EXTREME COMPOSITIONAL VARIATION OF PYROCHLORE-GROUP MINERALS AT THE OKA CARBONATITE COMPLEX, QUEBEC: EVIDENCE OF MAGMA MIXING?

The Huangyangshan A-type granitic pluton, distributed along the thrust fault in the Kalamaili region of East Junggar, Xinjiang, China, consists of alkaline granite containing abundant dioritic enclaves that formed via magma mixing. Both the host granite and the enclaves contain sodic amphiboles. The textural evidence indicates that amphiboles crystallized as a magmatic phase in both units. We determined major and trace element contents of amphiboles from both units to investigate the compositional variation of the amphiboles during the magma mixing process. The results show that cations of W- and C-site are influenced by chemical compositions of the magma whereas cations of A-, B- and T-site and Al3+ are controlled by crystal structure. Therefore, the variations of W- and C-site cations can reflect magma evolution. The core and rim of the amphiboles show similar trace element patterns, which also suggests that the amphiboles are late-stage phases. Furthermore, the amphibole-only thermometers yield reasonable estimates that are consistent with petrographic evidence. However, thermometers based on partition coefficients and all the currently available amphibole-based barometers that rely on Al contents or DAl cannot be applied to Fe-rich and Al-poor amphiboles.

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Compositional variation within granite suites of the Lachlan Fold Belt: its causes and implications for the physical state of granite magma

Earth and Environmental Science Transactions of the Royal Society of Edinburgh ◽

10.1017/s026359330000657x ◽

1996 ◽

Vol 87 (1-2) ◽

pp. 159-170 ◽

Cited By ~ 34

Author(s):

B. W. Chappell

Keyword(s):

Magma Mixing ◽

Volcanic Rocks ◽

Fractional Crystallisation ◽

Source Rocks ◽

Compositional Variation ◽

Fold Belt ◽

Significant Group ◽

Lachlan Fold Belt ◽

Compositional Variations ◽

Felsic Rocks

ABSTRACT:Granites within suites share compositional properties that reflect features of their source rocks. Variation within suites results dominantly from crystal fractionation, either of restite crystals entrained from the source, or by the fractional crystallisation of precipitated crystals. At least in the Lachlan Fold Belt, the processes of magma mixing, assimilation or hydrothermal alteration were insignificant in producing the major compositional variations within suites. Fractional crystallisation produced the complete variation in only one significant group of rocks of that area, the relatively high temperature Boggy Plain Supersuite. Modelling of Sr, Ba and Rb variations in the I-type Glenbog and Moruya suites and the S-type Bullenbalong Suite shows that variation within those suites cannot be the result of fractional crystallisation, but can be readily accounted for by restite fractionation. Direct evidence for the dominance of restite fractionation includes the close chemical equivalence of some plutonic and volcanic rocks, the presence of plagioclase cores that were not derived from a mingled mafic component, and the occurrence of older cores in many zircon crystals. In the Lachlan Fold Belt, granite suites typically evolved through a protracted phase of restite fractionation, with a brief episode of fractional crystallisation sometimes evident in the most felsic rocks. Evolution of the S-type Koetong Suite passed at about 69% SiO2 from a stage dominated by restite separation to one of fractional crystallisation. Other suites exist where felsic rocks evolved in the same way, but the more mafic rocks are absent. In terranes in which tonalitic rocks formed at high temperatures are more common, fractional crystallisation would be a more important process than was the case for the Lachlan Fold Belt.

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Causes of compositional diversity in a lobe of the Half Dome granodiorite, Tuolumne Batholith, Central Sierra Nevada, California

Earth and Environmental Science Transactions of the Royal Society of Edinburgh ◽

10.1017/s1755691009016065 ◽

2009 ◽

Vol 100 (1-2) ◽

pp. 173-183 ◽

Cited By ~ 6

Author(s):

R. C. Economos ◽

V. Memeti ◽

S. R. Paterson ◽

J. S. Miller ◽

S. Erdmann ◽

...

Keyword(s):

