Hydrogen and oxygen isotope evidence for fluid–rock interactions in the stages of pre- and post-UHP metamorphism in the Dabie Mountains

Hydrogen and oxygen isotope studies were carried out on high and ultrahigh pressure metamorphic rocks in the eastern Dabie Mountains, China. The δ¹⁸O values of eclogites cover a wide range of −4.2 to +8.8‰, but the δD values of micas from the eclogites fall within a narrow range of −87 to −71‰. Both equilibrium and disequilibrium oxygen isotope fractionations were observed between quartz and the other minerals, with reversed fractionations between omphacite and garnet in some eclogite samples. The δ¹⁸O values of −4 to −1‰ for some of the eclogites represent the oxygen isotope compositions of their protoliths which underwent meteoric water–rock interaction before the high to ultrahigh pressure metamorphism. Heterogeneous δ¹⁸O values for the eclogite protoliths implies not only the varying degrees of the water–rock interaction before the metamorphism at different localities, but also the channelized flow of fluids during progressive metamorphism due to rapid plate subduction. Retrograde metamorphism caused oxygen and hydrogen isotope disequilibria between some of the minerals, but the fluid for retrograde reactions was internally buffered in the stable isotope compositions and could be derived from structural hydroxyls dissolved in nominally anhydrous minerals.     

Temporal and tectonic evolution of the granulite-eclogite association from the Bergen Arcs, western Norway

The U-Pb and Sm-Nd dating of deep crustal rocks from the Bergen Arcs system helps resolve enigmatic aspects of the tectonic evolution of the Caledonian Orogen in western Norway and yields insights into the arrested stages of eclogite development within the granulites of the area. The U-Pb dating of zircon from one of the eclogite facies shear zones yields an upper intercept age of 945 ± 5 Ma [all errors two standard deviations (2σ)], which is similar to other zircon ages from the granulite facies protolith. The age is interpreted to represent the time of late Proterozoic (Sveconorwegian) granulite metamorphism. The U-Pb ages of sphene and epidote show that the eclogites formed early in the evolution of the Caledonian Orogen (pre-Scandian phase) at about 460 Ma. An eclogite facies quartz vein yields a Sm-Nd whole rock-garnet isochron of 440 ± 12 Ma that may reflect the onset of cooling immediately after peak eclogite facies conditions, although the Sm-Nd systematics reveal some isotopic disequilibrium within the sample. In tandem with previous ⁴⁰Ar/³⁹Ar age determinations from, an adjacent eclogite of 450 Ma for hornblende and 430 Ma for muscovite, these data indicate that < 30 Ma elapsed between formation of the eclogites and the initial stages of cooling and exhumation to at least mid-crustal levels. This corresponds to minimum cooling rates of 14 °C/m.y. The timing relations suggest that the formation and exhumation of these eclogites from the overlying Caledonian Nappe wedge in western Norway are related to an early phase of crustal subduction during or somewhat before the major phase of continent-continent collision.

The short period of time between the formation of the eclogites and the initial stages of exhumation and rapid cooling is consistent with the only partial and localized transformation of the granulite to eclogite. Isolated occurrences of eclogite within the granulite, the formation of eclogite along metasomatic fronts and the formation of hydrous eclogite facies minerals within the “dry” granulite all point to the importance of fluids in the transformation and re-equilibration of the granulite to eclogite. Together, field and isotopic data demonstrate that both the localized and limited access of fluids and the rapid cycling of continental crust through the deepest portions of the orogen to upper crustal levels resulted in the preservation of the arrested stages of eclogite formation and survival of the granulites metastably through eclogite facies conditions.     

Stable isotope geochemistry of marbles from the coesite UHP terrains of Dabieshan and Sulu, China

Marbles from Dabieshan and Sulu, China, suffered ultra high pressure (UHP) metamorphism in the coesite–eclogite facies at approximately 700°C and 30 kbars during Triassic continental collision and subduction. The marbles range in isotopic composition from +7 to +25 δ¹⁸O_VSMOW and from 0 to +6 δ¹³C_VPDB. High δ¹³C values are representative of unmodified protoliths and are similar to those of ¹³C-enriched Sinian carbonate rocks from the Yangtze craton. High oxygen isotope ratios reflect pristine protoliths but the low values may have been caused by infiltration of low ¹⁸O meteoric water during diagenesis and dolomitization, by fracture-controlled infiltration of water during subduction, by metamorphic mineral reactions, or by a combination of these processes. No evidence of regional isotopic transport during UHP metamorphism has been found. Sampling on scales of 1 to 100 m shows marbles to be inhomogeneous in both carbon and oxygen isotopes. Only samples separated by less than 10 cm have equilibrated oxygen and carbon isotope compositions. Limited isotopic equilibration between adjacent rocks is consistent with the preservation of unaltered UHP minerals and indicates that the metamorphic fluid–rock system was rock-dominated during and following peak metamorphism. A freely flowing, pervasive fluid phase was not present during UHP metamorphism. There is no evidence of isotopic exchange between marble and the upper mantle into which it was subducted. Correlation of geochemical similarities of UHP marbles with Sinian limestones implies that the subducted edge of the Yangtze craton extends at least as far north as the coesite–eclogite facies rocks of Dabieshan. Deposition of protolith carbonates may have taken place in a cold climate either preceding or following but not coincident with Neoproterozoic glaciation.     

