Abstract

The main event in the diagenetic history of the Hartford basin was the arrival of the rift-associated "heat wave" to its arkosic fill, in the form of both high heat flow and diabase dikes, sills, and basalt flows about 187 + or - 3 Ma. Pre-heat-wave (or pre-basalt) diagenetic minerals are widespread throughout the basin, and include hematite cement, quartz and albite overgrowths, and minor euhedral rutile. This widespread distribution suggests that they grew in a regime of generally downward-migrating meteoric water. The silica, aluminum, and sodium needed to make these cements were released probably by tropical weathering of the top of the arkose itself, a few hundred meters overhead. The heat wave of which basalts and dikes were part suddenly heated the basin, and radically determined the subsequent diagenesis of the arkoses: (1) it drove pore-water convection through sills and dikes and arkoses, erasing the earlier meteoric-water regime; (2) it caused quick growth of post-basalt diagenetic minerals (chert and mosaic albite cement, illite and chlorite cement, and fibrous laumontite cement with bipyramidal quartz euhedra) mostly at localized occurrences, the "updrafts" of pore-water convection cells; (3) it modified the texture of both quartz and albite cements from overgrowth to microcrystalline; and (4) the dikes, sills, and basalt flows probably provided much of the magnesium, ferrous iron, potassium, and copper needed to make the illite + chlorite cement and copper sulfide nodules found in the arkoses. The radiogenic age of the illite cement from one locality is 180 (+ or - 10) Ma, close to the age of the basalts and dikes. This temporal proximity warrants linking the growth of illite to the sudden heating and the igneous intrusion. High 18 O content of the diagenetic illite suggests an origin from relatively heavy water that could have been produced in surficial evaporative environments of the rift basin, and that later sank. Combined petrographic, K/Ar, geochemical, fission-track, and dynamic-modeling evidence and interpretations lead to a generalized time-temperature profile for the basin. Because of their distinctive tectono-thermal origin, rift basins probably have a distinctive diagenetic history. The diagenesis and dynamics sketched here for the Hartford rift basin, including some of the unusual mineralogical and textural details in its arkoses, may apply to other rift basins.

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