Sierra Nevada ◽

Magma Mixing ◽

Minimum Size ◽

Compositional Variation ◽

Isotopic Study ◽

High Silica ◽

Dominant Process ◽

Compositional Diversity ◽

Compatible Elements ◽

Host Rocks

ABSTRACTThe causes of compositional diversity in the Tuolumne Batholith, whether source heterogeneity, magma mixing, or fractional crystallisation, is a matter of longstanding debate. This paper presents data from detailed mapping and a microstructural and major element, trace element and isotopic study of an elongate lobe of the Half Dome granodiorite that protrudes from the southern end of the batholith. The lobe is normally zoned from quartz diorite along the outer margin to high-silica leucogranite in the core. Contacts are steep and gradational, except for the central leucogranite contact, which is locally sharp: magmatic fabrics overprint contacts. A striking feature of the lobe is the 18 wt SiO2 range comparable to that observed for the entire Tuolumne Batholith. Feldspar-compatible elements (Sr and Ba) decrease towards the centre, while Rb increases. Light and middle REEs show a smooth decrease towards the centre of the lobe. Calculated initial isotopic ratios of 87Sr/86Sr(i) and εNd(t) have identical values within error across the lobe, except in the central leucogranite, the most silica rich phase, which shows a slightly more crustal signature. Field, structural, geochemical and isotopic data suggest that fractionation was the dominant process causing compositional variation in this lobe. It is envisioned that this fractionation/crystal sorting occurred in a vertically flowing and evolving magma column with the present map pattern representing a cross-section of this column. Thus the areal extent of the lobe represents a minimum size of interconnected melt at the emplacement level of the Tuolumne Batholith and, given its marginal position, limited width and proximity to colder host rocks, implies that fractionation in larger chambers likely occurred in the main Tuolumne Batholith magma chamber(s).

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Pyrochlore from the Bailundo Carbonatite Complex (Angola): Compositional variation and implications to mineral exploration

Journal of African Earth Sciences ◽

10.1016/j.jafrearsci.2021.104154 ◽

2021 ◽

Vol 177 ◽

pp. 104154

Author(s):

I. Ribeiro da Costa ◽

J. Roseiro ◽

J. Figueiras ◽

P.C.R. Rodrigues ◽

A. Mateus

Keyword(s):

Mineral Exploration ◽

Compositional Variation ◽

Carbonatite Complex

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Compositional variation in pyrochlores of Amba Dongar carbonatite complex, Gujarat

Journal of the Geological Society of India ◽

10.1007/s12594-010-0048-2 ◽

2010 ◽

Vol 75 (3) ◽

pp. 495-502 ◽

Cited By ~ 9

Author(s):

Shrinivas G. Viladkar ◽

Ulrich Bismayer

Keyword(s):

Compositional Variation ◽

Carbonatite Complex ◽

Amba Dongar

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Pyrochlore Group Minerals from the Qaqarssuk Carbonatite Complex

Lanthanides, Tantalum and Niobium ◽

10.1007/978-3-642-87262-4_3 ◽

1989 ◽

pp. 80-99 ◽

Cited By ~ 2

Author(s):

C. Knudsen

Keyword(s):

Carbonatite Complex ◽

Pyrochlore Group

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Lueshite, pyrochlore and monazite-(Ce) from apatite-dolomite carbonatite, Lesnaya Varaka complex, Kola Peninsula, Russia

Mineralogical Magazine ◽

10.1180/002646198548151 ◽

1998 ◽

Vol 62 (6) ◽

pp. 769-782 ◽

Cited By ~ 51

Author(s):

A. R. Chakhmouradian ◽

R. H. Mitchell

Keyword(s):

Kola Peninsula ◽

Divalent Cations ◽

High Capacity ◽

Compositional Variation ◽

X Ray ◽

Degree Of Hydration ◽

Pyrochlore Group ◽

Primary Mineral ◽

High Degree

AbstractApatite-dolomite carbonatite at Lesnaya Varaka, Kola Peninsula, Russia, hosts intricate mineral intergrowths composed of lueshite in the core and pyrochlore-group minerals in the rim. Lueshite is a primary Nb-bearing phase in the carbonatite and ranges in composition from cerian lueshite to almost pure NaNbO3. For comparison, the compositional variation of lueshite from the Kovdor and Sallanlatvi carbonatites is described. At Lesnaya Varaka, lueshite is replaced by nearly stoichiometric Na-Ca pyrochlore due to late-stage re-equilibration in the carbonatite system. X-ray powder diffraction data for both minerals are presented. Barian strontiopyrochlore, occurring as replacement mantles on Na-Ca pyrochlore, contains up to 43% Sr and 8–18% Ba at the A-site, and shows a high degree of hydration and strong ionic deficiency at the A- and Y-sites. This mineral is metamict and, upon heating, recrystallises to an aeschynite-type structure. Monazite-(Ce) found as minute crystals in fractures, represents the solid solution between monazite-(Ce) CePO4, brabantite CaTh(PO4)2 and SrTh(PO4)2. Our data indicate the high capacity of the monazite structure for Th and accompanying divalent cations at low temperatures and pressures that has a direct relevance to solving the problem of long-term conservation of radioactive wastes. Monazite-(Ce) and barian strontiopyrochlore are products of low-temperature hydrothermal or secondary (hypergene) alteration of the primary mineral assemblage of the carbonatite.

