Genesis and geological implications of the Neoproterozoic A-type granite from the Lugu area, western Yangtze block

Previous studies have deeply examined the Neoproterozoic I-type and S-type granites on the western margin of the Yangtze Plate, whereas very insufficient attention has been paid to A-type granites, and hence their petrogenetic nature and tectonic implications are still controversial. In this paper, the authors report the results of a detailed study of the newly discovered Lugu Atype granites in the Yangtze Plate, using LA-ICP-MS zircon U-Pb geochronology, petrology, and whole-rock geochemistry, with emphasis placed on the petrogenesis and tectonic setting of the A-type granites. The Lugu granitoid consists of alkali feldspar granite and monzonitic granite that yielded zircon U-Pb ages of (806±5) Ma and (815±5) Ma by LA-ICP-MS, respectively. The Lugu granitoid is deduced as the Neoproterozoic magmatic product. The Lugu granitoid is characterized by high SiO₂ (71.2%-76.1%), high total alkali values (K₂O + Na₂O values from 6.73% to 8.95%), high K₂O/Na₂O ratios (1.89-3.60) and peraluminous features (A/CNK=1.07-1.49). BothFeO^T/(FeO^T+MgO) versus SiO₂ and(Na₂O+K₂O)-CaO versus SiO₂ plots indicate ferroan and alkali-calcic signatures typical of A-type granitoids. The content of rare earth elements of alkali feldspar granite and monzonitic granite is high (∑REE=221×10^-6-387×10^-6), and both rocks show enrichment of LREE rather than HREE with a right-inclined REE patterns for (La/Yb)_N ratios from 3.42 to 9.69. Both type rocks have obvious negative Eu anomaly (δEu=0.10-0.34). Meanwhile, the Lugu granitoid has relative high content of Zr, Nb, Ce and Y with high ratios of Ga/Al, Y/Nb, Yb/Ta and Ce/Nb, showing that it is a typical A₂-type granite. Combined with the data on geochemistry and regional geology, the authors hold that the Lugu Neoproterozoic A₂-type granite was derived from the partial melting of pelitic rock in early Neoproterozoic subduction orogeny.