DIAGENETIC MINERALOGY, GEOCHEMISTRY, AND DYNAMICS OF MESOZOIC ARKOSES, HARTFORD RIFT BASIN, CONNECTICUT, USA

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 h ows about 187 +/- 3 Ma. Pre-heat-wave (or pre-basalt) diagenetic miner als are widespread throughout the basin, and include hematite cement, quartz and albite overgrowths, and minor euhedral rutile. This widespr ead distribution suggests that they grew in a regime of generally down ward migrating meteoric water. The silica, aluminum, and sodium needed to make these cements were released probably by tropical weathering o f the top of the arkose itself, a few hundred meters overhead. The hea t 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, il lite and chlorite cement, and fibrous laumantite cement with bipyramid al quartz euhedra) mostly at localized occurrences, the ''updrafts'' o f pore-water convection cells; (3) it modified the texture of both qua rtz and albite cements from overgrowth to microcrystalline; and (4) th e dikes, sills, and basalt hows probably provided much of the magnesiu m, ferrous iran, potassium, and copper needed to make the illite + chl orite cement and copper sulfide nodules found in the arkoses. The radi ogenic age of the illite cement from one locality is 180 (+/- 10) Ma, close to the age of the basalts and dikes. This temporal proximity war rants linking the growth of illite to the sudden heating and the igneo us intrusion. High O-18 content of tbe diagenetic illite suggests an o rigin from relatively heavy water that could have been produced in sur ficial 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-temp erature pro file for the basin. Because of their distinctive tectono-t hermal origin, rift basins probably have a distinctive diagenetic hist ory. The diagenesis and dynamics sketched here for the Hartford rift b asin, including some of the unusual mineralogical and textural details in its arkoses, may apply to other rift basins.