Nature and origin of ultramafic lamprophyres and carbonatites from the borders of the Labrador Sea

• Main Author: Tappe, Sebastian
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Johannes Gutenberg-Universität Mainz 2005
• Subjects: ddc:550; Greenland; Labrador Sea
• Online Access: https://openscience.ub.uni-mainz.de/handle/20.500.12030/3455; https://hdl.handle.net/20.500.12030/3455; https://doi.org/10.25358/openscience-3453

Die Ränder des Labrador Meeres wurden während des späten Neoproterozoikums intensiv von karbonatreichen silikatischen Schmelzen durchsetzt. Diese Schmelzen bildeted sich bei Drucken zwischen ca. 4-6 GPa (ca. 120-180 km Tiefe) an der Basis der kontinentalen Mantel-Lithosphäre. Diese Magmengenerierung steht in zeitlichem und räumlichem Zusammenhang mit kontinentalen Extensionsprozessen, welche zu beiden Seiten des sich öffnenden Iapetus-Ozeans auftraten. Ultramafic lamprophyres (UML) are rare but widespread igneous rocks representing a silica undersaturated, potassic, volatile-rich magma type of upper mantle derivation. They occur as dyke swarms or in central complexes typically in association with carbonatites. Despite their potential for elucidating deep melting processes and the fact that they can be a primary source of diamonds, UML have commonly been ignored in igneous petrology as an oddity, which is manifested in the scarcity of petrogenetic studies and in their exclusion from the IUGS classification scheme in the year 2002. This thesis includes a method by which to correctly identify and classify UML within the IUGS system, thus giving them an appropriate place in the family of igneous rocks, which is a prerequisite to any systematic research on their origin. On the basis of this new scheme it is demonstrated that the borders of the Labrador Sea, which includes large parts of West Greenland, New Quebec and Labrador, have been the site of volumetrically significant UML magma production during the Late Neoproterozoic (~ 610-550 Ma). The carbonate-rich UML variety (aillikite), which is closest to a primary magma composition, shows Sr-Nd isotope signatures (87Sr/86Sri typically < 0.7045 and initial Nd values between +0.1 and +1.9) typical for asthenospheric convecting mantle. However, the required source assemblage is only stable at temperature conditions of the cold lithospheric mantle. It is therefore argued that potassic, carbonate-rich metasomatic agents derived from upwelling convective mantle ...