ABSTRACT. Avalonian arc volcanism around Boston, Massachusetts has mainly been recognized in caldera-related ash-flow tuffs of the 602-593 Ma Lynn-Mattapan Volcanic Complex. A new ID-TIMS U-Pb zircon date of 596.6 + 0.9 Ma (weighted mean ²⁰⁶Pb/²³⁸U age of four concordant single grain analyses) from volcanic ash at Worlds End in Hingham, MA now demonstrates that pyroclastic eruptions on land were accompanied by submarine eruptions of basalt in Mattapan time.

The dated ash contains 66.98 weight % SiO₂ and shows Zr/Ti and Nb/Y ratios linking it geochemically with ca. 593 Ma High Rock tuff of the Mattapan Volcanic Complex in Needham, MA. Physical links between subaerial and subaqueous volcanism are found in spectacular contact relationships at Worlds End. Here pale green ash settled to form discontinuous, laminated horizons draped over a submarine basaltic lava flow. The flow was repeatedly disrupted by bursting pillows and spalling blocks so that ash in some places is engulfed by chilled basalt, while elsewhere basalt forms breccia blocks in the ash. Wet ash also exploded in contact with hot lava to shoot dike-like stringers and ash-filled vesicles through neighboring basalt. Apart from ash and basalt, the section at Worlds End is composed of volcanic breccia and volcaniclastic conglomerate. Clasts include banded rhyolite, welded tuff and basaltic to andesitic varieties (identified via bulk geochemistry) indicating diverse earlier volcanic episodes as well. Granite clasts derived in part from underlying Dedham Granite (dating in progress) are also present, but quartzite clasts typical of conglomerate farther north in the Boston Basin are conspicuously absent.

The Worlds End sequence has traditionally been construed as an example of "Brighton" volcanic interbeds at the base of the "Roxbury" Conglomerate which together have been intepreted in terms of rift- or wrench-related tectonism late in the Avalonian cycle. This study, however, makes it clear that these particular basalts and associated conglomerates formed during the height of Avalonian magmatism in southeastern New England. Preliminary paleomagnetic results from the basalt are similar to directions previously obtained from the High Rock tuff suggesting that the Avalonian magmatic arc may have been located at low-to-mid latitudes at ~595 Ma.