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(a) Garnet compositions in the Sanqingge coesite eclogites. (b–g) Two representative garnets displayng the major chemical changes in sample Sqq08. (b–d) Show the chemical variations in Ca, Mg and Ti across garnet #1. (eÀg) Variation in the same elements across garnet #2. Py = pyrope; Alm + Sp = almandine + spessartine; Gros = grossular.
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Marbles and interlayered coesite-bearing eclogites near the village of Sanqingge in the Sulu ultrahigh-pressure (UHP) terrane of eastern China were studied to estimate their P-T evolution. Using garnet, omphacite and phengite as geothermobarometers, the coesite eclogites are calculated to have experienced P-T conditions of 3.4–3.7 GPa and ∼600°C (s...
Contexts in source publication
Context 1
... composition of garnet varies significantly in the studied samples (Table 1). Pyrope contents range between 20 and 43.5 mol.% (Fig. 4a). Garnet (Alm 30À38 Py 38À43.5 Gros 22.5À26 Sp 0.5À0.7 ) enclosed in omphacite shows greater pyrope contents than large garnet grains (Alm 28.5À52.5 Py 20À34.5 Gros 21.5À30 Sp 0.7À1.6 ). The garnets contain substantial Na (up to 0.13 wt.% Na 2 O) as a result of possible substitutions such as NaTi = MgAl. Sodium may also occur as the Na ...
Context 2
... may also occur as the Na 3 Al 2 (PO 4 ) 3 component in garnet as suggested by Brunet et al. (2006). These authors noted that the phosphorus content in pyrope is a potential pressure indicator when it coexists with phosphate minerals, which is the case here (see Fig. 3a). Garnet zoning is irregular. For example, one garnet grain in sample Sqq08 (Fig. 4bÀd) contains a Ca-rich core, a small Mg# (=Mg/ (Mg+Fe 2+ )) core and a large Mg# rim (analyses #56 and #196 respectively in Table 1); this is typical of prograde garnet zonation. In addition, the Ti contents in this garnet are very small (<0.006 a.p.f.u.) without any recognizable zona- tion in regard of this element (Fig. 4d). Conversely, ...
Context 3
... grain in sample Sqq08 (Fig. 4bÀd) contains a Ca-rich core, a small Mg# (=Mg/ (Mg+Fe 2+ )) core and a large Mg# rim (analyses #56 and #196 respectively in Table 1); this is typical of prograde garnet zonation. In addition, the Ti contents in this garnet are very small (<0.006 a.p.f.u.) without any recognizable zona- tion in regard of this element (Fig. 4d). Conversely, the core of another garnet grain in sample Sqq08 (Fig. 4eÀg) displays a small Ca, large Mg# core and also a relatively Ti-rich core (up to 0.020 a.p.f.u.). Some garnets contain oriented rutile rods 415 mm long and 1À2 mm thick (Fig. 5a) which may have been exsolved. Thus, early garnet compositions were possibly relatively ...
Context 4
... (=Mg/ (Mg+Fe 2+ )) core and a large Mg# rim (analyses #56 and #196 respectively in Table 1); this is typical of prograde garnet zonation. In addition, the Ti contents in this garnet are very small (<0.006 a.p.f.u.) without any recognizable zona- tion in regard of this element (Fig. 4d). Conversely, the core of another garnet grain in sample Sqq08 (Fig. 4eÀg) displays a small Ca, large Mg# core and also a relatively Ti-rich core (up to 0.020 a.p.f.u.). Some garnets contain oriented rutile rods 415 mm long and 1À2 mm thick (Fig. 5a) which may have been exsolved. Thus, early garnet compositions were possibly relatively Ti-rich (Ti >0.01 a.p.f.u. as is also observed for garnet inclusions in ...
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Citations
... UHP marble, a significantly carbon-rich rock type, is frequently found in coexistence with UHP eclogite in the Dabie Shan region (e.g., Liu et al., 2006Liu et al., , 2015. The presence of coesite inclusions within marble provides compelling evidence that carbon originating from the continental crust can be subducted to depths of at least 100 km (Zhang et al., 1995;Zhu et al., 2009). Meanwhile, the Dabie Shan region is also rich in postcollisional mafic magmatic rocks (Fig. 1), which have ages ranging from 130 Ma to 120 Ma and exhibit arc-like geochemical signatures. ...
