Pedogenesis of Late Triassic intrusive rocks in the northern Liaodong Peninsula related to decratonization of the North China Craton: Zircon U-Pb age and Hf-0 isotope evidence

摘要

Major and trace element, whole rock Sr-, Nd-and Hf-isotopes and zircon U-Pb age and Hf-0 isotope data have been determined for mafic to felsic intrusive rocks from the Late Triassic Mayihe (MYH), Longtou-Chaxinzi-Xiaoweishahe (LCX) and Nankouqian-Xidadingzi (NX) intrusions in the northern Liaodong Peninsula, NE China, in order to investigate their sources and petrogenesis related to decrationization of the North China Craton. The rocks include clinopyroxene diorite, morizodiorite, quartz diorite, granodiorite, monzogranite and mafic microgranular enclaves. Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) and secondary ion mass spectrometer (SIMS) U-Pb analyses of zircon from MYH, LXC to NX intrusions yield Late Triassic ages of 224-218 Ma, establishing that the mafic and felsic magmas were coeval. The clinopyroxene diorites from the MYH pluton have high MgO concentrations at low silica contents, with positive whole rock ε_(Nd)(t) (up to +3.8) and ε_(Hf)(t) (up to +6.2) and zircon ε_(Hf)(t) values and some mantle-like δ~(18)O values. They are enriched in large ion lithophile (ULEs) and light rare earth elements (LREEs) and depleted in high field strength elements (HFSEs), which can be explained as melts derived from a depleted mantle, with some subsequent crustal contamination. However, the diorites from the LXC and NX intrusions are high-K rocks and have high MgO concentrations and mantle-like δ~(18)O values, but with negative whole rock ε_(Nd)(t) ( — 12.9 to —7.6) and ε_(Hf)(t) (-14.0 to —5.6) and zircon ε_(Hf)(t) values, distinct from those of MYH pyroxene diorites, indicating that their parental magmas were derived from partial melting of an ancient Uthospheric mantle. The granitoids from three plutons have high SiO2 contents and low MgO concentrations, suggesting they were mainly derived from crustal sources. However, the MYH monzogranite and granodiorite have positive whole rock ε_(Nd)(t) and ε_(Hf)(t) (up to + 3.0) and zircon ε_(Hf)(t) values, indicating a juvenile crustal source, whereas, the LXC and NX granitoids have strong negative and variable whole rock ε_(Nd)( t) and ε_(Hf)(t) and zircon ε_(Hf)(t) values, indicating that they were derived from partial melting of ancient lower crustal materials with involvement of mantle components. Field observations, geochronology, geochemistry, Sr-Nd-Hf isotopic and zircon Hf-0 isotopic compositions point to a complex petrogenesis, where mantle- and crust-derived magma mixing was coupled with crystal fractionation, thus explaining the genetic link between mafic and felsic rocks. Identification of four components, i.e., depleted and enriched mantle and juvenile and ancient crust in the Late Triassic magmatism in the northern Liaodong Peninsula suggests a strong manfle-crust interaction process related to decratonization of the North China Craton.