Baddeleyite U-Pb geochronology and geochemistry of Late Paleoproterozoic mafic dykes from the Kongling complex of the northern Yangtze block, South China

Abstract

Baddeleyite LA-ICP-MS U-Pb geochronology, major and trace element geochemistry and Nd-Pb isotope analysis of Late Paleoproterozoic mafic dykes from the northern Yangtze block, South China craton are used to investigate their petrogenesis and tectonic implications. The dykes mainly consist of fine-grained dolerite. Baddeleyite U-Pb dating indicates that the dolerites formed at 1866 ± 21 Ma. These rocks are alkaline in composition with high TiO2 contents (2.21–3.17 wt%) and display LREE enrichments ((La/Yb)N = 6.0–6.4) with slight positive Eu anomalies (Eu/Eu* = 1.06–1.14). On primitive mantle normalized multi-element diagrams they generally show convex upward curves with weak negative Nb-Ta anomalies. All of the samples have similar Nd-Pb isotopic compositions with negative εNd(t) values ranging from −4.3 to −3.2, (206Pb/204Pb)i from 14.91 to 15.31, (207Pb/204Pb)i from 14.99 to 15.08, and (208Pb/204Pb)i from 34.75 to 35.58. Geochemical features such as EM1-like isotopic compositions suggest the dolerites were derived from partial melting of metasomatised subcontinental lithospheric mantle. The mafic dykes are inferred to have formed in a post-orogenic extensional setting. We suggest that the northern Yangtze block experienced a tectonic switch from convergence to extension at ~1.87 Ga. A comparison of Paleoproterozoic magmatic, metamorphic, and sedimentary events in the Yangtze block with the southern Siberian craton and northern Laurentia supports a Paleoproterozoic position of the Yangtze block adjacent to southern Siberia and northern Laurentia within the Columbia supercontinent.