c. 1.77 Ga. Zircons from these granites have epsilon(Hf)(t) values ranging from -5.9 to 8.7, corresponding to two-stage model ages of 1.9-2.7 Ga. These granites exhibit the following petrological geochemical characteristics that are typical of A- type granite: ( a) high content of SiO2 and alkalis (i. e. high K2O + Na2O with K2O/Na2O > 1), enrichment of high-fieldstrength elements (HFSE) and rare Earth elements (REE) (except for Eu) and extreme depletion of Ba, Sr, P, Ti and Eu; (b) 10 000xGa/Al ratios in the Dunhuang granites of 3.5-4.4, with an average value of 3.79 which is similar to the global average of 3.75 for A-type granites; (c) the presence of characteristic minerals such as amphibole, sphene and perthite; and (d) zirconium saturation temperature results indicate that the Dunhuang granites have high initial magmatic temperatures in the range 887- 950 degrees C, similar to those of typical of A-type granites. Whole-rock epsilon(Nd)(t) values range from -2.5 to -6.2 and T-DM model ages from 2.3 to 2.7 Ga. Nd-Hf isotopic and whole-rock geochemical data indicate that these granites were most likely derived from the late Archean crustal source in a post-collisional/postorogenic extensional tectonic environment. The late Palaeoproterozoic A- type granites in the TC could be correlated with those of the North China Craton (NCC), India and the Canadian Shield, thus demonstrating extensional tectonics and break-up of the Columbia supercontinent.