Abstract

Paleomagnetic analyses of the Nahant gabbro and an associated tonalite of eastern Massachusetts reveal five separate populations of paleomagnetic directions. The gabbro and tonalite, exposed in the northernmost portion of the Boston Basin, were selected for this study because of the lack of significant macroscopic evidence of penetrative metamorphism or deformation. The tonalite has not been radiometrically dated; however, an Ordovician date has been reported for the gabbro. Detailed petrographic data indicate that the five populations of paleomagnetic directions obtained for the gabbro and tonalite correspond to five styles of magnetic mineral alteration.A mean alteration index was calculated for each site in the Nahant gabbro and tonalite. There is an apparent relationship between magnetite alteration and normalized intensity at a given demagnetization level, with normalized intensity decreasing as magnetite alteration increases. This relationship illustrates the interdependence between paleomagnetic properties and magnetic minerals. Alteration of these minerals is probably associated with periods of magnetic overprinting.Autometasomatism is believed to have occurred in the gabbro. Fluid flushing initiated plagioclase decomposition, liberating Ca + Al + Si to form Al-rich (4–10 wt.% Al2O3) titanite (sphene, CaTiSiO5). Whether the metasomatism was deuteric or the result of postdeuteric hydrothermal activity has not been determined.The Nahant gabbro poles are interpreted as representing either a deuteric or a metasomatic chemical remanent magnetization (CRM). The tonalite pole is interpreted as a deuteric CRM. The gabbro and tonalite poles are similar to a number of poles from Ordovician to Devonian intrusives in the Acadia terrane, including the St. Stephen and St. George plutons.Petrographic examination of polished sections from the Devonian St. Stephen and St. George plutons reveals evidence for alteration of primary magnetic mineral assemblages. Similarities exist between these alterations and alterations observed in the Nahant suite, most notably, the formation of titanite from Fe–Ti oxides. The repeated determination of similar poles on intrusive rocks in the Acadia terrane suggests there may be some tectonic or geomagnetic significance to poles from intrusives; however, without more data pertaining to the timing of metasomatic events observed in the Nahant suite and the St. Stephen and St. George plutons, the age and significance of these poles cannot be interpreted with certainty.

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