The Neoarchean granitoids in the Yanlingguan area, western Shandong, North China Craton record the transition from TTG to K-rich granitoids

Abstract

The Neoarchean is an important period of continental crustal growth worldwide. Determining how the continental crust increases in size and was finally cratonized at the end of the Archean is therefore an important scientific issue in geology. This study involved field investigations, SHRIMP U-Pb zircon dating, zircon in situ Hf isotopic analysis, and whole-rock geochemistry of Neoarchean granitoids at Yanlingguan, a typical Neoarchean area in the western Shandong granite-greenstone belt, North China Craton. Three samples of diorite/TTG that occur as enclaves in monzogranite record the oldest magmatic zircon ²⁰⁷Pb/²⁰⁶Pb ages of 2738–2731 Ma. They have low ΣREE (total REE) contents (37.5–72.4 ppm), low (La/Yb)_n (8.9–25.5) and variable Eu/Eu* (0.90–1.86). The Xinfushan trondhjemite has a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2599 ± 4 Ma for magmatic concordant or near-concordant zircons from six samples. The trondhjemites have large REE variations, with ΣREE, (La/Yb)_n, and Eu/Eu* being 36.3–150.9 ppm, 23.9–91.1 and 0.57–2.00, respectively. The Lianhuashan monzogranite has a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2502 ± 6 Ma for magmatic concordant or near-concordant zircons from four samples. These rocks have ΣREE contents, (La/Yb)_n and Eu/Eu* of 42.9–573.7 ppm, 2.4–89.1 and 0.31–1.20, respectively. The geochemisal data indicate that the 2.7 Ga diorite/TTG rocks formed by fractional crystallization of mafic magma under low-pressure conditions. In contrast, the 2.6 Ga trondhjemites were most likely formed by a mixture of magmas derived from partial melting of low-K mafic and the earlier tonalitic rocks; whereas the 2.5 Ga monzogranites were derived by partial melting of the earlier TTG rocks, with addition of some sedimentary material. These events commenced with the subduction and melting of early Neoarchean oceanic crust, likely influenced by mantle plume activity, through an increase in crustal melting and consequent increase in the K₂O/Na₂O ratio in the 2.6 Ga TTG, and culminated in the production of the widespread generation of K-rich monzogranites at 2.5 Ga. Western Shandong thus underwent an increase in crustal maturity as it transitioned from TTG-dominated magmatism to K-rich granite, marking cratonization of the North China Craton at the end of the Neoarchean.