Remnants of premetamorphic fluid and oxygen isotopic signatures in eclogites and garnet clinopyroxenite from the Dabie-Sulu terranes, eastern China

A combined oxygen‐isotope and fluid‐inclusion study has been carried out on high‐ and ultrahigh‐pressure metamorphic (HP/UHPM) eclogites and garnet clinopyroxenite from the Dabie‐Sulu terranes in eastern China. Coesite‐bearing eclogites/garnet clinopyroxenite and quartz eclogites have a wide range in whole‐rock δ¹⁸O_VSMOW, from 0 to 11‰. The high‐T oxygen‐isotope fractionations preserved between quartz and garnet preclude significant retrograde isotope exchange during exhumation, and the wide range in whole‐rock oxygen‐isotope composition is thought to be a presubduction signature of the precursors. Aqueous fluids with variable salinities and gas species (N₂‐, CO₂‐, or CH₄‐rich), are trapped as primary inclusions in garnet, omphacite and epidote, and in quartz blebs enclosed within eclogitic minerals. In high‐δ¹⁸O HP/UHPM rocks from Hujialin and Shima, high‐salinity brine and/or N₂ inclusions occur in garnet porphyroblasts, which also contain inclusions of coesite, Cl‐rich blue amphibole and dolomite. In contrast, in low‐δ¹⁸O eclogites from Qinglongshan and Huangzhen, the Cl concentrations in amphibole are very low, < 0.2 wt.%, and low‐salinity aqueous inclusions occur in quartz inclusions in epidote porphyroblasts and in epidote cores. These low‐salinity fluid inclusions are believed to be remnants of meteoric water, although the fluid composition was modified during pre‐ and syn‐peak HP/UHPM. Eclogites at Houshuichegou and Hetang contain CH₄‐rich fluid inclusions, coexisting with high‐salinity brine inclusions. Methane was probably formed under the influence of CO₂‐rich aqueous fluids during serpentinisation of mantle‐derived peridotites prior to or during plate subduction. Remnants of premetamorphic low‐ to high‐salinity aqueous fluid with minor N₂ and/or other gas species preserved in the Dabie‐Sulu HP/UHPM eclogites and garnet clinopyroxenite indicate a great diversity of initial fluid composition in the precursors, implying very limited fluid–rock interaction during syn‐ and post‐peak HP/UHPM.     

Oxygen Isotopic Study on the Date‐Nagai Skarn‐Type Tungsten Deposit,Northeastern Japan

The skarn‐type tungsten deposit of the Date‐Nagai mine is genetically related to the granodiorite batholith of the Iidateyama body. Skarn is developed along the contact between pelitic hornfels and marble that remains as a small roof pendant body directly above the granodiorite batholith. Zonal arrangement of minerals is observed in skarn. The zonation consists of wollastonite, garnet, garnet‐epidote, and vesuvianite‐garnet zones, from marble to hornfels. Sheelite is included in garnet, garnet‐epidote, and vesuvianite‐garnet zones. The oxygen isotope values of skarn minerals were obtained as δ¹⁸O = 4.2–7.7‰ for garnet, 5.9–6.9‰ for vesuvianite, ?0.3–3.4‰ for scheelite, 6.0–10.9‰ for quartz, and 8.2‰ for muscovite. The temperature of skarn‐formation was calculated from oxygen isotopic values of scheelite‐quartz pairs to be 288°C. Calculated oxygen isotope values of fluid responsible for skarn minerals were 6.1–9.5‰ for garnet, 1.2–4.8‰ for scheelite, ?1.3‐3.6‰ for quartz, and 4.5‰ for muscovite. Garnet precipitated from the fluids of different δ¹⁸O values from scheelite, quartz, and muscovite. These δ¹⁸O values suggest that the origin of fluid responsible for garnet was magmatic water, while evidence for the presence of a meteoric component in the fluids responsible for middle to later stages minerals was confirmed.     

Petrography and geochemistry of eclogites from the Mir kimberlite,Yakutia, Russia

