Pub Date : 2023-09-29DOI: 10.5027/andgeov50n3-3650
Jorge E. Romero, Frederick J. Swanson, Julia A. Jones, Daniele Morgavi, Guido Giordano, Matteo Trolese, Felipe Aguilera, Tatiana Izquierdo, Diego Perugini
The 22-23 April 2015 eruption of the Calbuco volcano (Southern Andes, Chile) led to extensive pyroclastic density currents (PDCs) interactions with vegetation. We seek to describe the PDCs which affected both Tepu and Frío rivers, northern Calbuco, from their timing and deposition to cooling and erosion, as well as their impacts on forests. Our investigation is based on field stratigraphy, forest disturbance assessment, and geothermometry from degassing pipes and charcoal. These PDCs reached at least ~540-603 °C, as estimated from fumaroles, and consisted of both concentrated and dilute PDCs during the first pulse (22 April) at Tepu and mainly during the second pulse (23 April) at Frío. Effects of PDCs on forest vegetation recorded in Tepu consisted of heating, abrasion, burial, and impact force. On the valley floor, trees were buried with up to 4 m of deposits from the concentrated PDCs, and all trees in this deposition zone died with no subsequent sprouting. Conversely, in the margins of the valley, defoliated fallen trees and standing shrubs indicate scorching due to the passage of dilute PDCs, and some of them were later sprouting. Estimated impact forces required to produce toppling range from 1.5 to 3.7 kPa, and PDC velocities reached up to 36 m s-1. Charring of the buried wood involved an emplacement temperature of 400-550 °C within PDC deposits. The rapid watershed formation may have facilitated infiltration, decreasing the temperature in the basal part within the deposits at the Tepu river. Runoff during the subsequent months triggered lahars and caused the rivers to incise the deposits and transport sediment downstream. This set of observations provides valuable insights into how the interaction between volcanic phenomena and margine forest on the valley floors informs eruptive processes, dynamics, and impacts. Our study is also relevant to interpret the thermal history and potential hazards of PDCs.
{"title":"The April 2015 Calbuco eruption pyroclastic density currents: deposition, impacts on woody vegetation, and cooling on the northern flank of the cone","authors":"Jorge E. Romero, Frederick J. Swanson, Julia A. Jones, Daniele Morgavi, Guido Giordano, Matteo Trolese, Felipe Aguilera, Tatiana Izquierdo, Diego Perugini","doi":"10.5027/andgeov50n3-3650","DOIUrl":"https://doi.org/10.5027/andgeov50n3-3650","url":null,"abstract":"The 22-23 April 2015 eruption of the Calbuco volcano (Southern Andes, Chile) led to extensive pyroclastic density currents (PDCs) interactions with vegetation. We seek to describe the PDCs which affected both Tepu and Frío rivers, northern Calbuco, from their timing and deposition to cooling and erosion, as well as their impacts on forests. Our investigation is based on field stratigraphy, forest disturbance assessment, and geothermometry from degassing pipes and charcoal. These PDCs reached at least ~540-603 °C, as estimated from fumaroles, and consisted of both concentrated and dilute PDCs during the first pulse (22 April) at Tepu and mainly during the second pulse (23 April) at Frío. Effects of PDCs on forest vegetation recorded in Tepu consisted of heating, abrasion, burial, and impact force. On the valley floor, trees were buried with up to 4 m of deposits from the concentrated PDCs, and all trees in this deposition zone died with no subsequent sprouting. Conversely, in the margins of the valley, defoliated fallen trees and standing shrubs indicate scorching due to the passage of dilute PDCs, and some of them were later sprouting. Estimated impact forces required to produce toppling range from 1.5 to 3.7 kPa, and PDC velocities reached up to 36 m s-1. Charring of the buried wood involved an emplacement temperature of 400-550 °C within PDC deposits. The rapid watershed formation may have facilitated infiltration, decreasing the temperature in the basal part within the deposits at the Tepu river. Runoff during the subsequent months triggered lahars and caused the rivers to incise the deposits and transport sediment downstream. This set of observations provides valuable insights into how the interaction between volcanic phenomena and margine forest on the valley floors informs eruptive processes, dynamics, and impacts. Our study is also relevant to interpret the thermal history and potential hazards of PDCs.","PeriodicalId":50792,"journal":{"name":"Andean Geology","volume":"44 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-09-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135293812","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"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 : 2023-09-29DOI: 10.5027/andgeov50n3-3453
Evelyn Luz Bustos-Escalona, Adriana Cecilia Mancuso, Cecilia Andrea Benavente, Andrea Beatriz Arcucci
Spinicaudata are freshwater branchiopods. The growth and development of spinicaudata populations in freshwater environments depend on various physical, chemical, and biological factors. The action of volcanism modifies the limnological parameters in the process of fossilization of lacustrine deposits. This study identifies spinicaudatan assemblages in two Triassic lake systems in Argentina, and analyzes the dynamics of shell preservation according to the volcanic activity involved. Two taphonomic grades divided into three different second taphonomic grades were defined. The focus on taphonomic and chemistry studies addresses changes in spinicaudata abundance and richness through time, in different paleoenvironmental conditions, and shell preservation and chemical composition. The discussion highlights the importance of lava flow inflow and volcanic ash fall in the different lake systems. We emphasize the importance of taphonomic studies on the spinicaudata groups to understand their autochthonous and allochthonous populations in Triassic lake systems. In turn, we conclude that the post mortem processes experienced by the shells, the exposure time at the water-sediment interface, and the chemical alteration of the water into the paleolakes produced by the surrounding volcanic activity can explain the development and preservation of spinicaudata in these two Triassic lacustrine systems.
