Revisiting the Late Jurassic adakitic rocks in the Yanshan fold and thrust belts, North China Craton: Partial melts from thickened continental crust?

An integrated study of zircon U-Pb ages and Hf-O isotopes, whole rock and apatite geochemistry, and whole rock Sr-Nd-Hf isotopes were conducted for the Jurassic adakitic volcanic rocks from the Tiaojishan Formation in the Yanshan Fold and Thrust Belt (YFTB) of the North China Craton, to investigate their genesis, with attention to the role of crust thickness in achieving the adakitic traits. Zircon U-Pb dating gives eruption ages of 162-158 Ma for these rocks. These rocks have variable SiO2 (55.7-68.1 wt%) and MgO (0.35-4.0 wt%), and adakitic trace element signatures including high Sr/Y (27.9-100) and (La/Yb) N ratios (9.6-27.6). Chemical variations in whole rock and apatite suggest a feldspar-dominated fractional crystallization, superimposed by variable mineral accumulation on whole rock compositions. Together with isotopic compositions that include evolved whole rock (Sr-87/Sr-86)(i) = 0.70447-0.70591, epsilon(Nd)(t) = -17.5 to -12.1, and epsilon(Nf)(t) = -22.2 to -11.2, and zircon epsilon(Hf) (t) = -26.8 to -11.2 and delta O-18 = 5.7-7.0 parts per thousand, we attribute these rocks are sourced from mafic lower continental crust. By comparing the trace element signatures between the examined samples and modern continental arc magmatic rocks formed at variable crustal thickness, it is suggested that these rocks are most likely originated from a thickened crust. A further estimate based on whole rock Sr/Y and La/Yb gives a crust thickness of 56 +/- 7 km. The crust thickening and associated uplift of the YFTB could be attributed to the westward subduction of the Paleo-Pacific plate. This led to a high altitude (similar to 3.8 +/- 0.9 km) and cool climate habitat (a mean annual paleo-temperature of 7 +/- 7 degrees C) for the Yanliao Biota lived therein during this period, which may have played an important role in the earliest appearance of the feathered characteristic of dinosaurs.