Michael Tubrett

Identification of a subduction zone component in the Higganum dike, Central Atlantic Magmatic Province: A LA‐ICPMS study of clinopyroxene with implications for flood basalt petrogenesis

Determining the primary compositions of continental flood basalts has proven allusive because of the masking effects of crustal contamination. The Higganum dike (Connecticut, USA) that fed the Talcott basalt of the Central Atlantic Magmatic Province hosts strongly zoned clinopyroxenes. The cores are Mg- and Cr-rich, containing up to 1 wt % Cr2O3. The grains show optical and major, minor, and trace element continua between cores and their mantles and rims, indicating that the cores are early crystallizing phenocrysts, not xenocrysts. Laser ablation inductively coupled plasma mass spectrometry analyses of the cores were used to calculate liquid compositions of the most primitive component in the Higganum system and to make inferences about composition of continental flood basalt parental magmas. The Cr-rich cores yield calculated liquids with Mg numbers of 63. The high-Cr contents of these calculated liquids indicate the absence of significant clinoproxene or chromite fractionation. Olivine or orthopyroxene fractionation may have lowered the liquid Mg number but would not have significantly influenced the calculated incompatible trace element patterns. Extended rare earth element diagrams for the most Cr-rich liquids show enrichment in incompatible elements, similar to arc basalts. Prominent negative Nb anomalies are present as are distinct positive Pb anomalies indicative of the influence of subduction zone fluids in the mantle source. Calculated liquid Ba/La values range from 3 to 30, also suggestive of fluid input. These data indicate that the mantle source experienced subduction zone fluid metasomatism. Subsequent crustal contamination is evident in bulk-rock Nd and Sr isotopic compositions.

Age and provenance of a Paleoproterozoic to Devonian Canadian Cordilleran sequence of metasedimentary rocks, Thor–Odin dome, southeastern British Columbia

We present a U-Pb detrital zircon age and provenance study of a sequence of metasedimentary rocks in the northwestern Thor–Odin high-grade gneiss dome within the Omineca crystalline belt of the Canadian Cordillera. Despite strong overprint by deformation and metamorphism, we successfully analyzed the age and provenance of six samples collected at various structural levels, using U-Pb detrital zircon laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) analysis. The Thor–Odin dome consists of Paleoproterozoic basement and a metasedimentary cover sequence of previously unknown age and tectonic significance. Our results indicate that the oldest units of this sequence may be Paleoproterozoic, and some of the oldest known metasedimentary rocks in the Canadian Cordillera, originally deposited on top of Laurentian basement rocks. The youngest rocks are Devonian and deposited shortly before the onset of widespread Late Devonian to early Mississippian igneous activity in the Selkirk Domain or Kootenay arc. The cover sequence of the Thor–Odin dome thus preserves some of both the oldest and the youngest (meta)sedimentary rocks deposited between the formation of supercontinent Columbia and the onset of igneous activity and convergence that marked the beginning of Cordilleran deformation and metamorphism. Parts of as many as ∼1.4 b.y. of sedimentary history are preserved in the Thor–Odin dome, implying that much information on the sedimentary history of the Canadian Cordillera may be hidden in other Cordilleran gneiss domes.