The composition of garnet in garnet-rich rocks in the southern Proterozoic Curnamona Province, Australia: an indicator of the premetamorphic physicochemical conditions of formation

Heimann, Adriana; Spry, Paul G.; Teale, Graham S.; Conor, Colin; Pearson, Norman J.

doi:10.1007/s00710-010-0130-x

Cited by 24 publications

(10 citation statements)

References 62 publications

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“…Extrapolating these temperatures to the compact hematite investigated here, together with the aforementioned lines of evidence, it is reasonable to estimate temperatures of 300À350ºC for the spessartine and its host rock. The REE pattern of the compact-hematitehosted spessartine is analogous to that reported for garnet in garnetite from the Broken Hill deposit, Australia, the characteristics of which are very low LREE/HREE ratios and positive Eu anomalies (Schwandt et al, 1993;Spry et al, 2007;Heimann et al, 2011). In those rocks, which are considered to have originated from submarine hydrothermal processes, the positive Eu anomaly of garnet is thought to reflect reducing conditions of the fluids from which pre-metamorphic phases in the protolith precipitated (e.g.…”

Section: Discussionsupporting

confidence: 61%

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Spessartine in compact-hematite rock, southern Serra do Espinhaço, Minas Gerais, Brazil, and genesis of compact hematite

2013

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Fine-grained garnet (grains <50 μm across) is an accessory component of compact hematite, a rock consisting essentially of hematite; compact hematite is a variety of high-grade Fe ore. The garnet is characterized compositionally as spessartine (81–86 mol.%) with subordinate, but significant, amounts of calderite (5–11 mol.%) and “blythite” (up to ∼5 mol.%), as well as andradite (4–7 mol.%); pyrope and almandine endmembers are ≤∼2 and 1 mol.%, respectively. The recognition of spessartine in compact hematite indicates that oxidation state, rather than whole-rock chemical composition, controlled the garnet composition. The spessartine has a positive Eu anomaly, and a low Th/U ratio (0.13–0.65) compared to the average upper continental crust, Th/U = 3.9, as well as a positive linear correlation of Uvs.Li. The spessartine-hosting compact-hematite rock is a high-grade hematite ore that similarly shows a low Th/U ratio, but a convex, tetrad-like segment between Gd and Dy. Decoupling of Eu from other rare-earth elements and of U from Th in the spessartine, together with the anomalous geochemical behaviour of Gd, Tb and Dy in the compact hematite, and its low Th/U ratio, could have been achieved under oxidizing conditions at greenschist-facies metamorphic temperatures. The U and Li present in the spessartine could have been sourced from metamorphic fluids of continental, possibly evaporitic, origin. These interpretations are underpinned by the regional hematitization and tourmalinization observed in the southern Serra do Espinhaço.

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Section: Discussionsupporting

confidence: 61%

“…Some studies indicate that the composition of garnet reflects the whole-rock composition (e.g. Spry et al, 2007;Heimann et al, 2011). However, the whole-rock Mn content in the compact hematite is small, not exceeding 0.60 wt.% MnO (Table 1).…”

Section: Discussionmentioning

confidence: 99%

Spessartine in compact-hematite rock, southern Serra do Espinhaço, Minas Gerais, Brazil, and genesis of compact hematite

2013

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“…Chemical trends of alteration include (a) feldspar dissolution coupled with progressive enrichment of Fe-Mg-Mn from the white gneisses to the aluminous gneisses and to the garnetites, and (b) an argillic-type alteration in some white gneisses. In particular, the elevated Mn contents in garnet from the garnetites, combined with the presence of graphite in these rocks, are consistent with Mn and Fe contribution from a high-T and CO 2 -bearing fluid (see Spry and Wonder 1989;Large 1992;Heimann et al 2011). Variable degree of plagioclase dissolution by the hydrothermal fluid is also suggested by the differences in Eu between the precursor rocks (white gneiss: positive Eu anomalies), the aluminous gneisses (no anomalies), and the garnetites (negative Eu anomalies).…”

Section: Summary and Discussionsupporting

confidence: 55%

Hydrothermally altered volcanic rocks metamorphosed at granulite-facies conditions: an example from the Grenville Province

