ALTERATION MINERALOGY IN PILLOWS AND INTERPILLOW BRECCIA, TALCOTT BASALT, MERIDEN, CT, USA

The base of the Talcott Basalt in Meriden, CT is composed mainly of pillow basalts and an interpillow breccia, both formed when the basalt erupted into a lake. The contact between the Talcott Basalt and the underlying New Haven Arkose is a mixed zone of pillows and clastic sedimentary rocks. The basalt and the underlying sedimentary rocks were extensively altered during the eruption and in one or more episodes after cooling. The pillows themselves contain many vugs with multiple different minerals, some occurring in concentric layers. The purpose of this project is to analyze and identify these minerals in order to understand the hydrothermal alteration history of the rock, including the order of crystallization and the composition of hydrothermal fluids. Preliminary powder X-ray diffraction (powder XRD) results from representative samples collected at the Meriden outcrop show an abundance of quartz and calcite, along with other secondary minerals. Whole rock powder samples of the basalt indicate pervasive chlorite alteration in the matrix. Vug mineralogy includes quartz, calcite, and possibly zeolites such as heulandite. Secondary alteration of interpillow breccias produced quartz, calcite, and clay minerals such as chlorite and possibly corrensite.

Preliminary analysis of zonation of vug minerals indicates that silicate minerals were deposited first, with carbonate minerals filling in afterwards. This pattern is also present in the interpillow breccia. Samples of sedimentary rock in the breccia are surrounded by layers of quartz, indicating the presence of a silica-rich fluid. Calcite is visible crystallizing on this quartz, indicating that a carbonate-rich fluid later flowed through the outcrop. Analysis of sedimentary rock suggests that it is quartzite, which is likely a part of the New Haven sandstone that was altered by fluids and high temperatures. These relationships indicate a diverse hydrothermal alteration history.