Co-development of Jurassic I-type and A-type granites in southern Hunan, South China: Dual control by plate subduction and intraplate mantle upwelling

Two types of spatially and temporally associated Jurassic granitic rocks, I-type and A-type, occur as pluton pairs in several locations in southern Hunan Province, South China. This paper aims to investigate the genetic relationships and tectonic mechanisms of the co-development of distinct granitic rocks through petrological, geochemical and geochronological studies. Zircon LA-ICPMS dating results yielded concordant U-Pb ages ranging from 180 to 148 Ma for the Baoshan and Tongshanling I-type granodiorites, and from 180 to 158 Ma for the counterpart Huangshaping and Tuling A-type granites. Petrologically, the I-type granodiorites consist of mafic minerals such as hornblende whereas the A-type granites are dominated by felsic minerals (e.g., quartz, K-feldspar and plagioclase). Major and trace element analyses indicate that the I-type granodiorites have relatively low SiO2 (64.5-71.0%) and relatively high TiO2 (0.28-0.51%), Al2O3 (118-15.5%), total FeO (2.3-4.7%), MgO (1.3-2.6%) and P2O5 (0.10-0.23%) contents, and the A-type granites are characterized by high concentrations of Rb (212-1499 ppm), Th (18.3-52.6 ppm), U (11.8-33.6 ppm), Ga (20.0-36.6 ppm), Y (27.1-134.0 ppm) and HREE (20.3-70.0 ppm), with pronounced negative Eu anomalies (Eu/Eu* = 0.01-0.15). Moreover, the I-type granodiorites are classified as collision-related granites emplaced under a compressional environment, whereas the A-type granites are within-plate granites generated in an extensional setting. Zircon Hf isotopic compositions vary substantially for these granitic rocks. The I-type granodiorites are characterized by relatively young Hf model ages (T-DM1 = 1065-1302 Ma, T-DM(C) = 1589-2061 Ma) and moderately negative epsilon Hf(t) values (-5.9 to -11.5), whereas the A-type granites have very old model ages (T-DM1 = 1454-2215 Ma, T-DM(C) = 2211-2974 Ma) and pronounced negative epsilon Hf(t) values (-15.8 to -28.3). These petrochemical and isotopic characteristics indicate that the I-type granodiorites may have been derived from a deep source involving mantle-derived juvenile (basaltic) and crustal (pelitic) components, whereas the A-type granites may have been sourced from melting of meta-greywacke in the crust. This study proposes that the pressure and temperature differences in the source regions caused by combined effects of intra-plate mantle upwelling and plate subduction are the major controlling factors of the co-development of the two different types of magmas. Crustal anatexis related to litho-spheric delamination and upwelling of hot asthenosphere under a high pressure and temperature environment led to the formation of the I-type magmas. On the other hand, the A-type magmas were formed from melting of the shallower part of the crust, where extensional stress was dominant and mantle-crust interaction was relatively weak. Rifts and faults caused by mantle upwelling developed from surface to depth and successively became channels for the ascending I- and A-type magmas, resulting in the emplacement of magmas in adjacent areas from sources at different depths.