U-Pb Zircon Calendar for Namaquan (Grenville) Crustal Events in the Granulite-facies Terrane of the O'okiep Copper District of South Africa     

CLIFFORD  TOM N.; BARTON  ERIKA S.; STERN  RICHARD A.; DUCHESNE  JEAN-CLAIR 《Journal of Petrology》2004,45(4):669-691

The O'okiep Copper District is underlain by voluminous 1035–1210Ma granite gneiss and granite with remnants of metamorphosedsupracrustal rocks. This assemblage was intruded by the 1030Ma copper-bearing Koperberg Suite that includes jotunite, anorthosite,biotite diorite and hypersthene-bearing rocks ranging from leuconoriteto hypersthenite. New sensitive high-resolution ion microprobeage data demonstrate the presence of 1700–2000 Ma zirconas xenocrysts in all of the intrusive rocks, and as detritalzircon in the metasediments of the Khurisberg Subgroup. Thesedata are consistent with published Sm–Nd model ages ofc. 1700 Ma (T_CHUR) and c. 2000 Ma (T_DM) of many of the intrusivesthat support a major crust-forming event in Eburnian (Hudsonian)times. In addition, U–Th–Pb analyses of zirconsfrom all major rock units define two tectono-magmatic episodesof the Namaquan Orogeny: (1) the O'okiepian Episode (1180–1210Ma), represented by regional granite plutonism, notably theNababeep and Modderfontein Granite Gneisses and the Concordiaand Kweekfontein Granites that accompanied and outlasted (e.g.Kweekfontein Granite) regional tectonism [F₂(D₂)] and granulite-faciesmetamorphism (M₂); (2) the Klondikean Episode (1020–1040Ma), which includes the intrusion of the porphyritic RietbergGranite and of the Koperberg Suite that are devoid of regionalplanar or linear fabrics. Klondikean tectonism (D₃) is reflectedby major east–west-trending open folds [F₃(D_3a)], andby localized east–west-trending near-vertical ductilefolds [‘steep structures’; F₄(D_3b)] whose formationwas broadly coeval with the intrusion of the Koperberg Suite.A regional, largely thermal, amphibolite- to granulite-faciesmetamorphism (M₃) accompanied D₃. This study demonstrates, interalia, that the complete spectrum of rock-types of the KoperbergSuite, together with the Rietberg Granite, was intruded in ashort time-interval (<10 Myr) at c. 1030 Ma, and that therewere lengthy periods of about 150 Myr of tectonic quiescencewithin the Namaquan Orogeny: (1) between the O'okiepian andKlondikean Episodes; (2) from the end of the latter to the formalend of Namaquan Orogenesis 800–850 Ma ago. KEY WORDS: U–Pb, zircon; O'okiep, Namaqualand; granite plutonism; granulite facies; Koperberg Suite; Namaquan (Grenville) Orogeny  相似文献   

Iron-Titanium Oxide Minerals in the Bjerkrem-Sogndal Massif, South-western Norway   总被引：3，自引：0，他引：3  

DUCHESNE  JEAN-CLAIR 《Journal of Petrology》1972,13(1):57-81

Ilmenite and magnetite are investigated from the point of viewof their distribution, microtexture, and chemical composition(major and minor elements) in the Bjerkrem-Sogndal massif (Egersundarea, South-Rogaland, SW. Norway). This massif is an igneouslayered synkinematic lopolith made up of cumulates of the anorthosite-mangeritesuite. The lower part of the massif presents a rhythmic structure. The microtextures of ilmenite result from simple exsolutionof ilmenite-hematite solid solutions. Magnetite contains intergrowthsof ilmenite formed by oxidation-exsolution of ulv?spinel-magnetitesolid-solutions. In the stratigraphic sequence, on a large scale, ilmenite appearsfirst alone, and is then accompanied by magnetite; its hematitecontent decreases towards the top of the massif, while the titaniumcontent of the magnetite increases. On the scale of the rhythms,similar trends but of lesser amplitude are also observed. Evidence of deuteric readjustment of the orthomagmatic compositionof the two oxides is provided (1) by the observation of microtexturesat the contact between grains (zoning of primary ilmenite andrim of secondary ilmenite) (2) by the existence of differencesin chemical composition between isolated grains and grains incontact, and (3) by the determination of the equilibrium temperatureby means of the Buddington and Lindsley geothermometer. Reconstitution of the T-fo₂ orthomagmatic conditions in twoparticular levels of the massif shows that the reducing characterof the magma increases during differentiation. The sudden changesin the oxide assemblage at the base of the rhythms reflect asudden increase in the fo₂ of the magma. These increases, asshown by variation in Cr, Ni, and Co, are due to recurrencesof the basic character of the magma. The variations of the minor elements Mn, V, Ga, and Zn are interpretedin terms of the influence of the deuteric readjustment. It followsthat the ratios Mn/Fe²⁺, Ga/Fe³⁺, and Zn/Fe²⁺ increase and thatthe ratio V/Fe³⁺ decreases in the magma in the course of differentiation.The distribution of Mn between ilmenite and magnetite is discussed. Intermittent supplies of undifferentiated magma are proposedas the geological mechanism controlling the chemical recurrencesassociated with the rhythmic structure.  相似文献