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PETROGENETIC ASPECTS OF THE CONTAMINATION PROCESSES IN THE MAFIC-ULTRAMAFIC COMPLEX OF NIQUELANDIA (GOIÀS, BRASIL).
No full textThe mafic-ultramafic complex of Niquelandia is one of the three major layered intrusions occurring in the Goiàs State, Central Brasil.Previous studies on this complex [1] have revealed that the parent melts of the complex have suffered a significant crustal contamination.In a recent field work several lenses or septa of exotic metamorphic rocks were found striking parallel or sub-parallel to the complex layering: the total amount of these rocks has been estimated to represent the 13% of the volume of the whole complex.These lenses or septa were found starting from the intermediate portion of the LGZ unit upwards; however, they are mainly concentrated at the contact between LS and the US. They can be divided in three major lithotypes: quartzites, gneisses and calc-silicate rocks derived from sandstones, pelites and marls, respectively. The petrological and geochemical investigation of transects perpendicular to the complex layering provided the evidences that these rocks are the crustal contaminants of the complex parent melts: as the matter of fact they produced an enrichment in the hybrid melts of incompatible elements (i.e.: LREE, K, Ba and Rb).On the basis of their petrographic and geochemical characters, the contaminants are supposed to belong to the upper stratigraphic unit of the volcano-sedimentary sequence of Indaianopolis. This is also in agreement with geochronological data. In fact Pimentel and coworkers [2] dated one of the septa of contaminants occurring in US with the U/Pb method on zircons: they obtained an age of 1248±23 Ma and suggested that this age is referred as the US intrusion age, proposing that US and LS (their estimate of LS intrusion age is 797±10 Ma) were not coeval. This model is incongruent with the robust evidences that US and LS are cogenetic, as for instance the similare LILE patterns found in rocks of the two different units or the outcropping, in LS, of cumulus rock similar to the US ones.We present a simplified model of assimilation and fractional crystallization which tentatively supports the hypothesis that LS and US are genetically kindred. References. [1] Rivalenti, G., Correia, C.T., Girardi, V.A.V., Mazzuchelli, M., Tassinari, C.C., Bertotto, G.W. (2008): J. South Amer. Earth Sci., 25, 298-312; [2] Pimentel, M.M., Ferreira Filho, C.F., Amstrong, R.A. (2004): Precambrian Research, 132, 132-135
Equivocal carbonatite markers in the mantle xenoliths of the Patagonia backarc: the Gobernador Gregores case (Santa Cruz Province, Argentina)
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Origin and significance of late noritic dykes in the Baldissero Peridotite Massif (Ivrea-Verbano zone)
No full textThe Baldissero Peridotite Massif (Southern Ivrea-Verbano Zone) is a depleted lherzolite (ol 65/70, opx 16/20, cpx 10/11, sp 2/2.5, La(N)/Yb(N) 0.003/0.26, ε(Nd)270 = 6.4/14, 87Sr/86Sr = 0.7021/0.7035). It is cross-cutted by a swarm of 5/15 cm thick, fine grained (<1mm), NNE trending noritic dykes. Contacts peridotite-dyke are sharp. Dyke texture is hypidiomorphic and their mode consists of dominant plagioclase (31/45 %, An 40/48), orthopyroxene (25/37%, En = 78.9/87.8, Wo = 0.6/1.1), subordinate clinopyroxene (14/18%, En = 42.7/44.0, Wo = 47.9/52.4), interstitial pargasitic amphibole (10/15%) and opaques (1/2%, Fe-Ni sulfides). Re-equilibration conditions are 0.8/1 GPa and 800/900°C, like those of the ambient peridotite. Major element (wt%) dyke composition is characterized by high MgO (10/16) and high Mg values (0.81/0.89). In the observed MgO range, SiO2, CaO and K2O vary negligibly (52/53, 9/10 and <0.2, respectively). With decreasing MgO, TiO2 varies randomly from 0.8 to 0.4, Al2O3 increases from 15 to 21, Na2O from 3 to 4.5 and FeO decreases from 5 to 2. Bulk rock incompatible trace element patterns (Fig. 1) vary from weakly LREE depleted to weakly LREE enriched (La(N)/Yb(N) = 0.5/2.4) and have marked positive Sr anomaly (Sr/Sr(*) = 5.6/10.4), smaller Eu anomaly (Eu/Eu(*) 1.3/1.5) and negative Nb anomaly (La(N)/Nb(N) = 1.2/2.2). Incompatible element concentration increases, and the magnitude of the negative Nb and of the positive Sr anomalies decrease, with increasing MgO or Mg. Main incompatible trace element carriers are clinopyroxene and hornblende, which display patterns varying from slightly LREE depleted to enriched. Orthopyroxene is LREE depleted and plagioclase is LREE and Sr enriched. Clinopyroxene is Nb-depleted (La(N)/Nb(N) = 1.0/5.9) and amphibole is Nb enriched (La(N)/Nb(N) = 0.09/0.35). The latter phases have negative Zr spikes. Incompatible element concentration in the mafic phases show correlations with Mg similar to those of bulk rock (Fig. 2). The only available isotope determination refers to a Sm-Nd internal isochron which results in an age of 180Ma and ε(Nd) = 7.8. In spite of the contacts being sharp, peridotite mineral