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

Abstract

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)zirconU–Pbages,Lu–Hfisotopicdataandthewhole-rockgeochemicalandNdisotopic 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 ε 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) highcontentofSiO 2 andalkalis(i.e.highK 2 O+Na 2 OwithK 2 O/Na 2 O>1),enrichmentofhigh-field- strength elements (HFSE) and rare Earth elements (REE) (except for Eu) and extreme depletion of Ba, Sr,P,TiandEu;(b)10000×Ga/AlratiosintheDunhuanggranitesof3.5–4.4,withanaveragevalueof 3.79whichissimilartotheglobalaverageof3.75forA-typegranites;(c)thepresenceofcharacteristic 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°C, similar to those of typical of A-type granites. Whole-rock ε 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/post- orogenic 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.