Tectonic-hydrothermal pegmatites |
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| Authors: | Randall L Gresens |
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| Institution: | (1) Department of Geology, University of Washington, Seattle, Washington |
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| Abstract: | The proposed model for the genesis of complex zoned pegmatites depends on two basis conditions: (1) an alkali chloride intergranular fluid phase (of either magmatic or metamorphic origin) is in equilibrium with the solid phases in the host rock, and (2) deformation produces low pressure zones in the host rock. The phase relations of quartz-mica-feldspar in alkali chloride solutions predict that feldspar will form from quartz, muscovite and dissolved alkalies in low pressure zones in quartz-muscovite schist. The reaction is fed by diffusion of hydrothermal alkali chloride solutions into the low pressure zone. Low pH fluids produced during the reaction are out of equilibrium when lithostatic pressure is re-established so that muscovite forms as a late replacement.A critical lowering of pressure is necessary to force the reaction. If the rate of dilation is relatively slow, only quartz will migrate into the low pressure zone, and the critical low pressure will not be attained. A relatively higher rate of dilation will cause the reaction to proceed. This control can explain the quartz core and the occurrence of quartz veins that grade laterally into pegmatite cores. By this model, the quartz core is the first part of the pegmatite to crystallize.The rare elements concentrated in pegmatites are considered to come from four possible sources: (1) hydrothermally introduced solutions, (2) impurity elements adsorbed to mineral grains in the country rock, (3) elements released by recrystallization of the country rock, and (4) elements released by reactions.Other features of pegmatites are shown to be compatible with the proposed model. |
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