Reactive interaction between migmatite-related melt and mafic rocks: clues from the Variscan lower crust of Palmi (southwestern Calabria, Italy)
Authors: Renna Maria Rosaria 
Published: January 2023

Abstract. In the Variscan lower–intermediate crust exposed in the Palmi area (southwestern
Calabria, Italy), amphibolites occur as foliated, decimeter-thick layers
within migmatitic paragneiss and as a decametric main body adjacent to the
migmatites. The main body is mostly fine-grained and weakly to moderately
foliated; unfoliated medium-grained portions rarely occur. Amphibolites are
mainly composed of plagioclase (An80−91) frequently developing triple
junctions, amphibole consisting of cummingtonite rimmed by hornblende and
variable amounts of biotite. Minor quartz is present in amphibolite layers
within paragneiss. Accessory allanite occurs in amphibolite layers within
migmatites and in foliated, fine-grained portions from the main body. This
study mainly aims to achieve information about the effects triggered by the
migration of migmatite-related melts into associated mafic rocks and its
role in the re-distribution of major and trace elements out of the anatectic
source. On the basis of whole-rock major- and trace-element compositions, the
protolith of amphibolite is recognized as of cumulus origin, likely derived
from basic melt emplaced in the framework of the late-Variscan lithospheric
extension. The rocks experienced high-temperature subsolidus
re-equilibration (∼800 ∘C) in conjunction with the
development of amphibole. The origin of amphibole is attributed to a coupled
dissolution–precipitation process related to the reaction between migrating
SiO2-rich hydrous melt and precursor orthopyroxene (± plagioclase). Reactive melt migration also caused the crystallization of
biotite ± allanite ± quartz ± plagioclase. SiO2-rich
hydrous melt had REE (rare earth element) compositions similar to late-Variscan peraluminous
granites and could have been derived by partial melting of metasediments
akin to neighboring migmatitic paragneisses. Both whole-rock and amphibole analyses reveal a decrease in Mg# (Mg/(Mg+Fe2+)) from
amphibolite layers within paragneiss to fine- and medium-grained rocks of
the main body. Hornblende shows an increase in SiO2 and a decrease in
Al2O3 and K2O with increasing Mg#. Amphibolites
interlayered with paragneiss have higher K2O, Rb, Ba, Th, U and Zr
relative to those from the main body. Furthermore, amphibole from
amphibolites interlayered with paragneiss is distinct for relatively high
Rb, Ba, MREE (middle rare earth element) and HREE (heavy rare earth element) concentrations. Within the main body, foliated,
fine-grained rock has both the whole rock and amphibole enriched in Rb and Ba
and high Zr bulk-rock contents. Whole-rock and mineral chemistry
heterogeneity most likely reflects (i) variation of the composition of the
melt during its reactive migration, in response to dissolution of
pre-existing minerals and crystallization of new phases, and (ii) variable
modification of the original compositions. Foliated and fine-grained
amphibolites record the strongest modification, thereby suggesting that they
represent permeable pathway enabling effective interaction of the reacting
melt with precursor minerals and nucleation of new mineral phases.