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 (87 Sr/86Sr)i (0.7060-0.7153), ε Nd(t) (-0.8 to to -6.5) and ε Hf(t) (-2.7 to -7.8) values, and negative Δ ε 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 (87 Sr/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.