2017

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A 1.2 Ga association of aluminous gneisses, garnetites, and white felsic gneisses of andesitic composition in the southern Manicouagan area (central Grenville Province) provides evidence consistent with protolith formation and hydrothermal alteration in a submarine volcanic environment. In addition to field relations, potential relics of quartz phenocrysts in the aluminous gneisses, revealed by SEM–MLA (scanning electron microscope with a mineral liberation analysis software) imaging, are consistent with a volcanic precursor. Moreover, in these rocks, aluminous nodules and seams of sillimanite are considered to represent metamorphosed hydrothermal mineral assemblages and to reflect former pathways of hydrothermal fluid. These features are preserved despite the Grenvillian granulite-facies metamorphic overprint and evidence of partial melting. In addition, the garnetites are inferred to represent hydrothermally altered products of the white gneisses, based on the gradational contacts between the two rock types. The compositional ranges of minerals are generally similar to those of granulite-facies metapelites, but moderately elevated contents of Mn in garnet from the garnetites, and Zn in spinel from the aluminous gneisses, are consistent with hydrothermal addition of these elements to the protolith. The most prominent alteration trends are an increase in Fe–Mg–Mn from the white gneisses to the aluminous gneisses and the garnetites, and a trend of increasing alumina index in some white gneisses, suggesting mild argillic alteration. The new findings highlight the preservation of early hydrothermal alteration in high-grade metamorphic belts in the Grenville Province, and these altered rocks are potential targets for exploration.

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“…5E). The garnet patterns with low HREE abundances are similar to those of garnets associated with hydrothermal vent systems in the Broken Hill region of Australia (Heimann et al, 2011). The garnets in the hybrid tonalites are similar in bulk composition to some hydrothermal garnets, although the hydro thermal garnets range to higher spessartine contents and occur in garnet-rich assemblages (many are garnetites) that contain quartz, micas, grunerite, gahnite, and magne tite (Heimann et al, 2011).…”

Section: Mixing End Membersmentioning

confidence: 62%

“…The garnets in the hybrid tonalites are similar in bulk composition to some hydrothermal garnets, although the hydro thermal garnets range to higher spessartine contents and occur in garnet-rich assemblages (many are garnetites) that contain quartz, micas, grunerite, gahnite, and magne tite (Heimann et al, 2011). Heimann et al (2011) interpreted the fl at HREE patterns of the hydrothermal garnets to refl ect the bulk composition of the rock in which the garnets grew.…”

Section: Mixing End Membersmentioning

confidence: 99%

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et al. 2012

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The 465 Ma Svarthopen pluton in northcentral Norway was emplaced under middle-crustal conditions (~700 MPa) into metasedimentary rocks of the Helgeland Nappe Complex. The pluton is characterized by zones of mingling and mixing of gabbro/ diorite with peraluminous, garnet-bearing biotite granite. Variation in bulk-rock Sr and Nd isotope ratios are consistent with simple mixing; however, nonuniform enrichment of Zr and the rare earth elements (REEs) suggests that individual magma batches underwent postmixing fractionation. Hybrid intermediate rocks are characterized by Ca-rich garnet. Such garnet is absent in possible mafi c end members, and garnet in felsic end members is Ca poor. Evidently, the ferroan , peraluminous hybrid rocks promoted garnet stability, and we interpret these garnets to be igneous in origin. Garnets in the hybrids have low Zr contents, positive light REE slopes, and fl at to negative heavy REE slopes with lower REE abundances than in typical igneous garnet. These trace-element data combined with textural evidence suggest that garnet formed near the solidus, after fractionation of zircon, allanite, and possibly xenotime. The Svarthopen pluton is not unique: similar intermediate rocks with Ca-rich garnets crop out adjacent to three other plutons in the region.Formation of garnet-bearing hybrid rocks in the Svarthopen pluton provides an analog for mixing of peraluminous and ferroan endmember magmas in the deep crust, where such mixing should be widespread, particularly in continental arcs and zones of continental collision. Postmixing fractionation of hybrid magmas could greatly increase the diversity of major-and trace-element abundances yet retain an isotopic signature of mixing. More-over, formation of garnet-rich hybrids could result in lower-crustal rocks dense enough to delaminate from the arc crust.

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The composition of garnet in garnet-rich rocks in the southern Proterozoic Curnamona Province, Australia: an indicator of the premetamorphic physicochemical conditions of formation

Cited by 24 publications

References 62 publications

Spessartine in compact-hematite rock, southern Serra do Espinhaço, Minas Gerais, Brazil, and genesis of compact hematite

Spessartine in compact-hematite rock, southern Serra do Espinhaço, Minas Gerais, Brazil, and genesis of compact hematite

Hydrothermally altered volcanic rocks metamorphosed at granulite-facies conditions: an example from the Grenville Province

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