The Effects of Fe3? and Mn3? on Aluminum Silicate Phase Relations in North-Central New Mexico, U.S.A.

Aluminum silicate triple-point occurrences are common in metamorphicrocks of northern New Mexico. The three polymorphs show extensivesolid solution, with Fe and Mn substituting for Al. Mineraltextures, the spatial distribution of phases, and the systematicpartitioning of Fe and Mn indicate that the kyanite-andalusite-sillimaniteassemblages crystallized in equilibrium. The compositions ofminerals in the three-phase assemblage vary across the studyarea, recording regional variations in the pressures and temperaturesof metamorphism. The highest-pressure rocks, exposed at RioMora, contain kyanite at higher elevations and sillimanite atlower elevations. A sub-horizontal isograd separates the twominerals. Kyanite and sillimanite have nearly identical Fe contentwhich varies systematically with X_Fe2O3 in hematite or ilmenite.Andalusite occurs only along a single manganiferous layer, incrystals rich in MnAlSiO₅ and saturated in FeAlSiO₅. Triple-pointassemblages can be found wherever the folded manganiferous layercrosses the unfolded kyanite-sillimanite isograd. The TruchasRange, preserving slightly lower pressures of metamorphism,shows kyanite-andalusite-sillimanite in rocks with titaniferoushematite. Andalusite is enriched in Fe relative to kyanite andsillimanite, but no polymorphs contain Mn. Rocks with lowerX_Fe2O3 in hematite have kyanite and sillimanite without andalusite,whereas rocks with pure hematite contain only andalusite. Theshallowest erosional levels are preserved in the western PicurisRange where the three polymorphs occur as pure minerals in ilmenite-bearingrocks. Hematitic samples contain only andalusite which showsextensive solid solution of both Fe and Mn. The assemblage kyanite-andalusite-sillimanite is not invariant.Iron and manganese both add degrees of freedom. These transitionmetals have stabilized the three-phase assemblage, in apparentchemical equilibrium, across a P-T interval of 500-540 ?C, 3?8-4?6kb in rocks from New Mexico. The saturation level of FeAlSiO5in andalusite does not vary with Mn content but does vary withpressure and temperature. Calculations indicate that a 2-3 kbdecrease in pressure or a 25-50 ?C increase in temperature resultsin a 1 mole per cent increase in X_FeAlSiO5 in iron-saturatedandalusite.     

Tectonic evolution of Proterozoic rocks in the Cimarron Mountains, northern New Mexico, USA

Abstract Portions of three Proterozoic tectonostratigraphic sequences are exposed in the Cimarron Mountains of New Mexico. The Cimarron River tectonic unit has affinities to a convergent margin plutonic/volcanic complex. Igneous hornblende from a quartz diorite stock records an emplacement pressure of 2–2.6 kbar. Rocks within this unit were subsequently deformed during a greenschist facies regional metamorphism at 4–5 kbar and 330 ± 50° C. The Tolby Meadow tectonic unit consists of quartzite and schist. Mineral assemblages are indicative of regional metamorphism at pressures near 4 kbar and temperatures of 520 ± 20° C. A low-angle ductile shear zone separates this succession from gneisses of the structurally underlying Eagle Nest tectonic unit. Gneissic granite yields hornblende pressures of 6–8 kbar. Pelitic gneiss records regional metamorphic conditions of 6–7 kbar and 705 ± 15° C, overprinted by retrogression at 4 kbar and 530 ± 10° C. Comparison of metamorphic and retrograde conditions indicates a P–T path dominated by decompression and cooling. The low-angle ductile shear zone represents an extensional structure which was active during metamorphism. This extension juxtaposed the Tolby Meadow and Eagle Nest units at 4 kbar and 520° C. Both units were later overprinted by folding and low-grade metamorphism, and then were emplaced against the Cimarron River tectonic unit by right-slip movement along the steeply dipping Fowler Pass shear zone. An argon isotope-correlation age obtained from igneous hornblende dates plutonism in the Cimarron River unit at 1678 Ma. Muscovite associated with the greenschist facies metamorphic overprint yields a ⁴⁰ Ar/³⁹ Ar plateau age of 1350 Ma. By contrast, rocks within the Tolby Meadow and Eagle Nest units yield significantly younger argon cooling ages. Hornblende isotope-correlation ages of 1394–1398 Ma are interpreted to date cooling during middle Proterozoic extension. Muscovite plateau ages of 1267–1257 Ma appear to date cooling from the low-grade metamorphic overprint. The latest ductile movement along the Fowler Pass shear zone post-dated these cooling ages. Argon released from muscovites of the Eagle Nest/Tolby Meadow composite unit, at low experimental temperatures, yields apparent ages of c. 1100 Ma. Similar ages are not obtained north-east of the Fowler Pass shear zone, suggesting movement more recently than 1100 Ma.