蛇纹岩体中的硬玉岩与异剥钙榴岩

硬玉岩是一种稀少的岩类，常以构造岩包体和脉体的型式产出在蛇纹岩中。本文着重介绍与蛇纹岩化过程密切相关的硬玉岩。这类硬玉岩总是出现在洋壳俯冲碰撞带中，常常与蓝片岩、偶尔也与榴辉岩伴生。通过研究硬玉岩的矿物组合、化学成分和结构构造，综合世界各地硬玉岩产状，可将硬玉岩从广义的变质成因上分为三类，即：在蛇纹岩体中、或其它岩类包体中由交代作用形成的硬玉岩脉体；蛇纹岩中的构造岩成因岩石包体经固相变质作用后形成的块状硬玉岩：以及介于上述两者之间，由变质和交代的共同作用形成的硬玉岩。此外，还对交代成因的硬玉岩在蛇纹岩化过程中与异剥钙榴岩化作用的关系从流体及其化学成分演化的角度进行了分析和研究。提出了在海底和俯冲带上板片的构造背景下硬玉岩形成的模式。     

Jadeitites, albitites and related rocks from the Motagua Fault Zone, Guatemala

Jadeitites from Guatemala are found as weathered blocks in tectonized serpentinite in a 15-km zone north of the Motagua Fault Zone. Rock types found with jadeitite include albitites, albite-mica rocks, omphacite/taramitic amphibole-bearing metabasites, chlorite-actinolite schists, talc-carbonate rocks and antigorite schists. In addition to the predominant jadeitic (Jd₉₃_₁₀₀) pyroxene, common phases in jadeitite include micas (paragonite and/or phengite ± rarer phlogopite), omphacite, albite, titanite /Pm zircon, apatite and graphite. Conditions of jadeitite formation are 100-400d? C, 5-11 kbar with 0.0 > log₁₀a_sio2≥= 0.7. Fluid inclusions, coarse textures, vein structures, and rhythmic zoning of pyroxene indicate an aqueuos fluid was involved. Jadeitites are either (1) metasomatic modifications of former felsic-to-pelitic inclusions that have undergone silica depletion plus efficient soda exchange and enrichment, or (2) solution precipitations derived from such a source. The close spatial relationship of faults and shear zones, serpentinites, and jadeitites suggests jadeitites form in a relatively high-P/T setting with substantial flow of sodic fluid in a tectonized zone. Most Guatemalan jadeitites are extensively altered to analcime, albite, taramitic amphibole, (clino)zoisite ± nepheline and preiswerkite. This alteration reflects depressurization /Pm heating to below the jadeite + fluid = analcime reaction at high a_Na. With progressive alteration, analcime and nepheline are replaced by albite; the increase in silica content may result from fluid flowing up a tectonized zone reaching saturation with an albite assemblage. Albitite phases, albite, actinolite, zoisite, /Pm chlorite, phengite, K-feldspar and quartz, record conditions of c. 3-8 kbar at T < 400d? C, indicating a clockwise P-T trajectory of the blocks. Barium aluminosilicates—banalsite, celsian, cymrite and hyalophane—are common minor late-stage phases in jadeitites and albite-rich rocks. Barian phengite is common in albite-mica rocks.     

Amphibole and phlogopite in “hybrid” metasomatic bands monitor trace element transfer at the interface between felsic and ultramafic rocks (Eastern Alps,Italy)

