Greenstone belts in the central Godthåbsfjord region, southern West Greenland

In 2004 the Geological Survey of Denmark and Greenland (GEUS) initiated a study of the origin and tectono-metamorphic evolution of greenstone belts and important regional structures in the central Godthåbsfjord region, southern West Greenland (Fig. 1; Hollis et al. 2004). Like other Archaean belts worldwide, these greenstone belts are locally host to gold mineralisation. Their complexity requires a combination of detailed geological mapping, geochemistry, petrographic work and geochronological studies to develop models of their geological setting, evolution and gold mineralisation

A global geochemical database for environmental and resource management : recommendations for International Geochemical Mapping, final report of IGCP Project 259

Research conducted since 1988 as part of the International Geochemical Mapping (IGM) project has confirmed that the presently available data concerning the geochemical composition of the Earth’s surface are substantially incomplete and internally inconsistent. Many of the older data sets have serious deficiencies and do not meet basic requirements for establishing the range of natural geochemical background values. As a result of natural geological and environmental processes, element abundances in natural materials can vary by several orders of magnitude within short distances. These variations are inadequately documented and their existence is often overlooked in the setting of public policy. A high quality geochemical database is pertinent to a wide range of investigations in the earth and life sciences, and should be considered as an essential component of environmental knowledge. Detailed information about the natural variability of the geochemical background is pertinent to administrative and legal issues as much as to scientific research. Sustainable long-term management of environmental and mineral resources is dependent upon a comprehensive and reliable database. The International Geosphere-Biosphere Program on Global Change requires information on current conditions. Important aspects of change cannot be measured, or their consequences anticipated, unless the present composition of the earth’s surface materials is known. The International Geochemical Mapping project, which was endorsed in 1988 as a contribution to the IGBP (IGBP, 1989), is a multi-stage project established to consider how best to provide quantitative data to portray the geochemical diversity of the earth’s land surface. Participants in IGCP 259 have undertaken a comprehensive review of methods of regional and national geochemical mapping and examined the results obtained. Many problems have been identified and a variety of solutions discussed. Field and laboratory research has been carried out. The resulting recommendations are contained in this report. They are directed towards geochemists and those institutions, which have a mandate for providing an earth science and/or environmental database

Titanite petrochronology linked to phase equilibrium modelling constrains tectono-thermal events in the Akia Terrane, West Greenland

GeoHistory Facility instruments (part of the John de Laeter Centre) were funded via an Australian Geophysical Observing System (AGOS) grant provided to AuScope by the AQ44 Australian Education Investment Fund.The Mesoarchean Akia Terrane in West Greenland contains a detailed magmatic and metamorphic mineral growth record from 3.2 Ga to at least c. 2.5 Ga. This time span makes this region an important case study in the quest to track secular changes in geodynamic style which may ultimately inform on the development of plate tectonics as a globally linked system of lateral rigid plate motions. The common accessory mineral titanite has recently become recognised as a powerful high temperature geochronometer whose chemistry may chart the thermal conditions of its growth. Furthermore, titanite offers the potential to record the time-temperature history of mafic lithologies, which may lack zircon. Although titanite suffers from higher levels of common Pb than many other UPb chronometers, we show how measurement of 207Pb/206Pb in texturally coeval biotite may assist in the characterization of the appropriate common Pb composition in titanite. Titanite extracted from two samples of mafic gneisses from the Akia Terrane both yield UPb ages of c. 2.54 Ga. Although coeval, their chemistry implies growth under two distinctly different processes. In one case, the titanite has elevated total REE, high Th/U and grew from an in-situ partial melt, consistent with an identical date to granite dyke zircon. In contrast, the second titanite sample contains greater common Pb, lower total REE, lower Th/U, and grew from dominantly hydrothermal fluids. Zr-in-titanite thermometry for partial melt-derived titanite, with activities constrained by phase equilibrium modelling, indicates maxima of c. 690 °C. Elsewhere in the Akia Terrane, coeval metamorphism linked to growth of hydrothermal titanite is estimated at temperatures of c. 670 °C. These new results when coupled with existing findings indicate punctuated, repeated metamorphic events in the Akia Terrane, in which high temperature conditions (re)occurred at least three times between 3.0 and 2.5 Ga, but crucially changed in style across a c. 3.0 Ga change point. We interpret this change in metamorphism as reflecting a fundamental shift in geodynamic style in West Greenland at 3.0 Ga, consistent with other estimates for the onset of widespread plate tectonic-type processes.Publisher PDFPeer reviewe