{"title":"Taphonomic analysis of Spinicaudata from two Triassic lacustrine systems affected by different volcanic processes","authors":"Evelyn Luz Bustos-Escalona, Adriana Cecilia Mancuso, Cecilia Andrea Benavente, Andrea Beatriz Arcucci","doi":"10.5027/andgeov50n3-3453","DOIUrl":"https://doi.org/10.5027/andgeov50n3-3453","url":null,"abstract":"Spinicaudata are freshwater branchiopods. The growth and development of spinicaudata populations in freshwater environments depend on various physical, chemical, and biological factors. The action of volcanism modifies the limnological parameters in the process of fossilization of lacustrine deposits. This study identifies spinicaudatan assemblages in two Triassic lake systems in Argentina, and analyzes the dynamics of shell preservation according to the volcanic activity involved. Two taphonomic grades divided into three different second taphonomic grades were defined. The focus on taphonomic and chemistry studies addresses changes in spinicaudata abundance and richness through time, in different paleoenvironmental conditions, and shell preservation and chemical composition. The discussion highlights the importance of lava flow inflow and volcanic ash fall in the different lake systems. We emphasize the importance of taphonomic studies on the spinicaudata groups to understand their autochthonous and allochthonous populations in Triassic lake systems. In turn, we conclude that the post mortem processes experienced by the shells, the exposure time at the water-sediment interface, and the chemical alteration of the water into the paleolakes produced by the surrounding volcanic activity can explain the development and preservation of spinicaudata in these two Triassic lacustrine systems.","PeriodicalId":50792,"journal":{"name":"Andean Geology","volume":"38 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-09-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135294387","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"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 : 2023-09-29DOI: 10.5027/andgeov50n3-3640
Damián Groch, Verónica Gil, Marisa Gloria Cogliati, Micaela Virginia Medina, Michel Jaboyedoff
Because of their social, ecological, and economic impacts, landslides are an issue of global concern. Episodes occurring worldwide have been associated with natural and human causes, although the latter have been poorly explored. Despite recent scientific insight into intensive irrigation as a cause, research remains scarce. This paper focuses on determining the anthropogenic influence on the triggering of earth flows by long-term intensive irrigation, based on temporal and geospatial information. This analysis covers a sector of Northern Patagonia in Argentina, an area of transhumant pastoralism. Our results show that excessive water input creates favorable conditions for the triggering and evolution of earth flows in the region, even under water deficit conditions. As a result, on 22 September 2020, a collapse was recorded which mobilized ~19,000 m³ of earth debris along 360 m and produced economic losses estimated at 168,000 USD. Other landslides are expected to occur in the area if irrigation amounts remain at the same level.