Two types of gneisses associated with eclogite at Shuanghe in the Dabie terrane: carbon isotope, zircon U–Pb dating and oxygen isotope

There are two types of gneisses, biotite paragneiss and granitic orthogneiss, to be closely associated with UHP eclogite at Shuanghe in the Dabie terrane. Both concentration and isotope composition of bulk carbon in apatite and host gneisses were determined by the EA-MS online technique. Structural carbonate within the apatite was detected by the XRD and FTIR techniques. Significant ¹³C-depletion was observed in the apatite with δ¹³C values of −28.6‰ to −22.3‰ and the carbon concentrations of 0.70–4.98 wt.% CO₂ despite a large variation in δ¹⁸O from −4.3‰ to +10.6‰ for these gneisses. There is significant heterogeneity in both δ¹³C and δ¹⁸O within the gneisses on the scale of several tens meters, pointing to the presence of secondary processes after the UHP metamorphism. Considerable amounts of carbonate carbon occur in some of the gneisses that were also depleted in ¹³C primarily, but subjected to overprint of ¹³C-rich CO₂-bearing fluid after the UHP metamorphism. The ¹³C-depleted carbon in the gneisses is interpreted to be inherited from their precursors that suffered meteoric–hydrothermal alteration before plate subduction. Both low δ¹³C values and structural carbonate in the apatite suggest the presence of ¹³C-poor CO₂ in the UHP metamorphic fluid. The ¹³C-poor CO₂ is undoubtedly derived from oxidation of organic matter in the subsurface fluid during the prograde UHP metamorphism.

Zircons from two samples of the granitic orthogneiss exhibit low δ¹⁸O values of −4.1‰ to −1.1‰, demonstrating that its protolith was significantly depleted in ¹⁸O prior to magma crystallization. U–Pb discordia datings for the ¹⁸O-depleted zircons yield Neoproterozoic ages of 724–768 Ma for the protolith of the granitic orthogneiss, consistent with protolith ages of most eclogites and orthogneisses from the other regions in the Dabie–Sulu orogen. Therefore, the meteoric–hydrothermal alteration is directly dated to occur at mid-Neoproterozoic, and may be correlated with the Rodinia supercontinental breakup and the snowball Earth event. It is thus deduced that the igneous protolith of the granitic orthogneiss and some eclogites would intrude into the older sequences composing the sedimentary protoliths of the biotite paragneiss and some eclogites along the northern margin of the Yangtze plate at mid-Neoproterozoic, and drove local meteoric–hydrothermal circulation systems in which both ¹³C- and ¹⁸O-depleted fluid interacted with the protoliths of these UHP rocks now exposed in the Dabie terrane.     

Fluid inclusions in granulites, granulitized eclogites and garnet clinopyroxenites from the Dabie–Sulu terranes, eastern China

Minor granulites (believed to be pre-Triassic), surrounded by abundant amphibolite-facies orthogneiss, occur in the same region as the well-documented Triassic high- and ultrahigh-pressure (HP and UHP) eclogites in the Dabie–Sulu terranes, eastern China. Moreover, some eclogites and garnet clinopyroxenites have been metamorphosed at granulite- to amphibolite-facies conditions during exhumation. Granulitized HP eclogites/garnet clinopyroxenites at Huangweihe and Baizhangyan record estimated eclogite-facies metamorphic conditions of 775–805 °C and ≥15 kbar, followed by granulite- to amphibolite-facies overprint of ca. 750–800 °C and 6–11 kbar. The presence of (Na, Ca, Ba, Sr)-feldspars in garnet and omphacite corresponds to amphibolite-facies conditions. Metamorphic mineral assemblages and P–T estimates for felsic granulite at Huangtuling and mafic granulite at Huilanshan indicate peak conditions of 850 °C and 12 kbar for the granulite-facies metamorphism and 700 °C and 6 kbar for amphibolite-facies retrograde metamorphism. Cordierite–orthopyroxene and ferropargasite–plagioclase coronas and symplectites around garnet record a strong, rapid decompression, possibly contemporaneous with the uplift of neighbouring HP/UHP eclogites.

