Two-stage prograde and retrograde Variscan metamorphism of glaucophane-eclogites, blueschists and greenschists from Ile de Groix (Brittany, France)
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Keywords:
Glaucophane
Blueschist
Amphibole
Summary The sodic amphiboles possess two independent chemical substitution series (Fe 3+ -Al and Fe 2+ -Mg) that combine to provide a ‘plane’ of compositions. Yet at no single T and P are compositions covering the whole plane stable: (i) pure riebeckite exists under low- P conditions but breaks down in normal blueschists to give deerite; (ii) ferro-glaucophane is in competition at all except the lowest blueschist temperatures with almandine garnet; (iii) magnesio-riebeckite is stable at high- T and low- P but within the blueschist facies is replaced by the alternative higher density aegirine-talc assemblage; and (iv) glaucophane is stable only at high- P . At higher T and P than those of the blueschists, competition from NaCa pyroxenes, garnets, and deerite first erodes, and then removes, nearly all sodic amphibole compositions. At low- P the normal sodic amphibole-forming reactions from stilpnomelane and chlorite (in the presence of iron oxides, albite, etc.) produce an initial ‘rie-beckitic’ amphibole that subsequently becomes more glaucophanitic with increasing P . Under certain conditions, perhaps connected with hydrous fluid overpressures, these reactions become transposed such that crossitic compositions become replaced whilst ferroglaucophane to glancophane compositions remain stable.
Blueschist
Amphibole
Glaucophane
Almandine
Lawsonite
Phengite
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This study reports the discovery of lawsonite, which was for the first time identified in the Qingshuigou high-grade blueschist belt in the North Qilian Mountains and was also discovered in accompanying eclogite. It occurs as enclaves in quartz grains. There are no euhedra crystals observed, with size ranging from 9.5 to 4 μm. Using internally consistent mineral thermodynamic detabase of Holland and Powell (1998), and Domino/Theriak calculation software by de Capitani and Brown (1987), we calculated metamorphic p-T conditions of the HP/LT metamorphic belt: 2.24~2.39 GPa and 495~519℃ for the lawsonite-bearing eclogite in Qingshuigou; 1.35~2.15 GPa and 530~600℃ for the epidote blueschistin Baijingsi; 2.20~2.35 GPa and 520~545℃ for the epidote blueschist in Qingshuigou; 1.92~2.08 GPa and T=544~576℃ for the lawsonite-bearing blueschist in Arou coal mine. Results show that the Qingshuigou high-degree blueschist experienced metamorphic transformation from lawsonite blueschist to epodite blueschist, and has the similar p-T condition to that of lawsonite-bearing eclogite, indicating that they were the product of high temperature and low pressure metamorphism triggered by the northward subduction of the ancient Qilian Mountain Ocean. The high quality laser 40Ar/39Ar dating of the epidote blueschist yields the metamorphic ages of 447±1.7~447±5 Ma and 453±2~454±2 Ma. The age of 392±12~400±3 Ma can be interpreted as retrograde metamorphism age for low temperature eclogite. Combined with the lithological, geochemical and isotopic data above, Early Paleozoic tectonic evolution process of North Qilian orogeny was further discussed.
Blueschist
Lawsonite
Glaucophane
Phengite
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Based on coesite-bearing eclogites from Shuanghe of Dabieshan terrane,pseudosections including P-T、T-M (H2O) and P-M (H2O) were calculated in the NCKMnFMASHO system.Phase equilibria modeling predicts that the mineral assemblages in water-saturated metabasic rocks evolve via dehydration in cold crustal subduction zones (geothermal gradient of about 6℃·km-1).With increasing P-T conditions,chlorite is replaced by talc at about 2.2GPa (80km),and then glaucophane,talc and lawsonite disappear at 2.4GPa,2.5GPa and 2.9GPa (105km),respectively.Accordingly,high-ultrahigh pressure (HP-UHP) mineral assemblages such as lawsonite-bearing blueschist,glaucophane-lawsonite eclogite,lawsonite eclogite and phengite eclogite are modeled to be presented.Phase equilibria modeling also suggests that even the subducted basic rocks contain quite small quantities of water (such as 0.3%~0.5%),they can also be water-saturated under UHP lawsonite eclogite facies.Thus,lawsonite will be widely present in the metabasic rocks which have experienced the cold subduction.The exhumation of HP-UHP eclogites is controlled by the temperature and the peak mineral assemblage.When the peak eclogitic assemblage contains lawsonite (±glaucophane±talc) at T=540~600℃),it will involve two stages of exhumation:The early-stage exhumation is modeled to proceed in the lawsonite stability field,controlled by dehydration reactions,and thus,it will be difficult to preserve the peak mineral assemblage.The late-stage exhumation proceeds in the epidote stability field where the rock is fluid-absent and the HP eclogitic assemblage after the lawsonite disappearance is prone to be preserved.When the peak mineral assemblage includes chlorite,lawsonite and glaucophane at T540℃,the decompression-driven dehydration reactions only occur in the narrow area with the coexistence of lawsonite and epidote,and will not be happened either in the early exhumation with lawsonite stability and the late exhumation with epidote stability,where the rocks are fluid-absent.Under such conditions,although lawsonite formed in the peak stage is hard to be preserved during decompression,the other peak minerals may only suffer slight modification.After the disappearance of lawsonite at T600℃,the HP-UHP phengite eclogites contain very small amounts of water,which is modeled to be unchanged in a large range of pressure during the early exhumation stage.This favors the preservation of peak mineral assemblage.When HP-UHP eclogites are decompressed to pressures below 1.5GPa,they will be partially hydrated duo to external fluid-infiltration,producing sodic-calc or calc amphibole-bearing eclogites,which are generally not in an equilibrium state.The amount of fluid released by the namely anhydrous minerals during eclogite decompression can cause the partial hydration of eclogite,but is not enough to form water-saturated amphibolite.
