Metasomatic Reactions between Archean Dunite and Trondhjemite at the Seqi Olivine Mine in Greenland

Laura Whyatt,Stefan Peters,Tonci Balic-Zunic,Christopher L Kirkland,Andreas Pack,Kristoffer Szilas

doi:10.3390/min10010085

Laura Whyatt, Stefan Peters + Show 4 more

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https://doi.org/10.3390/min10010085

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Abstract

A metasomatic zone formed between the contact of a 2940 ± 5 Ma intrusive trondhjemite sheet in the Archean dunite of the Seqi Ultramafic Complex, SW Greenland, consists of three distinct mineral zones dominated by (1) talc, (2) anthophyllite, and (3) phlogopite. These zones supposedly resulted from a process of dissolution of olivine by silica rich fluid residual from the trondhjemite magma, with crystallization of secondary minerals along a compositional gradient in the fluid phase. A zircon crystal inclusion in a large (4 cm) olivine porphyroblast was dated in situ via LA-ICP-MS U–Pb isotope analysis, yielding a weighted mean 207Pb/206Pb age of 2963 ± 1 Ma, which coincides with granulite facies metamorphism and potential dehydration. Considering phase relations appropriate for the dunite composition, we deduced the talc forming conditions to be at temperatures of 600–650 °C and at a pressure below 1 GPa. This is supported by oxygen isotope data for talc, anthophyllite and phlogopite in the metasomatic zone, which suggests formation in the temperature range of 600–700 °C from fluids that had a δ18O of ~8‰ and a Δ’17O0.528 of about −40 ppm, i.e., from fluids that could have been derived from the late stage trondhjemite sheet.

Highlights

Metasomatism is the process of compositional change in a rock due to the removal and/or addition of chemical components during the interaction of a precursor rock with either melt or hydrous fluid.The main driver of metasomatic reactions is a chemical gradient between the rock and the fluid/melt phase, which leads to the exchange of components and potentially phase transitions.The Archean crust in the Nuuk region of SW Greenland ranges in age from 3.8 Ga tonalite to 2.5 Ga granite [1,2]
The sample used for the whole rock chemical analysis contained more quartz than the one used for the powder x-ray diffraction (PXRD) analysis
The investigated metasomatic zone at the Archean Seqi Ultramafic Complex, SW Greenland, consists of three distinct mineral zones dominated by (1) talc, (2) anthophyllite, and (3) phlogopite, which were developed at the contact of an intrusive trondhjemite sheet in dunite

Summary

Introduction

The Archean crust in the Nuuk region of SW Greenland ranges in age from 3.8 Ga tonalite to 2.5 Ga granite [1,2] Interspersed with such evolved continental crust, are scattered enclaves of both mafic and ultramafic rocks, which range in size from decimeter to kilometer scale (Figure 1). These remnants of older crustal components in the Archean continental crust are dominated by mafic metavolcanic rocks [3], with lesser amounts of plutonic rocks, including anorthosite, gabbro, norite, and peridotite [4].

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