Carbonic fluid (CO₂-rich) inclusions are predominant in both HP granulites and granulitized HP/UHP eclogites/garnet clinopyroxenites. They have low densities, having been reset during decompression. Minor amounts of CH₄ and/or N₂ as well as carbonate are present. In the granulitized HP/UHP eclogites/garnet clinopyroxenites, early fluids are high-salinity brines with minor N₂, whereas low-salinity fluids formed during retrogression. Syn-granulite-facies carbonic fluid inclusions occur either in quartz rods in clinopyroxene (granulitized HP garnet clinopyxeronite) or in quartz blebs in garnet and quartz matrices (UHP eclogite). For HP granulites, a limited number of primary CO₂ and mixed H₂O–CO₂(liquid) inclusions have also been observed in undeformed quartz inclusions within garnet, orthopyroxene, and plagioclase which contain abundant, low-density CO₂±carbonate inclusions. It is suggested that the primary fluid in the HP granulites was high-density CO₂, mixed with a significant quantity of water. The water was consumed by retrograde metamorphic mineral reactions and may also have been responsible for metasomatic reactions (“giant myrmekites”) occurring at quartz–feldspar boundaries. Compared with the UHP eclogites in this region, the granulites were exhumed in the presence of massive, externally derived carbonic fluids and subsequently limited low-salinity aqueous fluids, probably derived from the surrounding gneisses.     

Transition of UHP eclogites to gneissic rocks of low-amphibolite facies during exhumation: evidence from the Dabie terrane, central China

The Shuanghe ultrahigh-pressure (UHP) slab in the Dabie Mountains consists of layered coesite-bearing eclogite, jadeite quartzite, marble and biotite gneiss, and is fault bounded against hosting orthogneiss. Representative assemblages of eclogite are Grt+Omp+Coe+Rt±Ky±Phn±Mgs; it formed at P>27 kbar and 680–720±50 °C. During exhumation, these UHP rocks experienced multistage retrograde metamorphism. Coesite was overprinted by quartz aggregates, phengite by biotite±muscovite and rutile by titanite. Garnet was successively replaced by a thin rim of Amp, Amp+Pl, and Amp+Ep±Bt+Pl (minor). Omphacite and kyanite were replaced by Amp+Pl±Cpx (or ±Bt) and by Zo+Pl+Ms±Mrg±Bt, respectively. Secondary calcite occurs as irregular pockets in some layers. An outcrop near the UHP slab border is composed of 20 thin, concordant layers of foliated eclogites, amphibolite and gneissic rocks of variable bulk composition. These layers exhibit mineral assemblages and textures transitional from less altered through extensively retrograded eclogite to gneissic rock of low-amphibolite facies through hydration, metasomatism and recrystallization. Retrograde metamorphism has caused oxygen and hydrogen isotope disequilibria between some of the minerals, but the fluid for retrograde reactions was internally buffered in the stable isotope compositions. Retrograde metamorphism of variable extent may be attributed to selective infiltration of retrograde fluids of CO₂-rich and low-salinity aqueous, intensity of deformation and mineral resistance to alteration. The fluid phase for retrogression may have occurred either as discontinuous flow along grain boundaries in completely retrograded eclogites, and/or as isolated pockets in extensive or less altered eclogite layers.     

Retention of Precambrian Rb/Sr phlogopite ages through Caledonian eclogite facies metamorphism, Bergen Arc Complex, W-Norway

The Lindås Nappe, Caledonides W-Norway was affected by two major tectonometamorphic events. A Precambrian granulite facies event at T=800–900°C, P<10 kbar was followed by localized Caledonian eclogite facies (T=650–700°C and P>15 kbar) and localized amphibolite facies reworking. During the granulite–eclogite facies transition, anorthositic rocks were converted from garnet granulites to kyanite eclogites, while phlogopite-bearing spinel lherzolite reacted to garnet lherzolite. The eclogite and amphibolite facies reequilibration took place along shear zones and fluid pathways. In the unhydrated and undeformed parts, the minerals preserved their granulite facies composition with constant Fe/Mg ratios from core to rim, suggesting diffusional reequilibration. Rb/Sr age dating was carried out on relict granulite facies minerals from three lenses of ultramafites (Alvfjellet, Hundskjeften and Kvamsfjellet). Phlogopite from phlogopite lherzolite at Alvfjellet give 857±9 Ma, while clinopyroxene, amphibole, phlogopite and whole rock from a lherzolite at Hundskjeften yield an age of 842±12 Ma (MSWD=1.9). Clinopyroxene, feldspar, orthopyroxene phlogopite and whole rock from websterite, Kvamsfjellet, yield an age of 835±7 Ma (MSWD<1), while clinopyroxene, phlogopite and whole rock from a lherzolite from the same lens gives a result of 882±9 Ma. These results are interpreted as minimum ages for the granulite facies event and only slightly younger than, or overlap with previous U–Pb zircon ages (929±1 Ma) and Sm–Nd garnet–pyroxene ages (890–923 Ma) interpreted to date the end of the granulite facies event. By contrast, ages obtained for the eclogite and amphibolite facies range from 460 (U–Pb, sphene), 440 (Ar–Ar), 419 (U–Pb, zircon) to 410 Ma (Rb/Sr mineral ages).

