Metamorphic evolution of the Loma Marcelo skarn within the geotectonic context of the crystalline basement of the Ventania System (Argentina)

BALLIVIÁN JUSTINIANO, CARLOS A.; SATO, ANA MARÍA; AIGLSPERGER, THOMAS; LANFRANCHINI, MABEL E.; RECIO, CLEMENTE; PROENZA, JOAQUÍN A.; CURCI, MARCELA V.; LAJOINIE, MARÍA FLORENCIA; BASEI, MIGUEL A.S.; DE BARRIO, RAÚL E.; BALLIVIÁN JUSTINIANO, CARLOS A.; SATO, ANA MARÍA; AIGLSPERGER, THOMAS; LANFRANCHINI, MABEL E.; RECIO, CLEMENTE; PROENZA, JOAQUÍN A.; CURCI, MARCELA V.; LAJOINIE, MARÍA FLORENCIA; BASEI, MIGUEL A.S.; DE BARRIO, RAÚL E.

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2019 vol. 92 p. 56 - 76

0895-9811

This study describesthe mineralogical and isotopic changes that carbonate xenoliths experiencedunder multiple metamorphic events and hydrothermal fluid circulation during theevolution of the Ventania System basement. The high reactivity of carbonatexenoliths allowed the preservation of mineral assemblages corresponding to atleast three metamorphic events in the resulting Loma Marcelo skarn. TheVentania System basement is composed of Neoproterozoic granites andignimbrites, Early Cambrian granites, and Middle Cambrian rhyolites. Carbonatexenoliths were incorporated during the intrusion of a calc-alkaline granitewith an LA-ICP-MS U-Pb crystallization age of 621.6 ± 2.2 Ma. The intrusioninduced pyroxene-hornfels facies metamorphism in the carbonate xenoliths andthe associated metasomatism generated calcic and magnesian skarns depending onthe protolith composition. Garnet, clinopyroxene, wollastonite, bytownite, andmeionite were formed in the calcic skarn (CaS), whereas forsterite and spinelwere formed in the magnesian skarn (MgS). Crystallization of Early Cambrianalkaline granites was accompanied by intense hydrothermal activity that wasresponsible for low temperature (≤ 300°C) F-metasomatism in the skarn, asevidenced by the presence of F-rich vesuvianite (CaS) and chondrodite (MgS), amongother minerals. Vesuvianite was formed from calc-silicate mineral assemblagesof the previous metamorphic event, whereas chondrodite was formed byreplacement of forsterite. The low temperature formation of these typicalhigh-grade minerals could be an evidence of mineral formation underdisequilibrium conditions favoured by the high reactivity of hydrothermalfluids. Neopalaeozoic basement mylonitization under greenschist faciesmetamorphism was accompanied by hydrothermal fluid circulation. This event promotedextreme mobility of chemical elements in the basement rocks and epidotization(CaS) and serpentinization (MgS) in the Loma Marcelo skarn. The elongated andboudinaged shape of the skarn bodies, parallel to the mylonitic foliation, is aconsequence of dextral shearing that affected the basement rocks. Additionally,almost pure grossular crystallized post-tectonically in the CaS. Carbonates ofthe Loma Marcelo skarn are depleted in 13C and 18O (δ13CV-PDB= -2.5/-10.1 parts per thousand; δ18OV-SMOW = +7.3/+14.0 parts per thousand) relative tocarbonate sedimentary rocks. The δ13C and δ18O variationscan be attributed to the interaction between large amounts of hydrothermalfluids (W/R = 30-50) and Neoproterozoic carbonate sedimentary rocks.