VOLCANIC ASSOCIATIONS IN EASTERN KAZAKHSTAN: EVIDENCE FOR MANTLE

EFFECT ON FELSIC MELTS GENERATION

Khromykh S.V.

Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia, serkhrom@uiggm.nsc.ru

At last years many geophysical, geological and petrologic data prove the active role of mantle in evolution of accretion-collision orogens. Offered geodynamic scenarios for mantle effect on collisional tectogenesis differ but the fact that mantle may be a source for heat and matter for felsic melts generation is undoubted. Geologic and petrologic evidences for this were obtained under study of Late Paleozoic felsic volcanic associations in Ob’-Zaysan folded system (Eastern Kazakhstan).

Evolution of Ob’-Zaysan folded system (OZFS) began in Late Paleozoic in consequence of convergence Siberian and Kazakhstan paleocontinents and closing of Chara paleooceanic basin that finished to Middle of Carboniferous (Vladimirov et. al., 2003). At Late Carboniferous – Early Permian (300 – 270 Ma) time respond to collision stage of evolution within the OZFS the different magmatism appeared – from subalkaline gabbros and picrites to plagiogranites and granites-leucogranites composing largest in Eastern Kazakhstan Kalba and Zharma granite batholiths. Special attention would to devote on felsic volcanic associations. They occur in all formation zones of OZFS in the form of paleovolcanic central-type edifices in numbers about twenty.

Different eroded level of paleovolcanic edifices allows studying both a cover facies and a hypabyssal facies. Paleovolcanic edifices consist mainly of dacite-rhyodacite-rhyolite (granodiorite-granite-leucogranite) association with rhyolites (leucogranites) prevalence over dacites (granodiorites). Besides cover facies in some edifices extrusive stocks of volcanic breccia were mapped. The breccias have dacite or rhyolite matrix and contain much of fragments of different composition – from surrounding volcanic rocks to metamorphic rocks of amphibolite or granulite facies that indicate to magmas lifting from considerable depths. The character of dacites and granodiorites is presence of liquidus early-magmatic garnet (up to 3 vol. %). Together with felsic rocks in paleovolcanic edifices a stocks of andesibasalts and andesites with clinopyroxene phenocrysts, hypabyssal bodies of monzonites and syenites, and post-granite dolerite dikes are mapped.

Mineralogical study of garnet-bearing dacites shows that liquidus garnet contain about 40-45 wt.% of pyrope. Besides the grains of metamorphic garnet were found that are relics from metapelitic granulites involved to dacite melt on its generation level (Vladimirov et. al., 1985; Titov et. al, 2001). Thermometric study of melt inclusions in garnet phenocrysts shows the temperature of inclusions capture in 1030–1100°C and in quartz phenocrysts – temperature range from 1050°C in early generation to 900°C in late generation (Titov et. al, 2001) that prove to superheating of felsic melts. Glass compositions in melt inclusions in garnet respond to dacite-rhyodacite and in quartz – to rhyodacite-rhyolite that confirms the autochthonous of phenocrysts. Glass composition in melt inclusions and garnet compositions conform with anatectic melts and restite garnets taken from experimental malting of low-Na metapelite under 10 kbar and 950-1100°C (Douce, Johnston, 1991). Geochemical modeling of rare elements behavior in melting process (Kuibida et. al., 2004) confirm the conclusions about considerable depths of felsic melts generation both from metabasitic and metapelitic sources. The geochemical model calculation permits to think about high melting degrees (20 % for metabasites and 40 % for metapelites).

So investigation of rocks in paleovolcanic edifices ascertain that generation of felsic magmas happened in low-crust conditions under high temperatures. Forming of volcanic associations may be interpreted by intrusion of mantle melts into the basement of continental crust and following lower-crust anatexis in local focuses under volcanic edifices. Presence in paleovolcanic associations dacites and rhyolites – products of different degrees of melting – prove to high temperature gradient for anatectic process that is the heat source settled directly in continental crust basement. Nature and genesis of that mantle melts are not authentically ascertained yet but last geological and geochronological data allow supposing their forming with Tarim mantle plume activity (Vladimirov et. al., 2008)

Investigations was carried out with Financial Support of Presidium SB RAS (Integration project No 7.10.2) and Russian Fund of Basic Researches (project N 08-05-00974).

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