{"title":"Irrigación de largo plazo como desencadenante de flujos de tierra en Chenque Malal, Norpatagonia argentina","authors":"Damián Groch, Verónica Gil, Marisa Gloria Cogliati, Micaela Virginia Medina, Michel Jaboyedoff","doi":"10.5027/andgeov50n3-3640","DOIUrl":"https://doi.org/10.5027/andgeov50n3-3640","url":null,"abstract":"Because of their social, ecological, and economic impacts, landslides are an issue of global concern. Episodes occurring worldwide have been associated with natural and human causes, although the latter have been poorly explored. Despite recent scientific insight into intensive irrigation as a cause, research remains scarce. This paper focuses on determining the anthropogenic influence on the triggering of earth flows by long-term intensive irrigation, based on temporal and geospatial information. This analysis covers a sector of Northern Patagonia in Argentina, an area of transhumant pastoralism. Our results show that excessive water input creates favorable conditions for the triggering and evolution of earth flows in the region, even under water deficit conditions. As a result, on 22 September 2020, a collapse was recorded which mobilized ~19,000 m³ of earth debris along 360 m and produced economic losses estimated at 168,000 USD. Other landslides are expected to occur in the area if irrigation amounts remain at the same level.","PeriodicalId":50792,"journal":{"name":"Andean Geology","volume":"19 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-09-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135294412","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"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 : 2023-09-29DOI: 10.5027/andgeov50n3-3671
Diego Palma, Sebastián Jovic, Luciano López, Diego Guido, Gerardo Páez, Facundo De Martino, Gustavo Sotarello, Pablo Rolando
The Casposo District is located in the Cordillera Frontal, in the center-west of San Juan Province, Argentina. It is characterized by the presence of a low sulfidation epithermal system hosted in volcanic rocks of the Choiyoi Group, where the Mercado-Kamila and Julieta are the more important sectors, concentrating the largest resources of the district. Julieta consists of a quartz-calcite (Au-Ag) vein system, with colloform-banded and brecciated textures, as well as lattice-bladed replacements. In this study, a detailed analysis of the mineralogy and textures of the Julieta Vein system identifies 9 pulses grouped into 4 main events, 3 hydrothermal and 1 tectonic-hydrothermal. Pulses P1, P2 and P3 are barren and carbonated in nature. Pulses P4 and P5A are of chalcedony composition with a colloform-banded texture, where the last one shows cream-colored adularia bands and black ginguro bands, to which high Au (5,5 g/t) and Ag (52 g/t) values are associated. Pulse P5B is of silicic nature as well, although with a brecciated texture, whereas pulse P6 is siliceous carbonate. The composition of these pulses migrates to carbonates represented by P7 and P8, both of brecciated texture. Finally, the system closes with the tectonic-hydrothermal pulse P9. Julieta Vein has a dominant NW-SE strike, with minor subveins of E-W and N-S orientation, that can be divided into 3 segments: North, Central and South. The greater thicknesses, as well as the greater inclinations and high precious metals values are associated with inflections between the 3 segments. Concerning the ore shoots geometry, the structure has a main lithologically controlled subhorizontal ore shoot, while the secondary subvertical ore shoots are structurally controlled. Differences in the rheology of the host rock would control the vein morphology, forming a vertical jog. Factors such as the evolution of the mineralizing events, the vein morphology, and the precious metals distribution, allowed us to identify the importance of the lithological and structural controls in the development of mineralization in the Julieta Vein.
{"title":"Mineralización, control estructural y modelado 3D de clavos mineralizados en la veta Julieta, distrito Casposo (Cordillera Frontal, San Juan, Argentina)","authors":"Diego Palma, Sebastián Jovic, Luciano López, Diego Guido, Gerardo Páez, Facundo De Martino, Gustavo Sotarello, Pablo Rolando","doi":"10.5027/andgeov50n3-3671","DOIUrl":"https://doi.org/10.5027/andgeov50n3-3671","url":null,"abstract":"The Casposo District is located in the Cordillera Frontal, in the center-west of San Juan Province, Argentina. It is characterized by the presence of a low sulfidation epithermal system hosted in volcanic rocks of the Choiyoi Group, where the Mercado-Kamila and Julieta are the more important sectors, concentrating the largest resources of the district. Julieta consists of a quartz-calcite (Au-Ag) vein system, with colloform-banded and brecciated textures, as well as lattice-bladed replacements. In this study, a detailed analysis of the mineralogy and textures of the Julieta Vein system identifies 9 pulses grouped into 4 main events, 3 hydrothermal and 1 tectonic-hydrothermal. Pulses P1, P2 and P3 are barren and carbonated in