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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*
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: gvoldman@efn.uncor.edu

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.

Type
Original Article
Copyright
Copyright © Cambridge University Press 2008

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