Constraining the Magmatic Plumbing System in a Zoned Continental Flood Basalt Province

Transcription

1 G 3 Geochemistry, Geophysics, Geosystems Supporting Information for Constraining the Magmatic Plumbing System in a Zoned Continental Flood Basalt Province Tyrone O. Rooney 1, Susan R. Krans 1, Daniel Mège 2,3, Nicolas Arnaud 4, Tesfaye Korme 5, John Kappelman 6, Gezahegn Yirgu 5 1 Department of Earth and Env. Sciences, Michigan State University, East Lansing, Michigan 48824, USA 2 Space Research Centre, Polish Academy of Sciences, Bartycka 18A, Warsaw, Poland 3 Laboratoire de Planétologie et Géodynamique de Nantes, Université de Nantes, CNRS UMR6112, Nantes, France. 4 Géosciences Montpellier, Université de Montpellier 2, CNRS UMR5243, 34095, Montpellier, France. 5 School of Earth Sciences, Addis Ababa University, Addis Ababa, Ethiopia. 6 Department of Anthropology, University of Texas at Austin, Austin, Texas 78712, USA. Contents of this file Protocols for measuring ages, detailed age information, and an integrated structural figure which combined the manuscript figures 2a-e Additional Supporting Information (Files uploaded separately) Introduction Captions for Datasets : Supplementary Material 2, 3, 4, 5 This supplement contains: (1) the protocols used in the 40 Ar/ 39 Ar dating of the West Tana dikes. (2) Obtained ages and the spectrums of the dating process including argon release curves and isochrons. (3) an integrated structural map of the study area.

2 40 Ar/ 39 Ar ages of rocks in the West Tana area of Ethiopia 1. Protocol Whole rock mesostasis Ar Ar ages were obtained at the geochronology laboratory of Geosciences Montpellier (Université de Montpellier, France) following a protocol similar to that described in Garcia et al. (2003). Analyses were performed on the µm size fraction after separation with Frantz magnetic separator and hand picking under a binocular microscope. Except for samples MHK and KFT, samples was irradiated in the Ford Michigan reactor, University of Michigan, in the L67 position for 60 h during july The MHK and KFT samples were irradiated in MacMaster reactor, Ontario, during January 2005 in 5C position for 40h. Irradiation interference on K, Ca and Cl were corrected by irradiation of KCl and CaF 2 pure salts. The J factor was estimated by the use of duplicates of the Fish Canyon sanidine standard with an age of ± 0.16 Ma (Renne et al., 1998), with reproductive values within 0.6%. The samples were loaded in aluminum packets into a double vacuum Staudacher type furnace, the temperature of which is calibrated by means of a thermocouple, and step heated. The gas was purified by the means of cold traps with liquid air and Al Zr getters during 5 mn. Once cleaned, the gas was introduced into a VG3600 mass spectrometer, and 1 min was allowed for equilibration before analysis was done statically. Signals were measured by the mean of a Faraday cup with a resistor of 1011 ohm for 40 Ar and 39 Ar while 39 Ar, 38 Ar, 37 Ar and 36 Ar were analyzed with a photomultiplier after interaction on a Daly plate. Gain between both collectors was estimated by duplicate analysis of 39 Ar on both during each analysis, and also by statistical analysis over a period of several years. This gain was, on average, 54 and is known at better than 1.5%. This error is included in the age calculation, along with analytical errors on each signal and errors on the blank values. Detailed analytical results are available from the authors upon request. The age plateau given are weighted mean plateaus (Fleck et al., 1977), which takes the error on the J factor into account, but plateau criterion were rarely achieved, and sometimes simple mean age is given. The isochron ages are obtained in an inverse isochron diagram of 36 Ar/ 40 Ar versus 39 Ar/ 40 Ar (Roddick et al., 1980), which allows homogeneous excess components to be individualized in many occasions. Errors on age and intercept age include individual errors on each point and linear regression by York s method (York, 1969). The goodness of the data relative to individual errors is measured by Mean Square Weighted Deviation (MSWD). Classical furnace step heating was conducted and yielded age spectra from which plateau and isochron ages were calculated and are shown side by side to assess potential excess argon problems. If the inverse isochron age is close to the plateau age and 40 Ar/ 36 Ar is not significantly different from present day 40 Ar/ 36 Ar atmospheric ratio (295.5), we consider that the plateau age is reliable. When this is not the case, we prefer to rely on the inverse isochron age if this one is well determined. Garcia, S., Arnaud N., Angelier J., Bergerat, F., and Homberg, C., Rift jump in northern Iceland since 10 Ma from 40 Ar/ 39 Ar geochronology. Earth Planet. Sci. Lett. 214, Fleck R.J., Sutter J.F., and Elliot D.H., Interpretation of discordant 40 Ar/ 39 Ar age spectra of Mesozoic tholeiites from Antarctica. Geochim. Cosmochim. Acta 41, Renne P.R., Swisher C.C., Deino A. L., Karner D. B., Owens T. L., and DePaolo D. J., Intercalibration of standards, absolute ages and uncertainties in 40Ar/ 39Ar dating. Chemical Geology 145, Roddick J.C., R.A. Cliff, and D.C. Rex, The evolution of excess argon in alpine biotites a 40Ar/ 39Ar analysis. Earth Planet. Sci. Lett. 48, York D., Least square fitting o a straight line with correlated errors. Earth Planet. Sc. Lett. 5,

