HYDROTHERMAL ALTERATION OF BRECCIAS AT THE CENTRAL UPLIFT, CHESAPEAKE BAY IMPACT STRUCTURE

An 823-m deep U.S. Geological Survey test hole on the central uplift of the late Eocene Chesapeake Bay impact structure at Cape Charles, Va., yielded Exmore sediment-clast breccia from 355 to 655 m and suevitic crystalline-clast breccia containing brecciated gneiss megablocks below 655 m.  Core samples of suevitic breccia and gneiss blocks from 744 to 823 m depth were altered to a hydrothermal assemblage of albite, chlorite, white mica, quartz, and calcite.  Some chlorite is a pseudomorph of biotite, and strained calcite is overgrown by unstrained sparry calcite.  Survival of a shock-induced TiO₂ II phase suggests post-shock temperatures ≤550°C.  Fission-track data from a gneiss megablock indicate cooling through apatite closure at 37 ± 10 Ma (2σ).  Late-stage veins contain sparry calcite, smaller amounts of chlorite, quartz, and pyrite, and trace amounts of a Ca-REE carbonate, barite, and rutile with growth zones containing up to 2.3 weight percent WO₃; pyrite contains veins and inclusions of galena and chalcopyrite.  Homogenization temperatures (Th) of fluid inclusions in sparry calcite from a post-impact vein within a clast of impact-melt rock from the suevitic breccia range from 114°C to 257°C and have well-preserved fluid-inclusion associations up to 235°C; thus, hydrothermal fluids associated with the calcite vein certainly reached the boiling point of seawater (~220°C at 300 m water depth).  The ice-melting temperature (Tm) range is -1.1°C to -11.5°C, with an average from -1.8°C to -3.1°C, which indicates that paleosalinities of most aqueous inclusions approximate that of seawater (Tm of ice = -2.0°C), with some being more dilute and others more saline.  The coexistence of very saline fluid inclusions with fluid inclusions that homogenize to vapor suggests boiling.  In contrast, core samples from the overlying sediment-clast breccia at 427-433 m depth contain the authigenic mineral assemblage of illite, smectite having ≤20% interlayered illite, kaolinite, high-Fe chlorite, quartz, siderite, calcite, and pyrite, which, together with fission-track data, suggests only ~100°C maximum post-impact temperature at that level.  Fission-track ages from 428 m depth indicate cooling through zircon closure (~235°C) at 294 ± 46 Ma (2σ) and through apatite closure (~100°C) at 135 ± 25 Ma (2σ) without Eocene resetting.