These results demonstrate that the reopening temperature for the Rb/Sr system in phlogopite–biotite under dry and static high-pressure conditions is, in the given mineral assemblages, at least 650°C, considerably higher than the 300–400°C assumed as the closure temperature of this system. We ascribe this elevated reopening temperature to fluid absent conditions that prevented element transport and rehomogenization.     

Stable Isotope Investigations on the Granulite Facies Terrains of Brazil: A Review

Stable isotope data from three areas in the granulite facies terrains of Brazil are assembled and discussed. All the three areas (Jequié, Guaxupé and São José do Rio Pardo) are from the São Francisco Craton. The carbon isotope composition of the fluid inclusion CO₂ in the Archean granulite terrain of Jequié indicate the participation of two distinct sources, an upper mantle source and an internal source of Archean organic matter. The isotope data may be interpreted in terms of the granulite genesis due to the intrusion of magma in the lower crust. The singularly uniform carbon isotope data for CO₂ fluid inclusion of plutonic granulites is a clear indication of the magmatic addition of CO₂-rich volatiles from deeper crustal sources. In the São José do Rio Pardo area sulfur and carbon isotope data of scapolites from the granulites imply the derivation of volatiles from internal sources. The whole rock oxygen isotope data of the amphibolite and granulite facies gneisses from Guaxupé indicate a small scale variation of d¹⁸O values, compatible with the chemical data, suggesting the preservation of pre-metamorphic oxygen isotope composition. The isotope data of the granulites from São Francisco Craton indicate non-pervasive fluid flow during metamorphism.     

Petrotectonic evolution of the high- to ultrahigh-pressure Maksyutov Complex, Karayanova area, south Ural Mountains: structural and oxygen isotope constraints

The Maksyutov Complex consists of three fault-bounded lithologic units: a quartzofeldspathic gneiss containing mafic eclogite boudins (Unit #1); a metasedimentary blueschist-facies (Yumaguzinskaya) unit; and a meta-ophiolitic mélange (Unit #2). The geologic history of the high- to ultrahigh-pressure (HP–UHP) assembly of the Maksyutov Complex is complicated by several stages of prolonged retrograde metamorphism and deformation. The Sakmara River exposes all three units near the former village of Karayanova. A structural/petrologic cross-section through the area yields new quantitative data for the complex and, regionally, for the south Urals. Analysis of the Karayanova area has identified the major structures. Regional folding within the complex is parallel to the dominant foliation trending northeast–southwest. Stereonet data show that, during exhumation, this large-scale folding was refolded about axes trending southeast. Unit #1 and the Yumaguzinskaya are tectonically and petrologically distinct units juxtaposed by west-vergent thrusting and recrystallization within the same subduction zone. A shear zone developed later between Unit #2 and the Unit #1+Yumaguzinskaya tectonic package accompanying exhumation. Field relations and petrofabric demonstrate that blueschist-facies recrystallization overprinted an earlier eclogite-facies metamorphism. Thermobarometric measurements yield P–T values of 594–637°C, 1.5–1.7 GPa for eclogite, but these conditions may reflect annealing during the early-stage exhumation at 375 Ma. Cuboid graphite aggregates testify to precursor conditions for Unit #1 within the diamond stability field, if such textures are correctly interpreted. Measured ¹⁸O/¹⁶O partitioning between pairs of coexisting phases yield three main recrystallization temperature ranges: (1) 678±83°C, attending Unit #1 eclogite-facies metamorphism; (2) 453±17°C, during transitional blueschist/greenschist-facies metamorphism for the amalgamated Unit #1+Yumaguzinskaya+Unit #2 assembly; and (3) 250±68°C, reflecting late-stage hydrothermal alteration and exhumation. Oxygen isotope data for Units #1 and #2 indicate that garnet, blue amphibole, and pyroxene crystallized in isotopic equilibrium, validating previous thermobarometric calculations for a Unit #1 retrograde metamorphic event. Variations in δ¹⁸O values for phengites suggest the possibility of late metamorphic fluid infiltration. Retrograde recrystallization at high pressure in the presence of fluids and a calculated slow exhumation rate for the Maksyutov Complex account for the fact that inferred UHP coesite and diamond were completely back-reacted during decompression.     

