Quartz exsolution in clinopyroxene is not proof of ultrahigh pressures: Evidence from eclogites from the Eastern Blue Ridge, Southern Appalachians, U.S.A.
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Abstract:
Oriented quartz needles in clinopyroxene have become one of the diagnostic indicators of ultrahighpressure (UHP) metamorphism. The presence of apparently exsolved quartz is taken as evidence of decompression of a non-stochiometric Ca.Eskola component (Ca0.5⃞0.5AlSi2O6, CaEs) that is presumed to be stable only at UHP conditions. Eclogite from the Eastern Blue Ridge, North Carolina, contains clinopyroxene (Jd20CaTs5Ac5CaEs0Di65Hd5) with oriented needles of quartz and calcic amphibole that appear to have exsolved together. The quartz + amphibole intergrowths are surrounded by 1.5 µm haloes of neoformed pyroxene (Jd10CaTs10Ac5CaEs0Di70Hd5). The modes of quartz, amphibole, and clinopyroxene haloes were determined using BSE images, and reintegrated with the host clinopyroxene. Viewing the quartz and amphibole needles down the c-axis of the pyroxene host provides a better estimate of their proportions than in prismatic sections. Reintegrated pyroxene compositions were nearly identical to the analyzed host pyroxene with no CaEs component. Clinopyroxene with CaEs solid solution has been repeatedly synthesized at UHP conditions. However, examination of the phase equilibria usually cited as evidence for CaEs stability at conditions of ≥25 kbar shows that clinopyroxene with 10 mol% CaEs is stable well within the quartz field, and provides a pressure minimum similar to the albite = jadeite + quartz barometer. Exsolution of quartz and associated amphibole is commonplace in clinopyroxene from the Blue Ridge eclogite that lacks coesite or other evidence for UHP metamorphism. The presence of a diluted (5.10%) CaEs component in clinopyroxene does not require UHP conditions.Keywords:
Pyroxene
Amphibole
Omphacite
Coesite
Oriented quartz needles in clinopyroxene have become one of the diagnostic indicators of ultrahighpressure (UHP) metamorphism. The presence of apparently exsolved quartz is taken as evidence of decompression of a non-stochiometric Ca.Eskola component (Ca0.5⃞0.5AlSi2O6, CaEs) that is presumed to be stable only at UHP conditions. Eclogite from the Eastern Blue Ridge, North Carolina, contains clinopyroxene (Jd20CaTs5Ac5CaEs0Di65Hd5) with oriented needles of quartz and calcic amphibole that appear to have exsolved together. The quartz + amphibole intergrowths are surrounded by 1.5 µm haloes of neoformed pyroxene (Jd10CaTs10Ac5CaEs0Di70Hd5). The modes of quartz, amphibole, and clinopyroxene haloes were determined using BSE images, and reintegrated with the host clinopyroxene. Viewing the quartz and amphibole needles down the c-axis of the pyroxene host provides a better estimate of their proportions than in prismatic sections. Reintegrated pyroxene compositions were nearly identical to the analyzed host pyroxene with no CaEs component. Clinopyroxene with CaEs solid solution has been repeatedly synthesized at UHP conditions. However, examination of the phase equilibria usually cited as evidence for CaEs stability at conditions of ≥25 kbar shows that clinopyroxene with 10 mol% CaEs is stable well within the quartz field, and provides a pressure minimum similar to the albite = jadeite + quartz barometer. Exsolution of quartz and associated amphibole is commonplace in clinopyroxene from the Blue Ridge eclogite that lacks coesite or other evidence for UHP metamorphism. The presence of a diluted (5.10%) CaEs component in clinopyroxene does not require UHP conditions.
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The drill site of the 5000 m deep main hole of the Chinese Continental Scientific Drilling Project (CCSD) is located in the southern Sulu ultrahigh-pressure (UHP) metamorphic belt. The 0 to 2000 m interval of the CCSD main hole consists of eclogite, gneiss, garnet peridotite and minor schist and quartzite. The eclogites with a cumulative thickness of about 1000 m have different mineral assemblages and model contents, as well as variable petrochemical compositions. They can be divided into five types:Si-rich quartz eclogite, Ti-rich rutile eclogite, Al-rich phengite and kyanite eclogite, Mg-rich two-mineral eclogite and normal eclogite. Protoliths of eclogites include layered mafic to ultramafic cumulates and supracrustal rocks. The whole-rock composition of eclogites has an apparent controlling effect on the contents of some components in garnet, omphacite and phengite and directly influences the P-T estimates of UHP metamorphism. The extensive development of diffusional compositional zoning in the eclogitic garnet, omphacite and phengite indicates that these UHP minerals were re-equilibrated during the early-stage retrograde metamorphism. This fact and the existence of garnet porphyroblasts with growth zoning suggest that the eclogites formed at higher temperature (940℃) and pressure (4.5GPa) than those estimated by previous workers.
