Petrogenesis of the Huili Paleoproterozoic leucogranite in the Jiaobei Terrane of the North China Craton: A highly fractionated albite granite forced by K-feldspar fractionation

Abstract

A Paleoproterozoic leucogranite has been identified petrologically as highly fractionated granite near Huili in the Jiaobei Terrane of the North China Craton. This granite occurs as a 3×10km pluton in the metamorphic Paleoproterozoic Jingshan Group and was dated at 1.86Ga using zircon LA-ICP-MS U-Pb ages. The Huili leucogranite can be subdivided into three groups: groups I and II have normal leucogranite abundances with 2.22–2.57wt.% Na2O, 6.10–6.92wt.% K2O, and 71.68–73.07wt.% SiO2 and group III has a remarkably high Na2O concentration of 6.45–7.23wt.%, a low K2O content of 0.82–1.05wt.%, and 74.21–74.79%wt.% SiO2. Moreover, group III leucogranite comprises slightly lower Th, U, and Sr contents and considerably lower K-feldspar compatible elements, such as Rb, Ba, Pb, and Cs than those of groups I & II. All of these features strongly indicate the K-feldspar fractionation during the magma evolution from groups I and II to group III, which is strongly supported by the Pb isotope analysis. Group III leucogranite has a notably high U/Pb ratio of 0.30 to 0.45 with a normal U content of 1.42 to 2.21ppm and very low Pb content of 4 to 5ppm, indicating a significant K-feldspar fractionation of the magma. Group III leucogranite has high radiogenic Pb isotopes (i.e., 206Pb/204Pb=37.407–116.31), demonstrating that the rocks must have existed in a high U-Pb system for a notably long time. Fortunately, the whole rock Pb isotope values roughly define the Pb-Pb isochrons, not only when all three groups of samples are considered together but also when any individual group is considered. The ages are very close to the formation time of ~1.86Ga indicating the high U/Pb system required by these Pb isotopes must have been initiated during the formation of the Huili leucogranite, which has perfectly confirmed the K-feldspar fractionation during the magma evolution.All samples have consistent εNd values (t=1.86Ga) of −4.0 to −6.2 with two-stage Nd model ages (TDM2) of 2.67 to 2.85Ga. It is concluded that the 1.86Ga Huili leucogranite is highly fractionated, derived from ancient crustal material extracted from the depleted mantle at 2.7–2.8Ga, and experienced significant fractionation of K-feldspar in association with biotite and monazite fractionation during the magmatic evolution. Group III leucogranite is albite granite with albite phenocrysts, whereas groups I & II are K-feldspar granites with K-feldspar phenocrysts. It is here argued that the Huili albite granite (group III leucogranite) is derived from a highly differentiated residual melt separated from the K-rich magma represented by groups I and II leucogranite due to Kf fractionation during magma evolution.