LA-ICP-MS zircon U-Pb dating and geochemistry of the Late Paleozoic sanukitoids in the Hongshan area, west Junggar

Multi-stage andesitic dyke-like dark stripes are widely developed in the granitic Hongshan pluton, west Junggar, Central Asian Orogenic Belt (CAOB). The results of LA-ICP-MS zircon U-Pb dating, geochemical analyses, and isotopic tracing, indicates their formation at different time with diverse origins. LA-ICP-MS zircon U-Pb dating yields weighted average ages of 319.1±2.9 Ma and 313.3±2.4 Ma for the dark stripes with irregular and ring-like shapes, respectively. These two ages are largely older than the zircon U-Pb ages of the Hongshan pluton (305~301 Ma; Han et al., 2006; Feng et al., 2012; Xu et al., 2006), implying that the irregular and ring-like dark stripes are Late Carboniferous strata xenolithes captured by the Hongshan pluton during its emplacement. LA-ICP-MS zircon U-Pb dating yields a weighted average age of 295±2 Ma for the linear dark stripes, which are extensional dike swarms formed after the emplacement of the Hongshan pluton. All the andesitic dark stripes in the Hongshan pluton, such as the strata xenolithes and the dikes, have similar petrochemical compositions that are rich in SiO2 (56.48%~63.09%) and MgO (3.56%~6.31%), with high values of Mg# (51.74~62.40) and Na2O/K2O (1.34~3.43). All samples are enriched in light rare earth elements (LILEs, e.g. Rb, K, Ba, U, Sr) and large-ion lithophile elements, but strongly depleted in high field strength elements (HFSEs, e.g. Th, Nb, Ce, P) and heavy rare earth elements, with slight negative Eu anomalies. Additionally, the andesitic dikes have the same characteristics of Sr-Nd-Pb isotopes as typical sanukitoids ((87Sr/86Sr)i=0.703295~0.703520, (143Nd/144Nd)i=0.512612~0.512618, εNd(t)=6.91~7.62, (206Pb/204Pb)t=17.6883~17.9876, (207Pb/204Pb)t=15.5313~15.5686, (208Pb/204Pb)t=37.4460~38.0581). They have similar geochemical characteristics as sanukite, and generally exhibit geochemical characteristics related to the arc magma and may have a common source material. They are both products of different stages following the lithospheric plate subduction of the Junggar Ocean. The former was formed during the period of arc magma (319~313 Ma), and constitutes the andesitic dark strips like dykes associated with the island arc granitoids; the latter was formed in the late stage of the post-orogenic extension magmatism (305~295 Ma). The capture of andesitic dark strips (island arc volcanism), Hongshan rock mass (post-orogenic extension) and linear dyke swarms (intracontinental extension) recorded the whole process of ocean-land conversion in the Hongshan area of West Junggar. © 2020, Science Press. All right reserved.