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

Published online by Cambridge University Press:  27 March 2008

GUSTAVO G. VOLDMAN ,
GUILLERMO L. ALBANESI  and
MARGARITA DO CAMPO
GUSTAVO G. VOLDMAN*
Affiliation:
CONICET and Museo de Paleontología, Universidad Nacional de Córdoba, P.O. Box 1598, X5000FCO Córdoba, Argentina
GUILLERMO L. ALBANESI
Affiliation:
CONICET and Museo de Paleontología, Universidad Nacional de Córdoba, P.O. Box 1598, X5000FCO Córdoba, Argentina
MARGARITA DO CAMPO
Affiliation:
INGEIS (CONICET – UBA) and Universidad Nacional de Buenos Aires, 1428 Buenos Aires, Argentina
*
§Author for correspondence: [email protected]
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Abstract

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.

Keywords

conodontCAIclay mineralogycontact metamorphismOrdovicianPrecordilleraArgentina
Type
Original Article
Information
Geological Magazine , Volume 145 , Issue 4 , July 2008 , pp. 449 - 462
Copyright
Copyright © Cambridge University Press 2008

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