Geochemical constraints on the origin of Early Cretaceous alkaline intrusions and its tectonic implication, Sulu Orogenic Belt, Eastern North China Craton

Abstract

Post-orogenic alkaline intrusions from the Sulu Orogenic Belt of eastern North China Craton consist of A-type granites. In this study, we report U–Pb zircon ages, geochemical data, Sr–Nd–Pb, and zircon Hf isotopic data for these rocks. The LA-ICP-MS U–Pb zircon analyses yield consistent ages ranging from 127.1 ± 2.4 to 119.5 ± 4.8 Ma for four samples. The alkaline rocks are characterized by high total alkalis (K2O + Na2O = 8.32–10.11 wt%), light rare-earth element enrichment, and heavy rare-earth element depletion, with a wide range (La/Yb)N values (20–48), moderate negative Eu anomalies (Eu/Eu* = 0.50–0.74), enrichment in large-ion lithophile elements (LILEs, i.e., Rb, Th, U and Pb), and depletion in Ba, Sr and high field strength elements (HFSEs, i.e., Nb, Ta, and Ti), high (87Sr/86Sr)i ranging from 0.708 to 0.7089, low eNd (t) values from −19.4 to −16.8, (206Pb/204Pb)i = 16.751 – 16.935, (207Pb/204Pb)i = 15.381−15.535, (208Pb/204Pb)i = 37.472–37.838, negative eHf (t) values between −21.3 and −25.7 for the magmatic zircons, and larger TDM2 model ages from 2.5 to 2.8 Ga. These results suggest that the rocks were derived from a common enriched lithospheric mantle source that was metasomatized by foundered lower crustal eclogitic materials before magma generation. Furthermore, the geochemical and isotopic feature implies that the primary magma of these rocks originated through partial melting of ancient lithospheric mantle that was variably hybridized by melts derived from lower crust eclogite. These rocks in this study may have been generated by subsequent fractionation of potassium feldspar, plagioclase, ilmenite, and/or rutile. However, negligible crustal contamination occurred during the diagenesis process.