Abstract
The petrographic, lithogeochemical and U-Pb (LA-ICP-MS) geochronological studies were carried out to investigate the terrigenous rocks sampled from the lower part of the Ipsit suite of the Karagass series (Sayan segment of the Sayan-Baikal-Patom belt). These rocks include sandstones, aleurite sandstones and aleurolites, and their mineral compositions are close to that of arkose. Most of the studied rock samples show petrographic features typical of the epigenetic changes at the stage of catagenesis: regeneration of quartz clastic grains, pelitization of potassium-feldspar clastic grains, occurrence of clay-hydromica aggregate, sericitization of plagioclase, chloritization of biotite, and silicification of dolomite pieces, and occurrence of authigenous tourmaline. The above was confirmed by the analysis of the concentrations of petrogenic elements in the studied rocks from the lower part of the Ipsit suite. The analysis results show that the concentrations of K2O are elevated, while the concentrations of Na2O are relatively very low, which may be due to the redistribution of these elements during epigenetic transformations. According to the classification by genetic types on the basis of the system of petrochemical modules, the rocks of the lower part of the Ipsa suite are of the petrogenic nature. The acidic igneous rocks are dominant in the source area, as evidenced by the presence of granitoid and quartzite fragments in the clastogenic component, as well as the set of accessory minerals typical of the igneous rocks of the acidic composition, and the distribution pattern of rare and trace elements. According to the U-Pb (LA-ICP-MS) dating of detrital zircons from the aleurite sandstone sampled from the lower part of the Ipsit suite, the zircons are exclusively of the Archean-Early Proterozoic ages. Such ages correlate with the age of the granitoids of the Sayan complex and the felsic volcanites from the Maltsev layer of the Elash series (Biryusa block). Furthermore, the detrital-zircon age spectra of the aleurite sandstone of the lower part of the Ipsit suite are identical to the detrital-zircon age spectra of the terrigenous rocks from the underlying strata of the Shangulezh and Tagul suites of the Karagass series. This study suggests that sedimentation of the Ipsit suite of the Karagass series took place due to the influx of detrital material from the southern part of the Siberian craton into the sedimentation basin, and the acidic igneous rocks of the Biryusa block were one of the main sources of detrital material.
Highlights
Аннотация: В статье приводятся результаты петрографических, литогеохимических и U‐Pb (LA‐ICP‐MS) гео‐ хронологических исследований, проведенных для терригенных пород нижней части ипситской свиты кара‐ гасской серии
The petrographic, lithogeochemical and U‐Pb (LA‐ICP‐MS) geochronological studies were carried out to investigate the terrigenous rocks sampled from the lower part of the Ipsit suite of the Karagass series (Sayan segment of the Sayan‐Baikal‐Patom belt)
Accor‐ ding to the U‐Pb (LA‐ICP‐MS) dating of detrital zircons from the aleurite sandstone sampled from the lower part of the Ipsit suite, the zircons are exclusively of the Archean‐Early Proterozoic ages
Summary
Позднерифейско‐вендские осадочные толщи, прослеживающиеся вдоль южной окраины Сибир‐ ского кратона на расстоянии около 2000 км, отно‐ сятся к структуре Саяно‐Байкало‐Патомского пояса (СБПП) [Decisions..., 1983]. В саянском сегменте СБПП позднери‐ фейско‐вендские осадочные образования пред‐ ставлены отложениями карагасской и оселковой серии. Позднерифейский возраст карагасской серии установлен на основании изучения строматолитов и микрофитолитов [Khomentovsky et al, 1972; Bragin, 1986; Shenfil’, 1991], а также с учетом присутствия в разрезе шангулежской свиты долеритов нерсин‐ ского комплекса с возрастом ~740 млн лет [Glad‐ kochub et al, 2006]. Отложения шангулежской и тагуль‐ ской свит, судя по данным ряда исследователей [Bragin, 1986; Khomentovsky et al, 1972; Stanevich et al, 2007; и др.], образовались в мелководных морских условиях, в то время как породы ипсит‐ ской свиты формировались в результате штормо‐ вой переработки материала эолового происхож‐ дения [Sovetov et al, 2012]. 1 – basement of the Siberian craton; 2 – tillite; 3 – conglomerate; 4 – small‐pebble conglomerate and gravelite; 5 – sandstone; 6 – siliceous aleurolite and sandstone; 7 – argillite; 8 – aleurolite; 9 – sandy dolomite; – dolomite; – stromatolite and microfito‐ lite; – dolerite and gabbro‐dolerite
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