February 2008 LIP of the Month
February 2008 LIP of the Month
Reconnaissance Ar-Ar dating of Proterozoic dolerite dykes and sills in Siberia and the southern Urals: Identification of possible new Large Igneous Provinces (LIPs)
R.E. Ernst (1), J. A. Hanes (2), V.N. Puchkov (3), A.V. Okrugin (4), D.A. Archibald (2)
(1) Ernst Geosciences, 43 Margrave Ave., Ottawa, Canada K1T 3Y2; and Dept. Earth Sciences, Carleton University, Ottawa, Canada K1S 5B6;
Richard.Ernst@ErnstGeosciences.com
(2) Dept of Geological Sciences and Geological Engineering, Queen's University, Kingston, Ontario Canada K7L 3N6
(3) Ufimian Scientific Centre, Institute of Geology, Karl Marx Street 16/2, 450 000 Ufa, Russia
(4) Yakutian Institute of Geosciences, Russian Academy of Sciences, 39 Lenin Street, 677891 Yakutsk, Russia
(extracted from Extended Abstract presented at Moscow Tectonics Conference, Jan 29-Feb 2, 2008; http:/www.ginras.ru)
Herein we report reconnaissance Ar-Ar ages on hand-picked amphibole and biotite separates from dolerites from the Siberian Craton (Anabar and Aldan Shields) and from the southeast side of the East European Craton (Bashkirian Anticlinorium in the southern Urals). Our results define two probable new Large Igneous Provinces (LIPs) of ages ca. 1860 Ma and 1750 Ma, extend the 1500 Ma Kuonamka dyke swarm in the Siberian Craton, and expand the ca. 1395-1350 Ma Mashak Igneous (LIP) event in the East European Craton.
SIBERIAN CRATON, ANABAR SHIELD
Ar-Ar dating confirms that the 1503+/-5 Ma (U-Pb baddeleyite) age for an E-W trending dyke (Ernst et al. 2000) applies to at least 3 other dykes of this trend, confirming that all these dykes belong to the Kuonamka swarm (Okrugin et al. 1990). Additional members of this magmatic event may include Fomich sills in the northern Anabar Shield which have a Sm-Nd age of 1513+/-51 Ma (Veselovsky et al. 2006).
Additionally, one NW-trending dyke of the Eastern Anabar swarm (Okrugin et al. 1990; Ernst et al. 2000) has an Ar-Ar spectrum indicating a 1750 Ma age (see below for possible equivalents in Aldan shield). Also, a NW-trending Chieress dyke that previously gave a U-Pb baddelyite age of 1384+/-2 Ma (Ernst et al. 2000), (not shown) gives an Ar-Ar plateau age of 1337+/-11 Ma, thus providing a positive comparison of U-Pb and Ar-Ar ages.
SIBERIAN CRATON, ALDAN SHIELD
In the Aldan shield there are two dominant dyke-trends, NW and NE, but prior to this study, their ages were only broadly constrained by K-Ar dates between 1650 and 985 Ma (Okrugin et al. 1995). Herein we demonstrate that three NW-trending dykes of the Timptano-Algamaisky swarm have similar Ar-Ar ages with best determined values of 1748+/-5 Ma and 1725+/-12 Ma. These dykes are likely related to the ca. 1740-1700 Ma (1736+/-6, 1721+/-1, 1703+/-18 Ma; U-Pb) Bilyakchan-Ulkan volcanoplutonic belt in the southeastern Siberian Craton, and the 1734+/-4 Ma (U-Pb zircon) Anagar-Kan anorogenic complex in the southwestern Siberian Craton (Gladkochub et al. 2007; Bibikova et al. 2001; Larin et al. 1997), and together all these units define a possible major new ca. 1750-1700 Ma Ma LIP which is widespread in the southern Siberian Craton. As mentioned above, a ca. 1750 Ma dyke of the NW-trending Eastern Anabar swarm from northern Siberia (Anabar Shield) may also belong to this LIP and could suggest that this ca. 1750-1700 Ma LIP event extends throughout the Siberian Craton.
