GRANITIZATION OF BASIC VOLCANITES BY MANTLE FLUIDS IN CONTACT AUREOLE OF YURCHIKSKY GABBRO-NORITE INTRUSION (GANAL RIDGE, KAMCHATKA)

Tararin I.A.

Far East Geological Institute FED RAS, Vladivostok, Russia, itararin@mail.ru

The possibility of granitization of mafic and ultramafic rocks by the highly mineralized mantle fluids to produce of granitoids was theoretically justified by D.S. Korzhinsky (1952) and supported by numerous experimental researches (Zharikov, 1987; Zharikov et al., 1990; Gramenitskii, Larin, 1996; Marakushev, 1987; Sisson et al., 2005). According to these ideas, granitization of the mafic basement results from its replacement under the action of mantle fluids of the undercrust origin arisen through the mantle degassing process. The magmatic replacement presupposes that the granite formation and final melting of the primary rocks are preceded by their metasomatic alterations by alkaline-silicic fluids along with debasification and “clarification” going in parallel with increasing partial melting (formation of banded and shady migmatites).

Processes of granitization and magmatic replacement of mafic volcanites manifest themselves widely in the contact aureole of the Yurchiksky gabbro-norite intrusion of the Ganal Ridge in Kamchatka. This intrusive injected the sedimentary-volcanic rocks of the Ganalskaya series and represents a phacolithic body extends in outcrops for about 22 km. The greatest thickness of the intrusion is fixed above its northern part where a little thickness of the country rock “cap” of the Vakhtalkinskaya strata of the Ganalskaya series, injected by the gabbronorite small bodies, is responsible for intensive high-temperature processes of hornfels development accompanied by metasomatic alteration and magmatic replacement (with debasification and granitization) of the initial deposits.

The Vakhtalkinskaya strata 800-900 m thick is composed of mafic volcanites (similar in composition to the back-arc basalts) metamorphosed into amphibolites and clinopyroxene-amphibole schists containing the boudinaged beds of garnet-biotite ± cordierite plagiogneisses from the first to 20-50 m thick, silicic (dacitic) metavolcanites, and more rarely quartzites and marbles. In the inner parts of the contact aureole, the mafic volcanites were metamorphosed into amphibole, clinopyroxene-amphibole, and two-pyroxene-amphibole hornfels composed of the magnesio-hornblende or ferroan pargasite (X_Mg=0.60-0.67)², plagioclase (X_An=0.80-0.45), clinopyroxene (X_Mg=0.68-0.77), and more rarely orthopyroxene (X_Mg=0.70-0.72). Maximal temperature of the contact metamorphism reached 800^oC.

In the local areas, fluids metasomatize the amphibole, clinopyroxene-amphibole, and two-pyroxene-amphibole hornfels to produce the replacement of the hornblende crystals by orthopyroxene (X_Mg=0.58-0.63, more rarely X_Mg=0.45-0.56) associated with plagioclase (X_An= 0.35-0.50), and variable amount of biotite (X_Mg=0.45-0.70), apatite, and Fe-Ti-oxides. Further increasing intensity of the metasomatic processes resulted in complete replacement of amphibole and pyroxene-amphibole parageneses of hornfels by the biotite-orthopyroxene-plagioclase association of fine- and thin-grained metasomatites and appearance in them of thin leucocratic migmatite veinlets, lenses, and segregations of biotite-orthopyroxene-plagioclase±garnet composition characterized by hypidiomorphic-grained magmatic structures and coarser (1-2 and more millimeters) crystals of minerals. Characteristic of metasomatites and migmatite veinlets and segregations is the enhanced content of apatite.

Metasomatic alteration of hornfels accompanied by the magmatic replacement results in their intensive debasification manifested in increasing role of leucocratic minerals (mainly plagioclase) in the newly formed parageneses at the expense of dark-colored minerals. Simultaneously, the plagioclase feldspar became more sodic, and iron content of dark-colored minerals increase and, to a lesser degree, alumina content of orthopyroxene (reaching maximal values in garnet enderbites) increase.

Further intensification of the metasomatic processes and magmatic replacement of hornfels results in the formation in metasomatites of thin veinlets of garnet enderbites whose parageneses, besides orthopyroxene, biotite, and plagioclase contain quartz, cordierite, garnet, and more rarely potash feldspar. Large bodies of garnet enderbites and plagiogneisses, replacing the garnet enderbites with distance from the contacts of the Yurchiksky intrusion, are formed through the magmatic replacement of the boudinaged beds of the sedimentary rocks alternating with mafic volcanites hosted by the Vakhtalkinskaya strata. These processes answer the temperature of 700-800^oC and pressure of 3.2-4.8 kbar.

Comparison of the chemical composition of the initial mafic volcanites and their metamorphic products testifies that the metasomatic alterations and magmatic replacement correspond in chemistry to the silicic-alkaline metasomatosis (granitization) and are responsible for the successive and irregular supply of SiO₂, Al₂O₃, Na₂O, K₂O, Rb, Ba, Zr, Nb, and Cl into the rocks being replaced and removal of iron, magnesium, calcium, and some trace components – Cr, Co, Ti, Y, and S.

It is suggested that the metamorphic processes and magmatic replacement responsible for granitization of the initial mafic volcanites of the Vakhtalkinskaya strata in the contact aureole of the Yurchiksky gabbro-norite intrusion took place under the action of highly mineralized mantle fluids filtered through the magmatic channels along which the gabbroid magma ascended.

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