An AEM study of garnet clinopyroxenite from the Sulu ultrahigh-pressure terrane: formation mechanisms of oriented ilmenite, spinel, magnetite, amphibole and garnet inclusions in clinopyroxene (original) (raw)
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Minerals, 2022
The origin of ultramafic rocks, especially those in suture zones, has been a focus because they are not only important mantle sources of magma, but also provide substantial information on metamorphism and melt/fluid–peridotite interaction. Ultramafic rocks in Hujialin, in the central part of the Sulu orogen, include peridotite and pyroxenite. Although many papers on their origin and tectonic evolution have been published in the past few decades, these questions are still highly debated. Here, we present mineralogy, mineral composition, and bulk-rocks of these ultramafic rocks to evaluate their origin and tectonic evolution. The garnet clinopyroxenite is low in heavy rare-earth elements (HREE, 5.97–10.6 ppm) and has convex spoon-shaped chondrite-normalized REE patterns, suggesting the garnet formed later, and its precursor is clinopyroxenite. It is high in incompatible elements (i.e., Cs, Rb, Ba) and shows negative to positive U, Nb, and Ta anomalies, without pronounced positive Sr o...
Tectonophysics, 2009
Compared to other mafic and ultramafic rocks from the CCSD main borehole as well as from the outcrops, rocks from the 540-600 m section is extraordinary in terms of its geophysical as well as geochemical properties. It consists of N 70% hematite-ilmenite garnet pyroxenite (HI-GPX) and b 30% intercalated rutile garnet pyroxenite (R-GPX). Whole-rock geochemical data show that HI-GPXs have: (1) relatively high V with an average of 606 ppm, but lower Nb and Ta; (2) highest TFeO, Fe 2 O 3 /FeO ratio, and highly variable but strong positive Eu anomalies with Eu/Eu⁎ up to 2.9; (3) anomalously high V/Sc ratios ranging from 8.39 to 43.23, average 15.03; and (4) high amounts of hematite-ilmenite solid solutions with a very fine intergrowth structure down to nanometer scale. V/Sc ratios in the CCSD garnet pyroxenites are correlated negatively with MgO, but positively with Fe 2 O 3 /FeO ratios. Both suites of pyroxenites have similar rare earth elements and high field strength elements geochemistry. These features demonstrate that these pyroxenites were formed from metamorphism of high-Fe and/or -Ti gabbroic cumulates. This can account not only for low high field strength elements (HFSE) and rare earth elements (REE) but also low Nb/Ta and Zr/Hf ratios in these rocks. Seemingly negative correlation between Nb/Ta and Zr/Hf in the CCSD metabasites, not significantly affected by UHP metamorphism, is also consistent with the silicate differentiation trend in a basaltic magma chamber.
Petrology and U-Pb SHRIMP geochronology of a garnet peridotite, Sulu UHP terrane, east-central China
International Geology Review, 2007
Garnet clinopyroxenite at Rizhao occurs as a lens 100 x 225 m2 within a serpentinized peridotite body, in fault contact with felsic gneiss. Minor dunite also occurs within the same peridotite. The clinopyroxenite consists of porphyroblastic Grt-bearing (type 1), megacrystic Grt-bearing (type 2), porphyroblastic Cpx-bearing (type 3), and equigranular (type 4) garnet clinopyroxenite. Lamellaerich, coarse-grained Cpx occurs as inclusions in megacrystic Grt from type 2 clinopyroxenite, and as porphyroblasts in a matrix of Cpx + Grt + Ilm from type 3 clinopyroxenite; these clinopyroxenes contain abundant exsolution lamellae of Grt (Prp16-19Grs62Alm19-21Sps1) + Ilm ± Mag ± Amp (Mg#: 0.88-0.98; Na2O: 0.5-3.3 wt%; K2O: 0.7-1.0 wt%; TiO2: 0.1-0.2 wt%). Megacrystic and porphyroblastic garnet (Prp35-42Grs34-45Alm19-24Sps1) also occurs in a matrix of fine-grained Cpx + Grt + Ilm; spinel (Mg#: 0.60-0.62) is present as inclusions in both porphyroblastic Grt and lamellae-rich Cpx. Petrochemical data support a mantle origin involving crystallization of majoritic garnet. We propose that the protolith of the Rizhao clinopyroxenite initially was a spinel clinopyroxenite, and was convected to great depths (>450 km) to form a majoritic garnet that contains high CaO and considerable amounts of FeO and TiO2. During later decompression, the majoritic garnet broke down to form intergrowths of Cpx + Grt + Ilm ± Mag ± Amp at T = 700-800°C, P ≤ 3 GPa. It was then emplaced into a subduction zone and experienced UHP metamorphism at 620-880°C, 3 GPa during Triassic continental collision. Megacrystic/porphyroblastic garnet formed during this stage, and lamellaerich Cpx recrystallized to the matrix assemblage of Cpx + Grt + Ilm subsequently. The clinopyroxenite also experienced later retrogression during exhumation. Zircon U-Pb geochronology yields 215 ± 2 Ma for the Rizhao garnet clinopyroxenite; occurrence of garnet and clinopyroxene inclusions in zircon suggests that this age reflects the UHP metamorphism.
