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...
Guardado en:
| Autores principales: | , , |
|---|---|
| Formato: | JOUR |
| Materias: | |
| Acceso en línea: | http://hdl.handle.net/20.500.12110/paper_00167568_v145_n4_p449_Voldman |
| Aporte de: |
| Sumario: | 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. |
|---|