assemblage and composition varies over a distance of 7 cm. In the peridotite, toward the dyke contact, modal orthopyroxene increases and clinopyroxene decreases, whereas trace element concentration and La(N)/Yb(N) increase (from 0.003/0.017 to 0.1/0.26) and Mg decreases from 0.9 to 0.8. The dyke at the peridotite contact does not show sizeable chemical or modal variations. There is no clear relationship between the Mg variation of the dyke and the chemical trends observed in the contact peridotite. The high Mg numbers of the dykes and the Mg value of orthopyroxene, lower than that of bulk-rock, indicate that these norites are not melts, but rather represent mineral segregates possibly retaining some interstitial liquid. Estimates of the Mg values of the parental melts, made assuming (opx/liq)D(Mg) = 1.2, indicates a Mg range from 0.66 to 0.74, which is consistent with that of primary mantle-derived magmas. Since olivine does not occur even in the more MgO-rich dyke composition, the parent melt had to be, furthermore, SiO2-rich and hydrous, as indicated by the amphibole abundance. Whatever the set of solid/melt partition coefficient used for estimating trace elements in the parent melts, the calculated REE profiles would vary from slightly LREE depleted to variably LREE enriched. The HFSE characteristics of the primary melts cannot be precisely constrained because of the strong dependence of their partition coefficient on the melt composition. For example, using available partition coefficients for basaltic compositions, the estimated parental melts would have a weak Nb and Zr anomaly, which disappears using D sets for intermediate compositions and become positive if the D's are referred to high-silica compositions. In any case, the geochemical trends shown by bulk rock and mineral phases (i.e. the increase of incompatible element concentration with increasing Mg) are qualitatively maintained. We propose that this peculiar feature can be explained either by fluid-assisted melting of a depleted peridotite source, where melting degree increases with the increasing supply of hydrous fluid (which is the incompatible element carrier), or by fluid-peridotite reaction during porous flow percolation in a mantle column at a deeper and higher temperature level than that actually observed. If confirmed by new isotopic data, the occurrence of post-Hercynian gabbroic rocks in the Australpine and South Alpine domains will support the involvement of the marginal units of the Adria plate in the pre-oceanic extensional regime (Piccardo et al., 1994)
Interaction of mantle and crustal magmas in the southern part of the Ivrea Zone (Italy).
No full textGoing Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Carbonated peridotite xenoliths from the mantle wedge: The Patagonia case
No full textCarbonated mantle xenoliths are mainly documented in intraplate environments. Here we provide evidence that carbonated mantle also occurs in the wedge of the back-arc region related with the Andean subduction. Along the whole length of the Argentinean Andes, this region is characterised by alkali basalt volcanic centres containing abundant mantle xenoliths. The xenoliths of the easternmost volcanoes bear evidence of mantle interaction with a CO2-rich component. In the volcanic centre of Gobernador Gregores (Santa Cruz Province, Southern Patagonia) this interaction produces abundant carbonates. Here a large diatreme constituted by pyroclastic deposits contains mantle xenoliths up to 60 cm. The peridotites are dominantly lherzolites and subordinate harzburgites, wherlites and dunites, whose texture varies from secondary-protogranular (recrystallized) to weakly foliated. The relationships between the forming phases indicate the following events: an original mineral assemblage of olivine (ol1), clinopyroxene (cpx1), orthopyroxene and spinel (spl1) is overprinted by a subsequent mineral assemblage, constituted by new clinopyroxene (cpx2), pargasite and sometimes phlogopite; in turn, this mineral assemblage reacts with a further metasomatic agent which causes instability of cpx2 and of the hydrous phases. As an effect of the latter episode, cpx2 and hydrous phases are surrounded by pockets of carbonate plus silicate glass. Carbonate also occurs in veins at the crystal boundaries, as blebs in the silicate glass. Euhedral olivine (ol2), clinopyroxene (cpx3), spinel (spl2) and rarely rutile crystallise from the silicate glass. Apatite occurs both in the silicate glass and in the carbonates. Forsterite concentration in olivine varies from 87/91 in ol1 to 89/93 in ol2, which also contains high CaO (0.15/0.80 wt%). Cpx3 is characterised by higher Al2O3, CaO, TiO2 and lower Na2O with respect to cpx2. Spl2 contains higher Al2O3 and TiO2 with respect to spl1. Carbonate is calcite with MgCO3 concentrations up to 4%. Glass composition varies from trachiandesitic to tefriphonolitic. Trace element characteristics of clinopyroxenes vary markedly from cpx1 to cpx3 (Figs. 1a, b, c). Cpx1 is LREE depleted, whereas