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

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 382 degrees C. 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 (159-171 degrees C). A lower temperature calcite phase (107-123 degrees C) fills some voids. Fluid inclusions are saline (8.7 to 13.3 wt.% NaClequiv). The dolomite and calcite phases are characterized by very negative delta O-18(VPDB) (Vienna Peedee belemnite) values (between -19 and -14 parts per thousand) and negative delta C-13(VPDB) values (between -8 and -1 parts per thousand). The replacive saddle dolomite phase originated from a fluid with very positive delta O-18(VSMOW) (Vienna standard mean ocean water) values (+8 parts per thousand), whereas the following dolomite and calcite precipitated from fluids with lower delta O-18(VSMOW) values (+3.4 and +4.5 parts per thousand). 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.