Subduction is a fundamental geodynamic process that transfers carbon from Earth’s surface into the mantle. However, current understanding of the migration mechanisms, final storage region, and species involved in carbon recycling from continental crust remains limited. Here, we investigated the compositions of polyphasic inclusions and Mg isotopes in postcollisional mafic magmatic rocks from the Dabie Shan region of China. The main rock-forming minerals contained two distinct types of polyphasic inclusions, which displayed systematic differences in daughter mineral/gaseous phase assemblages, including host-like silicates ± carbonates (magnesite, dolomite, and calcite) + CH4 and carbonates + talc ± SiO2 (aqueous) + CH4, respectively. These inclusions indicate that carbon-rich silicate melts and carbon-rich magmatic fluids were trapped by host minerals during magmatic processes. The abundant carbonates and CH4 in both types of inclusions suggest that the mantle source of these postcollisional mafic magmatic rocks was rich in carbon, most likely existing in the forms of CO2 and CH4. Moreover, the studied postcollisional mafic magmatic rocks have mantle-like Mg isotope compositions, with δ26Mg values ranging from −0.23‰ to −0.16‰. The combined observations of polyphase inclusions and Mg isotopes indicate that a substantial carbon-rich mantle domain arose from the metasomatism of silicate melts derived from subducted continental slabs that had dissolved a certain quantity of CO2 and CH4. We proposed that continental subduction is an efficient pathway for transporting crustal carbon into an orogenic subcontinental lithospheric mantle wedge, where the recycled carbon can be stored for >100 m.y. and eventually released to the surface during postcollisional magmatism.
... Most HP-UHP marbles contain simple mineral assemblages, the stability of which is further controlled by fluid composition (e.g., X(CO 2 )) in many cases (e.g., Liu et al., 2017aLiu et al., , b, 2015Lü et al., 2013;Massonne, 2011;Castelli et al., 2007;Ogasawara et al., 1998). In addition, these rocks seem more reactive and easily loss eclogite-facies record during retrogression (e. g., Liu et al., 2015;Zhu et al., 2009;Castelli et al., 2007). Therefore, depicting metamorphic evolution and, especially, pressure-temperature (P-T) path of HP-UHP marbles is challenging. ...
In this study, a tremolite marble from the Dabie ultrahigh-pressure (UHP) terrane, east-central China was investigated for its metamorphic evolution by focusing on zircon. The marble contains an amphibolite-facies assemblage of dolomite, Mg-calcite, tremolite, biotite, and plagioclase, while zircon in the marble witnesses a complex recrystallization and growth history under both amphibolite-and eclogite-facies conditions. Cathodoluminescence reveals eight characteristic zones for zircon. As indicated by mineral inclusions in zircon, two zones formed no earlier than amphibolite-facies retrogression and are too thin to date. The other six zones contain inclusions of dolomite, aragonite, diopside (XNa=Na/(Na+Ca)=0.11–0.14), garnet (XCa=0.51–0.62, XMg=0.21–0.23, XFe=0.17–0.26, XMn=0.01), phengite and rutile, and formed under eclogite-facies conditions. Phase equilibria calculations illustrat that the Na-richest diopside formed under UHP conditions. Being an accessory eclogite-facies mineral in the marble, the analyzed chemistry of garnet inclusions cannot be reproduced by phase equilibria calculations because solid-solution models for many other minerals don’t incorporate Mn-endmembers. The eclogite-facies zircon zones show low HREE contents and flat MREE-HREE distribution patterns, which are interpreted to have been determined by the low bulk-rock HREE content instead of the presence of accessary garnet in the marble. U-Pb dating yielded a large age dataset ranging from about 250 to 210 Ma for the eclogite-facies zircon zones. Statistically, the eclogite-facies ages are characterized by a Gaussian distribution with a median peak at 232 Ma. We propose that zircon experienced a “protracted” recrystallization and/or growth history in the tremolite marble during the Triassic subduction and exhumation.
