U-Pb (SHRIMP-II) geochronology of Mukhalski alkaline massif,
Doroshkevich A.G.*, Ripp G.S.*, Sergeev S.A.**
*Geological Institute SB RAS, Russia;
** Centre of Isotopic Research of VSEGEI, Russia
The Central Asian fold belt was involved into large-scale intraplate processes at Late Paleozoic. These processes were accompanied a large magmatic areas formation like as Barguzinski, South-Mongolian, Khangayski and Khenteyski (Yarmoluyk, 2009). Barguzinski area is one of the earliest area and includes the Angaro-Vitimski granitic batholite, North-Baikalian and Vitim alkaline provinces. Massifs of alkaline ultrabasic and basic rocks, alkaline and nepheline syenites are situated within the provinces.
The Vitim province is 50 km wide and over 450 km long and includes more than 20 massifs of alkaline rocks. The rocks transected Neoproterozoic and early Paleozoic series and are covered by late Mesozoic-Cenozoic formations. The alkaline rocks have been incorporated in Sayzhenskii complex. Available geochronological data (Andreev, Scharakshinov, 1967; Konev, 1982; Zaguzin et al., 1976; Andreeva, 1982; Scharakshinov et al., 1991) for these rocks have wide variations. The variations probably are described by using of K-Ar methods of dating that are unstable to secondary processes. For example, age data for Sayzhenski massif range from 260 to 595 Ma, Verchne-Burulzayski - 205-394 Ma, Nizhne-Burulzayski - 261-510 Ma. À.À. Konev (Konev, 1982) has divided two groups of dating for rocks of Vitim province: Carboniferous (290-350 Ma) and Triassic (170-230 Ma); and À.Î. Scharakshinov (Scharakshinov et al., 1991) has sectioned the age data for 325-352 Ma, 241-265 Ma and 229-167 Ma.
The currently available dating for rocks of Sayzhenski complex are not sufficient to allow definite conclusion about age and duration of alkaline magmatic event in the region. Therefore, it is a necessary to use the more precise methods of rock dating. At first, we have dated the Mukhalski massif because of published K-Ar and Rb-Sr age data for the rocks and minerals of the massif are wide varying (Andreev, Scharakshinov, 1967; Konev, 1982; Zaguzin et al., 1976; Andreeva, 1982; Scharakshinov et al., 1991). Thus, pyroxene and nepheline from melteigite dated by K-Ar method gave 294 and 370 Ma, respectively (Zaguzin et al., 1976). Therefore, we performed a U-Pb SIMS SHRIMP-II dating of accessory zircon from rocks of Mukhalski massif.
The rocks of the massif are exposed in “erosion window” under covered Cenozoic basalts. Country rocks are Neoproterozoic limestones. The massif consists of urtite, ijolite and melteigite. There are dykes of nepheline and alkaline syenites. Chronological evolution of the massif according to T.T. Vrublevskaya (1988) can be summarized as follows: intrusions ijolites; emplacement urtites and melteigites; and intrusions of syenite dykes. Urtites, ijolites and melteigites have fine- and medium-grained structures, massive-banded and trachyte textures. The rocks consist of varying amounts of nepheline, aegerine-augite, hastingsite, ribekite and calcite. The syenites are composed of microcline, oligoclase and nepheline. Magnesium and calcareous skarns were formed on contacts of the rocks with limestones and their xenoliths. Metasomatic and postmagmatic processes are widespread and presented by albitization, nephelinization, microclinization.
Zircon from urtites forms crystals of prismatic habits 0.2-0.6 mm in size. The grains have two-three phase structures and sometimes show clear crystallization zoning (point 5.1) in cathodoluminescent image (fig. 1).
Fig. 1. Cathodoluminescent images of zircons from urtites of Mukhalski massif. White ellipses show a location of sampling.
All zircons contain fine light-colored edgings of secondary accretion (points 4.2, 12.1). Some parts of crystals (points 5.2, 9.1) have complicated inner domains with irregular luminescence and numerous solid inclusions. Zircons as a whole have high amounts of U and Th. Crystals of magmatic nature contain on average 1800 ppm of U at Th/U=0.4. The edgings of secondary accretion have around 200 ppm of U at Th/U=0.1, and heterogeneous domains are notable for anomalous Th/U rations, up to 2.8 (explaining by higher amount of Th (up to 4700 ppm), at U - 1200-1800 ppm). 14 analyses have been performed in 11 crystals. Results for 11 points on a diagram with concordia gave 294 Ma (Fig. 2, a). Furthermore, analytically significant differences in age have not been identified between magmatic domains and edgings of secondary accretion. This fact agrees with T.T. Vrublevskaya’s investigations (1988) that metasomatic processes accompanied a crystallization of melt. Obtained Sakmarian age most likely conforms to time of urtite crystallization. Three results with concordant values can be grouped in independent cluster with 324 Ma (Fig. 2, b). The data have been determined in interior of heterogeneous domains that can be considered as “seed crystals” of xenogenic nature.
Fig. 2. Diagram in Tera-Vasserburg’s coordinates for zircons from urtites of Mukhalski massif.
Therefore, the following conclusions can be deduced based on the obtained dating:
(1) Obtained U-Pb age of urtite crystallization in Mukhalski massif allows to specify the time of alkaline magmatic event in the region, in contrast to published K-Ar and Rb-Sr dating. (2) It is similar to age interval (310-275 Ma) of magmatic processes on Barguzinski area. The processes were accompanied by emplacement of Angaro-Vitimski granite batholite 305-275 Ma ago (Tsigankov et al., 2007) and formation of alkaline rocks of Synnyrski complex - 287-330 Ma (Pokrovski, Zhidkov, 1993; Kostyuk et al., 1990). According to V.V. Yarmolyuk (2009) a generation of above mentioned rocks characterized an early stage of Late Paleozoic rift system development in Central Asia that is related with activity of mantle superplume.
Undoubtedly, for precise determination of age and duration of alkaline magmatizm in Vitim province invites further geochronological investigations.
This study was financially supported by RFBR grant 08-05-98028, NSch-3848.2010.5 and ÌK-2873.2010.5.
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