Identification of the primary nature of granulite complexes with the use of geochemical tendencies of sedimentary and igneous differentiation

Kravchenko A.A., Beryozkin V.I.

Diamond and Precious Metal Geology Institute SB RAS, Yakutsk, Russia

[e-mail]

The rocks that undervent the granulite facies metamorphism are completely lacking petrographic and many geological features of the original magmatic and sedimentary rocks. That is why reconstruction of their primary nature is based on the geochemical characteristics. This can best be done by determining the relationships between elements reflecting specific characters of sedimentary and igneous processes in the course of protolith formation [1]. To show these processes on a binary diagram, geochemical indices may be used, e.g., the Chemical Index of Alteration (CIA) [2] in combination with the Femic Modulus (FM) [1]. With these indices plotted on the binary diagram we shall have two reference axes – Y and X, reflecting the trends of sedimentary and igneous differentiation, respectively (Fig. A). Other ratios between rock-forming oxides can also be used. Study of chemical composition of unmetamorphosed sedimentary and igneous rocks from different collections showed that the most useful for the rock identification are ratios (SiO₂+MgO)/(CaO+Na₂O) and K₂O/(Al₂O₃+FeO+Fe₂O₃) (Fig. B). The advantage of the diagram constructed with the use of these ratios is in its high resolving ability due to clear-cut distinctions in the composition of sedimentary and igneous

Fig. Discrimination diagrams for identification of the primary nature of rocks. In their construction a total of 2000 compositions of sandstones, siltstones, mudstones, peridotites, dunites, pyroxenites, gabbros, dolerites, diorites, syenites and various types of granitoids were used.

rocks. To avoid the errors that may arise from redistribution of Si, Na and K upon metamorphism and to control the analytical results, the ratios between immobile elements such as Fe, Ca, Al, Mg (Fig. C) and separately between other elements (Fig. D) can also be used. The diagrams (Fig. A,B,C,D) used in the analysis of the rocks from the Aldan granulite-gneiss area revealed geochemical trends of sedimentary differentiation for garnet-biotite gneisses, high-alumina gneisses, eulysites, marbles and calciphyres, and trends of igneous differentiation for ultramafic rocks, basic crystalline rocks (granulites), amphibolites, charnockite-, enderbite-, granite- and tonalite-trondhjemite gneisses within the correspoding fields. However, when constructing the diagrams we used data not for all types of sedimentary and igneous rocks. Also metasomatic rokcs were not studied. Thus there is a possibility that the diagrams can be incorrect for some cases.

The authors are indebted to G.V. Ivensen, V.A. Trunilina and A.V. Kostin who provided us with geochemical data.

The study was completed whith the financial support granted to research workers by the goverment of the Sakha Republic (Yakutia).

References:

1. Predovsky, A.A. (1970) Geochemical reconstruction of the primary composition of Precambrian metamorphosed volcanogenic-sedimentary rocks, Apatity, «Kirovskiy rabochiy» Publishing House, 115pp (in Russian)

2. Nesbitt, H.W. and Young, G.M. (1982) Early Proterozoic climates and plate motions inferred from major element chemistry of lutites, Nature, v. 299, p. 715-717

Дата добавления: 2015-10-29