Chapter 18 Characterization of mafic enclaves in the erupted products of Soufrière Hills Volcano, Montserrat, 2009 to 2010
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Abstract Lavas from the current eruption of the Soufrière Hills Volcano (SHV), Montserrat exhibit evidence for magma mingling, related to the intrusion of mafic magma at depth. We present detailed field, petrological, textural and geochemical descriptions of mafic enclaves in andesite erupted during 2009–2010, and subdivide the enclaves into three distinct types: type A are mafic, glassy with chilled margins and few inherited phenocrysts; type B are more evolved with high inherited phenocryst content and little glass, and are interpreted as significantly hybridized; type C are composite, with a mafic interior (type A) and a hybrid exterior (type B). All enclaves define tight linear compositional trends, interpreted as mixing between a mafic end member (type A) and host andesite. Enclave glasses are rhyolitic, owing to extensive crystallization during quenching. Type A quench crystallization is driven by rapid thermal equilibration during injection into the andesite. Conversely, type B enclaves form in a hybridized melt layer, which ponded near the base of the chamber and cooled more slowly. Vesiculation near the mafic–silicic interface resulted in disruption of the hybridized layer and the formation of the type B enclaves. The composite enclaves represent an interface between types A and B, suggesting multiple episodes of mafic injection.Metasomatism
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Hornblende
Fractional crystallization (geology)
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The Tulargen Cu-Ni sulfide deposit,whose discovery means a great breakthrough in Cu-Ni exploration in Xinjiang in the past ten years,is located in the eastern section of the Huangshan-Jingerquan ore belt.The Tulargen Cu-Ni deposit is a magmatic differentiation deposit related to mafic-ultramafic complex,which comprises three intrusions,namely complex Ⅰ,Ⅱ,Ⅲ,respectively.The hornblende pyroxene peridotite of complex Ⅰ is associated with abundant mafic enclaves,which include ore-bearing gabbro enclaves and plagioclasite enclaves without mineralization.Mafic enclaves and the host display different characteristics in such aspects as mineral species,types and intensities of alteration,contacts between minerals and forming sequence.There exists a distinct boundary,composed of squamiform chlorite,between the mafic enclaves and the host.The Harker diagram of major elements and figures of La,Nd,Zr,and Yb versus SiO2 have shown that samples of the Tulargen complex display a good correlation,whereas the mafic enclaves don't show such characteristics,which indicates that magmatic contamination played a significant role in the formation of the mafic enclaves.Nb/U and Ce/Pb ratios imply that,after crustal contamination,some elements of the mafic enclaves was mobilized and migrated by the late magmatic hydrous fluids.
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The marginal mafic granulites that locally border the Nain Plutonic Suite (NPS) have a range of initial Nd-isotope ratios that overlap with that of the NPS anorthosites and associated Nain dykes. The similarity in Nd-isotope data suggests that gneissic Archaean country rocks have contaminated all the anorthosites, marginal mafic granulites, and dykes. Sr-isotope data for the mafic granulites and dykes support a country rock contamination scenario but preclude wholesale assimilation of rocks such as the host Archaean tonalite gneisses as the sole contaminant. Initial epsilon Sr values of +10 to +403 and +0.9 to +242 for the mafic granulites and dykes, respectively, are significantly higher than values for NPS country rocks examined thus far. The elevated initial ε Sr values are therefore interpreted to result from the introduction of radiogenic Sr into the granulites and dykes via Sr-rich fluids, generated by the breakdown of Rb-rich mineral phases such as biotite in the country rocks during NPS.
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Abstract The Archaean Bastar craton is known for the presence of different generations of mafic dykes. Less studied many NW‐SE trending mafic dykes (intruded into the Archaean supracrustal rocks), encountered from the central part of the Bastar craton, are studied for their petrological and geochemical characteristics. There are many geological evidences which suggest emplacement of these mafic dykes in an intracratonic setting. Two distinct types of mafic dykes are recognized. Petrographically it is difficult to discriminate these two types. Many samples show metamorphic textures and amphibolites facies mineral assemblage but a few samples preserved original igneous texture and mineralogy. Major oxides show sub‐alkaline thoeliitic basalt/basaltic andesite nature. High‐Fe and high‐Mg contents classify them as high‐iron and high‐magnesium tholeiites. They fall in the gabbronorite field on R1‐R2 plot. Geochemical characteristics, particularly high‐field strength and rare‐earth elements, clearly distinguished them into two types: Group 1 and Group 2 mafic dykes. When compared with well‐studied mafic dyke swarms of the Southern Bastar craton, the Group 1 is recognized as Meso‐Neoarchaean sub‐alkaline mafic dykes (BD1‐CBC) and the Group 2 as Neoarchaean‐Palaeoproterozoic boninite–norite mafic dykes (BN‐CBC). The boninite–norite nature of the second group is also corroborated through immobile trace element ratios. BD1‐CBC dykes are characterized by a relatively higher concentration of HFSE and REE in comparison to BN‐CBC dykes. Some extent of crustal contamination is observed in BN‐CBC dykes but BD1‐CBC dykes do not show any indication of crustal contamination. Trace element modelling suggest that BD1‐CBC mafic dykes are derived from a melt originated through ∼20% melting of a depleted mantle source, whereas BN‐CBC mafic dykes are probably derived from a high‐Mg magma generated through ∼25% melting of a refractory mantle source. Both melts have undergone 30–40% olivine fractionation before the emplacement. Geochemistry also points out involvement of a plume in the genesis of these mafic dykes. Copyright © 2011 John Wiley & Sons, Ltd.
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We report the zircon Hf-O isotopic data for mafic enclaves from the Taihang Yanshanian intermediate to felsic plutons, and use them, along with the petrological, whole-rock chemical and Nd-Sr isotopic data, to reveal the petrogenesis of mafic enclaves. Mafic enclaves show magmatic textures and are finer-grained than host rocks. In places they are highly elongated due to stretching within the partially crystallized, convective felsic magma, but show no solid-state deformation. These data suggest that mafic enclaves and host rocks were co-existing, but compositionally distinct magmas. The mafic enclaves contain abundant hydrous minerals such as hornblende and biotite, with pyroxene relict being surrounded by hornblende reaction rim. Plagioclase xenocrysts from mafic enclaves show a complicated compositional and textural disequilibrium. Comparison between mafic enclaves and the immediate host rocks suggests that the two rock units are compositionally correlated. The eNd values of mafic enclaves are generally higher than the host rocks, though the Sr isotopic ratios of the two rock units are indistinguishable. Zircons from a single enclave sample show a significant variation in Hf isotopic compositions, with eHf = -10―-22, suggesting an origin through magma mixing between mafic and felsic magmas. This is supported by the relatively large variation of zircon O isotopic ratios (δ 18O = 5.5‰- 7.8‰) of the mafic enclaves. The petrogenesis of mafic enclaves could be described as below. Evolved basaltic magma (via fractionation of olivine and pyroxene) first mixed with crustally derived granitic melts in depths, forming a hybrid magma; then the hybrid magma broke up into discrete lumps upon entering the above felsic magma. Subsequently, the enclave-forming magma experienced a double mechanical transfer of plagioclase, and inward chemical transfer of fluid and Na, P, Y, Nb and Pb at the contacts with host felsic magmas.
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