--> Diagenesis and Hydrothermal Overprint in Permian Volcaniclastic Rocks, Northern Chile, by F. Breitkreuz, R. H. Goldstein, and A. W. Walton; #90986 (1994).

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Abstract: Diagenesis and Hydrothermal Overprint in Permian Volcaniclastic Rocks, Northern Chile

F. Breitkreuz, Robert H. Goldstein, Anthony W. Walton

Terrestrial basalto-andesitic to rhyolitic volcanic and sedimentary rocks of the Peine Formation and Chunar Beds E of the Salar de Atacama experienced moderate burial and hydrothermal activity. We infer this history from XRD and microprobe measurements of phyllosilicates, fluid inclusion measurements as well as field and thin section observations. Strong differential compaction and a zeolite facies assemblage of laumontite-prehnite-analcime-kaolinite presumably formed during burial not higher than advanced diagenesis. Reconnaissance measurements of fluid inclusions in analcime support this.

Evidence that significantly higher temperatures were superimposed on the lower temperature zeolite assemblage include: (1) homogenization temperatures from secondary fluid inclusions in calcite indicate heating to at least 230°C; other reconnaissance measurements suggesting heating to at least 260°C need confirmation. (2) formation temperatures of chlorite, calculated on the basis of the AIIV abundance. range from 178 to 251°C; (3) epidote is dominant on joints and dispersed in the rock throughout the 2 km thick succession. Thus, hydrothermal fluids affected the succession.

XRD measurements show 10% or less of smectite layers mixed with illite, chlorite and kaolinite. Chlorite (ChC) and illite (IC) crystallinities vary strongly within a stratigraphic level. The maximum ChC and IC plot near the diagenesis-anchizone transition. Paired analysis in the same samples show that the metamorphic grade indicated by crystallinity lags behind the temperatures

calculated from the chlorite geothermometer, the fluid inclusion data and inferred from the presence of epidote.

Disequilibrium, variable saturation with respect to phyllosilicates and permeability differences may explain the contrasting results between crystallinity on one hand and fluid inclusion data, presence of epidote and chlorite composition on the other.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994