 Diamond-bearing eclogites are an important component of the xenoliths that occur in the Mir kimberlite, Siberian platform, Russia. We have studied 16 of these eclogite xenoliths, which are characterized by coarse-grained, equigranular garnet and omphacite. On the basis of compositional variations in garnet and clinopyroxene, this suite of eclogites can be divided into at least two groups: a high-Ca group and a low-Ca group. The high-Ca group consists of high-Ca garnets in equilibrium with pyroxenes that have high Ca-ratios [Ca/(Ca+Fe+Mg)] and high jadeite contents. These high-Ca group samples have high modal% garnet, and garnet grains often are zoned. Garnet patches along rims and along amphibole- and phlogopite-filled veins have higher Mg and lower Ca contents compared to homogeneous cores. The low-Ca group consists of eclogites with low-Ca garnets in equilibrium with pyroxenes with a low Ca-ratio, but variable jadeite contents. These low-Ca group samples typically have low modal% of garnet, and garnets are rarely compositionally zoned. Three samples have mineralogic compositions and modes transitional to the high- and low-Ca groups. We have arbitrarily designated these samples as the intermediate-Ca group. The rare-earth-element (REE) contents of garnet and clinopyroxene have been determined by ion microprobe. Garnets from the low-Ca group have low LREE contents and typically have [Dy/Yb]_n < 1. The high-Ca group garnets have higher LREE contents and typically have [Dy/Yb]_n > 1. Garnets from the intermediate-Ca group have REE contents between the high- and low-Ca groups. Clinopyroxenes from the low-Ca group have convex-upward REE patterns with relatively high REE contents (ten times chondrite), whereas those from the high-Ca group have similar convex-upward shapes, but lower REE contents, approximately chondritic. Reconstructed bulk-rock REE patterns for the low-Ca group eclogites are relatively flat at approximately ten times chondrite. In contrast, the high-Ca group samples typically have LREE-depleted patterns and lower REE contents. The δ¹⁸O values measured for garnet separates range from 7.2 to 3.1‰. Although there is a broad overlap of δ¹⁸O between the low-Ca and high-Ca groups, the low-Ca group samples range from mantle-like to high δ¹⁸O values (4.9 to 7.2‰), and the high-Ca group garnets range from mantle-like to low δ¹⁸O values (5.3 to 3.1‰). The oxygen isotopic compositions of two of the five high-Ca group samples and four of the eight low-Ca group eclogites are consistent with seawater alteration of basaltic crust, with the low-Ca group eclogites representative of low-temperature alteration, and the high-Ca group samples representative of high-temperature hydrothermal seawater alteration. We interpret the differences between the low- and high-Ca group samples to be primarily a result of differences in the protoliths of these samples. The high-Ca group eclogites are interpreted to have protoliths similar to the mid to lower sections of an ophiolite complex. This section of oceanic crust would be dominated by rocks which have a significant cumulate component and would have experienced high-temperature seawater alteration. Such cumulate rocks probably would be LREE-depleted, and can be Ca-rich because of plagioclase or clinopyroxene accumulation. The protoliths of the low-Ca group eclogites are interpreted to be the upper section of an ophiolite complex. This section of oceanic crust would consist mainly of extrusive basalts that would have been altered by seawater at low temperatures. These basaltic lavas would probably have relatively flat REE patterns, as seen for the low-Ca group eclogites. Received: 10 July 1995 / Accepted: 17 May 1996     

Channelized fluids in subducted continental crust: constraints from δD–δ18O of quartz and fluid inclusions in quartz veins from the Chinese Continental Scientific Drilling Project

Fluid inclusions hosted by quartz veins in high-pressure to ultrahigh-pressure (HP-UHP) metamorphic rocks from the Chinese Continental Scientific Drilling (CCSD) Project main drillhole have low, varied hydrogen isotopic compositions (δD?=??97‰ to??69‰). Quartz δ¹⁸O values range from??2.5‰ to 9.6‰; fluid inclusions hosted in quartz have correspondingly low δ¹⁸O values of??11.66‰ to 0.93‰ (T _h?=?171.2～318.8°C). The low δD and δ¹⁸O isotopic data indicate that protoliths of some CCSD HP-UHP metamorphic rocks reacted with meteoric water at high latitude near the surface before being subducted to great depth. In addition, the δ¹⁸O of the quartz veins and fluid inclusions vary greatly with the drillhole depth. Lower δ¹⁸O values occur at depths of ～900–1000 m and ～2700 m, whereas higher values characterize rocks at depths of about 1770 m and 4000 m, correlating roughly with those of wall-rock minerals. Given that the peak metamorphic temperature of the Dabie-Sulu UHP metamorphic rocks was about 800°C or higher, much higher than the closure temperature of oxygen isotopes in quartz under wet conditions, such synchronous variations can be explained by re-equilibration. In contrast, δD values of fluid inclusions show a different relationship with depth. This is probably because oxygen is a major element of both fluids and silicates and is much more abundant in the quartz veins and silicate minerals than is hydrogen. The oxygen isotope composition of fluid inclusions is evidently more susceptible to late-stage re-equilibration with silicate minerals than is the hydrogen isotope composition. Therefore, different δD and δ¹⁸O patterns imply that dramatic fluid migration occurred, whereas the co-variation of oxygen isotopes in fluid inclusions, quartz veins, and wall-rock minerals can be better interpreted by re-equilibration during exhumation.

Quartz veins in the Dabie-Sulu UHP metamorphic terrane are the product of high-Si fluids. Given that channelized fluid migration is much faster than pervasive flow, and that the veins formed through precipitation of quartz from high-Si fluids, the abundant veins indicate significant fluid mobilization and migration within this subducted continental slab. Many mineral reactions can produce high-Si fluids. For UHP metamorphic rocks, major dehydration during subduction occurred when pressure–temperature conditions exceeded the stability of lawsonite. In contrast, for low-temperature eclogites and other HP metamorphic rocks with peak metamorphic P–T conditions within the stability field of lawsonite, dehydration and associated high-Si fluid release may have occurred as hydrous minerals were destabilized at lower pressure during exhumation. Because subduction is a continuous process whereas only a minor fraction of the subducted slabs returns to the surface, dehydration during underflow is more prevalent than exhumation even in subducted continental crust, which is considerably drier than altered oceanic crust.     