Ultramafic rocks in contact to gneisses in the Mt. Hochwart HP mélange (Eastern Italian Alps) preserve a series of metasomatic mineral zones. A phlogopitite with minor tremolite and accessory zircon and apatite forms close to the gneiss (Zone 1). Zone 2 consists of tremolite, phlogopite and anthophyllite followed by Zone 3 with anthophyllite plus minor chlorite and talc. Zone 3 grades into an amphibole–garnet peridotite lens. This reaction zone has been generated by infiltration of hydrous fluids at T of 660–700 °C and P < 1.2 GPa, which occurred during exhumation of coupled continental crust and mantle peridotites.The reaction zone between a trace element-rich (gneiss) and a trace element depleted reservoir (peridotite) allows assessment of local trace element mobility in aqueous fluids. We present the results of in situ LA-ICP-MS trace element analysis of minerals from the reaction zone. Phlogopite is the main host for Large Ion Lithophile Elements (LILE) and contributes significantly to the Li, Ti, Nb, Ta, Pb and Sc budget. Anthophyllite is the main host for Li whereas all other trace elements including Rare Earth Elements (REE) are preferentially incorporated into tremolite. Combined with the abundance of these minerals over the contact zone, the mineral trace element data suggests that the LILE and REE were mobile on a small scale of a few centimetres only. Limited mobility of Ta, which is generally regarded as barely mobile in fluids, is documented in elevated contents of Ta in anthophyllite coupled with low Nb/Ta. The high Li content in minerals throughout the reaction zone suggests that Li was the most mobile element.The studied metasomatic zones mirror geochemical processes occurring in subduction zones at the slab–mantle interface. Phlogopite crystallization at the slab–mantle interface is an efficient mechanism to filter LILE from the aqueous fluid. Thus, such reaction zones, forming at temperatures < 660–700 °C, likely prevents that the typical slab signature with enriched LILE is transported by aqueous fluids over long distances in the mantle wedge. However, if coupled to the downgoing slab, phlogopite- and tremolite-rich rocks from such reaction zones might be able to act as carriers of trace elements and water into deeper parts of the subduction zone.     

Geochemistry and age of the complex of alkaline metasomatic rocks and carbonatites of the Gremyakha-Vyrmes massif, Kola Peninsula

This paper presents new geochemical data on the complex of alkaline metasomatic rocks and carbonatites, which hosts the rare-metal mineralization of the Gremyakha-Vyrmes massif. The contents of major and trace, including rare-earth elements were determined in the albitites, aegirinites, and carbonatites. Two types of the rare-metal ores are distinguished: niobium albitite and zirconium aegirinite ores. It was shown that the albitites and aegirinites have similar trace element distribution patterns, being most geochemically close to the foidolites. The carbonatites, albitites, and aegirinites were dated by Rb-Sr and Sm-Nd methods at 1887 ± 58 Ma, which corresponds to the formation age of the Gremyakha-Vyrmes massif. The ultrabasic rocks, foidolites, alkaline metasomatic rocks, and carbonatites were formed successively within a relatively narrow range. The geological observations and geochemical data led us to conclude that the emplacement of the fluid-saturated carbonatite solutions-melts at the final stages of the massif formation against a background of fault tectonics caused a pervasive metasomatism of the ultrabasic and alkaline rock complexes and, as a result, the formation of the alkaline albitites and aegirinites. The carbonatites could be sources of rare-metals, while foidolites served as a geochemical barrier, and their metasomatic alteration led to the formation of Zr-Nb mineralization in the albitites and aegirinites.     

New Data on the Composition of Ophiolite Complexes on Karaginskii Island (Eastern Kamchatka)

The geochemistry and composition of peridotite rock-forming minerals from blocks in the serpentinite mélange of Karaginskii Island have been studied. The composition features of the rock-forming minerals are indicative of the fact that they represent abyssal peridotites of the mid-oceanic ridges that did not undergo remelting under suprasubduction conditions. According to the geochemical data, these rocks were subject to metasomatic alterations under mantle conditions in the suprasubduction setting, which were caused by metasomatizing melts and/or fluids generated in the subduction zone.     

The Palma Volcano-Sedimentary Supersuite,Precambrian Sul-Riograndense Shield,Brazil

Komatiites are generally found in Archean greenstone belts, but have been reported in some Mesozoic volcano-sedimentary sequences. The Palma Volcano-Sedimentary Supersuite (PVSS) is a Neoproterozoic sequence composed of meta-ultramafic volcanic rocks, mafic-to-felsic metavolcanic rocks, and metasedimentary rocks ranging from quartzites to chlorite-mica-quartz schists, marbles, and marls. Original PVSS structures were overprinted by two penetrative deformational surfaces (So//Sn and Sn+1); associated metamorphic assemblages are assigned to the upper greenschist facies. Basalts predominate over andesites and rhyodacites, and their major, minor, trace, and rare-earth elements display compatible geochemical trends characteristic of calc-alkaline magmas. Meta-ultramafic rocks are variable-thickness flows in the metasedimentary sequence. Metamorphism caused the development of a metasomatic tremolite zone around the serpentinite bodies, as well as chlorite and talc schist metasomatic zones. Evaluation of the geochemical mobility of the elements showed that the ratios between major and minor elements have not changed in the serpentinites. It was then possible to investigate the lithogeochemical nature of the serpentinites by studying the minor, trace, and rare-earth elements. The lithogeochemical investigation and the structures of the serpentinite bodies enable them to be regarded as komatiitic flows extruded under shallow water. These komatiite flows and calc-alkaline basalt to rhyodacite flows associated with shallow-water sediments suggest that this volcano-sedimentary sequence was developed in a Neoproterozoic arc-related tectonic setting.     

