Association of Neoproterozoic A- and I-type granites in South China: Implications for generation of A-type granites in a subduction-related environment

Abstract

Neoproterozoic magmatism in the Yangtze Block of South China produced voluminous S- and I-type granites, and sparse A-type granites. The Daxiangling A-type granitic pluton is spatially associated with the Shimian I-type pluton at the western margin of the Yangtze Block. Both plutons have similar SHRIMP zircon U-Pb ages of ∼ 800 Ma and are slightly younger than the tonalite-trondhjemite-granodiorite (TTG) gneisses in the area. The Shimian pluton is composed of granite and monzogranite with high SiO2 (69.3-76.6 wt.%), Na2O (2.79-3.80 wt.%) and K2O (3.94-5.87 wt.%), and low Fe2O3 (0.96-3.06 wt.%) and MgO (0.12-0.50 wt.%). The Daxiangling pluton consists of alkali-feldspar granites with higher SiO2 (76.3-79.3 wt.%) and lower Al2O3 (10.6-11.9 wt.%) and CaO (0.21-0.55 wt.%) than the Shimian granites. Both plutons are slightly peraluminous (A/CNK = 1.00-1.12) and belong to the high-K, calc-alkaline series. The Daxiangling alkali-feldspar granites have much higher Zr, Hf, Ga and HREE, and lower Sr than the Shimian granites. On the chondrite-normalized REE diagram, both plutons are enriched in LREE, but the rocks of the Daxiangling pluton show relatively flat patterns because of their higher HREE contents. Their primitive-mantle normalized spidergrams display negative Eu, Nb, Ta, Sr, P and Ti anomalies. Both plutons have nearly identical whole-rock Nd and zircon Hf isotopic compositions (εNd(t) = + 1 and εHf(t) = + 5 to + 9), similar to the TTG gneisses. We suggest that the I-type granites of the Shimian pluton were produced by dehydration melting of the TTG rocks as a result of underplating of mantle-derived mafic magmas. The rocks above the I-type magma source may have been converted to charnockites by heating and dehydration. With increasing temperatures, the charnockites underwent partial melting at temperatures > 900 °C to produce A-type magmas. Both the A- and I-type granites formed at an extensional active continental margin, perhaps in a back-arc environment. 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 suggests that aluminous A-type granite is most probably derived from a charnockite source heated by large-scale magmatic underplating, rather than a tonalite source. Crown Copyright © 2008.