Phase Equilibria of Hornblende‐Bearing Eclogite in the Western Dabie Mountain, Central China
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Abstract: The high‐pressure (HP) eclogite in the western Dabie Mountain encloses numerous hornblendes, mostly barroisite. Opinions on the peak metamorphic P‐T condition, PT path and mineral paragenesis of it are still in dispute. Generally, HP eclogite involves garnet, omphacite, hornblendes and quartz, with or without glaucophane, zoisite and phengite. The garnet has compositional zoning with X Mg increase, X Ca and X Mn decrease from core to rim, which indicates a progressive metamorphism. The phase equilibria of the HP eclogite modeled by the P‐T pseudosection method developed recently showed the following: (1) the growth zonation of garnet records a progressive metamorphic PT path from pre‐peak condition of 1.9–2.1 GPa at 508°C‐514°C to a peak one of 2.3–2.5 GPa at 528°C‐531°C for the HP eclogite; (2) the peak mineral assemblage is garnet+omphacite+glaucophane+quartz±phengite, likely paragenetic with lawsonite; (3) the extensive hornblendes derive mainly from glaucophane, partial omphacite and even a little garnet due to the decompression with some heating during the post‐peak stage, mostly representing the conditions of about 1.4–1.6 GPa and 580°C‐640°C, and their growth is favored by the dehydration of lawsonite into zoisite or epidote, but most of the garnet, omphacite or phengite in the HP eclogite still preserve their compositions at peak condition, and they are not obviously equilibrious with the hornblendes.Keywords:
Omphacite
Glaucophane
Phengite
Lawsonite
Hornblende
Paragenesis
Phase relations of basic rocks under high pressure (HP) and ultrahigh pressure (UHP) metamorphic conditions are modelled on the basis of a MORB composition. The calculated pseudosections predict that basic rocks will contain glaucophane, garnet, omphacite, lawsonite, phengite, quartz with or without talc under HP-lawsonite eclogite subfacies conditions (1.8-2.8 GPa, 500-600°C). In these assemblages, the pyrope content (Xpy) in garnet mainly increases with temperature rising, the grossular content (Xgr) chiefly decreases with pressure rising, and the silica content (Si-) in phengite increases linearly with increasing pressure; their contents are subtly affected by variations in bulk-rock composition. Thus, the isopleths of garnet and phengite compositions in P-T pseudosections potentially present a robust geothermobarometric method for natural glaucophane-bearing HP eclogites. Under low-T UHP conditions (>2.8 GPa, 550-650°C), a common assemblage for basic rocks is predicted to be garnet + omphacite + lawsonite + phengite + talc + coesite + phengite. In this assemblage, the Xpy steadily increases as temperature rises and the Si-phengite increases with pressure rising, whereas the Xgr is very sensitive as pressure changes. The peak P-T conditions for low-T UHP eclogites can be determined using the isopleths of maximum Xpy and Si-phengite in P-T pseudosections. Under medium-T UHP conditions (>2.8 GPa and >650°C), basic rocks are predicted commonly to contain garnet + omphacite + lawsonite + phengite + coesite. In this assemblage, the Xpy mostly depends on bulk-rock compositions, whereas the Xgr and Si-phengite regularly increase, respectively, as temperature and as pressure rises, and thus, can provide good thermobarometric constraints for medium-T UHP eclogites. The decompression of these HP and UHP assemblages are modelled to be dominated by lawsonite dehydration reactions, which will result in disappearance of lawsonite and for- mation of glaucophane, epidote and/or kyanite with releasing a large amount of bound fluid.