Metasomatic reaction bands at the Mt. Hochwart gneiss-peridotite contact (Ulten Zone, Italy): insights into fluid-rock interaction in subduction zones

We investigated the contact zone between peridotite lenses and host gneisses located on the northern side of the Hochwart peak, also known as Vedetta Alta (Ulten Zone, Alto Adige -Südtirol) where metasomatic contact bands occur. The country rocks are gneisses consisting mainly of quartz, K-feldspar, garnet, kyanite, biotite and muscovite. The ultramafic body consists of a hectometre-sized garnet peridotite and harzburgite lens. The reaction zone shows mineralogic zoning from phlogopite-rich to tremolite-anthophyllite-talc-rich rocks from the host gneiss towards the peridotite. In some cases, lenses of serpentine and talc in association with chlorite, and trondhjemitic pods develop at the ultramafic rocks border to the gneisses. Trondhjemite dikes with pegmatoid texture also crosscut the peridotite body. Phlogopite aggregates with accessory zircon, Cl-apatite and tourmaline and phlogopite-hornblende aggregates also occur. The combination of petrography, mineral chemistry and mass balance calculations constrains the gains and losses of elements during metasomatism. Reaction zone formation involved extensive addition of H₂O, K₂O and LILE from the fluid, whereas MgO, CaO and Al₂O₃ were removed from the peridotite. Thus, the formation of the reaction zones between the mantle rocks and the gneisses was triggered by considerable fluid/melt circulation, causing crystallisation of mainly phlogopite, anthophyllite and talc, and the release of a trondhjemitic residual melt. Field mapping provides evidence that the internal structures of the host migmatites (folds) and those of the peridotites (foliation, fluid texture) are discordant. Pseudosection calculations give insights into the P-T conditions (T 660–700°C; P 0.5–0.7 GPa) of metasomatism responsible for the formation of reaction zones, which is related to the retrograde path of the Ulten Zone peridotites. Our results suggest that the redistribution of major and trace elements in subduction zones is strongly influenced by metasomatic reactions occurring at the slab-mantle interface.     

南苏鲁造山带的超高压变质岩及岩石化学研究

在南苏鲁造山带核部，古老的表壳岩和花岗质侵人岩经历了三叠纪的超高压变质作用，在超高压变质岩石抬升过程中经历了强烈的角闪岩相退变质作用改造。据岩相学和岩石化学研究，可以区分出六大类典型超高压变质岩：榴辉岩、石榴石橄榄岩、石英硬玉岩、石榴石多硅白云母片岩、硬玉石英岩和石榴石绿辉石文石岩。这些岩石的角闪岩相退变质产物分别是斜长角闪岩、蛇纹岩、长英质片麻岩、长石石英云母片岩、石英岩和大理岩。地球化学研究揭示，榴辉岩的原岩很可能是形成在大陆内部构造环境的拉斑玄武岩，而石榴石橄榄岩可能是起源于亏损的残余地幔。石英硬玉岩原岩包括正变质的花岗岩和奥长花岗岩、副变质的酸性火山碎屑岩和长石石英砂岩。大面积分布的古老花岗岩很可能是形成在大陆或大陆边缘环境。长石石英云母片岩、石英岩和大理岩的原岩为沉积岩，与副变质的长英质片麻岩和基性火山岩—起构成了古老的表壳岩组合。双峰式的酸性和基性火山岩组合的存在也证明部分表壳岩是形成在大陆环境。因此，可以推测南苏鲁造山带核部的超高压变质岩原岩为形成在大陆板内环境的沉积岩—酸性和基性火山岩—花岗岩和奥长花岗岩建造。     

