Origin of Middle-Upper Cambrian dolomites in eastern Laurentia: A case study from Belle Isle strait, western Newfoundland

摘要

The Middle and Upper Cambrian Port au Port Group of western Newfoundland consists mainly of mixed carbonates and siliciclastics and is composed of the March Point, Petit Jardin, and Berry Head formations. Samples were collected from the core NF-02 that spans the uppermost Hawke Bay (40 m), the March Point (-83 m thick) and the lowermost Petit Jardin (similar to 47 m). Petrographic examination of carbonates of the March Point and Petit Jardin carbonates reveals three generations of dolomite: (i) micritic to near-micritic (D1, 4 mu m-35 mu m) fabricretentive dolomite, (ii) coarse crystalline sucrosic (D2, 60 mu m-150 mu m) fabric-destructive dolomite (sub-to euhedral zoned rhombs with clear rim and cloudy core), and (iii) coarse anhedral (D3, >0.5 mm) fracture-filling saddle dolomite. The microthermometric measurements of primary two-phased fluid inclusions retained in D2, and D3 samples yielded homogenization temperatures (T-h) showing a general increase with burial. The mean values of T-h and salinity are 89.1 +/- 6.4 degrees C vs 21.2 +/- 2.2 eq. wt% NaCl, and 110 +/- 8.2 degrees vs 21.1 +/- 4.7 eq. wt% NaCl, respectively. The investigated dolomites show a decrease of Sr from D1 (102 +/- 49 ppm) to D3 (47 +/- 15 ppm) which reflects the influence of progressive burial. The mean delta O-18 values decrease from D1 (-6.3 +/- 1.1 parts per thousand VPDB), to D3 (-9.7 +/- 1.7 parts per thousand VPDB) where D2 (-7.6 +/- 1.2 parts per thousand VPDB) is in the middle. The estimated delta O-18 values of the D1 parent fluid (-5 to-10 parts per thousand VSMOW) are bracketed by those suggested for the Middle Cambrian meteoric and seawaters. The fluid-inclusion gas analyses provided CO2/CH4 and N-2/Ar ratios coherent with the conclusions from the petrographic examination and geochemical compositions.