The Pleiades Volcanic Field (PVF) of northern Victoria Land (Antarctica) is made up of a dozen scoria cones whose erupted products present an unusually complete evolutionary trend from alkali-basalt to trachyte. With the aim of reconstructing the evolution of the PVF plumbing system, we have investigated the petrography and chemistry of main mineral phases using SEM-EDS coupled with major (EPMA-WDS) element analyses. We further focussed on clinopyroxene phenocrysts obtaining a more detailed characterisation by means of trace (LA-ICP-MS) element analyses coupled with machine learning thermobarometry. The results indicate that fractional crystallisation and magma mixing are the major processes determining the development of the complete evolution trend. While fractional crystallisation is a persistently active process in all parts of the plumbing system, mixing among differently evolved magma batches pertaining to the same association is responsible for the formation of intermediate compositions in the differentiation lineage at a specific pressure range (0.4-0.5 GPa). These processes are compatible with significant residence time of magmas at depth, resulting in multiple episodes of magma mixing, as testified by resorption and overgrowth textures in phenocryst assemblage occurring under isobaric conditions. The prolonged residence time likely increased the efficiency of the mixing process, leading to the formation of magmas with intermediate composition. In turn, the build-up of volatiles during the magma differentiation at depth could have favoured the eruption of these (variably differentiated) magmas. Considering that the PVF is situated in a glacial region, a process forcing long magma residence time can be envisaged associated with increased ice loading during glacial stages. This study specifically considers the ice fluctuations in the last 100 ka, theorising the possibility of a climate-controlled volcano plumbing system.
南极洲维多利亚陆地北部的昴宿星团火山场（PVF）由十几个火山灰锥组成，其喷发产物呈现出从碱性玄武岩到斜长岩的异常完整的演化趋势。为了重建 PVF 管道系统的演化过程，我们利用扫描电镜-电子显微镜和主要（EPMA-WDS）元素分析，对主要矿物相的岩相学和化学性质进行了研究。我们还通过痕量（LA-ICP-MS）元素分析和机器学习热压测量法，对陨石表晶进行了更详细的表征。结果表明，碎裂结晶和岩浆混合是决定完整演化趋势发展的主要过程。在管道系统的所有部分，碎裂结晶都是一个持续活跃的过程，而在特定的压力范围（0.4-0.5 GPa）内，属于同一联合体的不同演化岩浆批次之间的混合则是形成分异系中间成分的原因。这些过程与岩浆在深部的大量停留时间相适应，从而导致岩浆的多次混合，在等压条件下发生的表晶集合体的吸收和过度生长纹理就证明了这一点。停留时间的延长可能会提高混合过程的效率，从而形成具有中间成分的岩浆。反过来，岩浆在深部分化过程中挥发物的积累可能有利于这些（不同分化的）岩浆的喷发。考虑到滨海火山带位于冰川地区，可以设想在冰川期冰负荷增加的情况下，岩浆停留时间较长。本研究特别考虑了过去 100 ka 年的冰层波动，推测了气候控制火山管道系统的可能性。
{"title":"Textures and chemistry of crystal cargo of The Pleiades Volcanic Field, Antarctica: potential influence of ice load in modulating the plumbing system","authors":"Rocchi Irene, Tomassini Alice, Masotta Matteo, Petrelli Maurizio, Ágreda López Mónica, Rocchi Sergio","doi":"10.1093/petrology/egae027","DOIUrl":"https://doi.org/10.1093/petrology/egae027","url":null,"abstract":"The Pleiades Volcanic Field (PVF) of northern Victoria Land (Antarctica) is made up of a dozen scoria cones whose erupted products present an unusually complete evolutionary trend from alkali-basalt to trachyte. With the aim of reconstructing the evolution of the PVF plumbing system, we have investigated the petrography and chemistry of main mineral phases using SEM-EDS coupled with major (EPMA-WDS) element analyses. We further focussed on clinopyroxene phenocrysts obtaining a more detailed characterisation by means of trace (LA-ICP-MS) element analyses coupled with machine learning thermobarometry. The results indicate that fractional crystallisation and magma mixing are the major processes determining the development of the complete evolution trend. While fractional crystallisation is a persistently active process in all parts of the plumbing system, mixing among differently evolved magma batches pertaining to the same association is responsible for the formation of intermediate compositions in the differentiation lineage at a specific pressure range (0.4-0.5 GPa). These processes are compatible with significant residence time of magmas at depth, resulting in multiple episodes of magma mixing, as testified by resorption and overgrowth textures in phenocryst assemblage occurring under isobaric conditions. The prolonged residence time likely increased the efficiency of the mixing process, leading to the formation of magmas with intermediate composition. In turn, the build-up of volatiles during the magma differentiation at depth could have favoured the eruption of these (variably differentiated) magmas. Considering that the PVF is situated in a glacial region, a process forcing long magma residence time can be envisaged associated with increased ice loading during glacial stages. This study specifically considers the ice fluctuations in the last 100 ka, theorising the possibility of a climate-controlled volcano plumbing system.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"37 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140044320","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-03-01DOI: 10.1093/petrology/egae024
Gong-Jian Tang, Derek A Wyman, Wei Dan, Qiang Wang, Xi-Jun Liu, Ya-Nan Yang, Mustafo Gadoev, Ilhomjon Oimahmadov
Determining crustal melting in parallel with geodynamic evolution provides critical information on plateau crustal thickening and uplift. Here we investigate the timing and duration of crustal melting through in-situ analysis of zircon U-Pb ages, trace elements and Hf-O isotopes, and whole-rock elements and Sr-Nd-Hf isotopes for the granites and high-grade metamorphic rocks from the Pamir Plateau. Zircon dates record protracted crustal melting for both Central Pamir (43–33 Ma and 22–12 Ma) and South Pamir (28–10 Ma). The Pamir Cenozoic granites are characterized by significant elemental and isotopic heterogeneity. The elemental variability within the Pamir Cenozoic granites is attributed to fractional crystallization of dominantly K-feldspar and plagioclase with subordinate biotite from a variably fractionated melt, and the accumulation of early crystallized feldspar during magma ascent. Peritectic mineral entrainment and accessory mineral crystallization had some influence on the geochemical characteristics of the garnet-bearing leucogranite dikes. Zircon Hf isotopes and whole-rock Sr-Nd-Hf isotopes show secular variations for both Central and South Pamir granites. The Central Pamir granites show a mild decrease in whole-rock ƐNd(t) values from Eocene (-4.3 to -4.9) to Miocene (-6.2 to -7.7), and the zircon ƐHf(t) values decrease from c. 40 Ma (+2 to -5) to c. 10 Ma (-4 to -8). In contrast, the South Pamir granites have highly variable whole-rock Sr-Nd-Hf (87Sr/86Sr(i) = 0.70530 to 0.78302; ƐNd(t) = -31.5 to +0.2; ƐHf(t) = -40.0 to +8.2) and zircon Hf isotopes (ƐHf(t) = +7.5 to -31.7) and displays a strong decrease in ƐNd(t) and ƐHf(t) values from c. 25 Ma to c. 13 Ma. Geochemical and isotopic data indicate that both the Central and South Pamir experienced crustal melting from juvenile lower crust to ancient lower-middle crustal materials, and Indian crustal materials were incorporated into the melt region of the South Pamir leucogranites from c. 20 Ma. Our study highlights a causal link between a chain of events that includes magma underplating induced by lithosphere thinning and slab breakoff, lithosphere delamination and underthrusting of Indian lithosphere, and formation of the Cenozoic granites in Pamir. This series of processes are incorporated here into a comprehensive model for the geodynamic evolution of the Pamir during the India-Asia collision.