Two NE trending dykes of the Kalaro-Nimnyrsky swarm give overlapping Ar-Ar plateau ages (amphibole and biotite) of ca. 1860-1850 Ma. This swarm can be linked with extensional magmatism in the Akitkan belt of age 1866+/-6 and 1869+/-6 Ma (U-Pb zircon; Neymark et al. 1991, Larin et al. 2003), and the 1866+/-6 Ma (U-Pb zircon) Kengurak-Sergachi gabbro-anorthosite and related massifs located in the Selenga-Stanovoy superterrane in the southern Aldan Shield (Buchko et al. 2006). Thus, there also appears to be a ca. 1870-1860 Ma LIP event in the southern Siberian Craton.
EAST EUROPEAN CRATON (BALTICA): BASHKIRIAN ANTICLINORIUM OF THE SOUTHERN URALS
The Kusa sill which cuts the Satka formation has a Ar-Ar plateau ages (amphibole and biotite) of about 1350 Ma suggesting membership in the Mashak Igneous (LIP) Event which has ages ranging from ca. 1395 to 1350 Ma with the most precise ages being 1368.4+/-6.2 (Ronkin et al. 2005), 1385.3+/-1.4 Ma (Ernst et al. 2006) and 1366±12 Ma (Puchkov et al. 2007). Three additional dolerite intrusions that probably belong to the Mashak Igneous Event have disturbed Ar-Ar spectra with ca. 1000 Ma ages possibly reflecting Pre-Zilmerdak uplift and erosion causing unsealing of the sedimentary basin with relatively hot mineralised waters. The Kusa and related sills have a composition distinct from the main group of Mashak igneous rocks (Ernst et al. 2006), suggesting at least two magma groups (with unique geochemical histories) within the Mashak Igneous Event.
A dyke in the Inser region of the Southern Urals has an imprecise Ar-Ar age of ca. 400 Ma, suggesting it possibly belongs to a widespread Devonian (ca. 360 Ma) LIP in the East European Craton, which includes volcanic rocks in the Russian platform, in the western slope of the Urals, and along the Pripyat-Dnieper-Donets failed rift, and also mainly alkaline magmatism in the Kola Peninsula (event #45 in Ernst and Buchan 2001).
Figure 1: Generalized distribution of ages reported hereine and U-Pb ages from Ernst et al. (2000). Base map showing generalzied geology and dyke swrams after Ernst et al., (1996).
Figure 2: Ar-Ar spectra from this study.
Table 1:
ANABAR SHIELD ca. 1500 Ma Kuonamka Dyke swarm (E-W trend)
EQ94-04 has U-Pb baddeleyite age of 1503±5 Ma (Ernst et al., 2000)
| Map Label | Sample Number | Lab Number | Location | Mineral | Integrated Date (Ma) | Plateau Date (Ma) |
|---|---|---|---|---|---|---|
| 1 | EQ94-04 | JAH-424 | 69°45.8'N, 110°50.6'E | Amphibole | 1501 ± 11 | 1497 ± 11 |
| 1 | EQ94-04 | JAH-353 | “ | Amphibole | 1497 ± 11 | 1480 ± 13 |
| 1 | EQ94-04 | JAH-353/424 | “ | Amphibole | 1499 ± 09 | 1487 ± 09 |
| 2 | EQ94-13 | JAH-357 | 69°54.4'N, 112°35.9'E | Amph. Conc. | 1474 ± 13 | 1474 ± 13 (*) |
| 3 | EQ94-14 | JAH-354 | 69°53.8'N, 112°48.0'E | Amph. Conc. | 1427 ± 08 | 1450 ± 12 (*) |
(*) star = plateau segment, considered a minimum age
ANABAR SHIELD ca. 1750 Ma dyke (NW-trend)
| Map Label | Sample Number | Lab Number | Location | Mineral | Integrated Date (Ma) | Plateau Date (Ma) |
