Composition and Origin of Metamorphosed Ferromanganese Rocks in Metavolcanics of Eastern Kamchatka

摘要

This paper presents detailed mineralogical and geochemical data on the metamorphosed ferromanganeserocks observed as rare lenticular bodies among the metavolcanics of the Khavyven Highland and Ganal Ridge inEastern Kamchatka. They are specific porphyry-type banded Mn-garnet and amphibole rocks with large (up to1-5 mm) magnetite porphyroblasts. In the Khavyven Highland, ilmenohematite is associated with magnetite in themetamorphosed ferromanganese rocks of the greenschist facies. In the Ganal Ridge, magnetite in metamorphosedferromanganese rocks of the amphibolite facies shows decay structures and contains Mn-ilmenite lamellae sur-rounded by tiny crystals of ferroaluminous Zn-spinel. Comparison of Fe–Ti oxides in the ferromanganese rocks ofthe Khavyven Highland and Ganal Ridge shows that metamorphism of these structures differed not only in temper-ature and pressure levels but also in the oxygen fugacity. The rocks of the Khavyven Highland were metamorphosedunder 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 ferromanganeserocks of the Khavyven Highland are also characterized by progressive zonality: the crystal cores are composed ofMn-actinolite, whereas the marginal zones consist of Ca–Na amphibole (winchite and less common barroisite), tes-tifying to an increase of temperature, pressure, and oxygen fugacity during metamorphism. The higher-temperaturerocks of the Ganal Ridge usually contain Mg-hornblende and tschermakitic hornblende. The specific features of the ferromanganese rocks in Eastern Kamchatka—their lenticular morphology; confinementto volcanic-rich rocks; high contents of iron oxide (up to 20% or more) and manganese oxide (up to 7-10%); higherconcentrations of Ni and Cu; and lower concentrations of Ba, Sr, and Pb—allowed us to compare the studied rockswith the ferromanganese crusts of oceanic areas and suggest their sedimentary (hydrogenic) origin in a back-arc basinof island-arc systems of the Pacific-type active continental margin during the abrupt waning of the volcanic activityand intense underwater currents, which promoted the evacuation of the bulk of the lithogenic sedimentary material.