U–Pb geochronology of Riphean sandstone and gabbro from southeast Siberia and its bearing on the Laurentia–Siberia connection
Introduction
Siberia is included in several reconstructions of Rodinia, the Neoproterozoic supercontinent, and most workers agree that it was connected to Laurentia sometime during the Proterozoic era. Nevertheless, `the Laurentia–Siberia connection' is poorly constrained due to a paucity of comparative data. Most studies of Rodinia have focused on the relationship between Amazonia, Antarctica, Australia, China, Laurentia and Baltica e.g. 1, 2, 3. Similarities between Laurentia and Siberia were first recognized by Sears and Price [4], whose reconstruction (for 1700–1500 Ma) was based on congruence of tectonic grain between Archean cratons and shape of the craton margins. Other reconstructions connected Siberia to northeast Laurentia based on poorly constrained paleomagnetic data (e.g. 5, 6). With the critical acceptance of the SWEAT hypothesis 7, 8, 9, 10, which juxtaposes Australia and Antarctica against western Laurentia, the Sears and Price [4]reconstruction was seemingly supplanted as an option for placing Siberia on the western margin of Laurentia, unless Siberia rifted away from Laurentia before the amalgamation of Rodinia. A notable feature of the Rodinia reconstruction of Hoffman [8]is the inclusion of Siberia, which is linked to northern Laurentia based on continuity of ca. 2000–1900 Ma orogenic belts.
Condie and Rosen [11]produced a more detailed reconstruction of Laurentia–Siberia, the foundation for which is built upon linkage of the 2000–1900 Ma Thelon–Taltson tectonic zone with the Akitkan fold belt and on continuity of the Aldan Province with the Slave Province. Frost et al. [12]emphasize that the southeast extension of the Akitkan belt represents an interpretation beneath cover and its geochronology also is poorly defined, concluding that a strong linkage with the Thelon zone therefore would be tenuous. Instead, their isotopic and metamorphic studies in the western Aldan Province suggest similarities between the Slave Province and the Olekma terrane and that the Thelon–Taltson zone links better with the Aldan terrane (see [12], their fig. 14 for summary diagram illustrating previous reconstructions and locations of terrane and province boundaries). It is important to stress that all previous reconstructions utilize a ca. 2000–1900 Ma mobile belt as their piercing point, which requires continuity of the Slave and Aldan Province before the amalgamation of Laurentia (cf. [14]).
Condie and Rosen [11]offered several tests of their model, including a comparison of Proterozoic stratigraphy between southeast Siberia and northwest Laurentia. Lithostratigraphic comparison yields some first-order similarities 15, 16, which we attempt to build upon in this paper.
With a relatively well characterized absolute chronology for Proterozoic stratigraphic successions from northwest Laurentia (e.g. 17, 18) it seemed expedient to try to compare these rocks with correlative rocks from the conjugate margin of southeast Siberia as proposed by Condie and Rosen [11]. In this paper we present the U–Pb geochronology of detrital zircon grains from a late Riphean (ca. 1000–700 Ma) sandstone unit from the Sette–Daban fold belt of southeast Siberia and compare the results with previous work on correlative strata from northwest Canada. Included with this test are initial results from a search in southeast Siberia for remnants of voluminous and extensive 1267 Ma Mackenzie [19]and 723 Ma Franklin [20]magmatic events, which are so prominent across most of northern Laurentia. If Siberia was joined to Laurentia since 1900 Ma, as in the configurations proposed by both Condie and Rosen [11]and Frost et al. [12], and separation did not occur before 723 Ma, then both magmatic events should be represented in southern Siberia. Using these rationale we report U–Pb ages of potential equivalents to these mafic magmatic rocks, which intrude the Riphean supracrustal succession of southeast Siberia.
