Mathematics – Logic
Scientific paper
Sep 1988
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988metic..23..197m&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 23, Sept. 1988, p. 197-207. Research supported by DFG, Geological Survey of Canada, and Depar
Mathematics
Logic
27
Impact Melts, Metamorphism (Geology), Meteorite Craters, Sedimentary Rocks, Breccia, Canada, Crystallinity, Gneiss, Lithology, Stratigraphy
Scientific paper
About 100 cobble-sized samples collected from the surface of the central polymict breccia formation of Haughton impact crater, Canada, have been studied microscopically and chemically. Breccia clasts derived from the 1700 m deep Precambian basement consist of 13 rock types which can be grouped into sillimanite- and garnet-bearing gneiss; alkali feldspar-rich aplitic or biotite-hornblende-bearing gneiss; biotite and hornblende gneiss; apatite-rich biotite and biotite-hornblende gneiss; calcite-diopside gneiss; amphibolite; tonalitic orthogneiss; and basalts. The range of chemical compositions of these rocks is wide: e.g., SiO2 ranges from wt.%; Al2O3 from 7-20 wt.%; CaO from 0.01-25 wt.%; or P2O5 from < 0.01-5 wt.%. Nearly all samples of crystalline rocks are shock metamorphosed up to about 60 GPa. Most conspicuous is the absence of whole-rock melts and the very rare occurrence of unshocked rocks. The 45 samples examined can be classified into the following shock stages: stage O (< 5 GPa): 4.5%, stage Ia ( GPa): 9.0%, stage Ib ( GPa): 33%, stage II ( GPa): 29%, stage 111(45-55 GPa): 18%, stage III-IV ( GPa): 6.5%. Among Paleozoic sedimentary rock clasts higher degrees of shock than within crystalline rocks were observed such as highly vesiculated, whole-rock melts of sandstones and shales. Within the northern and eastern sectors of the allochthonous breccia no distinct radial variation of the cobble-sized lithic clasts regarding abundance, rock type, and degree of shock was observed, with the exception that clasts of shock-melted sedimentary rocks and of highly shocked basement rocks (stage III-IV) are strongly concentrated near the center of the crater. Based on our field and laboratory investigations we conclude that vaporization and melting due to the Haughton impact affected the lower section of the sedimentary strata from about 900 to 1700 m depth (Eleanor River limestones and dolomites, Lower Ordovician and Cambrian limestones, dolomites, shales, and sandstones). The 60-GPa shock pressure isobar reached only the uppermost basement rocks so that whole rock melting of the crystalline rocks was not possible.
Metzler A.
Ostertag Rolf
Redeker H.-J.
Stoeffler Dieter
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