Abstract

GARGIULO, M. Florencia; BJERG, Ernesto A.  and  MOGESSIE, Aberra. Characteristics and metamorphic evolution of the ultramafic rocks from Río de Las Tunas Belt, Frontal Cordillera of Mendoza. Rev. Asoc. Geol. Argent. [online]. 2011, vol.68, n.4, pp.571-593. ISSN 0004-4822.

The goal of this contribution is to present the petro-mineralogical and chemical study of metaperidotites and serpentinites from the río de Las Tunas belt in Frontal Cordillera, Mendoza province. The metaperidotites preserve relictic magmatic minerals Ol, Cpx, (brownish-red)Spls.l., scarce Opx with different modal proportions allowing to define the ultramafic protoliths: dunites, spinel-dunites, clinopyroxenitic-dunites, wehrlites, olivine-clinopyroxenites and clinopyroxenites. The crystallization sequence Srp+Mgt(±Brc±Chl)→Tr±Tlc→}Athà±(green)Spls.s./Ple±Opx evidence the metamorphic evolution of the ultramafic rocks. The initial serpentinization with pseudomorphic textures is followed by prograde metamorphism reaching the transition between the upper amphibolite facies and the granulite facies conditions. A second serpentinization process is evidenced by pseudomorphic textures after Amp and Chl and non-pseudomorphic and recrystallization textures, in agreement with the sequence a) FeChrà→CrMgt→àMgt and b) Spls.s./Ple(green)àMgt, which altogether indicate the low grade conditions during the retrogressive stage. During this stage reaction zones at metaperidotite-schist, -gneiss or -orthoamphibolite interfaces also were developed due to metasomatism between these lithologies. At the same time, local but pervasive carbonatization of the ultramafics near carbonatic levels of the Guarguaráz Complex took place. The emplacement of gondwanic igneous bodies in the study area occurred during the final stage of the metamorphic cycle. This magmatic event provides the physic-chemical conditions to allow elements mobilization, minerals concentration but also improves the development of the reaction zones. A new serpentinization event is restricted to shear zones where it concentrates the most extreme and pervasive hydrothermal alteration of the ultramafics with a later generation of talc crystallized as a consequence of the increase in the CO2 proportion in the fluid phase. This affected the stability of the serpentine minerals giving rise to its breakdown with formation of talc and carbonates.

Keywords : Metaperidotite; Serpentinite; Talc; Río de Las Tunas belt; Frontal Cordillera.

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