... (3) Now we have a better knowledge of carbon recycle in the carbonated MORB during deep subduction processes. Yet there is an increasing recognition that continental crust carrying sedimentary carbonate rocks can subduct to the upper mantle (Zhu et al., 2009). With depth, a certain amount of CO 2 is released from carbonatitic components via progressive metamorphism. ...
Earth carbon cycle shapes the evolution of our planet and our habitats. As a key region of carbon cycle, subduction zone acts as a sole channel transporting supracrustal carbonate rocks down to the mantle, balancing carbon budget between the Earth’s surface and the interior, and regulating CO2 concentration of the atmosphere. How carbonates evolve at depth is thus, a most fundamental issue in understanding carbon flux and carbon sequestration mechanism in the Earth. This study reviews prominent progresses made in the field of crystal chemistry of carbonates along subduction geotherms. It clearly finds that, in addition to common carbonates in the Earth’s crust, several new polymorphs of carbonates have been discovered to be stable under high-pressure and high-temperature conditions. This opens possibilities for oxidized carbon species in the deep Earth. However, metamorphic decarbonatation and reduction reactions restrict subducting carbonates to the top-mid region of the lower mantle. Specifically, subsolidus decarbonatation in the form of carbonates reacting with silicates has been proposed as an efficient process releasing CO2 from slabs to the mantle. Besides, carbonate reduction in the metal-saturated mantle likely results in generation of super-deep diamonds and a considerable degree of carbon isotope fractionation. Review of these novel findings leads us to consider three issues in the further studies, including 1) searching for new chemical forms of carbon in the mantle, 2) determining the reduction efficiency of carbonates to diamonds and the accompanying carbon isotope fractionation and 3) concerning carbon cycle in subduction of continental crust.
... Fragments of the lower crust of the overriding plate travelled on the surface of the subducting oceanic plate to great depth. The dominant rock types of this lower crust might have been amphibolites and basic granulites, concluded on the basis of the abundance of eclogites among the exposed UHP rocks of the Dabie-Sulu belt, but (meta)granitoids (e.g., Wallis et al., 1997) and metasedimentary rocks, such as quartzites (e.g., Liou et al., 1997b) and banded limestone-marl, transformed to banded marble-eclogite rocks (e.g., Wang and Liou, 1993;Zhu et al., 2009) during subduction, were among the tectonically eroded fragments. Alternatively, the sediments could have been on top of the oceanic plate before subduction and were then transported to great depths. ...
... The thick light grey bar at about 865 °C represents the limit for reliable discrimination of eclogite types because of the expected effect by significant cation diffusion (intracrystalline, but perhaps also intercrystalline). . However, evidence for such a sedimentary cover on the subducted oceanic plate is very limited for the Dabie-Sulu belt as such metasedimentary rocks (eventually coesite-bearing paragneisses reported by are virtually lacking and garnet in the studies by Wang and Liou (1993) and Zhu et al. (2009) point to type II eclogite in the banded marble-eclogite rocks. ...
Eclogites are witnesses of geodynamic processes that are commonly related to subduction of oceanic crust. Information on the part of these processes that refers to the burial of this rock type is rarely published but stored in the eclogitic garnet core and inclusions therein. To better understand general aspects of the burial process, a literature search on the chemical characteristics of garnet in worldwide occurrences of eclogite was undertaken. In most cases extended garnet cores show either a prograde growth zoning with increasing Mg, starting at a few percent of pyrope component, and decreasing Mn contents (type I eclogite) or a (nearly) constant chemical composition frequently with pyrope contents significantly above 10 percent (eclogites of types II and III). Only in minor cases, it is difficult to assign the reported garnet core to an eclogite type. The growth zoning of garnet was thermodynamically modelled for the chemical composition of a basalt following different burial paths. These paths are characterized either by a trajectory along a low geothermal gradient (type I eclogite), as expected for the subducting upper portion of oceanic crust, or a one characterized by nearly isothermal burial at temperatures above 500 °C reaching peak pressures up to 2.1 GPa (type III eclogite), as possibly due to crustal thickening during continent-continent collision, or more (type II eclogite) when basic rocks are tectonically eroded from the overriding continental plate before deep subduction. In addition, diffusion modelling was undertaken on mm-sized garnet demonstrating that the characteristics of the core zoning are not fully obliterated even during residence at temperatures of 800-850 °C within 10 million years. The scrutiny of more than 200 eclogites reported in the literature led to the following result: about half of them are type II eclogites; a third and a sixth can be related to type I and type III, respectively. Among type III are almost all of the few Proterozoic eclogites considered.
