Geochronology and Geochemical Characteristics of Amphibolite in Guandi Complex, Zhoukoudian Area and Its Geological Significance

The Guandi complex outcrops on both the north and south sides of the Fangshan pluton, being composed offelsic gneiss, plagioclase amphibolite and migmatite. The amphibolites are dispersed throughout the gneiss in Guandi complex. Metamorphic zircons from the amphibolite dike yield a LA-ICP-MS U-Pb upper intercept age of (2 551±37)Ma, which have been interpreted as the crystallization age. According to the geochemical results, these amphibolites are characterized by low TiO₂ and high MgO contents, with Mg-number of 70. These amphibolites display the enriched LREE, depleted HREE pattern with pronounced negative Eu anomaly. Element variation diagrams and petrography characteristics indicate that the protolith consists of clinopyroxene and plagioclase. Hf isotopic compositions indicate that they are sourced from an enriched lithospheric mantle. Data of trace elements illustrates that these amphibolitic dikes are characterized by enriched LILE (Rb, Ba, K), and have depleted HFSE patterns with pronounced negative Nb, Ta and Zr anomalies, which are consistent withan arc-related enriched mantle source region. Therefore, it's proposed that the amphiboliticdikes were formed during the east-dipping subduction. The enriched mantle signatures are consistent with the metasomatization of melts and fluids derived from the subducted slab. Combining previous research results, these results help elucidate important components about the crustal growth of the North China Block during the Neoarchean.