Editorial

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Open AccessEditorial

Editorial for the Special Issue “Mineral Textural and Compositional Variations as a Tool for Understanding Magmatic Processes”

by Silvio Mollo, Flavio Di Stefano and Francesca Forni

Minerals 2021, 11(2), 102; https://doi.org/10.3390/min11020102 - 21 Jan 2021

Viewed by 1048

Abstract

This Special Issue of Minerals collects seven different scientific contributions highlighting how magma chamber processes and eruption dynamics studied either in the laboratory or in nature may ultimately control the evolutionary histories and geochemical complexities of igneous rocks [...] Full article

(This article belongs to the Special Issue Mineral Textural and Compositional Variations as a Tool for Understanding Magmatic Processes)

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23 pages, 6899 KiB

Open AccessArticle

Significance of Calcite Trace Elements Contents and C-O Isotopic Compositions for Ore-Forming Fluids and Gold Prospecting in the Zhesang Carlin-Like Gold Deposit of Southeastern Yunnan, China

by Jiasheng Wang, Jinyang Chang, Chao Li, Zhenchun Han, Tao Wang and Huanhuan Han

Minerals 2020, 10(4), 338; https://doi.org/10.3390/min10040338 - 09 Apr 2020

Cited by 9 | Viewed by 3269

Abstract

The Zhesang gold deposit of southeastern Yunnan is an important component of the Dian-Qian-Gui (Yunnan, Guizhou, and Guangxi) “Golden Triangle”, which hosts a multitude of Carlin-like gold deposits (CLGDs). Calcite is one of the most common gangue minerals in Zhesang. The calcites that [...] Read more.

The Zhesang gold deposit of southeastern Yunnan is an important component of the Dian-Qian-Gui (Yunnan, Guizhou, and Guangxi) “Golden Triangle”, which hosts a multitude of Carlin-like gold deposits (CLGDs). Calcite is one of the most common gangue minerals in Zhesang. The calcites that have been found in the mining area are classified as ore-stage and post-ore calcites. The ore-stage calcite exhibits a clear paragenetic relationship with gold-bearing arsenopyrite and with an alteration halo that has been cut by the post-ore calcite. To elucidate the origin of the ore-forming fluids of the Zhesang gold deposit and to investigate the possibility of utilizing calcite geochemistry as prospecting indicators, the rare earth elements (REEs), Y, Fe, Mn and Mg contents, and C-O isotopic compositions of calcites from Zhesang have been analyzed. The ore-stage calcite is enriched in middle rare earth elements (MREEs) relative to light rare earth elements (LREEs) and heavy rare earth elements (HREEs) (MREE/LREE = 1.11–1.61, MREE/HREE = 6.12–8.22), whereas post-ore calcite exhibits an enrichment in LREE (LREE/HREE = 4.39–14.93, MREE/LREE = 0.35–0.71). The ore-stage and post-ore calcites were both formed by hydrothermal fluids; however, these hydrothermal fluids may have different sources. The Fe contents of the ore-stage calcite are significantly higher than those of post-ore calcite (4690–6300 μg/g versus 2030–2730 μg/g). Ore-stage calcite also has significantly lower δ¹⁸O_V-SMOW values than post-ore calcite (11.03–12.49‰ versus 16.48–17.14‰). These calcites with an MREE/LREE ratio greater than 0.92, MREE/HREE ratio greater than 5.69, Fe content greater than 3827 μg/g, and δ¹⁸O_V-SMOW value less than 14.40‰ represent ore-stage calcites and are important prospecting guidelines. According to the REE, C-O isotopic characteristics of the calcites and the previous findings, it is inferred that the ore-forming fluids of the Zhesang gold deposit were a mixture of crustal fluid by meteoric water leaching wall rocks and a small amount of basic magmatic fluid. The formation of post-ore calcite might be derived from meteoric water and marine carbonates interaction. The ore-forming fluids of the Zhesang gold deposit may be associated with the intrusion of diabase that outcrops in the mining area, and that the basic magmatic activities of the Indosinian period also provided some of the ore-forming materials and heat for gold mineralization. Full article

(This article belongs to the Special Issue Mineral Textural and Compositional Variations as a Tool for Understanding Magmatic Processes)

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25 pages, 9096 KiB

Open AccessArticle

Mesozoic Northward Subduction Along the SE Asian Continental Margin Inferred from Magmatic Records in the South China Sea

by Guanqiang Cai, Zhifeng Wan, Yongjian Yao, Lifeng Zhong, Hao Zheng, Argyrios Kapsiotis and Cheng Zhang

Minerals 2019, 9(10), 598; https://doi.org/10.3390/min9100598 - 30 Sep 2019

Cited by 18 | Viewed by 3958

Abstract

During the Mesozoic, Southeast (SE) Asia (including South China and the South China Sea (SCS)) was located in a transitional area between the Tethyan and Pacific geotectonic regimes. However, it is unclear whether geodynamic processes in the SE Asian continental margin were controlled [...] Read more.