{"title":"Protracted and progressive crustal melting during continental collision in the Pamir and plateau growth","authors":"Gong-Jian Tang, Derek A Wyman, Wei Dan, Qiang Wang, Xi-Jun Liu, Ya-Nan Yang, Mustafo Gadoev, Ilhomjon Oimahmadov","doi":"10.1093/petrology/egae024","DOIUrl":"https://doi.org/10.1093/petrology/egae024","url":null,"abstract":"Determining crustal melting in parallel with geodynamic evolution provides critical information on plateau crustal thickening and uplift. Here we investigate the timing and duration of crustal melting through in-situ analysis of zircon U-Pb ages, trace elements and Hf-O isotopes, and whole-rock elements and Sr-Nd-Hf isotopes for the granites and high-grade metamorphic rocks from the Pamir Plateau. Zircon dates record protracted crustal melting for both Central Pamir (43–33 Ma and 22–12 Ma) and South Pamir (28–10 Ma). The Pamir Cenozoic granites are characterized by significant elemental and isotopic heterogeneity. The elemental variability within the Pamir Cenozoic granites is attributed to fractional crystallization of dominantly K-feldspar and plagioclase with subordinate biotite from a variably fractionated melt, and the accumulation of early crystallized feldspar during magma ascent. Peritectic mineral entrainment and accessory mineral crystallization had some influence on the geochemical characteristics of the garnet-bearing leucogranite dikes. Zircon Hf isotopes and whole-rock Sr-Nd-Hf isotopes show secular variations for both Central and South Pamir granites. The Central Pamir granites show a mild decrease in whole-rock ƐNd(t) values from Eocene (-4.3 to -4.9) to Miocene (-6.2 to -7.7), and the zircon ƐHf(t) values decrease from c. 40 Ma (+2 to -5) to c. 10 Ma (-4 to -8). In contrast, the South Pamir granites have highly variable whole-rock Sr-Nd-Hf (87Sr/86Sr(i) = 0.70530 to 0.78302; ƐNd(t) = -31.5 to +0.2; ƐHf(t) = -40.0 to +8.2) and zircon Hf isotopes (ƐHf(t) = +7.5 to -31.7) and displays a strong decrease in ƐNd(t) and ƐHf(t) values from c. 25 Ma to c. 13 Ma. Geochemical and isotopic data indicate that both the Central and South Pamir experienced crustal melting from juvenile lower crust to ancient lower-middle crustal materials, and Indian crustal materials were incorporated into the melt region of the South Pamir leucogranites from c. 20 Ma. Our study highlights a causal link between a chain of events that includes magma underplating induced by lithosphere thinning and slab breakoff, lithosphere delamination and underthrusting of Indian lithosphere, and formation of the Cenozoic granites in Pamir. This series of processes are incorporated here into a comprehensive model for the geodynamic evolution of the Pamir during the India-Asia collision.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"29 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140018338","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-02-29DOI: 10.1093/petrology/egae019
Stephen J Barnes, Morgan Williams
According to the “trapped liquid” paradigm, whole-rock chemical compositions of cumulate rocks in layered intrusions can be represented as closed-system multicomponent mixtures of the cumulus phases and the liquids from which they crystallised. Alternative open-system models for cumulate solidification assert that intercumulus liquid is continuously mobile as a result of compaction of crystal mushes. In the closed-system models, all excluded elements, i.e. those incompatible in the cumulus phases, should be correlated with one another, whereas in open systems the more incompatible elements should be decoupled from the more compatible ones and correlations should be poor. These alternative hypotheses are tested using a database of more than 63,000 whole-rock analyses of mostly ultramafic cumulates from a single package of layers across the entire width of the Mirabela layered intrusion, a 2.5 km wide by >2 km thick intrusion with Great Dyke-like stratigraphy. Variably compatible elements Al, Na, Ti and P are found to show strong correlations with one other in olivine-pyroxene cumulates across the spectrum from mesocumulate to orthocumulate rocks, where drill-core sample intervals are composited over 3m; weaker correlations are found over 1m sample intervals. The closed-system trapped liquid model is robust for this suite of rocks at the scale of 3m lengths of drill core (approx. 1 kg of sample). Furthermore, a very smooth and symmetrical variation is observed from dominantly ad- to mesocumulate rocks in the geometrical centre of the intrusion towards orthocumulates at both margins, attributable to increase in cooling rate towards the margins. The most adcumulate rocks are gabbronorites at the base of the mafic sequence in the centre of the intrusion. There is no corresponding decrease in layer thickness towards the centre that would be predicted by a compaction model. The only incompatible element showing partial decoupling is K, which appears to have been mobile at a very late stage, probably in an aqueous fluid phase. There is no evidence for mobilisation of PGEs or other chalcophile elements by this fluid despite the presence of disseminated sulfide throughout the sampled interval.
