The Amphibole in ultramafic rocks from southern Mariana trench inner slope:Implication for metasomatism in forearc setting

This study investigates the ultrahigh pressure (UHP) metamorphic and metasomatic processes involved in the transient subduction-exhumation of continental crust to over 100km depths during a collisional orogeny and the implications for the evolution of the continental crust and crust-mantle interaction at depth. The study focuses on garnet websterites (orthopyroxene eclogites) and zoned, clinopyroxenite-garnetite veining features present in a range of eclogite-facies, crustal metamafic-ultramafic bodies hosted within the Western Gneiss Region (WGR), western Norway. The structural occurrences and textures of some of these crustal garnet websterites are seemingly unique to the WGR but little research has focused on their origin or from a metasomatic perspective. Based on field and petrographic observations, a metasomatic origin is attributed to vein-associated garnet websterites at Kolmannskog, Myrb�rneset and Svartberget. A metamorphic origin is attributed to body domains at Nyb�, Kolmannskog, B�tneset and Myrb�rneset and a combined metamorphic-metasomatic origin is attributed to garnet websterite body domains at �rsheimneset and Rem�ysunde and inferred at Grytting and Eiksunddal. UHP P-T conditions are obtained from garnet websterites of ~3.7GPa, 740�C at Nyb�, ~3.1GPa, 670�C at Grytting, ~3.5GPa, 700�C at �rsheimneset, ~3.6GPa, 815�C at Rem�ysunde, ~3.0GPa, 750�C at Kolmannskog and ~3.85GPa, 790�C at Svartberget. On this basis, it is proposed the Nord�yane UHP domain be extended eastwards to incorporate the Kolmannskog locality which lies outside its currently defined boundary. Constructed P-T paths suggest the northern Nord�yane UHP domain experienced ~100�C higher temperatures than the southern Nordfjord-Stadlandet UHP domain but experienced similar pressures implying a lower regional P-T gradient than previously established. P-T paths also suggest UHP, vein-forming metasomatism occurred prior to peak temperatures. U-Pb isotopic dating of zircon and monazites in garnetite vein cores dates UHP metasomatism at 414�5.6Ma at �rsheimneset and 410�2.6Ma at Svartberget. The fluid responsible for UHP metasomatism is considered to be a Si-Al-K-H2O-rich supercritical liquid produced in the surrounding country rock associated with the breakdown of phengite with a Na-LILE-LREE-HFSE-P enrichment signature. The major element composition of the fluid added to the Svartberget body is calculated to be 48-60% SiO�2, 17-27% Al2O3, 3-11% K2O, <10% MgO, CaO and FeO, 3-6% Na2O, <4% P2O5�, <1% TiO2 and MnO with an overall, undersaturated-saturated sialic, syenitic character hybridised through interaction with the garnet peridotite body margins. The continental fluid-mafic-ultramafic rock systems studied imply a zoned metasomatic unit forms at the interface between subducted continental crust and above mantle wedge at depths of ?120-130km and along any fluid pathways penetrating into the mantle transferring abundant alkalis, water and trace elements into the mantle. Fluid-mantle interaction is proposed to form abundant biotite and amphibole and zones of garnet websterite, biotite websterite and biotite clinopyroxenite with lenses of eclogite and/or accessory phase (rutile, zircon, monazite, apatite, xenotime)-rich garnetite �glimmerite selvages where residual fluids accumulate. Subcontinental mantle metasomatism may be associated with UHP, supercritical liquids derived from subducted, eclogite-facies, continental crust rather than oceanic crust as the continental crust is a greater source of the Si, alkalis, trace elements and water which characterise mantle metasomatism.