Geochemistry of xenolithic eclogites from West Africa, part 2: origins of the high MgO eclogites

Oxygen isotope, mineral trace element, and measured and reconstructed whole-rock compositions are reported for the high MgO eclogite xenolith suite (16 to 20 wt% MgO in the whole rock) from the Koidu Kimberlite complex, Sierra Leone. In contrast to the previously published data for low MgO eclogites (6 to 13 wt% MgO) from this area, high MgO eclogites equilibrated at higher temperatures (1080 to 1130°C vs. 890 to 930°C) have only mantlelike δ¹⁸O and show variable degrees of light rare earth element (REE) enrichment. Analyses of multiple mineral generations suggest that the heterogeneous REE patterns of the high MgO eclogites reflect variable degrees of metasomatic overprinting. High MgO and Al₂O₃ contents of the eclogites suggest a cumulate origin, either as high-pressure (2 to 3 GPa) garnet-pyroxene cumulates or low-pressure (<1 GPa) plagioclase-pyroxene-olivine cumulates. Trace element modeling suggests a low-P origin for eclogites with flat heavy REE patterns and a high-P origin for eclogites with fractionated heavy REE. Flat heavy REE patterns, the presence of Sr anomalies, and low to moderate transition element contents in the low-P group are consistent with a low-pressure origin as metamorphosed olivine gabbros and troctolites. These metagabbroic high MgO eclogites either could represent the basal section of subducted oceanic crust or foundered mafic lower continental crust. In the former case, the metagabbroic high MgO eclogites may be genetically related to the Koidu low MgO suite. Crystal fractionation trends suggest that the metapyroxenitic high MgO eclogites formed at lower pressures than their current estimated equilibrium pressures (>4 GPa).     

Separate or shared metamorphic histories of eclogites and surrounding rocks? An example from the Bohemian Massif

Eclogite boudins occur within an orthogneiss sheet enclosed in a Barrovian metapelite‐dominated volcano‐sedimentary sequence within the Velké Vrbno unit, NE Bohemian Massif. A metamorphic and lithological break defines the base of the eclogite‐bearing orthogneiss nappe, with a structurally lower sequence without eclogite exposed in a tectonic window. The typical assemblage of the structurally upper metapelites is garnet–staurolite–kyanite–biotite–plagioclase–muscovite–quartz–ilmenite ± rutile ± silli‐manite and prograde‐zoned garnet includes chloritoid–chlorite–paragonite–margarite, staurolite–chlorite–paragonite–margarite and kyanite–chlorite–rutile. In pseudosection modelling in the system Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O (NCKFMASH) using THERMOCALC, the prograde path crosses the discontinuous reaction chloritoid + margarite = chlorite + garnet + staurolite + paragonite (with muscovite + quartz + H₂O) at 9.5 kbar and 570 °C and the metamorphic peak is reached at 11 kbar and 640 °C. Decompression through about 7 kbar is indicated by sillimanite and biotite growing at the expense of garnet. In the tectonic window, the structurally lower metapelites (garnet–staurolite–biotite–muscovite–quartz ± plagioclase ± sillimanite ± kyanite) and amphibolites (garnet–amphibole–plagioclase ± epidote) indicate a metamorphic peak of 10 kbar at 620 °C and 11 kbar and 610–660 °C, respectively, that is consistent with the other metapelites. The eclogites are composed of garnet, omphacite relicts (jadeite = 33%) within plagioclase–clinopyroxene symplectites, epidote and late amphibole–plagioclase domains. Garnet commonly includes rutile–quartz–epidote ± clinopyroxene (jadeite = 43%) ± magnetite ± amphibole and its growth zoning is compatible in the pseudosection with burial under H₂O‐undersaturated conditions to 18 kbar and 680 °C. Plagioclase + amphibole replaces garnet within foliated boudin margins and results in the assemblage epidote–amphibole–plagioclase indicating that decompression occurred under decreasing temperature into garnet‐free epidote–amphibolite facies conditions. The prograde path of eclogites and metapelites up to the metamorphic peak cannot be shared, being along different geothermal gradients, of about 11 and 17 °C km^?1, respectively, to metamorphic pressure peaks that are 6–7 kbar apart. The eclogite–orthogneiss sheet docked with metapelites at about 11 kbar and 650 °C, and from this depth the exhumation of the pile is shared.     