{"title":"Postcumulus processes recorded in whole-rock geochemistry: a case study from the Mirabela layered intrusion, Brazil","authors":"Stephen J Barnes, Morgan Williams","doi":"10.1093/petrology/egae019","DOIUrl":"https://doi.org/10.1093/petrology/egae019","url":null,"abstract":"According to the “trapped liquid” paradigm, whole-rock chemical compositions of cumulate rocks in layered intrusions can be represented as closed-system multicomponent mixtures of the cumulus phases and the liquids from which they crystallised. Alternative open-system models for cumulate solidification assert that intercumulus liquid is continuously mobile as a result of compaction of crystal mushes. In the closed-system models, all excluded elements, i.e. those incompatible in the cumulus phases, should be correlated with one another, whereas in open systems the more incompatible elements should be decoupled from the more compatible ones and correlations should be poor. These alternative hypotheses are tested using a database of more than 63,000 whole-rock analyses of mostly ultramafic cumulates from a single package of layers across the entire width of the Mirabela layered intrusion, a 2.5 km wide by >2 km thick intrusion with Great Dyke-like stratigraphy. Variably compatible elements Al, Na, Ti and P are found to show strong correlations with one other in olivine-pyroxene cumulates across the spectrum from mesocumulate to orthocumulate rocks, where drill-core sample intervals are composited over 3m; weaker correlations are found over 1m sample intervals. The closed-system trapped liquid model is robust for this suite of rocks at the scale of 3m lengths of drill core (approx. 1 kg of sample). Furthermore, a very smooth and symmetrical variation is observed from dominantly ad- to mesocumulate rocks in the geometrical centre of the intrusion towards orthocumulates at both margins, attributable to increase in cooling rate towards the margins. The most adcumulate rocks are gabbronorites at the base of the mafic sequence in the centre of the intrusion. There is no corresponding decrease in layer thickness towards the centre that would be predicted by a compaction model. The only incompatible element showing partial decoupling is K, which appears to have been mobile at a very late stage, probably in an aqueous fluid phase. There is no evidence for mobilisation of PGEs or other chalcophile elements by this fluid despite the presence of disseminated sulfide throughout the sampled interval.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"38 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-02-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140008027","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The major- and trace-element compositions of amphiboles in andesite from Quaternary Yufu Volcano, northeastern Kyushu, Japan were analysed to investigate the generation processes of andesitic magma from Yufu Volcano. The amphiboles in andesite from Yufu volcano can be divided into two groups based on major-element composition: pargasite and magnesio-hornblende. To estimate temperature, pressure, and major- and trace-element compositions of melts in equilibrium with amphiboles, we used the recently proposed methods that can calculate temperature, pressure, major element compositions, and partition coefficients of trace-element between amphibole and melt using only the major-element compositions of amphibole. The estimated temperature, pressure, and major-element composition of melt in equilibrium with the amphibole phenocrysts indicate that each group crystallised under different conditions. These differences suggest that two magma chambers at different depths existed beneath Yufu Volcano and that the andesitic magma of Yufu Volcano was formed by mixing of the two magmas. The trace-element compositions of melts in equilibrium with the pargasite and magnesio-hornblende, estimated by applying the partition coefficients calculated from major-element compositions of amphibole to trace-element compositions of amphiboles, indicate magma derived from slab melt and the partial melting of crustal material, respectively. Because magma is a mixture of minerals and melt, we estimate the chemical compositional ranges of the two end-member magmas on the Y versus SiO2 diagram from the mixing relationship between amphibole and estimated melt, as well as phenocrysts of plagioclase, clinopyroxene, and orthopyroxene. The overlap of the estimated compositional range with the whole-rock composition represents the chemical compositions of the end-members of magma mixing, yielding estimates of the mafic (SiO2 ≈ 45 wt%) and felsic (SiO2 ≈ 68 wt%) end-member magmas. Furthermore, we estimate the concentrations of other elements in the end-member magmas by substituting the estimated SiO2 concentrations of the magmas into linear regression equations between the whole-rock contents of other elements and SiO2. The trace-element compositions of the mafic and felsic end-member magmas, as estimated in this study, have similar features to those of gabbroids and Cretaceous granitic rocks, respectively, that are presumed to lie beneath Yufu Volcano. These similarities could be explained by the possibility that the compositions of the end-member magmas were influenced by basement rocks.
{"title":"Genesis of andesitic magma erupted at Yufu Volcano, Kyushu Island, Southwest Japan arc: Evidence from the chemical compositions of amphibole phenocrysts","authors":"Ikuo Okada, Tomoyuki Shibata, Masako Yoshikawa, Hidemi Ishibashi, Takeshi Sugimoto, Yasutaka Hayasaka","doi":"10.1093/petrology/egae018","DOIUrl":"https://doi.org/10.1093/petrology/egae018","url":null,"abstract":"The major- and trace-element compositions of amphiboles in andesite from Quaternary Yufu Volcano, northeastern Kyushu, Japan were analysed to investigate the generation processes of andesitic magma from Yufu Volcano. The amphiboles in andesite from Yufu volcano can be divided into two groups based on major-element composition: pargasite and magnesio-hornblende. To estimate temperature, pressure, and major- and trace-element compositions of melts in equilibrium with amphiboles, we used the recently proposed methods that can calculate temperature, pressure, major element compositions, and partition coefficients of trace-element between amphibole and melt using only the major-element compositions of amphibole. The estimated temperature, pressure, and major-element composition of melt in equilibrium with the amphibole phenocrysts indicate that each group crystallised under different conditions. These differences suggest that two magma chambers at different depths existed beneath Yufu Volcano and that the andesitic magma of Yufu Volcano was formed by mixing of the two magmas. The trace-element compositions of melts in equilibrium with the pargasite and magnesio-hornblende, estimated by applying the partition coefficients calculated from major-element compositions of amphibole to trace-element compositions of amphiboles, indicate magma derived from slab melt and the partial melting of crustal material, respectively. Because magma is a mixture of minerals and melt, we estimate the chemical compositional ranges of the two end-member magmas on the Y versus SiO2 diagram from the mixing relationship between amphibole and estimated melt, as well as phenocrysts of plagioclase, clinopyroxene, and orthopyroxene. The overlap of the estimated compositional range with the whole-rock composition represents the chemical compositions of the end-members of magma mixing, yielding estimates of the mafic (SiO2 ≈ 45 wt%) and felsic (SiO2 ≈ 68 wt%) end-member magmas. Furthermore, we estimate the concentrations of other elements in the end-member magmas by substituting the estimated SiO2 concentrations of the magmas into linear regression equations between the whole-rock contents of other elements and SiO2. The trace-element compositions of the mafic and felsic end-member magmas, as estimated in this study, have similar features to those of gabbroids and Cretaceous granitic rocks, respectively, that are presumed to lie beneath Yufu Volcano. These similarities could be explained by the possibility that the compositions of the end-member magmas were influenced by basement rocks.