International Geological Journal - Official Journal of the Carpathian-Balkan Geological Association

Bulk-rock and mineral-scale geochemical evidence of open-system magmatic processes in the felsic rocks of the northern part of the Ditrău Alkaline Massif (Eastern Carpathians, Romania)

Published: Aug 2023

Pages: 297 - 323



Abstract: The felsic suite in the northern part of the Ditrău Alkaline Massif has previously been considered as a homogeneous, uniform unit of the igneous complex. However, these felsic rocks exhibit remarkable diversity on the micro-­scale. Petrographic observations were complemented by bulk-rock and mineral-scale geochemical data to further support this hypothesis. Whole-rock composition of the studied rocks validates the presence of two, compositionally-different felsic cumulate piles in the massif (characterised by Eu/Eu* = 2.5–6.7, ΣREE = 125–206 ppm and Eu/Eu* = 1.0–1.1, ΣREE = 358–688 ppm, respectively). Mineral chemical data (amphibole, clinopyroxene, and plagioclase) were acquired from crystals of different textural positions (clot-forming and isolated) to understand which of the ferromagnesian minerals and plagioclase have the highest potential for recognising distinct magmatic environments and open-system magmatic processes. The composition of clinopyroxenes (diopside with mg# = 0.77 and 0.85, respectively) overlaps with the formerly published data. Moreover, a new, cumulus-phase occurrence of the clinopyroxenes with a convex REE pattern has been identified. Abrupt changes of Ba, Sr, TiO2 and ΣREE concentrations in different domains of the zoned amphibole crystals and variations in the MgO, FeO, TiO2, Sr, Ba, La, Ce, and anorthite content in plagioclase crystals, together with the bulk-rock data point to the hybrid nature of the majority of the investigated rocks. Our recent geochemical data support the previous, predominantly petrography-based assumptions that the felsic suite in the northern area of the massif was formed in a dynamic, open-system magmatic environment, wherein crystal settling, magma mixing, crystal/mush transfer, and recycling as well as country rock assimilation, prevailed.

Keywords: Eastern Carpathians, Ditrău Alkaline Massif, felsic cumulate, hybridisation, magma mixing, mineral chemistry

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