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Mineralogy and genesis of pyrochlore apatitite from The Good Hope Carbonatite, Ontario: A potential niobium deposit

Mineralogical Magazine ◽

10.1180/mgm.2019.64 ◽

2019 ◽

Vol 84 (1) ◽

pp. 81-91 ◽

Cited By ~ 1

Author(s):

Roger H. Mitchell ◽

Rudy Wahl ◽

Anthony Cohen

Keyword(s):

Magma Mixing ◽

Alkaline Rock ◽

Potassium Feldspar ◽

Oscillatory Zoning ◽

Scattered Electron ◽

Carbonatite Complex ◽

Northwestern Ontario ◽

Compositional Zonation ◽

Phase Relationships

AbstractThe Good Hope carbonatite is located adjacent to the Prairie Lake alkaline rock and carbonatite complex in northwestern Ontario. The occurrence is a heterolithic breccia consisting of diverse calcite, dolomite and ferrodolomite carbonatites containing clasts of magnesio-arfvedsonite + potassium feldspar, phlogopite + potassium feldspar together with pyrochlore-bearing apatitite clasts. The apatitite occurs as angular, boudinaged and schlieren clasts up to 5 cm in maximum dimensions. In these pyrochlore occurs principally as euhedral single crystals (0.1–1.5 cm) and can comprise up to 25 vol.% of the clasts. Individual clasts contain compositionally- and texturally-distinct suites of pyrochlore. The pyrochlores are hosted by small prismatic crystals of apatite (~100–500 μm × 10–25 μm) that are commonly flow-aligned and in some instances occur as folds. Allotriogranular cumulate textures are not evident in the apatitites. The fluorapatite does not exhibit compositional zonation under back-scattered electron spectroscopy, although ultraviolet and cathodoluminescence imagery shows distinct cores with thin (<50 μm) overgrowths. Apatite lacks fluid or solid inclusions of other minerals. The apatite is rich in Sr (7030–13,000 ppm) and rare earth elements and exhibits depletions in La, Ce, Pr and Nd (La/NdCN ratios (0.73–1.14) relative to apatite in cumulate apatitites (La/NdCN > 1.5) in the adjacent Prairie Lake complex. The pyrochlore are primarily Na–Ca pyrochlore of relatively uniform composition and minor Sr contents (<2 wt.% SrO). Irregular resorbed cores of some pyrochlores are A-site deficient (>50%) and enriched in Sr (6–10 wt.% SrO), BaO (0.5–3.5 wt.%), Ta2O5 (1–2 wt.%) and UO2 (0.5–2 wt.%). Many of the pyrochlores exhibit oscillatory zoning. Experimental data on the phase relationships of haplocarbonatite melts predicts the formation of apatite and pyrochlore as the initial liquidus phases in such systems. However, the texture of the clasts indicates that pyrochlore and apatite did not crystallise together and it is concluded that pyrochlores formed in one magma have been mechanically mixed with a different apatite-rich magma. Segregation of the apatite–pyrochlore assemblage followed by lithification resulted in the apatitites, which were disrupted and fragmented by subsequent batches of diverse carbonatites. The genesis of the pyrochlore apatitites is considered to be a process of magma mixing and not simple in situ crystallisation.

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Hybrid magma generation preceding Plinian silicic eruptions at Hekla, Iceland: evidence from mineralogy and chemistry of two zoned deposits

Geological Magazine ◽

10.1017/s0016756807003470 ◽

2007 ◽

Vol 144 (4) ◽

pp. 643-659 ◽

Cited By ~ 32

Author(s):

GUDRUN SVERRISDOTTIR

Keyword(s):

Basaltic Andesite ◽

Surface Composition ◽

Magma Mixing ◽

Major And Trace Elements ◽

Silicic Magma ◽

Compositional Variation ◽

Rhyolitic Magma ◽

Partial Melt ◽

Dyke Injection ◽

Reverse Zoning

Hekla is a Holocene volcanic ridge in southern Iceland, which is notable for the link between repose periods and the composition of the first-erupted magma. The two largest explosive silicic eruptions, H4 and H3, erupted about 4200 and 3000 years ago. Airfall deposits from these eruptions were sampled in detail and analysed for major and trace elements, along with microprobe analyses of minerals and glasses. Both deposits show compositional variation ranging from 72 % to 56 % SiO2, with mineralogical evidence of equilibrium crystallization in the early erupted rhyolitic component but disequilibrium in the later erupted basaltic andesite component. The eruptions started with production of rhyolitic magma followed by dacitic to basaltic andesite magma. Sparse crystallization of the intermediate magma and predominant reverse zoning of minerals, trending towards a common surface composition, indicate magma mixing between rhyolite and a basaltic andesite end-member. The suggested model involves partial melting of older tholeiitic crust to produce silicic magma, which segregated and accumulated in deep crustal reservoir. Silicic magma eruption is triggered by basaltic andesite dyke injection, with a proportion of the dyke magma contributing to the production and eruption of a mixed hybrid magma. Both the volume of the silicic partial melt, and the proportion of the hybrid magma depend on the pre-eruptive repose time.

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