Granitization and magmatic replacement in the contact aureole of the Yurchik gabbronorite massif (Ganal Ridge, Kamchatka)

摘要

It is shown that formation of high-temperature granulite-like rocks in the contact aureole of the Yurchik gabbronorite intrusion of the Ganal Ridge in East Kamchatka was caused by contact metamorphism, metasomatism, and local melting of the primary sedimentary-volcanogenic rocks of the Vakhtalkinskaya Sequence of the Ganal Group. The temperature in the inner part of the aureole reached 700-800A degrees C and caused transformation of basic rocks into two-pyroxene-plagioclase, clinopyroxene-amphibole-plagioclase, and amphibole-plagioclase rocks, while sedimentary rocks were replaced by garnet-biotite and garnet-cordierite-biotite hornfelses. Locally, basic volcanic hornfelses were subjected to metasomatic alteration with the formation of bodies of biotite-orthopyroxene-plagioclase metasomatites. In the zones of the most intense fluid filtration, the metasomatites experienced local magmatic replacement resulting in the formation of biotite-orthopyroxene-plagioclase +/- garnet migmatite veinlets and patches. Bodies of garnet enderbites were formed after sedimentary interlayers at temperatures of 700-800A degrees C and a lithostatic pressure of 3.2-4.8 kbar. The comparison of the chemical composition of the Vakhtalkinskaya basic volcanics and the products of their transformation indicates that, in terms of chemistry, the metasomatic alterations and magmatic replacement correspond to siliceous-alkaline metasomatism (granitization) causing a subsequent and uneven influx of Si, Al, Na, K, Rb, Ba, Zr, Nb, and Cl and removal of Fe, Mg, Mn, Ca, and some trace elements (Cr, Co, Ti, Y, and S). The processes of metamorphism and metasomatism were presumably provoked by highly mineralized mantle fluids that were filtered through magmatic channels that served as pathways for gabbroid magma.