Association of A- and I-type granites at the western Yangtze Block, SW China: Implications for generation of A-type granite

Abstract

Neoproterozoic magmatism in the Yangtze Block produced voluminous S- and I-type granites, and sparse Atype granites. The Shimian and Daxiangling plutons, which are composed of granite and alkaline-feldspar granite (AFG) respectively, are spatially associated along the western margin of the Yangtze Block. Zircon SHRIMP U-Pb dating shows that both plutons formed at 800 Ma and are roughly contemporaneous with the TTG gneisses in the area. Geological evidence indicates that TTG rocks are the oldest and the AFG are the youngest. The granites have high SiO2 (69.3-76.6 wt%), Na2O (2.79-3.80 wt%), K2O (3.94-5.87 wt%), and low Fe2O3 (0.96- 3.06 wt%) and MgO (0.12-0.50 wt%). The AFG have higher SiO2 (76.3-79.3 wt%) and lower Al2O3 (10.6-11.9 wt%) and CaO (0.21-0.55 wt%). The AFG also have much higher Zr, Hf, Ga, HREE, and lower Sr than the granites. Both granite suites are slightly peraluminous (A/CNK = 1.00-1.12), and show similar and subparallel patterns on chondrite-normalized REE diagrams and primitive-mantle normalized spidergrams. They all show negative Eu, Nb, Ta, Sr, P, and Ti anomalies. They also have identical whole-rock Nd and zircon Hf isotopic compositions (εNd(t) = +1 and εHf(t) = +5 to +9), similar to those of the TTG. Geochemical features indicate that the granites are I-type, whereas AFG are A-type. We suggest both the I- and A-type granites were derived from the TTG but under different pressure and temperature conditions. The I-type granites were produced by dehydration melting of TTG rocks as a result of underplating by mantlederived mafic magmas. The rocks above the I-type magma source may have been converted to charnockites by heating and dehydration. With increasing temperatures, partial melting of the charnockites at temperatures >900°C could have produced the A-type magmas. These granites formed at an extensional active continental margin. A-type granites with mantle-like isotopic signatures may be reworked or recycled from juvenile crustal rocks. The association of A- and I-type granites described here suggests that A-type