西南大别榴辉岩和麻粒岩的氧同位素地球化学

报道了大别造山带西南部湖北红安榴辉岩和罗田麻粒岩的氧同位素组成，并讨论了氧扩散作用对矿物氧同位素平衡的影响，结果得到，红安榴辉岩的全岩δ^18O值为6．4－7．3‰，罗田黄土岭麻粒岩的全岩δ^18O值为6．6－7．8‰，罗田惠兰山麻粒岩的全岩δ^18O值为3．9‰，这些榴辉岩和麻粒岩全岩的氧同位素组成保持了峰期变质条件下的平衡分馏特征，得到的氧同位素温度对于红安榴辉岩425－620度，对于罗田麻粒岩为740－945度。根据快速颗粒边界扩散模型计算的矿物对氧同位素温度不仅与大多数实测氧同位素温度一致，而且与岩石学测温结果相吻合，因此，这些岩石与东大别榴辉岩一样在形成后经历了快速冷却过程，退变质反应过程中没有外来流体加入。     

Phanerozoic high-pressure eclogite and intermediate-pressure granulite facies metamorphism in the Gyeonggi Massif, South Korea: Implications for the eastward extension of the Dabie–Sulu continental collision zone

Petrological analysis, zircon trace element analysis and SHRIMP zircon U–Pb dating of retrogressed eclogite and garnet granulite from Bibong, Hongseong area, SW Gyeonggi Massif, South Korea provide compelling evidence for Triassic (231.4 ± 3.3 Ma) high-pressure (HP) eclogite facies (M1) metamorphisms at a peak pressure–temperature (P–T) of ca. 16.5–20.0 kb and 775–850 °C. This was followed by isothermal decompression (ITD), with a sharp decrease in pressure from 20 to 10 kb and a slight temperature rise from eclogite facies (M1) to granulite facies (M2), followed by uplift and cooling. Granitic orthogneiss surrounding the Baekdong garnet granulite and the ophiolite-related ultramafic lenticular body near Bibong records evidence for a later Silurian (418 ± 8 Ma) intermediate high-pressure (IHP) granulite facies metamorphism and a prograde P–T path with peak P–T conditions of ca. 13.5 kb and 800 °C. K–Ar ages of biotite from garnet granulites, amphibolites, and granitic orthogneisses in and around the Bibong metabasite lenticular body are 208–219 Ma, recording cooling to about 310 °C after the Early Triassic metamorphic peak. Neoproterozoic zircon cores in the retrogressed eclogite and granitic orthogneiss provide evidence that the protoliths of these rocks were  800 and  900 Ma old, respectively, similar to the ages of tectonic episodes in the Central Orogenic Belt of China. This, and the evidence for Triassic HP/UHP metamorphism in both China and Korea, is consistent with a regional tectonic link within Northeast Asia from the time of Rodinia amalgamation to Triassic continent–continent collision between the North and South China Blocks, and with an eastward extension of the Dabie–Sulu suture zone into the Hongseong area of South Korea.     

造山带中成对出现的高压麻粒岩与榴辉岩及其地球动力学意义

在一些典型碰撞造山带中,高压麻粒岩与榴辉岩在空间和时间上密切相关,它们之间的关系对揭示碰撞造山带的造山过程和造山机制具有重要意义.本文以中国西部的南阿尔金、柴北缘及中部的北秦岭造山带为例,详细陈述了这3个地区榴辉岩和相关的高压麻粒岩的野外关系、变质演化和形成时代,目的是要建立大陆碰撞造山带中榴辉岩和相关高压麻粒岩形成的地球动力学背景模式.南阿尔金榴辉岩呈近东西向分布在江尕勒萨依,玉石矿沟一带,与含夕线石副片麻岩、花岗质片麻岩和少量大理岩构成榴辉岩一片麻岩单元,榴辉岩中含有柯石英假象,其峰期变质条件为P=2.8～3.0GPa,T=730～850℃,并在抬升过程中经历了角闪岩-麻粒岩相的叠加;大量年代学研究显示其峰期变质时代为485～500Ma.南阿尔金高压麻粒岩分布在巴什瓦克地区,包括高压基性麻粒岩和高压长英质麻粒岩,它们与超基性岩构成了一个大约5km宽的构造岩石单元,与周围角闪岩相的片麻岩为韧性剪切带接触.长英质麻粒岩和基性麻粒岩的峰期组合均具有蓝晶石和三元长石(已变成条纹长石),形成的温压条件为T=930～1020℃,P=1.8～2.5GPa,并在退变质过程中经历了中压麻粒岩相变质作用叠加.锆石SHRIMP测定显示巴什瓦克高压麻粒岩的峰期变质时代为493～497Ma.都兰地区的榴辉岩分布柴北缘HP-UHP变质带的东端,在榴辉岩和围岩副片麻岩中均发现有柯石英保存,形成的峰期温压条件为T=670～730℃和P=2.7～3.25GPa,退变质阶段经过了角闪岩相的叠加;榴辉岩相变质时代为420～450Mao都兰地区的高压麻粒岩分布在阿尔茨托山西部,高压麻粒岩包括基性麻粒岩长英质麻粒岩,基性麻粒岩的峰期矿物组合为Grt+Cpx+Pl±Ky±Zo+Rt±Qtz,长英质麻粒岩的峰期矿物组合为：Grt+Kf+Ky+Pl+Qtz.峰期变质条件为T=800～925℃,P=1.4～1.85GPa,退变质阶段经历了角闪岩-绿片岩的改造,高压麻粒岩的变质时代为420～450Ma.北秦岭榴辉岩分布在官坡-双槐树一带,榴辉岩的峰期变质组合为Grt+Omp±Phe+Qtz+Rt,所计算的峰期温压条件为T=680～770℃和P=2.25～2.65GPa,年代学数据显示榴辉岩的变质时代为500Ma左右.北秦岭高压麻粒岩分布在含榴辉岩单元的南侧松树沟一带,包括高压基性麻粒岩和高压长英质麻粒岩,与超基性岩在空间上密切伴生,高压麻粒岩的峰期温压条件为T=850～925℃,P=1.45～1.80GPa,锆石U-Pb年代学研究显示其峰期变质时代为485～507Ma.以上三个实例显示,出现在同一造山带、在空间上伴生的高压麻粒岩和榴辉岩有各自不同的变质演化历史,但榴辉岩中的榴辉岩相变质时代和相邻的高压麻粒岩中的高压麻粒岩相变质作用时代相同或相近,这种成对出现的榴辉岩和高压麻粒岩代表了它们同时形成在造山带中不同的构造环境中,即榴辉岩的形成于大陆俯冲带中,而高压麻粒岩可能形成在俯冲带之上增厚的大陆地壳根部.     