Clinopyroxene?a mineral telescoped through the processes of blueschist facies metamorphism
Journal of Metamorphic Geology, 1987
Textural evolution and compositional variation of clinopyroxenes in Ward Creek metabasites are described. Pyroxenes change, with increasing grade, from finegrained aggregates through fan-shaped medium-grained prisms to blocky coarse crystals. Characteristic features of metamorphic pyroxenes include: (1) the occurrence of coexisting pyroxene pairs, the compositions of which are used to delineate compositional gaps; (2) the existence of large compositional variations of pyroxenes, within a single specimen, which record a considerable span of P andlor T for crystallization; and, (3) the development of compositional trends in single specimens and in three metamorphic zones which are progressive in nature. The first formed clinopyroxene (Jd2&u&, AcI5) in the lower lawsonite zone mimics the composition of relict igneous augite. It changes continuously, with increasing grade, at nearly constant low XJd content towards acmite. At a composition around JdzoAug,,,AcS0, the trend turns towards jadeite and intersects a solvus to form two coexisting clinopyroxenes in the middle lawsonite zone. At higher grade, the compositional gap becomes restricted towards the jadeite-omphacite join and clinopyroxene increases in XJd toward jadeite. A reversed compositional trend occurs at higher grade; clinopyroxenes decrease in jadeite component at nearly constant AuglAc ratio of 50150 and finally become omphacite in the uppermost pumpellyite and epidote zones. The Na-Ca pyroxenes, close to the binary join Jd-Ac, occur in the lawsonite-and pumpellyite-zones, ranging from XJd = 1.0-0.30 together with Ab and Qz. The ubiquitous occurrence of aragonite at temperature estimates of 170-240°C by Taylor & Coleman (1968) for these zones does not support the low-temperature extrapolation
Geochimica et Cosmochimica Acta, 1992
Clinopyroxenes and garnets from gabbroic rocks of the Ivrea Verbano mafic complex have been analyzed by electron microprobe for major elements and by ion microprobe for REE, SC, Cr, Ti, V, Zr, Na, and Sr content. The samples represent two petrographic types: in the first, garnet is formed by subsolidus reaction and occurs in coronas (c-type); in the other, garnet occurs as large porphyroblasts (p-type) and may have been a phase on the liquidus. Clinopyroxenes and garnets are unzoned (with one exception) for major and trace elements, suggesting that, in general, equilibrium has been attained under granulite facies conditions as indicated by the geothermometers. Clinopyroxene, although affected in its HREE and SC content by the coexistence with garnet, has REE patterns which vary, along with the bulk rock patterns, stratigraphically upwards from LREE-depleted to LREE-enriched. Trace element distribution coefficients (D) between clinopyroxene and garnet, as measured in the ptype assemblages, vary systematically with major-element compositional parameters such as FeO, MgO, FeO/MgO, A1203, Na*O, and apparent equilibration temperature. In addition, the overall pattern of REE partitioning, D(Ce) to D(Yb), is significantly steeper than those found in previously published estimates, except when these were determined on exceptionally carefully prepared mineral separates. The D values determined on c-type assemblages are comparatively erratic and appear to depend on the modal gnt/cpx ratio. This feature is tentatively attributed to failure to achieve complete equilibrium during slow cooling when the corona structures were formed. Subsolidus reequilibration between phases has generally obliterated the igneous phase chemistry of the rocks sufficiently so that the composition of the parent liquid cannot be determined from those of the constituent minerals even when these represent original "phenocrysts."
