Metamorphic evolution and partial melting of metapelites in Eastern Cordillera, Central Andes, NW Argentina: new insights from equilibrium phase diagrams (pseudosections)

SOLA, ALFONSO M.; HASALOVÁ, PAVLINA; BECCHIO, RAÚL A.

Heidelberg

Simposio; 23rd Latin American Colloquium LAK 2014; 2014

Heidelberg University

We apply thermodynamic modeling on pelitic rock compositions to study in detail and quantify the metamorphic evolution and the partial melting process in the Sierra de Molinos (Salta Province) during Famatinian orogen. We interpret the metamorphic evolution of Sierra de Molinos with reference to pseudosections in NCKFMASH system that are based on an average subaluminous pelites considered to be representative of the presumed protolith (samples mo40 and mo10408; see Sola et al. 2013). Metamorphism in the Sierra de Molinos is the subject of ongoing research and some preliminary results were briefly summarized by (Sola et al. 2013). The sedimentary protolith of the metamorphic rocks consists of a turbiditic sequence assigned to the Puncoviscana Formation (Neoproterozoic to Early Cambrian) composed by metapelites, metapsammites and minor calc-silicates. The metasediments show a continuous transition from low and medium metamorphic grade to anatectic migmatites (metatexites and diatexites). On the base of mineral assemblages recognized in metapelites the study area is divided into four metamorphic zones that according to the increase in the grade are: (1) chlorite-white mica (2) muscovite-biotite (3) sillimanite-K-feldspar and (4) cordierite-K-feldspar. The metamorphic grade increases from W to E, from sub-greenschist facies in the west to upper amphibolite facies migmatites on the east. The mineral assemblages appear to be the result of a progressive increase in temperature without a significant change in pressure (Sola and Becchio 2012; Sola et al. 2013). Due to the lack of assemblages suitable for conventional geobarometry, the integration of the results obtained in pseudosections, the Ti in biotite thermometry (Henry et al. 2005) along with the observation of mineral assemblages, provide key information to recreate the tectonothermal history of this region.-The high content of Mg in pelites stabilized cordierite with small increment of temperature.-The sillimanite becomes stable within the subsolidus field (3 to 4 kbar/500-650ºC). This reaction is observed in zone (2) where fibrolite aggregates replace cordierite porphyroblasts.-The predicted amount of melt produced by crossing the wet solidus is small (5 vol %) and the first major melting reaction encountered is related to the breakdown of muscovite which produce 15-16 vol % of melt over a small temperature interval, equilibrating melt, K-feldspar along with sillimanite in metatexite migmatites of zone (3). The highest temperatures obtained for this stability field (bt+melt+qtz+pl+kfs+sill) yielded 650-660ºC.-The microstructures and equilibrium modeling reveal that cordierite replace biotite and sillimanite increasing temperature through reaction: bt + sil + pl + qtz = crd + kfs + melt.- At the peak conditions (700-710ºC) the sillimanite disappears and the mineral assemblage in migmatites is dominated by peritectic cordierite and K-feldspar (bt+melt+qtz+crd+pl+kfs). The stability field for this assemblage is limited to pressures below 4 - 4.5 kbar. There is no record in migmatites of Sierra de Molinos of assemblages involving garnet and cordierite which marks the transition to the granulite facies above 730-740ºC.- The abundance of late muscovite in some migmatites require the retrograde P-T path of the Sierra de Molinos to have passed at pressures above 3.5 kbar.