nature. Pulses P4 and P5A are of chalcedony composition with a colloform-banded texture, where the last one shows cream-colored adularia bands and black ginguro bands, to which high Au (5,5 g/t) and Ag (52 g/t) values are associated. Pulse P5B is of silicic nature as well, although with a brecciated texture, whereas pulse P6 is siliceous carbonate. The composition of these pulses migrates to carbonates represented by P7 and P8, both of brecciated texture. Finally, the system closes with the tectonic-hydrothermal pulse P9. Julieta Vein has a dominant NW-SE strike, with minor subveins of E-W and N-S orientation, that can be divided into 3 segments: North, Central and South. The greater thicknesses, as well as the greater inclinations and high precious metals values are associated with inflections between the 3 segments. Concerning the ore shoots geometry, the structure has a main lithologically controlled subhorizontal ore shoot, while the secondary subvertical ore shoots are structurally controlled. Differences in the rheology of the host rock would control the vein morphology, forming a vertical jog. Factors such as the evolution of the mineralizing events, the vein morphology, and the precious metals distribution, allowed us to identify the importance of the lithological and structural controls in the development of mineralization in the Julieta Vein.","PeriodicalId":50792,"journal":{"name":"Andean Geology","volume":"27 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-09-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135294410","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"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 : 2023-09-29DOI: 10.5027/andgeov50n3-3656
Ángel R. Miño-Boilini, Diego Brandoni
The restudy of a sloth specimen from the Honda Group (La Venta, Department of Huila, Colombia), more precisely from the upper levels of the La Victoria Formation and the lower levels of the Villavieja Formation (Serravallian, middle Miocene), suggests its assignment to Nematherium sp. (Mammalia, Folivora). This is the first record of Nematherium for Colombia, and given that until now the genus had been recorded from the Burdigalian (early Miocene) of southern South America, the record presented herein extends its geographic and chronological distribution.
对来自Honda Group (La Venta, Department of Huila, columbia)的树懒标本的重新研究，更准确地说是来自La Victoria组的上层和Villavieja组的下层(Serravallian，中中新世)，表明它属于Nematherium sp.(哺乳动物，Folivora)。这是哥伦比亚的第一个Nematherium记录，并且考虑到到目前为止该属的记录来自南美洲南部的burdigian(早中新世)，本文提出的记录扩展了其地理和时间分布。
{"title":"Nematherium (Xenarthra, Folivora) from the Serravallian of La Venta, Department of Huila, Colombia; chronological and biogeographical implications","authors":"Ángel R. Miño-Boilini, Diego Brandoni","doi":"10.5027/andgeov50n3-3656","DOIUrl":"https://doi.org/10.5027/andgeov50n3-3656","url":null,"abstract":"The restudy of a sloth specimen from the Honda Group (La Venta, Department of Huila, Colombia), more precisely from the upper levels of the La Victoria Formation and the lower levels of the Villavieja Formation (Serravallian, middle Miocene), suggests its assignment to Nematherium sp. (Mammalia, Folivora). This is the first record of Nematherium for Colombia, and given that until now the genus had been recorded from the Burdigalian (early Miocene) of southern South America, the record presented herein extends its geographic and chronological distribution.","PeriodicalId":50792,"journal":{"name":"Andean Geology","volume":"19 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-09-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135293813","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"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 : 2023-09-29DOI: 10.5027/andgeov50n3-3653
Guillermo E. Alvarado, Theofilos Toulkeridis, Yasuo Miyabuchi, Wendy Pérez
Pyroclastic density currents with an abundance of cauliflower-shaped bombs are an uncommon type of deposit, called bomb and ash flow (BoAF) deposits in several papers. Although they are similar to block and ash flow (BAF) deposits (e.g., rich in juvenile blocks and breadcrusted bombs), they are often related to eruptions of mafic to intermediate magmas. In the current study, we analyze and compare historical and prehistorical BoAF-generating eruptions at Asama and Aso (Japan), Mayon (Philippines), Mt. Spurr (United States), Fuego (Guatemala), Arenal (Costa Rica), Cotopaxi and Tungurahua (Ecuador), and Láscar and Tilocálar volcanoes (Chile). Our review indicates that BoAFs show a substantial contribution of juvenile rounded material but with different rheologies and fragmentation mechanisms. This juvenile material is typically basaltic or andesitic, as it is more susceptible to form volcanic bombs with scoriaceous cauliflower textures. Thus, BoAFs could be a subset of the BAF deposits. The study and recognition of this type of deposit in volcanic sequences could be misinterpreted as a ballistic bomb deposit or even a hot bomb-rich lahar deposit, therefore, its appropriate interpretation is fundamental for volcanic hazard assessment.