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"5 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-02-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140007894","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-02-29DOI: 10.1093/petrology/egae022
Marie A Kieffer, Sarah A S Dare, Olivier Namur, Eduardo T Mansur
Mafic layered intrusions constitute a natural laboratory to investigate petrogenetic processes using trace element variations in apatite chemistry. Although these intrusions are related to large igneous provinces, there is a wide range of parameters that can affect the chemistry of the primary melt (i.e., composition of the source, pressure, temperature, oxygen fugacity), followed by possible crustal contamination. In this study, we use a comprehensive dataset of analyses of cumulus and intercumulus apatite from a variety of mafic layered intrusions to demonstrate the use of apatite as a powerful petrogenetic indicator. The dataset (determined in this study and compiled from the literature) comprises electron microprobe and LA-ICP-MS analyses, as well as in-situ LA-MC-ICP-MS analyses of Sr isotopes in apatite from well documented layered intrusions (Sept-Iles, Skaergaard, Bushveld, Panzhihua) and the Sudbury Igneous Complex. For the first time, we show that high values of (La/Nd)N, Th, U, Pb, and As in apatite correlate with high (87Sr/86Sr)initial and are related to contamination with continental crust. An elevated (Gd/Yb)N ratio might indicate melting of a mantle source deep enough to retain Yb in garnet. We also confirm that increasingly negative Eu anomaly and decreasing Sr/Y ratio in apatite are indicators of fractional crystallisation of plagioclase, and that high Sr/Y is indicative of early saturation of apatite and/or delayed crystallisation of plagioclase. The reversal to more primitive compositions caused by magma mixing is expressed by higher Sr, V, Mg and Sr/Y ratio, and lower REE+Y, As and Na concentrations in apatite following magma replenishment. Lastly, we show that apatite signature can efficiently distinguish a mafic from a felsic intrusion using its REE and Sr content coupled to its Eu anomaly. It is also possible to further identify the more primitive from the more evolved parts of a mafic layered intrusion, using the Lu, Th, V and volatile (F/Cl) content to distinguish intercumulus from cumulus apatite, respectively. Finally, identifying a mafic magmatic system using detrital apatite in till will prove useful for provenance and mineral exploration studies.
镁质层状侵入体是利用磷灰石化学痕量元素变化研究成岩过程的天然实验室。虽然这些侵入体与大型火成岩带有关，但影响原生熔体化学性质的参数范围很广（即源的成分、压力、温度、氧富集度），其次还可能受到地壳的污染。在这项研究中，我们利用对来自各种黑云母层状侵入体的积云和积云间磷灰石进行分析的综合数据集，证明磷灰石是一种强有力的岩石成因指标。该数据集（由本研究确定，并从文献中汇编）包括电子微探针和 LA-ICP-MS 分析，以及对来自有据可查的层状侵入体（Sept-Iles、Skaergaard、Bushveld、攀枝花）和萨德伯里火成岩群的磷灰石中硒同位素的原位 LA-MC-ICP-MS 分析。我们首次发现磷灰石中(La/Nd)N、Th、U、Pb和As的高值与高(87Sr/86Sr)初始值相关，并且与大陆地壳的污染有关。(Gd/Yb)N比率的升高可能表明地幔源的熔融深度足以将Yb保留在石榴石中。我们还证实，磷灰石中Eu异常的负值增加和Sr/Y比值的降低是斜长石部分结晶的指标，而高Sr/Y则表明磷灰石的早期饱和和/或斜长石的延迟结晶。岩浆补充后，磷灰石中的Sr、V、Mg和Sr/Y比值升高，REE+Y、As和Na浓度降低，表明岩浆混合导致成分向更原始的方向逆转。最后，我们表明，磷灰石特征可以利用其 REE 和 Sr 含量及其 Eu 异常有效地区分黑云岩侵入体和长岩侵入体。此外，我们还可以利用Lu、Th、V和挥发物（F/Cl）的含量来区分层间磷灰石和层积磷灰石，从而进一步识别黑云母层状侵入体的原始部分和演化部分。最后，利用沉积物中的非晶质磷灰石识别岩浆系统将证明对矿源和矿产勘探研究非常有用。
{"title":"Apatite chemistry as a petrogenetic indicator for mafic layered intrusions","authors":"Marie A Kieffer, Sarah A S Dare, Olivier Namur, Eduardo T Mansur","doi":"10.1093/petrology/egae022","DOIUrl":"https://doi.org/10.1093/petrology/egae022","url":null,"abstract":"Mafic layered intrusions constitute a natural laboratory to investigate petrogenetic processes using trace element variations in apatite chemistry. Although these intrusions are related to large igneous provinces, there is a wide range of parameters that can affect the chemistry of the primary melt (i.e., composition of the source, pressure, temperature, oxygen fugacity), followed by possible crustal contamination. In this study, we use a comprehensive dataset of analyses of cumulus and intercumulus apatite from a variety of mafic layered intrusions to demonstrate the use of apatite as a powerful petrogenetic indicator. The dataset (determined in this study and compiled from the literature) comprises electron microprobe and LA-ICP-MS analyses, as well as in-situ LA-MC-ICP-MS analyses of Sr isotopes in apatite from well documented layered intrusions (Sept-Iles, Skaergaard, Bushveld, Panzhihua) and the Sudbury Igneous Complex. For the first time, we show that high values of (La/Nd)N, Th, U, Pb, and As in apatite correlate with high (87Sr/86Sr)initial and are related to contamination with continental crust. An elevated (Gd/Yb)N ratio might indicate melting of a mantle source deep enough to retain Yb in garnet. We also confirm that increasingly negative Eu anomaly and decreasing Sr/Y ratio in apatite are indicators of fractional crystallisation of plagioclase, and that high Sr/Y is indicative of early saturation of apatite and/or delayed crystallisation of plagioclase. The reversal to more primitive compositions caused by magma mixing is expressed by higher Sr, V, Mg and Sr/Y ratio, and lower REE+Y, As and Na concentrations in apatite following magma replenishment. Lastly, we show that apatite signature can efficiently distinguish a mafic from a felsic intrusion using its REE and Sr content coupled to its Eu anomaly. It is also possible to further identify the more primitive from the more evolved parts of a mafic layered intrusion, using the Lu, Th, V and volatile (F/Cl) content to distinguish intercumulus from cumulus apatite, respectively. Finally, identifying a mafic magmatic system using detrital apatite in till will prove useful for provenance and mineral exploration studies.