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Multistage metasomatism in ultrahigh-pressure mafic rocks from the North Dabie Complex (China)

Malaspina, N; Hermann, Joerg; Scambelluri, Marco; Compagnoni, Roberto

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Release of metamorphic fluids within the slab and/or from the slab to the mantle wedge in subduction environments can produce important metasomatic effects. Ultrahigh-pressure (UHP) metasomatised rocks represent ideal materials to study the element exchange at pressures corresponding to sub-arc depths in subduction zones. We present a petrologic and geochemical study of eclogites (s.l.) from the Dabie Mountains (China). The investigated samples were collected in the North Dabie Complex, where...[Show more]

dc.contributor.authorMalaspina, N
dc.contributor.authorHermann, Joerg
dc.contributor.authorScambelluri, Marco
dc.contributor.authorCompagnoni, Roberto
dc.date.accessioned2015-12-07T22:27:46Z
dc.identifier.issn0024-4937
dc.identifier.urihttp://hdl.handle.net/1885/22047
dc.description.abstractRelease of metamorphic fluids within the slab and/or from the slab to the mantle wedge in subduction environments can produce important metasomatic effects. Ultrahigh-pressure (UHP) metasomatised rocks represent ideal materials to study the element exchange at pressures corresponding to sub-arc depths in subduction zones. We present a petrologic and geochemical study of eclogites (s.l.) from the Dabie Mountains (China). The investigated samples were collected in the North Dabie Complex, where eclogite-facies rocks are significantly overprinted by granulite-facies metamorphism and partial melting. The studied eclogites are included in meta-lherzolitic bodies, which are in turn hosted by leucocratic gneisses. The textural relations among the various rock-forming minerals enabled us to identify several re-crystallisation stages. The peak (UHP) paragenesis consists of garnet, clinopyroxene and rutile. UHP garnet and clinopyroxene display oriented inclusions of polycrystalline rods of rutile + ilmenite and of albite, K-Ba-feldspar and quartz, respectively. Garnet and clinopyroxene are both rimmed by an inclusion free zone that formed after the peak, still at high-pressure conditions. Such optical zoning does not correspond to a difference in major element concentrations between garnet core and rim. This observation provides evidence that the major element composition of garnet was reset during exhumation, thus preventing thermobarometric determination of peak metamorphic conditions. Further decompression is documented by the formation of limited ilmenite + amphibole and granulite-facies coronas consisting of clinopyroxene, orthopyroxene, plagioclase and amphibole around garnet. In order to investigate the stability of observed mineral parageneses, a series of reconnaissance piston cylinder synthesis experiments were carried out in an identical bulk composition. The experimental study indicates that the peak metamorphic paragenesis is stable at P∼3.5 GPa and T ≥ 750-800 °C. The petrological study, combined with bulk-rock and mineral trace element analyses, provides evidence of intense metasomatism affecting these eclogites. The bulk-rock major and trace element compositions indicate that the eclogites derive from basaltic protoliths with MORB and E-MORB affinity. Compared with such basalts, the studied rocks show strong depletion in SiO2 and alkalis and enrichment in MgO and FeO. These features likely derive from element exchange with ultramafic rocks prior to subduction, possibly related with the influx of Si-depleted and Mg-enriched fluids produced during the serpentinisation of the associated lherzolitic rocks. On the other hand, the trace element bulk-rock compositions show strong enrichment in Cs, Ba and Pb. The same characteristic enrichment and fractionation is recorded by peak metamorphic clinopyroxene but not in retrograde amphibole. Therefore, influx of crustal fluids transporting LILE and light elements must have occurred during subduction at UHP conditions. This stage likely records the tectonic coupling of the mafic-ultramafic rocks with the associated crustal rock units and provides evidence of LILE mobility between different slab components.
dc.publisherElsevier
dc.sourceLithos
dc.subjectKeywords: eclogite; geochemistry; metasomatism; trace element; ultrahigh temperature metamorphism; ultramafic rock; Asia; China; Dabie Mountains; Eurasia; Far East China; Mafic and ultramafic rocks; Metasomatism; North Dabie Complex; Subduction; Trace element mobility; Ultrahigh-pressure fluids
dc.titleMultistage metasomatism in ultrahigh-pressure mafic rocks from the North Dabie Complex (China)
dc.typeJournal article
local.description.notesImported from ARIES
local.identifier.citationvolume90
dc.date.issued2006
local.identifier.absfor040202 - Inorganic Geochemistry
local.identifier.absfor040304 - Igneous and Metamorphic Petrology
local.identifier.ariespublicationu9503261xPUB19
local.type.statusPublished Version
local.contributor.affiliationMalaspina, N, Universita di Genova
local.contributor.affiliationHermann, Joerg, College of Physical and Mathematical Sciences, ANU
local.contributor.affiliationScambelluri, Marco, Universita di Genova
local.contributor.affiliationCompagnoni, Roberto, University di Torino
local.description.embargo2037-12-31
local.bibliographicCitation.startpage19
local.bibliographicCitation.lastpage42
local.identifier.doi10.1016/j.lithos.2006.01.002
dc.date.updated2015-12-07T09:56:39Z
local.identifier.scopusID2-s2.0-33746296274
CollectionsANU Research Publications

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