CRUSTAL-MANTLE INTERACTION IN THE PETROGENESIS OF PALAEOPROTEROZOIC POST-COLLISIONAL GRANITOIDS AND FELSIC VOLCANICS OF THE BAIKAL AREA

Donskaya T.V., Gladkochub D.P., Mazukabzov A.M.

Institute of the Earth’s Crust SB RAS, Irkutsk, Russia, tanlen@crust.irk.ru

The post-collisional stage of the Siberian craton assembly was accompanied by extensive magmatic activity. Numerous post-collisional granitoids with ages of 1.88 – 1.84 Ga are widespread within the basement salients along the southern cratonic margin. At the same time the granitoids and volcanic rocks of the North-Baikal volcano-plutonic belt were formed.

Palaeoproterozoic post-collisional magmatic complexes in the Baikal area are presented by 1.86 Ga Primorsk rapakivi-like granites (Donskaya et al., 2003) and 1.87 – 1.85 Ga granitoids and volcanic rocks of the North-Baikal volcano-plutonic belt (Larin et al., 2003; Donskaya et al., 2008). According to Whalen et al. (1987) all these rocks belong to A-type granites. The studied granitoids and felsic volcanics have the similar age and some similarity of geochemical features. Moreover, they were produce in the same tectonic setting. However, these rocks were derived from compositionally different sources.

The Primorsk rapakivi-like granites occurs in the western Baikal area within a narrow band along the western shore of Lake Baikal. The granites intrude the Precambrian metamorphic rocks of the Goloustnaya salient and the Palaeoproterozoic rocks of the Sarma Group. All analyzed granites are enriched in Ba (up to 3450 ppm), Sr (up to 335 ppm), Zr (up to 450 ppm). First phase granites are characterized by high Eu/Eu* ratios (0.92 – 1.62). The Primorsk rapaki-like granites could be classify as the least oxidized (“dry”) granitoids based on high FeO*/(FeO*+MgO) values, iron-rich compositions of amphiboles and biotites, the occurrence of ilmenite as the only Fe–Ti oxide, and oxygen fugacity below the QFM buffer. The initial _Nd value of the Primorsk granite is -6.0. It is close to _Nd value in host gneiss (Gladkochub et al., 2008). We believe that a possible source of the Primorsk granites is the Neoarchean infracrustal gneisses, which were characterized by low oxygen fugacity. The contribution of juvenile mantle material into the crustal source responsible for the origin of the Primorsk granites could not be recognized.

The North-Baikal volcano-plutonic belt is located northward from the Primorsk granites. The Irel granitoids and the Khibelen felsic volcanics of the southern part of the North-Baikal belt demonstrate some variations of geochemical and isotopic compositions within the different locations within the belt. However, neighbour granites and felsic volcanics have similarity in isotopic and geochemical characteristics. It can testify to their genetic relationship. Maximal and minimal initial _Nd values for the felsic volcanics and comagmatic granitoids of the different locations within the belt lie between -1.7…-2.8 and -8.0…-9.2. Their potential crustal source (Mesoarchaean tonalites of the basement of the North-Baikal volcano-plutonic belt) is less radiogenic. These Mesoarchaean tonalites show _Nd(1.85 Ga) = -15.0. According to geochemical and isotopic characteristics of the studied felsic volcanics and comagmatic granitoids, we assume that they could be derived from Mesoarchaean crustal source of tonalitic compositions with the addition of juvenile mantle materials. The estimated input of juvenile mantle materials into the sources vary from 33-40 to 77-86% (Donskaya et al., 2008).

Thus, we conclude that Palaeoproterozoic post-collisional magmatic complexes in the Baikal area having geochemical characteristics close to A-type granites were produced by melting of contaminated mantle-derived sources and by melting of pure crustal source as well. This conclusion is contradicted to model of Bonin (2007), who reported that all A-type granites could be derived from mantle-derived sources only.

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