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Magma – Enclaves interactions in the East Carpathian subvolcanic zone, Romania: Petrogenetic implications

Domenii publicaţii > Ştiinţele pământului şi planetare + Tipuri publicaţii > Articol în revistã ştiinţificã

Autori: Eugenia Nitoi, Marian Munteanu, Stefan Marincea and Viorel Paraschivoiu

Editorial: Cas and Wildner - New Zealand, Journal of Volcanology and Geothermal Research, in press, 2002.

Rezumat:

Zona subvulcanica a Carpatilor estici separa doua sectoare extrusive ale arcului vulcanic est carpatic si consta in mici intruziuni amplasate, fie in fundamentul metamorfic precambrian fie in depozitele sedimentare Paleogene in timpul Panonian-Pontian.Toate intruziunile contin enclave magmatice,metamorfice sau sedimentare.Aceasta lucrare bazata pe o multitudine de date asupra enclavelor si rocilor gazda aduce argumente asupra existentei proceselor petrogenetice care au determinat diferentierea magmei .Rocile magmatice din zona subvulcanica pot fi impartite in trei grupe:grupul I constituit numai din enclave mafice „cognate” cu trasaturi mineralogice si petrografice de cumulate;grupul II de roci (andezite bazaltice,andezite, microdiorite si dacite) sunt cele mai raspindite intilnite fiind ca enclave dar si roci gazda.Analizele de elemente minore si majore executate pe enclavele ‘cognate” dar si pe rocile lor gazda pot fi utilizate ca indicatori in evolutia magmei din zona subvulcanica a Carpatilor estici.Investigatiile microscopice ale relatiilor dintre enclave si rocile gazda pot fi utilizate in descifrarea proceselor care au loc in timpul diferentierii magmei ,cum ar fi asimilarea si amestec de magme.Trasaturile diferentierii si compozitia mineralogica a enclavelor „cognate” si a rocilor gazda indica fractionarea clinopiroxenului si a amfibolului dintr-o topitura cu compozitie bazalt-andezitica .Topitura bazalt-andezitica rezulta probabil din cristalizarea fractionata a olivinei si piroxenului ,indicata prin petrografia enclavelor „cognate”Procesele de asimilare au avut loc mai ales intre componentii lichid-lichid si interdifuzii solid-lichid.Procesele de mixing sunt evidente in dacite prin existenta zonelor reverse si in cuart-andezite unde coexista doua populatii difeite de plagioclaz. The East Carpathian Subvolcanic Zone separates the two extrusive sectors of the East Carpathian volcanic arc and consists of shallow intrusions 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. This work tried to put together data about the intrusions and their enclaves in order to gain evidence for the processes that determined magma differentiation. The magmatic rocks in the East Carpathian subvolcanic zone have been divided in 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 and rhyodacitic host rocks.
Major and trace element analyses have been done on both cognate enclaves and their host rocks, and their compositions have been used to indicate the evolution of the magmas in the East Carpathian subvolcanic zone. Microscopic investigations of the relations between the enclaves and their host rocks have been used to decipher the processes that took place during magma differentiation, like assimilation and mixing.
Differentation trends and mineralogical composition of cognate enclaves and host rocks indicate that clinopyroxene and amphibole fractionation from a melt with basaltic andesite composition represented the most important evolutive process. The basaltic andesite melt presumably resulted from olivine and pyroxene fractional crystallization from a basic magma as locally indicated by the petrography of cognate enclaves. Assimilation processes took place mostly by liquid-liquid and solid-liquid interdiffusion and locally may be quantitatively important. Mixing processes have been evidenced in dacites by plagioclase reverse zonation and in quartz-bearing andesites due to the coexistence of two different plagioclase populations.

Cuvinte cheie: magme calco-alcaline,enclave,asimilare,cristalizare fractionata, granati magmatici,Carpatii estici // calc-alkaline magmas, enclaves, assimilation, fractional crystallization, magmatic garnet, Eastern Carpathians

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