Sebastian Tappe

Craton formation in Late Archean subduction zones revealed by first Greenland eclogites

It is now well established that the early continental crust was formed by melting of basaltic lithologies such as amphibolite and eclogite. However, considerable uncertainty surrounds the geologic environment in which melting took place. Commonly invoked options range between melting at the underside of oceanic plateaus above mantle plumes or melting of oceanic lithosphere during shallow subduction. Distinguishing between these scenarios has important implications for the early evolution of continents. We use the first eclogites discovered from the North Atlantic craton (NAC) to constrain the formation of the deep root to this continent. Late Archean eclogite xenoliths (2.7 ± 0.3 Ga) from a kimberlite in West Greenland are broadly coeval with a major regional episode of tonalite-trondhjemite-granodiorite (TTG) magmatism. Major and trace element systematics of the eclogites reveal a highly refractory character that is mirrored by NAC peridotites. Moreover, the refractory eclogites define a complementary relationship to the Late Archean TTG granitoids from the NAC, and their elevated garnet δ 18 O values along with negative Eu anomalies suggest seafloor-altered oceanic crust as the most viable eclogite protolith. These results from Greenland provide strong support for a model in which early continental crust grew by melting of basaltic slabs in subduction zones, where tectonic stacking of down-going oceanic lithosphere provided the mechanism that coupled formation of cratonic crust and mantle.

Late Cretaceous–Palaeocene continental rifting in the High Arctic: U–Pb geochronology of the Kap Washington Group volcanic sequence, North Greenland

Abstract: The Kap Washington Group volcanic sequence is exposed on the north coast of Greenland. The sequence was erupted in a continental rift setting during the opening of the Arctic Ocean and provides important geological constraints on the timing of this event. In this study we present the first isotope dilution thermal ionization mass spectrometry U–Pb zircon ages from 10 samples of trachytic to rhyolitic composition. Samples from Lockwood Island (n = 4) gave concordant ages between 67.2 ± 0.5 and 61.0 ± 0.3 Ma (2σ). Lavas from Kap Kane (n = 5) yielded concordant ages between 70.5 ± 0.3 and 69.7 ± 0.2 Ma. An ignimbrite from the Kap Washington peninsula yielded a concordant age of 68.5 ± 0.3 Ma. The Kap Washington Group sequence and similar occurrences on Ellesmere Island record a prolonged period of continental rift volcanism lasting from 92 to 58 Ma. We propose that this volcanism occurred along the margins of a nascent Eurasia Basin undergoing east–west extension linked to rifting in the Labrador Sea–Baffin Bay system.

A Multidisciplinary Approach to the Attawapiskat Kimberlite Field, Canada: Accelerating the Discovery-to-Production Pipeline

Victor Mine is one of 21 known kimberlites within the Attawapiskat kimberlite field, Ontario, Canada. Victor was discovered in 1988 and commercial production began in 2008. In 2008, it was identified that subsequent resource evaluation programs targeting satellite kimberlites within the Attawapiskat kimberlite field required effective integration of multidisciplinary data to identify areas of high prospectivity within the field and identify high-interest pipes early in the exploration pipeline. Systematic relationships were revealed between diamond data and various other data sets (petrography, whole-rock major and trace element compositions, mineral trace element compositions, geophysics, and volcanology) both within the kimberlite field and within individual pipes. The most valuable datasets were identified, gaps in knowledge were determined, and economically relevant projects were formulated. One such project has revealed new emplacement ages for Victor and Uniform kimberlites, which suggest that kimberlite magmas erupted within a relatively narrow time span between ~180 and 170 Ma in the Attawapiskat kimberlite field. The integration of large and complex datasets and the communication between investigators from different fields within the geosciences is highly beneficial to accelerate the discovery-to-production pipeline in the diamond industry.

Volatile-rich Metasomatism in the Cratonic Mantle beneath SW Greenland: Link to Kimberlites and Mid-lithospheric Discontinuities

