Ca. 2.5Ga granodioritic gneiss in Annanba area of southeastern Tarim and its petrogenesis

The newly-discovered granodioritic gneiss is mainly composed of plagioclase, alkali feldspar, quartz, amphibolite and biotite in Ananab area, southeastern Tarim, characterized by high SiO₂(>70%), Al₂O₃(>15%) and Na₂O(3.56%~4.15%), and low MgO(0.39%~0.59%), Fe₂O₃(0.23%~0.36%), FeO(0.76%~1.11%), K₂O/Na₂O(0.64~0.81)and Mg^#(19~27). Meanwhile, the granodioritic gneiss has lower content of ∑ REE, with strong fractionation of LREE/HREE((La/Yb)_N=46.27~98.27); the chondrite-normalized REE patterns show right-inclined patterns with obvious positive Eu(δEu=1.57~2.00) anomalies. The rocks are enriched in LILE(such as Rb, Ba and Sr)and depleted in HFSE(such as Nb and Ta); in addition, the values of compatible elements(Cr, Ni)are relatively low. On the basis of the above data, the geochemical analyses indicate that granodioritic gneiss is characteristic of high-Al TTG series and low-HREE TTG. It is therefore held that the rocks might have been formed by partial melting of thickened mafic lower crust under the pressure condition of eclogite facies, with garnet, rutile and amphibole left in the residual magmas. LA-ICP-MS zircon U-Pb dating of granodioritic gneiss from Annanba area of Aksai yielded the formation time of 2555±11Ma, basically consistent with the age of magmatism of southeastern Tarim(2.5~2.6Ga), showing the significant continental crust accretion in late Neoarchean. In addition, the metamorphic ages of ca.2.44Ga and ca.1.96Ga were obtained for granodioritic gneiss, which implies that the basement rocks from southeastern Tarim were superimposed upon and reconstructed by two episodes of tectonic thermal events in Palaeoproterozoic.