3 2. Obtained ages All the samples, except BEL 02, are Cenozoic dikes. BEL 02 is the neo Proterozoc host rock in the southernmost part of the study area (see Table 1 for location). ANG 06 The age spectrum is slightly disturbed with major steps around 30Ma, which is confirmed by the inverse isochron, suggesting no excess argon with an acceptable MSWD. The preferred age is Ma and is suggested to date the emplacement of this dike.

4 BEL 02 Tulu Dimtu Ophiolite Belt [1] (host rock [2]) Although degassing of this sample happened rather suddenly, the ages are quite clustered and nicely aligned on the inverse isochron with a good MSWD showing excess argon, and suggesting that the alignment is not controlled only by low radiogenic steps. The age of 562 Ma is not very well defined, with an error of at last 8Ma, but represents a probable estimate of the last cooling of that unit, possibly close to emplacement age. [1] Vail, J.R. (1985) Pan African (late Precambrian) tectonic terrains and the reconstruction of the Arabian Nubian Shield. Geology 13, [2] Mège, D. and Korme, T. (2004) Dyke swarms in the Ethiopian Large Igneous Province: not only a matter of stress. J. Volc. Geotherm. Res., 132/4, , doi: /s (03)

5 BEL 03a This sample shows a nice plateau at 31.4 Ma, confirmed by the inverse isochron in which the absence of excess argon strongly suggests that it dates dike emplacement.

6 BEL 06 This sample is very scattered with an inverse isochron mainly defined by steps with the lowest content in radiogenic argon. This could come from some alteration of the groundmass, and suggests possible argon loss. The inverse isochron age of 29.4 Ma is probably a minimum for the emplacement age.

7 BEL 10 This sample is quite scattered and degassed abruptly. Whole rock age of ca 29.6Ma seems plausible given the geologic context, but is not strongly supported.

8 BEL 13 Two replicates of the sample were analysed, showing lightly or strongly concave spectra that are effectively corrected by the inverse isochron approach. The two inverse isochron ages of Ma and are indistinguishable, and results in a good MSWD Ma is taken as the reference age.

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10 BEL 16 This sample shows decreasing ages for which only steps associated with the highest K/Ca ratio seems meaningful at ca 30 Ma. The inverse isochron is badly defined. An age of ca 30 Ma seems plausible given the geologic context.

11 BEL 17a This sample plateau has a humped back shape with a total fusion age of ca 30 Ma, partially confirmed by an isochron of poor quality. An age of ca 30 Ma seems plausible given the geologic context.

12 BEL 21b The last steps for this sample offers a small plateau confirmed by the inverse isochron. The degassing pattern and grouping of the points in the isochron suggest that the age of is probably a maximum, though close to the real age.

13 KFT 01 This sample is obviously dominated by a strong component of argon in excess, which is sometimes found in dikes intruding a very old basement. The age of 24 Ma probably represents a maximum. The isochron evidences excess argon but is too badly resolved to be useful statistically.

14 MKH 01 Sample MKH 01 shows a credible pseudo plateau at Ma, which is taken as close to the real dike emplacement age. The inverse isochron is badly resolved.

