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Abstract
Journal of Sedimentary Research, Section
A: Sedimentary Petrology and Processes
Vol. 65A (1995)No.
3. (July), Pages 541-546
Carbonate Botryoids in Lower Devonian Amygdaloidal Basalts: Evidence for
Precipitation of High-Magnesium Calcite from Heated and Volcanic CO2-Buffered
Marine Waters
Denis Lavoie
ABSTRACT
Amygdaloidal basalts are found in the Lower Devonian Upper Gaspe Limestones
in the Gaspe Peninsula. These basalts were extruded on a distal outer shelf,
as evidenced by field relationships, embedded fragments of metazoans, and
high vacuole content. The lava flows were fractured and brecciated; the
network of fractures allowed connections of some vesicles with seawater.
Vesicles in the nonfractured basalt are filled by silicates, whereas those
opened to seawater are filled by carbonates. The latter comprise, in stratigraphic
order, geopetal micrite, botryoidal calcite, and coarse anhedral calcite.
Petrographic attributes, including microdolomite inclusions and trace-element
distribution in the botryoids, indicate former high-magnesium calcite mineralogy.
The d18OPDB (-9.2
to -10.0
and d13CPBD (-3.0
to -5.3)
ratios support botryoid precipitation from heated marine waters that were
contaminated by magmatic CO2 volatiles. High radiogenic Sr values
originated from compaction waters expelled from the host succession. Submarine
high-magnesium calcite agrees with the aragonite-inhibiting stage for the
Early Devonian. The botryoid fabric should be the norm for early calcite
in such an odd diagenetic setting.
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