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

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