15 SHA 04 Sample analysis suggests slight excess argon, with a maximum age of ca 28 Ma. Excess seems satisfactorily resolved by the inverse isochron, with an acceptable age of ca 27 Ma, but with a large error.

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17 3. Integrated structural map of the study area Structural map of the study area, with location of the sampling sites. This figure puts together figures 2a, 2b, 2c, 2d, and 2e. The tectonic structures are classified in three categories: dikes, unspecified structures, which may include dikes and other fractures (field and satellite imagery usually reveals linear features parallel to identified dikes, usually underlined by stream beds and/or vegetation lines), and normal faults (when fractures have identifiable offsets). The 1500 m elevation contour line marks the elevation corresponding to a change in Oligocene dike chemical composition discussed in the text. This figure illustrates that locally, tectonic structures associated with two different dike swarms are parallel. Since not all the swarms are well dated, a large uncertainty exists as to the origin of such structures. Uncertainty is especially large in four areas: (1) west of the Angareb ring dike complex, circum-angareb dikes are parallel to the Doka Dike Swarm; (2) southwest of the Angareb complex, the Tana-Gondar structures are parallel to the circum-angareb structures, and may have reactivated some of them; (3) west of Aykel, structures radiating from the Angareb complex become parallel to some circum-tana structures, and might have been similarly reactivated; (4) east of the Dangur ring dike complex, radiating structures may exist and be parallel to the God Serpent Dike Swarm. In addition to field work information, the datasets used for mapping are ESA/Sentinel-2 images, particularly as RGB color composites of bands or (resolution 10 m) on which principal component analysis was conducted, and A (resolution 20 m), complemented by sub-meter scale Digital Globe satellite images available on Google Maps and Bing Maps and the geologic map of the Angareb ring dike complex by Hahn et al. (1977). The stratigraphic boundaries were determined from Sentinel imagery with the help of the 1/2,000,000 geological maps of Sudan (Ministry of Geology and Mines, Sudan, 1981) and Ethiopia (Second Edition, Geological Survey of Ethiopia, 1996).

18 37 00 SU DA N ETH IOP IA areb Angareb Ring Dike Complex a Go A Ang 2800 ng 32 h T Wad Arud Gallabat Metema 3000 R1a had Gonder ,29 Dib aba Aykel Kwara Plateau T A B Galegu 4 Din der ,10 16a 9Aym Lake Tana T Dengel Ber Shawira B Mount Baghilti Bahr Dar 38 0 Blue 2600 Mount Dangur Nile Mount Belaya Pawi 00 Choke Mountain 44 STRATIGRAPHY/LITHOLOGY Quaternary alluvial deposits Quaternary lava flows Miocene stratovolcano Be les T B DIKE SWARMS AND RELATED STRUCTURES TANA PLATEAU MARGIN TECTONICS GONDAR GRABEN Tana-Gondar Angareb Ayma Tana Serpent Cenozoic lava flows Mesozoic sediments MORPHOLOGY Plateau edge N Contour (spacing 200 m) E-W Dinder Precambrian fabric Dangur Contour 1500 m Main river Doka Precambrian Cenozoic volcanic edifice Cenozoic collapse structure Cenozoic plug Intrusive, ring complex, previously documented Intrusive, ring complex, previously undocumented Dike Dike or other fracture Normal fault Dense tension fracture array (true spacing ca. 100 m) Foliation km Sample Number ID on map A A A A A 19 SHA04A 20 D11 21 D12 22 D13 23 KFT KFT MTM-2 30 MTM-5 31 ANG ANG ANG BEL-03A 35 BEL BEL BEL BEL BEL-17a 40 BEL BEL-21B 42 MKH BEL-02 44

19 Data Set Supplementary Material 2: Table of major element, trace element, and metadata on dikes used in this study. Location information is provided at Latitude and longitude in WGS84 datum. IGSN is the international geologic serial number. Orientation is in degrees. Additional comments are made as to the argon dates presented. Analytical data from this table are derived from the methods noted in the manuscript. Data Set Supplementary Material 3: Table providing all values used to construct isochrons for 40 Ar/ 39 Ar geochronology. Data Set Supplementary Material 4: Detailed table of ICPMS output per day. Standards, gas blanks, detection limits, calibration bounds, and replicates are presented. Data Set Supplementary Material 5: Detailed table of XRF standard information by day and full procedural replicate information (new disk made from powder and analyzed by XRF).