The cratonic part of Greenland has been a hotspot of scientific investigation since the discovery of some of the oldest crust on Earth and of significant diamond potential in the underlying lithospheric mantle, the characterization of which remains, however, incomplete. We applied a detailed petrographic and in situ analytical approach to a new suite of fresh kimberlite-borne peridotite xenoliths, recovered from the North Atlantic craton in SW Greenland, to unravel the timing and nature of mantle metasomatism, and its link to the formation of low-volume melts (e.g. kimberlites) and to geophysically detectible discontinuities. Two types of mineralogies and metasomatic styles, occurring at two depth intervals, are recognized. The first type comprises lherzolites, harzburgites and dunites, some phlogopite-bearing, which occur from 100–170 km depth. They form continuous trends towards lower mineral Mg# at increasing TiO2, MnO and Na2O and decreasing NiO contents. These systematics are ascribed to metasomatism by a hydrous silicate melt precursor to c. 150 Ma kimberlites, in the course of rifting, decompression and lithosphere thinning. This metasomatism was accompanied by progressive garnet breakdown, texturally evident by pyroxene–spinel assemblages occupying former coarse grains and compositionally evident by increasing concentrations of elements that are compatible in garnet (Y, Sc, In, heavy rare earth elements) in newly formed clinopyroxene. Concomitant sulphide saturation is indicated by depletion in Cu, Ni and Co. The residual, more silica-undersaturated and potentially more oxidizing melts percolated upwards and metasomatized the shallower lithospheric mantle, which is composed of phlogopitebearing, texturally equilibrated peridotites, including wehrlites, showing evidence for recent pyroxene-breakdown. This is the second type of lithology, which occurs at 90–110 km depth and is inferred to have highly depleted protoliths. This type is compositionally distinct from lherzolites, with olivine having higher Ca/Al, but lower Al and V contents. Whereas low Al may in part reflect lower equilibration temperatures, low V is ascribed to a combination of intrinsically more oxidizing mantle at lower pressure and oxidative metasomatism. The intense metasomatism in the shallow cratonic mantle lithosphere contrasts with the strong depletion recorded in the northwestern part of the craton, which at 590–550 Ma extended to >210 km depth, and suggests loss of 40 km of lithospheric mantle, also recorded in the progressive shallowing of magma sources during the breakup of the North Atlantic craton. The concentration of phlogopite-rich lithologies in a narrow depth interval ( 90–110 km) overlaps with a negative seismic velocity gradient that is interpreted as a mid-lithospheric discontinuity beneath western Greenland. This is suggested to be a manifestation of small-volume volatile-rich magmatism, which paved the way for Mesozoic kimberlite, ultramafic lamprophyre, and carbonatite emplacement across the North Atlantic craton. VC The Author(s) 2018. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com 2311 J O U R N A L O F P E T R O L O G Y Journal of Petrology, 2017, Vol. 58, No. 12, 2311–2338 doi: 10.1093/petrology/egy009 Advance Access Publication Date: 14 February 2018

U–Pb geochronology of the Eocene Kærven intrusive complex, East Greenland: constraints on the Iceland hotspot track during the rift-to-drift transition

Several major tholeiitic (e.g. the Skaergaard intrusion) and alkaline (e.g. the Kangerlussuaq Syenite) intrusive complexes of the North Atlantic Large Igneous Province are exposed along the Kangerlussuaq Fjord in East Greenland. The Kaerven Complex forms a satellite intrusion to the Kangerlussuaq Syenite and includes early tholeiitic gabbros and a series of cross-cutting alkaline intrusions ranging from monzonite to alkali granite. The alkaline intrusions cut the gabbros, and are cut by the outer nordmarkite zone of the Kangerlussuaq Syenite. This study presents the first U–Pb zircon ages from the alkaline units of the Kaerven Complex. Fourteen multi-grain zircon fractions have been analysed by thermal ionization mass spectrometry (TIMS). Absolute age differences could not be resolved between the different units, suggesting a relatively rapid succession of intrusions between c . 53.5 and 53.3 Ma. Our compilation of precise radiometric age data shows that most of the alkaline magmatism in the Kangerlussuaq Fjord occurred prior to 50 Ma. Moreover, pre-50 Ma alkaline intrusions and lavas show a SSE-younging trend, which is interpreted as the track of the Iceland hotspot during the rift-to-drift transition of the North Atlantic.

Reassembly of the Dharwar and Bastar cratons at ca. 1 Ga: Evidence from multiple tectonothermal events along the Karimnagar granulite belt and Khammam schist belt, southern India

The northern part of the Nellore–Khammam schist belt and the Karimnagar granulite belt, which are juxtaposed at high angle to each other have unique U–Pb zircon age records suggesting distinctive tectonothermal histories. Plate accretion and rifting in the eastern part of the Dharwar craton and between the Dharwar and Bastar craton indicate multiple and complex events from 2600 to 500 Ma. The Khammam schist belt, the Dharwar and the Bastar craton were joined together by the end of the Archaean. The Khammam schist belt had experienced additional tectonic events at

Craton reactivation on the Labrador Sea margins: 40Ar/39Ar age and Sr-Nd-Hf-Pb isotope constraints from alkaline and carbonatite intrusives

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Geochemistry of hypabyssal kimberlites from Lac de Gras, Canada: Comparisons to a global database and applications to the parent magma problem

∼1900 and