Abstract

TIBALDI, AlinaM. et al. Physical conditions for the formation ofgabbros and migmatites derived from mafic rocks in the center of Sierra deValle Fértil, San Juan. Rev. Asoc. Geol. Argent. [online]. 2009, vol.65, n.3, pp.487-503. ISSN 0004-4822.

A sequence of plutonicmafic rocks inter-stratified with both mafic- and metasedimentary-derivedmigmatites is found along the San Juan valley in the center of the Sierra deValle Fértil. This natural example shows the transition from igneous tometamorphic petrologic processes which occurred during the crystallization ofmafic magmas and the subsequent partial melting of crystallized gabbroic rocks.This work studies the mineralogical changes associate to this petrologictransition. Thermobarometric estimates based on amphibole-plagioclase indicatethat the mafic magmas crystallized at around 1100ºC and 5 ± 0.5 kbar. Theconditions under which gabbroic rocks were partially melted are estimated usingtwo pyroxenes thermometry and amphibole-plagioclase thermobarometry. Similarphysical conditions in the range between 740 and 840ºC and 5 to 6.5 kbar arerecovery from mineral assemblages in the mesosomes and leucosomes of maficmigmatites. The main mineral compositional changes that accompanied the partialprocess of the gabbroic rocks are: 1) depletion of aluminium content and Mg#ratio in pyroxenes; 2) depletion of anorthite mole fraction of theplagioclases; and 3) depletion of the Mg# ratio in amphiboles. These mineralcompositional variations are consistent with those found by experimentallymelting mafic protoliths. Experimental results showed that the temperature forpromoting amphibole-dehydration melting in mafic protoliths is of around 850ºC,whereas the addition of an aqueous fluid would lower the melting temperature toaround 800ºC. Combining field relationships, textural features, mineralcompositional variations and thermo-barometric estimates with experimentalresults, we demonstrated that the transition from igneous mafic rocks to maficmigmatites was due to isobaric cooling. Moreover, repetitive intrusion of maficmagmas sustained the sequence at granulite-facies conditions, so avoiding rapidcooling. The natural case from the centre of the Sierra de Valle Fértil isinterpreted to expose an example of the plutonic-metamorphic core of theFamatinian magmatic arc, where the abnormally high metamorphic gradient provideevidence that large volume of mafic magmas reached and emplaced at paleodepthsof between 16 and 20 kilometres.

Keywords : Famatinian arc; Thermobarometry; Mafic rocks; Maficmigmatites; Magmatic metamorphism.

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