Andalusite and Na- and Li-rich cordierite in the La Costa pluton, Sierras Pampeanas, Argentina: textural and chemical evidence for a magmatic origin

摘要

The La Costa pluton in the Sierra de Velasco (NW Argentina) consists of S-type granitoids that can be grouped into three igneous fades: the alkali-rich Santa Cruz facies (SCF, SiO_2 ～67 wt%) distinguished by the presence of andalusite and Na- and Li-rich cordierite (Na_2O = 1.55-1.77 wt% and Li_2O = 0.14-0.66 wt%), the Anillaco facies (SiO_2 ～74 wt%) with a significant proportion of Mn-rich garnet, and the Anjullon facies (SiO_2 ～75 wt%) with abundant albitic plagioclase. The petrography, mineral chemistry and whole-rock geochemistry of the SCF are compatible with magmatic crystallization of Na- and Li-rich cordierite, andalusite and muscovite from the peraluminous magma under moderate P-T conditions (～1.9 kbar and ca. 735℃). The high Li content of cordierite in the SCF is unusual for granitic rocks of intermediate composition.