The widespread Triassic volcanic rocks of Greece, dismembered during
the Hellenide orogeny,
are used to interpret the nature of Triassic rifting. Four assemblages
of
volcanic rocks are distinguished on
geochemical criteria: (1) a predominant subalkaline basalt–andesite–dacite
series with a high proportion of
pyroclastic rocks; (2) minor shoshonites; (3) alkali basalt and (4) MORB.
The stratigraphic and palaeogeographic
distribution of these rock types is synthesized. New Pb and Nd isotopic
data are used to discriminate
between hypotheses suggesting that either subduction or extension was
responsible for the Triassic volcanism.
In the subalkaline basalt assemblage, εNd is negative
with depleted mantle model ages >1.5 Ga. Pb isotopic
compositions are mostly close to the very distinctive compositional field
of Cenozoic extensional
rocks of the Aegean area, with very high 207Pb/204Pb
for relatively low 206Pb/204Pb ratios. These
isotopic data
confirm interpretations based on trace elements that subalkaline basalts
were predominantly derived from
melt-depleted peridotite in the sub-continental lithospheric mantle as
a
result of extension. Small areas of
enriched hydrous mantle partially melted to yield shoshonitic magmas.
Nd and Pb isotopic compositions of the
alkali basalts are quite different from those in other rock types and
suggest a HIMU mantle source component
derived from a small plume, which also influenced MORB compositions.
Distribution of these various
rock types is used to constrain palaeogeographic reconstruction of
Triassic micro-continental blocks.