The Molloy Oceanic Core Complex: Petrological Studies of Mantle Rocks

Abstract

Seafloor spreading at ultraslow spreading rates causes uplift of the underlying crustal structure and mantle, forming oceanic core complexes (OCC). At the edge of the perennial Arctic sea ice cover, the Fram Strait forms a narrow pathway between Svalbard and Greenland where the Eurasian and North American plates are spreading apart. Here, in a region of incipient seafloor spreading on the oceanic margin, tectonic extension is exhuming the mantle as an OCC at the ultraslow-spreading Molloy Ridge. This ridge is notable for its short length (~60 km), unique tectonic setting, ultraslow spreading rates (<10 mm yr-1), and the resulting extensive uplift of mantle peridotites, offering valuable insights into the mantle composition and accretion processes at ultraslow-spreading segments. The lack of evidence of volcanic activity suggests tectonic spreading along the Molloy Ridge, likely focused on the eastern side of the OCC.

This thesis presents findings derived from geochemical and petrographic analyses of 25 peridotites sampled from three locations on the Molloy OCC to understand the composition of the Molloy mantle, the petrology of the OCC, and the spreading type. Analyses of bulk rock and relict orthopyroxenes, clinopyroxenes, and Cr-spinels show that 1) despite its proximity to Svalbard and Greenland, the Molloy mantle is primarily suboceanic rather than subcontinental, 2) its lherzolitic composition shows that it has experienced low degrees of partial melting, and 3) the mantle is enriched in both Al2O3 and REE compared to the Mohns and Knipovich Ridges.

The enriched composition of the Molloy mantle suggests post-depletion refertilization by fragments of subcontinental lithospheric mantle from the nearby continents, indicating a comparable geological context to the Lena Trough to the north. Melt segregation and melt-rock interaction are suggested to influence the geochemical composition of the peridotites, clinopyroxenes, and Cr-spinels of the investigated samples from the shear zone.

These findings indicate that many factors, including the interaction between suboceanic and subcontinental mantle during the later stages of continental rifting, influence the petrology of mantle rocks in the Fram Strait. Moreover, the findings support the theorized importance of the emplacement of mantle onto the seafloor as the principal mechanism for the accretion of oceanic crust along non-volcanic ultraslow-spreading ridges and thus contribute to the broader understanding of geological processes in these unique tectonic settings.