Abstract:

The Hongshan complex is an important component of the Mesozoic intrusive rocks in the North Taihang Mountains, and is especially significant in alkaline rocks. Previous research restricted our overall understanding of Hongshan complex for lack in the volcanic rocks which are found on both sides of the Hongshan complex. In this paper, we firstly present the zircon U-Pb age data, whole-rock geochemistry and zircon Hf-isotope data of the Hongshan volcanic rocks. According to the U-Pb dating results, the eruption of tuffs on north of the Hongshan complex is dated at (132.8±1.1) Ma, indicating that the age is also close to the emplacement timing of the Hongshan complex and the intrusive rocks in North Taihang Mountains, even in other North China region. The tuffs are mainly composed of detritus, vitric, little crystal fragment and plastic lava. They are generally rich in Al₂O₃ (15.13%-16.78%), TFeO (5.61%-8.69%) and alkalis (K₂O+Na₂O,9.72%-11.13%), with normal Eu (δEu,0.97-1.02) and obvious fractionation between LREE and HREE (LREE/HREE,8.61-12.30). These rocks are all rich in LREE and LILEs (Rb, Sr, Th, K, etc), depleted in HFSEs (Nb, Ta, Ti, etc). The ε_Hf (t) values of the selected zircons vary between -20.51 and -12.34,with wide range of Hf-isotope crustal model ages from 1.9 to 2.5 Ga. Based on the above data, we suggest that the Hongshan volcanic rocks were most likely generated in a setting associated with westward subduction of the paleo-Pacific Plate to Eurasian Plate in Late Mesozoic, and originated from partial melting of an EMI-type mantle source, where mantle wedge-derived magmas may have provided the heat and material for the anatexis of ancient crust of the southern Taihang Mountains, resulted in hybrid melts. Such hybrid melts followed by a coupled fractionation of ferromagnesian phases are ultimately evolved into the Hongshan volcanic rocks.

Key words: zircon U-Pb dating, zircon Hf isotope, geochemistry, Hongshan volcanic rock, southern Taihang Mountains