Palaeomagnetism, rock magnetism and geochemistry of Jurassic dykes and correlative redbeds, Massachusetts, USA

Abstract

Jurassic diabase dykes, sills and sedimentary rocks in central Massachusetts were sampled for palaeomagnetic analysis. The intrusions fall into three of the chemical types for eastern North American diabases: high TiO2 quartz‐normative (Holden); low TiO2 quartz‐normative (Ware); and high Fe2O3 quartz‐normative (Pelham–Loudville). The characteristic magnetizations in the majority of intrusive samples unblock between 550 °C and 580 °C, with Curie temperatures in a discrete interval between 556 °C and 580 °C. The dominant remanence in the diabases is carried by C1 to C3 oxidation‐exsolved titanomagnetite occurring as euhedral grains, as fine needles or dust in the matrix, as devitrifed glass, and as fine magnetite‐ilmenite‐silicate symplectite. In some dykes, titanomagnetite was further modified by deuteric oxidation during post‐magmatic cooling, creating titanomaghematite and/or a granulation of the magnetite. Palaeopoles for the three diabase groups are: Holden, 60.1°N, 80.5°E, A95 = 4.1°; Ware, 73.5°N, 85.8°E, A95 = 3.9°; and Pelham–Loudville, 65.3°N, 95.6°E, A95 = 4.1°. These data are combined with samples from two stratigraphic sections through the Early Jurassic part of the Sugarloaf Formation in the Deerfield Basin representing both fine‐grained mudstones and coarser arkoses. These haematite‐dominated rocks reveal several components of magnetization, a steep recent field direction, an intermediate secondary diagenetic overprint direction in both mudstones and arkoses, and a high‐temperature shallow primary direction found only in the mudstones. Palaeopoles for the Sugarloaf Formation are: mudstones, 57.7°N, 81.3°E, A95 = 9.1°; and arkoses, 75.1°N, 131.6°E, A95 = 5.9°. Based on the new palaeomagnetic data reported here, the North American plate in the Middle Jurassic was at higher palaeolatitudes than indicated by the present North American apparent polar wander path.

Paleomagnetism and magnetostratigraphy of the lower Glen Canyon and upper Chinle Groups, Jurassic‐Triassic of northern Arizona and northeast Utah

“…This would imply instantaneous emplacement of these lavas across the entire volcanic province. Prèvot and McWilliams argument for a short‐lived event is based on the absence of “anomalous” directions in intrusive suites of the Newark Supergroup, but recently, McEnroe and Brown [2000] report distinct directions for dike subsets of Early Jurassic age in New England and Kodama et al [1994] report a pole similar to that of the intermediate volcanic flows for sediments baked by a diabase intrusion in the Culpeper basin. The high dispersion of earliest Jurassic poles is well documented in North America, and is also evident in Europe [ Sichler and Perrin , 1993].…”

“…The high dispersion of earliest Jurassic poles is well documented in North America, and is also evident in Europe [ Sichler and Perrin , 1993]. As in the case of the Newark extrusive series, dike subsets reported by McEnroe and Brown [2000] are grouped according to their geochemistry. By inference, they are interpreted as temporally distinct groups.…”

Early Jurassic magnetostratigraphy and paleolatitudes from the Hartford continental rift basin (eastern North America): Testing for polarity bias and abrupt polar wander in association with the central Atlantic magmatic province

“…Normal and reverse polarity directions were reported by DuBois [1957], but to our knowledge, there has been no follow‐up paleomagnetic work on sediments of the Hartford basin until the present study. From the contiguous Deerfield basin, paleomagnetic results from two sedimentary sites (from the Fall River Formation, a unit correlative to the Shuttle Meadow Formation of the Hartford basin) were reported by McEnroe and Brown [2000].…”

Asymmetrical magnetic fabrics in the Egersund doleritic dike swarm (SW Norway) reveal sinistral oblique rifting before the opening of the Iapetus

“…It usually contains lamellae of ilmenite resulting from an oxidationexsolution (oxy-exsolution) phenomenon (Fig. 3a, b;McEnroe and Brown, 2000;Dunlop and € Ozdemir, 2009). Ilmenite forms both interstitial subhedral grains and amoeboid to very skeletal ( Fig.…”

“…The latter secondary fine-grained opaque is likely maghemite, a classic alteration product of magnetite in doleritic dikes (e.g. McEnroe and Brown, 2000;Silva et al, 2010).…”