Metasomatism

Orogeny

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Citations (1)

Metasomatic Mantle Xenoliths from the Bismarck Microplate (Papua New Guinea)--Thermal Evolution, Geochemistry and Extent of Slab-induced Metasomatism

Journal of Petrology (2002)

Leander Franz

A suite of ultramafic mantle xenoliths from the TUBAF and EDISON seamounts in the Bismarck Archipelago NE of Papua New Guinea was sampled by video-guided grab. The xenoliths, which were transported to the sea floor by rift-related, Quaternary trachybasalts, mainly represent part of the oceanic mantle. Mineral zoning in peridotite xenoliths testifies to slow cooling after mantle formation at a mid-ocean ridge. Cooling rates in the range of 1°C/Ma were calculated from zoning of Ca in olivine using the Lasaga algorithm. Subsequent to this cooling, a strong metasomatism affected the mantle peridotites when metasomatic agents emerged from the underlying slab of a subduction zone, which was stalled about 15 my ago. This resulted in the formation of orthopyroxene-, clinopyroxene-, phlogopite- and hornblende-bearing veins crosscutting spinel peridotites and olivine clinopyroxenites, as well as pervasively metasomatized plagioclase lherzolites. The metasomatic xenoliths reveal strong chemical disequilibria between the metasomatic minerals and the adjacent, unaltered host rock minerals, which are especially prominent in the veined samples. Temperatures during the metasomatic overprint, estimated using spinel–olivine thermometry, range between 660 and 950°C. Oxygen barometry reveals an elevated oxygen fugacity, with Δlog(fo2)FMQ values of 0·4 to >4. A geochemical study of the ultramafic rocks shows that all types of xenoliths have been metasomatized. Pervasively metasomatized plagioclase lherzolites and cumulate olivine clinopyroxenites have high contents of middle and heavy rare earth elements compared with veined peridotites. Cryptic metasomatism, indicated by increased light rare earth elements and Nd concentrations, results from LREE-rich hydrous fluid circulation. The investigated peridotites underwent a three-stage evolution from depleted oceanic ridge residues via repeated depletion to metasomatic imprint within a supra-subduction-zone setting.

Metasomatism

Xenolith

Ultramafic rock

Peridotite

10.1093/petrology/43.2.315

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Citations (95)

Origin of fine‐grained peridotite xenoliths from Iraya volcano of Batan Island, Philippines: deserpentinization or metasomatism at the wedge mantle beneath an incipient arc?

Island Arc (2000)

Shoji Arai Megumi Kida

Abstract Peridotite xenoliths from the subarc mantle, which have been rarely documented, are described from Iraya volcano of the Luzon arc, the Philippines, and are discussed in the context of wedge‐mantle processes. They are mainly harzburgite, with subordinate dunite, and show various textures from weakly porphyroclastic (C‐type) to extremely fine‐grained equigranular (F‐type). Textural characteristics indicate a transition from the former to the latter by recrystallization. The F‐type peridotite has inclusion‐rich fine‐grained olivine and radially aggregated orthopyroxene, being quite different in texture from ordinary mantle‐derived peridotites previously documented. Despite their strong textural contrast, the two types do not show any systematic difference in modal composition. The harzburgite of C‐type has ordinary mantle peridotite mineralogy; olivine is mostly Fo91–92 and chromian spinel mostly has Cr#s (= Cr/[Cr + Al] atomic ratios) from 0.3 to 0.6. Olivine is slightly more Fe‐rich (Fo89–91) and spinel is more enriched in Cr (the Cr#, 0.4–0.8) and Fe 3+ in F‐type peridotites than in C‐type harzburgite. Orthopyroxene in F‐type peridotites is relatively low in CaO (<1 wt%), Al 2 O 3 (<2 wt%) and Cr 2 O 3 (<0.4 wt%). The F‐type peridotite was possibly formed from the C‐type one by recrystallization including local dissolution and precipitation of orthopyroxene assisted by fluid (or melt) of subduction origin. Textural characteristics, however, indicate a deserpentinization origin from abyssal serpentinite of which protolith was a C‐type peridotite. In this scenario the initial abyssal serpentinite was possibly dehydrated due to an initiation of magmatic activity beneath an incipient oceanic arc like Batan Island. The F‐type peridotite is characteristic of the upper mantle of island arc, especially of incipient arc.

Peridotite

Metasomatism

Xenolith

10.1111/j.1440-1738.2000.00294.x

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Citations (50)

Slab melt and intraplate metasomatism in Kapfenstein mantle xenoliths (Styrian Basin, Austria)

Lithos (2006)

Massimo Coltorti Costanza Bonadiman Barbara Faccini T. Ntaflos Franca Siena

Amphibole

Peridotite

Metasomatism