Genesis of mineralized cavities (Miaroles) in granitic pegmatites and granites |
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| Authors: | I S Peretyazhko |
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| Institution: | (1) Geology Department, Central Michigan University, 314 Brooks Hall, Mount Pleasant, MI 48859, USA;(2) Present address: Wolverine Gas and Oil Corporation, 1 Riverfront Plaza, 55 Campau NW, Grand Rapids, MI 49503, USA |
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| Abstract: | Analysis the development of large fluid segregations in a flux of small fluid bubbles during the degassing of granitic (pegmatitic)
melts indicates that the velocity of the buoyant ascent of fluid bubbles depends on their sizes, the viscosity and density
of the melts, and the duration of melt flow. Possible variants of the primary and secondary boiling of magma are discussed
depending on the P-T conditions and concentrations of H2O, F, B, and other components dissolved in the magma. The possible density ranges of the fluid phases are considered, along
with the viscosity and density of granitic (pegmatitic) melts, velocities of the buoyant ascent of fluid bubbles in them,
and the processes of their coalescence and accumulation in the temperature range of 650–850°C. Provisional evaluates are obtained
for the duration of melt crystallization and the development of intrusive massifs and dikes of granites and syngenetic intragranite
and epigenetic (intruded into the host rocks) granite pegmatites. Simulation data and geological observations suggest that
large fluid segregations were formed already in the magma chambers in which the heterogeneous granite (pegmatitic) magma was
derived, before its emplacement into the host rocks. These generation regions could be magma chamber areas within granite
intrusions, in which melts enriched in volatiles were accumulated and then degassed with the release of fluid phases of various
composition and density. The crystallization of fluid-rich melts under favorable conditions gives rise to granites with miarolitic
structures. The emplacement of heterogeneous pegmatitic magma (which consists of immiscible silicate melts and large fluid
segregations) into the host rocks results in that these segregations (would-be miaroles) occur in any part of the pegmatite-hosting
chamber. This explains why miaroles of significantly different composition and with broadly varying proportions of their filling
minerals may occur in various parts of pegmatite veins or their swells, as well as near contacts with the host rocks. |
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