Geochronology of very low-grade Mesozoic Andean metabasites; an approach through the K–Ar, ⁴⁰ Ar/ 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.

El volcanismo jurásico superior de la Formación Río Damas-Tordillo (33°-35,5°S): antecedentes su sobre petrogénesis, cronología, proveniencia e implicancias tectónicas.

“…Central portion of lava flows or restricted domains within the sedimentary rocks are often less affected by the alteration processes. The observed alteration minerals are: prehnite, pumpeyllite, chlorite, epidote, titanite, actinolite, calcite, quartz, white mica (sericite), albite and K-feldspar, defining a typical mineralogy of prenhite-pumpeyllite facies in concordance with the reported by others authors for this Andean region (Calderón, 2008;Levi et al, 1989;Oliveros et al, 2008;Robinson et al, 2004). The fine grained volcanic litharenites from the easternmost outcrops of the Río Damas Formation, in the head of the Maipo river valley at 34º15'S, bear scarce calcite and chlorite as secondary mineral phases, indicating that the alteration degree diminishes eastward (Charrier, 1981).…”

“…All the volcanic and sedimentary rocks of the Río Damas-Tordillo Formation are altered to some extent, as a consequence of burial and contact metamorphism (Calderón, 2008;Levi et al, 1989;Oliveros et al, 2008;Robinson et al, 2004). The alteration in general is very penetrative, partly hiding the primary features of the rock, particularly at the contact between beds, vesicular portions or highly fractured zones of the lava flows and in the fine matrix of volcanic and sedimentary rocks.…”

“…The alteration that affects the Mesozoic and Cenozoic volcano-sedimentary rocks cropping out in the Andes of Central Chile, which can be very pervasive, is the result of the combination of very low-grade burial metamorphism (Prenhite-Pumpellyte facies), and contact metamorphism and hydrothermal alteration related to the intrusion of numerous granitic stocks of Miocene age (Calderón, 2008;Levi et al, 1989;Muñoz et al, 2009;Oliveros et al, 2008;Robinson et al, 2004;Thiele, 1980).…”

Crustal‐scale structural architecture in central Chile based on seismicity and surface geology: Implications for Andean mountain building

“…However, the knowledge on the tectonic evolution of the western side of the range in this region has increased during the last years due to a more accurate mapping and dating of tectonic and depositional events. This has been possible due to a more systematic search for unaltered levels [ Fuentes , 2004; Muñoz et al , 2006], the use of more penetrative radioisotopic dating [ Charrier et al , 1996, 2002, 2005; Fock et al , 2006; Montecinos et al , 2008; Oliveros et al , 2008], the discovery of abundant Cenozoic mammal fauna [ Wyss et al , 1990; Flynn et al , 1995; Croft et al , 2003; Flynn et al , 2003], and the use of quantitative geomorphology [ Farías et al , 2008a] and low‐temperature thermochronology [ Farías et al , 2008a; Maksaev et al , 2009]. These new studies have allowed to establish that the contractive development of the Andes in central Chile occurred fundamentally since the latest Oligocene–earliest Miocene [e.g., Charrier et al , 2002, 2005] (similar to the central Argentinean Andes [ Ramos et al , 1996; Giambiagi and Ramos , 2002; Giambiagi et al , 2003a; Ramos et al , 2004]), and that the main stage of surface uplift occurred during the late Miocene–early Pliocene [ Farías et al , 2008a].…”

White mica K–Ar ages from lawsonite–blueschist facies Hakoishi sub–unit and from prehnite–pumpellyite facies Tobiishi sub–unit of the Kurosegawa belt, Kyushu, Japan

“…A limited number of K-Ar dating for alumino-celadonite has been published by previous workers. Oliveros et al (2008) applied the K-Ar method for metamorphic celadonite and U-Pb method for the titanite in metabasites suffered the PrP facies metamorphism in the Andes of central Chile. Celadonite is identified from amygdules along with titanite and quartz [see Figs.…”