Stable isotopic and trace element evidence for restricted fluid migration in 2 GPa eclogites

Mineral stable isotopic and trace element studies in 2 GPa banded eclogites of the Tauern Window, eastern Alps, record mm- to cm-scale heterogeneities that reflect compositional variations in the accompanying metamorphic fluids. A close correlation between dolomite mode and dolomite δ¹⁸O is consistent with equilibrium partitioning among coexisting minerals and fluids. Small variations in dolomite δ¹³C values correspond with δ¹⁸O variations, but an overall decrease in dolomite δ¹³C by c. 1%o across a 12-cm sample is a relict feature that pre-dates eclogite equilibration. Garnet, omphacite, and clinozoisite rims show little systematic mineral-mineral partitioning behaviour for Ti, V, Cr, Y, Sr, or Zr; major elements, however, are well equilibrated among these same minerals. Despite the apparent lack of mineral-mineral trace element equilibration, most of the trace elements vary systematically with water activity calculated in each layer. Trace element behaviour during the eclogite metamorphism thus appears to have been controlled largely by mineral-fluid interactions along grain boundaries. Shallow structural levels in other subduction complexes (c. 10-45 km) typically exhibit fracture-controlled permeability and extensive metasomatism, but there is no field or geochemical evidence for extensive fluid advection during high-pressure metamorphism in the Tauern eclogites. Because most dewatering and devolatilization during tectonic burial occurs prior to eclogite conditions, the volumetric fluid/rock ratio in eclogites should generally be low. Low fluid/rock ratios, coupled with the possible non-wetting nature of the fluids, permits the production and preservation of fine-scale chemical heterogeneities in deeply subducted eclogites and associated fluids. However, the eventual breakdown at greater depth of volatile-bearing dolomite, phengite, clinozoisite, zoisite, or amphibole could lead to renewed fracture-controlled fluid release from the subducted rocks to regions appropriate for arc magma generation.     

Trace element distribution in Central Dabie eclogites

Coesite-bearing eclogites from Dabieshan (central China) have been studied by ion microprobe to provide information on trace element distributions in meta-basaltic mineral assemblages during high-pressure metamorphism. The primary mineralogy (eclogite facies) appears to have been garnet and omphacite, usually with coesite, phengite and dolomite, together with high-alumina titanite or rutile, or both titanite and rutile; kyanite also occurs occasionally as an apparently primary phase. It is probable that there was some development of quartz, epidote and apatite whilst the rock remained in the eclogite facies. A later amphibolite facies overprint led to partial replacement of some minerals and particularly symplectitic development after omphacite. They vary from very fine-grained dusty-looking to coarser grained Am + Di + Pl symplectites. The eclogite facies minerals show consistent trace element compositions and partition coefficients indicative of mutual equilibrium. Titanite, epidote and apatite all show high concentrations of REE relative to clinopyroxene. The compositions of secondary (amphibolite facies) minerals are clearly controlled by local rather than whole-rock equilibrium, with the composition of amphibole in particular depending on whether it is replacing clinopyroxene or garnet. REE partition coefficients for Cpx/Grt show a dependence on the Ca content of the host phases, with D _REE ^Cpx/Grt decreasing with decreasing D _Ca . This behaviour is very similar to that seen in mantle eclogites, despite differences in estimated temperatures of formation of 650–850 °C (Dabieshan) and 1000–1200 °C (mantle eclogites). With the exception of HREE in garnet, trace elements in the eclogites are strongly distributed in favour of minor or accessory phases. In particular, titanite and rutile strongly concentrate Nb and Zr, whilst LREE–MREE go largely into epidote, titanite and apatite. If these minor/accessory minerals behave in a refractory manner during melting or fluid mobilisation events and do not contribute to the melt/fluid, then the resultant melts and fluids will be strongly depleted in LREE–MREE. Received: 11 February 1999 / Accepted: 31 January 2000     

Dehydration of metapelites during high‐P metamorphism: The coupling between fluid sources and fluid sinks

Polymetamorphic metapelites and embedded eclogites share a complex, episodic interplay of dehydration and fluid infiltration at the eclogite type‐locality (Saualpe–Koralpe, Eastern Alps, Austria). The metapelites inherited a fluid content (i.e. mineral‐bound OH expressed in terms of mol.% H₂O) of ～6–7 mol.% H₂O from high‐T–low‐P metamorphism experienced during the Permian. At or near P_max of the subsequent Eoalpine event (～20 kbar and 680^°C), the breakdown of paragonite to Na‐rich clinopyroxene and kyanite in metapelites released a discrete pulse of hydrous fluid. Prior to the dehydration event, the rocks were largely fluid absent, allowing only limited re‐equilibration during the prograde Eoalpine evolution. Similarly, Permian‐aged gabbros have persisted metastably due to the absence of a catalyst prior to fluid‐induced re‐equilibration. The fluid triggered partial to complete eclogitization along a fluid infiltration front partially preserved in metagabbro. Near‐isothermal decompression to ～7.5–10 kbar and 670–690°C took place under fluid‐absent conditions. After decompression, a second breakdown of phengitic white mica and garnet produced muscovite, biotite, plagioclase and ～0.1–0.7 mol.% H₂O that enhanced extensive fluid‐aided re‐equilibration of the metapelites. Potential relicts of high‐P assemblages were largely obliterated and replaced by the recurrent amphibolite facies assemblage garnet+biotite+staurolite+kyanite+muscovite+plagioclase+ilmenite+quartz. The hydrous fluid originating from the metapelites infiltrated the embedded eclogites at these P–T conditions and induced the local breakdown of the peak assemblage omphacite and garnet to fine‐grained symplectites of diopside and plagioclase. Further fluid infiltration led to the formation of hornblende–quartz poikiloblasts at the expense of the symplectites. The metapelites re‐equilibrated until the growth of retrograde staurolite consumed any remaining free fluid, thereby terminating the process. Further re‐equilibration is inhibited by both the lack of a catalytic fluid and H₂O as a reactant essential for rehydration reactions. The interplay between fluid sources and fluid sinks describes a closed cycle for the rocks at the eclogite type‐locality. Final, near‐isobaric cooling is indicated by a slight increase of X_Fe in garnet rims. Post‐decompression dehydration and fluid‐aided re‐equilibration arrested by the introduction of staurolite might explain the apparently homogeneous retrogression conditions as well as the notorious absence of diagnostic high‐P assemblages in metapelites at the eclogite type‐locality.     

