Unraveling the petrogenesis of the Miocene La Peña alkaline intrusive complex, Mendoza, Argentina: Insights from the study of the disregarded late dykes

The La Peña Complex (LPC) is a silica-undersaturated alkaline potassic intrusive system, with a subduction-related signature, linked to the early Miocene retroarc magmatism of the Southern Central Andes, in the flat slab segment. The LPC is composed of several intrusions, predominantly plutonic (clinopyroxenite, malignite and syenite), cross cut by a voluminous swarm of radial and annular dikes with mostly volcanic-subvolcanic textures and variable compositions (foid-bearing alkali feldspar trachyte, trachyte, benmoreite, ledmorite, syenite, tephrite, tephriphonolite and alkaline lamprophyre). In the TAS classification these rocks plot in the alkaline series covering a wide spectrum of compositions following two different trends: 1) alkaline (potassic) strongly silica-undersaturated series, from tephrite, phonotephrite to tephra-phonolite, and 2) mid-alkaline, less silica-undersaturated series, ranging from basaltic trachyandesite to trachyandesite (benmoreite), and trachyte. Dikes from the alkaline series show higher K2O/Na2O ratios and Sr, La, Ce, contents compared to those from the mid-alkaline series. Rocks of the alkaline series are richer in K-feldspar, sodalite, leucite (pseudoleucite), biotite, potassic-ferro-pargasite and garnet than the less silica-undersaturated (trachytic) rocks, reflecting a stronger alkaline potassic affinity. A review of geochemical, isotopic and mineralogical data, and a new geochemical modeling performed on the LPC dikes, suggests that both trends represent separated magmatic series that evolved from two different parental magmas lodged ∼30 km deep in the crust. Our results suggest that the compositional variations observed in LPC dikes, cannot be explained by a simple magmatic evolution via fractional crystallization from a unique parental magma, and that an assimilation and fractional crystallization (AFC) process is required to explain some compositional differences. Our results suggest an upper crustal contaminant (evolved rocks) with a Grenvillian isotope signature. On the other hand, analyses of feldspar crystals from the tephriphonolitic dikes indicate local mixing effects, between an evolved tephriphonolitic melt and a less evolved and hotter mafic magma.