Computer Science
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30s.559o&link_type=abstract
Meteoritics, vol. 30, no. 5, page 559
Computer Science
2
Dating, Geochemistry, Impact Melting, Isotopes, Oxygen, Rare Earth Elements
Scientific paper
The Sudbury impact structure [1] contains an unique crater-centered system of Offset Dikes. Their origin and relation to the coherent impact melt sheet ("Sudbury Igneous Complex" SIC) is still a matter of debate. Previous petrographical studies have demonstrated that the dioritic Foy Offset Dike is connected with North Range norites of the SIC [2]. A close genetic association is further strengthened by U-Pb ages of 1.850 Ga for zircons and baddeleyites from both, the Foy Offset and North Range norites, which are identical within errors [3, 4]. Zircons from the local country rocks (Levack Gneiss), however, display distinct shock features and a U-Pb age of ~ 2.7 Ga, being in accordance with T^(Nd)(sub)(DM) of 2.71 Ga for these rocks. Age data and zircon morphology substantiate that the Offset Dike material originated by batch melting at T >= 1700 degrees C, and set tight temporal limits for emplacement. To model relative contributions of the different precursor rock components to the impact melt now forming the Offset Dikes, whole rock samples from Foy Offset, Levack country rocks, and the Main Mass of the SIC were analyzed for REE, Sr, Nd, and oxygen isotope ratios. SIC samples were collected at several localities to cover the whole geochemical spectrum of the impact melt sheet. Geochemical characteristics of the central Foy Offset match, in general, those of the North Range norites with epsilon(sub)(Nd)^(T=1.85 Ga) of -9.5, and epsilon(sub)(Sr)^(T=1.85 Ga) ranging from +20 to +30, T^(Nd)(sub)(DM) around 2.71 Ga, and high REE contents with steep distribution patterns ([La/Yb](sub)(cn) = 9 to 22) and minor Eu anomalies. REE and oxygen isotope data indicate that the marginal facies of the Foy Offset contains an enhanced but variable contribution from local country rocks: At the centre of the Foy Offset, delta^(18)O is + 6.8 per mil, similar to values for the the North Range norites [5]; whereas, the marginal facies has delta^(18)O of + 7.9 per mil, close to values for Levack material. The data show that Archean gneisses are the main precursor rocks for the Offset Dike impact melt, contributions of partial melts from sub-lithospheric mantle could not be detected [6]. Uptake of country rock material could be related to the slow cooling of the impact melt in the Foy Offset, which is indicated by the internal Rb-Sr isochron age of 1814 +/- 18 (2 sigma) Ma. Mixing calculations will be performed to model the geochemical characteristics in more detail. Together with pseudotachylites, dikes with unusual geochemical properties such as the Offsets could provide the only indications for the former presence of an impact structure in deeply eroded shield areas. Acknowledgments: We thank B.O. Dressler (OGS) and R.A.F. Grieve (Can. Geol. Surv.) for providing samples. References: [1] Grieve R. A. F. et al. (1991) JGR, 96, 22753-22764. [2] Grant R. W.and Bite A. (1984) Ontario Geol. Surv., Spec. Vol. 1, 275-300. [3] Ostermann M. et.al. (1994) Meteoritics, 29, 513. [4] Krogh T. E. et al. (1984) Ontario Geol. Surv., Spec. Vol. 1, 431-446. [5] Ding T. P. and Schwarcz H. P. (1984) Can. J. Earth Sci., 21, 305-318. [6] Deutsch A. (1994) GSA Spec. Pap. 293, 289.
Agrinier Pierre
Deutsch Alex
Ostermann Matthias
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