Magma–enclave interactions in the East Carpathian Subvolcanic Zone, Romania: petrogenetic implications

Abstract

The East Carpathian volcanic arc is made up of two extrusive sectors separated by a subvolcanic zone. The East Carpathian Subvolcanic Zone (ECSZ) consists of calc–alkaline shallow intrusions (sills, laccoliths and dykes) emplaced in the Precambrian metamorphic basement or in its Paleogene cover during the Pannonian–Pontian time span. All intrusions contain enclaves of magmatic, metamorphic or sedimentary origin. The magmatic rocks in the ECSZ have been divided into three groups. Group I rocks are ubiquitous but occur only as mafic cognate enclaves showing cumulate features. Group II rocks (basaltic andesites, andesites, microdiorites and dacites) are the most widespread occurring both as enclaves and host rocks. Group III comprises only enclave-poor rhyolitic host rocks. Major and trace element analyses of both cognate enclaves and their host rocks, have been done in order to get information about the evolution of the magmas in the ECSZ. Microprobe analyses of the main mineralogical phases and microscopic investigation of the relationships between the enclaves and their host rocks have been used to decipher the processes that took place during magma differentiation. Differentiation trends and mineralogical composition of cognate enclaves and host rocks indicate that clinopyroxene and amphibole fractionation in mid-crustal magmatic chambers represented the most widespread evolutionary process in the study area. Xenolith assimilation, evidenced at microscopic scale, took place mostly by liquid–liquid and solid–liquid interdiffusion and locally may be quantitatively important. Mixing processes have been inferred for dacites and for garnet-bearing andesites.