Composition and origin of metamorphosed ferromanganese rocks in metavolcanics of Eastern Kamchatka

This paper presents detailed mineralogical and geochemical data on the metamorphosed ferromanganese rocks observed as rare lenticular bodies among the metavolcanics of the Khavyven Highland and Ganal Ridge in Eastern Kamchatka. They are specific porphyry-type banded Mn-garnet and amphibole rocks with large (up to 1–5 mm) magnetite porphyroblasts. In the Khavyven Highland, ilmenohematite is associated with magnetite in the metamorphosed ferromanganese rocks of the greenschist facies. In the Ganal Ridge, magnetite in metamorphosed ferromanganese rocks of the amphibolite facies shows decay structures and contains Mn-ilmenite lamellae surrounded by tiny crystals of ferroaluminous Zn-spinel. Comparison of Fe-Ti oxides in the ferromanganese rocks of the Khavyven Highland and Ganal Ridge shows that metamorphism of these structures differed not only in temperature and pressure levels but also in the oxygen fugacity. The rocks of the Khavyven Highland were metamorphosed under greater oxidizing conditions relative to the analogous rocks of the Ganal Ridge. The garnets in the studied rocks demonstrate progressive zonality from the center to the rims of crystals: the Fe, Mg, and Ca concentrations are increased, while the Mn concentration is decreased. Amphibolites in the ferromanganese rocks of the Khavyven Highland are also characterized by progressive zonality: the crystal cores are composed of Mn-actinolite, whereas the marginal zones consist of Ca-Na amphibole (winchite and less common barroisite), testifying to an increase of temperature, pressure, and oxygen fugacity during metamorphism. The higher-temperature rocks of the Ganal Ridge usually contain Mg-hornblende and tschermakitic hornblende.