To demonstrate the benefit of our study, we link the core zoning of eclogitic garnet from various (ultra)high-pressure terranes in Phanerozoic orogenic belts to the geodynamics shaping corresponding orogens. The eclogites in these belts are dominated by type II. Thus, we propose that some of the material of the lower portion of the overriding continental crust was tectonically eroded by a subducted oceanic plate and brought to great depth. Afterwards, this material was exhumed first in a deep subduction channel and then in an exhumation channel during continent-continent collision where a contact with the upper continental plate was re-established. Furthermore, we suggest that type II eclogite can also occur in extrusion wedges as far as oblique subduction took place.
... Marble is a major rock unit coexisting with UHP eclogites in the Dabie-Sulu orogen (Fig. 2b). The occurrence of coesite in marbles from Shuanghe area (Liu et al., 2006) and the coexistence of UHP magnesite and dolomite in marbles from Sanqingge area (Zhu et al., 2009) indicate that marbles in Dabie-Sulu orogen have also undergone coesiteeclogite-facies UHP metamorphism. ...
... Thirty-five UHP marble samples were collected from six locations: 14 from the Dabie UHP terrane, including the Changpu region (6), Shuanghe area (5), Shima area (2), and Xindian area (1); 9 from the Sulu UHP terrane, including 4 from an open quarry 1 km northwest of the village of Sanqingge and 5 marble samples studied by Zhu et al. (2009); other 12 UHP marble samples were from the southern Susong Group in the Liuping area, comprising gneiss, mica schist, amphibolite, and dolomitic marble (Wei and Shan, 1997). Lithium diffuses from Lienriched to -depleted end-members (i.e., from eclogite to marble), so all samples were collected away from their coexisting eclogites. ...
... Exsolution textures in minerals are formed either by spinodal decomposition or by heterogeneous nucleation and growth (Wenk 1976;Robinson 1980;Degraef and Mchenry 2012). Decrease in pressures generally plays an important role in the formation of exsolution textures in metamorphic rocks (Schmadicke and Muller 2000;Zhu and Ogasawara 2002;Zhang et al. 2005;Proyer et al. 2009;Zhu et al. 2009;Colás et al. 2016), whereas temperature and fO2 changes may be the most important factors controlling the processes of exsolution development in igneous rocks (Rajesh 2006;Mccallum et al. 2006;Lenaz et al. 2016). Exsolution textures can thus provide critical information about changing P (pressure), T (temperature), and fO 2 (oxygen fugacity) conditions in the mantle (Rajesh 2006;Lenaz et al. 2016), and about the uplift and exhumation history of their host peridotites within the mantle. ...
