Abstract:

The middle segment of northern margin of the North China Plate was influenced by intense tectonic and magmatic
activity during the Permian. Dozens of intrusions formed an E-W trending magmatic belt that extended from the Dongshenmiao to
Xianghuang Qi, Inner Mongolia, with a length of 400 km and width of 30 km. As a representative, Bayinzhurihe Pluton consists
of quartz diorite and hornblende gabbro, which belong to high-potassium calc-alkaline series. They are characterized by low SiO2
(49.72%~59.34%) and high Al2O3 (15.79%~19.50%) contents, with Mg#=38~44, enrichment in large ion lithophile elements
(LILE, e.g., Ba=829×10-6~1104×10-6, Sr=502×10-6~726×10-6), relative depletion in high field strength elements (HFSE, e.g.,
Nb, Ta) in primitive mantle-normalized spidergrams, and right-inclined rare earth element patterns, mostly with slightly negative
Eu anomalies (δEu=0.63~1.09). Their low Sr/Nd ratios (12~24) and La/Nb ratios (1.33~2.39) may reflect intraplate magmatism
involved. Hornblende gabbro shows non-arc-like (Nb/Th =9.6-18.4) basalt affinity compositions. LA-ICP-MS U-Pb zircon
dating of the Pluton yielded an intrusive age of 265±2Ma. Its zircons have εHf(t )= -13.0~-17.2 and t DM
C=2.12~2.38 Ga, mainly show the characteristics of lower continental crust. These features lead us to conclude that Bayinzhurihe Pluton was generated bymixing of lower crustal-derived mafic melt and minor enriched mantle-derived mafic magma, followed by fractional crystallization,due to crustal extension in post-collisional setting.

Key words: zircon LA-ICP-MS U-Pb dating, Hf isotopic compositions, post-collision, northern margin of the North China Plate