Abstract

The distribution and crystal-chemical characteristics of chlorite, corrensite, and mica in samples from a stratigraphic profile in the Cameros basin are controlled by changes in the sedimentary facies. The lacustrine marls and limestones from the base and the top of the profile contain quartz ± calcite ± illite ± dolomite ± chlorite ± albite ± paragonite ± Na, K-rich mica. Chlorite is rich in Mg, with Fe/(Fe + Mg) ratios ranging between 0.18–0.37. A formation mechanism involving reaction between Mg-rich carbonate and dioctahedral phyllosilicates is proposed for these Mg-rich chlorites, on the basis of the mutually exclusive relationship found between Mg-rich chlorite and dolomite, together with the relative increase in the proportion of calcite in samples containing chlorite.

The mudrocks from the middle part of the profile are composed of quartz + albite +illite +corrensite (with a mean coefficient of variability of 0.60%) ± chlorite. Corrensite and chlorite are richer in Fe2+than those from the base or top of the profile, with mean Fe/(Fe +Mg) ratios of 0.51 and 0.56, respectively. Textural and compositional features suggest a formation mechanism for the corrensite, chlorite, and chlorite-mica crystals through replacement of detrital igneous biotite. Whether or not corrensite occurs with chlorite appears to be related to redox conditions. The presence of corrensite alone is apparently favored by oxidizing conditions, whereas the occurrence of corrensite + chlorite is related to more reducing conditions. Corrensite shows higher Si and Na + K +Ca contents, and slightly lower Fe/(Fe + Mg) ratios than chlorite. The presence of corrensite and the lack of random chlorite-smectite interlayering is discussed in terms of the fluid/rock ratio; the occurrence is related to the hydrothermal character of metamorphism in the Cameros basin.

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