We investigated lherzolitic peridotites in the Cretaceous Purang ophiolite along the Yarlung Zhangbo suture zone (YZSZ) in SW Tibet to constrain their mantle-melt evolution history. Coarse-grained Purang lherzolites contain orthopyroxene (Opx) and olivine (Ol) porphyroclasts with embayments filled by small olivine (Ol) neoblasts. Both clinopyroxene (Cpx) and Opx display exsolution textures represented by lamellae structures. Opx exsolution (Opx1) in clinopyroxene (Cpx1) is made of enstatite, whose compositions (Al 2 O 3 = 3.85-4.90 wt%, CaO = <3.77 wt%, Cr 2 O 3 = 0.85-3.82 wt%) are characteristic of abyssal peridotites. Host clinopyroxenes (Cpx1) have higher Mg#s and Na 2 O, with lower TiO 2 and Al 2 O 3 contents than Cpx2 exsolution lamellae in Opx, and show variable LREE patterns. Pyroxene compositions of the lherzolites indicate 10-15% partial melting of a fertile mantle protolith. P-T estimates (1.3-2.3 GPa, 745-1067°C) and the trace element chemistry of pyroxenes with exsolution textures suggest crystallization depths of ~75 km in the upper mantle, where the original pyroxenes became decomposed, forming exsolved structures. Further upwelling of lherzolites into shallow depths in the mantle resulted in crystal-plastic deformation of the exsolved pyroxenes. Combined with the occurrence of microdiamond and ultrahigh-pressure (UHP) mineral inclusions in chromites of the Purang peridotites, the pyroxene exsolution textures reported here confirm a multi-stage partial melting history of the Purang lherzolites and at least three discrete stages of P-T conditions in the course of their upwelling through the mantle during their intra-oceanic evolution.
... Pyroxenes generally contain a variety of microscopic or submicroscopic exsolution features that arise from the attempts of individual crystals to reduce their total free energies upon cooling and/or decompression (Feinberg et al. 2004;McCallum et al. 2006;Liu et al. 2007;Zhu et al. 2009a;He et al. 2013;Alifirova et al. 2015). High-resolution transmission electron microscope (HRTEM) observations and selected area electron diffraction (SAED) patterns have been used to resolve the fine structural detail of intergrowths and defects in minerals (Bursill and Smith 1984;Nord and Lawson 1989;Chen and Xu 2005). ...
... Exsolution is initiated either by spinodal decomposition as has been observed in Lunar clinopyroxene decomposed by rapid continuous cooling (Lally et al. 1975;Wenk 1976), or by heterogeneous nucleation and growth (Robinson 1980;DeGraef and;McHenry 2012). Pressure decrease usually plays an important role in the formation of exsolution textures in metamorphic rocks (Schmadicke and Muller 2000;Zhu and Ogasawara 2002;Zhang et al. 2005;Proyer et al. 2009;Zhu et al. 2009a;Colás et al. 2016), whereas temperature and fO 2 might be the most important factors controlling the exsolution processes in igneous rocks (Rajesh 2006;McCallum et al. 2006;Zhu and Xu 2007;Lenaz et al. 2016). Exsolution texture provides useful information about P, T, and fO 2 corresponding to geological events after the formation of a homogeneous mineral phase in magma. ...
The Haladala pluton, consisting of troctolite, olivine gabbro and gabbro with zircon SHRIMP U-Pb age of 309 ± 2 Ma (MSWD = 0.72), intruded the Devonian-Carboniferous arc segments in the middle Tianshan. Amphibole, coexisting with magnetite, amphibole, and phlogopite, crystallized in a magma chamber at depth of ~ 20 km (6.9–7.4 kbar, 934–943 °C) based on various thermobaramoters. Two kinds of exsolution textures (spinel rods in clinopyroxene, orthopyroxene lamellae in clinopyroxene) occur in troctolite and olivine gabbro. We describe oriented spinel rods and orthopyroxene lamellae exsolved from the host clinopyroxene based on optical and high-resolution transmission electron microscope (HRTEM) observations. The spinel rods (100) are parallel to their host clinopyroxene (010). Orthopyroxene lamellae (010) are coherent and strictly parallel to their host clinopyroxene (010). Exsolution of spinel rods from the host clinopyroxene is controlled by the reaction of (Ca0.5M2+0.5)Fe3+[AlSiO6]in clinopyroxene → (Ca0.86−0.17M2+0.14−0.17)(M2 + 1.00−0.96Al0−0.04)[Al0.17−0.10Si1.83−1.90O6] + Fe3O4 + O2.
... They display typical eclogite-facies assemblage of garnet and omphacite (Fig. 3a,b), with minor amounts of quartz, phengite, rutile, calcite, dolomite, magnetite, apatite and zircon. Coesite inclusions occur within zircon and garnet from the eclogite (Liu et al., 2007;Zhu and Zhu, 2007;Zhu et al., 2009), demonstrating that such eclogite experienced UHP metamorphism. For the purpose of comparison, P-type eclogite was also sampled from an outcrop in Rongcheng. ...
