Mineralogical and petrochemical characteristics and genesis of Laoniugou gneiss in Jiapigou gold mine, Jilin Province

Abstract

Detailed mineralogical and petrochemical studies show that the Laoniugou gneiss of the Jiapigou gold mine is composed mainly of plagioclase gneiss and irregular to lentiform plagioclase amphibolite melanic enclaves. The major element contents show an obvious bimodal and trondhjemitic series evolutional trend. This situation is significantly different from that encountered in bimodal calc-alkalic volcanic rocks in the rift-type Archaean greenstone belt. The contents of Rb, Sr and Ba are 7–21 ppm, 153–363 ppm and 201–1451 ppm respectively, close to those of common Archaean grey gneisses. AU the samples of plagioclase gneisses show positive Eu anomalies (even up to 4.6). The protoliths of the plagioclase gneiss are high-Al2O3 trondhjemitic series rocks, belonging to typical TTG of Archaean high-grade metamorphic terrain. The gneiss is quite similar to the B-type Amitsoq gneiss of W. Greenland. The authors believe that the plagioclase amphibolite enclaves are the relics of ancient oceanic crust while the plagioclase gneiss is the TTG ancient intrusive rock resulting from partial melting of the oceanic crust.