Mineral and whole-rock geochemistry of high-Al podiform chromitites in the Fizh Massif of the Cretaceous Oman ophiolite: origin of hydrous N-MORB melts in a nascent forearc setting
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Abstract:
The Oman ophiolite is one of the largest and best exposed ophiolites in the world and has >450 chromitite deposits. We report here a newly identified chromitite deposit in the Wadi Rajmi, Oman. This deposit occurs within a dunitic envelope, which is surrounded by harzburgite, and consists of both massive and disseminated chromitite types. The Rajmi peridotites represent depleted upper mantle rocks that underwent >20% partial melting and experienced metasomatism by melts and fluids derived from a subducting slab. They demonstrate geochemical affinities similar to those of the Izu–Bonin–Mariana forearc peridotites, supporting their formation in a forearc environment. The Rajmi chromitites have low Cr# values and are classified as high-Al chromitites. They have geochemical compositions comparable with those of chromitites crystallized from mid-ocean ridge basalt (MORB)-type melts. However, the chromites in these high-Al chromitites contain various silicate inclusions (e.g. amphiboles and micas), indicating the hydrous and atypical MORB nature of their parental magmas. Combined with the mineralogical and geochemical characteristics of the country rocks, we posit that the parental melts of the Rajmi high-Al chromitites have a MORB-like affinity derived from partial melting of a nascent forearc mantle. Supplementary material: The whole-rock and mineral data of the Rajmi harzburgites, dunites and chromitites are available at https://doi.org/10.6084/m9.figshare.c.6795689 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschistsKeywords:
Chromitite
Forearc
Massif
Amphibole
Ultramafic rock
Metasomatism
Peridotite
Systematic investigations of compositional variations in peridotite infiltrated by a small volume of metasomatic agent allow a better understanding of both the scales and processes of mantle metasomatism (Nielsen et al., 1993; Nielsen and Wilshire, 1993). Insights into liquid-rock mantle reactions have been provided by recent whole-rock (Bodinier et al., 1990) and clinopyroxene (Takazawa et al., 1992) investigations, from which the composition of most altered and uumetasomatized end-members as well as reaction gradients can be evaluated. This study focuses on amphibole directly deposited by the reacted melt, as it is the most sensitive phase to changes in liquid composition. Variations in amphibole crystal-chemistry can be, therefore, applied in monitoring P-T-X gradients. We used a combination of microanalytical techniques (EPMA and SIMS) and single-crystal structure refinements (SREF) to investigate compositional variations in amphiboles in proximity (m-scale) of an alkaline vein infiltrated in the ambient peridotite.
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Distinct equilibration temperatures, deformation and trace element characteristics are observed in amphibole-bearing and amphibole-free peridotite xenoliths from Nushan, Sino-Korean Craton, eastern China. Amphibole-free peridotites are predominantly deformed, fine-grained (∼1 mm) and equilibrated at 990–1110°C. Their cpx are characterized by either light rare earth element (LREE)-depleted or relatively flat REE patterns with only a slight depletion in high field strength elements (HFSE). LREE enrichment is generally associated with Fe-rich samples, consistent with 'wall-rock' metasomatism adjacent to basaltic veins. In contrast, amphibole-bearing peridotites are less deformed, coarse-grained (>3 mm) and display chemical zonation in the pyroxenes suggesting cooling from 1050 to 850°C. Their cpx show a large variation in LREE (Cen = 1·7–68) and almost constant heavy rare earth element (HREE) contents (Ybn = 9·8–11·6). The highest LREE contents occur in cpx from amphibole-rich samples, coupled with Fe enrichment, strong enrichment in Th and U, and marked depletion in the HFSE. These characteristics may be accounted for by combined 'wall-rock' and 'diffuse' metasomatism involving an agent rich in volatiles and incompatible elements. As such the Nushan xenoliths could represent samples from two spatially separate metasomatic aureoles. Conversely, the cryptic and modal metasomatism could be genetically related, because the amphibole-peridotites and Fe-rich amphibole-free samples show similar Sr–Nd isotopic ranges that are indistinguishable from those of the Cenozoic basalts from eastern China. The different metasomatic assemblage and the trace element composition can be accounted for in terms of P–T control on amphibole stability and progressive chemical evolution of asthenosphere-derived melts during upward migration. Trace element signatures and metasomatic assemblages, together with the fertile composition of the Nushan peridotites, suggest an origin as newly accreted lithosphere rather than as relic cratonic mantle. Metasomatism may have occurred after late Mesozoic lithospheric thinning, which marked a dramatic change in lithospheric architecture beneath the Sino-Korean Craton.
