Carboniferous subalkaline volcanites of Beysikhan volcanotectonic structure (West Transbaikalia)
The Institute of Geology SB RAS, Ulan-Ude, The Russian Federation
The active collisional and intraplate tectonic-magmatic processes, which resulted in formation of the great Selenga-Vitim volcano-plutonic belt of rifting type and forming the Angara-Vitim granitoid batholith at the territory of West Transbaikalia on Late Paleozoic occured [Gordienko, Kuzmin, 1999].
The Selenga-Vitim volcano-plutonic belt was formed in the second period of Carboniferous within Ider, Dzhida, Uda-Vitim zones of caledonides along the great sutural deep-earth structure where the Central-Mongol microcontinent collided with the continental margin of the Siberian craton in the end of Early Paleozoic [Gordienko, 1987]. The belt includes in Zhelturin depression, Gunzun-Borgoy-Tugnuy volcanotectonic structure (further as VTS), Tamir VTS, Verkhnekudun VTS and Beysikhan VTS. Beysikhan VTS is one of the large structure of this belt is situated on south-west of the Vitim plateau where sedimentary, volcanic and carboniferous plutonic formations were found [Gordienko et al., 2003].
Beysikhan VTS (S ca. 100 km2) is situated on left bank Vitim River made up homonymous ridge in the Atalanga – Vitim interfluve. The VTS basement consists of Vend-Early Cambrian sedimentary-metamorphic rocks. The volcanites are presented by acid rocks and middle rocks. Its major part is represented by acid volcanites, particularly, trachydacites, dacites, trachyrhyodacites, trachyrhyolites, rhyolites and also pyroclastic analogues including ignimbrites. Trachyandesites, andesites and tuffs are presented in minor degree. It is occured transitional varieties of andesite-dacites composition which is very close to andesites and dacites in mineral composition. In general, volcanites of middle composition undergo the postmagmatic changes as chlorization, epidotization both phenocrystes and matrix. In rocks of acid composition devitrification of primary glass and following intense recrystallization of felsitic matrix are observed.
Trachydacites represent greenish deep-grey massive rocks. The porphyric varieties of the rare phenocryst (1-3%) represented by plagioclase, K-Na feldspar (commonly by pertite structure) and chloritized biotite. Feldspar consist from 0,7 up to 10% anortite minal, and mictopertites include albite interposition (20-30 % of the whole). The main structure is pilotaxitic, thin-grained and composed of quartz-feldspar aggregate, formed after recrystallisation of primary, vitreous matrix. The subvolcanic analogue is characterized by glomerophyric growth of plagioclase up to 1 mm and chloritised biotite. Trachyrhyolites are of felsitic, spherolite, micrograined structure and characterized by high crystallinity. Rare phenocrysts are represented by plagioclase and K-feldspar and also pseudomorphs on mafic mineral. Judge by crystal morphology and by substitution products (chlorite, epidote) we assume that colored mineral’s relicts are amphiboles. The major part of the isometric quartz grain and albite are micrograined.
Trachyandesites are massive rocks with pilotaxitic, microlite and micrograined in mass. Plagioclase phenocrysts’ composition is the following An58-42Or4Ab39-52. The albite and orthoclase ratio in K-feldspar is equal to 1,1-1,3 (Ab/Or=1,1-1,3). Alkaline amphibole-kaersutite, titanium rich (TiO2 3,08-12,2.%) and biotite occur in phenocryst of subvolcanic analogues. Ore minerals are represented by titanomagnetite (ÒiO2 - 12,2%, FeO - 80,3%).
The rocks’ chemical composition was defined in GIN SB RAS (Ulan-Ude), petrogenic oxides and Cr, Ni, Co, V were defined by atomic absorption and flame-photometric methods (analysts: V.A. Ivanova, I.V. Borzhonova, M.G. Egorova); microelements concentration by - X-ray-fluorescent method (analysts: B.Z. Zhalsaraev, Z. Rinchinova). Rare-earth elements were defined by ICP-AES method on OPTIMA 2000 DV (analysts: T.I. Kazantsev, L.A. Levantueva).
