164 / 2015-08-31 09:28:14

Geochronology and petrogenesis of mafic dikes in the Guandi Complex, Zhoukoudian area and its Geological Significance

Geochronology and petrogenesis of mafic dikes in the Guandi Complex, Zhoukoudian area and its Geological Significance
Dongyang Yuan12, Dewei Li1234*, Demin Liu123, Qi Chen12, Ping Zeng12 and Bin Huang12
1. Faculty of Earth Science, China University of Geosciences, Wuhan 430074, China
2. Institute of Geological Survey, China University of Geosciences, Wuhan 430074, China
3.Center for Tibetan Plateau Studies, China University of Geosciences, Wuhan 430074, China
4. Disaster Research Center, China University of Geosciences, Wuhan 430074, China
Abstract
The Guandi Complex outcrops on both the north and south sides of the Fangshan pluton, the total area of about 0.375 Km2, and consists of orthogneiss and mafic amphibolite. Although previous studies have demonstrated that the orthogneiss of Guandi Complex yield a 207Pb/206Pb age of 2521~2548Ma, which is interpreted as the crystallization age or hypomigmatization age, there are unresolved issues about the origin of the Guandi Complex owing to the lack of geodynamic setting constraints. As important geological markers, mafic dike can be formed in varieties of tectonic settings, can help constrain the Precambrian tectonic evolution of these Archean terranes. Mafic amphibolite are dispersed in the gneiss of Guandi complex in the three styles: deformed amphibolitic boudins, lenticle in ductile shear zone and mafic amphibolite dike. Igneous zircons from the mafic amphibolite dike yield a 207Pb/206Pb upper intercept age of 2551±37Ma, which is interpreted as the crystallization age. It provides strong evidence that the mafic dikes in the North China block intruded during the Neoarchean. According to the geochemical results, mafic amphibolite characterized by low contents of TiO2, along with high MgO, CaO, and a Mg-number of 70. This rock displays an enriched LREE depleted HREE pattern with a pronounced negative Eu anomaly. element variation diagrams show that SiO2, Al2O3 and TiO2 show negative correlations with MgO, combined with a positive correlation between Zr and trace elements, consistent with fractional crystallization. Ni and Co correlate positively with MgO consistent with olivine fractionation. Combined with Residual clinopyroxene in the petrography, the Protolith may consist of olivine and clinopyroxene. Hf isotopic composition (ɛHf(t) = -9.0 to+14.5) indicate an enriched lithospheric mantle source. We also show that the mafic amphibolitic dikes are characterized by LREE-enriched, and HFSE depleted trace element patterns with pronounced negative Nb, Ta and Zr anomalies consistent with an arc-related mantle source region, rather than an OIB-like source. Therefore, the mafic amphibolitic dikes are proposed to have been formed in a subduction-related environment and their enriched mantle source was metasomatized by the melts and fluids derived from the subducted slab. The east-dipping subduction beneath the newly accreted arc and Eastern Block formed arc-affinity mafic dikes. According to previous research results, ocean-continent transformation have been the one of main modes for the continental crustal growth. The continental crustal growth of North China block during the Neoarchean time (2.6~2.5 Ga) is an important period. This study constraints the origin of Guandi Complex by Geochronology, Hf isotopic and geodynamic setting. And, It is significant to the Precambrian geological evolution of NCC and continental crust growth.
Keywords: Guandi Complex, Mafic amphibolite dikes, U-Pb dating, Geochemistry, Hf isotopic, Metasomatized mantle wedge