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Primitive mantle
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Metasomatism
Tremolite
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Peridotite
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The garnet websterites and garnet clinopyroxenites from the Beni Bousera peridotite massif contain varying amount of amphibole plogophite. The presence of amphibole in both pyroxenites indicates that post-formation metasomatism has affected these pyroxenites. Textural observations, associated with major and trace element compositions of minerals allow distinguishing two metasomatic episodes, occurring at two different periods of the evolution of the Beni Bousera massif. Garnet websterites record interactions with LREE-rich silicate melt before the uplift of the massif, and crystallization of interstitial amphibole and phlogopite. Partition coefficients between amphibole and matrix clinopyroxene and garnet indicate that the former is in chemical equilibrium with the pyroxenite minerals. The fluid responsible for the metasomatic episode is an alkali silicate melt originating from the partial melting of a peridotitic source similar to the source of the recent basaltic magmas erupted in Morocco. Garnet clinopyroxenites show the crystallization of amphibole and plagioclase at low P-T conditions during the ascent of the Beni Bousera massif. Amphiboles are in chemical disequilibrium with the pyroxenite matrix minerals. The melt responsible for this event is depleted in LREE and highly enriched in HREE. Such a melt can be produced by partial melting of a garnet-bearing source, with garnet as a melting phase, similar to the garnet pyroxenites from the Beni Bousera massif.
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Phlogopite
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Amphibole
Xenolith
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Incompatible element
Fractional crystallization (geology)
Peridotite
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A suite of mantle peridotite xenoliths from the Malaitan alnoite display both trace element enrichment and modal metasomatism. Pargasitic amphibole is present in both garnet- and spinelbearing xenoliths, formed by reaction of a metasomatic fluid (represented by H2O and Na2O) with the peridotite assemblage. Two pargasite-forming reactions are postulated, whereby spinel is totally consumed: 6MgAl2O4 + 8CaMgSi2O6 + 7Mg2Si2O6 + 4H2O + 2Na2O = 4NaCa2Mg4Al3Si6O12(OH)2 + 6Mg2SiO4 or spinel is both a reactant (low Cr) and a product (high Cr): 24MgAlCrO4 + 16CaMgSi2O6 + 14Mg2Si2O6 + 8H2O + 4Na2O = 8NaCa2Mg4Al3Si6O12(OH)2 + 12MgCr2O4 + 12Mg2SiO4 Seven garnet—spinel-peridotites display cryptic metasomatism as demonstrated by the LREE enrichment in clinopyroxenes. The LREE enrichment correlates positively with 143ND/144ND (0·512771–0·513093) which defines a mixing line between a mantle MORB source and a metasomatic fluid. Isotopic evidence (Sr and Nd) from garnet, clinopyroxene, and amphibole demonstrate this fluid has not originated in the alnoite sensu stricto. Calculated amphibole equilibrium liquids show a range in La/Yb and Ce/Yb ratios similar to those calculated for the augite and subcalcic diopside megacrysts. Sr and Nd isotope analyses from amphibole are within error of the augite (PHN4074) and subcalcic diopside megacrysts (CRN2I6, PHN4069, and PHN4085). It is concluded that fluids emanated from a proto-alnoite magma throughout megacryst fractionation, and the mixing line was generated during the crystallization of the subcalcic diopsides. This study demonstrates that metasomatism represented in these xenoliths is not a prerequisite for alnoite magmatism, but is a consequence of it.
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Amphibole
Peridotite
Xenolith
Fractional crystallization (geology)
Diopside
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