Early Cretaceous continental delamination in the Yangtze Block: Evidence from high-Mg adakitic intrusions along the Tanlu fault, central Eastern China

Abstract

Early Cretaceous high-Mg adakitic rocks from central Eastern China provide important insights into the thinning mechanism of the over-thickened lithosphere in the Yangtze Block (YB) as well as the North China Block (NCB). The Tanlu fault (TLF), located between the North China and Yangtze Blocks, and has been considered as a prominent pathway of magmas and fluids that resulted in lithosphere thinning of the YB during the Mesozoic–Cenozoic. Here we report the petrology, whole-rock geochemistry, zircon U–Pb geochronology, in situ Hf isotopes, and whole-rock Sr–Nd–Pb isotopes of four high-Mg adakitic intrusions along the TLF in northeastern Langdai. These adakitic intrusions consist of monzodiorite, quartz monzonite porphyry, and quartz monzodiorite. Zircon LA-MC-ICPMS analyses of five samples yield weighted mean 206Pb/238U ages of 127.58±0.80, 126.90±0.81, 120.71±0.64, 122.75±0.57, and 129.2±1.1Ma, indicating their emplacement during the Early Cretaceous. The intrusions have intermediate SiO2 (53.18–65.48wt%) and high potassium (K2O=3.07–3.95wt%; Na2O/K2O=1.02–1.26) and are classified as shoshonitic to high-K calc-alkaline series. They are characterized by high MgO (1.80–7.35wt%), Mg# (50–65), Sr (591–1183ppm), Ni (20.3–143.0ppm), and Cr (51.40–390.0ppm) contents, high (La/Yb)N (11.60–28.33) and Sr/Y (27.9–113.5) ratios, and low Y (7.79–22.4ppm) and Yb (0.60–2.01ppm) contents, comparable with high-Mg adakites. The samples are enriched in light rare earth elements but depleted in heavy rare earth elements and high field strength elements with slightly negative to positive Eu anomalies (δEu=0.81–1.30), resembling the features of high-Mg adakitic rocks. Their whole-rock εNd(t)=−16.2 to −15.0, initial (87Sr/86Sr)i=0.7060–0.7074, low radiogenic Pb (206Pb/204Pb(t)=16.208–16.509, 207Pb/204Pb(t)=15.331–15.410, and 208Pb/204Pb(t)=36.551–36.992), and zircon εHf(t)=−36.6 to −16.6 suggest magma derivation from a continental crustal source. The geochemical and isotopic features, in combination with existing geological data, suggest that the intrusions are high-Mg adakites and, by comparison with contemporaneous intrusions in Eastern China, were likely to have been formed by partial melting of over-thickened basaltic lower crust following the delamination of eclogitic lithosphere during the Late Jurassic to Early Cretaceous. It is possible that the TLF played a key role in lithospheric delamination and thinning and in the generation of the high-Mg adakitic rocks.