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"15 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-02-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140007886","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Carbonate-bearing sediments, containing calcite, dolomite or magnesite as major carbonate components, are important components of sedimentary sequences deposited on passive margins through Earth’s history. When involved in collisional orogenic processes, these sediments are metamorphosed at variable temperatures and pressures, and undergo decarbonation reactions. While the orogenic metamorphism of some of these lithologies (i.e., impure limestones and dolostones, marls sensu strictu and calcareous pelites) is relatively well understood, very little is known about the metamorphic evolution and decarbonation history of mixed carbonate-silicate rocks in which either dolomite or magnesite is the dominant carbonate component. Here we present the results of a petrologic study of representative samples of metasediments from Central Nepal, derived from Proterozoic dolomitic and magnesitic protoliths metamorphosed during the Himalayan orogeny. The main metamorphic assemblages developed in sediments originally containing different amounts of dolomite or magnesite are characterised in detail. Forward thermodynamic modelling applied to seven samples allows constraints to be placed on: (i) the main decarbonation reactions, (ii) the P-T conditions under which these reactions took place, (iii) the composition of the fluids, and (iv) the amounts of CO2 released. We conclude that the CO2 productivity of dolomitic and magnesitic pelites and marls originally containing 15-40% carbonate is significant (>5.5 ±1.0 CO2 wt% and up to 10.5 ±1.5 CO2 wt%), whereas for carbonate contents above 60-70%, CO2 productivity is negligible unless aqueous fluids infiltrate from the outside and trigger decarbonation reactions. Since the dolomitic and magnesitic protoliths are significantly abundant in the sedimentary sequences involved in the still active Himalayan orogen, the decarbonation processes described here could contribute to the diffuse CO2 degassing currently observed at the surface. Furthermore, we propose for the first time that the peculiar magnesium-rich assemblages investigated in this study may derive from evaporitic protoliths, and that the whole Upper Lesser Himalayan Sequence may therefore represent the metamorphic product of a Proterozoic sequence consisting of alternating layers of carbonatic, evaporitic and pelitic sediments.
含碳酸盐沉积物的主要碳酸盐成分为方解石、白云石或菱镁矿，是地球历史上被动边缘沉积序列的重要组成部分。在参与碰撞造山过程时，这些沉积物在不同的温度和压力下发生变质，并发生脱碳反应。虽然对其中一些岩性（即不纯灰岩和白云石、普通泥灰岩和钙质辉长岩）的造山变质作用了解得相对较多，但对以白云石或菱镁矿为主要碳酸盐成分的碳酸盐-硅酸盐混合岩的变质演化和脱碳历史却知之甚少。本文介绍了对尼泊尔中部变质岩代表性样本的岩石学研究结果，这些样本来自喜马拉雅造山运动期间变质的新生代白云岩和菱镁矿原岩。详细描述了最初含有不同数量白云石或菱镁矿的沉积物中形成的主要变质组合。通过对七个样本进行前向热力学建模，可以对以下方面进行限制：(i) 主要的脱碳反应，(ii) 发生这些反应的 P-T 条件，(iii) 流体的成分，以及 (iv) 释放的二氧化碳量。我们得出的结论是，白云质和镁质辉长岩和泥灰岩原本含有 15-40% 的碳酸盐，其二氧化碳生产率很高（>5.5 ±1.0 CO2 wt%和高达 10.5 ±1.5 CO2 wt%），而当碳酸盐含量超过 60-70% 时，除非水性流体从外部渗入并引发脱碳反应，否则二氧化碳生产率可以忽略不计。由于白云石和菱镁矿原岩在喜马拉雅造山带仍处于活动状态的沉积序列中非常丰富，这里描述的脱碳过程可能是目前在地表观测到的弥漫性二氧化碳脱气的原因。此外，我们首次提出，本研究调查的奇特富镁集合体可能来自蒸发态原岩，因此整个上小喜马拉雅序列可能代表了新生代序列的变质产物，该序列由碳酸盐岩、蒸发态和辉绿岩沉积层交替组成。
{"title":"Metamorphism of dolomitic and magnesitic rocks in collisional orogens and implications for orogenic CO2 degassing","authors":"Tamang Shashi, Groppo Chiara, Girault Frédéric, Perrier Frédéric, Rolfo Franco","doi":"10.1093/petrology/egae021","DOIUrl":"https://doi.org/10.1093/petrology/egae021","url":null,"abstract":"Carbonate-bearing sediments, containing calcite, dolomite or magnesite as major carbonate components, are important components of sedimentary sequences deposited on passive margins through Earth’s history. When involved in collisional orogenic processes, these sediments are metamorphosed at variable temperatures and pressures, and undergo decarbonation reactions. While the orogenic metamorphism of some of these lithologies (i.e., impure limestones and dolostones, marls sensu strictu and calcareous pelites) is relatively well understood, very little is known about the metamorphic evolution and decarbonation history of mixed carbonate-silicate rocks in which either dolomite or magnesite is the dominant carbonate component. Here we present the results of a petrologic study of representative samples of metasediments from Central Nepal, derived from Proterozoic dolomitic and magnesitic protoliths metamorphosed during the Himalayan orogeny. The main metamorphic assemblages developed in sediments originally containing different amounts of dolomite or magnesite are characterised in detail. Forward thermodynamic modelling applied to seven samples allows constraints to be placed on: (i) the main decarbonation reactions, (ii) the P-T conditions under which these reactions took place, (iii) the composition of the fluids, and (iv) the amounts of CO2 released. We conclude that the CO2 productivity of dolomitic and magnesitic pelites and marls originally containing 15-40% carbonate is significant (>5.5 ±1.0 CO2 wt% and up to 10.5 ±1.5 CO2 wt%), whereas for carbonate contents above 60-70%, CO2 productivity is negligible unless aqueous fluids infiltrate from the outside and trigger decarbonation reactions. Since the dolomitic and magnesitic protoliths are significantly abundant in the sedimentary sequences involved in the still active Himalayan orogen, the decarbonation processes described here could contribute to the diffuse CO2 degassing currently observed at the surface. Furthermore, we propose for the first time that the peculiar magnesium-rich assemblages investigated in this study may derive from evaporitic protoliths, and that the whole Upper Lesser Himalayan Sequence may therefore represent the metamorphic product of a Proterozoic sequence consisting of alternating layers of carbonatic, evaporitic and pelitic sediments.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"5 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-02-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140008075","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-02-03DOI: 10.1093/petrology/egae011
Ahmadreza Malekpour-Alamdari
Azizi et al. (2023) have attributed the E-W-oriented mafic dike swarm in the Biarjmand metamorphic core complex to an Eocene extensional event which is much younger than a previously suggested Late Jurassic-Early Cretaceous age (Malekpour-Alamdari et al., 2017). They proposed that the emplacement of these dikes occurred in a rapid extensional regime coeval with the exhumation of the core complex after gravitational instability in the Central Iran/Eurasia collision zone. I appreciate the opportunity this paper provides to shed light on specific aspects of the Late Mesozoic-Early Cenozoic continental extension within the Eurasian sector of the Neotethys subduction system. However, I here bring to attention certain discrepancies within Azizi et al.'s (2023) publication. Specifically, the assignment of an Eocene age to the emplacement of the E-W-oriented dike swarm, even though purportedly supported by U-Pb zircon dating, appears to be at odds with field observations and previously published geochronological data. Furthermore, the paper contains internal contradictions in its presentation of the core complex model for the study area. It is important to note that Malekpour-Alamdari (2017) and Malekpour-Alamdari et al. (2017) previously documented the geochronological-based metamorphic core complex model of the area. Regrettably, despite its direct relevance, these earlier works have not been acknowledged in Azizi et al.'s (2023) paper. In this comment, I outline the problems with the structural and regional geology, the zircon U-Pb age of the dike samples, the age of the dike swarm, and the geodynamic interpretations in Azizi et al.'s (2023) work.
