Geochemistry of charnockite rock-forming minerals (Nimnyrsky block of Aldan shield)
I. S. Sedova, V. A. Glebovitsky, S. G. Skublov
Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2015. V. 144. N 1. P. 3-29
Geochemistry of orhtopyroxene, biotite, plagioclase and potassium feldspar has been studied with aid of the ionic microprobe instrument Cameca IMS-4f in three samples: autochthonous charnockite-gneiss (chgn), para-autochthonous charnockite (ch1) aged 2.45—2.4 Ga, and the fourth generation anatectic migmatite leucosom (Lc4) dated 1.9—1.8 Ga. Charnockite-gneisses and charnockites had appeared due to metasomatic granitization, anatectic migmatization and diatexis, Lc4 — after considerable interval, under repeated migmatization of chgn è ch1. During these processes acidity and potassium alkalinity of rocks have increased, their maficity decreased, with significant increasing contents of Rb, Ba, Pb and LREE. Closed in oxides compositions of minerals assume the similarity of PT parameters while their formation in granulites facies. Contents of HREE and LREE in Pl and Kfs are decreasing within the range chgn → ch → Lc4, and for Opx in consequence chgn → ch; for Bt any regularity is absent. The lower Kd values in pairs Opx -Pl, Opx-Bt, Opx-Kfs within charnockites, as compared with charnockite-gneisses, are determined by the lower REE concentrations in orhtopyroxenes of charnockites. In pairs Pl-Kfs, the tendency of decreasing/increasing Kd values in the chgn → ch1 → Lc4 consequence is traced more often than in pairs Pl-Bt and Kfs-Bt, in connection with unbalanced composition of biotite. Considerable differences of Opx, PI, Bt, Kfs compositions in garnet-free and garnet-bearing rocks, by Al, #Fe contents and La/Yb ratio, are connected with peculiarities of chemical composition of initial rocks and acidity-alkalinity regime of the mineral-forming environment.
Key words: charnockite, charnockite-gneiss, leucosom, garnet-free and garnet-bearing rocks, orhtopyroxene, biotite, plagioclase, potassium feldspar, mineral-mineral pair, distribution coefficient, concentration spectra.