Age and Geochemistry of Basite-Ultrabasite Intrusions of Saltych Anticlinorium (West Azov)

In the Saltych anticlinorium of the West Azov, small intrusions of rocks of basic and ultrabasic composition are known in the middle reaches of the Obitochna, Kiltychiya and Burtychiya rivers and in the upper reaches of the Obitochna river. They are composed from amphibolized peridotites, pyroxenites, actinolithites, tremolithites, hornblendites, amphibole and other shales. These intrusions are considered "fractured intrusions", which are rooted in the already deployed rocks of the Archean age. In the stratigraphic scheme they are referred to the Novoselytske association. Intrusions break through the veins of syenites, pegmatites and aplite-like granites. Metaultrabasites, according to I.S. Usenko and G.L. Kravchenko, these are ultrabasic rocks of the normal series, which were formed, probably, because of differentiation of basaltic magma. The main and ultrabasic rocks of these intrusions have not been studied enough yet — their age, geodynamic conditions of formation have not been determined, the potential for minerals has not been fully revealed. For this purpose, we determined the microelement composition of meta-ultrabasites, metabasites and acid rocks by the ICP MS method and determined the age of monazite from veins of aplite-like granites that break through the metapyroxenites of the Kamyshuvatka intrusion. According to the obtained geochemical and mineralogical data, the vein rocks of medium and acid composition, which break through the intrusions of the ultrabasite-basite composition at the Saltych anticlinorium, have a core genesis. Zircon in them is relict, and syngenetic monazite was found only in the veins of aplite-like granites that break through the metapyroxenites of the Kamyshuvatka intrusion. According to the dating of this monazite, it has been determined that the upper age limit for the formation of intrusions of the ultrabasite-basite composition of the Novoselytske association is 2051.8±3.8 Ma. Peridotites and pyroxenites of these intrusions could be formed from melts of the depleted upper mantle, which were fractionated in intermediate magmatic chambers. In peridotites (#mg = 0.74-0.86) of the Kamyshuvatka intrusion and intrusion near the Reva farm, the Ni content (510-826 ppm) is lower than that of Clark for ultrabasic rocks, and the chromium content at the level of Clark and below it is Cr — 1250-2260 ppm. Positive anomalies of Ti and Eu indicate the processes of fractionation of the original magma of pyroxenites. The formation of intrusions of the ultrabasite-basite composition of the Saltych anticlinorium of the Azov megablock is probably connected with the activation of the Earth’s mantle about 2050 million years ago.

Geoquímica de las rocas plutónicas del batolito cambro-ordovícico del Cordón de Lila y de la Sierra de Almeida, Región de Antofagasta, Chile

An Upper Cambrian-Ordovician (490-460 Ma) batholith crop out along the Cordón de Lila and Sierra de Almeida, in the Antofagasta region, northern Chile, which belongs to the Famatinian arc. Two plutonic rock types can be distinguished: granodiorites and monzogranites. The first lithology is positioned along an occidental fringe in as much the second is located along an oriental part of the batholith. The main geochemical difference between the two lithologies is the enrichment in silica and K of the second with respect of the first one. Most of the granodiorites are metaluminous or are located between the metaluminous and peraluminous fields. The monzogranites are mostly peraluminous. The chemical composition of the plutons (mayor and trace elements and rare earth elements) allow to infer a common origin for both groups, from a mafic magma that evolved by fractional crystallization. The difference in composition between both lithologies, specially the meta or peraluminous character, can be explained by means a different degree of interaction of the original magma with the felsic crustal rocks.

Geochemistry of Cretaceous subduction initiation related cumulate gabbros in a forearc setting from Chaldoran ophiolite, NW Iran

<p>The Neo-Tethys-related Chaldoran ophiolite in NW Iran and at the Turkish border is a part of the larger Khoy ophiolite. Cumulate and isotropic gabbro along with serpentinized peridotite, pillow basalt, pelagic limestone, rare radiolarites, and volcano-sedimentary units are the main rock types in the area. The gabbros occur as lenses with ultramafic rocks, or as relatively large exposures with fault contact with ultramafic rocks. In this study, we provide new whole-rock geochemistry, mineral chemistry and zircon U/Pb age for the cumulate gabbros from the Chaldoran area. Gabbros have tholeiitic composition and are highly depleted. Chondrite normalized rare earth elements (REE) pattern for gabbros are comparative with REE patterns for N-MORB, but overall with more depleted features. The N-MORB normalized multi-elements pattern shows high depletion in HREE and HFSE and enrichment in some LREE and LILEs. Negative anomaly for some HFSE relative to N-MORB, along with enrichment in LILE for the samples indicates the source region as subduction influenced mantle. The cumulated gabbro whole rock and Clinopyroxenes geochemistry indicate an intra-oceanic forearc setting for the studied samples. They also have many similarities to boninite in mineral and whole rock geochemistry. U-Pb zircon dating of the gabbro samples indicates 95.3-114.1 Ma ages for the generation of the gabbros parent magma. The original magma was related to the later stages of the forearc setting in the subduction initiation (SI) stage. This ‘SI’ related Albian-Cenomanian the Chaldoran depleted gabbro likely are the continuation of Taurus SI related late Cretaceous ophiolite complexes in Turkey.</p>