{"title":"Comment on ‘The Generation of Eocene Mafic Dike Swarms During the Exhumation of a Core Complex, Biarjmand Area, NE Iran’ by Azizi et al. (2023), Journal of Petrology, 64, 1-18","authors":"Ahmadreza Malekpour-Alamdari","doi":"10.1093/petrology/egae011","DOIUrl":"https://doi.org/10.1093/petrology/egae011","url":null,"abstract":"Azizi et al. (2023) have attributed the E-W-oriented mafic dike swarm in the Biarjmand metamorphic core complex to an Eocene extensional event which is much younger than a previously suggested Late Jurassic-Early Cretaceous age (Malekpour-Alamdari et al., 2017). They proposed that the emplacement of these dikes occurred in a rapid extensional regime coeval with the exhumation of the core complex after gravitational instability in the Central Iran/Eurasia collision zone. I appreciate the opportunity this paper provides to shed light on specific aspects of the Late Mesozoic-Early Cenozoic continental extension within the Eurasian sector of the Neotethys subduction system. However, I here bring to attention certain discrepancies within Azizi et al.'s (2023) publication. Specifically, the assignment of an Eocene age to the emplacement of the E-W-oriented dike swarm, even though purportedly supported by U-Pb zircon dating, appears to be at odds with field observations and previously published geochronological data. Furthermore, the paper contains internal contradictions in its presentation of the core complex model for the study area. It is important to note that Malekpour-Alamdari (2017) and Malekpour-Alamdari et al. (2017) previously documented the geochronological-based metamorphic core complex model of the area. Regrettably, despite its direct relevance, these earlier works have not been acknowledged in Azizi et al.'s (2023) paper. In this comment, I outline the problems with the structural and regional geology, the zircon U-Pb age of the dike samples, the age of the dike swarm, and the geodynamic interpretations in Azizi et al.'s (2023) work.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"58 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-02-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139677647","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-02-03DOI: 10.1093/petrology/egae008
C L Urueña, C Möller
The Sveconorwegian orogen in Scandinavia and the Grenville orogen in Canada are both remnants of large and hot orogens that formed part of the supercontinent Rodinia around 1 billion years ago. Formerly deeply buried portions of crust in these orogens are exposed and offer insights into the tectonic dynamics of the basement within large orogens. The Eastern Segment of the Sveconorwegian Province hosts a ~30 000 km2 crustal portion that was buried to c. 40 km depth at a late stage of the orogeny, 980–960 Ma ago, and is bound towards the foreland in the east by a ~25 km wide zone of step anastomosing deformation, the Frontal wedge. This zone represents the outermost ductile deformation that developed within the crystalline basement in the orogen. We investigated a heterogeneously deformed and recrystallised syenodiorite with the aim to understand the character of the deformation-related metamorphism within the Frontal wedge. Field relations, microtextures, and mineral reactions show that the metamorphic recrystallisation was governed by hydrous fluid infiltration along the ductile deformation zones. Equilibrium was attained on a millimetre scale only and metamorphic recrystallisation was dependent on the introduction of hydrous fluid. The metamorphism reached high-pressure epidote-amphibolite-facies; geothermobarometric estimates suggest 540–600 °C and 9–12 kbar. Metamorphic zircon formed during the breakdown of Zr-bearing igneous phases, primarily baddeleyite. SIMS U–Pb analyses of igneous zircon and baddeleyite date the igneous crystallisation of the syenodiorite at 1230 ± 6 Ma. Metamorphic zircon grains are <20 μm and too small for precise dating, but yielded ages around 1 Ga. Collectively, the metamorphic data indicate that subvertical movements along steep planes within the Frontal wedge allowed for the regional-scale tectonic burial to ~40 km depth of the Eastern Segment to the west. Some of the same steep deformation structures were re-utilised as discrete movement planes during later exhumation.
