Boulders from the basement, the trace of an ancient crust?
Published:January 01, 1980
Z. NIKIC, H. BAADSGAARD, R.E. FOLINSBEE, J. KRUPICKA, A. PAYNE LEECH, A. SASAKI, 1980. "Boulders from the basement, the trace of an ancient crust?", Selected Studies of Archean Gneisses and Lower Proterozoic Rocks, Southern Canadian Shield, G. B. Morey, Gilbert N. Hanson
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A tabular diatreme encountered on the 1,450-, 1,800-, and 3,500-ft (442-, 549-, and 1,067-m) levels of the Con gold mine cuts steeply dipping Yellowknife pillow basalts almost at right angles and lies parallel to the gold-bearing Campbell and Con shear zones. Rounded gray gneissic tonalite boulders 6 in. to 2 ft (15 to 61 cm) in diameter predominate and are encased in a dark, pyritized, carbonatized, highly altered hornblende-biotite schist matrix, rich in apatite, which appears to have been a fluidized phase at the time of diatreme emplacement. Sulfur-isotope measurements deviate little from the meteoritic standard; they range between −9.5‰ 34S from pyrite in a tonalite boulder to + 7.2‰ 34S from a pyrite-free anhydrite portion of the vein material. Zircons from the tonalite are highly metamict, as they are rich in uranium and lead. Zircons from one boulder yield the oldest 207Pb-206Pb date of 3,210 m.y. B.P.; three fractions from the same boulder yield younger dates of 3,030 to 3,040 m.y. B.P.; a denser fourth fraction, possibly annealed, gives a date of 2,900 m.y. B.P. Zircons from a quartz monzonite boulder in the diatreme yield a typical Kenoran date of 2,570 m.y. B.P., and this rock may be derived from the hood zone of the nearby diapiric Stock Lake quartz monzonite that was intruded closely following the extrusion of silicic phases of the Yellowknife volcanics 2,600 m.y. ago. The diatreme is believed to have transported, from a deep crustal source, fragments of a previously unrecognized basement gneiss of early Archean age into the 11,000-m-thick overlying Yellowknife volcanic pile.