Geología e interpretación petrológica de los granitos y pegmatitas de la Sierra de Mazán, La Rioja, Argentina

GEOLOGIC AND PETROLOGIC INTERPRETATION OF GRANITES AND PEGMATITES OF SIERRA DE MAZÁN, LA RIOJA, ARGENTINA. The Sierra de Mazán is located in Northern Sierras Pampeanas, nearby the Sierra de Velasco and the Sierra de Ambato. The Sierra is tilted to the west by Cenozoic regional faults, which control the NS elongation. A low metamorphic basement is represented by the Cambrian La Cébila Formation which is intruded by Lower Ordovician, strongly peraluminous S-type porphyritic cordierite-bearing granite, that makes up most of the Sierra de Mazán. Two minor felsic phases are close related with the S-type granite: The one is an equigranular peraluminous leucogranite that contains mineralized quartz veins with cassiterite and wolframite, known as La Quebrada Granite. The other barren felsic phase shows a textural layering defined by alternating bands of pegmatite, composed of megacrysts of perthitemicrocline. Quartz, muscovite, minor albite and tourmaline, make up the remainder of the coarse layer. An alternated medium-grained layer, display an equigranular texture composed of quartz, albite, muscovite, scarce microcline and tourmaline. Graphic and symplectitic textures are common in this layer. The cordierite-granite and its two felsic phases display plastic deformation, and are genetically interpretated as synchronous and continuous during the Ordovician times. The magmatic history of different granitic phases is interpreted following these steps: 1) An S-type granitic melt originated deep in the crust by incongruent partial melting of fluid – absent aluminous greywackes. These conditions allowed the rise and crystallization of magma and its location at shallow levels. 2) The remaining melt after the principal crystallization consisted of a felsic silicate melt with some S and W complexes. 3) The water rich last melt produced a layered barren pegmatite rich in boron. The geochemistry of the different stages from the cordierite granite to pegmatite phases suggest, an evolution by fractional crystallization of cordierite granite, where the major oxide elements Fe, Mg, and Ca decrease while the SiO2 increases. Ba, Sr, Zr and Ti trace elements are strongly depleted in the pegmatitic melt. The fractionation of zircon, monazite and apatite produced REE depletion and a strong negative Eu anomaly in the residual pegmatitic melt, in equilibrium with a boron-rich aqueous phase. This process produced the tetrad effect displayed by the REE patterns. 1, 2) CONICET, Facultad de Ciencias Naturales e Instituto Miguel Lillo, Universidad Nacional de Tucumán. E-mail: ajtoselli@yahoo.com. ar 3) Instituto de Geociencias. Universidade de São Paulo, Rua do Lago. São Paulo. Brasil. E-mail: baseimas@spider.usp. br La historia magmática de las diferentes fases graníticas pudo desarrollarse según las siguientes etapas: 1) el magma granítico “tipo-S” se debe haber originado profundamente en la corteza, por fusión parcial incongruente de grauvacas aluminosas, en ausencia de fluidos. Estas condiciones permitieron el ascenso del magma y su emplazamiento en niveles someros. 2) El residuo remanente, después de la cristalización principal consistió en una fase silicática félsica con algunos complejos de Sn y W, que resultó en la fase granítica mineralizada. 3) El último fundido, rico en agua y boro produjo la pegmatita bandeada estéril. La geoquímica de las distintas fases, desde máficas en el granito cordierítico, terminando en fases félsicas y desarrollo de pegmatitas, sugiere una evolución por cristalización fraccionada del granito porfiroide cordierítico, en el que los óxidos de elementos mayores: Fe, Mg y Ca disminuyen, con el aumento en SiO2 y de igual forma, los elementos trazas Ba, Sr, Zr y Ti sufren una disminución notable en las pegmatitas. El fraccionamiento de circón, monacita y apatita produjo empobrecimiento de las Tierras Raras del fundido félsico residual pegmatítico, que ha debido estar en equilibrio con una fase acuosa rica en boro, que se manifiesta por el marcado efecto tetrada en las Tierras Raras, la fuerte anomalía negativa del Eu.