REE geochemistry of early Precambrian charnockites and tonalitic-granodioritic gneisses of the Qianan Region, eastern Hebei, North China

Abstract

The charnockites and gneisses from the Qianan area, eastern Hebei Province, China, were investigated for their isotope and trace element abundances.The majority of the charnockites are acid in composition, only a few samples have a composition corresponding to intermediate to basic rocks. A Rb-Sr whole-rock isochron of the charnockites gives an age of 2.65 ± 0.05 Ga. The highly fractionated REE patterns and the variable (Ce/Yb)N ratios in these rocks are interpreted as being a result of igneous processes. It is believed that the charnockitic rocks are related to each other by fractional crystallization under high temperatures and low PH2O conditions; the major precipitating phases are thought to be orthopyroxene and plagioclase.By contrast, the Qianan gneisses are mainly of tonalitic and granodioritic composition. However, a few samples have composition corresponding to granite (s.s.), probably produced by metasomatic processes. A Rb-Sr whole-rock isochron of the Qianan gneisses yields an age of 2.13 ± 0.06 Ga and this age probably represents a remobilization event. The Qianan gneisses do not contain hypersthene; they have strongly fractionated REE patterns similar to those found in many Archaean tonalitic rocks of the world. In addition, the total REE abundances and the magnitudes of the Eu anomaly show regular variation with the SiO2 content. This may suggest that the Qianan gneisses might have evolved under hydrous conditions with hornblende exerting a major control on their trace element chemistry.