Research Article| September 01, 1987 Archean cratons, diamond and platinum: Evidence for coupled long-lived crust-mantle systems David I. Groves; David I. Groves 1Department of Geology, University of Western Australia, Nedlands 6009, Western Australia Search for other works by this author on: GSW Google Scholar Susan E. Ho; Susan E. Ho 1Department of Geology, University of Western Australia, Nedlands 6009, Western Australia Search for other works by this author on: GSW Google Scholar Nicholas M.S. Rock; Nicholas M.S. Rock 1Department of Geology, University of Western Australia, Nedlands 6009, Western Australia Search for other works by this author on: GSW Google Scholar Mark E. Barley; Mark E. Barley 1Department of Geology, University of Western Australia, Nedlands 6009, Western Australia Search for other works by this author on: GSW Google Scholar Maureen T. Muggeridge Maureen T. Muggeridge 1Department of Geology, University of Western Australia, Nedlands 6009, Western Australia Search for other works by this author on: GSW Google Scholar Author and Article Information David I. Groves 1Department of Geology, University of Western Australia, Nedlands 6009, Western Australia Susan E. Ho 1Department of Geology, University of Western Australia, Nedlands 6009, Western Australia Nicholas M.S. Rock 1Department of Geology, University of Western Australia, Nedlands 6009, Western Australia Mark E. Barley 1Department of Geology, University of Western Australia, Nedlands 6009, Western Australia Maureen T. Muggeridge 1Department of Geology, University of Western Australia, Nedlands 6009, Western Australia Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1987) 15 (9): 801–805. https://doi.org/10.1130/0091-7613(1987)15<801:ACDAPE>2.0.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation David I. Groves, Susan E. Ho, Nicholas M.S. Rock, Mark E. Barley, Maureen T. Muggeridge; Archean cratons, diamond and platinum: Evidence for coupled long-lived crust-mantle systems. Geology 1987;; 15 (9): 801–805. doi: https://doi.org/10.1130/0091-7613(1987)15<801:ACDAPE>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Diamondiferous intrusions and magmatic Pt-Pd deposits are both concentrated on, or adjacent to, the oldest cratons, those with >3.0 Ga high-grade gneiss terranes and/or greenstone belts. Given the old age (>3.0 Ga) of peridotitic inclusions both in diamonds and in kimberlites, diamonds presumably grew in mantle depleted in basaltic major elements near the base of thickened lithosphere and below early sialic nuclei. Most genetic models for magmatic Pt-Pd deposits require a Pt-Pd–enriched, high–Mg-Si melt generated from analogously depleted mantle. The depleted mantle was most likely formed by removal of basaltic melts that contributed to Archean intracratonic greenstone belts. Extensive melting also decreased density and increased rigidity beneath ancient cratons, favoring stabilization and preservation of thick Archean continental lithosphere. Overall, these considerations suggest that localized, thick sialic crust and rigid lithosphere developed before 3.0 Ga, forming enduring, coupled crust-mantle systems below ancient sialic nuclei, the sites of selectively preserved greenstone belts. The data confirm that Early Archean terranes are highly prospective for post-Archean magmatic ore deposits. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Synopsis Small outcrops of folded calcareous metasediments, surrounded by otherwise lime-poor Moine rocks, occur along both sides of the Great Glen: (from SW to NE) in Ardgour, beside Loch Lochy, in Gleann Liath and Glen Urquhart, and near Inverness, Rosemarkie and Nigg. Further examples lie in Glen Dessarry to the NW. Outcrops cover from a few m 2 to several km 2 , and vary from pure limestones, with much rarer dolostones, to Ca-Mg-Fe-silicate ‘skarns’. The few previously described examples have been contradictorily assigned to the Lewisian, Moine, Dalradian, and even Durness Limestone. The mineralogical range in each outcrop is generally similar, indicating amphibolite-facies metamorphism. Several limestones are intimately associated with graphitic, kyanite or sillimanite-rich pelites (also atypical of the local Moine), and with the margins of synformal (D 2 or D 3 ), foliated intrusions. Chemically, the calcareous rocks are strikingly homogeneous, as revealed by cluster analysis, and by very high correlation coefficients between ‘immobile’ trace elements (Ti, P, Y, Zr, Nb, Ce, Th), between Cr, Co and Ni, and between K and Rb, Ba or Pb. Mg also correlates inversely with Sr, as the dolomite/calcite ratio varies. This homogeneity suggests that most of the calcareous rocks belong to a single, now dismembered lithostratigraphical formation. Over 100 analyses of Lewisian ‘limestones’, and over 400 covering most of the Dalradian limestone formations, show quite different and far less homogeneous geochemistries. In particular, the Great Glen limestones cannot be equated with the geographically nearest Dalradian limestones, in the Appin–Ballachulish–Lismore area, as revealed by discriminant and correlation analysis. Field and regional considerations argue that the limestones are unlikely to be either Cambro-Ordovician, or an integral part of the Moine succession. As a working hypothesis, they should be assigned to a separate, provisional lithostratigraphical assemblage, whose relationship to the Lewisian, Moinian and Dalradian remains to be established.
Alkaline dykes tentatively dated at ∼1.3 Ga cut the Vestfold Hills in a consistent N–S to N15°E direction. They form a spectrum between more abundant ultramafic lamprophyres (UML) corresponding broadly to H 2 O–CO 2 -rich nephelinites, and alkaline lamprophyres (AL), representing H 2 O–CO 2 -rich basanites. Olivine (Fo 46–93 , averaging Fo 75 ) is abundant only in the UML, but both types carry primary diopsidic clinopyroxene with complex zoning; amphibole (pargasite, hastingsite, kaersutite with up to 8.6% TiO 2 ); titanian phlogopite (up to 10% TiO 2 ); feldspars (orthoclase, anorthoclase, albite and andesine), nepheline (K-poor and Si-rich), ilmenite (up to 1% MgO and MnO), chrome titanomagnetite, and carbonate (magnesian calcite, ferroan dolomite, breunnerite). Lamprophyric peculiarities include the local coexistence of three feldspars, extremely Ti-rich amphiboles and micas, and the presence of globular structures and possibly primary carbonates. Some dykes carry small but abundant lherzolite xenoliths, others carry chromian diopside (1% Cr 2 O 3 ) and En 58–76 orthopyroxene xenocrysts. The dykes represent primitive, mantle-derived magmas which have undergone varying but generally low degrees of polybaric fractionation, together perhaps with mixing of more primitive and fractionated batches, during their ascent through the crust.
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