Authors:	Tappe, Sebastian
Title:	Nature and origin of ultramafic lamprophyres and carbonatites from the borders of the Labrador Sea
Online publication date:	12-Dec-2005
Year of first publication:	2005
Language:	english
Abstract:	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 infiltrated the cold base of the cratonic lithosphere (4-6 GPa) where they stabilized as phlogopite- and carbonate-dominated vein assemblages. Asthenospheric upwelling continued and successively converted the veined lithosphere into part of the hotter convective mantle thereby causing remelting of the veins (controlling Sr-Nd signature) and volatile-fluxed melting of the lithospheric wall-rock peridotites (controlling compatible element contents of the melt). This multi-stage veined mantle melting model can account for the highly incompatible element enriched nature, the high MgO contents and long-term isotopic depletion of the parental UML magma and, moreover, the comparatively long time scale of UML magmatism to either side of the Labrador Sea. A variety of low-pressure processes have modified the primary UML magma during its ascent leading to the diversity of UML and secondary dolomite-bearing carbonatite types seen at the surface. The envisaged geodynamic scenario under which UML magma production occurred is one of incipient rifting with progressive thinning of thick cratonic lithosphere in the present-day Labrador Sea area. This continental extension at the eastern Laurentian margin occurred in response to plate reorganization which resulted in the breakup of the supercontinent Rodinia during the Late Neoproterozoic at ~ 600-580 Ma. Similar old and compositionally identical UML occurrences are known from other parts of former Rodinia (i.e. the Fen and Alnö complexes of Baltica), which emphasizes that this tectonomagmatic process operated on a global scale.
DDC:	550 Geowissenschaften 550 Earth sciences
Institution:	Johannes Gutenberg-Universität Mainz
Department:	FB 09 Chemie, Pharmazie u. Geowissensch.
Place:	Mainz
ROR:	https://ror.org/023b0x485
DOI:	http://doi.org/10.25358/openscience-3453
URN:	urn:nbn:de:hebis:77-9013
Version:	Original work
Publication type:	Dissertation
License:	In Copyright
Information on rights of use:	https://rightsstatements.org/vocab/InC/1.0/
Appears in collections:	JGU-Publikationen