Hydration of eclogite, Pam Peninsula, New Caledonia
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Abstract:
Garnet glaucophanite and greenschist facies assemblages were formed by the recrystallization of barroisite‐bearing eclogite facies metabasites in northern New Caledonia. The mineralogical evolution can be modelled by calculated P–T and P–X H2O diagrams for appropriate bulk rock compositions in the model system CaO–Na 2 O–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O. The eclogites, having developed in a clockwise P–T path that reached P ≈19 kbar and T ≈590 °C, underwent decompression with the consumption of free H 2 O as the volume of hydrous minerals increased. Eclogite is preserved in domains that experienced no fluid influx following the loss of this fluid. Garnet glaucophanite formed at P ≈16 kbar during semi‐pervasive fluid influx. Fluid influx, after further isothermal decompression, was focused in shear zones, and resulted in chlorite–albite‐bearing greenschist facies mineral assemblages that reflect P ≈9 kbar.Keywords:
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Omphacite
Blueschist
Recrystallization (geology)
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The eclogite from central Qiangtang is located in the central part of Longmucuo–Shuanghu suture zone in Pianshishan area of Gumu Town, Gerze County. Major minerals of eclogite include garnet, omphacite, phengite, rutile and amphibole, and their sorrounding rocks are garnet phengite schist and blueschist. Garnet phengite schist is mainly composed by garnet, phengite and quartz, and blueschist is mainly composed by garnet, amphibolite and phengite. Petrology and mineralogy data suggest that the eclogite has experienced three stages of metamorphism. Stage Ⅰ: Peak eclogite metamorphism featured by Grt + Omp + Phen + Rt, the p-T conditions of the eclogite are 500 °C and ~ 2.3 GPa. Stage Ⅱ: Epidote-amphibolite facies metamorphism is characterized by assemblages of Bar + Ab + Ep of later stage alteration on the early eclogite minerals. Stage Ⅲ: Green schist facies metamorphism is featured by fibrous actinolites. The evolution of eclogite is an evidence of the subduction of Paleo-Tethys and collision between Gondwana and Laurasia in the Northern Tibetan plateau.
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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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Lawsonite
Blueschist
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Glaucophane
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Abstract Lawsonite eclogite (metabasalt and metadolerite) and associated metasedimentary rocks in a serpentinite mélange from an area just south of the Motagua fault zone (SMFZ), Guatemala, represent excellent natural records of the forearc slab–mantle interface. Pseudosection modelling of pristine lawsonite eclogite reproduces the observed predominant mineral assemblages, and garnet compositional isopleths intersect within the phase fields, yielding a prograde P – T path that evolves from 20 kbar, 470 °C (M1) to 25 kbar, 520 °C (M2). The dominant penetrative foliation within the eclogite blocks is defined by minerals developed during the prograde evolution, and the associated deformation, therefore, took place during subduction. Thermometry using Raman spectra of carbonaceous material in metasedimentary rocks associated with the SMFZ eclogites gives estimates of peak‐ T of ∼520 °C. Barometry using Raman spectroscopy shows unfractured quartz inclusions in garnet rims retain overpressures of up to ∼10 kbar, implying these inclusions were trapped at conditions just below the quartz/coesite transition, in agreement with the results of phase equilibrium analysis. Additional growth of Ca‐rich garnet indicates initial isothermal decompression to 20 kbar (M3) followed by hydration and substantial cooling to the lawsonite–blueschist facies (M4). Further decompression of the hydrated eclogite blocks to the pumpellyite–actinolite facies (3–5 kbar, 230–250 °C) is associated with dehydration and veining (M5). The presence of eclogite as m‐ to 10 m‐sized blocks in a serpentinite matrix, lack of widespread deformation developed during exhumation and derived prograde P – T path associated with substantial dehydration of metabasites within the antigorite stability field suggest that the SMFZ eclogites represent the uppermost part of the forearc slab crust sampled by an ascending serpentinite diapir in an active, moderate‐ T subduction zone.
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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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Hornblende
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Lawsonite is an abundant hydrous mineral in oceanic crust, sediments, and metasomatic rocks at depths of ~45 to 300 km in most subduction zones, but it is rarely preserved in the geologic record because it commonly transforms to epidote and other minerals during prograde or retrograde metamorphism. Owing to the significance of lawsonite for water and element cycling in subduction zones, occurrences of fresh lawsonite in blueschist and, more rarely, eclogite provide important opportunities to determine lawsonite composition, zoning, and inclusion suites and to use this information to reconstruct reaction history during subduction and exhumation. In this review, we use new and published data to document lawsonite composition in eight of the nine known lawsonite eclogite localities in which fresh lawsonite coexists with garnet + omphacite in the rock matrix, as well as the composition of lawsonite inclusions in six of seven known sites in which lawsonite occurs only as inclusions in garnet in eclogite-facies rocks that lack matrix lawsonite. As lawsonite blueschist is much more common than lawsonite eclogite, we survey the composition of lawsonite in representative localities of blueschist, including blueschist associated with eclogite (lawsonite-bearing, epidote-bearing), and blueschist not associated with eclogite at current exposure levels. Included in this review are metabasaltic rocks, silica- and carbonate-rich metasedimentary rocks, metasomatic rocks, and lawsonite-rich veins. This dataset demonstrates that lawsonite composition is a sensitive indicator of reaction history during subduction and exhumation, and specifically of fluid–rock interaction, with implications for element cycling in subduction zones. Furthermore, most exhumed lawsonite eclogite records slab-surface conditions that correspond to the location where the slab-mantle interface transitions from decoupled to coupled, and therefore provides key insights into the thermal history and dynamics of subduction zones.
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Blueschist
Omphacite
Phengite
Glaucophane
Metasomatism
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Blueschist
Omphacite
Phengite
Protolith
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