Magmatic complexes of the Urals as suspect parts of Large Igneous Provinces

Petrogenetic, geochemical studies and isotope age determinations of flood basalts, dolerites, trachybasalts, picrite-basalts, rapakivi granites, layered mafic-ultramafic intrusions and also alkaline and carbonatite magmatic complexes of the Urals permit to put forward a preliminary list of objects - “candidates” at being attributed to Large Igneous Provinces (LIPs) - manifestations of superplume activity. Their petro-geochemical properties distinguish them from spreading and subduction types, and are closer to epicontinental rift zones. They are characterized by wide areas of development and very short periods of activity. In the Southern Urals near the base of the Lower Riphean (Uppermost Paleoproterozoic and Lower Mesoproterozoic) there are volcanic deposits of the Navysh Subformation, represented by trachybasalts. The age of the unit was determined as 1752 ± llMa. Volcanic rocks of the age level of 1750-1780 Ma are developed not only in some other places of Baltica, but also in the Northern Africa, Siberia, Laurentia (parts of Nuna supercontinent). Therefore, they may belong to a LIP. Higher up the section of the Riphean, at the base of the Middle Riphean (Mid-Mesoproterozoic), rhyolites of the basalt-rhyolite Mashak Formation were dated as 1380-1385 Ma. The same ages have also rapakivi granites, layered gabbro, carbonatites and dolerite dykes developed in the Southern Urals and encountered in boreholes of the East European platform; magmatic rocks of the same age are traced to Laurentia and Siberian cratons and date the beginning of Nuna supercontinent break-up. Less confidently we may speak of the younger Neoproterozoic magmatic complexes of the Southern Urals as LIPs, dated as ca. 720 Ma and 680 Ma (Arshinian and Kiryabinka complexes); they need a further study. The next in the succession of magmatic episodes, represented by subalkaline volcanics, is connected with a rift process that started at ca. 490 Ma, that led to oceanic spreading and formation of the Paleouralian ocean. The second Paleozoic episode was marked by an eruption of trachytes and carbonatites and is dated between 435 and 455 Ma. The younger complex is Devonian in age and is traced along the western slope of the Urals to Pay-Khoy and Novaya Zemlya. They belong to the LIP called Kola-Dnieper. The last but not the least are the Lower Triassic flood basalts and dykes traced from the easternmost parts of the Southern and Middle Urals to the western margin of the Polar Urals.


Introduction
The analysis of plume activity in history of ancient orogenic belts encounters some specific difficulties. The ancient plumes have cooled a long time ago, and therefore it is impossible to apply seismotomographic methods to unravel them. The magmatic complexes, the main manifestations of plumes, are partly eroded and partly concealed under younger sediments; very often they are subjected to strong deformations. Such classical features of individual plumes as time-progressive volcanic chains are as a rule not revealed in orogens. Although the bulk of magmatic rocks of plumes is represented by flood basalts and dolerite dykes, plume magmatism is variable and may include trachybasalts, picrite-basalts, rapakivi granites, layered mafic-ultramafic intrusions and also alkaline, carbonatite and kimberlite magmatiс complexes [1]. Fortunately, enough the petro-geochemical properties of plume complexes distinguish them from spreading and subduction types, while many epicontinental rifts, accompanied by specific volcanism, may indicate plume activity. In case of largescale plume events called superplimes they occupy wide areas called LIPs (Large Igneous Provinces) and are characterized by very short periods of activity. These features help to reveal ancient plumes even in foldbelts.

Problems of plume identification in the Urals
Very important innovation that appeared in [2] and some more recent publications of this author was a theme of probable plume events in the Urals, a point that was not raised until the early years of the new century. During the last decade, a considerable progress was achieved in the study of petrogeochemistry of magmatic rocks of the western slope of the Urals, and their isotopic ages became more accurate owing to the application of modern methods and new instrumental base. In many cases, these data do not contradict the idea of their plume nature. Of course, the areas of their development (mostly some parts of the western slope of the Urals) are not so wide to attribute them to LIPs, but they can be correlated with simultaneous magmatic events in some other regions. LIPs and corresponding superplumes are characterized by short life times, and the maximum of their activity corresponds to 10−15 Ma [1,3,4]. In addition, the identified even-aged magmatic complexes can be placed on reconstructions of ancient supercontinents (Pangea, Rodinia, Nuna), demonstrating their relatively compact primary positions above hypothetical superswells.

Suspect plumes in the Urals
The most promising candidates to plumes are magmatic complexes of the western slope of the Urals and its northern prolongation -foldbelts of Pai-Khoy and Novaya Zemlya. Eight complexes can be established provisionally as material witnesses of plume and superplume episodes in the history of this part of the Urals.

Navysh episode
In the Southern Urals near the base of the Lower Riphean (Uppermost Paleoproterozoic and Lower Mesoproterozoic), covering the crystalline Taratash complex dated as Archean and Lower Paleoproterozoic, there are volcanic deposits of the Navysh Subformation, represented mostly by trachybasalts ( Fig.1). The age of the unit was determined as 1752±11Ma (SHRIMP, zircons) [5]. It is shown that volcanic rocks of the age range of 1750−1780 Ma are developed in some other places of Baltica, and also in the Northern Africa, Siberia, Laurentia, parts of the Nuna supercontinent at that time [3], [6]. Therefore, they may belong to a LIP.

Kiryabinka episode
Somewhat younger is the Proterozoic Kiryabinka layered peridotite-pyroxenite-gabbro intrusion, situated at the North-Eastern margin of the Bashkirian meganticlinorium (680±3.4 Ma, zircons, U−Pb method) [9]. The even-aged magmatic rocks are found toward South from this point in the Bashkirian meganticlinorium (Krivaya Luka) and toward North, in the Middle Urals − Schegrovitsk trachybasalt, Zhuravlik wehrlite-gabbro-granodiorite and Troitsk granitoid formations. In the East European platform, comparable ages belong to basites of the Onega graben. They may be the parts of a concealed LIP.

Kidryas episode
It is represented by subalkaline volcanics connected with a rift process that started at ca. 490 Ma, and led to oceanic spreading and formation of the Paleouralian ocean. This accompanied the formation of the Baltica passive margin [10] and can be attributed to a plume-connected volcanogenic type [11]. The comparable and contemporaneous rifting events, accompanied by volcanism, took place in the Lower−Middle Ordovician along the eastern (in modern co-ordinates) margin of the Siberian continent [12]. As it is shown by paleomagnetic data (e.g. [13], [14]), the "upside-down" position of the Siberia, and sub−longitudinal strike of the Uralian margin could suggest close, vis-a-vis positions of the margins, and their volcanism may belong to the same superplume episode, occurring above the same superswell.

Devonian magmatic series
Probably it consists of several episodes, and the Frasnian is the strongest one.

Uralo−Siberian LIP and superplume
Last, but not least, are the Lower Triassic flood basalts, dolerite and rhyolite dikes traced from the easternmost parts of the Southern and Middle Urals to the western margin of the Polar Urals. It became evident that they belong to the Uralo-Siberian LIP, developed above the "African" superswell ( [2], [19]).

Results and conclusions
The main result of the present research at this stage is a perspective of examination of the new idea of a considerable role of plume tectonics in geological processes of the Urals. The idea is certainly worth of a further study, and may be fruitful not only for a general theory of the Earth development, but also for the understanding of regularities in genesis and localization of mineral deposits [4].