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| Titel |
Rejuvenation of deeper magma reservoirs during differentiation: an example recorded in a post-plutonic dyke swarm S-Adamello (N-Italy) |
| VerfasserIn |
N. Hurlimann, R. Nandedkar, O. Müntener, P. Ulmer |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250061372
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| Zusammenfassung |
| The Adamello Batholith (42-29 Ma) is located in the Northern Italian Alps. It intruded into
the Southern Alpine basement and its Permo-Triassic cover. In the Southern part a
series of late dykes intruded into gabbroic and mainly tonalitic plutonic bodies.
Models for volcanic eruptions of intermediate to silicic magmas are associated with
the intrusion of fresh mafic magma into a magma reservoir, triggering eruption
of the resident more evolved magma. Here we present an example of a series of
post-plutonic dykes ranging from picro-basalts to dacites that indicates rejuvenation
processes.
Distinct phases of magma intrusion are documented by compositionally distinct dykes.
Various degrees of magma mingling are widespread in this sequence. Primitive primary
magmas occur as picro-basaltic dykes that are consistent with mantle-separation conditions of
28 kbar and 1370Ë C constrained, by multiple saturation experiments (Ulmer 1988). More
evolved magmas in the same dyke swarm are homogeneous amphibole-plagioclase-phyric
basaltic-andesites to dacites, which are almost devoid of mingling textures. Textures and
mineral zoning patterns indicate a continuous differentiation process for these intermediate
magmas at mid-crustal depth. In contrary, the mingled magmas display amphibole textures
from pristine to complete pseudomorphs now present as clinopyroxene. Plagioclase and
amphibole show reverse zoning and the common occurrence of bands or cores with sieve
textures, indicating resorption processes of crystals related to the interaction with
high-temperature mafic magma.
Chilled margins and liquid apophyses of the mingled basalts and basaltic-andesites show
crystallization of microphenocrysts of olivine+clinopyroxene+plagioclase at the
emplacement level. In strongly mingled parts of the dykes resorbed amphibole is overgrown
by fresh amphibole corresponding in composition to the microphenocrysts. Trace-element
partition-coefficients between the cryptocrystalline matrix and these microphenocrysts in the
chilled margins of the same conduits were determined resulting in Kd’s that are
consistent with conditions close to the upper temperature stability of amphibole
as constrained by experimentally determined partition coefficients (~1020Ë C).
In contrast, amphibole-plagioclase thermometry of the quenched matrix of more
evolved basaltic-andesites/andesites reveals temperatures of 920±35Ë C. Textures and
mineral-chemistry of mingled magmas imply rejuvenation of an evolved magma reservoir by
interaction with a replenishing hotter mafic magma.
On a regional scale the occurrence of basaltic-andesites/andesites is confined to the NE
part of the entire dyke swarm within the S-Adamello and vanishes northwards. This pattern
appears to define a zone at mid-crustal depth where sustained fractionation and/or
assimilation is more widespread, leading to a more evolved composition of the
mingled products and a potential source for feeding the dyke swarm located in the
SW.
References:
Ulmer, P., High pressure phase equilibria of a calc-alkaline picro-basalt: Implications for
the genesis of calc-alkaline magmas. Carnegie Institution of Washington Yearbook,
Annual Report of the Director of the Geophysical Laboratory, 88, 28-35 (1988). |
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