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K-metasomatism of the upper Lemitar and Hells Mesa silicic ignimbrites near Socorro, New Mexico is thought to be the result of downward percolation of alkaline, saline brines in a hydrologically closed basin [Chapin, C.E., Lindley, J.I., 1986. Potassium metasomatism of igneous and sedimentary rocks in detachment terranes and other sedimentary basins: economic implications. Arizona Geological Society Digest, XVI, 118–126.]. During the chemical changes associated with metasomatism, Na-rich phases, primarily plagioclase, are replaced by secondary mineral phases. Adularia, a low-temperature K-feldspar, is the dominant mineral formed during K-metasomatic alteration, but mixed-layer I/S, discrete smectite, and kaolinite can also be present in the assemblage. The formation of discrete smectite within the assemblage during K-metasomatism may have occurred during periods of low cation/H+ ratios in the alkaline, saline brine. Upon an increase in the cation/H+ ratio, such as could occur during evaporation of the alkaline lake, the solution may have become sufficiently concentrated to cause illitization of smectite resulting in the formation of mixed-layer I/S within the assemblage. Distribution of phases in the alteration assemblage strongly suggests a dissolution–precipitation reaction for K-metasomatism in the Socorro area as indicated by the presence of dissolution embayments in plagioclase crystals, the presence of euhedral adularia, and the common occurrence of authigenic clay minerals in the assemblage. K-metasomatism causes significant chemical modification of the silicic ignimbrite, particularly increases in K2O and Rb and depletion of Na2O and Sr with increasing adularia abundance. The correlation between Rb and K2O suggests that Rb is enriched during alteration due to substitution for K in adularia. The effect of hydrothermal activity, either prior to, or following metasomatism, is also observed in some samples, as shown by high concentrations of elements such as Ba, As, Sb, Pb and Cs. Enrichment of middle and HREE in the upper Lemitar Tuff samples and depletion of middle and HREE in Hells Mesa Tuff samples suggests attempted re-equilibration between the secondary alteration assemblage and the metasomatizing fluid. Preliminary data indicate clay minerals within the secondary assemblage may have played an important role in the incorporation of REE during redistribution. |
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