Paleoproterozoic I-type granites and their implications for the Yangtze block position in the Columbia supercontinent: Evidence from the Lengshui Complex, South China

Abstract

The early history of the Yangtze Block is not well constrained owing to scarce outcrops of the Archean to Paleoproterozoic rocks. In this paper, we report an integrated study of major and trace element data, SHRIMP and LA–ICP–MS zircon U–Pb age data, and Hf isotopic data for the granites from the Lengshui Complex in the northern Yangtze Block. Zircon U–Pb dating yields a 207Pb/206Pb age of 1960±19Ma for gneissic granites, and 1936±32Ma for K-granites, which indicates that these granites were almost contemporaneously emplaced. The gneissic granites have SiO2 contents ranging from 71.11% to 75.10%, K2O contents from 4.61% to 5.95%, with relatively high differentiation indexes of 90.91–95.28, and they have alkalinity ratios of 3.31–4.48 and A/CNK values of 1.14–1.29. Also, low Sr, Ta, and Nb contents but relatively high Y, Rb, Nd, and Yb contents are found. The K-feldspar granites have higher contents of trace and rare earth elements. All the samples display typical I-type evolutionary trends in chemical variation diagrams, which are plotted into the syn-collision granite fields in the tectonic discrimination diagrams. The zircon ɛHf (t) value of −15.77 and the TDM2 age dated at ca. 3.6Ga for the Lengshui Complex suggest that the Complex could be derived from the old crustal partial melting of the old crustal material such as ca. 2.9Ga TTGs in the Kongling terrain. Taking nearby rapakivi and A-type granites into consideration, the Lengshui Complex may represent part of the rock records of the progression from plate convergence to continental extension and rifting in the Yangtze Block, implying that the Yangtze Block could be an important part of the Paleoproterozoic Columbia supercontinent.