Clinopyroxenes Multi-stage metasomatism Happo-O'ne Japan The metaperidotites found in the Happo-O'ne region of central Japan are mostly lherzolites to harzburgites with subordinate dunites. Their protoliths originated as a series of refractory residues by near-fractional melting, 15–25% melting in the spinel field, which is confirmed by a HREE melting model of the apparently primary clinopyroxene. These metaperidotites display U-shaped primitive mantle (PM)-normalized REE patterns (0.02–0.5 times PM) and are highly enriched in LILE (0.2–20 times PM) relative to HFSE (b 0.2 times PM), providing evidence for mantle-wedge metasomatism. In-situ analyses confirmed that clinopyroxenes and tremolite are enriched in fluid-mobile elements (B, Sr, Pb, Li, Cs, Ba and Rb; 0.1–100 times PM) coupled with depletion of HFSE (Ta, Hf, Th, Zr, Ti and Nb; b 0.7 times PM) + U; these chemical features of clinopyroxenes and tremolite are similar to those of their whole rocks, and reflect slab-fluid metasomatism. We recognized three stages of clinopyroxenes that are different in morphology and REE patterns, but show the same major-element chemistry. Nearly euhedral clinopyroxene (Cpx1) displays a spoon-shaped REE pattern (0.2–3 times PM), reflecting the first stage (stage 1) of metasomatism. Acicular clinopyroxene (Cpx2) after tremolite +olivine shows a U-shaped REE pattern (0.06–1 times PM) (stage 2), whereas fine-grained clinopyroxene (Cpx3) derived from orthopyroxene shows very low concentrations of REE (stage 3). These three successive stages of retrogressive clinopyroxene formation possibly denote a retrogressive chemical change of the involved fluids during multi-stage metasomatism. The fluid of stage 1 is highly enriched in LREE (0.2–2 times PM), MREE, Pb, Sr, Li and Rb relative to the stage-3 fluid that is very low in MREE, Pb, LREE and Sr, but has high Na, Mn, Ba, B and Cs. The stage-2 fluid, which is low in LREE (0.06–1 times PM), Li and Pb, and very low in Cs, Rb and Hf relative to the fluid of stage 1, is considered to be a transitional fluid between stages 1 and 3. The retrogressive change of the fluid composition is possibly equivalent to a transversal change of the slab-derived fluid within the mantle wedge due to a change of metamorphic condition, depth of the subducting slab and an episode of metasomatism during the exhumation of the Happo-O'ne peridotites.
Journal of Metamorphic Geology, 2013
Regularly oriented orthopyroxene (opx) and forsterite (fo) inclusions occur as opx + rutile (rt) or fo + rt inclusion domains in garnet (grt) from Otrøy peridotite. Electron diffraction characterization shows that forsterite inclusions do not have any specific crystallographic orientation relationships (COR) with the garnet host. In contrast, orthopyroxene inclusions have two sets of COR, that is, COR-I: <111> grt ⁄ ⁄ <001> opx and {110} grt ⁄ ⁄ {100} opx (13°off) and COR-II: <111> grt ⁄ ⁄ <011> opx and {110} grt ⁄ ⁄ {100} opx (14°off), in four garnet grains analysed. Both variants of orthopyroxene have a blade-like habit with one pair of broad crystal faces parallel ⁄ sub-parallel to {110} grt plane and the long axis of the crystal, <001> opx for COR-I and <011> opx for COR-II, along <111> grt direction. Whereas the lack of specific COR between forsterite and garnet, along with the presence of abundant infiltrating trails ⁄ veinlets decorated by fo + rt at garnet edges, provide compelling evidence for the formation of forsterite inclusions in garnet through the sequential cleaving-infiltrating-precipitatinghealing process at low temperatures, the origin of the epitaxial orthopyroxene inclusions in garnet is not so obvious. In this connection, the reported COR, the crystal habit and the crystal growth energetics of the exsolved orthopyroxene in relict majoritic garnet were reviewed ⁄ clarified. The exsolved orthopyroxene in a relict majoritic garnet follows COR-III: {112} grt ⁄ ⁄ {100} opx and <111> grt ⁄ ⁄ <001> opx. Based on the detailed trace analysis on published SEM images, these exsolved orthopyroxene inclusions are shown to have the crystal habit with one pair of broad crystal faces parallel to {112} grt ⁄ ⁄ {100} opx and the long crystal axis along <111> grt ⁄ ⁄ <001> opx. Such a crystal habit can be rationalized by the differences in oxygen sub-lattices of both structures and represents the energetically favoured crystal shape of orthopyroxene inclusions in garnet formed by solid-state exsolution mechanism. Considering the very different COR, crystal habit, as well as crystal growth direction, the orthopyroxene inclusions in garnet of the present sample most likely had been formed by mechanism(s) other than solid-state exsolution, regardless of their regularly oriented appearance in garnet and the COR specification between orthopyroxene and garnet. In fact, the crystallographic characteristics of orthopyroxene and the similar chemical compositions of garnet at opx + rt inclusion domains, fo + rt inclusion domains ⁄ trails and garnet rim suggest that the orthopyroxene inclusions in the garnet are most likely formed by similar cleaving-infiltration process as forsterite inclusions, though probably at an earlier stage of metamorphism. This work demonstrates that the oriented inclusions in host minerals, with or without specific COR, can arise from mechanism(s) other than solid-state exsolution. Caution is thus needed in the interpretation of such COR, so that an erroneous identification of exhumation from UHP depths would not be made.