Trace element geochemistry of coesite-bearing eclogites from the Roberts Victor kimberlite, Kaapvaal craton

Trace element characteristics of seven coesite-bearing eclogitic xenoliths from the Roberts Victor kimberlite demonstrate that this suite of eclogites originated as gabbroic cumulates in oceanic crust that was subsequently subducted. All but one of the garnets show positive Eu anomalies, accompanied by a flat heavy rare earth pattern, which is atypical of garnet, but characteristic of plagioclase, arguing for a considerable amount of plagioclase in the protoliths. Forward modelling of the accumulation of liquidus minerals from primitive komatiitic, picritic, and basaltic liquids suggests that at least some of the eclogite protoliths were not derived from basaltic parental liquids, whereas derivation from either komatiitic or picritic liquids is possible. The reconstructed eclogite bulk rocks compare favourably with oceanic gabbros from ODP hole 735B (SW Indian Ridge), even to the extent that oxygen isotopic systematics show signs of low-temperature seawater alteration. However, the oxygen isotope trends are the reverse of what is expected for cumulates in the lower section of the oceanic crust. These new findings show that δ¹⁸O values in eclogitic xenoliths, despite being sound indicators for their interaction with hydrothermal fluids at low pressure, do not necessarily bear a simple relationship with the inferred oceanic crustal stratigraphy of the protoliths.     

Geochemical constraints of the eclogite and granulite facies metamorphism as recognized in the Raobazhai complex from North Dabie Shan,China

A combined study of major and trace elements, fluid inclusions and oxygen isotopes has been carried out on garnet pyroxenite from the Raobazhai complex in the North Dabie Terrane (NDT). Well‐preserved compositional zoning with Na decreasing and Ca and Mg increasing from the core to rim of pyroxene in the garnet pyroxenite indicates eclogite facies metamorphism at the peak metamorphic stage and subsequent granulite facies metamorphism during uplift. A P–T path with substantial heating (from c. 750 to 900 °C) after the maximum pressure reveals a different uplift history compared with most other eclogites in the South Dabie Terrane (SDT). Fluid inclusion data can be correlated with the metamorphic grade: the fluid regime during the peak metamorphism (eclogite facies) was dominated by N₂‐bearing NaCl‐rich solutions, whereas it changed into CO₂‐dominated fluids during the granulite facies retrograde metamorphism. At a late retrograde metamorphic stage, probably after amphibolite facies metamorphism, some external low‐salinity fluids were involved. In situ UV‐laser oxygen isotope analysis was undertaken on a 7 mm garnet, and impure pyroxene, amphibole and plagioclase. The nearly homogeneous oxygen isotopic composition (δ¹⁸O_VSMOW = c. 6.7‰) in the garnet porphyroblast indicates closed fluid system conditions during garnet growth. However, isotopic fractionations between retrograde phases (amphibole and plagioclase) and garnet show an oxygen isotopic disequilibrium, indicating retrograde fluid–rock interactions. Unusual MORB‐like rare earth element (REE) patterns for whole rock of the garnet pyroxenite contrast with most ultra‐high‐pressure (UHP) eclogites in the Dabie‐Sulu area. However, the age‐corrected initial ε_Nd(t) is ? 2.9, which indicates that the protolith of the garnet pyroxenite was derived from an enriched mantle rather than from a MORB source. Combined with the present data of oxygen isotopic compositions and the characteristic N₂ content in the fluid inclusions, we suggest that the protolith of the garnet pyroxenite from Raobazhai formed in an enriched mantle fragment, which has been exposed to the surface prior to the Triassic metamorphism.     