During the Mesozoic, Southeast (SE) Asia (including South China and the South China Sea (SCS)) was located in a transitional area between the Tethyan and Pacific geotectonic regimes. However, it is unclear whether geodynamic processes in the SE Asian continental margin were controlled by Tethyan or paleo-Pacific Ocean subduction. Herein, we report ~124 Ma adakitic granodiorites and Nb-enriched basalts from the Xiaozhenzhu Seamount of the SCS. Granodiorites have relatively high Sr/Y (34.7–37.0) and (La/Yb)_N (13.8–15.7) ratios, as well as low Y (9.67–9.90 μg/g) and Yb (0.93–0.94 μg/g) concentrations, typical of adakites. Their Sr/Y and (La/Yb)_N values coupled with their relatively low initial ⁸⁷Sr/⁸⁶Sr ratios (0.70541–0.70551), relatively high K₂O contents (3.31–3.38 wt%), high Th/La ratios (0.33–0.40), negative ε_Nd(t) values (−3.62 to −3.52), and their variable zircon ε_Hf(t) values (−3.8 to +5.2) indicate that these rocks were formed by melting of subducted oceanic crust and sediments. The Nb-enriched basalts show enrichment in high field strength elements (HFSE) and have ε_Nd(t) values of +2.90 to +2.93, as well as relatively low initial ⁸⁷Sr/⁸⁶Sr ratios of 0.70341–0.70343, demonstrating that they were derived from a depleted-mantle (DM) source metasomatized by silicate magmas originating from melting of a subducted oceanic lithospheric slab. By combining our findings with data from other Late Mesozoic arc-related magmatic rocks and adakites from the broader study area, we propose a geotectonic model involving subduction of young oceanic lithosphere during the Late Jurassic and northward subduction of the proto-South China Sea (PSCS) along the SE Asian continental margin during the Early Cretaceous. This conceptual model better explains the two-period Mesozoic magmatism, commonly reported for the SE Asian continental margin. Full article

(This article belongs to the Special Issue Mineral Textural and Compositional Variations as a Tool for Understanding Magmatic Processes)

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10 pages, 1975 KiB

Open AccessArticle

Singularity Analysis of Volcanic Ages and Implications for Tectonic Setting in the Mesozoic, Great Xing’an Range, Northeast China

by Pingping Zhu and Qiuming Cheng

Minerals 2019, 9(7), 419; https://doi.org/10.3390/min9070419 - 09 Jul 2019

Cited by 3 | Viewed by 2558

Abstract

Frequency distribution of zircon U–Pb ages has been commonly utilized to interpret the age of a magmatic event. Anomalies in age peaks are related to plate movement caused by mantle convection during the formation of supercontinents and continent crust growth. In this paper, [...] Read more.

Frequency distribution of zircon U–Pb ages has been commonly utilized to interpret the age of a magmatic event. Anomalies in age peaks are related to plate movement caused by mantle convection during the formation of supercontinents and continent crust growth. In this paper, a singularity analysis method (frequency anomalies) is used to analyze a dataset (n = 823, discordance lower than 10%) of zircon U–Pb ages from the Great Xing’an Range (GXR), in order to characterize the causal relationship between age transitions and Pacific Plate subduction. The number-age plot result shows that there is a peak around at 125 Ma, and the log–log plot reveals that there are two transitional ages (knee points) at 125 Ma and 145 Ma. The age densities of the peak at 125 Ma and the transition at 145 Ma can both be fitted by power law functions, which indicate transitional ages have the characteristic of singularity. Combined with the subduction geological background in the late Mesozoic, the possible singularity mechanisms corresponding to the age peak at 125 Ma and the transition at 145 Ma are slab rollback and slab breakoff of the Pacific Plate, which is consistent with conclusions from geology and geochemistry. This result suggests that singularity analysis can be used as a new method to quantitatively characterize volcanic activities and tectonic setting in geological processes. Full article

(This article belongs to the Special Issue Mineral Textural and Compositional Variations as a Tool for Understanding Magmatic Processes)