cpx2 is the richest in REE and has a fractionated REE pattern (La(N)/Yb(N) = 9.6/14.8) with a maximum at Ce. Cpx3 has a lower REE concentration with respect to cpx2; its REE pattern (La(N)/Yb(N) = 1.8/2.9) shows a maximum at MREE. HFSE anomaly are negative and variable in cpx2 and cpx3 (La(N)/Nb(N), Ti/Ti(*) and Zr/Zr(*) range 6.77/52.13, 0.02/0.24, 0.08/0.34 respectively in cpx2 and 0.76/0.88, 0.10/0.16, 0.14/0.22 respectively in cpx3). Amphibole has REE patterns (Fig. 1d) similar to cpx2 ((parg/cpx2)D(REE) ~1), but with marked positive Nb spikes ((parg/cpx2)D(Nb) = 42.3/148.4) and positive Ti and Sr anomalies ((parg/cpx2)D(Ti) = 5.4/8.2; (parg/cpx2)D(Sr) = 1.7/1.9). Zr is preferentially partitioned into amphibole ((parg/cpx2)D(Zr) = 1/1.5). Glass is LREE enriched and has smoothly fractionated patterns from La to Yb (La(N)/Yb(N) = 10.0/29.6). It has remarkable positive Nb anomalies (La(N)/Nb(N) = 0.1/0.6) - much higher respect to the associated amphibole ((parg/glass)D(Nb) = 0.4/0.7) - and generally negative Zr and Ti spikes (Fig. 1e). Apatite has a very high LREE concentration (La(N) ~ 4300) and La(N)/Yb(N) ~3500 (Fig. 1f). The last metasomatic episode is mass balanced by the following reaction: 1.00anf + 0.26opx + 0.02carb = 0.41cpx3 + 0.05spl2 + 0.36ol2 + 0.46glass. The relationships above described indicate that litospheric spinel facies mantle was affected by metasomatic episodes consistent with the former addition of a hydrous component followed by a CO2-rich component. The P-T range of these metasomatic events is 8.3 - 22.4 kbar and 890 - 1230°C (Fig. 2). Amphibole and cpx2 instability increases with increasing T and P, so that the more marked carbonatation occurs in the higher T-P range. Although the metasomatic agents, including the CO2-rich one, are possibly related with dehydration and decarbonation of the slab, their provenience needs to be isotopically constrained, since Gorring et al. (1997) propose that at this latitude the slab had windows through which the mantle underlying the slab interacted with the wedge
The mantle lithosphere in Northeastern Brazil and Fernando de Noronha. Plume-related mantle metasomatism?
No full textCenozoic alkali basalts in the Rio Grande do Norte State and the Fernando de Noronha Island contain abundant spinel facies mantle xenoliths. At Pico Cabuji (NE Brazil) both protogranular and porphyroclastic xenoliths occur. The former mainly consists of lherzolites and the latter of harzburgites, with average equilibrium temperatures of 1064±6°C and 1238±7°C, respectively. Pressure is assumed essentially equal in the two groups (~1.8 GPa). Porphyroclastic xenoliths have more refractory bulk rock and mineral phase compositions than the progranular ones, in keeping with their mode. Both groups show chemical variation trends consistent with fractional melting and basalt extraction. REE profiles of clinopyroxenes vary from LREE enriched (spoon shaped) to LREE depleted in the progranular group, whereas they are dominantly slightly convex upwards in the porphyroclastic clinopyroxenes. HFSE (Ti and Zr) anomalies are in general modest in the clinopyroxenes of both groups. At Fernando de Noronha, xenoliths are variably-textured lherzolites and harzburgites. Their chemical variation trends overlap and extend those of the Pico Cabuji samples. Average temperature is 1056±87°C and pressure 1.6±0.2 GPa. Clinopyroxenes have spoon-shaped to LREE depleted profiles similar to those of the Pico Cabuji protogranular type, but have higher REE concentrations and more marked negative HFSE spikes (Fig. 1). Nd and Sr isotopes of the Pico Cabuji porphyroclastic clinopyroxenes (143Nd/144Nd = 0.51339-0.51255, 87Sr/86Sr = 0.70275-0.70319) and of Fernando de Noronha (143Nd/144Nd = 0.51323-051285, 87Sr/86Sr = 0.70323-0.70465) plot on distinct arrays originating from a similar, isotopically depleted composition and trending to lowNd-lowSr and lowNd-highSr, respectively. 143Nd/144Nd decreases, and 87Sr/86Sr increases, with increasing Mg of clinopyroxene and with all the parameters indicative of previous melting episodes (modal clinopyroxene, bulk rock MgO). The opposite correlations are observed with LREE concentration, La(N)/Sm(N), and with all the parameters reflecting metasomatic enrichment indicating different isotope and geochemical characteristics of the Pico Cabuji and Fernando de Noronha metasomatic agents. In both occurrences the premetasomatic lithosphere was isotopically similar to DMM (depleted MORB mantle) whereas the metasomatic component was EMI-like and EMII-like at Pico Cabuji and Fernando de Noronha respectively. The EMII component of Fernando de Noronha may be related with a crustal reservoir present at Fernando de Noronha and not at Pico Cabuji. The diversity of the metasomatic components between Pico Cabuji and Fernando de Noronha indicates considerable heterogeneity in their respective sources, difficult to reconcile with their supposed genetic relationship with the same plume
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