EGU21-13269, updated on 04 Mar 2021
EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Petrogenesis and emplacement depths of the Petite Pluton during the closure of the Rocas Verdes basin, southern Patagonia - preliminary results

Gianfranco Gregorina1, Maria Fernanda Torres García1, Mauricio Calderón1, Thomas Theye2, Francisco Hervé1,3, Cristobal Ramírez de Arellano1, and Francisco Fuentes4
Gianfranco Gregorina et al.
  • 1Carrera de Geología, Universidad Andrés Bello, Santiago, Chile
  • 2Institut für Anorganische Chemie, Universität Stuttgart, Stuttgart, Germany
  • 3Departamento de Geología, Universidad de Chile, Santiago, Chile
  • 4Escuela de Geología, Facultad de Ciencias, Universidad Mayor, Santiago, Chile

The Petite Pluton is a Cretaceous intrusion covering an area of nearly 136 km2 located in Isla Capitán Aracena, southernmost Patagonia, Chile. This pluton and other stocks are located outside of the margins the Early Cretaceous-Paleogene Fuegian Batholith. The Petite Pluton intrudes the Capitán Aracena ophiolitic complex, interpreted as supracrustal remnants generated during the rifting stage of the Rocas Verdes marginal basin (Late Jurassic- Early Cretaceous; cf. Calderón et al., 2013, Geochem. J.) overlain by hemi-pelagic sedimentary basin infill (Yahgan Formation). These units are locally deformed and exposed in the southern limit of the NW-SE-trending Magallanes fold-and-thrust belt. The satellite plutons consist of amphibole-bearing diorites and quartzdiorites (48-55 wt.% SiO2) with calc-alkaline compositional trends consistent with their generation in a subduction environment. On N-MORB normalized incompatible elements pattern, the rocks show peaks in LILE (Rb, Ba, Sr) and subtle throughs in Ti, Zr, Nb, Ta and Y. Chondrite-normalized REE pattern is concave upwards with enrichment of LREE relative to HREE without Eu anomaly. The mineral compositions of diorites of Petite pluton consist of amphibole (magnesio-hornblende and tschermakitic hornblende), plagioclase is labradorite and andesine (An44-59), with Ca-rich composition in small grains included within poikilitic amphibole, biotite (annite), quartz, minor contents of K-feldspar, titanite, magnetite-ilmenite pairs and traces of apatite and zircon. Amphibole composition can be used as a proxy of the amount of H2O-rich fluids involved in magma evolution and could potentially be used to constrain the crustal depths of pluton emplacement in magmatic plumbing systems (Yavuz & Döner, 2017, P. di Mineralogia; Torres García et al., 2020, Lithos). The calculated pressure and temperature of 3 kbar and 800-850°C, indicate the emplacement and crystallization of magma batches in the upper crust. Oxygen fugacity [log (ƒO2)] varies between -9.9 and -10.7 (NNO), indicating amphibole crystallization from basaltic-andesitic melts under moderately oxidizing conditions. The moderately Mg# (60-72) of amphibole is consistent with their crystallization from mafic-intermediate melt-dominated crystal mushes with residual melts generated after the fractionational crystallization of olivine and clinopyroxene at deeper crustal depths. The amphibole composition constraint an amount of 6 wt% of H2O in the residual melts. The subtle negative Eu anomaly in amphibole indicates its partially simultaneous fractionation with plagioclase, suggesting rapid undercooling. The emplacement of the Petite Pluton at ~10 km depth occurred during and/or lately after the tectonic emplacement of ophiolitic complexes within an accretionary wedge, governed by a NE tectonic transport (Muller et al., 2021, Tectonophysics). Late Cretaceous satellite plutons suggest a continentward migration of the magmatic arc, related to the flattening of the subducted oceanic lithosphere of the proto-Pacific Ocean.

Acknowledgements. The study is supported by  Fondecyt grant 1161818.

How to cite: Gregorina, G., Torres García, M. F., Calderón, M., Theye, T., Hervé, F., Ramírez de Arellano, C., and Fuentes, F.: Petrogenesis and emplacement depths of the Petite Pluton during the closure of the Rocas Verdes basin, southern Patagonia - preliminary results, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13269,, 2021.