Ca. 1.04 Ga hot Grenville granites in the western Yangtze Block, southwest China

Abstract

The late Mesoproterozoic to early Neoproterozoic magmatism in the western Yangtze Block, though minor in volume, has offered a rare opportunity for understanding the role of the Yangtze Block in the Grenville orogeny. The Huidong K-feldspar granites have SIMS zircon U-Pb ages of 1048.5 ± 4.9 Ma and 1043.1 ± 5.1 Ma, whereas the Yuanmou biotite granites were dated at 1041 ± 12 Ma by LA-ICP-MS. Their ages are indistinguishable within error, suggesting that they were emplaced contemporaneously. Both Huidong and Yuanmou rocks show geochemical features typical of A-type granites, including high FeO*/MgO, Ga/Al ratios and elevated contents of high-field-strength elements (HFSE). The Huidong granites exhibit positive whole-rock εNd(t) (+0.58 to +4.4) and zircon εHf(t) (+6.0 to +8.3) values close to those of regional coeval mafic rocks. They have a narrow range of zircon δ18O from 6.2‰ to 7.2‰, which is only slightly higher than the ‘mantle zircon’ value of 5.3 ± 0.6‰. High Nb/La and Nb/Th ratios in these rocks overlap with those of regional coeval mantle-derived rocks. Along with high zircon saturation temperatures of >1000 °C, we suggest that the Huidong granites could have formed mainly by extreme fractionation from basaltic parental magmas, or alternatively by partial melting of newly emplaced tholeiite at high temperatures. By contrast, the Yuanmou granites have lower whole-rock εNd(t) (−2.0 to +0.59) and zircon εHf(t) (−1.5 to +5.1) values and a Hf isotope crustal model age of ∼1.56–1.97 Ga, suggesting that its parental magmas may be generated by partial melting of old crust during intrusion of mantle-derived magmas. Ca. 1.04 Ga granites in the region show geochemical features similar to A-type granites in orogenic belts, e.g., the aluminous A-type granites in the Lachlan Fold Belt, Southeast Australia. They are also characterized by high-extremely high (>1000 °C) temperature in their genesis, similar to coeval hot Grenville granites globally. Temporal correlation of metamorphic and magmatic records between South China and the Grenville Province leads us to conclude that the ∼1.04 Ga A-type granites in the western Yangtze Block form parts of the hot Grenville granites emplaced during post-orogenic crustal extension.