Barren Miocene granitoids in the Central Andean metallogenic belt, Chile: Geochemistry and Nd-Hf and U-Pb isotope systematics

Abstract

ABSTRACT. Four Middle-to-Late Miocene barren plutonic complexes that occur between the giant porphyry copper deposits of the central Chilean Andes were selected for U-Pb LA-ICPMS geochronology and Hf-isotope systematics on single zircon grains. Major and trace elements and Sr-Nd-Hf isotope whole rock geochemical studies were undertaken to compare with slightly younger or coeval barren and fertile intrusive rocks between 32° and 34°S. The studied granitoids yield resolvable crystallization ages of 11.3±0.1 Ma (Cerro Mesón Alto massif), 10.3±0.2 Ma (La Gloria pluton), 14.9±0.2 Ma/14.9±0.1 Ma (Yerba Loca stock) and 11.2±0.1 Ma/14.7±0.1 Ma (San Francisco Batholith). Major and trace elements discard an adakitic signature as suggested for coeval porphyric intrusions at 32°S, slightly younger mineralized porphyries at Río Blanco-Los Bronces deposit and other Cenozoic adakites. Volcanic host rocks are less fractionated than the intrusive rock units. The same observation can be made for the unmineralized northern plutons compared to the southern ones. Initial Sr-Nd isotope data show insignificant variation (0.703761-0.704118 and 0.512758-0.512882), plotting in the mantle array. Trace element enrichment can be explained by addition of subducted-slab fluids and/or terrigenous sediments to the mantle wedge prior to and/or slight crustal input during magma ascent. Zircon grains separated from these barren intrusives share a similar initial εHf-data variation for the younger age group (10-12 Ma; 7.04-9.54) and show a more scattered range for the older one (14-15 Ma; 8.50-15.34); both sets plot between the DM and CHUR evolution lines. There is evidence that magma evolution was slightly distinct through time from older to younger barren magmatism, compared to a few fertile porphyritic rocks from Río Blanco-Los Bronces porphyry copper deposit. It is suggested that chronological inconsistencies within these complexes might be related to differential shortening across the NE-SW-trending Yeso Valley transfer fault, assumed as coeval, which also explains the local lack of easterly magmatic arc front migration. Francisco). Elementos mayores y trazas excluyen una característica adakítica, como ha sido sugerido para los intrusivos porfídicos a los 32°S, los pórfidos mineralizados ligeramente más jóvenes de Río Blanco-Los Bronces y otras adakitas del Cenozoico. Las rocas volcánicas de caja son menos fraccionadas que las unidades de rocas intrusivas. La misma conclusión se obtiene con relación a los plutones de la zona norte comparados a los del sur. Valores isotópico iniciales Sr-Nd muestran una variación insignificante (0,703761-0,704118 y 0,512758-0,512882) en el campo del manto. El enriquecimiento de elementos traza puede ser explicado a través de una adición de fluidos de la placa subducida y/o por sedimentos terrígenos transportados hacia la cuña de manto y/o por un leve aporte cortical durante el ascenso del magma. Granos de circón separados de los intrusivos estériles del grupo más joven (10-12 Ma; 7,04-9,54) comparten valores i...

Nuevas observaciones sobre el origen del Depósito Mesón Alto, Valle del Yeso, Chile central: un depósito compuesto de procesos glaciares y de remoción en masa?

“…These three intrusive units are the Cerro Mesón Alto Massif (CMA [samples FA-1, -3]; Deckart et al, 2010), Yeso Valley; La Gloria Pluton (LGP [FA-4, -5, -6]; Deckart et al, 2010), Colorado Valley; and Cerro Aparejo Intrusion (CAI, Yeso Valley, this study) upstream of the Cerro Mesón Alto Massif (Fig. 1, Table 1).…”

“…In addition, in the study area there is a strip of granite-to-quartzmonzonite plutons dated between 10 and 12 Ma (La Gloria Pluton, Cerro Mesón Alto Massif) intruding the Oligo-Miocene units ( Fig. 1) (e.g., Thiele, 1980;Cornejo y Mahood, 1997;Kurtz et al, 1997;Fock, 2005;Deckart et al, 2010). These plutons are located on the Laguna Negra and Chacayes-Yesillo Faults, which are west-vergent reverse faults (Fock, 2005;Fock et al, 2006).…”

Cenozoic structural evolution of the Argentinean Andes at 34º40’S: A close relationship between thick and thin-skinned deformation

“…Piquer et al (2010) dated a granodiorite at 7.8±0.4 Ma (K-Ar in biotite) and interpreted the emplacement of intrusives rocks as being controlled by the Infiernillo-Los Cipreses fault system, a set of Late Eocene-Middle Miocene normal faults that acted as reverse faults during an episode of major tectonic contraction in the Late Miocene, which induced the ascent of the intrusive bodies. Barren Miocene granitoids located between 33º and 34ºS, with arc-like signature and crystallization ages from 14.9±0.2 Ma to 10.2±0.2 Ma (U-Pb LA-ICPMS), suggest that in this region magmatism does not reflect eastward arc front migration (Deckart et al, 2010).…”

“…and some of these granitoids constitute very important porphyry Cu (-Mo) deposits (Maksaev et al, 2003;Deckart et al, 2010;Piquer et al, 2010). Geodetic models based on GPS estimations reveal that the current convergence rate between the Nazca and South American plates in this segment of the Andes is around 70 mm/a (Norabuena et al, 1999;Kendrik et al, 2003).…”

“…15e). Magmatic activity was intense during the Miocene and Pliocene in Chile, where widespread volcanic and plutonic rocks are exposed approximately between 33º and 35ºS (Maksaev et al, 2003;Deckart et al, 2010;Piquer et al, 2010). Fission track analysis in apatite reveal cooling ages from 21 to 3.1 Ma and suggest an important period of Andean uplift and denudation between 5.6 and 3.1 Ma (Maksaev et al, 2003).…”