Calculated phase equilibria for MORB compositions: a reappraisal of the metamorphic evolution of lawsonite eclogite
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Abstract:
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.Keywords:
Lawsonite
Glaucophane
Omphacite
Blueschist
Phengite
Grossular
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 Glaucophane‐bearing ultrahigh pressure (UHP) eclogites from the western Dabieshan terrane consist of garnet, omphacite, glaucophane, kyanite, epidote, phengite, quartz/coesite and rutile with or without talc and paragonite. Some garnet porphyroblasts exhibit a core–mantle zoning profile with slight increase in pyrope content and minor or slight decrease in grossular and a mantle–rim zoning profile characterized by a pronounced increase in pyrope and rapid decrease in grossular. Omphacite is usually zoned with a core–rim decrease in j(o) [=Na/(Ca + Na)]. Glaucophane occurs as porphyroblasts in some samples and contains inclusions of garnet, omphacite and epidote. Pseudosections calculated in the NCKMnFMASHO system for five representative samples, combined with petrographic observations suggest that the UHP eclogites record four stages of metamorphism. (i) The prograde stage, on the basis of modelling of garnet zoning and inclusions in garnet, involves P – T vectors dominated by heating with a slight increase in pressure, suggesting an early slow subduction process, and P – T vectors dominated by a pronounced increase in pressure and slight heating, pointing to a late fast subduction process. The prograde metamorphism is predominated by dehydration of glaucophane and, to a lesser extent, chlorite, epidote and paragonite, releasing ∼27 wt% water that was bound in the hydrous minerals. (ii) The peak stage is represented by garnet rim compositions with maximum pyrope and minimum grossular contents, and P – T conditions of 28.2–31.8 kbar and 605–613 °C, with the modelled peak‐stage mineral assemblage mostly involving garnet + omphacite + lawsonite + talc + phengite + coesite ± glaucophane ± kyanite. (iii) The early decompression stage is characterized by dehydration of lawsonite, releasing ∼70–90 wt% water bound in the peak mineral assemblages, which results in the growth of glaucophane, j(o) decrease in omphacite and formation of epidote. And, (iv) The late retrograde stage is characterized by the mineral assemblage of hornblendic amphibole + epidote + albite/oligoclase + quartz developed in the margins or strongly foliated domains of eclogite blocks due to fluid infiltration at P–T conditions of 5–10 kbar and 500–580 °C. The proposed metamorphic stages for the UHP eclogites are consistent with the petrological observations, but considerably different from those presented in the previous studies.
Omphacite
Glaucophane
Grossular
Coesite
Phengite
Pyrope
Lawsonite
Almandine
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Phengite
Omphacite
Coesite
Pyrope
Lawsonite
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Amphibole
Glaucophane
Almandine
Dalradian
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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.
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Omphacite
Lawsonite
Coesite
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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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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.
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Glaucophane
Omphacite
Blueschist
Phengite
Grossular
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Blueschist
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Abstract Low‐ T eclogites in the North Qilian orogen, NW China share a common assemblage of garnet, omphacite, glaucophane, epidote, phengite, quartz and rutile with or without paragonite. Phase relations for the low‐ T eclogites can be modelled well in the system NCKFMASHO with the updated solid‐solution models for amphibole and clinopyroxene. Garnet in the eclogite typically exhibits growth zonations in which pyrope increases while grossular somewhat decreases from core to rim, which is modelled as having formed mainly in the P – T conditions of lawsonite‐eclogite facies at the pre‐peak stage. Omphacite shows an increase in jadeite component as aegirine and also total FeO decrease in going from the inclusions in garnet to grains in the matrix, and from core to rim of zoned crystals, reflecting an increase in metamorphic P – T conditions. Glaucophane exhibits a compositional variation in X (gl) (= Fe 2+ /(Fe 2+ + Mg)) and F (gl) (= Fe 3+ /(Fe 3+ + Al) in M2 site), which decrease from the inclusions in garnet to crystals in the matrix, consistent with an increase in P – T conditions. However, for zoned matrix crystals, the X (gl) and F (gl) increase from core to rim, is interpreted to reflect a late‐stage decompression. Using composition isopleths for garnet rim and phengite in P – T pseudosections, peak P – T conditions for three samples Q5–45, Q5–01 and Q7–28 were estimated as 530–540 °C at 2.10–2.25 GPa, 580–590 °C at 2.30–2.45 GPa and 575–590 °C at 2.50–2.65 GPa, respectively, for the same assemblage garnet + omphacite + glaucophane + lawsonite (+ phengite + quartz + rutile) at the peak stage. The eclogites suggest similar P – T ranges to their surrounding felsic–pelitic schists. During post‐peak decompression of the eclogites, the most distinctive change involves the transformation of lawsonite to epidote, releasing large amount of water in the rock. The released fluid promoted further growth of glaucophane at the expense of omphacite and, in appropriate bulk‐rock compositions, paragonite formed. The decompression of eclogite did not lead to pronounced changes in garnet and phengite compositions. Peak P – T conditions of the North Qilian eclogite are well constrained using both the average P – T and pseudosection approaches in Thermocalc. Generally, the conventional garnet–clinopyroxene geothermometer is too sensitive to be used for constraining the temperature of low‐ T eclogite because of the uncertainty in Fe 3+ determination in omphacite and slight variations in mineral compositions because of incomplete equilibration.
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Abstract In contrast to low‐ T eclogites (garnet growth zoning preserved) or high‐ T eclogites (garnet diffusionally homogenized at peak conditions), medium‐temperature eclogites pose additional challenges to P–T determinations due to the partial preservation of garnet zoning. The Dulan area, in the southeastern part of North Qaidam ultrahigh‐pressure terrane, exposes minor eclogites hosted by ortho‐ and paragneisses. Four fresh, medium‐temperature eclogites contain the paragenesis Grt+Omp+Rt+Qz/Coe+Ph±Ky±Zo. In all samples, garnet X Mg shows little zoning, suggesting diffusional modification, and precludes the use of pyrope+almandine+grossular isopleth intersections to determine a P–T path. However, in one sample, sharp zoning in grossular content suggests that grossular growth compositions are preserved. Since garnet pyrope+almandine compositions appear to be modified, we instead use the intersections of grossular and garnet volume isopleths to define a prograde P–T path. This approach yields a path from ~17 kbar and ~410°C to ~35 kbar and ~625°C with a gradient of ~5–9°C/km through the lawsonite stability field. Peak P–T conditions determined from the intersection between Si pfu in phengite and Zr‐in‐rutile isopleths are ~26–33 kbar and ~625–700 °C for the four eclogites. These conditions are close to the limit of the lawsonite stability field, suggesting that fluid released from lawsonite breakdown may have promoted re‐equilibration at these conditions. These peak conditions are also in good agreement (within 3 kbar and 50°C) with garnet–omphacite–phengite (±kyanite) thermobarometry in three of the four samples. We regard the phengite–rutile constraints as more reliable, because they are less sensitive to uncertainties associated with ferric iron compared to conventional thermobarometry. Phase equilibrium modelling predicts that the retrograde assemblage of amphibole+zoisite formed at ~60 km. Infiltration of external fluids was likely required for the growth of these hydrous minerals. Based on the comparison of P–T estimation methods applied in this study, we propose that the garnet grossular+volume isopleths can recover the prograde P–T path of medium‐temperature eclogites, and that the combination of phengite+rutile isopleths represents a more robust approach to constrain peak P–T conditions.
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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.
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Phengite
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Hornblende
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