The dikes of the Miocene La Peña complex, Mendoza, central Andes, Argentina: two alkaline magmatic series

ENRIQUEZ ELIEL; MARQUEZ-ZAVALÍA MARÍA FLORENCIA; IBAÑES OSCAR; PAGANO DIEGO; GALLISKI MIGUEL ANGEL; MARTINA FERNANDO; MOROSINI AUGUSTO; COLOMBO FERNANDO; D'ERAMO FERNANDO

Córdoba

Congreso; XIII CONGRESO DE MINERALOGÍA, PETROLOGÍA ÍGNEA Y METAMÓRFICA, Y METALOGÉNESIS; 2019

Asociación Mineralógica Argentina

The Miocene, (18.7 ± 0.5 Ma; Zappettini et al. 2005) La Peña Complex (LPC), is a silicaundersaturated alkaline potassic intrusive (Zappettini et al. 2013) recently linked to the Early Miocene arc magmatism of the Central Andes (Pagano et al. 2016). The LPC consists of a plutonic massif (clinopyroxenite and syenites) intruded by a voluminous radial and annular dike swarm of variable compositions. The dikes (0.20-10 m thick) radiate from trachytic bodies located in the central part of the LPC following NW-SE, E-W, NE-SO and N-S trends.They have porphyritic to aphyric textures and a fluidal structure. In the TAS classification (Le Bas et al. 1986) these rocks plot in the alkaline field following two distinctive trends: 1) an alkaline (potassic) silica-undersaturated series, from tephrite to tephriphonolite, and 2) a mid-alkaline, more silica-saturated series, from trachybasalt to trachyte. Dikes from thealkaline series show higher K2O/Na2O ratios and Sr, La, Ce, U, Th and Nb 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 suggests that both trends represent separated magmatic series that evolved from two different parental magmas. Fractional crystallization modeling performed on the LPC dikes using MELTS software and mass balance calculations suggest that only the alkaline suite is related to the basanite tephrite melt. The best fit is obtained assuming an H2O content of ≈2 wt.%. Variations within each series possibly respond to assimilation and fractional crystallization, and local mixing processes.In other world occurrences, closely associated rocks belonging to two different silicaundersaturated lineages (basanite to phonolite and alkali basalt to trachyte), similar to those cropping out at La Peña, have been interpreted as the result of evolution from two distinct parental magmas of basanitic and alkali basaltic compositions, respectively (e.g., Panter et al. 1997).At the LPC the origin of both parental melts could be explained by different degrees of partial melting of a depleted mantle source previously metasomatized by a slab-derived fluid.