Devonian rodingite from the northern margin of the North China Craton: mantle wedge metasomatism during ocean–continent convergence

The northern margin of the North China Craton (NCC) was an active convergent margin during Palaeozoic and preserves important imprints of magmatic and metasomatic processes associated with oceanic plate subduction. Here, we investigate the mafic–ultramafic rocks in the Xiahabaqin–Sandaogou complexes from the northern NCC including pyroxenite, hornblendites, hornblende gabbro, and their rodingitized counterparts within a serpentinite domain. We present petrological, zircon U–Pb geochronological, and geochemical data to constrain the nature and timing of the magmatic and metasomatic processes in the subduction zone mantle wedge. The rock suites investigated in this study are characterized by low contents of SiO₂, Na₂O, and K₂O, with high CaO, FeO, Fe₂O₃, and MgO. The rodingitized rocks show markedly high CaO and lower MgO compared to their ultramafic protolith, suggesting extensive post-magmatic infiltration of Ca-rich, Si-poor fluids derived by serpentinization of mantle peridotite. The enrichment of large ion lithophile and light rare earth elements such as Ba, Sr, K, La, and Ce with relative depletion of high field strength elements like Nb, Ta, Zr, and Hf in the ultramafic rocks collectively suggest metasomatism of a fore-arc mantle wedge by fluids released through dehydration of subducted oceanic slab and subduction-derived sediments. Dehydration and decarbonation leading to metasomatic fluid influx and serpentinization of mantle wedge peridotite account for the enriched geochemical signatures for the rodingitized rocks. The zircon grains in these rocks show textures indicating magmatic crystallization followed by fluid-controlled dissolution–precipitation. Magmatic zircons from altered pyroxenite, hornblendite, and rodingitized pyroxenite in Xiahabaqin yield protolith crystallization ages peaks at 396 Ma and 392 Ma and metasomatic grains show ages of 386 Ma, 378 Ma, and 348 Ma. The zircons from hornblendite and basaltic trachyandesite indicate protolith emplacement during 402–388 Ma. Metasomatic zircon grains from rodingitized hornblende gabbro in Sandaogou complex show a wide range of ages as 412 Ma, 398 Ma, 383 Ma, and 380 Ma. The common magmatic zircon ages peaks at 398–388 Ma in most of the rocks suggest a similar time for magma crystallization in the Xiahabaqin and Baiqi during Middle Devonian. Subsequently, repeated pulses fluids and melts resulted in metasomatic reactions in mantle wedge until early Permian. The Lu–Hf analysis of the zircon grains from these rocks display markedly negative εHf(t) values ranging from ?22.4 to ?7.7, suggesting magma derivation from an enriched, hydrated lithospheric mantle through fluid–rock interaction and mantle wedge metasomatism. Rodingitization processes are associated with exhumation of ultramafic mantle wedge rocks within a serpentinized subduction channel close to the subducted slab in response to slab roll back in a long-lasting subduction regime. This study offers insights into magmatic and metasomatic processes of ultramafic rocks in the fore-arc mantle wedge which were exhumed and accreted to an active continental margin during the southward subduction of the Palaeo-Asian oceanic lithosphere beneath the NCC.     

A study of natural and experimental metasomatic assemblages in an ultramafic-quartzofeldspathic metasomatic system from the haast schist,South Island,New Zealand