The volcanites of Beysikhan structure are high-alumina, alkali enriched rocks and presented by trachyandesites, trachydacites and rhyolites. It is notable that trachyandesites slightly differ from trachyrhyolites by their total alkalinity. However, silica growth evokes the sharp potassium increase. In middle volcanites Na2O/K2O ratio are 0,79-1,49, in acid rocks - 0,51-0,99 that allows refer them to kali-sodic series. The agpaitic coefficient in middle volcanites varies 0,51-0,78, in acid - 0,73-0,91. Judgings by these features the volcanites of Beysikhan structure can be refered to subalkaline trachyandesite-trachyrhyolite assemblage [Magmatic..., 1987]. According to the Mg coefficient (mg=41-53) and low Ni/Co ratio (0,8-1,3) trachyandesites are deep differentiated rocks.
On spider-diagrams of incoherent elements, normalized to N-MORB, middle and acid rocks have positive anomalies of Ba, La, Ce and sharp minimums of Nb and Ti that is characteristic for island arc magmas. However indicated ratios Ba/Nb (50-61), Ti/V (24-36), high Zr concentration are natural for active continental margins volcanites [Kononova et al., 1993]. Volcanites is notable for the high total content and the differentiated ratio of REE distribution. REE concentration level varies wide range with prevalence of light lanthanides that is defined by the relation ((La/Yb)N = 4,9-9,8) in trachyandesites and ((La/Yb)N = 3,4-11,6) in trachydacites and trachyrhyolites. Characteristic of volcanites are distinct minimum of europium that reflects extraction Eu from melt at fractionation concerning rich with calcium plagioclase.
The Beysikhan VTS volcanites, located in the right bank of the Khuludiy, are trachybasalts and andesite basalts. Trachybasalts are characterised by amygdaloidal texture and porphyritic structure. In general, phenocrysts are presented by plagioclase, subcalcic augite and the titan-augite (Wo24-28En36-39Fs19-23) (f=34-37, ÑàÎ=9,8-16,5 mas.%) are seen rarely. The individual grains have hornblende: actinolite and a biotite-phlogopite (f=28-30). The groundmass is sericitic and albitized plagioclase, submerged in devitrification glass, replaced by chlorite and epidote.
The volcanites sampled from the left bank of the Khuludiy are trachyrhyolites and ignimbrites. Trachyrhyolites represent massive fluidal or thinbanded rocks of porphyritic structure with micrograined, felsitic, micropoikilitic, and in more crystallized varieties - microspherulitic structure. Phenocrysts (5-10 % from ground mass) are presented plagioclase (albite) and potassic feldspar (sanidine). Trachyrhyolites contain glomerophyric plagioclase and potassic feldspar (K-feldspar) growths of 1-2 mm. K-feldspar phenocrysts partially pelitizated, and grains plagioclase sericitized.
Tabular feldspar phenocrysts form the roundish (melted off) formations of 0,3-3 mm. The groundmass consists of K-feldspar fine grains (0,05-0,1 mm), quartz and rare smallest kernels of a hornblende and scales of biotite. The rocks are densely covered by sericite and an ore mineral. Ignimbrites are characterised by fluidization and considerable relicts quantity of primary glassy lava (welded tuff). Basic and acid rocks differ in potassium content, the sums of alkalis and potassium/sodium ratio. The prevailing part of volcanites refers to calci-alkaline kali-sodic series.
The volcanites are characterized by moderate REE content and its weak fractionation. Volcanites REE concentration in samples of basalts (right bank the Khuludiy) is 32,9-36,47 ppm. The REE distribution spectrum is poorly differentiated (LaN/YbN = 2,0-2,8). The acid rocks from its left bank shows 53,2 ppm. REE distribution spectra are more differentiated (LaN/YbN = 4,8). All volcanites show negative europium anomaly. Incoherent elements concentration in all basalts and andesite basalts is the following (in ppm): Zr 37-160, Nb l-4, Y 8-21. The incoherent elements concentration in acid rocks is rather high (in ppm): Zr 220, Nb 10, Y 25.
Isotope Rb-Sr age of volcanic association Beysikhan VTS, received on mass-spectrometer Finnigan ÌÀÒ-252 (Analytical center of collective using, Irkutsk), gives 323± 46 Ma with primary 87Sr/86Sr ratio equal to 0,706±0,001. Obviously, low accuracy is connected with isotope heterogeneity of trachyrhyolites. Trachybasalts Àr-Ar age (the right bank of the Khuludiy) is 306,6±3,2 Ma, U-Pb zircon age (sampling from acid volcanites thicknesses (trachyrhyolites), left bank of the Khuludiy, is 290 Ma.
Thus, the Beysikhan VTS volcanites’ isotope age can be defined as Middle - Late Carboniferous - Early Permian.
This study was financially supported by RFBR grant ¹ 05-05-64035, 08-05-00290.
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