{"title":"Review of bomb and ash flows: Genesis and case studies of a subset of block and ash flow deposits","authors":"Guillermo E. Alvarado, Theofilos Toulkeridis, Yasuo Miyabuchi, Wendy Pérez","doi":"10.5027/andgeov50n3-3653","DOIUrl":"https://doi.org/10.5027/andgeov50n3-3653","url":null,"abstract":"Pyroclastic density currents with an abundance of cauliflower-shaped bombs are an uncommon type of deposit, called bomb and ash flow (BoAF) deposits in several papers. Although they are similar to block and ash flow (BAF) deposits (e.g., rich in juvenile blocks and breadcrusted bombs), they are often related to eruptions of mafic to intermediate magmas. In the current study, we analyze and compare historical and prehistorical BoAF-generating eruptions at Asama and Aso (Japan), Mayon (Philippines), Mt. Spurr (United States), Fuego (Guatemala), Arenal (Costa Rica), Cotopaxi and Tungurahua (Ecuador), and Láscar and Tilocálar volcanoes (Chile). Our review indicates that BoAFs show a substantial contribution of juvenile rounded material but with different rheologies and fragmentation mechanisms. This juvenile material is typically basaltic or andesitic, as it is more susceptible to form volcanic bombs with scoriaceous cauliflower textures. Thus, BoAFs could be a subset of the BAF deposits. The study and recognition of this type of deposit in volcanic sequences could be misinterpreted as a ballistic bomb deposit or even a hot bomb-rich lahar deposit, therefore, its appropriate interpretation is fundamental for volcanic hazard assessment.","PeriodicalId":50792,"journal":{"name":"Andean Geology","volume":"39 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-09-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135294411","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"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 : 2023-01-31DOI: 10.5027/andgeov50n1-3468
Fernando Guillermo Sardi, Márcia Elisa Boscato Gomes, Silvana Elizabeth Marangone
The Reflejos de Mar Li-pegmatite, located in northwestern Argentina, is part of the Villismán pegmatite group, Ancasti District, Pampean Pegmatite Province. Four garnet crystals from the outermost part of the pegmatite were analyzed by major and minor elements (SiO2, TiO2, Al2O3, Cr2O3, MgO, CaO, MnO, FeO) using electron microprobe. The pegmatite belongs to the rare-element class, spodumene type, LCT (Li-Cs-Ta) petrogenetic family. Based on their Mn and Fe contents, the analyzed garnet can be assigned to the spessartine-almandine serie. The cores and rims of the analyzed garnets show significant differences for the divalent components in VIII-coordination, especially, Mn and Fe. The average MnO and FeO contents in the cores of the grains is 34.6 and 8.29 wt%, respectively, while in the rims is 29.31 and 12.95 wt%, respectively. The Fe/Mn ratio at the core of the grains is 0.24 while at the rims it is 0.44. Cr2O3 and TiO2 contents are very low (˂0.17 wt%) and the values of SiO2 and Al2O3 are ~36 and ~21 wt%, respectively. The mean chemical and molecular formulas of the core can be expressed: [(Mn2.40 Fe0.57 Ca0.02 Mg0.02)3.01 (Al1.99 Cr0.002)2.00 (Si2.99 Ti0.01)3.00 O12]; {Sps79.8 Alm18.9 Grs0.7 Prp0.5}; and the rim as: [(Mn2.04 Fe0.89 Ca0.05 Mg0.04)3.02 (Al2.00 Cr0.002)2.00 (Si2.98 Ti0.005)3.00 O12]; {Sps67.6 Alm29.5 Grs1.6 Prp1.3}. The chemical composition of garnet from the Reflejos de Mar pegmatite is similar to other worldwide examples in similar rocks, especially LCT pegmatites, which are highly evolved and associated with Li mineralization. Therefore, its composition could be used as an additional tool in the exploration of Li-bearing pegmatites in the Pampean Pegmatite Province. The differences in Fe-Mn contents between core and rim of the crystals would be controlled by variations in composition of the pegmatitic melt and, in addition, by the simultaneous precipitation of other mineral phases, for example, schorl and Mn-Fe-bearing phosphates.
