Transport of mafic magma through the crust and sedimentary basins: Jameson Land, East Greenland

Igneous sheet-complexes transport magma through the crust, but most studies have focused on single segments of the magma-transport-system or have low resolution. In the Jameson Land Basin in East Greenland, reflection-seismic data and extensive outcrops give unparalleled constraints on mafic intrusions down to 15 km. This dataset shows how sill-complexes develop and how magma is transported from the mantle through sedimentary basins. The feeder zone of the sill-complex is a narrow zone below basin, where a magmatic underplate body impinges on thinned crust. Magma was transported through the crystalline crust through dykes. Seismic data and published geochemistry indicate magma was supplied from a magmatic underplate, without perceptible storage in crustal magma-chambers and crustal assimilation. As magma entered the sedimentary basin, it formed distributed, bowl-shaped sill-complexes throughout the basin. Large magma volumes in sills (4-20 times larger than the Skaergaard Intrusion), and few dykes highlight the importance of sills in crustal magma-transport. On scales smaller than 0.2 km, host-rock lithology, and particularly mudstone tensile strength-anisotropy, controls sill-architecture in the upper 10km of the basin, whereas sills are bowl-shaped below the brittle-ductile transition zone. On scales of kilometres and towards basin margins, tectonic stresses and lateral lithological changes dominate architecture of sills.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5670470

Solidification timescale for the Dufek Intrusion, Antarctica determined by U-Pb zircon ages

<p>The 8-9 km thick Dufek layered mafic intrusion of Antarctica was emplaced at approximately 182 Ma associated with the Ferrar dolerites and the breakup of the supercontinent Gondwana.  It is rivaled in thickness only by the Bushveld Complex of South Africa and shows a similar progression in mineral compositions all the way to the uppermost contact with an overlying granophyre layer.  This progression in mineral composition suggests that it crystallized from the bottom to the top and did not form an upper solidification front (a.k.a., Upper Border Series) typical of smaller intrusions such as the Skaergaard Intrusion.  Unlike the Bushveld Complex, however, the Dufek Intrusion is exposed in only two ~1.8 km thick sections: the lowermost Dufek Massif, and the uppermost Forrestal Range, which are separated from one another by a ~50km wide snowfield.  The remainder of the stratigraphy is inferred from geophysics, evolution of mineral compositions, and projection of the dip of the layering through the snowfield. </p><p> </p><p>            We obtained precise CA-ID-TIMS U-Pb zircon ages from samples from the Dufek Massif and Forrestal Range in order to determine the timescale of solidification of a large layered mafic intrusion.  What we found is surprising - zircons from the bottom of the intrusion record younger ages than those from the top of the intrusion.  Two samples from the Dufek Massif have zircon U-Pb ages of 182.441±0.048 Ma and 182.496±0.057 Ma; whereas three samples from the Forrestal Range have zircon U-Pb ages of 182.601±0.064 Ma, 182.660±0.10 Ma, 182.78±0.21 Ma.  Thus, the lower section of the Dufek Intrusion solidified approximately 160,000 years after the upper.  We explore two possibilities for this reverse-age stratigraphy, (1) that the ages reflect the solidification of interstitial melt in a single magma chamber cooling from the top down, or (2) that the Dufek Massif and Forrestal Range are two separate magma chambers that are not connected at depth.  Our results have implications for the stratigraphic thickness estimates of the Dufek Intrusion as well as the duration of magmatism associated with continental breakup.</p><p> </p><p> </p>

Bulk Liquid for the Skaergaard Intrusion and Its PGE-Au Mineralization: Composition, Correlation, Liquid Line of Descent, and Timing of Sulphide Saturation and Silicate–Silicate Immiscibility

Preferred and modelled bulk composition of the Skaergaard intrusion are compared to coeval basaltic compositions in East Greenland and found to relate to the second evolved cycle of Geikie Plateau Formation lavas and coeval Skaergaard-like dikes in major and trace element (Mg# ∼45, Ce/Nb ∼2·5, (Dy/Yb)N ∼1·35), and precious metal composition (Pd/Pt ∼3, Au/Pt ∼2) as well as in age (∼56 Ma). Successful comparisons of precious metal compositions only occur with Skaergaard models based on mass balance. The bulk liquid of the intrusion evolved along the liquid line of descent to immiscibility between Si- and Fe-rich silicate liquids after ∼90% of crystallization (F = ∼0·10) in agreement with experimental constraints. Immiscibility led to accumulation and fractionation of the Fe-rich silicate melt in the mushy floor of the intrusion and continued accumulation of granophyre component in the remaining bulk liquid. The composition of plagioclase in the precious metal mineralized gabbro and modelling of Pd/Pt and Au/Pt in first formed droplets of sulphide melt suggest that sulphide saturation was reached in interstitial melts in crystal mushes in the floor and roof and in bulk liquid with a composition equivalent to that of the bulk liquid at lower UZa times and after crystallization of 82–85% of the bulk liquid (F = 0·19–0·16). Prior to sulphide saturation in UZa type melt, the precious metals ratios of the bulk liquid were controlled by the loss of Pt relative to Pd and Au in agreement with the low empirical and experimental solubility of Pt of ∼9ppb compared to a much higher value for Pd and Au. The relative timing between sulphide saturation (F = ∼0·18) and immiscibility between silicate melts (F = ∼0·10) and modelled precious metal ratios underpin the proposed multi-stage model for the mineralization, advocating initial accumulation in the mushy floor of the magma chamber controlled by sulphide saturation in mush melts rather than bulk melt, followed by redistribution of precious metals in a macro-rhythmic succession of gabbroic layers of the upward migrating crystallization zone.

LAWRENCE RICKARD WAGER (1904–1965): A DISTINGUISHED GEOLOGIST WHO HELPED TO PIONEER AERIAL PHOTOGRAPHIC INTERPRETATION FOR ALLIED FORCES IN WORLD WAR II

ABSTRACT ‘Bill’ Wager, after undergraduate and postgraduate studies at the University of Cambridge, became a lecturer at the University of Reading in southern England in 1929. He was granted leave in the 1930s to participate in lengthy expeditions that explored the geology of Greenland, an island largely within the Arctic Circle. With friends made on those expeditions, he became in June 1940 an early recruit to the Photographic Development Unit of the Royal Air Force that pioneered the development of aerial photographic interpretation for British armed forces. He was quickly appointed to lead a ‘shift’ of interpreters. The unit moved in 1941 from Wembley in London to Danesfield House in Buckinghamshire, known as Royal Air Force Medmenham, to become the Central Interpretation Unit for Allied forces—a ‘secret’ military intelligence unit that contributed significantly to Allied victory in World War II. There Wager led one of three ‘shifts’ that carried out the ‘Second Phase’ studies in a three-phase programme of interpretation that became a standard operating procedure. Promoted in 1941 to the rank of squadron leader in the Royal Air Force Volunteer Reserve, he was given command of all ‘Second Phase’ work. Sent with a detachment of photographic interpreters to the Soviet Union in 1942, he was officially ‘mentioned in a Despatch’ on return to England. By the end of 1943 the Central Interpretation Unit had developed into a large organization with an experienced staff, so Wager was allowed to leave Medmenham in order to become Professor of Geology in the University of Durham. He resigned his commission in July 1944. Appointed Professor of Geology in the University of Oxford in 1950, he died prematurely from a heart attack in 1965, best remembered for his work on the igneous rocks of the Skaergaard intrusion in Greenland and an attempt to climb Mount Everest in 1933.