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

VOLDMAN G. G; ALBANESI, G. L.; DO CAMPO, M.

GEOLOGICAL MAGAZINE

Cambridge University Press

Lugar: Reino Unido; Año: 2008 vol. 145 p. 1 - 14

0016-7568

 The Yerba Loca Formation (Middle-Upper Ordovician), exposed in western Precordillera, San Juan, Argentina, is made up of clastic-carbonate turbidites, and basic-ultrabasic rocks. It is affected by regional Siluro-Devonian low-grade metamorphism that locally reaches greenschist facies. At Ancaucha creek, 42 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 anomaly of about 4.5 times intrusion thickness. Hydrothermalism at Ancaucha creek was restricted to a few layers, in particular close to the intrusions, as indicated by conodont textures and rock fabric. One-dimensional thermal computer simulation clearly contradicts empirical data, which point to largely developed thermal aureoles; though conodont CAI temperatures from the contact zone also suggest temperatures >600ºC. The clay mineral assemblage of most of the analysed samples (chlorite   illite   smectite   I/S mixed-layers) is complex and probably derives from several superimposes processes. The coexistence of smectite and I/S, plus illite in some cases, even in samples with KI values corresponding to anchizone metamorphism are interpreted as non-equilibrium assemblages. In turn, CIS values (0.27-0.32) indicate anchizone metamorphism, underestimating the effect of intrusive heat, probably reflecting the smaller sensitivity of inorganic materials to geologically short metamorphic heating.