A GEOLOGICAL PERSPECTIVE TO SOME BASALTIC AND SILICIC MAGMA INTERACTIONS IN ARC RELATED ASSOCIATIONS

Common features observed in a number of Lower Paleozoic arc related magmatic rocks help to interpret the environment and emplacement mechanism of their parental magmas. The analyzed rocks are part of the Western Puna Eruptive Belt and Western Pampean Ranges —Sierra de Los Llanos, Igneous Chepes Complex in Central and Northwestern Argentine. Mineralogy, textures, and space relationships between granites and diorites/gabbros, either as marginal phases or enclaves of the latter in the more sialic components show the simultaneous presence of two melts: a mafic one and above it a granitic one. As the melting of mantle rocks in the arc wedge progress, small volumes of basaltic magma lodge in temporary magmatic chambers at the base of the crust. Heating of the overlying crust also produce melting: at deep crustal levels limited interaction between these two kinds of magmas are produced. Blending between both melts can generate homogeneous and hybrid magmas. Moreover, as seen in related field relationships big volumes of hybridization are not expected. They can only grow small bodies.

In the same way, limited geochemical and isotopic exchange between melts is expected. In crustal settings the scenery is a little different. It is improbable that the basaltic magma intrude into a silicic one because both melts have different densities and rheologies. But the heat transfer is the essential energy to keep the silicic magmas melted and potentially to move upwards. The basaltic magma is in a dynamic differentiation process; and continuously receives new batches of melt. Colder, highly viscous and less dense than basaltic ones, silicic magmas pond over the mafic ones. Field relations strongly suggest that mafic magma could only move upwards at the border of silicic magmatic chambers. There is no easy way for the mafic magma to intrude a less dense and highly viscous melt, except in peripherical places. Petrographical evidence suggest that basic magma was chilled in contact with colder melt and built irregularly shaped or pillowy structured of basic magma in hybridized melt or granitoid compositions. The principal process is the mechanical mixing of both melts.