Sources and Petrogenesis of Late Triassic Dolerite Dikes in the Liaodong Peninsula: Implications for Post-collisional Lithosphere Thinning of the Eastern North China Craton

Abstract

A combination of major and trace element, whole-rock Sr, Nd and Hf isotope, and zircon U–Pb isotopic data are reported for a suite of dolerite dikes from the Liaodong Peninsula in the northeastern North China Craton. The study aimed to investigate the source, petrogenesis and tectonic setting of the dikes. Sensitive high-resolution ion microprobe U–Pb zircon analyses yield a Late Triassic emplacement age of ∼213 Ma for these dikes, post-dating the collision between the North China and Yangtze cratons and consequent ultrahigh-pressure metamorphism. Three geochemical groups of dikes have been identified in the Liaodong Peninsula based on their geochemical and Sr–Nd–Hf isotope characteristics. Group 1 dikes are tholeiitic, with high TiO₂ and total Fe₂O₃ and low MgO contents, absent to weak negative Nb and Ta anomalies, variable (⁸⁷Sr/⁸⁶Sr)_i (0·7060–0·7153), ε_Nd(t) (− 0·8 to −6·5) and ε_Hf(t) (−2·7 to −7·8) values, and negative Δε_Hf(t) (−1·1 to −7·8). They are inferred to be derived from partial melting of a relatively fertile asthenospheric mantle in the spinel stability field, with some upper crustal assimilation and fractional crystallization. Group 2 dikes have geochemical features of high-Mg andesites with (⁸⁷Sr/⁸⁶Sr)_i values of 0·7063–0·7072, and negative ε_Nd(t) (−3·0 to −9·5) and ε_Hf(t) (−3·2 to −10·1) values, and may have originated as melts of foundered lower crust, with subsequent interaction with mantle peridotite. Group 3 dikes are shoshonitic in composition with relatively low (⁸⁷Sr/⁸⁶Sr)_i values (0·7061–0·7063), and negative ε_Nd(t) (−13·2 to −13·4) and ε_Hf(t) (−11·0 to −11·5) values, and were derived by partial melting of an ancient, re-enriched, refractory lithospheric mantle in the garnet stability field. The geochemical and geochronological data presented here indicate that Late Triassic magmatism occurred in an extensional setting, most probably related to post-orogenic lithospheric delamination.