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Abstract Nyböite occurs as porphyroblasts in the Jianchang eclogite in the Donghai area, northeastern Jiangsu Province, eastern China. The Jianchang eclogite contains some inclusions of quartz after coesite in clinopyroxene, garnet and epidote. It has colourless to pale-violet pleochroism. A thin rim with violet pleochroism often develops around nyb6ite and is taramitic. It is further retrogressed by the symplectite which is mainly composed of hornblende, aegirine-augite and albite. Nyböite is associated with jadeitic pyroxene in the Jianchang eclogite, although other porphyroblastic amphiboles in other Donghai eclogites are barroisitic to katophoritic and are associated with omphacite. Fe-Mg partitioning between garnet and clinopyroxene and the presence of coesite pseudomorphs indicate P-T conditions in the Jianchang eclogite of about 740 ± 60°C and more than 28 kbar. Similar P-T conditions were estimated for other porphyroblastic amphibole-bearing eclogites in the Donghai area. Nyböite can occur in the Na-Al-Fe-rich local bulk composition under the medium to high temperature and very high-pressure conditions. Retrograde rim amphibole is poorer in Na B , variable in Si content, and richer in Na A variable than the porphyroblastic amphibole in the Donghai area. This roughly implies a P-T path where P decreases without a large decrease of T.
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Eclogite from the Northern Dabie Mountain is a new finding by the authors. These eclogites in foliated perdotite are enveloped by banded gneiss and occur in the mafic-ultramafic rock belt. They are mainly composed of omphacite, garnet, diopside, orthopyroxene, amphibole, plagioclase and magnetite, and a small amount of rutile, spinel, olivin and 鏾rundum. The mineral association of peak metamorphism of the eclogite is omphacite+garnet+rutile. The existence of eclogite in the Northern Dabie Mountain implies that there was an eclogitic metamorphism prior to the granulitic facies one in the mafic-ultramafic rock belt.
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Coesite-bearing eclogite samples from the Chinese Continental Scientific Drilling main hole (100-2000 m) were examined. Three major evolutionary stages are distinguished from mineral assemblages and textural relations. (1) The pre-peak stage is indicated by inclusion assemblages in rutile eclogite such as amphibole and paragonite, with pre-peak P-T conditions of ~13 kbar and ~910°C. (2) The peak ultrahigh-pressure (UHP) metamorphic stage is characterized by the mineral assemblage garnet-omphacite-phengite-rutile-apatite-coesite/quartz pseudomorph ± kyanite; P-T conditions reached ~910°C and ~37 kbar for rutile eclogite, and ~850°C and ~35 kbar for phengite eclogite. (3) The retrograde stage produced decompression textures via back reactions, such as thin coronas of amphibole or zoisite on garnet, fine-grained amphibole-plagioclase symplectites on omphacite and biotite-plagioclase, or K-feldspar-albite on phengite, ilmenite or titanite on rutile, at P-T conditions of 530-560°C and 7-9 kbar. The presence of K-feldspar-albite symplectite on phengite and minor interstitial K-feldspar implies that post-peak decompressional partial melting occurred locally during rapid exhumation of the subducted UHP slab. Two alternative P-T paths may be constructed: (1) the first possesses a different pre-peak stage but similar peak UHP to post-peak decompression cooling for rutile and phengite eclogite; (2) the second shows a consistent evolution from a pre-peak stage via peak UHP stage to a post-peak stage of near-isothermal decompression, followed by near-isobaric cooling for both rutile and phengite eclogite. We prefer the first P-T path, because the inferred paths show a clear increase in pressure and temperature from near-peak to peak UHP stage, and this may be associated with subductionrelated tectonism as a result of continental collision between the North China and Yangtze blocks. Rapid subduction and fast retrograde exhumation is likely responsible for preservation of some pre-peak inclusion assemblages and prograde mineral growth zoning.
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