{"title":"Fluid-induced metamorphism and deformation at the eastern boundary of the Sveconorwegian Province","authors":"C L Urueña, C Möller","doi":"10.1093/petrology/egae008","DOIUrl":"https://doi.org/10.1093/petrology/egae008","url":null,"abstract":"The Sveconorwegian orogen in Scandinavia and the Grenville orogen in Canada are both remnants of large and hot orogens that formed part of the supercontinent Rodinia around 1 billion years ago. Formerly deeply buried portions of crust in these orogens are exposed and offer insights into the tectonic dynamics of the basement within large orogens. The Eastern Segment of the Sveconorwegian Province hosts a ~30 000 km2 crustal portion that was buried to c. 40 km depth at a late stage of the orogeny, 980–960 Ma ago, and is bound towards the foreland in the east by a ~25 km wide zone of step anastomosing deformation, the Frontal wedge. This zone represents the outermost ductile deformation that developed within the crystalline basement in the orogen. We investigated a heterogeneously deformed and recrystallised syenodiorite with the aim to understand the character of the deformation-related metamorphism within the Frontal wedge. Field relations, microtextures, and mineral reactions show that the metamorphic recrystallisation was governed by hydrous fluid infiltration along the ductile deformation zones. Equilibrium was attained on a millimetre scale only and metamorphic recrystallisation was dependent on the introduction of hydrous fluid. The metamorphism reached high-pressure epidote-amphibolite-facies; geothermobarometric estimates suggest 540–600 °C and 9–12 kbar. Metamorphic zircon formed during the breakdown of Zr-bearing igneous phases, primarily baddeleyite. SIMS U–Pb analyses of igneous zircon and baddeleyite date the igneous crystallisation of the syenodiorite at 1230 ± 6 Ma. Metamorphic zircon grains are &lt;20 μm and too small for precise dating, but yielded ages around 1 Ga. Collectively, the metamorphic data indicate that subvertical movements along steep planes within the Frontal wedge allowed for the regional-scale tectonic burial to ~40 km depth of the Eastern Segment to the west. Some of the same steep deformation structures were re-utilised as discrete movement planes during later exhumation.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"8 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-02-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139680102","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-02-03DOI: 10.1093/petrology/egae009
Thomas Grocolas, Othmar Müntener
The volcanic–plutonic connection plays a fundamental role for magmatic systems, linking crystallising plutons, volcanic activity, volatile exsolution and ore deposits. Nonetheless, our understanding of the nature of these links is limited by the scarcity of continuous outcrops exhibiting clear relationships between the plutonic roots that feed its volcanic counterpart. One way to better characterise the volcanic–plutonic connection is to quantify the amount of melt segregation within crystallising tonalitic to granodioritic plutonic rocks, and to compare those with recent silicic eruptions. Here we investigate the processes of interstitial melt segregation in the calc-alkaline Western Adamello pluton (Italy). The Western Adamello tonalite exhibits a coarse-grained, equigranular texture and is mainly composed of hornblende partially replaced by biotite, plagioclase, quartz and alkali feldspar. Within the tonalites, several types of schlieren textures, crystal accumulation zones and dikes are found, comprising: (i) hornblende-biotite-gabbros, spatially-related to (ii) plagioclase- and quartz-rich leucotonalites; and (iii) quartz-, albite- and alkali-feldspar-rich domains forming aplitic to pegmatitic dikes indicative of melt segregation and extraction. Hornblende, biotite and plagioclase phenocrysts have essentially the same compositional range in the tonalites, gabbros and leucotonalites. Together with field observations, this indicates that deformation-driven crystal–melt segregation controls the modal variation within the host tonalite. The calculated melt in equilibrium with the primitive amphiboles has the same trace element composition as the host tonalite to within 5–10 %, indicating that the tonalite did not experience substantial melt loss. Quantitative modal compositions and crystallisation–differentiation calculations suggest that the evolution of the tonalite is controlled by plagioclase and hornblende crystallisation followed by a biotite-forming peritectic reaction. This peritectic reaction can be written as melt1 + amphibole = melt2 + biotite + quartz + plagioclase and decreases the remaining interstitial melt fraction from 40 to 15 % in a small temperature interval (~50 °C), therefore reducing the temperature window for large-scale melt segregation. The biotite-forming reaction initiates in weakly corundum-normative compositions in low to intermediate K calc-alkaline differentiation (e.g., Western Adamello and Peninsular Ranges batholith, California), whereas it seems absent in intermediate to high K, clinopyroxene-normative melts (e.g., Tuolumne intrusive suite, California). This difference is likely controlled by the initial aluminium saturation index and the differentiation path of the parental melt within the middle to lower crust. Textural observations and mass balance models indicate that 75–88 % plagioclase and quartz and 30–70 % interstitial melt was mechanically removed from the Western Adamello tonalite to form hornblende-
火山与板岩的联系对岩浆系统起着根本性的作用，它将结晶板岩、火山活动、挥发性溶解和矿床联系在一起。然而，我们对这些联系的性质的了解受到限制，因为很少有连续的露头显示出为火山岩提供养分的柱状根之间的明确关系。要更好地描述火山与深成岩之间的联系，一种方法是量化结晶的黑云母岩到花岗闪长岩深成岩中的熔体偏析量，并将其与近期的硅质喷发进行比较。在这里，我们研究了意大利钙碱性 Western Adamello 暴长岩中的间隙熔体偏析过程。Western Adamello辉长岩呈粗粒等粒状构造，主要由角闪石组成，部分被斜长石、斜长石、石英和碱性长石所取代。在碳酸盐岩中，发现了几种裂隙纹理、晶体堆积带和岩峰，包括：(i) 角闪石-生物辉石-辉长岩，在空间上与(ii) 富含斜长石和石英的白云石相关；(iii) 富含石英、白云石和碱性长石的区域，形成阳起石-伟晶岩岩峰，表明熔体分离和提取。在辉长岩、辉长岩和白云母中，角闪石、斜长石和斜长石表晶的成分范围基本相同。结合实地观测结果，这表明变形驱动的晶体-熔体偏析控制着主英安岩内部的模态变化。计算得出的与原始闪长岩处于平衡状态的熔体，其微量元素组成与寄主辉长岩相同，含量在5-10%以内，这表明辉长岩没有经历大量的熔体损失。定量模态成分和结晶分异计算表明，辉长岩的演化受控于斜长石和角闪石的结晶，然后是生物岩形成的围岩反应。这种围岩反应可以写成熔体1 +闪石 = 熔体2 + 斜长石 + 石英 + 斜长石，在一个较小的温度区间（约 50 °C）内将剩余的间隙熔体部分从 40% 降低到 15%，从而降低了大规模熔体偏析的温度窗口。在中低 K 值的钙碱性分异（如加利福尼亚州的 Western Adamello 和 Peninsular Ranges 岩床）中，生物玢岩形成反应在弱刚玉常态成分中开始，而在中高 K 值的挛辉石常态熔体（如加利福尼亚州的 Tuolumne 侵入岩套）中似乎不存在。这种差异可能受初始铝饱和度指数和母体熔体在中下地壳内的分化路径控制。纹理观察和质量平衡模型表明，75-88%的斜长石和石英以及30-70%的间隙熔体被机械地从Western Adamello辉长岩中移除，形成角闪石-生物辉长岩，而白云母则是由40-80%的斜长石和石英堆积而成。在300-400立方公里的西阿达梅洛碳酸盐岩中，只有约0.8-2.4立方公里的岩石类型与物理偏析过程有关，这表明熔体萃取有限。在全球范围内都可以观察到这种在辉长岩到花岗闪长岩柱岩中的晶体-熔体偏析过程，它有助于花岗岩液体的提取。在 Western Adamello tonalite 中观察到的这种机制可能有助于晶体贫乏的流纹岩的堆积和富含金属的盐水的偏析。