Differentiating between inherited and autocrystic zircon in granitoids

The Maniitsoq map project is supported by the Ministry of Mineral Resources, Government of Greenland. The LA-ICP-MS instruments in the JdLC were funded via an Australian Geophysical Observing System grant provided to AuScope Pty Ltd. by the AQ44 Australian Education Investment Fund program.Inherited zircon, crystals that did not form in situ from their host magma but were incorporated from either the source region or assimilated from the wall-rock, is common but can be difficult to identify. Age, chemical and/or textural dissimilarity to the youngest zircon fraction are the primary mechanisms of distinguishing such grains. However, in Zr-undersaturated magmas, the entire zircon population may be inherited and, if not identifiable via textural constraints, can lead to erroneous interpretation of magmatic crystallization age and magma source. Here, we present detailed field mapping of cross-cutting relationships, whole-rock geochemistry and zircon textural, U-Pb and trace element data of trondhjemite, granodiorite and granite from two localities in a complex Archean gneiss terrane in southwest Greenland, which reveal cryptic zircon inheritance. Zircon textural, U–Pb and trace element data demonstrate that, in both localities, trondhjemite is the oldest rock (3011 ± 5 Ma, 2σ), which is intruded by granodiorite (2978 ± 4 Ma, 2σ). However, granite intrusions, constrained by cross-cutting relationships as the youngest component, only contain inherited zircon derived from trondhjemite and granodiorite based on ages and trace element concentrations. Without age constraints on the older two lithologies, it would be tempting to consider the youngest zircon fraction as recording crystallization of the granite but this would be erroneous. Furthermore, whole-rock geochemistry indicates that the granite contains only 6 µg g-1 Zr, extremely low for a granitoid with ∼77 wt. % SiO2. Such low Zr concentration explains the lack of autocrystic zircon in the granite. We expand on a differentiation tool that uses Th/U ratios in zircon versus that in the whole rock to aid in the identification of inherited zircon. This work emphasizes the need for field observations, geochemistry, grain characterization, and precise geochronology to accurately determine igneous crystallization ages and differentiate between inherited and autocrystic zircon.PostprintPeer reviewe

Building Mesoarchaean crust upon Eoarchaean roots : the Akia Terrane, West Greenland

The Maniitsoq project is supported by the Ministry of Mineral Resources and Labour, Government of Greenland. NJG thanks Curtin University and Australian Research Council grant FL160100168 for financial support.Constraining the source, genesis, and evolution of Archaean felsic crust is key to understanding the growth and stabilization of cratons. The Akia Terrane, part of the North Atlantic Craton, West Greenland, is comprised of Meso-to-Neoarchaean orthogneiss, with associated supracrustal rocks. We report zircon U–Pb and Lu–Hf isotope data, and whole-rock geochemistry, from samples of gneiss and supracrustals from the northern Akia Terrane, including from the Finnefjeld Orthogneiss Complex, which has recently been interpreted as an impact structure. Isotope data record two major episodes of continental crust production at ca. 3.2 and 3.0 Ga. Minor ca. 2.7 and 2.5 Ga magmatic events have more evolved εHf, interpreted as reworking of existing crust perhaps linked to terrane assembly. Felsic rocks from the Finnefjeld Orthogneiss Complex were derived from the same source at the same time as the surrounding tonalites, but from shallower melting, requiring any bolide-driven melting event to have occurred almost simultaneously alongside the production of the surrounding crust. A simpler alternative has the Finnefjeld Complex and surrounding tonalite representing the coeval genesis of evolved crust over a substantial lithospheric depth. Hafnium isotope data from the two major Mesoarchaean crust-forming episodes record a contribution from older mafic Eoarchaean crust. Invoking the involvement of an Eoarchaean root in the growth of younger Mesoarchaean crust puts important constraints on geodynamic models of the formation of the discrete terranes that ultimately assembled to form Earth’s cratons.Publisher PDFPeer reviewe

Regional zircon U-Pb geochronology for the Maniitsoq region, southwest Greenland

The Ministry of Mineral Resources, Government of Greenland, funded this project. Analyses in the JdLC GeoHistory Facility were enabled by instrumentation supported by AuScope (auscope.org.au) and the Australian Government via the National Collaborative Research Infrastructure Strategy. The Tescan Mira3 FEG-SEM was funded through the Australian Research Council LIEF program.Zircon U-Pb geochronology places high-temperature geological events into temporal context. Here, we present a comprehensive zircon U-Pb geochronology dataset for the Meso- to Neoarchean Maniitsoq region in southwest Greenland, which includes the Akia Terrane, Tuno Terrane, and the intervening Alanngua Complex. The magmatic and metamorphic processes recorded in these terranes straddle a key change-point in early Earth geodynamics. This dataset comprises zircon U-Pb ages for 121 samples, including 46 that are newly dated. A principal crystallization peak occurs across all three terranes at ca. 3000 Ma, with subordinate crystallization age peaks at 3200 Ma (Akia Terrane and Alanngua Complex only), 2720 Ma and 2540 Ma. Metamorphic age peaks occur at 2990 Ma, 2820-2700 Ma, 2670-2600 Ma and 2540 Ma. Except for one sample, all dated metamorphic zircon growth after the Neoarchean occurred in the Alanngua Complex or within 20 km of its boundaries. This U-Pb dataset provides an important resource for addressing Earth Science topics as diverse as crustal evolution, fluid-rock interaction and mineral deposit genesis.Publisher PDFPeer reviewe