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La asociacion Monzonitica del Domo Anatectico del Tormes: geoquimica y petrogenesis

By  and L&#243 and L&#243

Abstract

In certain peripheral zones of the Anatectic Tormes Dome there exists a group of rocks, which are from basic to acid as well as rich in alkali feldspar, apatite and allanite. Such an unusual mineralogy makes the rocks to be enriched in K20, P2O5, LREE and other elements of low ionic potential. Their geochemical signature is related to the volcanic shoshonites, to the vaugnerites of the French Central Massif as well as to the monzonitic series of Ballon, being, in turn, almost exceptional in the iberian variscan. The least evolved terms are rather fractionated, therefore, they are not primary melts. During the differentiation, the saturation of accessory phases such as apatite, allanite and zircon exerts an important effect in the evolution of the residual liquid giving rise to an inflection point in the Harker diagrams. From the good adjustments obtained in mass balance and Rayleigh fractionation modelling, we can infer that the process of fractional crystallization is the dominant one. In this respect, the porphyritic granitoids can be derived from a liquid of monzodioritic composition, being rocks genetically to the associated acid rocks. The corroded cores of plagioclase and the spatial relations suggest the existence of different pulses of magma from basic to acid coming from the same magmatic chamber.<br><br>En dos bandas del Domo anat&#233;ctico del Tormes aparece un conjunto de rocas plut&#243;nicas desde b&#225;sicas a &#225;cidas ricas en feldespato pot&#225;sico, apatito y allanita. Esta mineralog&#237;a tan particular hace que est&#233;n enriquecidas en K20, P205, LREE y otros elementos de bajo potencial i&#243;nico. Su quimismo es af&#237;n a las shoshonitas volc&#225;nicas, a las vaugneritas del Macizo Central franc&#233;s y a las series monzon&#237;ticas de Ballon, siendo, a su vez, casi excepcional en el varisco ib&#233;rico. Los t&#233;rminos menos evolucionados est&#225;n bastante fraccionados por lo que no se trata de fundidos primarios. Durante la diferenciaci&#243;n existen fases accesorias como apatito, allanita y circ&#243;n cuya saturaci&#243;n ejerce un importante efecto en la evoluci&#243;n del l&#237;quido residual originando evoluciones quebradas en los diagramas de Harker. A partir de los buenos ajustes obtenidos con las modelizaciones por balance de masas y fraccionaci&#243;n Rayleigh se infiere que el proceso de cristalizaci&#243;n fraccionada es el dominante. En este sentido, se pueden hacer derivar los granitoides porf&#237;dicos a partir de un l&#237;quido de composici&#243;n monzodior&#237;tica, por lo que no quedan desconectadas las rocas b&#225;sicas de las &#225;cidas asociadas. Los n&#250;cleos corro&#237;dos de plagioclasa y las relaciones espaciales parecen indicar que han existido distintos pulsos de magma desde b&#225;sicos a &#225;cidos procedentes de la misma c&#225;mara magm&#225;tica

Topics: monzonitic association, shoshonites, fractional crystallization, Tormes Dome, asociaci&#243;n monzon&#237;tica, shoshonitas, cristalizaci&#243;n fiaccionada, Domo del Tormes, Geology, QE1-996.5, Science, Q, DOAJ:Earth Sciences, DOAJ:Earth and Environmental Sciences
Publisher: Consejo Superior de Investigaciones Científicas
Year: 2001
DOI identifier: 10.3989/egeol.01573-4130
OAI identifier: oai:doaj.org/article:c2f15ef3743f412eb1ac94686db7509e
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