Phengite
Omphacite
Lawsonite
Coesite
Grossular
Glaucophane
Pyrope
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Abstract The Sivrihisar Massif, Turkey, is comprised of blueschist and eclogite facies metasedimentary and metabasaltic rocks. Abundant metre‐ to centimetre‐scale eclogite pods occur in blueschist facies metabasalt, marble and quartz‐rich rocks. Sivrihisar eclogite contains omphacite + garnet + phengite + rutile ± glaucophane ± quartz + lawsonite and/or epidote. Blueschists contain sodic amphibole + garnet + phengite + lawsonite and/or epidote ± omphacite ± quartz. Sivrihisar eclogite and blueschist have similar bulk composition, equivalent to NMORB, but record different P–T conditions: ∼26 kbar, 500 °C (lawsonite eclogite); 18 kbar, 600 °C (epidote eclogite); 12 kbar, 380 °C (lawsonite blueschist); and 15–16 kbar, 480–500 °C (lawsonite‐epidote blueschist). Pressures for the Sivrihisar lawsonite eclogite are among the highest reported for this rock type, which is rarely exposed at the Earth's surface. The distribution and textures of lawsonite ± epidote define P–T conditions and paths. For example, in some lawsonite‐bearing rocks, epidote inclusions in garnet and partial replacement of matrix epidote by lawsonite suggest an anticlockwise P–T path. Other rocks contain no epidote as inclusions or as a matrix phase, and were metamorphosed entirely within the lawsonite stability field. Results of the P–T study and mapping of the distribution of blueschists and eclogites in the massif suggest that rocks recording different maximum P–T conditions were tectonically juxtaposed as kilometre‐scale slices and associated high‐ P pods, although all shared the same exhumation path from ∼9–11 kbar, 300–400 °C. Within the tectonic slices, alternating millimetre–centimetre‐scale layers of eclogite and blueschist formed together at the same P–T conditions but represent different extents of prograde reaction controlled by strain partitioning or local variations in f O 2 or other chemical factors.
Lawsonite
Blueschist
Omphacite
Phengite
Glaucophane
Allochthon
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Abstract Phase relations of basic rocks under high pressure ( HP ) and ultrahigh pressure ( UHP ) metamorphic conditions are modelled on the basis of a MORB composition. The calculated pseudosections predict that basic rocks will contain glaucophane, garnet, omphacite, lawsonite, phengite, quartz with or without talc under HP ‐lawsonite eclogite subfacies conditions (1.8–2.8 GPa , 500–600°C). In these assemblages, the pyrope content ( X py ) in garnet mainly increases with temperature rising, the grossular content ( X gr ) chiefly decreases with pressure rising, and the silica content ( S i‐) in phengite increases linearly with increasing pressure; their contents are subtly affected by variations in bulk‐rock composition. Thus, the isopleths of garnet and phengite compositions in P – T pseudosections potentially present a robust geothermobarometric method for natural glaucophane‐bearing HP eclogites. Under low‐ T UHP conditions (>2.8 GPa , 550 – 650°C), a common assemblage for basic rocks is predicted to be garnet + omphacite + lawsonite + phengite + talc + coesite + phengite. In this assemblage, the X py steadily increases as temperature rises and the S i‐phengite increases with pressure rising, whereas the X gr is very sensitive as pressure changes. The peak P – T conditions for low‐ T UHP eclogites can be determined using the isopleths of maximum X py and S i‐phengite in P – T pseudosections. Under medium‐ T UHP conditions (>2.8 GPa and >650°C), basic rocks are predicted commonly to contain garnet + omphacite + lawsonite + phengite + coesite. In this assemblage, the X py mostly depends on bulk‐rock compositions, whereas the X gr and S i‐phengite regularly increase, respectively, as temperature and as pressure rises, and thus, can provide good thermobarometric constraints for medium‐ T UHP eclogites. The decompression of these HP and UHP assemblages are modelled to be dominated by lawsonite dehydration reactions, which will result in disappearance of lawsonite and formation of glaucophane, epidote and/or kyanite with releasing a large amount of bound fluid.
