The mineralogy and metamorphic geology of low-grade metasediments, Northern Range, Trinidad

Abstract

Forty-one low-grade metasediments from the Northern Range of Trinidad and from the neighbouring island of Tobago, West Indies, were investigated by microscopic, X-ray powder diffraction, Guinier camera technique and electron microprobe methods. The following minerals were encountered (in order of decreasing abundance): quartz, muscovite, chlorite, paragonite/muscovite mixed-layer, paragonite, organic material, calcite, pyrophyllite, dolomite, albite, hematite, chloritoid, pyrite, rutile, titanite, rectorite, stilpnomelane. The chemical composition of muscovite is dependent on the mineral assemblage, with Si = 6.1–6.2 per formula unit in two high alumina assemblages, versus Si = 6.6 in a low alumina assemblage. Microprobe analyses indicate that muscovite, paragonite and pyrophyllite are sometimes intergrown on a micrometre scale. Chlorite composition estimated from X-ray data shows good correspondence with microprobe data for two Al-poor samples but is less satisfactory for an Al-rich sample. Observed mineral assemblages in AKNa are indicative of the pyrophyllite-paragonite-albite fades. For the first time, chemical data for coexisting pyrophyllite, chlorite (M = MgO/(MgO+ FeO) = 0.377) and chloritoid (M = 0.110) are given. This assemblage is indicative of the low or medium pressure range of the lower greenschist facies. Pressure conditions during metamorphism are not well constrained but a minimum pressure of 2 kbar, corresponding to 5–6 km of overburden, can be estimated from the Na M4 content of actinolite. Temperatures derived from the calcite-dolomite geothermometer range between 300 and 350°C and are consistent with the presence of the assemblage pyrophyllite + calcite + quartz.