Abstract
A titanian clinohumite-bearing dunite was recently found in the Ulamertoq ultramafic body within the 3.0 Ga Akia Terrane of southern West Greenland. Titanian clinohumite occurs as disseminated and discrete grains. Titanian clinohumite contains relatively high amounts of fluorine, reaching up to 2.4 wt.%. The high-Fo content of olivine (Fo93) coupled with low Cr/(Cr + Al) ratio of orthopyroxene implies that the dunite host is not of residual origin after melt extraction by partial melting of the primitive mantle. Olivine grains are classified into two types based on abundances of opaque mineral inclusions: (1) dusty inclusion-rich and (2) clear inclusion-free olivines. Opaque inclusions in coarse-grained olivines are mainly magnetite. Small amounts of ilmenite are also present around titanian clinohumite grains. The observed mineral association indicates partial replacement of titanian clinohumite to ilmenite (+magnetite) and olivine following the reaction: titanian clinohumite = ilmenite + olivine + hydrous fluid. The coexistence of F-bearing titanian clinohumite, olivine, and chromian chlorite indicates equilibration at around 800–900 °C under garnet-free conditions (<2 GPa). Petrological and mineralogical characteristics of the studied titanian clinohumite-bearing dunite are comparable to deserpentinized peridotites derived from former serpentinites. This study demonstrates the importance of considering the effects of hydration/dehydration processes for the origin of ultramafic bodies found in polymetamorphic Archaean terranes.
Highlights
Archaean rocks are widely exposed in southern West Greenland
Two conflicting origins are proposed for the ultramafic rocks in the Archaean terranes in southern West Greenland: (1) residue after partial melting, and (2) ultramafic cumulate formed by mafic mineral accumulation or crystallization from komatiitic melts [11,15]
The present study demonstrates that titanian clinohumite-bearing peridotites occur, at least locally in the Akia Terrane, and are distributed in several Archaean terranes in southern
Summary
Archaean rocks are widely exposed in southern West Greenland. Several tectonomagmatic terranes comprise southern West Greenland, which can be sub-divided based on age and metamorphic evolution: from north to south the Akia, Akulleq, and Tasiusarsuaq terranes [1,2] (Figure 1a). West Greenland [3,4,5,6,7,8,9,10] These ultramafic rocks could provide direct information on the evolution of the Earth’s mantle. The origins of these ultramafic rocks and their tectonic settings are, still enigmatic [4,6,8,9,10,11,12,13,14]. Two conflicting origins are proposed for the ultramafic rocks in the Archaean terranes in southern West Greenland: (1) residue after partial melting, and (2) ultramafic cumulate formed by mafic mineral accumulation or crystallization from komatiitic melts [11,15]. Whole rock and mineral compositions are similar to abyssal peridotites [6], whereas other chemical indicators suggest a potential subarc origin [12,14]
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