The mineralogy of a metasomatic sequence formed between ultramafic and quartzofeldspathic protoliths from the Southern Alps of New Zealand consists of a forsterite-antigorite core surrounded concentrically by zones of antigorite-magnesite, magnesite-talc, talc, tremolite, chlorite and muscovite with discontinuous pods of albite associated with the muscovite zone. On the basis of trace element data the original ultramafite-schist contact is positioned between the present tremolite and chlorite zones.An experimental study of a metasomatic system was undertaken in an attempt to clarify diffusion relationships during the metasomatic event. To simulate the natural event, ultramafic and quartzofeldspathic natural starting materials were tightly packed in a gold tube with a graphite layer between to allow later identification of the original lithological interface. Run conditions were 450° C at 2 kb for 40 days. Phase dissolution and formation were analysed petrographically and component migration was examined with the electron-microprobe. The following hierarchical scheme of component migration, phase dissolution and phase formation is delineated: CO₂ migrates from the schist into the ultramafite forming first the antigorite-magnesite zone and then the magnesite-talc zone at higher values of CO₂. These zones are then partially overprinted by the formation of talc due to SiO₂ metasomatism. The SiO₂ is supplied from the schist by the dissolution of quartz and albite in the region adjacent to the ultramafite. The tremolite zone forms at the expense of the metasomatic talc zone upon the introduction of CaO from the schist into the ultramafite. Concurrent with tremolite formation, MgO migrates from the ultramafite into the schist to form the chlorite metasomatic zone. The growth of the chlorite zone causes dissolution of the pre-metasomatic micas and displaces K₂O from the chlorite zone further into the schist. Displaced K₂O and Na₂O are responsible for the formation of the muscovite zone and the albitite pods.     

Subduction metamorphism of serpentinite‐hosted carbonates beyond antigorite‐serpentinite dehydration (Nevado‐Filábride Complex,Spain)

At sub‐arc depths, the release of carbon from subducting slab lithologies is mostly controlled by fluid released by devolatilization reactions such as dehydration of antigorite (Atg‐) serpentinite to prograde peridotite. Here we investigate carbonate–silicate rocks hosted in Atg‐serpentinite and prograde chlorite (Chl‐) harzburgite in the Milagrosa and Almirez ultramafic massifs of the palaeo‐subducted Nevado‐Filábride Complex (NFC, Betic Cordillera, S. Spain). These massifs provide a unique opportunity to study the stability of carbonate during subduction metamorphism at P–T conditions before and after the dehydration of Atg‐serpentinite in a warm subduction setting. In the Milagrosa massif, carbonate–silicate rocks occur as lenses of Ti‐clinohumite–diopside–calcite marbles, diopside–dolomite marbles and antigorite–diopside–dolomite rocks hosted in clinopyroxene‐bearing Atg‐serpentinite. In Almirez, carbonate–silicate rocks are hosted in Chl‐harzburgite and show a high‐grade assemblage composed of olivine, Ti‐clinohumite, diopside, chlorite, dolomite, calcite, Cr‐bearing magnetite, pentlandite and rare aragonite inclusions. These NFC carbonate–silicate rocks have variable CaO and CO₂ contents at nearly constant Mg/Si ratio and high Ni and Cr contents, indicating that their protoliths were variable mixtures of serpentine and Ca‐carbonate (i.e., ophicarbonates). Thermodynamic modelling shows that the carbonate–silicate rocks attained peak metamorphic conditions similar to those of their host serpentinite (Milagrosa massif; 550–600°C and 1.0–1.4 GPa) and Chl‐harzburgite (Almirez massif; 1.7–1.9 GPa and 680°C). Microstructures, mineral chemistry and phase relations indicate that the hybrid carbonate–silicate bulk rock compositions formed before prograde metamorphism, likely during seawater hydrothermal alteration, and subsequently underwent subduction metamorphism. In the CaO–MgO–SiO₂ ternary, these processes resulted in a compositional variability of NFC serpentinite‐hosted carbonate–silicate rocks along the serpentine‐calcite mixing trend, similar to that observed in serpentinite‐hosted carbonate‐rocks in other palaeo‐subducted metamorphic terranes. Thermodynamic modelling using classical models of binary H₂O–CO₂ fluids shows that the compositional variability along this binary determines the temperature of the main devolatilization reactions, the fluid composition and the mineral assemblages of reaction products during prograde subduction metamorphism. Thermodynamic modelling considering electrolytic fluids reveals that H₂O and molecular CO₂ are the main fluid species and charged carbon‐bearing species occur only in minor amounts in equilibrium with carbonate–silicate rocks in warm subduction settings. Consequently, accounting for electrolytic fluids at these conditions slightly increases the solubility of carbon in the fluids compared with predictions by classical binary H₂O–CO₂ fluids, but does not affect the topology of phase relations in serpentinite‐hosted carbonate‐rocks. Phase relations, mineral composition and assemblages of Milagrosa and Almirez (meta)‐serpentinite‐hosted carbonate–silicate rocks are consistent with local equilibrium between an infiltrating fluid and the bulk rock composition and indicate a limited role of infiltration‐driven decarbonation. Our study shows natural evidence for the preservation of carbonates in serpentinite‐hosted carbonate–silicate rocks beyond the Atg‐serpentinite breakdown at sub‐arc depths, demonstrating that carbon can be recycled into the deep mantle.     

