Geochronology of very low-grade Mesozoic Andean metabasites; an approach through the K-Ar, ^sup 40^Ar/^sup 39^Ar and U-Pb LA-MC-ICP-MS methods

Abstract:

Multiple geochronological methods using different metamorphic minerals were combined to date the regional, very low-grade metamorphism affecting Upper Jurassic-Lower Cretaceous volcano-sedimentary successions in the Andes of central Chile. Early Late Cretaceous metamorphic ages (between 82 and 108 Ma) were obtained by the K-Ar and U-Pb methods for celadonite and titanite. A much younger thermal event is responsible for actinolite formation at 8 Ma, most probably related to the intrusion of proximal Miocene granitoids. Previous models for the metamorphism should be reinterpreted taking into account the absence of a greenschist-facies event. The combination of different metamorphic minerals and chronometers is regarded as a powerful analytical tool to date the very low-grade metamorphism associated with the Mesozoic extensional regime developed within the Andes.

A recent advance in the study of very low-grade metabasites is the quantitative approach to the intensive P-T-X conditions using internally consistent thermodynamic databases (e.g. Powell et al. 1993; Robinson et al. 2004, 2005; Day & Springer 2005). However, the accurate timing of the very low-grade metamorphic processes has been a subject scarcely investigated, largely as a consequence of the small number and size of newly formed minerals suitable for dating. In burial-type processes, dating of metamorphic minerals would record the time of the different stages in the development of a basin, as shown by Aguirre et al. (1999) and Fuentes et al. (2005).

Present knowledge of the geodynamic evolution of the Andes during the Late Jurassic and Early Cretaceous is poorly constrained because of the absence of a precise chronostratigraphic framework. However, these epochs are of the utmost importance in the development of the orogen, as they coincide with major planetary events; for example, intense volcanic activity and formation of several large igneous provinces accompanied the opening of the Atlantic Ocean. Widespread rifting occurred along the western border of South America during the Late Jurassic-Early Cretaceous (Aberg et al. 1984), and has been attributed to asthenospheric mantle upwelling leading to extension, crustal attenuation and subsidence (Aguirre et al. 1999).

During those epochs, large volumes of volcanic rocks were deposited in central Chile between 25° and 36°S along a 1200 km string of ensialic basins characterized by alternating marine and terrestrial conditions (Vergara et al. 1995). The Upper Jurassic-Lower Cretaceous successions are displayed as two parallel belts at the western and eastern flanks of a Mesozoic synclinorium (Fig. 1); the western belt along the Coastal Range and the eastern one along the Andes, near the border with Argentina. The rocks in both belts are affected by burial metamorphism at upper zeolite, prehnite-pumpellyite and lower greenschist facies (Levi et al. 1989; Fig. 1). In the central Chilean Andes (33-35°S), these epochs are characterized by alternating cycles of marine transgression, basin generation and continental uplift, which have given rise to thick successions of volcaniclastic rocks, limestone-shale units, and intermediate to basic lava flows. Palaeontological ages are known for the marine carbonate units but isotopie dates do not exist for documenting the episodes of volcanism, or the very low-grade metamorphism. Two different metamorphic events took place, one during the early Late Cretaceous and the other in early Miocene; these events explain the metamorphic pattern found in the Upper Jurassic-Lower Cretaceous and the Cenozoic volcanic sequences (Fig. 1), which unconformably overlie the Mesozoic units (Robinson et al. 2004).

Here we report K-Ar, Ar-Ar, and laser ablation inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) U-Pb ages that suggest the occurrence of at least two metamorphic events. These new data provide insights into the geodynamic evolution of this segment of the Andean Cordillera.

Geological framework

A typical section of the Upper Jurassic-Lower Cretaceous volcano-sedimentary successions is well exposed in the Andes of central Chile, east of Santiago (Fig. 1). The Rio Damas Formation consists of c. 3 km of red continental volcanogenic sedimentary rocks and porphyritic lavas of basic composition. A Late Jurassic age has been assigned to this formation based on biostratigraphic data relating to both the underlying and overlying marine sequences (Thiele 1980). Ac. 1.4 km thick unit, the Lo Valdés Formation, concordantly overlies the Rio Damas Formation. The former consists of a 700 m basal section of basic lavas, partly pillowed, andesitic hyaloclastites, and a marine fossiliferous sequence of limestones and shales. Based on its palaeontological record, the age of the Lo Valdés Formation covers the interval Early-Mid-Tithonian to Hauterivian (Hallam et al. 1986). Both formations have been affected by regional, very low-grade, burial-type metamorphism. Common secondary mineral assemblages in metabasites of the Rio Damas Formation are: (1) epidote + actinolite + chlorite ± titanite; (2) chlorite + epidote ± pumpellyite ± calcite; (3) actinolite + prehnite + epidote with ubiquitous chalcedony. Previous workers have assigned these mineral associations to the pumpellyite-actinolite facies and, based on mineral chemistry and thermodynamic modelling on the Na^sub 2^O-CaO-MgO-Al^sub 2^O^sub 3^-SiO^sub 2^-H2O (NCMASH) basaltic system, a minimum value for the invariant CHEPPAQ (chlorite-water-epidoteprehnite-pumpellyite-actinolite-quartz) point was established as 260 °C and 1.1 kbar (Robinson et al. 2004). Metabasites of the Lo Valdés Formation contain assemblages with pumpellyite, prehnite, chlorite, mixed-layer smectite-chlorite, celadonite, titanite, quartz, K-feldspar, calcite and minor epidote (actinolite absent), which are characteristic of the prehnite-pumpellyite facies (Levi et al. 1989). Based on facies distribution and regional tectonics, the metamorphism of the Upper Jurassic-Lower Cretaceous successions described here has been attributed to an early Late Cretaceous event (Levi et al. 1989; Robinson et al. 2004). Age determinations for the main secondary minerals from this section have not been determined; however, the occurrence of a Cretaceous event is supported by ages of 105 ± 3 Ma and 101.3 ± 2.9 Ma obtained by K-Ar analysis of celadonite from basic lavas of the Rio Damas Formation at its type locality in Termas del Flaco, some 200 km south from the Maipo area (Belmar 2000).