Phengite
Omphacite
Lawsonite
Coesite
Grossular
Pyrope
Glaucophane
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High‐pressure metamorphic rocks exposed in the Bantimala area, c . 40 km north‐east of Ujung Pandang, were formed as a Cretaceous subduction complex with fault‐bounded slices of melange, chert, basalt, turbidite, shallow‐marine sedimentary rocks and ultrabasic rocks. Eclogites, garnet–glaucophane rocks and schists of the Bantimala complex have estimated peak temperatures of T =580–630 °C at 18 kbar and T =590–640 °C at 24 kbar, using the garnet–clinopyroxene geothermometer. The garnet–omphacite–phengite equilibrium is used to estimate pressures. The distribution coefficient K D1 =[( X pyr ) 3 ( X grs ) 6 /( X di ) 6 ]/[(Al/Mg) M2,wm (Al/Si) T2,wm ] 3 among omphacite, garnet and phengite is a good index for metamorphic pressures. The K D1 values of the Bantimala eclogites were compared with those of eclogites with reliable P–T estimates. This comparison suggests that peak pressures of the Bantimala eclogites were P =18–24 kbar at T =580–640 °C. These results are consistent with the P–T range calculated using garnet–rutile–epidote–quartz and lawsonite–omphacite–glaucophane–epidote equilibria.
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Omphacite
Phengite
Lawsonite
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Pseudosections calculated with thermocalc predict that lawsonite-bearing assemblages, including lawsonite eclogite, will be common for subducted oceanic crust that experiences cool, fluid-saturated conditions. For glaucophane–lawsonite eclogite facies conditions (500–600 °C and 18–28 kbar), MORB compositions are predicted in the NCKMnFMASHO system to contain glaucophane, garnet, omphacite, lawsonite, phengite and quartz, with chlorite at lower temperature and talc at higher temperature. In these assemblages, the pyrope content in garnet is mostly controlled by variations in temperature, and grossular content is strongly controlled by pressure. The silica content in phengite increases linearly with pressure. As the P–T conditions for these given isopleths are only subtly affected by common variations in bulk-rock compositions, the P–T pseudosections potentially present a robust geothermobarometric method for natural glaucophane-bearing eclogites. Thermobarometric results recovered both by isopleth and conventional approaches indicate that most natural glaucophane–lawsonite eclogites (Type-L) and glaucophane–epidote eclogites (Type-E) record similar peak P–T conditions within the lawsonite stability field. Decompression from conditions appropriate for lawsonite stability should result in epidote-bearing assemblages through dehydration reactions controlled by lawsonite + omphacite = glaucophane + epidote + H2O. Lawsonite and omphacite breakdown will be accompanied by the release of a large amount of bound fluid, such that eclogite assemblages are variably recrystallized to glaucophane-rich blueschist. Calculated pseudosections indicate that eclogite assemblages form most readily in Ca-rich rocks and blueschist assemblages most readily in Ca-poor rocks. This distinction in bulk-rock composition can account for the co-existence of low-T eclogite and blueschist in high-pressure terranes.
Lawsonite
Glaucophane
Omphacite
Blueschist
Phengite
Grossular
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Blueschist
Glaucophane
Omphacite
Phengite
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Abstract: The high‐pressure (HP) eclogite in the western Dabie Mountain encloses numerous hornblendes, mostly barroisite. Opinions on the peak metamorphic P‐T condition, PT path and mineral paragenesis of it are still in dispute. Generally, HP eclogite involves garnet, omphacite, hornblendes and quartz, with or without glaucophane, zoisite and phengite. The garnet has compositional zoning with X Mg increase, X Ca and X Mn decrease from core to rim, which indicates a progressive metamorphism. The phase equilibria of the HP eclogite modeled by the P‐T pseudosection method developed recently showed the following: (1) the growth zonation of garnet records a progressive metamorphic PT path from pre‐peak condition of 1.9–2.1 GPa at 508°C‐514°C to a peak one of 2.3–2.5 GPa at 528°C‐531°C for the HP eclogite; (2) the peak mineral assemblage is garnet+omphacite+glaucophane+quartz±phengite, likely paragenetic with lawsonite; (3) the extensive hornblendes derive mainly from glaucophane, partial omphacite and even a little garnet due to the decompression with some heating during the post‐peak stage, mostly representing the conditions of about 1.4–1.6 GPa and 580°C‐640°C, and their growth is favored by the dehydration of lawsonite into zoisite or epidote, but most of the garnet, omphacite or phengite in the HP eclogite still preserve their compositions at peak condition, and they are not obviously equilibrious with the hornblendes.
