关键词: 地球化学, 锆石Lu-Hf同位素, 岩石成因, 岩浆混合作用, 花岗质岩石, 暗色包体, 古太平洋, 延边地区

Abstract:  The Yanbian region of eastern Jilin Province, located on the southeastern margin of the Xingmeng orogenic belt and western Paleo-Pacific tectonic domain, provides a critical area for understanding their superposition and transformation. Dark microgranular enclaves within granitic rocks serve as valuable probes into deep Earth dynamics, recording crucial information on magma source characteristics and evolution. This study investigates Early Jurassic Yueshandong and Mingnandong granitic plutons in the Yanbian area, focusing on the genesis of the granitic rocks and their enclaves by petrogeochemical and zircon Lu-Hf isotope analyses to ascertain the tectonic background of Early Jurassic magmatism. The w(SiO2) of the dark enclaves in the Yueshandong pluton and the Mingnandong pluton are 51.09% to 53.52% and 49.53% to 51.96% respectively; The w(Cr) are 1 192.00×10-6 to 3 219.00×10-6 and 72.68×10-6 to 194.90×10-6 respectively; The w(Ni) are 330.60×10-6 to 693.20×10-6 and 58.61×10-6 to 81.06×10-6 respectively; The w(V) are 595.10×10-6 to 800.70×10-6 and 599.80×10-6 to 619.60×10-6 respectively; The w(SiO2) of the host rocks in the Mingnandong pluton is 65.75% to 68.19%, the w(Cr) is 33.20×10-6 to 57.54×10-6, the w(Ni) is 19.25×10-6 to 30.45×10-6, and the w(V) is 181.80×10-6 to 229.20×10-6. All the samples show the characteristics of being rich in sodium and poor in potassium, relatively enriched in light rare earth elements, relatively depleted in heavy rare earth elements, and being enriched in large ion lithophile elements (LILE) such as Rb, Th, U, and K, and relatively depleted in high field strength elements (HFSE) such as Nb, P, and Ti. The values of Nb/Ta and Rb/Sr are close to the mantle average values. The petrogeochemistry indicates that the magmatic source regions of the quartz diorite, granodiorite and their enclaves have mantle-source characteristics. The εHf(t) values of the zircons in the Yueshandong pluton are -3.2 to 2.4, with single-stage model age (TDM1) and two-stage model age (TDM2) of 1 004 to 746 Ma and 1 991 to 1 442 Ma, respectively, and its initial magma originated from the depleted mantle or the newly formed lower crust. The elliptical to circular shape and lack of baking rims of the dark microgranular enclaves, coupled with weak fractional crystallization trends, point to magma mixing. It is speculated that these dark enclaves in the Yueshandong and the Mingnandong plutons represent residues of the magma mixing process. The granitic magmatism in the study area during the Early Jurassic was likely formed in an active continental margin setting related to the subduction of the Paleo-Pacific plate.

Key words: geochemistry, zircon Lu-Hf isotopes, petrogenesis, magmatic mixing, granitic rocks, dark enclave, Paleo-Pacific, Yanbian area