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23 pages, 8124 KiB

Open AccessArticle

Modeling the Crystallization and Emplacement Conditions of a Basaltic Trachyandesitic Sill at Mt. Etna Volcano

by Manuela Nazzari, Flavio Di Stefano, Silvio Mollo, Piergiorgio Scarlato, Vanni Tecchiato, Ben Ellis, Olivier Bachmann and Carmelo Ferlito

Minerals 2019, 9(2), 126; https://doi.org/10.3390/min9020126 - 21 Feb 2019

Cited by 9 | Viewed by 4361

Abstract

This study documents the compositional variations of phenocrysts from a basaltic trachyandesitic sill emplaced in the Valle del Bove at Mt. Etna volcano (Sicily, Italy). The physicochemical conditions driving the crystallization and emplacement of the sill magma have been reconstructed by barometers, oxygen [...] Read more.

This study documents the compositional variations of phenocrysts from a basaltic trachyandesitic sill emplaced in the Valle del Bove at Mt. Etna volcano (Sicily, Italy). The physicochemical conditions driving the crystallization and emplacement of the sill magma have been reconstructed by barometers, oxygen barometers, thermometers and hygrometers based on clinopyroxene, feldspar (plagioclase + K-feldspar) and titanomagnetite. Clinopyroxene is the liquidus phase, recording decompression and cooling paths decreasing from 200 to 0.1 MPa and from 1050 to 940 °C, respectively. Plagioclase and K-feldspar cosaturate the melt in a lower temperature interval of ~1000–870 °C. Cation exchanges in clinopyroxene (Mg-Fe) and feldspar (Ca-Na) indicate that magma ascent is accompanied by progressive H₂O exsolution (up to ~2.2 wt. %) under more oxidizing conditions (up to ΔNNO + 0.5). Geospeedometric constraints provided by Ti–Al–Mg cation redistributions in titanomagnetite indicate that the travel time (up to 23 h) and ascent velocity of magma (up to 0.78 m/s) are consistent with those inferred for other eruptions at Mt. Etna. These kinetic effects are ascribed to a degassing-induced undercooling path caused principally by H₂O loss at shallow crustal conditions. Rare earth element (REE) modeling based on the lattice strain theory supports the hypothesis that the sill magma formed from primitive basaltic compositions after clinopyroxene (≤41%) and plagioclase (≤12%) fractionation. Early formation of clinopyroxene at depth is the main controlling factor for the REE signature, whereas subsequent degassing at low pressure conditions enlarges the stability field of plagioclase causing trace element enrichments during eruption towards the surface. Full article

(This article belongs to the Special Issue Mineral Textural and Compositional Variations as a Tool for Understanding Magmatic Processes)

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13 pages, 11560 KiB

Open AccessArticle

Unravelling the Crustal Architecture of Cape Verde from the Seamount Xenolith Record

by Abigail K. Barker, Thor H. Hansteen and David Nilsson

Minerals 2019, 9(2), 90; https://doi.org/10.3390/min9020090 - 01 Feb 2019

Cited by 5 | Viewed by 4012

Abstract

The Cape Verde oceanic plateau hosts 10 islands and 11 seamounts and provides an extensive suite of alkaline lavas and pyroclastic rocks. The volcanic rocks host a range of crustal and mantle xenoliths. These xenoliths provide a spectrum of lithologies available to interact [...] Read more.

The Cape Verde oceanic plateau hosts 10 islands and 11 seamounts and provides an extensive suite of alkaline lavas and pyroclastic rocks. The volcanic rocks host a range of crustal and mantle xenoliths. These xenoliths provide a spectrum of lithologies available to interact with magma during transport through the lithospheric mantle and crust. We explore the origin and depth of formation of crustal xenoliths to develop a framework of magma-crust interaction and a model for the crustal architecture beneath the Cape Verde oceanic plateau. The host lavas are phononephelinites to phonolites and the crustal xenoliths are mostly mafic plutonic assemblages with one sedimentary xenolith. REE profiles of clinopyroxene in the host lavas are light rare-earth element (LREE) enriched whereas clinopyoxene from the plutonic xenoliths are LREE depleted. Modelling of REE melt compositions indicates the plutonic xenoliths are derived from mid-ocean ridge basalt (MORB)-type ocean crust. Thermobarometry indicates that clinopyroxene in the host lavas formed at depths of 17 to 46 km, whereas those in the xenoliths formed at 5 to 20 km. This places the depth of origin of the plutonic xenoliths in the oceanic crust. Therefore, the xenoliths trace magma-crust interaction to the MORB oceanic crust and overlying sediments located beneath the Cape Verde oceanic plateau. Full article