Cretaceous isochron ages from K–Ar and Ar / Ar dating of eclogitic rocks in the Tso Morari Complex, western Himalaya, India

The Tso Morari Complex, which is thought to be originally the margin of the Indian continent, is composed of pelitic gneisses and schists including mafic rock lenses (eclogites and basic schists). Eclogites studied here have the mineral assemblage Grt + Omp + Ca-Amp + Zo + Phn + Pg + Qtz + Rt. They also have coesite pseudomorph in garnet and quartz rods in omphacite, suggesting a record of ultrahigh-pressure metamorphism. They occur only in the cores of meter-scale mafic rock lenses intercalated with the pelitic schists. Small mafic lenses and the rim parts of large lenses have been strongly deformed to form the foliation parallel to that of the pelitic schists and show the mineral assemblages of upper greenschist to amphibolite facies metamorphism. The garnet–omphacite thermometry and the univariant reaction relations for jadeite formation give 13–21 kbar at 600 °C and 16–18 kbar at 750 °C for the eclogite formation using the jadeite content of clinopyroxene (X_Jd = 0.48).

Phengites in pelitic schists show variable Si / Al and Na / K ratios among grains as well as within single grains, and give K–Ar ages of 50–87 Ma. The pelitic schist with paragonite and phengite yielded K–Ar ages of 83.5 Ma (K = 4.9 wt.%) for paragonite–phengite mixture and 85.3 Ma (K = 7.8 wt.%) for phengite and an isochron age of 91 ± 13 Ma from the two dataset. The eclogite gives a plateau age of 132 Ma in Ar/Ar step-heating analyses using single phengite grain and an inverse isochron age of 130 ± 39 Ma with an initial ⁴⁰Ar / ³⁶Ar ratio of 434 ± 90 in Ar/Ar spot analyses of phengites and paragonites. The Cretaceous isochron ages are interpreted to represent the timing of early stage of exhumation of the eclogitic rocks assuming revised high closure temperature (500 °C) for phengite K–Ar system. The phengites in pelitic schists have experienced retrograde reaction which modified their chemistry during intense deformation associated with the exhumation of these rocks with the release of significant radiogenic ⁴⁰Ar from the crystals. The argon release took place in the schists that experienced the retrogression to upper greenschist facies metamorphisms from the eclogite facies conditions.     

Oxygen and carbon isotope composition of cold-water Berriedale Limestone (Lower Permian), Tasmania, Australia

The Berriedale Limestone formed at about 80°S paleolatitude and contains many glacial dropstones. It formed during a period of major Gondwana deglaciation.

The Berriedale Limestone contains mostly bryozoans, brachiopods and bivalves, with some intraclasts and rare pellets. The faunal diversity is low and the fauna are similar to the modern cold-water foramol faunal assemblage. Micrite, microspar and spar occur as equant to well developed rhombs of calcite. The coarse spar cements are bored and are ruptured by dropstones, indicating submarine origin of low-Mg calcite at water-temperatures of around 3°C. The mixing zone cementation was preceded by erosion of early formed crystals. The eroded crystals occur as inclusions in mixing zone cements.

The fauna are characterized by heavy δ¹³C and light δ¹⁸O. The whole-rock field of δ¹⁸O-δ¹³C falls at the edge of “Normal Marine Limestone” and deviates to lighter δ¹⁸O values (down to −16.7‰ PDB). Lightest δ¹⁸O values ( −22‰ PDB) of fresh-water sparry calcite cement are similar to those in the Early Permian continental tillites, suggesting that the Permian sea was diluted by isotopically light melt waters. Micrite δ¹⁸O values (−9.2 to −12.6‰ PDB) are within the range of whole-rock values. The δ¹⁸O values of calcite in shales are lighter than limestone values.

The δ¹⁸O values of the fauna give an unrealistic range of sea-water temperatures because the fauna have equilibrated with variable amounts of melt waters. However, calculated original δ¹⁸O values of the fauna indicate temperatures < 4°C. The heaviest δ¹⁸O of fauna gives cold temperatures of 9°C (with δ_w −2.8‰) and −3°C (with δ_w −6‰). The lightest values of sparry calcite cements (−22‰ PDB) indicate that the limestone reacted with cold melt waters.

The δ¹⁸O of Permian sea is estimated to be about +1.2‰ and was diluted by melt waters as light as −27‰ SMOW.     

