Timing of magmatism of the Ditrău Alkaline Massif, Romania – A review based on new U–Pb and K/Ar data

Abstract

The timing of Triassic magmatism of the Ditrău Alkaline Massif (Eastern Carpathians, Romania) is important for constraining the tectonic framework and emplacement context of this igneous suite during the closure of Paleotethys and coeval continental rifting, as well as formation of back-arc basins.Our latest geochronological data refine the previously reported ages ranging between 237.4 ± 9.1 and 81.3 ± 3.1 Ma. New K/Ar and U–Pb age data combined with all recently (post-1990) published ages indicate a relatively short magmatic span (between 238.6 ± 8.9 Ma and 225.3 ± 2.7 Ma; adding that the most relevant U–Pb ages scatter around ∼230 Ma) of the Ditrău Alkaline Massif. The age data complemented by corresponding palinspastic reconstructions shed light on the paleogeographic environment wherein the investigated igneous suite was formed.The magmatism of the Ditrău Alkaline Massif could be associated with an intra-plate, rift-related extensional tectonic setting at the southwestern margin of the East European Craton during the Middle–Late Triassic (Ladinian–Norian) period.

Petrochronological Evidence for a Three-Stage Magmatic Evolution of the Youngest Nepheline Syenites from the Ditrău Alkaline Massif, Romania

“…In the following, a short delineation of the published absolute age data on DAM rocks is given. An extensive list of published radiometric age data can be found in [6]. The first published radiometric age data from DAM rocks are from [37].…”

“…230 Ma were presented by [27]. From the northern part of the complex near Jolotca, [6] reported amphibole and biotite K-Ar ages from a cumulitic nepheline syenite and three granite samples, respectively. The reported ages range from 238.6 ± 8.9 Ma to 196.3 ± 7.4 Ma.…”

“…Around 20% of the crystals are dominated by a crystallographically oriented, CL-bright to relatively CL-dark, fine-to coarse-scaled oscillatory zonation, typical for crystal growth from a melt (Figure 5; e.g., crystals 6, 7, 12a, 25b). Some crystals show CL-bright, featureless small recrystallization internal domains and/or rims (Figure 5; e.g., crystals 6,11,19). All these zircons are xenomorphic to hypidiomorphic and do not show any significant inclusions.…”

“…Our new age data and recent published zircon and titanite U-Pb age data strongly contradict these findings. The authors of [6] [31,43]).…”

“…However, the dynamics behind intrusive alkaline complexes, especially their chronological evolution (i.e., the longevity of heating sources, plumbing and storage systems, duration of igneous activity from first melt generation to late-and postmagmatic hydrothermal-metasomatic overprinting, temporal aspects of the geochemical and isotope geochemical evolution) remain largely enigmatic, especially compared to the huge data set and knowledge base on granitic and basaltic igneous systems. Although there exists a number of well-studied alkaline complexes with an extensive data set of absolute age data (e.g., [1][2][3][4][5][6]), none of them has revealed its temporal evolution in such detail as has been found for more silicic complexes (e.g., [7][8][9]). From a geochronological point of view, the main problem arises during the integration of accurate and precise absolute age data for specific geological events and processes with the alkali complex's evolution for the following reasons: (a) Highly fractionated alkaline and ultra-alkaline igneous rocks, especially of silica-undersaturated and/or ultrabasic composition, often do not contain significant amounts of minerals (e.g., zircon, phosphate minerals, titanite), which potentially can provide accurate and precise growth ages.…”