Sill and lava geochemistry of the mid‐Norway and NE Greenland conjugate margins

Abstract

[1] This paper presents major, trace-elements, and Sr-Nd isotopes for two prominent sills formed during the opening of the North Atlantic, sampled by the Utgard borehole (6607/5-2) in the Vïring Plateau. The Utgard sills are compared to opening-related lavas recovered from ODP Leg 104 Hole 642E farther west on the Vïring Plateau and on the NE Greenland conjugate margin. The Utgard sills (3.6-5.9 wt % MgO) are enriched in strongly relative to moderately incompatible trace elements and have 87 Sr/ 86 Sr and 143 Nd/ 144 Nd ratios of 0.70380-70387 and 0.51292-0.51293, respectively, in the Upper Utgard Sill, and 0.70303-0.70306 and 0.51297-0.51299 in the Lower Sill. Alteration is minor. The Utgard melts originated by partial melting of an asthenospheric, depleted mantle source (DMM or Iceland Rift Zone, IRZ, type) with chemical characteristics similar to the source that gave rise to NE Greenland lavas. The Utgard magmas underwent extensive fractional crystallization in the lower crust (Upper Sill: >70%; Lower Sill: >55%) with removal mainly of olivine and pyroxenes, accompanied by 1% assimilation of crustal melts. This crystallization formed significant masses of dense cumulates ($3.25 g/cm 3 ) (underplating). Assuming an areal extent similar to that of the two sills, we estimate a composite layer of ultramafic cumulates mixed with less dense country rocks to be >320 m thick beneath the two Utgard sills and >8.8 km beneath the thickest part of the Vïring Plateau lavas. Opening-related cumulates may thus account for a significant part of the lower crustal high-velocity, high-density bodies (average density 3.1 g/cm 3 ) along the Norwegian margin.

The T‐Reflection and the Deep Crustal Structure of the Vøring Margin, Offshore mid‐Norway

“…In the Vøring Basin, exploration well 6607/5‐2 at the Utgard High penetrated two microgabbro sills intruded into Upper Cretaceous mudstone and sandstone lithologies (Neumann et al, ). Sonic logs from sills show an average velocity of 7 km/s (Planke et al, , ).…”

“…Geochemical analyses of the Utgard High sills demonstrate that the LCB can be explained as a heterogeneous mixture of cumulates associated with the opening related magmatism and less dense rocks such as old continental basement (Neumann et al, ). In this context, the “cumulate” could be defined as a rock that forms via partial crystallization of a melt, after which the remaining melt is removed from the system.…”

Deep structure of the Lofoten‐Vesterålen segment of the Mid‐Norwegian continental margin and adjacent areas derived from 3‐D density modeling

“…Moreover, the Utgard High already affected the deformation pattern, and therefore, formation occurred much earlier than the Eocene breakup. Therefore, the presence of this central horst is most likely related to inherited high‐density Caledonian basement which could have been partly intruded by breakup‐induced intrusions [e.g., Neumann et al ., ].…”

Pre-breakup magmatism on the Vøring Margin: Insight from new sub-basalt imaging and results from Ocean Drilling Program Hole 642E

“…The sill geochemistry (Neumann et al, 2013) shows that these magmas are significantly more differentiated compared to the already fractionated Vøring SDR melts Westerhold et al, 2008;or 55.53 Ma after Westerhold et al, 2012) and is more likely early Eocene in age.…”