Amphibole-plagioclase fractional crystallisation and magma mixing as majordifferentiation processes in the Akrotiri Volcanic Complex, Santorini, Greece
E. Gartzos et al., Amphibole-plagioclase fractional crystallisation and magma mixing as majordifferentiation processes in the Akrotiri Volcanic Complex, Santorini, Greece, SCHWEIZ MIN, 79(2), 1999, pp. 231-262
Citations number
68
Categorie Soggetti
Earth Sciences
Journal title
SCHWEIZERISCHE MINERALOGISCHE UND PETROGRAPHISCHE MITTEILUNGEN
The Pleistocene Akrotiri Volcanic Complex (AVC) builds up the southern part
of the island of Thera and comprises the oldest volcanic rocks of the Sant
orini volcanic islands with ages probably ranging from 1.6 to 0.5 Ma. The l
ower volcanic sequence starts mainly with rhyodacitic magmas, continuing wi
th the production of heterogeneous dacites and terminating this first cycle
with andesites.
The rhyodacitic magmas are results of fractional crystallisation of amphibo
le-and plagioclase-rich cumulates from a water-bearing basaltic melt with i
sland are characteristics. The andesitic to dacitic magmas, in contrast, ar
e mainly products of mechanical mixing (mingling) between rhyodacitic and b
asaltic to andesitic melts. These differentiation processes can be inferred
from phenocryst and xenocryst chemistry as well as from basic to intermedi
ate inclusions, which occur in various proportions in the dacitic and rhyod
acitic lavas. The inclusions are interpreted as undercooled and quenched ("
pillowed") products, which were formed by injection of vapor-rich hybrid-me
lts of basaltic to andesitic composition into cooler dacitic to rhyodacitic
magmas. Such injections are seen as trigger mechanism, which started the a
scent of silicic magmas from deep-seated crustal magma reservoirs.
The upper and younger volcanic sequence of the AVC is composed of high-alum
ina basalts and basaltic andesites and erupted after a period of magmatic q
uiescence. Complete magma mixing is present in the late Pleistocene Cape Ma
vrorachidi eruptive centre, producing cinder, scoriae and lavas of basaltic
andesite composition. Textural patterns and mineral chemistry of the large
r phenocrysts (xenocrysts) exhibit significant evidence of magma mixing bet
ween an evolved andesitic melt and a basaltic melt.