Lawsonite
Glaucophane
Blueschist
Phengite
Coesite
Omphacite
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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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Lawsonite
Blueschist
Glaucophane
Omphacite
Phengite
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Abstract The sodic amphibole glaucophane is generally considered as indicative of blueschist-facies metamorphism. However, sodic amphiboles display a large range in chemical compositions, owing principally to the Fe2+Mg–1 and Fe3+Al–1 substitutions. Therefore, the whole-rock composition (namely its Na2O and FeO* content, and the Fe2+–Fe3+ ratio), strongly controls the stability field of the sodic amphiboles at the transition from greenschist- to blueschist-facies conditions. Neglecting these variables can lead to erroneous estimates of the metamorphic conditions and consequently the tectonic framework of the rocks. This paper explores the mechanisms that control the development of sodic amphibole and sodic pyroxene within the basement of the Dent Blanche Tectonic System (Western Alps), as a result of the Alpine metamorphic history. Field, petrographic and geochemical data indicate that sodic amphibole and sodic pyroxene form in different rock types: (1) in undeformed pods of ultramafic cumulates (hornblendite), sodic amphibole (magnesioriebeckite) forms coronas around magmatic pargasite; (2) metatonalite displays patches of radiating sodic (magnesioriebeckite) and calcic (actinolite) amphiboles; (3) sodic amphibole (magnesioriebeckite–glaucophane) occurs with high-Si potassic white mica (phengitic muscovite) in fine-grained (blue) schists; (4) in mylonitized granitoids (amphibole-gneiss) metasomatized along the contact with ultramafic cumulates, sodic amphibole (magnesioriebeckite–winchite) mainly forms rosettes or sheaves, generally without a shape-preferred orientation. Only locally are the needles aligned parallel to the stretching lineation. Pale green aegirine–augite is dispersed in an albite–quartz matrix or forms layers of fine-grained fibrous aggregates. The bulk-rock chemical composition of the different lithologies indicates that sodic amphibole and sodic pyroxene developed in Na- and Fe-rich systems or in a system with high Fe3+/Fe*. Thermodynamic modelling performed for different rock types (taking into account the measured Fe2O3 contents) reveals that sodic amphibole appears at ∼8 ± 1 kbar and 400–450 °C (i.e. at the transition between the greenschist- and blueschist-facies conditions) about 5 kbar lower than previous estimates. To test the robustness of our conclusion, we performed a review of sodic amphibole compositions from a variety of terranes and P–T conditions. This shows (1) systematic variations of composition with P–T conditions and bulk-rock chemistry, and (2) that the amphibole compositions reported from the studied area are consistent with those reported from other greenschist- to blueschist-facies transitions.
Amphibole
Blueschist
Glaucophane
Greenschist
Pyroxene
Actinolite
Ultramafic rock
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Blueschist
Glaucophane
Omphacite
Seismic anisotropy
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Abstract The potential seismological contributions of metamorphosed and deformed oceanic crust in a subduction zone environment were studied in a detailed petro‐fabric analysis of blueschist and eclogite in the North Qilian suture zone, NW China. The calculated whole‐rock seismic properties based on the measured lattice preferred orientations of the constituting minerals show increasing P‐wave and S‐wave velocities and decreasing seismic anisotropies from blueschist to eclogite, mainly due to the decreasing volume proportion and deformation extent of glaucophane. The low velocity of the upper layer in the subducting oceanic crust can be explained by the existence of blueschist and foliated eclogite, which induces a 3–12% reduction in velocity compared to that induced by the surrounding mantle rocks. This low‐velocity layer may gradually disappear when blueschist and foliated eclogite are replaced by massive eclogite at a depth in excess of 60–75 km for the paleo North Qilian subduction zone. Trench‐parallel seismic anisotropy with a moderate delay time (0.1–0.3 s) can only effectively contribute to deformed blueschist and eclogite in a high‐angle (>45–60°) subducting slab, regardless of the direction of slab movement. The calculated reflection coefficients ( R c = 0.04–0.20) at the lithologic interfaces between eclogite and blueschist imply that it may be possible to detect eclogite bodies in shallow subduction channels using high‐resolution seismic reflection profiles. However, the imaging of eclogite bodies located in deep subduction zones could be challenging.
Blueschist
Glaucophane
Slab
Cataclastic rock
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Carbonates of ultrahigh-pressure metamorphic (UHPM) origin were discovered in eclogites from western Tianshan, China. In the eclogites, relict magnesite (XMg = 0.79) occurs as rounded to subidiomorphic inclusions (0.01-0.1 mm) within matrix dolomite, and also as rounded inclusions with thin reaction rims of dolomite in glaucophane. Based on the textural evidence and calculated phase relationship, these eclogites record an early UHPM assemblage overprinted by a blueschist assemblage, which can be explained by the reaction:
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Blueschist
Magnesite
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