Abstract
On the island of Seriphos, Greece, the shallow intrusion of a
granodiorite pluton into a series of previously regionally
metamorphosed gneisses, marbles and marble-bearing schists produced a
contact metamorphic aureole and extensive deposits of Ca-Fe-Mg skarns
and Fe-ores. Structural and petrological investigations show that the
contact metamorphic aureole was formed as a result of the
... read more
(dry) thermal
heating accompanying the emplacement and gradual crystallization of the
magma. At or just after the time of its final solidification the
granodiorite was affected by a major stage of (auto-) brecciation that
caused an intense fracturing of the plutonic body as well as of the
surrounding country rocks. As a consequence of the associated increase
in rock permeabilities intense metasomatic-hydrothermal activities
along the newly formed transport channels caused a leaching of the
granodiorite of its mafic components (Fe, Mg, Mn, etc.) and a
deposition of the same (Fe, Mg, Mn-) components in the skarns and ore
formations in the country rocks.
Thermodynamic analyses of the observed mineral parageneses in the
skarn and ore deposits show that at Seriphos metasomatism continuously
took place under gradually decreasing temperatures and under constant
maintenance of local equilibrium between Fe-saturated, hydrothermal
solutions and the solids locally present along the transport channels.
Fluid inclusion studies indicate that the metasomatic 'juices' were
saline NaCI-KCI-CaCI 2-MgCI 2-(FeCI 2?)- brines with a dominantly magmatic
origin. The fluid inclusion studies also point out that, as
metasomatism progressed and temperatures dropped, fluid pressures
decreased in proportion. Oxygen isotope ratios of quartzes and other
mineral phases from the metasomatic formations indicate that the total
amount of fluid in the hydrothermal fluid flow system must have been
small relative to the total amount of solid phases that equilibrated
with the metasomatic solutions. Apparently a limited amount of
dominantly magma-derived, saline, hydrous solutions percolated along
the cracks and fractures in the permeable solid rock system,
transporting both heat and matter from the cooling, plutonic heat and fluid source into the adjacent country rocks. Quantitative comparison
of the mass exchanges between the granodiorite pluton and the
surrounding country rocks confirms that, as a whole, the metasomatic
system at Seriphos must have been a largely closed system, and that
there were no sources of material outside the granodiorite and its
immediate environment.
On the basis of the presented evidence a mathematical model is
developed, describing the coupled transfer of heat and mass during the
thermal evolution of the intrusive system, both in the magmatic,
intrusive stages and in the post-magmatic, hydrothermal cooling stages
of the intrusive event. The magmatic stages are modelled by assuming
convection in the magma melt and the conductive transfer of heat into
the surrounding contact aureole. The post-magmatic, hydrothermal stages
of the intrusion are encountered by simulating the advective outflow of
metasomatic solutions from the HT-HP plutonic heat and fluid source
into its LT-LP environment. Although the simplified model cannot
describe in detail the complex interactions of all the processes
accompanying the intrusion of a large body of magma into the upper
levels of the earth's crust, the model results, in general, are well in
agreement with the field observations at Seriphos. The total amount of
Fe, for instance, that is modelled to precipitate in the contact
metasomatic skarn and ore deposits is in good accordance with the field
estimations
show less