Journal of Metamorphic Geology, 2003
The Xugou garnet peridotite body of the southern Sulu ultrahigh-pressure (UHP) terrane is enclosed in felsic gneiss, bounded by faults, and consists of harzburgite and lenses of garnet clinopyroxenite and eclogite. The peridotite is composed of variable amounts of olivine (Fo 91), enstatite (En 92)93), garnet (Alm 20)23 Prp 53)58 Knr 6)9 Grs 12)18), diopside and rare chromite. The ultramafic protolith has a depleted residual mantle composition, indicated by a high-Mg number, very low CaO, Al 2 O 3 and total REE contents compared to primary mantle and other Sulu peridotites. Most garnet (Prp 44)58) clinopyroxenites are foliated. Except for rare kyanite-bearing eclogitic bands, most eclogites contain a simple assemblage of garnet (Alm 29)34 Prp 32)50 Grs 15)39) + omphacite (Jd 24)36) + minor rutile. Clinopyroxenite and eclogite exhibit LREE-depleted and LREE-enriched patterns, respectively, but both have flat HREE patterns. Normalized La, Sm and Yb contents indicate that both eclogite and garnet clinopyroxenite formed by high-pressure crystal accumulation (+ variable trapped melt) from melts resulting from two-stage partial melting of a mantle source. Recrystallized textures and P-T estimates of 780-870°C, 5-7 GPa and a metamorphic age of 231 ± 11 Ma indicate that both mafic and ultramafic protoliths experienced Triassic UHP metamorphism in the P-T forbidden zone with an extremely low thermal gradient (< 5°C km)1), and multistage retrograde recrystallization during exhumation. Develop of prehnite veins in clinopyroxenite, eclogite, felsic blocks and country rock gneiss, and replacements of eclogitic minerals by prehnite, albite, white mica, and K-feldspar indicate low-temperature metasomatism.
Earth and Planetary Science Letters, 2006
The ultrahigh-pressure (UHP) Maowu Ultramafic Complex (Dabie Shan, China) is hosted by coesite-bearing gneisses. Textural and geochemical data demonstrate that garnet-orthopyroxenites within the ultramafic complex derive from garnet-harzburgite precursors that have been metasomatised at peak UHP conditions (4.0 ± 1.0 GPa, 750 ± 50°C) by the addition of a silica-and incompatible trace element-rich fluid phase (hydrous melt), sourced from the associated crustal rocks. This metasomatism produced poikilitic orthopyroxene with high LREE and Ni contents and inclusion-rich garnet porphyroblasts. Solid polyphase primary inclusions within peak metamorphic garnet display negative crystal shapes and constant volume ratios of infilling mineral phases. Experimental homogenisation of these inclusions at conditions close to the estimated metamorphic peak demonstrates that the polyphase inclusions derive from trapped solute-rich aqueous fluids. The trace element characteristics of the experimentally re-homogenised inclusions include high LREE contents, a pronounced enrichment in LILE, with spikes of Cs, Ba, Pb and high U/Th.
Garnet compositions as recorders of P–T–t history of metamorphic rocks
Gondwana Research, 2010
On the basis of tests made in recent studies, we conclude that the simultaneous solutions of the equilibrium conditions of garnet-biotite Fe-Mg exchange and Grs = Plag + Aluminosilicate + Qtz (GASP) reactions, as formulated on the basis selected experimental data and well constrained thermodynamic mixing properties of garnet and plagioclase, offer robust estimates of P-T conditions of metapelitic assemblages that consist of the above minerals. Additional calculations are presented to show the compatibility of the retrieved P-T conditions of natural assemblages with the aluminosilicate phase diagram. We also calculate the minimum grain sizes of garnets that should be used for the calculation of the peak metamorphic P-T (P p , T p ) conditions, using reasonable initial guessed values of minimum cooling rate and maximum T p . To retrieve the thermal history of metapelites, we have developed a finite difference scheme for modeling multicomponent diffusion profiles in garnet, incorporating provisions for continuous nucleation and growth. This has been interlinked with a genetic algorithm that permits retrieval of the temperature vs. time path of metapelites through modeling of the zoning profiles in garnets, keeping several model parameters as floating variables. The numerical code has been applied to retrieve the T-t history of the low pressure-high temperature Royke metamorphic belt, Japan, by modeling the concentration profiles of Fe, Mn, Mg and Ca of garnets of different sizes that have been inferred to have developed by a process of continuous nucleation and growth.