Petrographic traces of open-system magmatic processes in the felsic suite of the Ditrău Alkaline Massif (Eastern Carpathians, Romania)

The Ditrău Alkaline Massif (DAM; Eastern Carpathians, Romania) is a unique pluton with a presently tilted vertical cross-section of a pre-existing alkaline magma storage system, which consists of various rock types from ultramafic cumulates to granitoid rocks, crosscut by lamprophyre, syenite, ijolite and tinguaite dykes. The igneous event took place in an intra-plate, rift-related tectonic environment during the Middle–Late-Triassic.

During the roughly 190 years of scientific exploration of the DAM, felsic rocks were considered as homogeneous, uniform units of the massif. However, recently identified structural, textural and geochemical features revealed open-system magmatic processes that operated during the crystallization of the DAM (e.g., Batki et al., 2018). The heterogeneity of the felsic rocks can only be revealed on micro-scale and is mostly determined by the presence of mafic mineral aggregates. However, felsic minerals are also characterised by disparate microtextural traits. The felsic rocks in the northern part of the DAM were classified into two groups based on their field occurrence and microtextural features: (1) felsic rocks (lacking or containing scant mafic phases) spatially related to mafic rocks and (2) felsic rocks (with mafic minerals and clusters) spatially unrelated to mafic rocks (Kiri et al., 2022).

Distinct attributes (e.g., idiomorphic–hypidiomorphic feldspars aligned parallel to their crystal faces, contact melting and embayment, feldspar aggregates) of Group 1 suggest the settling and accumulation of the rock-forming minerals. Such textural features can also be observed in Group 2. Isolated mafic phases are scant in rocks of the latter group; however, different variants of clusters containing identical or different mafic minerals are prevalent. There are other particular textural properties: feldspar megacrysts, adjacent feldspars with contrasting zoning sequences and biotite clusters in the metamorphic country rock and xenoliths.

The formation of mafic clusters could be associated with: (1) mineral accumulation, (2) magma mixing–mingling and (3) entrainment of exotic (crustal or restitic) materials. Petrologic and geochemical evidences of such processes have already been reported from the DAM (e.g., Batki et al., 2018). Some of the clots are polycrystalline pseudomorphs after antecrysts and/or xenocrysts (e.g., clinopyroxene, green amphibole, garnet) that were entrained by the interplay between different magma batches or by country rock contamination. All cluster varieties revealed a transition from fresh aggregated crystals through imperfect to complete replacement. Hence, disparate clots could imply different phases of hybridisation of the incorporated materials.

Micro-scale characteristics of felsic crystal accumulation, mafic clusters and flow fabrics as well as metamorphic wall rock xenoliths indicate that the studied rocks were formed under dynamic magmatic circumstances. Crystal settling, shear flow, convection, along with numerous open-system igneous processes (e.g., magma mixing and mingling, magma recharge, crystal/mush transfer and recycling, wall rock assimilation) played an important role in the petrogenesis of the felsic suite of the DAM.

 

References

Batki, A., Pál-Molnár, E., Jankovics, M.É., Kerr, A.C., Kiss, B., Markl, G., Heincz, A., Harangi, Sz. (2018). Lithos, 300–301, 51–71.

Kiri, L., Szemerédi, M., Pál-Molnár, E. (2022). Central European Geology, 65, 1, 49–76.