Section snippets
Regional geology of SE Siberia
We chose the Riphean–Vendian (ca. 1600–540 Ma) succession of southeast Siberia for study because, in the reconstruction of Condie and Rosen [11], it is situated immediately opposite the comparative Proterozoic succession of northwest Laurentia. This region, on the eastern margin of the Siberian platform (Fig. 1), has well established lithostratigraphy and biostratigraphy, and shares a comparable tectonic history with western North America [21]. The study area is located within the Sette–Daban
Mayamkan Formation sandstone
The detrital zircon sample discussed in this paper was collected in the Uchur–Maya depression from the Mayamkan Formation, uppermost terrigenous clastic unit of the Uy Group (Fig. 1). The Mayamkan Formation is exposed southward from the Maya River in a belt of about 90 km in length. It is important to note that this unit is considered to be facies equivalent to Uy Group strata in Allakh-Yun section (Kandyk Formation), where the gabbro sills were sampled (Section 1 in Fig. 1). A specimen of the
Mayamkan Formation sandstone
A sandstone sample of ∼2 kg was pulverized to fragments ∼0.25 mm size, washed and dried. After removal of the highly magnetic minerals, bromoform was used for separation of the heavy mineral concentrate, which was subsequently washed in deionized water, dried and separated according to paramagnetic behavior. Approximately 100 grains were selected at random from the least-magnetic, 62–105 μm diameter fraction (maximum zircon grain diameter ∼100 μm). Detrital zircon grains were mounted in a 2.5
Mayamkan Formation sandstone
Of 31 single grain analyses, 27 yield 207Pb/206Pb ages that group between 1500 and 1050 Ma (Fig. 2). The remaining four analyses are 1798±25 Ma, 2074±39 Ma, 2660±14 Ma and 2709±8 Ma. 1σ errors in the 207Pb/206Pb ages generally are 2–4%, and 70% of analyses are less than 5% discordant and therefore considered to approximate the true crystallization age of the zircons. The youngest grain, MM1-27, was analyzed in six locations providing a weighted mean 206Pb/238U age of 1057±28 Ma (2σ). This is
Discussion
The relatively immature composition of the Mayamkan Formation sandstone, including abundant angular lithic fragments, implies a local source for some of the detritus that presently may be hidden by the late Paleozoic–Mesozoic Verkhoyansk Fold Belt or Mesozoic Okhotsk–Chukotka Volcanic Belt (Fig. 1). However, the youngest U–Pb zircon ages from granite and crystalline basement of adjacent areas are about 1700 Ma from the Aldan Province [13], 1740 Ma from the Okhotsk Massif [32]and 1800 Ma from
New reconstruction
Our data partly accord with the reconstruction of Frost et al. [12], based on U–Pb and Sm–Nd studies in the western Aldan Province, in which northern Laurentia is connected to the Olekma and Aldan terranes via the Slave Province and Thelon–Taltson tectonic zone, respectively. However, we propose a continental reconstruction with more significant anticlockwise rotation of Siberia with respect to its modern position and placement of southeast Siberia opposite the Greenland-Caledonides segment of
Conclusions
A significant conclusion of our study is that the lowermost Uy Group can be no younger than 1010 Ma because it is intruded by a 1005±4 Ma (U–Pb baddeleyite) diabase sill. Previous work indicated that the Uy Group and underlying Lakhanda Group are of upper Riphean age (1000–650 Ma; 22, 23). As well, the youngest detrital zircon from the Mayamkan Formation provides a maximum age of ca. 1070±40 Ma for the upper Uy Group in the Uchur–Maya depression.
Our geochronological data offer somewhat
Acknowledgements
Ken Buchan and Richard Ernst from the paleomagnetic laboratory of the GSC are thanked for stimulating discussions, for comments on an early version of the manuscript and help with interpreting paleomagnetic data. The manuscript was improved will the help of thoughtful reviews by Kevin Ansdell, Wouter Bleeker, Charlie Jefferson, Oleg Rosen, Dave Scott, Mikhail Semikhatov and Cees van Staal. We thank Carol Frost and Ron Frost for providing us with a preprint of their paper [12]. We also thank
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