Conodont palaeothermometry of contact metamorphism in Middle Ordovician rocks from the Precordillera of western Argentina

VOLDMAN, GUSTAVO GABRIEL; ALBANESI, GUILLERMO LUIS; DO CAMPO, MARGARITA

GEOLOGICAL MAGAZINE

Cambridge University Press

Año: 2008 vol. 154 p. 449 - 462

0016-7568

The Yerba Loca Formation (Middle–Upper Ordovician), exposed in the WesternPrecordillera, 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 valuesof 4; consequently, the occurrence of smectite and I/S probably resulted from retrograde diagenesis processes.