... In general, Dabie-Sulu M-type eclogites occur as thin bands or layers within marbles, and show syntectonic deformation textures with the hosted marble (Wu et al., 2006a;Liu et al., 2007;Zhu et al., 2009 (Rumble et al., 2000;Chu et al., 2003;Zheng et al., 2003;Wu et al., 2006a;this study). These observations indicate that their protoliths were volcano-sedimentary interlayers. ...
... These observations indicate that their protoliths were volcano-sedimentary interlayers. Because these M-type eclogites show mafic composition, their protoliths are probably basaltic tuff (Wu et al., 2006a;Zhu et al., 2009). Their deposition may be synchronous with the protolith of marbles (sedimentary carbonates), and then they underwent low-T waterrock interaction during diagenesis to acquire the high δ 18 O values. ...
Marble-hosted eclogite is volumetrically minor in collisional orogens, but its
geochemistry has great bearing on the origin of deeply subducted crustal rocks and the fluid mobility of subduction zones. This paper presents a combined study of whole-rock major-trace elements and Sr-Nd isotopes, mineral O isotopes, carbonate C and O isotopes, and zircon U-Pb ages and Lu-Hf isotopes for marble-hosted ultrahigh-pressure metamorphic eclogites from Rongcheng and Sanqingge in the Sulu orogen. The results provide insights into the protolith nature of eclogites and the fluid mobility of subduction zones. Zircon U-Pb dating yields consistent middle Triassic ages for the two occurrences of eclogites, indicating new growth of metamorphic zircon during continental collision. The Sanqingge eclogite shows LREE-enriched patterns and negative εNd(t) of -16.6 to -14.3 for whole-rock and negative εHf(t) of -27.1 to -15.2 for metamorphic zircon. A few relict zircon domains show middle Neoproterozoic U-Pb ages and negative εHf(t) of -35.2 to -15.5. Thus, the Sangqingge eclogite was metamorphosed from a mafic rock that was derived from partial melting of an anciently enriched mantle source. In contrast, the Rongcheng eclogite exhibits flat or even LREE-depleted patterns with negative εNd(t) values of -12.2 to -1.0 for whole-rock but positive εHf(t) values of 5.4 to 10.4 for zircon. The occurrence of interstitial and highly cuspate plagioclase along grain boundaries indicates the presence of partial melting in the eclogite. Thus, its positive zircon εHf(t) values are ascribed to the eclogite protolith of juvenile origin, whereas the LREE depletion is due to extraction of LREE-rich anatectic melt from the eclogite during the Triassic continental collision. As such, the Rongcheng eclogite was metamorphosed from a mafic rock that was derived from partial melting of a less enriched mantle source. All the eclogites from both areas show variably high d18O values of 9.4‰ to 19.5‰. Oxygen isotope fractionations between mineral pairs mostly yield eclogite-facies temperature of 600 to 800 oC, suggesting that the high d18O signature was inherited from their protoliths before the Triassic subduction. In combination with the field relation between the eclogite and marble, it is inferred that the eclogite protolith is probably basaltic tuff and its high d18O value would be acquired together with the marble protolith during their deposition from the surface water. Therefore, there would be the limited isotopic exchange between marble and eclogite during continental collision.
... However, our continuous work on the Ganjialing marbles has only identified dolomite and aragonite within different metamorphic zircon domains (unpublished data), which at least attests to the rare occurrence of magnesite in the host marbles. More serious explanations come from the petrogenetic researches of Proyer et al. (2013), Zhu (2005) and Zhu et al. (2009) on the Sanqingge magnesite-bearing marbles. If one reviews their petrological descriptions, it can be easily found that (1) magnesite and aragonite could be hardly demonstrated to be ever in textural equilibrium, although both phases have been identified in these marbles; (2) these marbles show a much higher magnesite abundance than those in our study; and (3) the common occurrence of magnesite comes along with that of talc. ...
