Petrogenesis and geological significance of the Paleoproterozoic Jianyuling granitic gneiss in the southern margin of the North China Craton

The Paleoproterozoic Jianyuling granitic gneiss exposed in the Xiaoqinling area along the southern margin of the North China Craton provides crucial insights into Early Precambrian crustal evolution. This study conducted petrological, zircon LA-ICP-MS U-Pb geochronological, and whole-rock geochemistry to constrain the emplacement age, petrogenesis, and tectonic setting of these rocks. The results indicate that the upper intercept age of the discordia line between 206Pb/238U and 207Pb/235U and the concordia line for the granitic gneiss massif in Jianyuling is 2 293±11 Ma, with a weighted average 207Pb/206Pb age of 2 283±6 Ma (MSWD = 1.3), constrain the crystallization age between 2 293 Ma and 2 283 Ma, indicating emplacement during the Paleoproterozoic. Geochemical data exhibit typical features of C-type adakites, characterized by high SiO2 (69.33%~71.57 %), Na2O (3.51%~4.45%), and K2O (2.72%~4.14%) contents, and Na2O/K2O ratios of 0.9~1.39, indicating a sodic affinity. The rocks belong to high-K calc-alkaline series (Rittmann index σ = 1.43~2.43) and exhibit strong LREE/HREE fractionation (ΣLREE = 454×10-6~605×10-6, ΣHREE = 39.3×10-6~45.6×10-6). Characteristic trace element patterns show enrichment in large-ion lithophile elements (such as Rb, Ba, Th) and high-field-strength elements (such as Zr, Hf), coupled with marked depletion in Nb, P, and Ti. Diagnostic adakitic characteristics include elevated Mg# (an average of 42.24), Sr/Y (58.39~68.03), and La/Yb (82.39~94.38) ratios, combined with low Y (6.24×10-6~7.24×10-6) and Yb (0.44×10-6~0.56×10-6) contents. It indicates that it belongs to C-type adakite. In summary, the Jianyuling granitic gneiss originated from partial melting of the Taihua Group TTG basement rocks with limited low-pressure mantle interaction, and formed in a back-arc extensional environment triggered by subduction slab retreat during the Early Paleoproterozoic.