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Pub Date : 2024-02-03DOI: 10.1093/petrology/egae007
Ross Chandler, Ganesh Bhat, John Mavrogenes, Brad Knell, Rhiannon David, Thomas Leggo
The paleoregolith overlying the c. 2.06 Ga Mt Weld carbonatite (eastern Yilgarn Craton, Western Australia) hosts one of the largest Rare Earth Element (REE) deposits globally. Historic exploration and research has focussed on this weathered material, with a lack of unweathered samples preventing a thorough investigation into the nature of the underlying carbonatite. Recent deep drilling has allowed this first in-depth study into the primary geology, mineralogy and geochemistry of the carbonatite complex. Mt Weld shares a similar lithological architecture to other global carbonatite complexes such as Ngualla (Tanzania, Bonga (Angola), Chilwa Island (Malawi) and Mirima Hill (Malawi), displaying a central (~600 m diameter) unit of magnesio- to ferrocarbonatite is surrounded by a broad (~1.2 km) annulus of calciocarbonatite, itself surrounded by a fenitic halo. Primary REE mineralisation occurs within the central magnesio- to ferrocarbonatites (~2% Total Rare Earth Oxides in bulk rock) and is dominated by fine-grained monazite hosted within fractures and voids, as well as late magmatic synchysite/bastnäsite. Both high and low phosphourous ferrocarbonatites occur within this central magnesio- to ferrocarbonatite unit that display (respectively) monazite or synchysite/bastnäsite mineralisation with textural evidence for these REE-bearing carbonates occasionally forming as polycrystalline pseudomorphs of earlier burbankite group minerals. Magnesio- to ferrocarbonatite dykes with zhonghuacerite/olekminskite/ancylite assemblages occur throughout the otherwise REE-poor calciocarbonatites (~0.2% Total Rare Earth Oxides in bulk rock). Late hydrothermal events strongly influenced the modern-day mineral assemblages with much of the existing ore mineral textures interpreted as hydrothermal reworkings of pre-existing REE-bearing minerals. A comparison of the fresh carbonatite and the paleoregolith geochemistry suggest minimal horizontal migration of ore elements during paleoregolith formation, with the overlying paleoregolith material broadly reflecting the underlying carbonatite trace and minor element signatures. This allows the inference of an approximately 5x upgrade in REE (and Nb) concentrations from the primary carbonatite to the overlying paleoregolith. Mt Weld shows distinct geological, mineralogical and isotopic differences to other currently mined carbonatite-associated REE deposits such as Bayan Obo (China), Mountain Pass (USA) and the Mianning-Dechang belt (China), which suggests that fundamentally different carbonatite mantle sources and evolutionary paths can form world-class REE ore bodies.
{"title":"The Primary Geology of the Paleoproterozoic MT Weld Carbonatite Complex, Western Australia","authors":"Ross Chandler, Ganesh Bhat, John Mavrogenes, Brad Knell, Rhiannon David, Thomas Leggo","doi":"10.1093/petrology/egae007","DOIUrl":"https://doi.org/10.1093/petrology/egae007","url":null,"abstract":"The paleoregolith overlying the c. 2.06 Ga Mt Weld carbonatite (eastern Yilgarn Craton, Western Australia) hosts one of the largest Rare Earth Element (REE) deposits globally. Historic exploration and research has focussed on this weathered material, with a lack of unweathered samples preventing a thorough investigation into the nature of the underlying carbonatite. Recent deep drilling has allowed this first in-depth study into the primary geology, mineralogy and geochemistry of the carbonatite complex. Mt Weld shares a similar lithological architecture to other global carbonatite complexes such as Ngualla (Tanzania, Bonga (Angola), Chilwa Island (Malawi) and Mirima Hill (Malawi), displaying a central (~600 m diameter) unit of magnesio- to ferrocarbonatite is surrounded by a broad (~1.2 km) annulus of calciocarbonatite, itself surrounded by a fenitic halo. Primary REE mineralisation occurs within the central magnesio- to ferrocarbonatites (~2% Total Rare Earth Oxides in bulk rock) and is dominated by fine-grained monazite hosted within fractures and voids, as well as late magmatic synchysite/bastnäsite. Both high and low phosphourous ferrocarbonatites occur within this central magnesio- to ferrocarbonatite unit that display (respectively) monazite or synchysite/bastnäsite mineralisation with textural evidence for these REE-bearing carbonates occasionally forming as polycrystalline pseudomorphs of earlier burbankite group minerals. Magnesio- to ferrocarbonatite dykes with zhonghuacerite/olekminskite/ancylite assemblages occur throughout the otherwise REE-poor calciocarbonatites (~0.2% Total Rare Earth Oxides in bulk rock). Late hydrothermal events strongly influenced the modern-day mineral assemblages with much of the existing ore mineral textures interpreted as hydrothermal reworkings of pre-existing REE-bearing minerals. A comparison of the fresh carbonatite and the paleoregolith geochemistry suggest minimal horizontal migration of ore elements during paleoregolith formation, with the overlying paleoregolith material broadly reflecting the underlying carbonatite trace and minor element signatures. This allows the inference of an approximately 5x upgrade in REE (and Nb) concentrations from the primary carbonatite to the overlying paleoregolith. Mt Weld shows distinct geological, mineralogical and isotopic differences to other currently mined carbonatite-associated REE deposits such as Bayan Obo (China), Mountain Pass (USA) and the Mianning-Dechang belt (China), which suggests that fundamentally different carbonatite mantle sources and evolutionary paths can form world-class REE ore bodies.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"86 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-02-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139677766","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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