Paragonite eclogites from Dabie Shan,China: re-equilibration during exhumation?

Abstract Paragonite in textural equilibrium with garnet, omphacite and kyanite is found in two eclogites in the ultrahigh-pressure metamorphic terrane in Dabie Shan, China. Equilibrium reactions between paragonite, omphacite and kyanite indicate a pressure of about 19 kbar at c . 700° C. However, one of the paragonite eclogites also contains clear quartz pseudomorphs after coesite as inclusions in garnet, suggesting minimum pressures of 27 kbar at the same temperature. The disparate pressure estimates from the same rock suggest that the matrix minerals in the ultrahigh-pressure eclogites have recrystallized at lower pressures and do not represent the peak ultrahigh-pressure assemblages. This hypothesis is tested by calibrating a garnet + zoisite/clinozoisite + kyanite + quartz/coesite geobarometer and applying it to the appropriate eclogite facies rocks from ultrahigh- and high-pressure terranes. These four minerals coexist from 10 to 60 kbar and in this wide pressure range the grossular content of garnet reflects the equilibrium pressure on the basis of the reaction zoisite/clinozoisite = grossular + kyanite + quartz/coesite + H₂O. The results of the geobarometer agree well with independent pressure estimates from eclogites from other orogenic belts. For the paragonite eclogites in Dabie Shan the geobarometer indicates pressures in the quartz stability field, confirming that the former coesite-bearing paragonite-eclogite has re-equilibrated at lower pressures. On the other hand, garnets from other coesite-bearing but paragonite-free kyanite-zoisite eclogites show a very wide variation in grossular content, corresponding to a pressure variation from coesite into the quartz field. This wide variation, partly due to a rimward decrease in grossular component in garnet, is caused by partial equilibration of the mineral assemblage during the exhumation.     

Oxygen isotope composition of syngenetic inclusions in diamond from the Finsch Mine,RSA

Oxygen isotope data have been obtained for silicate inclusions in diamonds, and similar associated minerals in peridotitic and eclogitic xenoliths from the Finsch kimberlite by laser-fluorination. Oxygen isotope analyses of syngenetic inclusions weighing 20–400 μg have been obtained by laser heating in the presence of ClF₃. ¹⁸O/¹⁶O ratios are determined on oxygen converted to CO₂ over hot graphite and, for samples weighing less than 750 μg (producing <12 μmoles O₂) enhanced CO production in the graphite reactor causes a systematic shift in both δ¹³C and δ¹⁸O that varies as a function of sample weight. A “pressure effect” correction procedure, based on the magnitude of δ¹³C (CO₂) depletion relative to δ¹³C (graphite), is used to obtain corrected δ¹⁸O values for inclusions with an accuracy estimated to be ±0.3‰ for samples weighing 40 μg.Syngenetic inclusions in host diamonds with similar δ¹³C values (−8.4‰ to −2.7‰) have oxygen isotope compositions that vary significantly, with a clear distinction between inclusions of peridotitic (+4.6‰ to +5.6‰) and eclogitic paragenesis (+5.7‰ to +8.0‰). The mean δ¹⁸O composition of olivine inclusions is indistinguishable from that of typical peridotitic mantle (5.25 ± 0.22‰) whereas syngenetic purple garnet inclusions possess relatively low δ¹⁸O values (5.00 ± 0.33‰). Reversed oxygen isotope fractionation between olivine and garnet in both diamond inclusions and diamondiferous peridotite xenoliths suggests that garnet preserves subtle isotopic disequilibrium related to genesis of Cr-rich garnet and/or exchange with the diamond-forming fluid. Garnet in eclogite xenoliths in kimberlite show a range of δ¹⁸O values from +2.3‰ to +7.3‰ but garnets in diamondiferous eclogites and as inclusions in diamond all have values >4.7‰.     