Fluid-controlled crustal metasomatism within a high-pressure subducted mélange (Mt. Hochwart, Eastern Italian Alps)

The Nonsberg–Ultental Region of northern Italy contains a Palaeozoic mélange that was partially subducted during the Variscan orogeny. This mélange is constituted mainly by metapelites characterized by shale-type REE-patterns, displaying partial melting which began under high-pressure conditions. The resulting migmatites enclose minor slivers of mantle-wedge peridotites that have been incorporated into the mélange during subduction. Peridotites display important large ion lithophile elements (LILE) enrichment consequent to amphibole recrystallization contemporaneously with metapelite migmatization at P ≈ 2.7 GPa and T ≈ 850 °C in the garnet–peridotite field. Crustal and mantle (ultramafic) rocks of the mélange display the same Sm–Nd ages of about 330 ± 6 Ma, which dates both the metamorphic peak and the migmatization event. The zircon U–Pb age of the metasomatic amphibolitic contact between garnet peridotite and migmatite is identical (333.3 ± 2.4 Ma) within analytical errors. Therefore, metasomatism, migmatization and peak metamorphism are constrained to the same event. The presence of Cl-rich apatite and ferrokinoshitalite in the contact amphibolite, together with the trace-element patterns of peridotites, suggest that metasomatism was driven by Cl- and LILE-rich fluids derived from ocean water transported into the subduction zone by sediments and crustal rocks. These fluids interacted with the crust, prompting partial melting under water oversaturated conditions and partitioning LILE from the crust itself. Peridotites, which were well below their wet solidus temperature, could not melt but they recrystallized in the crustal mélange under garnet-facies conditions. Crustal fluids caused extensive hydration and LILE-enrichment in peridotites and severe Sm–Nd isotope disequilibrium between minerals, especially in the recrystallized peridotites. The proposed scenario suggests massive entrapment of crustal aqueous fluids at high-pressure conditions within subduction zones.     

The importance of talc and chlorite “hybrid” rocks for volatile recycling through subduction zones; evidence from the high-pressure subduction mélange of New Caledonia

The transfer of fluid and trace elements from the slab to the mantle wedge cannot be adequately explained by simple models of slab devolatilization. The eclogite-facies mélange belt of northern New Caledonia represents previously subducted oceanic crust and contains a significant proportion of talc and chlorite schists associated with serpentinite. These rocks host large quantities of H₂O and CO₂ and may transport volatiles to deep levels in subduction zones. The bulk-rock and stable isotope compositions of talc and chlorite schist and serpentinite indicate that the serpentinite was formed by seawater alteration of oceanic lithosphere prior to subduction, whereas the talc and chlorite schists were formed by fluid-induced metasomatism of a mélange of mafic, ultramafic and metasedimentary rocks during subduction. In subduction zones, dehydration of talc and chlorite schists should occur at sub-arc depths and at significantly higher temperatures (∼ 800°C) than other lithologies (400–650°C). Fluids released under these conditions could carry high trace-element contents and may trigger partial melting of adjacent pelitic and mafic rocks, and hence may be vital for transferring volatile and trace elements to the source regions of arc magmas. In contrast, these hybrid rocks are unlikely to undergo significant decarbonation during subduction and so may be important for recycling carbon into the deep mantle. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Geochemical evidence for arc-type volcanism in the Aegean Sea: the blueschist unit of Siphnos, Cyclades (Greece)

Blueschists, eclogites, chlorite–actinolite rocks and jadeite-gneisses of the blueschist unit of Siphnos have been investigated for their geochemical composition. Their protolith nature is characterised and a geodynamic model for the pre-metamorphic evolution of these metavolcanic rocks is proposed on the basis of immobile elements, especially trace elements and rare earth elements (REE).

