Geochronology and geochemistry of the granitoids in the Diancangshan-Ailaoshan fold belt: Implications on the Neoproterozoic subduction and crustal melting along the southwestern Yangtze Block, South China

Abstract

The Neoproterozoic magmatic and tectonic history of the South China Craton (SCC) is critical for the reconstruction of Rodinia break-up and subsequent Gondwana assembly. Nonetheless, the tectonic attribution of the Neoproterozoic igneous rocks in the southwestern Yangtze is contentious, and the mechanism for crustal anatexis remains unclear. Here, we report mineralogical, whole-rock geochemical and Sr-Nd isotope data, and zircon U-Pb-Hf-O isotope data from the newly found Neoproterozoic trondhjemites and granites in the Diancangshan-Ailaoshan (DCS-ALS) fold belt along the southwestern Yangtze. Both the trondhjemites (ca. 771–762 Ma) and granites (ca. 767 Ma) have high SiO2 and low MgO contents. The trondhjemites have relatively high CaO, Na2O and Sr contents, but low K2O contents and Rb/Sr ratios, and show positive to slightly negative Eu anomalies and heavy rare earth elements (HREEs) depletions, resulting in high Sr/Y and La/Yb ratios. In contrast, the granites have lower CaO, Na2O, and Sr contents, but higher K2O and Rb/Sr, and display negative Eu anomalies and flat HREEs patterns. The trondhjemites have negative εNd(t) (-4.12 to −3.85), positive to negative εHf(t) (-2.50 to + 5.28) and low δ18O (4.37 ‰–7.27 ‰) values. In comparison, the granites have positive εNd(t) (0.09 to 0.26), εHf(t) (+1.24 to + 9.86) and higher δ18O (6.50 ‰–7.24 ‰) values. The distinct elemental and isotopic compositions between the trondhjemite and granite samples reflect two different types of crustal anataxis within different crustal levels. The trondhjemites were formed by water-fluxed melting of the juvenile mafic lower continental arc crust (with addition of ancient Yangtze basement). The granites were derived from dehydration melting of juvenile felsic middle crust. Our results imply that the trondhjemites and granites were formed in a continental arc setting. The melting processes revealed here should be applicable to decipher the widespread Neoproterozoic crustal melting along the southwestern Yangtze, which was most likely fluid-controlled via subduction-induced asthenosphere upwelling. Combined with available regional geological data, we suggest that a long-lived Neoproterozoic (∼880–730 Ma) subduction zone may have extended from the Panxi-Hanna region in the northern/western Yangtze Block to the DCS-ALS fold belt in the southwestern Yangtze. Thus, the SCC may have located on the northwestern margin of Rodinia.