Reversal of Fe-Mg Partitioning Between Garnet and Staurolite in Eclogite-facies Metapelites from the Champtoceaux Nappe (Brittany, France)

This paper concentrates on the petrology of eclogite-faciesmetapelites and, particularly, the significance of staurolitein these rocks. A natural example of staurolite-bearing eclogitic micaschistsfrom the Champtoceaux nappe (Brittany, France) is first described.The Champtoceaux metapelites present, in addition to quartz,phengite, and rutile, two successive parageneses: (1) chloritoid+staurolite+garnetcores, and (2) garnet rims+kyanite?chloritoid. Detailed microprobe analyses show that garnet and chloritoidevolve towards more magnesian compositions and that stauroliteis more Fe-rich than coexisting garnet. A comparison of thestudied rocks with other known occurrences of eclogitic metapelitesshows that whereas staurolite is always more Fe-rich than garnetin high-pressure eclogites, the reverse is true in low- to medium-pressuremicaschists. Phase relations between garnet, staurolite, chloritoid, biotite,and chlorite are analysed in the KFMASH system (with excessquartz, phengite, rutile, and H₂O). The topology of univariantreactions is depicted for a normal and a reverse Fe-Mg partitioningbetween garnet and staurolite. Mineral compositional changesare also predicted for varying bulk-rock chemistries. In the studied micaschists, the zonal arrangement of garnetinclusions and the progressive compositional changes of ferromagnesianphases record part of the prograde P–T path, before theattainment of ‘peak’ metamorphic conditions (atabout 65O–7OO?C, 18–20 kb). The retrograde path,which records the uplift of the Champtoceaux nappe, occurs underdecreasing temperatures.     

西藏松多榴辉岩的矿物原位微量元素地球化学研究

对松多榴辉岩中单矿物进行的LA-ICP-MS原位微区微量元素分析研究结果表明,石榴石主要富集中、重稀土元素和Y,同时具有高丰度的Sc、V、Cr和Co等元素;绿辉石中的微量元素以中稀土元素、Sr、Sc、V、Cr、Co、Ni和Ti为主,含有一定量的Zr、Hf等。石榴石、绿辉石、角闪石和绿帘石中均显示轻稀土元素亏损的特点,表明在退变质过程中没有发生明显的富轻稀土元素的外来流体交代作用,因而其微量元素矿物地球化学的某些特点不同于苏鲁地区的榴辉岩。石榴石变斑晶中某些元素(如Ti、Zr)的分带性暗示了榴辉岩在紧随峰期变质之后的折返过程中发生了降压增温过程。榴辉岩主要变质矿物中微量元素的分配显然受到矿物主量元素的分配所控制,如MgO在石榴石和绿辉石之间的分配对Ni、Co、Ti分配的控制以及CaO的分配对Sr、Y、REE分配的控制等。退变质过程中矿物的形成或分解以及物理化学条件的改变都可以引起矿物间微量元素的重新分配。由绿辉石退变质而形成的角闪石,较之原先的绿辉石,其微量元素配分曲线总体特征会发生变化,但元素总体丰度相近,某些元素特点相似,又反映了绿辉石和角闪石之间的成生联系。金红石是Ti、Nb、Ta、Zr、Hf的主要赋存矿物,而与之共生的绿帘石所表现出来的高场强元素的亏损特征表明了金红石的存在所带来的影响。     

REE concentrations of garnet and omphacite in eclogites from the Dabie Mountain, central China

Distributions of the rare-earth elements (REE) in omphacite and garnet and REE behaviors during metamorphic processes were discussed. The REE concentrations of garnet and omphacite in six eclogite samples from the Dabie Mountain, central China, were measured by inductively coupled plasma-mass spectrometry (ICP-MS). The correlation of δEu ratios between garnet and omphacite indicated that chemical equilibrium of REE distribution between garnet and omphacite could be achieved during ultra-high pressure (UHP) metamorphism. Most of the partition coefficients (Kd=CiOmp/CiGrt) of light rare-earth elements (LREE) are higher than 1. However the partition coefficients of heavy rare-earth elements (HREE) are lower than 1. This indicated that the LREE inclined to occupy site M2 in omphacite, but the HREEs tended to occupy eightfold coordinated site in garnet during the eclogite formation. The REE geochemistry of the eclogites indicated that LREE could be partially lost during the prograde metamorphic process of protolith, but be introduced into the rocks during the symplectite formation. LREE are more active than HREE during the UHP metamorphism. The results are favorable to highlighting the REE behavior and evolution of UHP metamorphic rocks.     

Characteristic Features of REE and Pb–Zn–Ag Mineralizations in the Na Son Deposit,Northeastern Vietnam

