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Autori: Damian Floarea, Damian Gheorghe & Constantina Ciprian
Editorial: Carpathian Journal of Earth and Environmental Sciences, Vol. 4, No. 2, , p.101-122, 2009.
The intrusive rock from Nistru zone represents the subvolcanic suite of the igneous activity from the southern part of the Gutâi Mountains. The subvolcanic magmatic rocks from Nistru zone follow, in succession, the phase of Sarmatian pyroxene andesites. They are spatially associated to the volcanic rocks, being distinguished by morphological and structural aspects, as well as by petrographic types. The relations between the main types of magmatic rocks emphasize their sequential emplacement: the first sequences form stocks of small sizes with irregular forms represented by the porphyry facies of the quartz-microdiorites with lateral transitions to andesitic facies and by porphyry quartz-micromonzodiorites. These stocks are accompanied by apophyses from which the ones of the quartz-micromonzodioritic outcrop on Nistru Valley; the second sequence of magmatic subvolcanic rocks is represented by dykes, apophyses and sills of a few meters size that pierce the stocks. These are represented by equigranular quartz-microdiorites under the form of a dyke that pierces the porphyry quartz-microdiorite stock, and porphyry microdiorites under the form of intrusive apophyses included in the porphyry quartz-microdioritic stock. The porphyry microgabbro forms a small sill occurring within the quartz-micromonzodioritic stock. The magma of the subvolcanic rocks suite from Nistru zone has an andesitic, calc-alkaline and metaluminous with low tendencies towards peraluminous character. The primary magma of the subvolcanic magmatic rocks suite was generated from the mixing of a mantle melt with the crustal material. Significant for crustal contamination, in its ascendant movement, is the high value of the Rb/Sr = 0.3 ratio. Under reducing environment various rock types were emplaced after the differentiation processes in the shallow chamber. The large ion lithofile elements (LILE): Rb, Cs, Ba, K, Sr/light rare earth elements (LREE):La, Ce, Nd, Pr, Ne, Sm and LILE/HFSE (high-field strengh elements: Ti, Y, Sc, Hf, Zr, U, Th, Ta, Nb, La, Lu ratios are characteristic for the contribution of the different mantle sources in generating the parental magma. The Rare Earth Elements (REE) patterns distribution within a Nistru intrusive subvolcanic rocks suite, evaluated by chondrite and N-MORB normalizing are characteristic of generated magmas in subduction conditions. The concentration of the large ion lithophile elements (LILE, Rb, Ba, Cs) and enrichments in light rare earth elements (LREE, La, Ce, Nd, Pr, Ne, Sm) indicate as important source for magma partial melting of ascending mantle wedge and fluid addition from dehydration of the subducting oceanic crust and associated sediments. The low content in Sr is related to the implication of the oceanic crust in the melting of the subducted plate. The heavy rare earth elements (HREE), Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y show abundances similar to those for N-MORB. The enrichment in the complete line of the elements from Ba-Hf, the strong fractioning of LREE, the Nb and Eu negative anomaly supports the fractional crystallization and the assimilation as predominant petrogenetic processes within the magma evolution and at the same time with the accumulation in upper magmatic chambers. Magma with subduction-related geochemical signatures can also be generated in syn-collisional tectonic setting.
Cuvinte cheie: Subvolcanic, magmatism, subduction, major elements, trace and rare earth elements, fractional crystallisation, assimilation.