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 from intra-granitic pegmatites have been analyzed to constrain the last stages of magmatic differentiation in the Devonian Las Chacras-Potrerillos batholith, central Argentina. Based on accessory phase assemblages and mineral chemistry, the pegmatites are assigned to the NYF (Nb – Y – F) family. They were emplaced in all six units of the batholith but there is an increase in pegmatite abundance, size and complexity from north to south in the batholith. A similar N–S trend in pegmatite melt evolution is defined by fractionation indicators in pegmatitic K-feldspar (e.g., K/Rb: 68–301, Rb/Sr: 1–47, Cs: 8–71 ppm, Ba: 66–800 ppm), in white mica (e.g., K/Rb: 36–99, Rb/Sr: 40–826, Cs: 30–206 ppm, Ba 18–1889 ppm) and in tourmaline (Fetot + Mgtot: 2.6–4.0 apfu, Altot: 5.0–6.4 apfu). The regional fractionation patterns of pegmatite mineral chemistry are consistent with a general southward increase in the fractionation degree of the host granite units, confirming that compositional characteristics of the parental magmas are passed on to the pegmatitic phases. A new zircon U–Pb age of 388.3 ± 1.9 Ma from the southern granites constitutes the oldest crystallization age of the batholith. This resolves the previously-debated emplacement sequence and has important implications for the magmatic evolution of the largest and most evolved pegmatites, which cluster around the contact zone between the southern granites and the younger, central unit (biotite porphyritic granite). The age constraints combined with chemical data from the pegmatitic minerals show that pegmatites in the southern region of the batholith share a common magma source, which we suggest is the central unit of biotite porphyritic granite. The increased degree of fractionation in the southern pegmatites is attributed to elevated concentrations of volatiles in the parental magmas, which derive from dehydration partial melting and assimilation of fluids from metaclastic rocks in the crust.