Geochemistry of Muscovite from Precambrian Metamorphic Rocks of Northern New Mexico

Abstract

More than 200 mica samples (primarily muscovite) and 65 whole-rock samples from Precambrian rocks of northern New Mexico were analyzed for Si, Al, K, Fe, Na, Mn, Mg, Ca, Ti, Li, Rb, Cr, Zn, Cu, Ba, and F by atomic absorption spectrophotometer and electron microprobe. Two major stratified units were sampled: an older complex of dominantly rhyolitic metavolcanic rocks with subordinate metasedimentary rocks, and an overlying metasedimentary unit. The rocks are exposed in the Las Tablas area and also about 40 km distant, in the Picuris Range. Mass-balance calculations support field and textural evidence for metasomatic derivation of quartz-muscovite schist from metarhyolite—a hypothesis that conflicts with published mapping of quartz-muscovite schist as metasediment. During metasomatism, H and small amounts of Mg, Li, and Ti(?) were added to the rock, whereas Na, Ca, and Cu(?) were removed. The tendency was for concentrations of Si, Al, Fe, Mn, and Zn to remain unchanged, and also for K and Rb to remain, but only because muscovite was formed; generally, the mobile K and Rb were partly removed in solution. Geochemical data support a “dilution model” for progressive metasomatic growth of muscovite in metarhyolite. Whole-rock concentrations of Zn, Li, Mn, Mg, and Ti— elements taken up by muscovite—remained nearly unchanged during metasomatism. Consequently, as the amount of muscovite grew larger, the weight fraction of each of these elements in muscovite grew smaller. A pervasive pore fluid of rather fixed Na + activity created a uniform value of Na concentration in metasomatic muscovite in metarhyolite. Sodium in isochemically grown muscovite in the metasedimentary unit showed a greater variation of Na values. A generally higher Na concentration in muscovite from the Picuris Range (paragonite solid solution) supports mineralogic evidence of the attainment of a higher metamorphic grade. (This might also be reflected by higher Al and lower Fe and Mg values for muscovite from the Picuris Range.) Na values in muscovites from pegmatites and surrounding schists are similar, as would be expected if the pegmatites were emplaced during metamorphism. Interelemental correlations reflect the different modes of origin for muscovite. Only six significant correlations were found for metasomatically grown muscovite, consistent with the “dilution model” for muscovite growth under open system conditions. There are 29 significant correlations for isochemically grown metasedimentary muscovite, many of them predictable from classical geochemical affinities. A “trace element stratigraphy” based on muscovite chemical composition can be demonstrated only if there are sufficient samples to adequately represent the mean concentration and standard deviation of the elements in each stratigraphic unit. The best discriminators in this study were Mn, Li, Rb, Cr, and Zn. Attempts at finer correlation within major units were less satisfactory, but Mn and Cu concentration of muscovite from the metavolcanic unit possibly shows stratigraphically controlled variation. A special note of caution to geochronologists is based on the apparent geochemical mobility of Rb during metasomatic alteration, which can go unrecognized if metasomatic quartz-muscovite schists are erroneously interpreted as isochemically metamorphosed metasedimentary rocks.