(This article belongs to the Special Issue Mineral Textural and Compositional Variations as a Tool for Understanding Magmatic Processes)

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17 pages, 2336 KiB

Open AccessArticle

Impulsive Supply of Volatile-Rich Magmas in the Shallow Plumbing System of Mt. Etna Volcano

by Cristina Perinelli, Silvio Mollo, Mario Gaeta, Serena Pia De Cristofaro, Danilo Mauro Palladino and Piergiorgio Scarlato

Minerals 2018, 8(11), 482; https://doi.org/10.3390/min8110482 - 25 Oct 2018

Cited by 12 | Viewed by 2643

Abstract

Magma dynamics at Mt. Etna volcano are frequently recognized as the result of complex crystallization regimes that, at shallow crustal levels, unexpectedly change from H₂O-undersaturated to H₂O-saturated conditions, due to the impulsive and irregular arrival of volatile-rich magmas from [...] Read more.

Magma dynamics at Mt. Etna volcano are frequently recognized as the result of complex crystallization regimes that, at shallow crustal levels, unexpectedly change from H₂O-undersaturated to H₂O-saturated conditions, due to the impulsive and irregular arrival of volatile-rich magmas from mantle depths. On this basis, we have performed hydrous crystallization experiments for a quantitative understanding of the role of H₂O in the differentiation of deep-seated trachybasaltic magmas at the key pressure of the Moho transition zone. For H₂O = 2.1–3.2 wt %, the original trachybasaltic composition shifts towards phonotephritic magmas never erupted during the entire volcanic activity of Mt. Etna. Conversely, for H₂O = 3.8–8.2 wt %, the obtained trachybasalts and basaltic trachyandesites reproduce most of the pre-historic and historic eruptions. The comparison with previous low pressure experimental data and natural compositions from Mt. Etna provides explanation for (1) the abundant release of H₂O throughout the plumbing system of the volcano during impulsive ascent of deep-seated magmas; (2) the upward acceleration of magmas feeding gas-dominated, sustained explosive eruptions; (3) the physicochemical changes of gas-fluxed magmas ponding at shallow crustal levels; and (4) the huge gas emissions measured at the summit craters and flank vents which result in a persistent volcanic gas plume. Full article

(This article belongs to the Special Issue Mineral Textural and Compositional Variations as a Tool for Understanding Magmatic Processes)

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35 pages, 6183 KiB

Open AccessArticle

The Merensky Cyclic Unit, Bushveld Complex, South Africa: Reality or Myth?

by Emma J. Hunt, Rais Latypov and Péter Horváth

Minerals 2018, 8(4), 144; https://doi.org/10.3390/min8040144 - 03 Apr 2018

Cited by 25 | Viewed by 9354

Abstract

The Merensky Unit, Bushveld Complex, is commonly described using genetic terms such as “cyclic unit”, typically without careful consideration of the connotations. We suggest that this contributes to the debate on processes forming the unit. This study integrates an extensive field study with [...] Read more.

The Merensky Unit, Bushveld Complex, is commonly described using genetic terms such as “cyclic unit”, typically without careful consideration of the connotations. We suggest that this contributes to the debate on processes forming the unit. This study integrates an extensive field study with detailed petrographic and textural analyses of the Merensky Unit to determine whether it is a “cyclic unit” sensu stricto. The study indicates that the bulk of the platinum-bearing chromitite-feldspathic orthopyroxenite developed through heterogeneous nucleation and in situ growth during multiple replenishment events. The overlying leuconorite developed above a gradational boundary, reflecting mixing following replenishment by a relatively more evolved magma. The bulk of this unit also formed in situ. The uppermost poikilitic anorthosite formed above a distinct boundary through a subsequent injection of a plagioclase-saturated magma, which crystallised in situ. Processes of gravitational settling and local remobilisation of crystals cannot be discounted from contributing to the development of the unit. The final textures throughout the unit developed through pervasive textural equilibration, with extensive fluid-mediated textural equilibration forming the megacrystic feldspathic orthopyroxenite. The evidence for at least five replenishment events indicates that the Merensky Unit is not a cyclic unit; therefore, the genetic term, “Merensky Cyclic Unit”, is misleading and its use should be carefully considered in future work. Full article

(This article belongs to the Special Issue Mineral Textural and Compositional Variations as a Tool for Understanding Magmatic Processes)

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