The timing of the Middle Triassic magmatism in the Dolomites area (Southern Alps, Italy): U-Pb geochronology of zircon and titanite hosted in plutonic rocks and phonolite dykes
Journal
LITHOS
ISSN
0024-4937
Date Issued
2025-12-04
Author(s)
Nardini, Nicolò
Department of Physics and Earth Sciences, University of Ferrara, Via Saragat 1, 44121 Ferrara, Italy
Department of Lithospheric Research, University of Vienna, Josef-Holaubek-Platz 2 (UZA II), 1090 Wien, Austria
Ickert, Ryan
Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, Indiana 47907, USA
Department of Earth and Planetary Sciences, ETH Zürich, Clausiusstrasse 25, 8092 Zurich, Switzerland
Okhai, Disha
Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, Indiana 47907, USA
Department of Lithospheric Research, University of Vienna, Josef-Holaubek-Platz 2 (UZA II), 1090 Wien, Austria
Dellantonio, Elio
Museo Geologico delle Dolomiti, Piazza SS. Filippo e Giacomo 1, 38037 Predazzo, Italy
Department of Lithospheric Research, University of Vienna, Josef-Holaubek-Platz 2 (UZA II), 1090 Wien, Austria
DOI
10.1016/j.lithos.2024.107894
Abstract
The Middle Triassic magmatic event in the Southern Alps (Italy) is characterised by the emplacement and
eruption of multiple magma batches with different geochemical affinities in a short time span. In this work, we
present the first geochemical and geochronological data on highly differentiated alkaline dykes (phonolites)
exposed in and near the Middle Triassic Predazzo magmatic complex (Southern Alps, Italy) and a first complete
geochronological survey of the Predazzo pluton. The phonolites, cutting the Middle Triassic volcanic sequences,
are sodalite- and aegirine-bearing and have titanite, apatite and magnetite as accessory phases. Their trace
element concentration reflects a geochemical signature similar to the orogenic-like signature typical of the
Middle Triassic magmatic rocks of the Predazzo complex. New in situ U–Pb dating on titanite from the phonolite
dykes and U–Pb dating on zircon separates from all the magmatic units of the Predazzo pluton show that the
phonolite dykes were emplaced between 233.8 ± 3.1 Ma and 238.1 ± 4.5 Ma, i.e., overlapping to and postdating
the emplacement and crystallisation of plutonic rocks, which yield ages ranging from 237.97 ± 0.15 Ma to
238.31 ± 0.13 Ma. The ages obtained for the diverse suites of the pluton are in good agreement with previous
dating and show that silica-saturated to -undersaturated rocks were placed within the same context in a short
time span. Thermometry and hygrometry on titanite, clinopyroxene and K-feldspar revealed that phonolites
represent volatile- and REE-rich (H2O = 8 ± 0.6 wt%) highly differentiated alkaline melts intruding at relatively
low T (752–868 ◦C), which easily caused the spatially related Cu–W mineralisation. Comparisons with subvolcanic
rocks exposed in the Central (Edolo) and Western Alps (Finero mafic complex) indicate that the phonolites
still preserve the orogenic signature of the Middle Triassic event in the Southalpine, that characterise all
magmas emplaced before the opening of the Alpine Tethys in the Late Triassic. These new geochronological
constraints on the Predazzo pluton and phonolite dykes enable us to outline the complete sequence of magmatic
events associated with the emplacement of the Predazzo volcano-plutonic complex during Mid-Triassic and to
speculate about the genesis of Cu–W mineralisation.
eruption of multiple magma batches with different geochemical affinities in a short time span. In this work, we
present the first geochemical and geochronological data on highly differentiated alkaline dykes (phonolites)
exposed in and near the Middle Triassic Predazzo magmatic complex (Southern Alps, Italy) and a first complete
geochronological survey of the Predazzo pluton. The phonolites, cutting the Middle Triassic volcanic sequences,
are sodalite- and aegirine-bearing and have titanite, apatite and magnetite as accessory phases. Their trace
element concentration reflects a geochemical signature similar to the orogenic-like signature typical of the
Middle Triassic magmatic rocks of the Predazzo complex. New in situ U–Pb dating on titanite from the phonolite
dykes and U–Pb dating on zircon separates from all the magmatic units of the Predazzo pluton show that the
phonolite dykes were emplaced between 233.8 ± 3.1 Ma and 238.1 ± 4.5 Ma, i.e., overlapping to and postdating
the emplacement and crystallisation of plutonic rocks, which yield ages ranging from 237.97 ± 0.15 Ma to
238.31 ± 0.13 Ma. The ages obtained for the diverse suites of the pluton are in good agreement with previous
dating and show that silica-saturated to -undersaturated rocks were placed within the same context in a short
time span. Thermometry and hygrometry on titanite, clinopyroxene and K-feldspar revealed that phonolites
represent volatile- and REE-rich (H2O = 8 ± 0.6 wt%) highly differentiated alkaline melts intruding at relatively
low T (752–868 ◦C), which easily caused the spatially related Cu–W mineralisation. Comparisons with subvolcanic
rocks exposed in the Central (Edolo) and Western Alps (Finero mafic complex) indicate that the phonolites
still preserve the orogenic signature of the Middle Triassic event in the Southalpine, that characterise all
magmas emplaced before the opening of the Alpine Tethys in the Late Triassic. These new geochronological
constraints on the Predazzo pluton and phonolite dykes enable us to outline the complete sequence of magmatic
events associated with the emplacement of the Predazzo volcano-plutonic complex during Mid-Triassic and to
speculate about the genesis of Cu–W mineralisation.