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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

¹ Área de Cristalografía y Mineralogía, Facultad de Ciencias, Universidad de Extremadura, Avda. de Elvas s/n, 06071 Badajoz, Spain, ² Departamento de Mineralogía y Petrología y I.A.C.T., Universidad de Granada-CSIC, Avda. Fuentenueva s/n, 18002, Granada, Spain, and ³ Departamento de Geología, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Plaza Ercilla 803, Santiago de Chile, Chile

^* E-mail: munguira{at}unex.es

(Received 5 April 2001; revised 22 September 2001)

Chlorites from Cambrian basaltic vesicular lavas, characterized by relatively high X_c (0.81–0.98), gave temperatures of formation of 285 to 350°C (Cathelineau’s empirical geothermometer). Both the X_c 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 X_c 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/MgO ratios (3.2831.66) 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 X_c calculation and empirical geothermometric calculations are inadequate.

KEYWORDS: chlorite, metabasites, very low-grade metamorphism, chemical composition, thermometry

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