20 Mesozoic Magmatism, Postrift Tectonics, and Passive Margin Development of Northeastern North America
The northeastern region experienced protracted Mesozoic (Triassic-Cretaceous) magmatism associated with continental rifting and subsequent passive margin development. We welcome contributions related to the occurrence, tectonics, or driving mechanisms of these syn- and postrift events.
Monday, 23 March 2026
8:00 AM - 12:00 PM
CCC, Room 27
Heterogeneity and varying degrees of enrichment and depletion of the sub-continental lithospheric mantle of New England as documented by lamprophyres and spinel-bearing mantle xenoliths
Paper number: 20-8
Program: Technical Sessions
Time: 10:20 AM – 10:40 AM
Authors
Abstract
The character of the subcontinental lithospheric mantle (SCLM) plays an important role in generating magma at all stages from the collisional to the rifting setting and beyond. Evidence for enrichment and refertilization of the New England SCLM The Westerly, Rhode Island, lamprophyre dikes differ from the majority of other mafic dikes in New England in their composition, but more critically in the entrainment of large numbers of mantle xenoliths that range from lherzolite, wehrlite, and harzburgite; all of them are spinel bearing as well. These monchiquite-type lamprophyres intrude into the Permian Narragansett Pier and Westerly Granites at around 175 mya based on an older K-Ar date (Hermes et al., 1984). These rocks are thus ideal for understanding the SCLM during the transition from continental rifting to oceanic rifting. Bulk rock geochemistry of the lamprophyres and xenoliths indicate differences in trace element abundances and patterns. The lamprophyres in particular show more significant signs of enrichment in incompatible elements. Sr-Nd-Pb isotopic data also indicates that the lamprophyres, lherzolites, and wehrlites are different as well. Both the lamprophyre and wehrlite show a lower 87Sr/86Sr and higher 206Pb/204Pb (and 207Pb as well) than lherzolite, more indicative of a high μ system. Olivine, orthopyroxene, clinopyroxene, and spinel mineral trace element patterns also show significant differences between them from the different rock types. Some of those minerals present in the lamprophyre are similar to the xenolith ones, which reflects them being xenocrysts. Geothermobarometers indicate a more narrow range regardless of the sample type. Modelling suggests that no amount of partial melting or fractional crystallization can produce any genetic relationships between the lamprophyres or any of their included xenoliths. This suggests that all three of these rock types define a highly variable SCLM that has been fertilized in some places and depleted in others with those effects being present after continental rifting has occurred.
Geological Society of America Abstracts with Programs. Vol. 58, No. 2
doi: 10.1130/abs/2026NE-411491
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