Late Triassic high-Mg diorites and associated mafic dikes from the southern Zhangguangcai Range (NE China): Melting of juvenile arc crust and subarc mantle lithosphere

摘要

The Lesser Xing'an-Zhangguangcai Range is located in the eastern part of the Central Asian Orogenic Belt (CAOB). Early Mesozoic igneous rocks in this area can place constraints on the crustal growth mechanism in the Northeastern China. This paper reports a suite of Late Triassic high-Mg diorites and mafic dikes from the southern Lesser Xing'an-Zhangguangcai Range. The mafic dikes have younger zircon U-Pb ages of 195 +/- 2 Ma. They display intermediate SiO2 (50.58% to 58.20%) and high Na2O/K2O ratios of 1.90 to 3.20, low A/CNK ratios of 0.75 to 0.84, low (Sr-87/Sr-86)(i) ratios of 0.7045 to 0.7046, and high epsilon(Nd)(t) values of +3.1 to +3.5. Most zircons in the mafic dikes exhibit positive epsilon(Hf)(t) values from +8.6 to +16.0, with single-stage Hf model ages ranging from 0.2 to 0.5 Ga in combination with their enrichment in LREE and LILEs and depletion in HFSEs. They are considered to be formed by melting of metasomatized subarc lithosphere mantle. The diorite and quartz diorite have older zircon U-Pb ages of 200 +/- 3 and 202 +/- 2 Ma, respectively. They have SiO2 = 54.45% to 61.18%, Al2O3 = 17.16% to 19.10%, TiO2 = 0.74% to 1.13%, and Mg-# = 46.6 to 49.5, and they also display moderately depleted Sr-Nd (epsilon(Nd)(t) = +2.3 to +4.3) and zircon Lu-Hf (epsilon(Hf)(t) = +8.6 to +16.0) isotopic compositions. These features suggest that they were formed by melting of juvenile arc crust in active continental margin. Considering the occurrence of the contemporaneous and latter igneous rocks in the Zhangguangcai Range, we argue that the diorite, quartz diorite, and mafic dikes from the Lengshan area were also related to the westward subduction of Palaeo-Pacific Plate (Izanagi Plate) beneath the Eurasian Plate. The strike-like fault along the eastern margin of the Songneng-Zhuangguangcai Block may play an important role in the genesis of diorite melt.