MINERALOGICAL FEATURES AND P-T CONDITIONS OF METAMORPHISM OF NEOPROTEROZOIC METABASITES FROM THE TYRADA AND MALAYA KADRA RIVER BASINS, YENISEI RIDGE

Abstract

The relevance of the work. Today it is considered that metabasite rock associations widespread in the Trans-Angara part of the Yenisei Ridge are products of rifting processes caused by activity of plume and breakdown of the Rodinia supercontinent during ~1100–700 Ma. In this case, data on their absolute age are the subject of discussion. The paper considers the Neoproterozoic metamorphic events using thermodynamic features of formation of the studied amphibolite manifestations as an example and their place in geodynamic model of formation of the Yenisei Ridge, as well as the role of syn/postcollisional granitoid magmatism in age rejuvenation of Transangar amphibolites. The aim of the research is to establish the main mineral paragenesis of amphibolites and their secondary mineralization; determine the thermodynamic conditions of metamorphism and its main stages; correlate the nature of metamorphism with the metamorphic events of the region and determine its role in the formation of the modern appearance of the Yenisei Ridge; identify the reasons responsible for the mismatch between the age of the studied amphibolites according to the Ar-Ar isotopy and the accepted dating the Zaangarya metabasalts. Objects: amphibolites of the Shumikhinsky metapicritic-basalt complex, which is a part of the malogarevskaya metamorfic series of the Late Archean age (?) occurring among carbonate rocks and crystalline schists in the valleys of the Tyrad and Malaya Kadra river basins and the Tei and Panimba tributaries respectively. Methods. Chemical composition of the mineral phases was obtained using an analytical complex for scanning electron microscopy and microanalysis based on a TESCAN VEGA II LMU scanning electron microscope, combined with an Oxford INCA Energy 350 in the Center for Collective Use «Analytical Center of the Geochemistry of Natural Systems» of the National Research Tomsk State University. Based on the data the crystal chemical formulas of minerals were calculated, classification and geothermobarometric diagrams were constructed. Results. The authors established general paragenesis of amphibolites of the studied manifestations: Hbl+Pl (XAn=0,28–0,38)+Qz+Ilm (Sph)+Fsp with secondary local development of Act, Zo, Bt, Ms and Chl. Thermodynamic parameters of metamorphism of metabasites correspond to the conditions of low-temperature zones of the amphibolite facies (555–475 °С, 5–7,5 kbar) with the dynamics of pressure increase against the general background of gradual cooling. Amphibolites are derivatives of collision metamorphism (~880–800 Ma), which appeared in the accretionary conditions of previously split tectonic blocks of the Siberian craton. The local development of potassium feldspar, actinolitization, and chloritization (344–199 °С) refers to superimposed processes under reheating conditions and may be associated with the final stage of the Grenville tectonic events accompanied by syn/postcollisional granitoid magmatism (γR3ta, 760–630 Ma). It could become a cause of age rejuvenation of the studied metabasites (707–675 Ma).

Keywords

• amphibolites
• neoproterozoic collision metamorphism
• epidote-amphibolite facies
• p-t conditions
• age of metamorphism
• yenisei ridge