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AAPG Bulletin

Abstract

AAPG Bulletin, V. 94, No. 4 (April 2010), P. 513531.

Copyright copy2010. The American Association of Petroleum Geologists. All rights reserved.

DOI:10.1306/09080909083

Massive dolomitization of a pinnacle reef in the Lower Devonian West Point Formation (Gaspe Peninsula, Quebec): An extreme case of hydrothermal dolomitization through fault-focused circulation of magmatic fluids

D. Lavoie,1 G. Chi,2 M. Urbatsch,3 W. J. Davis4

1Geological Survey of Canada-Quebec, 490 de la Couronne, Quebec City, QC G1K 9A9, Canada; [email protected]
2Department of Geology, University of Regina, Regina, SK S4S 0A2, Canada; [email protected]
3Department of Geology, University of Regina, Regina, SK S4S 0A2, Canada; [email protected]
4Geological Survey of Canada-Ottawa, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada; [email protected]

ABSTRACT

Devonian pinnacle reefs of the West Point Formation in Gaspe Peninsula (eastern Canada) were built on paleotectonic highs in a foreland basin. Of the nine pinnacles known in outcrop, one is dolomitized and occurs at the junction of two Acadian faults.

The petrography of the dolomitized facies has revealed the presence of three dolomite phases and one late calcite cement. A first dolomite phase of small crystals is volumetrically minor; the following dolomite phase dominates and consists of centimeter-size replacive saddle dolomite crystals that contain fluid inclusions with homogenization temperatures ranging between 301 and 382degC. The third dolomite consists of millimeter- to centimeter-size saddle dolomite crystals that fill late fractures; this phase is characterized by lower temperature fluid inclusions (159–171degC). A lower temperature calcite phase (107–123degC) fills some voids. Fluid inclusions are saline (8.7 to 13.3 wt.% NaClequiv). The dolomite and calcite phases are characterized by very negative delta18OVPDB (Vienna Peedee belemnite) values (between minus19 and minus14permil) and negative delta13CVPDB values (between minus8 and minus1permil). The replacive saddle dolomite phase originated from a fluid with very positive delta18OVSMOW (Vienna standard mean ocean water) values (+8permil), whereas the following dolomite and calcite precipitated from fluids with lower delta18OVSMOW values (+3.4 and +4.5permil). We propose that fault-focused circulation of magmatic fluids is responsible for the very high-temperature massive dolomite replacement of the calcite host, and high-temperature burial fluids later used fractures to circulate in the dolomitized host to precipitate late dolomite and calcite. Regionally, hydrocarbon migration is recognized at the time of late calcite cementation.

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