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Chlorite composition and geothermometry: a comparative HRTEM/AEM-EMPA-XRD study of Cambrian basic lavas from the Ossa Morena Zone, SW Spain

Published online by Cambridge University Press:  09 July 2018

A. López-Munguira ,
F. Nieto  and
D. Morata
A. López-Munguira*
Affiliation:
Área de Cristalografía y Mineralogía, Facultad de Ciencias, Universidad de Extremadura, Avda. de Elvas s/n, 06071 Badajoz, Spain
F. Nieto
Affiliation:
Departamento de Mineralogía y Petrologa y I.A.C.T., Universidad de Granada-CSIC, Avda. Fuentenueva s/n, 18002, GranadaSpain
D. Morata
Affiliation:
Departamento de Geología, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Plaza Ercilla 803, Santiago de Chile, Chile
*
*E-mail: munguira@unex.es
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Abstract

Chlorites from Cambrian basaltic vesicular lavas, characterized by relatively high Xc (0.81-0.98), gave temperatures of formation of 285 to 350°C (Cathelineau's empirical geothermometer). Both the Xc and temperature calculations gave results which were too high for the diagenetic conditions indicated by the interbedded shale mineralogy. The HRTEM and XRD studies indicate the absence of smectite layers in these chlorites; i.e. according to lattice images, the actual value of Xc is 1. The chlorite composition in these basaltic lavas must, therefore, be explained in relation to their whole-rock geochemistry, with which a good correlation has been found. The basaltic lavas are characterized by relatively high FeO/M ratios (3.28±1.66) and must be considered as an Fe-rich system, with similar chemical behaviour to pelitic rocks. In these cases, direct chlorite formation, without a previous smectite precursor, is a normal occurrence and the Xc calculation and empirical geothermometric calculations are inadequate.

Keywords

chloritemetabasitesvery low-grade metamorphismchemical compositionthermometry
Type
Research Article
Information
Clay Minerals , Volume 37 , Issue 2 , June 2002 , pp. 267 - 281
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2002

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