Conodont palaeothermometry of contact metamorphism in Middle ordovician rocks from the precordillera of western Argentina
The Yerba Loca Formation (Middle-Upper Ordovician), exposed in the Western Precordillera, San Juan, Argentina, is made up of clastic-carbonate turbidites, and basic-ultrabasic rocks. It is affected by regional Siluro-Devonian very low-grade metamorphism that locally reaches greenschist facies. At An...
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2008
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| Acceso en línea: | https://bibliotecadigital.exactas.uba.ar/collection/paper/document/paper_00167568_v145_n4_p449_Voldman http://hdl.handle.net/20.500.12110/paper_00167568_v145_n4_p449_Voldman |
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paper:paper_00167568_v145_n4_p449_Voldman2023-06-08T14:38:50Z Conodont palaeothermometry of contact metamorphism in Middle ordovician rocks from the precordillera of western Argentina Argentina CAI Clay mineralogy Conodont Contact metamorphism Ordovician Precordillera aureole computer simulation conodont contact metamorphism contact zone greenschist facies low grade metamorphism metasomatism Ordovician paleotemperature petrography thermal alteration turbidite Argentina Precordillera San Juan [Argentina] South America Croton ovalifolius Lenodus variabilis Paroistodus horridus The Yerba Loca Formation (Middle-Upper Ordovician), exposed in the Western Precordillera, San Juan, Argentina, is made up of clastic-carbonate turbidites, and basic-ultrabasic rocks. It is affected by regional Siluro-Devonian very low-grade metamorphism that locally reaches greenschist facies. At Ancaucha creek, 45 conodont samples were taken from two sections that include 30 to 50 m thick sills. In order to analyse the thermal alteration patterns produced by these intrusive bodies, conodont Colour Alteration Index (CAI) is contrasted with optical petrography and X-ray diffraction analyses of clay minerals. The intrusions are dated as post-Darriwilian (Da2), as determined by conodont biostratigraphy of the host rock, which indicates the Paroistodus horridus Subzone of the Lenodus variabilis Zone. The distribution of CAI values defines a thermal aureole of about 2.5 times intrusion thickness that prevailed over the later very low-grade metamorphism. Metasomatism at Ancaucha creek is recorded by CAI values of 4 to 7, particularly restricted to a few layers close to the intrusions, as indicated by conodont textures and rock fabric. One-dimensional thermal computer simulation conforms to empirical data indicating temperatures greater than 600 °C for the contact zone, although it points out slightly narrower thermal aureoles. The clay mineral assemblage of most of the analysed samples (chlorite, illite, smectite and I/S mixed-layers) is complex and probably derives from several superimposed processes, thus representing non-equilibrium assemblages. In turn, KI values (0.27-0.32) indicate anchizone metamorphism, in agreement with regional CAI values of 4; consequently, the occurrence of smectite and I/S probably resulted from retrograde diagenesis processes. Copyright © Cambridge University Press 2008. 2008 https://bibliotecadigital.exactas.uba.ar/collection/paper/document/paper_00167568_v145_n4_p449_Voldman http://hdl.handle.net/20.500.12110/paper_00167568_v145_n4_p449_Voldman |
| institution |
Universidad de Buenos Aires |
| institution_str |
I-28 |
| repository_str |
R-134 |
| collection |
Biblioteca Digital - Facultad de Ciencias Exactas y Naturales (UBA) |
| topic |
Argentina CAI Clay mineralogy Conodont Contact metamorphism Ordovician Precordillera aureole computer simulation conodont contact metamorphism contact zone greenschist facies low grade metamorphism metasomatism Ordovician paleotemperature petrography thermal alteration turbidite Argentina Precordillera San Juan [Argentina] South America Croton ovalifolius Lenodus variabilis Paroistodus horridus |
| spellingShingle |
Argentina CAI Clay mineralogy Conodont Contact metamorphism Ordovician Precordillera aureole computer simulation conodont contact metamorphism contact zone greenschist facies low grade metamorphism metasomatism Ordovician paleotemperature petrography thermal alteration turbidite Argentina Precordillera San Juan [Argentina] South America Croton ovalifolius Lenodus variabilis Paroistodus horridus Conodont palaeothermometry of contact metamorphism in Middle ordovician rocks from the precordillera of western Argentina |
| topic_facet |
Argentina CAI Clay mineralogy Conodont Contact metamorphism Ordovician Precordillera aureole computer simulation conodont contact metamorphism contact zone greenschist facies low grade metamorphism metasomatism Ordovician paleotemperature petrography thermal alteration turbidite Argentina Precordillera San Juan [Argentina] South America Croton ovalifolius Lenodus variabilis Paroistodus horridus |
| description |
The Yerba Loca Formation (Middle-Upper Ordovician), exposed in the Western Precordillera, San Juan, Argentina, is made up of clastic-carbonate turbidites, and basic-ultrabasic rocks. It is affected by regional Siluro-Devonian very low-grade metamorphism that locally reaches greenschist facies. At Ancaucha creek, 45 conodont samples were taken from two sections that include 30 to 50 m thick sills. In order to analyse the thermal alteration patterns produced by these intrusive bodies, conodont Colour Alteration Index (CAI) is contrasted with optical petrography and X-ray diffraction analyses of clay minerals. The intrusions are dated as post-Darriwilian (Da2), as determined by conodont biostratigraphy of the host rock, which indicates the Paroistodus horridus Subzone of the Lenodus variabilis Zone. The distribution of CAI values defines a thermal aureole of about 2.5 times intrusion thickness that prevailed over the later very low-grade metamorphism. Metasomatism at Ancaucha creek is recorded by CAI values of 4 to 7, particularly restricted to a few layers close to the intrusions, as indicated by conodont textures and rock fabric. One-dimensional thermal computer simulation conforms to empirical data indicating temperatures greater than 600 °C for the contact zone, although it points out slightly narrower thermal aureoles. The clay mineral assemblage of most of the analysed samples (chlorite, illite, smectite and I/S mixed-layers) is complex and probably derives from several superimposed processes, thus representing non-equilibrium assemblages. In turn, KI values (0.27-0.32) indicate anchizone metamorphism, in agreement with regional CAI values of 4; consequently, the occurrence of smectite and I/S probably resulted from retrograde diagenesis processes. Copyright © Cambridge University Press 2008. |
| title |
Conodont palaeothermometry of contact metamorphism in Middle ordovician rocks from the precordillera of western Argentina |
| title_short |
Conodont palaeothermometry of contact metamorphism in Middle ordovician rocks from the precordillera of western Argentina |
| title_full |
Conodont palaeothermometry of contact metamorphism in Middle ordovician rocks from the precordillera of western Argentina |
| title_fullStr |
Conodont palaeothermometry of contact metamorphism in Middle ordovician rocks from the precordillera of western Argentina |
| title_full_unstemmed |
Conodont palaeothermometry of contact metamorphism in Middle ordovician rocks from the precordillera of western Argentina |
| title_sort |
conodont palaeothermometry of contact metamorphism in middle ordovician rocks from the precordillera of western argentina |
| publishDate |
2008 |
| url |
https://bibliotecadigital.exactas.uba.ar/collection/paper/document/paper_00167568_v145_n4_p449_Voldman http://hdl.handle.net/20.500.12110/paper_00167568_v145_n4_p449_Voldman |
| _version_ |
1768542582237822976 |