Kyanite eclogite xenolith from the orthogneiss terrane of the Tisza Megaunit, Jánoshalma area, crystalline basement of southern Hungary

New evidence for high-pressure, eclogite facies metamorphism in the crystalline basement of the Tisza Megaunit (southern Hungary) is reported. The retrogressed mafic eclogite forms a small lens in the orthogneiss and it was found in the borehole near Jánoshalma. The carbonated eclogite contains the peak metamorphic assemblage omphacite + garnet + phengite + kyanite + clinozoizite + rutile + K-feldspar + quartz. Omphacite (Xjd_0.40–0.41Xdio_0.52–0.53Xhd_0.05Xaug_1.55–2.85) occurs in the matrix and as inclusions in garnet (Xpy_0.37–0.38Xgrs_0.21–0.22Xalm_0.39–0.40Xsps_0–0.01Xadr_0–0.01) and kyanite. Thermobarometry based on net-transfer reactions between garnet, omphacite, kyanite and phengite yields P–T conditions of 710 ± 10 °C and 2.6 ± 0.75 GPa. Retrogression during decompression is manifested by formation of symplectites; the most typical are diopside + plagioclase after omphacite, corundum + spinel + plagioclase after kyanite and biotite + plagioclase after phengite. Carbonatization along the veins of the retrogressed eclogite was probably coeval with formation of these symplectites. At places where carbonate is absent the rock was completely hydrated and retrogressed down to the greenschist facies with the development of actinolite. Similar eclogites together with abundant orthogneisses occur mainly in the eastern parts of the Tisza Megaunit, suggesting the existence of an ancient (possibly Variscan) subduction/accretionary complex.     

PTtD Path and Fluid Evolution of HighPressure Eclogites from the Dabie Mountains,Central East China

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Constraints on element mobility associated with the conversion of granulite to eclogite along fractures in an anorthositic complex on Holsnøy, Norway

On Holsnøy, an island off the coast of Western Norway, an anorthositic complex metamorphosed to granulite facies was partially overprinted by a later eclogite facies metamorphism. Eclogite facies rocks (containing omphacite, garnet, kyanite and hydrous phases such as mica and zoisite) occur in shear zones of various scales and adjacent to veins. Previous studies of shear zones on Holsnøy reported evidence for substantial element mobility (Jamtveit et al ., 1990; Mattey et al ., 1994). In this work, we compare chemical compositions of granulite and its undeformed eclogitized equivalent adjacent to veins in locations where a single band of granulite can be traced and sampled as it approaches the vein. This tracing is crucial because the pre-granulite rocks cover a substantial compositional range, indicative of a petrologically variable protolith consisting of anorthosite, gabbro and jotunite. We analysed multiple core samples collected across nine separate granulite-eclogite transition zones located at veins in anorthositic, jotunitic and gabbroic protoliths for major and trace elements. For each transition, no compositional difference between the average granulite and average eclogite composition was found at the 90% confidence level except for LOI (loss on ignition), which was consistently significantly higher in the eclogite samples. Although not significant at the 90% confidence level for any single traverse, the average eclogite concentrations of SiO₂ , Na₂O, Cs, As and Br exceed the average granulite concentrations for eight or all nine of the traverses. For most traverses, statistical analysis of the data limits any gain of SiO₂ in the eclogites to no more than a few relative per cent. Other than the introduction of volatile substances, presumably an H₂O-rich fluid, eclogitization associated with vein formation was essentially isochemical.     

Wollastonite and scapolite in Precambrian calc-silicate granulites from Australia and Antarctica

Abstract Scapolite, wollastonite, calcite, diopside, grossular-andradite garnet and sphene occur in calc-silicate rocks in the granulite terrain of the Arunta Block, central Australia. This assemblage buffers the CO₂ activity at a low value, so that any coexisting fluid phase must be H₂O rich and CO₂ poor ( X _co2 = 0.2-0.3). In contrast, the H₂O activity in the surrounding felsic and mafic granulites was low. Thus fluid activities during granulite facies metamorphism were locally buffered in various rock units and fluid flow appears to have been restricted or fluid may have been absent. Late retrograde rims of garnet and garnet-quartz separate phases formed in the high-grade stage. Formation of these rims would have required either an influx of water-rich fluid or a decrease in pressure. Evidence from the surrounding granulites shows that in one locality, the calc-silicate rocks had undergone late isobaric hydration; in another locality, minor uplift had occurred soon after peak P-T conditions. In both, scapolite had partly broken down to plagioclase-calite. A calc silicate rock from the granulite terrain of Enderby Land, Antarctica, contains scapolite, wollastonite, calcite, diopside, quartz and sphene; this assemblage also indicates low CO₂ activities. In this rock, wollastonite has broken down to calcite-quartz, to indicate isobaric cooling without influx of hydrous fluid.