The protoliths of the eclogites are characterised as calc-alkaline basalts, andesites and Fe-rich tholeiites evolving in an island-arc setting. Trace element data indicate that subducted marine sediments were assimilated in the magma chamber, enriching the protoliths in LILE and Pb. Produced in the early stage of back-arc basin opening, a protolith with affinities to both island-arc and MORB formed the precursor of the chlorite–actinolite rocks. They were created by low degrees of partial melting of very primitive magmas, akin to spinel-peridotites and have affinities to boninites, probably through melting of the peridotitic mantle wedge. Tholeiitic basalts and andesites with N-MORB affinity, especially in their REE-patterns, were then produced by partial melting, possibly in an embryonic back-arc basin. These rocks were the protoliths of the blueschists of Siphnos. Their enrichment in some LILE and Pb indicates a N-MORB source contaminated by marine sediments, probably shales or other Pb-rich sediments. Because the jadeite-gneisses show affinities to MOR-granites and volcanic arc granites, intrusion of their protoliths in a back-arc environment is likely. The protoliths of the quartz-jadeite gneisses are rhyodacites/dacites and rhyolites, those of the glaucophane-jadeite gneisses were andesites.

The proposed geodynamic model, solely based on geochemical data, is consistent with geochemical data from neighbouring islands, though those rock units show much higher chemical variability. Consistent with geotectonic models, which are based on structural and geophysical data, the volcanic protoliths of the Siphnos blueschist unit reflect the transition from subduction to spreading environment and record in detail: subduction, formation of an island-arc, and the evolution of a back-arc basin.     

危地马拉硬玉岩与硬玉化绿辉石岩的岩石学特征及其地质意义

位于北美-加勒比板块俯冲带内的危地马拉硬玉岩区产有硬玉岩、榴辉岩、钠长岩等岩石类型,但绿辉石岩还没有详细报导过。研究样品为该区的硬玉岩和绿辉石岩,前者主要由Jd端元含量较高的硬玉组成,具有粒、柱状镶嵌结构。硬玉晶体具有显著的成分振荡环带,其核部及其背散射电子图像的浅色区Jd端元含量为94.81%～95.48%;暗色区Jd含量大于97.92%。后者绿辉石岩具有交代结构,主要由绿辉石和硬玉组成。其中绿辉石的CaO(9.01%～10.80%)和MgO(6.09%～7.94%)含量较高,FeO(2.84%～4.89%)含量较低;硬玉的CaO、MgO和FeO含量变化范围较大,分别为0.59%～4.30%,0.26%～3.05%,0.76%～2.87%。硬玉岩中流体包裹体的存在,以及绿辉石岩中显著的硬玉交代绿辉石现象说明其成因与缅甸硬玉一样,是硬玉质流体通过结晶-交代作用形成。硬玉岩中硬玉振荡的环带结构反映形成时温度-压力-组分体系存在振荡变化,平直连续的环带边界反映结晶时P-T条件位于硬玉稳定的低温高压区域。该区绿辉石岩的硬玉化作用清楚,能够观察到三期硬玉岩化作用现象,说明绿辉石岩可能是在硬玉岩的形成过程中,硬玉质流体与原岩辉石岩作用的结果,这进一步阐明了俯冲带内硬玉岩区硬玉质流体的广泛透入性与结晶交代作用的多样性。     

The Wadi Zaghra metasediments of Sinai,Egypt: new constraints on the late Cryogenian–Ediacaran tectonic evolution of the northernmost Arabian–Nubian Shield

