Data di Pubblicazione:
2010
Citazione:
Fluid/mineral interaction in mantle wedge garnet peridotites / N. Malaspina, M. Scambelluri, J. Hermann. - In: GEOPHYSICAL RESEARCH ABSTRACTS. - ISSN 1607-7962. - 12:(2010), pp. 8082-8082. ((Intervento presentato al convegno EGU General Assembly 2010 tenutosi a Vienna nel 2010.
Abstract:
We present two case studies of metasomatised garnet peridotite from the Sulu (Zhimafang) and of garnet orthopyroxenite
from the Dabie Shan (Maowu) ultrahigh-pressure terranes (Eastern China). The mantle derived peridotite
from Zhimafang shows two ultrahigh-pressure (UHP) mineral assemblages. The older one is made of porphyroclastic
garnet rich in inclusions (Grt1), coarse exsolved clinopyroxene (Cpx1) and coarse phlogopite flakes (Phl1).
The younger paragenesis consists of fine-grained olivine + clinopyroxene (Cpx2) + orthopyroxene magnesite
Phl2 equilibrated with neoblastic garnet (Grt2). The inclusions inside porphyroclastic Grt1 are polyphase secondary
inclusions related to microfractures cutting the garnet core. They display irregular shapes and contain
microcrystals of calcic-amphibole, chlorite, phlogopite and rare talc, associated with pyrite and/or spinel. The low
Al2O3 content (<0.2 wt.%) in orthopyroxene coexisting with garnets and clinopyroxenes indicates equilibration
at P=4.0–6.0 GPa and T=700–1000 °C. The trace element composition of Cpx1 and Phl1 combined with previous
petrologic and isotopic data suggests that the Zhimafang garnet peridotite experienced metasomatism by a melt
with alkaline character at high-temperature conditions (T=1000 °C and P>5.0 GPa). The microtextural identification
of pseudosecondary inclusions in the porphyroclastic garnet core and their geochemical characterisation
indicate that an incompatible element- and silicate-rich fluid subsequently metasomatised the garnet peridotite and
equilibrated with the newly formed Cpx2 probably during Triassic UHP metamorphism.
Ultramafic metasomatic layers at Maowu Ultramafic Complex (Dabie Shan) consist of layered websterite and
orthopyroxenite which preserve an old olivine + orthopyroxene (Opx1) + garnet (Grt1) Ti-clinohumite paragenesis,
overgrown by poikilitic Opx2. Grt2 is associated with Opx2 + phlogopite along the foliation, together
with fine-grained idiomorphic clinopyroxene. Grt2 cores contain disseminated primary polyphase inclusions. The
textural and geochemical analyses of the primary polyphase inclusions indicate that they derive from a homogeneous
fluid characterised by high LILE concentrations with spikes in Cs, Ba, Pb and high U/Th. These inclusions
are interpreted as remnants of the LILE- and LREE-enriched residual fluid produced when a crust derived Si-rich
metasomatic agent reacted with a previous harzburgite to form garnet orthopyroxenite. The in situ trace element
analyses of the major phases garnet, clinopyroxene and phlogopite that formed at the same time as the polyphase
inclusions at Maowu, permit the determination of empirical mineral/fluid partitioning at pressures relevant for element
recycling in subduction zones. Our estimated D(Cpx/fluid) suggests that all LILE are highly incompatible,
Th and U are moderately incompatible, Pb is close to unity and Sr is moderately compatible. Phlogopite preferentially
incorporates Rb and K with respect to Ba and Cs, and Th with respect to U. The similarity between the
residual Maowu fluid with the secondary inclusions in the UHP wedge-type garnet peridotite from Sulu, indicates
that the fluids produced from reactions at the slab–mantle interface may be effective metasomatic agents in the
mantle wedge. Such reactions may produce phlogopite, which plays an important role in controlling the LILE
characteristics of the slab-derived fluid in subduction zones.
Tipologia IRIS:
01 - Articolo su periodico
Elenco autori:
N. Malaspina, M. Scambelluri, J. Hermann
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