The Na Son deposit is a small‐scale Pb–ZnPb–Zn–Ag deposit in northeast Vietnam and consists of biotite–chlorite schist, reddish altered rocks, quartz veins and syenite. The biotite–chlorite schist is intruded by syenite. Reddish altered rocks occur as an alteration halo between the biotite–allanite‐bearing quartz veins and the biotite–chlorite schist. Allanite occurs in the biotite–allanite‐bearing quartz veins and in the proximal reddish altered rocks. Rare earth element (REE) fluorocarbonate minerals occur along fractures or at rim of allanite crystals. The later horizontal aggregates of sulfide veins and veinlets cut the earlier reddish altered rocks. The earlier Pb–Zn veins consist of a large amount of galena and lesser amounts of sphalerite, pyrite and molybdenite. The later Cu veins cutting the Pb–Zn veins include chalcopyrite and lesser amounts of tetrahedrite and pyrite. The occurrences of two‐phase H₂O–CO₂ fluid inclusions in quartz from biotite–allanite‐bearing quartz veins and REE‐bearing fluorocarbonate minerals in allanite suggest the presence of CO₂ and F in the hydrothermal fluid. The oxygen isotopic ratios of the reddish altered rocks, biotite–chlorite schist, and syenite range from +13.9 to +14.9 ‰, +11.5 to +13.3 ‰, and +10.1 to +11.6 ‰, respectively. Assuming an isotopic equilibrium between quartz (+14.6 to +15.8 ‰) and biotite (+8.6 ‰) in the biotite–allanite‐bearing quartz vein, formation temperature was estimated to be 400°C. At 400°C, δ¹⁸O values of the hydrothermal fluid in equilibrium with quartz and biotite range from +10.5 to +11.7 ‰. These δ¹⁸O values are consistent with fluid that is derived from metamorphism. Assuming an isotopic equilibrium between galena (+1.5 to +1.7 ‰) and chalcopyrite (+3.4 ‰), the formation temperature was estimated to be approximately 300°C. The formation temperature of the Na Son deposit decreased with the progress of mineralization. Based on the geological data, occurrence of REE‐bearing minerals and oxygen isotopic ratios, the REE mineralization is thought to result from interaction between biotite–chlorite schist and REE‐, CO₂‐ and F‐bearing metamorphic fluid at 400°C under a rock‐dominant condition.     

Low δ18O eclogites from the Kokchetav massif, northern Kazakhstan

Oxygen isotopic compositions of silicates in eclogites and whiteschists from the Kokchetav massif were analyzed by whole‐grain CO₂‐laser fluorination methods. Systematic analyses yield extremely low δ¹⁸O for eclogites, as low as ?3.9‰ for garnet; these values are comparable with those reported for the Dabie‐Sulu UHP eclogites. Oxygen isotopic compositions are heterogeneous in samples of eclogite, even on an outcrop scale. Schists have rather uniform oxygen isotope values compared to eclogites, and low δ¹⁸O is not observed. Isotope thermometry indicates that both eclogites and schists achieved high‐temperature isotopic equilibration at 500–800 °C. This implies that retrograde metamorphic recrystallization barely modified the peak‐metamorphic oxygen isotopic signatures. A possible geological environment to account for the low‐δ¹⁸O basaltic protolith is a continental rift, most likely subjected to the conditions of a cold climate. After the basalt interacted with low δ¹⁸O meteoric water, it was tectonically inserted into the surrounding sedimentary units prior to, or during subduction and UHP metamorphism.     

Fluids in deeply subducted continental crust: Petrology, mineral chemistry and fluid inclusion of UHP metamorphic veins from the Sulu orogen, eastern China

The complex vein associations hosted in southern Sulu ultrahigh-pressure (UHP) eclogites contain quartz ± omphacite (or jadeite) ± kyanite ± allanite ± zoisite ± rutile ± garnet. These minerals have chemical compositions similar to those of host eclogites. Inclusions of polycrystalline quartz pseudomorphs after coesite were identified in vein allanite and garnet, and coesite inclusions were found in vein zircon. These facts suggest that the veins together with host eclogites have been subjected to synchronous UHP metamorphism. The vein minerals contain relatively high concentrations of rare earth elements (REE), high-field-strength elements (HFSE) and transition metal elements (TME). A kyanite-quartz vein has a whole-rock composition similar to adjacent UHP metamorphic granitic gneisses. Abundant primary multi-solid fluid inclusions trapped within UHP vein minerals contain complex daughter minerals of muscovite, calcite, anhydrite, magnetite, pyrite, apatite, celestite and liquid and gas phase of H₂O with solids up to 30-70% of the inclusion volume. The presence of daughter minerals anhydrite and magnetite indicates the subduction fluids were oxidizing, and provides a possible interpretation for the high oxygen fugacity of subduction zone magmas. These characteristics imply that the UHP vein minerals were crystallized from supercritical silicate-rich aqueous fluids that were in equilibrium with peak-UHP minerals, and that the fluids in deeply subducted continental crust may contain very high concentrations of silicate as well as HREE, HFSE and TME. Such fluids might have resulted in major fractionation between Nb and Ta, i.e. the UHP fluids have subchondritic Nb/Ta values, whereas the host eclogites after extraction of the fluids have suprachondritic Nb/Ta values. Therefore, voluminous residual eclogites with high Nb/Ta ratios may be the complementary suprachondritic reservoir capable of balancing the subchondritic depleted mantle and continental crust reservoirs.     

