1490-1450 Ma) deposition and orogenesis in a reconstructed orogenic belt, northern New Mexico, USA: Defining the Picuris orogeny. Geophysics, Geomagnetism 8a) yields relatively young Pb model ages in a tight range of 1.5–1.7 Ga with µ values equal to or slightly higher than the evolution curve suggested by Stacey & Kramers, (1975). The bimodal age distribution of the Karnataka craton and the ‘transition zone’ is only visible after rejection of samples with high Sm/Nd. (1984). Continental crust is tertiary crust, formed at subduction zones through recycling of subducted secondary (oceanic) crust. 9g) which all fall into the category (3), although these have distinct age spectra. Numbers show sample locations (Nd and Sr whole-rock isotopes and Pb isotope composition of leached feldspars). For correlations on a larger scale, ∼380 whole-rock Nd results were compiled (Table B1, Appendix B) from the available literature on Precambrian rocks of other fragments of central Gondwana (East Africa, Madagascar, South India, Sri Lanka, Antarctica). 7, U–Pb, single zircon Pb-evaporation, and Sm–Nd isotopic study of a granulite domain in SE Madagascar, Sr, Nd, and Pb isotopic systematics in the Archean low- to high-grade transition zone of southern India: syn-accretion vs post-accretion granulites, Age of younger tonalitic magmatism and granulitic metamorphism in the South Indian transition zone (Krishnagiri area); comparison with older Peninsular gneisses from the Gorur–Hassan area, The Mozambique Belt in northern Mozambique: Neoproterozoic (1100–850 Ma) crustal growth and tectogenesis, and superimposed Pan-African (800–550 Ma) tectonism, Isotopic age determinations on granitic and gneissic rocks from the Ubendian–Usagaran system in southern Tanzania, Summary of the geology of Tanganyika, Part 2: Geological map, Petrology of granulite-facies metapelites of the Highland Complex, Sri Lanka: implications for the metamorphic zonation and the, A model discussing the early history of the Earth based on a study of lead isotope ratios from veins in some Archean cratons of Africa, Bulletin of the Geological Survey of Tanganyika, Pan-African charnockite formation in East Gondwana: geochronologic (Sm–Nd and Rb–Sr) and petrogenetic constraints, Bulletin of the Indian Geological Association, Annual Reports on Scientific Results, University of Leeds, Sm–Nd and Rb–Sr ages of metamorphic rocks from the Sør Rondane Mountains, East Antarctica, Recent Progress in Antarctic Earth Science, Cambrian orogenic belt in East Antarctica and Sri Lanka: implications for Gondwana assembly, Sm–Nd and Rb–Sr isochron ages for meta-trondhjemites from Cape Hinode, East Antarctica, Proceedings of the NIPR Symposium, Vol. Table 2 lists the Pb isotope data obtained on feldspars in this study. Physics, Solar Various models have been proposed for the evolution of the depleted mantle (e.g. Spatiotemporal reconstruction of Late Mesozoic silicic large igneous province and related epithermal mineralization in South China: Insights from the Zhilingtou volcanic‐intrusive complex. Continental growth seen through the sedimentary record. (b) 208Pb/204Pb vs 206Pb/204Pb diagram for leached feldspar from high-grade gneisses from Tanzania with the Pb isotope growth curve of Stacey & Kramers, (1975) for reference. Cretaceous ongonites (topaz-bearing albite-rich microleucogranites) from Ongon Khairkhan, Central Mongolia: Products of extreme magmatic fractionation and pervasive metasomatic fluid: rock interaction. Planets, Magnetospheric Ratios for Rb–Sr are calculated from modern 87Sr/86Sr of 0.720 for cc and 0.702 for dm (Hofmann, 1997). 9. In the 208Pb/204Pb vs 206Pb/204Pb diagram (Fig. Therefore only the time-integrated effects are displayed in a conventional isochron diagram (Fig. These samples fall on the younger (740 Ma) Sr-errorchron and have normal crustal or higher Rb/Sr ratios, which also yield Sr model ages <2 Ga (Table 1, Fig. Department of Applied Geology, University of New South Wales. 8) from the Nyanzian volcanics of the Archean Tanzania Craton (Robertson, 1973) are similar to the three most primitive Pb isotopic compositions obtained from the granitoid gneisses of the craton presented above (Fig. The combination of the Pb isotopes with the intermediate Nd model ages and the large spread in the Nd model ages indicates that these rocks from the Uluguru Mts formed as a result of mixing of crustal material of different residence ages. The chemistry of Karoo-age andesitic lavas along the northern Mozambique coast, southern Africa and possible implications for Gondwana breakup. Crustal accretion and reworking processes of micro-continental massifs within orogenic belt: A case study of the Erguna Massif, NE China. Only Sr model ages which fall in the range of Nd model ages calculated for the respective crustal province and are not younger than the metamorphic age that is known to have affected the sample are considered to be meaningful here. These complexes exhibit striking similarities in lithology, structure and grade of metamorphism (Coolen, 1980;,Appel et al., 1998). The high-grade metasediments on the other hand display 87Rb/86Sr values which are consistent with average continental crust or higher; none show signs of depletion or values envisaged for the lower continental crust. Calculated with the Isoplot software of Ludwig, (1994). The 207Pb/204Pb vs 206Pb/204Pb distribution of the other southern Ubendian samples may be explained by evolution during the time from the metamorphic event in the craton (at 2.5 Ga) and the 2.0 Ga event in the Ubendian–Usagaran along growth curves with widely different µ values. These may be distal parts of Proterozoic orogenic belts girdling Archean cratons or domains which include Proterozoic accreted terranes not spatially resolved. It is also important to note that metasediments and metaigneous rocks of some domains can exhibit distinct age characteristics, e.g. An appropriate size fraction was sifted off and the sample was washed with distilled water to remove surface dust. This deviation may be attributed either to derivation from an evolved crustal source or alternatively to Rb depletion by an event late enough to leave time for the development of the high 87Sr/86Sr ratio, such as a Pan-African metamorphic overprint. Oxford University Press is a department of the University of Oxford. Newly recognized granulite occurrences in the Mozambique Belt after Appel et al., (1998). Extension of this approach to larger parts of Gondwana will allow a more robust reconstruction of ancient terrane boundaries and will lead to a reliable model for the evolution and the geodynamic setting of Precambrian crustal domains. 7b), where the data points follow the S&K evolution curve. Three cycles of metamorphism were derived from this database and partly later refined by U–Pb data. Physical and Engineering Sciences. Strictly Archean domains with Nd model ages >2.6 Ga (e.g. In contrast to the Sm–Nd system, changes in Th/Pb and U/Pb occur mostly during crustal processes such as metamorphism, hydrothermal alteration, sedimentation and weathering, as a result of the different solubilities of U, Th and Pb in crustal fluids (Faure, 1986). Between each step the samples were washed with pure H2O. In such old and eroded belts, Pb and Nd isotopes supply particularly valuable information on crustal genesis, evolution and terrane amalgamation and can be used to distinguish between old reworked and juvenile crust. Hf and Nd isotopes in Early Ordovician to Early Carboniferous granites as monitors of crustal growth in the Proto-Andean margin of Gondwana. 6). Other explanations without mixing of older components include the derivation from a mantle with curvilinear evolution (DePaolo, 1981) as shown in Fig. Although it cannot be assumed that this subset (consisting of the granulites from the Pare and Usambara Mts, the Umba Steppe, Wami River and the eastern Uluguru Mts) rigorously fulfills the criteria for geological significance of whole-rock isochrons, it can be observed that all samples with Nd model ages <1.6 Ga plot close to this Pan-African reference line.


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