|---|---|---|---|---|---|---|
| 4 | EQ94-11 | JAH-412 | 69°59.2'N, 111°50.7'E | Biotite | 1732 ± 23 | 1754 ± 27 |
| 4 | EQ94-11 | JAH-412/432 | “ | Biotite | 1734± 18 | 1755± 22 |
ALDAN SHIELD ca. 1750 Ma Timptano-Algamaisky Dyke Swarm (NW-trend)
| Map Label | Sample Number | Lab Number | Location | Mineral | Integrated Date (Ma) | Plateau Date (Ma) |
|---|---|---|---|---|---|---|
| 5 | EQ98-0710 | JAH-200 | 57.241°N, 126.137°E | Amphibole | 1743 ± 06 | 1749 ± 07 |
| 5 | EQ98-0710 | JAH-200/339 | “ | Amphibole | 1747 ± 05 | 1748 ± 05 |
| 5 | EQ98-0710 | JAH-214 | “ | Amphibole | 1752 ± 06 | 1742 ± 06 |
| 5 | EQ98-0710 | JAH-252 | “ | Biotite | 1715 ± 06 | 1767 ± 06 |
| 5 | EQ98-0710 | JAH-203 | “ | Biotite | 1727 ± 06 | 1766 ± 06 |
| 6 | EQ98-1502 | JAH-349 | “ | Amphibole | 1696 ± 09 | 1725 ± 12 |
| 6 | EQ98-1502 | JAH-411 | ~57.7°N, ~125.4°E | Biotite | 1695 ± 23 | 1720 ± 35 (*) |
| 7 | EQ98-1311 | JAH-347 | 57.928°N, 125.488°E | Amphibole | 1614 ± 08 | 1697 ± 14 (*) |
(*) star = plateau segment, considered a minimum age
ALDAN SHIELD ca. 1870 Ma Kalaro-Nimnyrsky Dyke Swarm (NE-trend)
| Map Label | Sample Number | Lab Number | Location | Mineral | Integrated Date (Ma) | Plateau Date (Ma) |
|---|---|---|---|---|---|---|
| 8 | EQ98-0406 | JAH-346_1 | 57.052°N, 125.803°E | Amphibole | 1832 ± 19 | 1869 ± 22 |
| 8 | EQ98-0406 | JAH-346_2 | “ | Amphibole | 1818 ± 08 | 1866 ± 10 |
| 9 | EQ98-0207 | JAH-377 | 56.988°N, 125.484°E | Biotite | 1835 ± 10 | 1849 ± 11 |
| 9 | EQ98-0207 | JAH-375 | “ | Biotite | 1849 ± 21 | 1865 ± 22 |
BASHKIRIAN ANTICLINORIUM, S. URALS ca. 1350 Ma Kusa Sill
| Map Label | Sample Number | Lab Number | Location | Mineral | Integrated Date (Ma) | Plateau Date (Ma) |
|---|---|---|---|---|---|---|
| 10 | EQ03-1007 | JAH-376 | 55.3218°N, 059.4437°E | Biotite | 1338 ± 07 | 1348 ± 09 |
| 10 | EQ03-1007 | JAH-367 | “ | Biotite | 1330 ± 06 | 1338 ± 07 |
BASHKIRIAN ANTICLINORIUM, S. URALS overprinted sill of Mashak Igneous Event
| Map Label | Sample Number | Lab Number | Location | Mineral | Integrated Date (Ma) | Plateau Date (Ma) |
|---|---|---|---|---|---|---|
| 11 | EQ03-0302 | JAH-355 | 55.5267°N, 059.7947°E | Amph. Conc. | 1565 ± 12 | 1143 ± 16 (*) |
(*) star = plateau segment, considered a minimum age
BASHKIRIAN ANTICLINORIUM, S. URALS overprinted dyke (NE-trend) of Mashak Igneous Event
(U-Pb baddeleyite age is 1385.3±1.4 Ma: Ernst et al., 2006)
| Map Label | Sample Number | Lab Number | Location | Mineral | Integrated Date (Ma) | Plateau Date (Ma) |
|---|---|---|---|---|---|---|
| 12 | EQ03-0701 | JAH-352 | 54.9190°N, 058.7873°E | Amphibole | 969 ± 43 | 891 ± 40 |
BASHKIRIAN ANTICLINORIUM, S. URALS ca. 400 Ma dyke (NNE-trend) in the Inser Region
| Map Label | Sample Number | Lab Number | Location | Mineral | Integrated Date (Ma) | Plateau Date (Ma) |
|---|---|---|---|---|---|---|
| 13 | EQ03-2301 | JAH-350 | 54.2182°N, 057.5902°E | Amphibole | 402 ± 17 | 405 ± 14 |
| 13 | EQ03-2301 | JAH-416 | “ | Biotite | 400 ± 06 | 395 ± 06 |
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