Reflejos de Mar li -伟晶岩位于阿根廷西北部，是Pampean伟晶岩省Ancasti地区Villismán伟晶岩群的一部分。采用电子探针对伟晶岩最外层的4颗石榴石晶体进行了主微量元素(SiO2、TiO2、Al2O3、Cr2O3、MgO、CaO、MnO、FeO)分析。伟晶岩属稀土类，锂辉石型，LCT (Li-Cs-Ta)产岩族。根据所分析的石榴石的Mn和Fe含量，可将其归属于spessarine -almandine系列。所分析的石榴石芯和边缘在viii -配位中的二价成分，特别是Mn和Fe，存在显著差异。晶粒芯部MnO和FeO的平均含量分别为34.6%和8.29 wt%，轮辋中MnO和FeO的平均含量分别为29.31 wt%和12.95 wt%。晶粒核心处的Fe/Mn比值为0.24，边缘处的Fe/Mn比值为0.44。Cr2O3和TiO2的含量很低(小于0.17 wt%)， SiO2和Al2O3的含量分别为~36和~21 wt%。核的平均化学分子式为[(Mn2.40 Fe0.57 Ca0.02 Mg0.02)3.01 (Al1.99 Cr0.002)2.00 (Si2.99 Ti0.01)3.00 O12];{Sps79.8 Alm18.9 Grs0.7 Prp0.5};边缘为[(Mn2.04 Fe0.89 Ca0.05 Mg0.04)3.02 (Al2.00 Cr0.002)2.00 (Si2.98 Ti0.005)3.00 O12];{Sps67.6 Alm29.5 Grs1.6 Prp1.3}。Reflejos de Mar伟晶岩中石榴石的化学成分与世界上其他类似岩石中的石榴石相似，特别是LCT伟晶岩，其高度演化并与锂矿化有关。因此，其组成可作为潘潘伟晶岩省含锂伟晶岩勘探的附加工具。晶体核心和边缘之间铁锰含量的差异将由伟晶质熔体组成的变化和其他矿物相的同时沉淀控制，例如，含铁和含锰磷酸盐。
{"title":"Garnet composition from the Reflejos de Mar LCT-pegmatite, Ancasti district, Argentina and its implication for exploration of primary deposits of lithium","authors":"Fernando Guillermo Sardi, Márcia Elisa Boscato Gomes, Silvana Elizabeth Marangone","doi":"10.5027/andgeov50n1-3468","DOIUrl":"https://doi.org/10.5027/andgeov50n1-3468","url":null,"abstract":"The Reflejos de Mar Li-pegmatite, located in northwestern Argentina, is part of the Villismán pegmatite group, Ancasti District, Pampean Pegmatite Province. Four garnet crystals from the outermost part of the pegmatite were analyzed by major and minor elements (SiO2, TiO2, Al2O3, Cr2O3, MgO, CaO, MnO, FeO) using electron microprobe. The pegmatite belongs to the rare-element class, spodumene type, LCT (Li-Cs-Ta) petrogenetic family. Based on their Mn and Fe contents, the analyzed garnet can be assigned to the spessartine-almandine serie. The cores and rims of the analyzed garnets show significant differences for the divalent components in VIII-coordination, especially, Mn and Fe. The average MnO and FeO contents in the cores of the grains is 34.6 and 8.29 wt%, respectively, while in the rims is 29.31 and 12.95 wt%, respectively. The Fe/Mn ratio at the core of the grains is 0.24 while at the rims it is 0.44. Cr2O3 and TiO2 contents are very low (˂0.17 wt%) and the values of SiO2 and Al2O3 are ~36 and ~21 wt%, respectively. The mean chemical and molecular formulas of the core can be expressed: [(Mn2.40 Fe0.57 Ca0.02 Mg0.02)3.01 (Al1.99 Cr0.002)2.00 (Si2.99 Ti0.01)3.00 O12]; {Sps79.8 Alm18.9 Grs0.7 Prp0.5}; and the rim as: [(Mn2.04 Fe0.89 Ca0.05 Mg0.04)3.02 (Al2.00 Cr0.002)2.00 (Si2.98 Ti0.005)3.00 O12]; {Sps67.6 Alm29.5 Grs1.6 Prp1.3}. The chemical composition of garnet from the Reflejos de Mar pegmatite is similar to other worldwide examples in similar rocks, especially LCT pegmatites, which are highly evolved and associated with Li mineralization. Therefore, its composition could be used as an additional tool in the exploration of Li-bearing pegmatites in the Pampean Pegmatite Province. The differences in Fe-Mn contents between core and rim of the crystals would be controlled by variations in composition of the pegmatitic melt and, in addition, by the simultaneous precipitation of other mineral phases, for example, schorl and Mn-Fe-bearing phosphates.","PeriodicalId":50792,"journal":{"name":"Andean Geology","volume":"43 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-01-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135356816","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"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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