Petrology of ultrahigh-pressure eclogites from the ZK703 drillhole in the Donghai, eastern China

The 558-m-deep ZK703 drillhole located near Donghai in the southern part of the Sulu ultrahigh-pressure metamorphic belt, eastern China, penetrates alternating layers of eclogites, gneisses, jadeite quartzites, garnet peridotites, phengite–quartz schists, and kyanite quartzites. The preservation of ultrahigh-pressure metamorphic minerals and their relics, together with the contact relationship and protolith types of the various rocks indicates that these are metamorphic supracrustal rocks and mafic-ultramafic rock assemblages that have experienced in-situ ultrahigh-pressure metamorphism. The eclogites can be divided into five types based on accessory minerals: rutile eclogite, phengite eclogite, kyanite–phengite eclogite, quartz eclogite, and common eclogite with rare minor minerals. Rutile eclogite forms a thick layer in the drillhole that contains sufficient rutile for potential mining. Two retrograde assemblages are observed in the eclogites: the first stage is characterized by the formation of sodic plagioclase+amphibole symplectite or symplectitic coronas after omphacite and garnet, plagioclase+biotite after garnet or phengite, and plagioclase coronas after kyanite; the second stage involved total replacement of omphacite and garnet by amphibole+albite+epidote+quartz. Peak metamorphic P–T conditions of the eclogites were around 32 to 40 kbar and 720°C to 880°C. The retrograde P–T path of the eclogites is characterized by rapidly decreasing pressure with slightly decreasing temperature. Micro-textures and compositional variations in symplectitic minerals suggest that the decompression breakdown of ultrahigh-pressure minerals is a domainal equilibrium reaction or disequilibrium reaction. The composition of the original minerals and the diffusion rate of elements involved in these reactions controlled the symplectitic mineral compositions.     

Nitrogen-bearing,aqueous fluid inclusions in some eclogites from the Western Gneiss Region of the Norwegian Caledonides

Minerals in eclogites from different localities in the Western Gneiss Region of the Norwegian Caledonides (age 425 Ma) contain a variety of fluid inclusions. The earliest inclusions recognized are contained in undeformed quartz grains, protected by garnet, and consist of H₂O+N₂ (with ). The reconstructed P-V-T-X properties of these fluid inclusions are compatible with peak or early-retrograde metamorphic conditions. Matrix minerals (quartz, garnet, apatite, plagioclase) contain a complex pattern of mostly truly secondary inclusions, dominated by CO₂ and N₂. The textural patterns and P-V-T-X properties of these inclusions are incompatible with the high pressures of the eclogite-forming metamorphic event, but suggest that they were formed during uplift, by a combination of remobilization of preexisting inclusions and influx of external fluids. The fluid introduced at a late stage was dominated by CO₂, and did not contain N₂. The present data agree with theoretical predictions of eclogite fluids from mineral equilibria, and highlight the differences between granulite (CO₂) and eclogite (H₂O+N₂) fluid regimes. The provenance of the nitrogen in the eclogite fluid inclusions represents an important, but unsolved question in the petrology of high-pressure metamorphic rocks.Contribution no. 68 to the Norwegian programme of the International Lithosphere Project     

Oxygen isotope thermometry using quartz inclusions in garnet

Oxygen isotope ratios of quartz inclusions (QI) within garnet from granulite and amphibolite facies gneisses in the Adirondack Mountains, NY were analysed and used to determine metamorphic temperatures. Primary QI for eight of 12 samples have δ¹⁸O values significantly lower than matrix quartz (MQ). The primary QI retain δ¹⁸O values representative of thermal conditions during garnet crystallization, whereas the δ¹⁸O values of MQ were raised by diffusive exchange with other matrix minerals (e.g. mica and feldspar) during cooling. The δ¹⁸O differences between QI and MQ show that garnet (a mineral with slow diffusion of oxygen) can armour QI from isotopic exchange with surrounding matrix, even during slow cooling. These differences between δ¹⁸O in MQ and QI can further be used to test cooling rates by Fast Grain Boundary diffusion modelling. Criteria for identifying QI that preserve primary compositions and are suitable for thermometry were developed based on comparative tests. Relations between δ¹⁸O and inclusion size, distance of inclusion to host–garnet rim, core–rim zonation of individual inclusions, and presence or absence of petrological features (healed cracks in QI, inclusions in contact with garnet cracks lined by secondary minerals, and secondary minerals along the inclusion grain boundary) were investigated. In this study, 61% of QI preserve primary δ¹⁸O and 39% were associated with features that were linked to reset δ¹⁸O values. If δ¹⁸O in garnet is homogeneous and inclusions are removed, laser‐fluorination δ¹⁸O values of bulk garnet are more precise, more accurate, and best for thermometry. Intragrain δ¹⁸O(Grt) profiles measured in situ by ion microprobe show no δ¹⁸O zonation. Almandine–rich garnet (Alm_60–75) from each sample was measured by laser‐fluorination mass‐spectrometry (LF‐MS) for δ¹⁸O and compared with ion microprobe measurements of δ¹⁸O in QI for thermometry. The Δ¹⁸O(Qz–Grt) values for Adirondack samples range from 2.66 to 3.24‰, corresponding to temperatures of 640–740 °C (A[Qz–Alm] = 2.71). Out of 12 samples that were used for thermometry, nine are consistent with previous estimates of peak temperature (625–800 °C) based on petrological and carbon–isotope thermometry for regional granulite and upper amphibolite facies metamorphism. The three samples that disagree with independent thermometry for peak metamorphism are from the anorthosite–mangerite–charnockite–granite suite in the central Adirondacks and yield temperatures of 640–665 °C, ~100 °C lower than previous estimates. These low temperatures could be interpreted as thermal conditions during late (post‐peak) crystallization of garnet on the retrograde path.