Intra-granitic pegmatites of the Las Chacras-Potrerillos batholith (Sierra de San Luis, Argentina): Geochemistry, dating, and a new model of magma evolution

Abstract

The chemical composition of primary minerals and fine-grained border zones fromintra-granitic pegmatites have been analyzed to constrain the last stages of magmaticdifferentiation in the Devonian Las Chacras-Potrerillos batholith, central Argentina.Based on accessory phase assemblages and mineral chemistry, the pegmatites areassigned to the NYF (Nb - Y - F) family. They are emplaced in all six units of thebatholith and show an increased abundance, size and complexity in the granitic unitsof the southern region. Fractionation indicators in pegmatitic K-feldspar (e.g., K/Rb: 68- 301, Rb/Sr: 1- 47, Cs: 8 - 71 ppm, Ba: 66 - 800 ppm), white mica (e.g., K/Rb: 36 - 99,Rb/Sr: 40 - 826, Cs: 30 - 206 ppm, Ba 18 - 1889 ppm) and tourmaline (Fetot +Mgtot:2.6 - 4.0 apfu, Altot: 5.0 - 6.4 apfu) define a consistent pattern with the most evolvedcompositions in the southern pegmatites. This regional pattern of pegmatite complexityand mineral chemistry reflects compositional variations in the different parentalmagmas as well as variations in magmatic evolution and suggests higherconcentrations of fluids and fluxes in the southern area of the batholith. Although thesouthern granites are the host for the most complex pegmatites in the batholith, thetiming of emplacement was not constrained. A new zircon U-Pb age of 388.3 ± 2 Mafrom the southern granites constitute the oldest crystallization age of the batholith andresolves the previously debated emplacement sequence, which has importantimplications for the pegmatite evolution. The initial ascent and emplacement of thelower crustal derived southern granites heated the middle to upper crust and releasedfluids into the basement. The volatile-rich basement and the subsequent intrusion ofthe central biotite porphyritic granite, caused high volumes of partial melting andenhanced crustal contamination. Increased assimilation of volatile-rich metaclasticcrustal material into this subsequent central magma enabled the formation of highlyevolved melts and more complex pegmatites. We suggest that the intrusion of thebiotite porphyritic granite represent the parental magma for the complex southernpegmatites and caused the formation of a contact zone in the previously emplacedsouthern granites.