Omphacite
Glaucophane
Phengite
Lawsonite
Hornblende
Paragenesis
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从核心减少到边界，它显示进步变态。最近开发的磅假节方法建模的 HP eclogite 的阶段 equilibria 显示出下列：(1 ) 石榴石的生长带状配列记录一条进步的变形的磅路径从预先达到顶点在到一座山峰的 508 ° C -514 ° C 的 1.9 2.1 GPa 的状况在为 HP eclogite 的 528 ° C -531 ° C 的 2.3 2.5 GPa 之一；(2 ) 山峰矿物质集合是 garnet+omphacite+glaucophane+quartz ± p hengite，与 lawsonite 基因的可能的帕拉；( 3 )广泛的角闪石由于解压缩主要源于 glaucophane ，部分 omphacite 甚至一点石榴石，一些在山峰以后的阶段期间加热，主要代表大约 1.4 1.6 GPa 和 580 °C -640°C,和他们的生长的条件被 lawsonite 的脱水赞成进黝帘石或绿帘石，但是大多数在 HP eclogite 的石榴石， omphacite 或 phengite 仍然以山峰状况保存他们的作文，并且他们不显然是有角闪石的 equilibrious 。
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Abstract Coexisting garnet blueschist and eclogite from the Chinese South Tianshan high‐pressure ( HP )–ultrahigh‐pressure ( UHP ) belt consist of similar mineral assemblages involving garnet, omphacite, glaucophane, epidote, phengite, rutile/sphene, quartz and hornblendic amphibole with or without paragonite. Eclogite assemblages generally contain omphacite >50 vol.% and a small amount of glaucophane (<5 vol.%), whereas blueschist assemblages have glaucophane over 30 vol.% with a small amount of omphacite which is even absent in the matrix. The coexisting blueschist and eclogite show dramatic differences in the bulk‐rock compositions with higher X (CaO) [=CaO/(CaO + MgO + FeO total + MnO + Na 2 O)] (0.33–0.48) and lower A/ CNK [=Al 2 O 3 /(CaO + Na 2 O + K 2 O)] (0.35–0.56) in eclogite, but with lower X (CaO) (0.09–0.30) and higher A/ CNK (0.65–1.28) in garnet blueschist. Garnet in both types of rocks has similar compositions and exhibits core–rim zoning with increasing grossular and pyrope contents. Petrographic observations and phase equilibria modelling with pseudosections calculated using thermocalc in the NCKM n FMASHO system for the coexisting garnet blueschist and eclogite samples suggest that the two rock types share similar P–T evolutional histories involving a decompression with heating from the P max to the T max stage and a post‐ T max decompression with slightly cooling stage, and similar P–T conditions at the T max stage. The post‐ T max decompression is responsible for lawsonite decomposition, which results in epidote growth, glaucophane increase and omphacite decrease in the blueschist, or in an overprinting of the eclogitic assemblage by a blueschist assemblage. Calculated P–X (CaO), P –A/ CNK and P–X ( CO 2 ) pseudosections indicate that blueschist assemblages are favoured in rocks with lower X (CaO) (<0.28) and higher A/ CNK (>0.75) or fluid composition with higher X ( CO 2 ) (>0.15), but eclogite assemblages preferentially occur in rocks with higher X (CaO) and lower A/ CNK or fluid composition with lower X ( CO 2 ). Moreover, phase modelling suggests that the coexistence of blueschist and eclogite depends substantially on P–T conditions, which would commonly occur in medium temperatures of 500–590 °C under pressures of ~17–22 kbar. The modelling results are in good accordance with the measured bulk‐rock compositions and modelled temperature results of the coexisting garnet blueschist and eclogite from the South Tianshan HP – UHP belt.
Omphacite
Glaucophane
Blueschist
Lawsonite
Phengite
Amphibole
Pigeonite
Pyrope
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