Impure marbles from ultra-high pressure (UHP) metamorphic belts bear significant information on the metamorphic evolution and carbon cycling during continental subduction and exhumation. In this study, detailed petrological data are presented and a P-T-X(CO2) path is constructed for the impure marbles from the Dabie UHP terrane. Coesite relicts are discovered as inclusions within dolomite from the selected samples, which have a peak assemblage of dolomite, aragonite, garnet, omphacite, phengite, coesite, allanite and rutile. Estimated with the compositions of peak minerals, a P-T condition of 4.05-4.45 GPa at 740-820 °C is obtained by conventional geothermobarometry. The modeled fluid compositions have a low X(CO2) (0.01-0.02) at the peak conditions, while the X(CO2) firstly increased during isothermal exhumation and then decreased at later retrogression. The discovery of coesite within dolomite underscores the role of the “pressure vessel” models and highlights the significance of fluid unavailability in preserving coesite in UHP rocks. Neither petrological evidence nor independent peak P-T estimations support the breakdown of dolomite in the studied marbles, which contests recent suggestions. Analysis on the phase relations in the CaO-MgO-SiO2-H2O-CO2 system shows that the bulk rock compositions have a large control on the stable UHP carbonate associations in carbonate-bearing rocks. The low X(CO2) in the peak fluids indicates a weak decarbonation of the impure marbles under sub-arcs. In the last, a large fraction of CO2 is shown to be sequestrated during regional retrogression of clinopyroxene marbles, which has a profound influence and must be considered for the global carbon cycling.
... In addition, lithologies related to sedimentary protoliths, that experienced UHP metamorphism, were reported such as marble (e.g. Zhu et al., 2009) and metamorphosed clastic sediments (e.g. Negulescu et al., 2009;Zhang et al., 2009). ...
Near the coastal village of As Sifah, NE Oman, eclogite-facies rocks
occur in the Saih Hatat window. We investigated a metapelite from this
area, which is composed of mm-sized garnet and greenish phengite and
minor epidote, blue amphibole, paragonite, albite, quartz, rutile,
opaque phases, barite, and carbonate. Garnet exhibits a chemical
zonation with
Gro17Alm66Pyr6Spe11,
Gro22Alm72.5Pyr5Spe0.5, and
Gro25Alm65Pyr8Spe2 as inner
core, mantle and outermost rim compositions. Inner portions of phengite
have Si contents of up to 3.6 per formula unit (pfu), whereas rims are
poorer in Si (3.2–3.4 pfu). We constructed a P-T pseudosection in
the system
Na2O-K2O-CaO-FeO-O2-MnO-MgO-Al2O3-SiO2-TiO2-H2O
for the bulk-rock composition of the studied metapelite and contoured it
by isopleths of various parameters such as the molar fractions of garnet
components. Based on this contouring a P-T path was derived that starts
at ultrahigh-pressure conditions. Garnet began to form at 25 kbar and
490 °C. Subsequently, temperatures increased and pressures decreased
to finally reach P-T conditions of 13 kbar and 565 °C at which
low-Si phengite, the outermost rim of garnet, Na-amphibole, epidote,
quartz, magnetite, and rutile were in equilibrium. The P-T path is
related to events in a subduction channel where the top of subducted
oceanic crust, including the studied metasediments, was involved in an
upwards-directed mass flow, resulting in the release of about 3 wt.%
H2O by garnet formation from hydrous minerals such as
chlorite and lawsonite. In order to get hints at the interaction of such
hydrous fluids, we have analyzed the trace and minor elements in
phengite. The contents of B, Rb, Cs, and Tl (20, 397, 6.7 and 1.7 ppm,
respectively) are nearly constant over the entire Si range of potassic
white mica. In contrast, the contents of Ba and Sr increase from 900 and
0.5 to 10500 and 14 ppm, respectively, with decreasing Si content in
phengite. We hypothesize that this result reflects early leaching of
mobile elements during subduction and the later approach of the rock to
a barite deposit within a melange ascending in the subduction channel.
