ALBERT

Notes: 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.

Notes: Abstract Schreyer and Chinner (1966) described staurolite-quartzite bands of unusual bulk chemical composition associated with the Big Rock kyanite deposit in northern New Mexico. They discussed two possible hypotheses-syngenetic or metasomatic-for the origin of the bands. This paper discusses additional evidence in support of the metasomatic hypothesis. Quartz-muscovite schist and quartz-kyanite rock are believed to be metarhyolite that was altered metasomatically by hydrothermal activity during metamorphism. Experimentally demonstrable reactions that involve ionic exchange equilibria between pore fluid and solid phases best explain the observed alteration. Staurolite-quartzite bands are postulated to be basalts or amphibolites that were hydrothermally altered in a manner analogous to alteration of the enclosing rocks. The process would deplete a basaltic parent rock in alkali and alkaline earth elements and produce a chemical composition similar to the bulk chemical composition of staurolite-quartzite. Schreyer and Chinner used textural evidence to prove that staurolite formed at the expense of chloritoid. Based on the chemical compositions of staurolite and relict chloritoid reported by them, several chemical equations were constructed, using the principle of ionic equilibria between pore fluid and solid phases, in an attempt to realistically portray the reaction. It is suggested that, under open-system metasomatic conditions, the ratio aMg+2/aH+ in the supercritical aqueous fluid is an important parameter that controls the stability of chloritoid vs. staurolite.

Notes: Abstract The Kiawa pegmatites, New Mexico, are thought to have formed by the process outlined in part I of this paper, i.e., by the reaction, Quartz + Muscovite + (Na+, K+) ⇀ Perthite + H+. The pegmatites and country rocks were mapped and sampled in detail and geochemical data were obtained by spectrochemical analysis of separated minerals for 17 major and trace elements. Structural compatibility with the model is demonstrated by the occurrence of the pegmatites in structurally low pressure zones. Geochemical evidence indicates that quartz and muscovite, the raw materials for the reaction, were available before formation of the pegmatites. The distribution of major and trace elements in the minerals of the pegmatites and country rocks supports the model. Elements that were released by major reactions are found in secondary minerals in the country rocks and/or in pegmatite minerals. The distribution of barium in the minerals is that which would be predicted from its known geochemical behavior. Geologic evidence suggests that the pegmatites formed synkinematically during regional metamorphism. Other features of the Kiawa pegmatites, such as albitization, structural control of element distributions, and possibly an older generation of pegmatites, are shown to be in accord with the proposed model. Certain features of the pegmatites are not amenable to a magmatic explanation.

Notes: Abstract The petrogenesis of Franciscan-type blueschists is controversial or paradoxial in regard to the possible significance of the serpentinite-blueschist association, the isochemical vs. allochemical character of blueschist metamorphism, the significance of “high pressure” mineralogy, and the physical-geologic-tectonic conditions existing during metamorphism. A model for blueschist alteration during serpentinization is presented that departs from conventional treatments of metamorphism because the reaction path rather then the thermodynamically lowest energy state is considered to be a controlling factor. Alteration of ultramafic rocks to serpentinites requires oxidation of iron in the rock and selective withdrawal of water from saline pore fluids derived from surrounding eugeosynclinal rocks. Pore fluids become concentrated and chemically reducing in the vicinity of serpentinites. The activated pore fluids may react with surrounding rocks via reactions that include a reduction step. The pore fluids may also affect the reaction path through surface chemical effects existing between mineral surfaces and/or “growth units” and the reducing pore fluid. The degree of polarization of oxygen ions in the silicate structural types, a function of polymerization and aluminum substitution, may control the surface effects and result in the preferential growth of chain silicates, and, more generally, of silicates with low amounts of tetrahedral aluminum. The ratio Mg+2/H+ in the pore fluid can change during serpentinization depending on the extent to which magnesium is lost from the original mafic rock. This ratio may be an important control on the growth of jadeite vs. glaucophane in the presence of excess quartz. The reduction reactions and those involving conventional fluid-solid equilibria cause a change in pore fluid chemistry as the reaction proceeds. Such reactions may explain short-range metasomatic transitions observed in some blueschists. The kinetic controls involving surface chemical effects are catalytic and may explain isochemical phenomena. The ultimate “drive” for the process is the large negative free energy change of serpentinization that results when ultramafic rocks are emplaced in eugeosynclinal rocks with which they are not in equilibrium. Removal of this overwhelming disequilibrium may induce secondary disequilibrium and activation of pore fluids, producing Franciscan-type blueschists.