Modes of Occurrences and Geochemical Characteristics of Ilmenite and Magnetite of the North Suoi Cun Lherzolite and Gabbroid

Modes of occurrences and geochemical characteristics of ilmenite and magnetite in the North Suoi Cun ultramafic rocks are studied to understand source, formation and evolution of magma chamber. The North Suoi Cun ultramafic block consist of plagiolherzolite and olivine melano-gabbro. The Plagiolherzolite and olivine melano-gabbro consist of large amounts of olivine and pyroxene, lesser amounts of plagioclase and sulfide minerals and small amounts of biotite, chromspinel, ilmenite and magnetite. The results suggest that: a/ There are two types of ilmenite, that were formed in two distinct stages: Early Mg-rich ilmenite was formed at intermediate magma chamber and latter Mn-rich ilmenite was formed at crystallized magma chamber; b/ Original magma of Suoi Cun plagiolherzolite and melano gabbroid was derived from sub-continental lithospheric mantle; c/ magnetite was formed at latter stage by two processes: serpentinization and alteration of early sulfide minerals.

Geochemistry and Sr, Nd isotopic composition of the Hronic Upper Paleozoic basic rocks (Western Carpathians, Slovakia)

The paper presents new major and trace element and first Sr-Nd isotope data from selected lavas among the Permian basaltic andesite and basalts of the Hronicum Unit and the dolerite dykes cutting mainly the Pennsylvanian strata. The basic rocks are characterized by small to moderate mg# numbers (30 to 54) and high SiO2 contents (51-57 wt. %). Low values of TiO2 (1.07-1.76 wt. %) span the low-Ti basalts. Ti/Y ratios in the dolerite dykes as well as the basaltic andesite and basalt of the 1st eruption phase are close to the recommended boundary 500 between high-Ti and low-Ti basalts. Ti/Y value from the 2nd eruption phase basalt is higher and inclined to the high-Ti basalts. In spite of this fact, in all studied Hronicum basic rocks Fe2O3* is lower than 12 wt. % and Nb/La ratios (0.3-0.6) are low, which is more characteristic of low-Ti basalts. The basic rocks are characterized by Nb/La ratios (0.56 to 0.33), and negative correlations between Nb/La and SiO2, which point to crustal assimilation and fraction crystallization. The intercept for Sr evolution lines of the 1st intrusive phase basalt is closest to the expected extrusions age (about 290 Ma) with an initial 87Sr/86Sr ratio of about 0.7054. Small differences in calculated values ISr document a partial Sr isotopic heterogeneity source (0.70435-0.70566), or possible contamination of the original magma by crustal material. For Nd analyses of the three samples, the calculated values εCHUR (285 Ma) are positive (from 1.75 to 3.97) for all samples with only subtle variation. Chemical and isotopic data permit us to assume that the parental magma for the Hronicum basic rocks was generated from an enriched heterogeneous source in the subcontinental lithospheric mantle.

Geochemistry of the Little Dal basalts: continental tholeiites from the Mackenzie Mountains, Northwest Territories, Canada

Analyses of the Little Dal lavas and the 779 Ma Tsezotene sills, both of the Mackenzie Mountains Supergroup, Northwest Territories, Canada, show them to be continental tholeiites that are geochemically related. The plagioclase- and clinopyroxene-phyric lavas are geochemically evolved and enriched in iron (up to 20.2 wt.% as Fe2O3). Two magmatic lineages are identifiable and may represent different degrees of partial melting in the same source region, but cannot be related by fractional crystallization. Within each lineage, geochemical variation can be explained by fractional crystallization involving up to 60% crystallization of the original magma. The most evolved lavas occur at the base of the pile; less fractionated lavas occur toward the top of the sequence. The Nd isotopic composition of the Little Dal lavas averages ε = 1.4 at 780 Ma. Trace element and isotopic compositions are permissive of contamination by continental crust, but do not require a crustal component. The preserved volume of the Little Dal basalts is anomalously low (−100 km3) compared with other Proterozoic continental tholeiites. There is considerable basaltic detritus in the sedimentary rocks of the overlying Coates Lake and Rapitan groups, and much of the original lava sequence may have been eroded. The Little Dal magmatic event is interpreted to be an early manifestation of rifting of North America from Australia.

