Geochemistry and Genesis of the Sodium-Charnockitic Gneisses, Eastern Hebei Province, China

High in sodium and low in potassium (Na_2O/ K_2O>1), the charnockitic gneiss series in theSantunying- Taipingzhai area, eastern Hebei province, consists of hypersthene- quartz- diorite,hypersthene-granodiorite and hypersthene-plagioclase-granite. Geological, petrological and large ion lithophileelement(LILE), high field strength element (HFSE) and REE geochemical studies suggest that themedium-coarse-grained hypersthene-granodiorite is the product of crystallization of anatectic magmas of thesame composition. Under granulite facies conditions, the equilibrium crystallization differentiation of themagmas yielded the early crystallization phase-high-SiO_2, LILE-depleted, low-∑REE, positive Eu anomalyand REE- saturated hypersthene- plagioclase- granite. The residual phase, coarse- grained to pegmatitichypersthene- granodiorite, is marked by low SiO_2, LILE-enrichment, high ∑REE and REE-undersaturation.These rocks and hypersthene-quartz-diorite enclaves constitute the sodium-charnockitic gneiss series in easternHebei province. Model calculation for trace elements in the granitoids was applied. On the basis of a systematicgeological study, the equation for calculation was chosen, the source magma was determined and the partitioncoefficients were obtained. The resulting curves are entirely consistent with those observed in the patterns of ac-tual rocks. The study indicates that whole-rock REE patterns can not be used directly in the comparison of thesources and genesis of granitoids.

Geochemistry and Genesis of the Sodium-Charnockitic Gneisses, Eastern Hebei Province, China

Abstract High in sodium and low in potassium (Na₂O/K₂O > 1), the charnockitic gneiss series in the Santunying- Taipingzhai area, eastern Hebei province, consists of hypersthene- quartz- diorite, hypersthene-granodiorite and hypersthene-plagioclase-granite. Geological, petrological and large ion lithophile element (LILE), high field strength element (HFSE) and REE geochemical studies suggest that the medium-coarse-grained hypersthene-granodiorite is the product of crystallization of anatectic magmas of the same composition. Under granulite facies conditions, the equilibrium crystallization differentiation of the magmas yielded the early crystallization phase—high- SiO₂, LILE-depleted, low-ΣREE, positive Eu anomaly and REE- saturated hypersthene- plagioclase- granite. The residual phase, coarse- grained to pegmatitic hypersthene-granodiorite, is marked by low SiO₂, LILE-enrichment, high ΣREE and REE-undersaturation. These rocks and hypersthene-quartz-diorite enclaves constitute the sodium-charnockitic gneiss series in eastern Hebei province. Model calculation for trace elements in the granitoids was applied. On the basis of a systematic geological study, the equation for calculation was chosen, the source magma was determined and the partition coefficients were obtained. The resulting curves are entirely consistent with those observed in the patterns of actual rocks. The study indicates that whole-rock REE patterns can not be used directly in the comparison of the sources and genesis of granitoids.