CONDITIONS OF MIAROLES (POCKETS OR CAVITIES) FORMATION IN GRANITES AND

GRANITIC PEGMATITES

Peretyazhko I.S.

Institute of Geochemistry SB RAS, Irkutsk, Russia, pgmigor@igc.irk.ru

The degassing or first boiling of silicate melts occurs in P-T conditions that are mainly determined by quantity of the fluid dissolved in them. Formation and increase of size in fluid bubbles can be started, when total partial pressure of the volatiles included in the melt will exceed lithostatical pressure (P_fl > P_lith). Free fluid phase is released both at intrusion of melt due to decrease in outer pressure and temperature, and as a result of magmatic chamber decompression. Melts lose only a part of fluid phase before start of crystallization in process of first boiling. Dynamics of degassing is often deviates from equilibrium way, and significant amount of fluid remains in the melt in excess of its equilibrium solubility. A release of volatiles will continue or can be started after crystallization of some amount of silicate minerals, being mostly anhydrous. In the partially crystallized silicate melt, the molar part of volatile components increases that results in its saturation in fluid and second boiling.

Conditions of liquidus determine minimally possible water amount in melt for equilibrium : melt L + crystals ↔ melt L. Maximally possible water amount in melt in certain P-T conditions corresponds to its solubility, if the condition melt L ↔ melt L + fluid V is realized. A region of the granite melt saturated in volatiles and free fluid (L + V) significantly widens due to decreasing temperature and increasing pressure at increase of water, fluorine and boron solved in it. Presence of fluorine and boron increases water solubility in melt, decreases its density and viscosity, effects the dynamics of magma degassing and phase ratios of minerals.

Miaroles (pockets or cavities) with various mineralization are mostly found in granitic pegmatites - syngenetic that occur among the source granites or those intruded into host rocks. Significant variations in compositions of veined body in fields of miarolitic granitic pegmatites are associated with intrusion of chemically heterogenous magma in host rocks. Each portion of that magma evolved in the autonomous regime. The miaroles from several tens cubic centimeters to several cubic meters by volume occur in any zone and near contacts of pegmatite bodies as well. Calculations of bubble moving rate and bubble coalescence in granite melt show that accumulation of fluid large isolations is possible in cases: (1) when period between intrusion of melt and its complete crystallization is thousands years; (2) at increase in rate of bubble (fluid foam) moving caused by decrease in melt viscosity; (3) as a result of significant increase in average size of bubbles due to their coalescence.

The first case is realized in syngenetic (intergranite) pegmatites, as their formation being related to processes of granite intrusion crystallization that continue for hundreds thousand years. At usual viscosity of granitic melt (10⁵-10⁶ Pa·s) this time appears to be sufficient for bubble moving on many hundreds meters and fluid accumulation in apical and still liquid parts of intrusive bodies. If the host rocks are deformed enough to allow an increase in volume of the system (∆V) that is caused by accumulation of fluid phase, then formation of pockets is possible in granites. At presence of prenetrating zone in apical part of granitic body the larger part of fluid flows leaves into roof rocks, and pockets in granites and syngenetic pegmatites do not form.

At increase in rate of bubbles moving and enlargement of their size in the second and third cases can be in rare-metal granitic melts that are enriched in water, F, B and other volatiles. Such melts form either in hearths of pegmatite magma accumulation or in final stages of pegmatite body crystallization. Due to limited spread of rare-metal complexes, amounts of such melts are much less than “usual” alumosilicate ones, from which quartz-feldspar zones crystallize composing the main volume of granitic pegmatite bodies. It should also be mentioned that miaroles in rare-metal complexes occur much more rarely than in quartz-feldspar zones. There are also no reasons to consider that processes of coalescence resulted in a great number of bubbles in size much more than 200-300 μm at magma voluminous boiling. Thus, fluid large bubbles (future miaroles) up to several (sometimes many tens) cubic meters by volume do not succeed to form for the time after intrusion in host rocks and crystallization of pegmatite bodies that equals years – first decades.

The data of calculations and geological observations allow to suggest that large fluid isolations are formed in hearths of pegmatite magma accumulation in the above-liquidus conditions. After magma saturation in volatiles the degassing or first boiling started and small fluid bubbles emanated in the hearth. As a result of fluid-magmatic interaction, colloidal solutions and/or jellous environments (melt-like gels) containing significant amounts of silicate and volatile components could form in some bubbles. During the protracted time, small bubbles floated and were combined into larger isolations. Fluid pressure largely exceeded P_lith that could cause roof ruptures of granite intrusion and host rocks. As a result of such intrusion, heterogenous granitic magma can contained large fluid isolations. Time interval of magma ascend from hearth of its accumulation to location of pegmatite body formation cannot be protracted (i.e. much less than time of granite melt crystallization in veined bodies that equals years – decades). Ascent of pegmatite magma will stop, when P_fl will be balanced by lithostatic pressure. As a result, fluid bubbles could appear in any part of pegmatite body and close to contacts with host rocks as well. Subsequently, large bubbles floated rather rapidly, since the rate of their moving is directly proportional to squared diameter. Decompression at intrusion and partial crystallization of magma saturated in volatiles could also result in its boiling and emanation of numerous small bubbles. Emanation of bubbles will stop or continue up to complete crystallization of pegmatite bodies in dependence on P-T conditions and fluid initial content in magma. Some part of “new” small bubbles is trapped by the growing crystals as fluid inclusions or combined with large bubbles still formed in the hearth. Not large fluid isolations can form as well in flow of small bubbles in favorable conditions that are related to decrease in viscosity of melt and increase in time of its crystallization (particularly, in pegmatite bodies of significant thickness). Part of free fluid emanated in various stages of pegmatite magma degassing after its intrusion and complete crystallization, is likely to leave for host rocks.

This work is supported by Russian Foundation for Basic Research, grant 08-05-00471.