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Three rock types are distinguished in this study: (a) the layered rocks, (b) the Reef pegmatites which are parallel to the layering and form prominent marker horizons and (c) the iron rich ultramafic (IRUM) pegmatites which are transgressive to the layered rocks.Reaction textures are observed in the layered rocks adjacent to Reef pegmatites and are compared with similar reaction textures from alteration...
The constituent primary and secondary minerals in a suite of fifteen samples of eclogite, amphibole-eclogite and garnet-amphibolite, scattered as boudins and pods in metapelitic schists and gneisses on Isla Margarita, Venezuela, have been analysed by electron microprobe to augment existing bulk-rock chemical data. The large sample population available allows spurious effects of stoichiometric Fe...
Compositions of melts and coexisting olivine, chromite and Ca-poor pyroxene have been determined in the temperature range 1242°-1405°C at 1 atm and at 10 -10 -10 -5 atm oxygen fugacity ( O 2 ). Experiments were also conducted at 5 kbar to examine the effect of pressure on the composition of the melts. Starting materials are mixtures of olivine, enstatite,...
Volcano Mountain, Yukon Territory, a Quaternary edifice located near the junction of the Pelly and Yukon Rivers, erupted two Recent nephelinite lava flows between 3000 and 7300 years BP and a third Recent lava flow that is probably younger. The porphyritic nephelinites have olivine phenocrysts, Fo 91 -Fo 79 , and Ti-rich diopside microphenocrysts. The groundmass consists...
Gorgona Island, Colombia is remarkable not only because it contains the only Phanerozoic komatiites, but also because it has mafic to ultramafic lavas with a wide range of compositions, from moderately enriched to extremely depleted (relative to Bulk Earth). The komatiite flows are, in many respects similar to Archaean komatiites; they formed from MgO-rich (18%) liquids and have upper spinifex zones...
Unequivocal examples of oceanic plateaux older than about 2.1 Ga have yet to be identified from the geological record. The most likely explanation for this is the partial dismemberment of ancient plateaux at convergent plate margins, and their subsequent juxtaposition in accretionary complexes with arc volcanic, sedimentary and plutonic rocks. A review of Cretaceous plateaux in the eastern Indian...
Magmatism occurred intermittently in the Parana region of South America between 240-50 Ma, with the peak of magmatic activity between 138-126 Ma, comprising the Parana continental flood basalts (CFB). Detailed chemical subdivision of the Parana basalts into six units allowed stratigraphic correlations to be made, implying a northward migration of magmatism following the rifting of the South Atlantic...
Greenstone belts that are older than 0.18 Ga formed under circumstances that generate a buoyant, garnet-poor root. Because they do not generate enough depletion in the lithospheric mantle, juvenile arcs involving average temperature Early Precambrian mantle were not a significant source of Early Precambrian greenstone belts. Instead, Early Precambrian greenstone belts originated from mantle plumes...
The 2.9 Ga-old Forrestania greenstone belt is intermediate in character between typical > 3.0 Ga and 2.7 Ga belts in containing both Al-depleted and Al-undepleted komatiites. The extensive nickel exploration database on the belt provides a unique opportunity to examine geochemical variations in the context of volcanological evolution. Within each of the six komatiitic units in the greenstone...
A 3.1-3.3 Ga Mid-Archean accretionary complex has been identified in the Cleaverville area, Pilbara granite-greenstone terrain, Western Australia, by using a well-defined duplex structure and by reconstructing the oceanic plate stratigraphy. Archean mid-oceanic ridge basalts (AMORBs) which are part of this sequence were selected for detailed petrochemical analysis to infer Archean divergent plate...
Large igneous provinces are conspicuous features of late Phanerozoic geology, and include continental flood basalts, rifted continental margin volcanic sequences and oceanic plateaus. The latter are formed in an environment which typically recycles back into the mantle on a time scale of < 200 m.y., but because comparisons have recently been made between oceanic plateaus and Precambrian greenstone...
The southern portion of the Chile Ridge is one of few sites where active subduction of a spreading center and its consequences for ridge axis magmatism can be investigated. New major element, trace element, and isotopic data for lavas recovered from the ridge axis between 43°S and 46°20 S of the southern Chile Ridge have revealed a suite of mid-ocean ridge basalts which possess typical major element...
The xenoliths from the Southeast Province of the Kerguelen Archipelago derived from the lower crust or the upper mantle, can contribute to define the characteristics of the mantle sources below Kerguelen and improve the constraints on the formation of the Kerguelen Islands and plateau. Our petrographic, geochemical and isotopic (Sr, Nd and Pb) study focuses on peridotites (Type Iα: harzburgite/clinopyroxene-poor...
There are very few alkaline rocks in Precambrian terrains. The oldest well-documented examples are 2.7 Ga trachytes and leucite phonolites from the Kirkland Lake region of Canada. These rocks are highly potassic, with major- and trace-element characteristics closely resembling those of shoshonitic lavas in modern island arcs. Other examples of Archean alkaline rocks are limited to rare, volumetrically...
Embedded within the vast granitoid terrane of the Minto block of northeastern Superior Province are Late Archean greenstone belts of the Goudalie domain that preserve a long-lived record of continent-ocean interaction. The Vizien greenstone belt is one such belt and it contains four fault-bounded structural panels. The 2786 Ma mafic-ultramafic sequence is an allochthonous package of pillowed basaltic...
Processes of carbonate formation have been related to C and O isotopic compositions in the Mesozoic alkali basalt (Mecsek Mts.) and lamprophyre (Transdanubian Range) suites of Hungary. In the studied magmatic rocks, carbonates are present as ocelli, amygdales, xenoliths, veins and groundmass carbonate. C and O isotope studies of these types of carbonate have yielded information on the origin of...
Moinian (Proterozoic) gneisses in Easter Ross-shire, Scotland, exhibit pronounced alteration where they have been subjected to the flow of hydrocarbon-bearing fluids from an adjacent Devonian sedimentary basin. Comparison of the petrography of altered and unaltered gneiss indicates that element redistribution accompanied the alteration process. The alteration involved creation of secondary porosity...
Mn-rich beds occur within the Brecce di Seravezza Formation (BSFm) intercalated between Norian metadolostone-Megalodontic marble and Hettangian marble in the Alpi Apuane, (Italy). The BSFm is a rockfall talus deposit that accumulates at the base of the footwall scarp of normal faults bordering extensional marine coastal basins. The type sequence of metabreccia ends with a bed of chloritoid schist...
Most models of crustal assimilation assume that the amount of assimilant added to the magma is proportional to each infinitesimally small amount of solid removed during crystallization (AFC). In some magmatic systems, however, assimilation and crystallization are not strictly related and the mass assimilated is decoupled from, and therefore varies independently of, the mass crystallized (FCA). The...
Pressure-temperature estimates on a group of well-equilibrated garnet pyroxenites from the Qilin locality, southeastern China have been used to construct the first detailed, locality-specific paleogeotherm for eastern China. This xenolith-defined geothermal gradient is comparable to that for northwestern Spitsbergen, Norway, and higher than that derived for southeastern Australia. The strong curvature...
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