Chemistry of chromian spinel in volcanic rocks as a potential guide to magma chemistry

Chromian spinel in volcanic rocks is a potential discriminant for magma chemistry. The TiO2 content of spinel, compared at similar Fe3+/(Cr + Al + Fe3+) ratios, can distinguish island arc basalts from intraplate basalts. MORB spinels are low in this ratio and are intermediate for the TiO2 level at comparable Fe3+ ratios. Spinels from back-arc basin basalts, although similar in TiO2/Fe3+ ratio, are more enriched in Fe3+ than the MORB spinels. Spinels in the oceanic plateau basalts are distinctly lower in TiO2 than other intraplate basalt spinels and even slightly lower in TiO2 than the MORB spinels. The data were successfully applied to estimate the kind of the magma from which spinelbearing cumulates, especially dunites, were formed. Original magma chemistry of altered or metamorphosed volcanics in which spinels survive can also be estimated by the chemistry of relict spinel alone. It is possible to estimate the magma type of source volcanics for detrital spinel particles of volcanic derivation.

The Wedza–Mimosa platinum deposit, Great Dyke, Zimbabwe: layering and stratiform PGE mineralization in a narrow mafic magma chamber

The Wedza–Mimosa platinum deposit is a loosely stratiform PGE-rich sulphide zone (MSZ) set within a complexly layered, shallow synclinal sequence of pyroxenites and gabbros. Both the MSZ and its host rocks display systematic stratigraphic, lithological and compositional variations between the margins and the axis of the deposit. A well preserved marginal facies is characterized by discordant layering, stratigraphic complexity, evidence of non-equilibrium crystallization, magmatic erosion and extensive pegmatoids, as well as by irregular MSZ development and metal contents. The principal transverse variations and marginal phenomena are interpreted in terms of the high transverse heat flux and heat gradient caused by the narrow trumpet-like cross-sectional structure of the original magma chamber, the magma replenishment process, the angular relationship between the chamber floor and the stratified magma, and the increase in magma density caused by the onset of plagioclase crystallization.

Petrology and geochemistry of post-obduction dykes of the Ballantrae complex, SW Scotland

ABSTRACTDoleritic intrusions known to post-date the obduction of the Ballantrae complex during Arenig times record changing magma provenance during the cooling and serpentinisation of obducted peridotite. There are two groups of dolerites with different petrographical and geochemical characteristics.The earlier emplaced group, which is subordinate, is characterised by amphibole formed under low metamorphic facies conditions and the virtual absence of Fe–Ti oxides. Chemically these rocks are of primitive tholeiitic character and are similar to modern island-arc basalts. They were derived from a strongly depleted mantle source region. The source region of the later emplaced group, which is predominant, was much less depleted. Clinopyroxene and plagioclase, much of it albitised, are the dominant minerals. Fe–Ti oxides also are common and there are pronounced chilled margins and well-preserved ophitic textures. Although showing mildly alkaline tendencies, these rocks originated from a tholeiitic parental magma of “within-plate” type.Both groups show evidence of rodingitisation and associated alteration related to serpentinisation of the peridotite. During the metasomatic activity, only some elements were mobile, while Al2O3, total Fe, MgO, TiO2, Cr, Ni, Nb, Y and Zr remained almost constant. Assessment of original magma type and geotectonic environment, and demonstration that both groups of doleritic rocks show the products of fractionation, has relied heavily on data for the apparently immobile elements and on petrographical study which identified those rocks least affected by alteration.

Compositional variation in the pyroxenes and amphiboles of the Kangerdlugssuaq intrusion, East Greenland: further evidence for the crustal contamination of syenite magma

Pyroxenes and amphiboles from the under-saturated to over-saturated syenites of the Kangerdlugssuaq intrusion have been examined to see what light they throw on the two contrasting petrogenetic models for the intrusion. Pyroxene crystals are strongly zoned outwards from augitic to more acmitic compositions, with the most calcic cores present in the foyaite, contrary to the expected pattern if the foyaite is the most evolved rock type as previously postulated. Amphiboles, which are absent in the foyaites, show an exceptionally wide compositional field varying from actinolite through richterite and katophorite to arfvedsonite. Many are manganoan and potassian varieties. However, there is no consistent variation throughout the intrusion as previous work has suggested. These results are not favourable to the idea of a crystal fractionation model for the intrusion and we suggest that the foyaite is closest to the original magma which has hybridized with the enclosing gneisses and basalts to produce the over-saturated rocks. Such a model is consistent with the existing isotopic data.