ABSTRACT

The La Tinta mélange is a small but singular ultramafic mélange sheet that crops out in eastern Cuba. It is composed of dolerite-derived amphibolite blocks embedded in a serpentinite matrix. The amphibolite blocks have mid-ocean ridge basalt (MORB)-like composition showing little if any imprint of subduction zone component, similar to most forearc and MOR basalts worldwide. Relict Cr-spinel and olivine mineral chemistry of the serpentinized ultramafic matrix suggest a forearc position for these rocks. These characteristics, together with a hornblende ⁴⁰Ar/³⁹Ar age of 123.2 ± 2.2 Ma from one of the amphibolite blocks, suggest that the protoliths of the amphibolite blocks correspond to forearc basalt (FAB)-related rocks that formed during the earlier stage of subduction initiation of the Early Cretaceous Caribbean arc. We propose that the La Tinta amphibolites correspond to fragments of sills and dikes of hypoabyssal rocks formed in the earlier stages of a subduction initiation scenario in the Pacific realm (ca. 136 Ma). The forearc dolerite-derived amphibolites formed by partial melting of upwelling fertile asthenosphere at the beginning of subduction of the Proto-Caribbean (Atlantic) slab, with no interaction with slab-derived fluids/melts. This magmatic episode probably correlates with Early Cretaceous basic rocks described in Hispaniola (Gaspar Hernandez serpentinized peridotite-tectonite). The dikes and sills cooled and metamorphosed due to hydration at low pressure (ca. 3.8 kbar) and medium to high temperature (up to 720ºC) and reached ca. 500ºC at ca. 123 Ma. At this cooling stage, serpentinite formed after hydration of the ultramafic upper mantle. This process might have been favoured by faulting during extension of the forearc, indicating an early stage of dike and sill fragmentation and serpentinite mélanges formation; however, full development of the mélange likely took place during tectonic emplacement (obduction) onto the thrust belt of eastern Cuba during the latest Cretaceous.     

Metaultramafic schists and dismembered ophiolites of the Ashe Metamorphic Suite of northwestern North Carolina,USA

ABSTRACT

Metaultramafic rocks (MUR) in the Ashe Metamorphic Suite (AMS) of northwestern North Carolina include quartz ± feldspar-bearing QF-amphibolites and quartz-deficient, locally talc-, chlorite-, and/or Mg-amphibole-bearing TC-amphibolites. Some workers divide TC-amphibolites into Todd and Edmonds types, based on mineral and geochemical differences, and we provisionally add a third type – olivine ± pyroxene-rich, Rich Mountain-type rocks. Regionally, MUR bodies range from equant, Rich Mountain- to highly elongate, Todd-TC-amphibolite-type bodies. The MURs exhibit three to five mineral associations containing assemblages with olivine, anthophyllitic amphibole, Mg-hornblende, Mg-actinolite, cummingtonite, and serpentine representing decreasing eclogite to greenschist facies grades of metamorphism over time. MUR protoliths are difficult to determine. Southwestern MUR bodies have remnant olivine ± pyroxene-rich assemblages representing ultrabasic-basic, dunite-peridotite-pyroxenite protoliths. Northeastern TC-amphibolite MURs contain hornblende and actinolitic amphiboles plus chlorites – aluminous and calcic assemblages suggesting to some that metasomatism of basic, QF-amphibolites yields all TC-amphibolites. Yet MgO-CaO-Al₂O₃ and trace element chemistries of many TC-amphibolites resemble compositions of plagioclase peridotites. We show that a few AMS TC-amphibolites had basaltic/gabbroic protoliths, while presenting arguments opposing application of the metasomatic hypothesis to all TC-amphibolites. We establish that MUR bodies are petrologically heterolithic and that TC-amphibolites are in contact with many rock types; that those with high Cr, Ni, and Mg have olivine- or pyroxene-dominated protoliths; that most exhibit three or more metamorphic mineral associations; and that contacts thought to be metasomatic are structural. Clearly, different MUR bodies have different chemistries representing various protoliths, and have different mineral assemblages, reflecting both chemical composition and metamorphic history. Spot sampling of heterolithic MUR bodies does not reveal MUR body character or history or allow ‘type’ designations. We recommend that the subdivision of MUR bodies into ‘types’ be abandoned and that the metasomatic hypothesis be carefully applied. AMS MURs and associated metamafic rocks likely represent fragments of dismembered ophiolites from various ophiolite types.     

Element mobility in mafic and felsic ultrahigh-pressure metamorphic rocks during continental collision

In order to decipher element mobility in ultrahigh-pressure (UHP) eclogite-facies metamorphic rocks during subduction and exhumation of continental crust, major-trace elements and Sr-Nd isotopes were systematically investigated for two continuous core segments of about 3 m length from the Chinese Continental Scientific Drilling (CCSD) project in the Sulu orogen. The segments are composed of lithological transitions between UHP eclogite and granitic gneiss. The eclogite exhibits a large variation in major and some trace elements such as LILE (e.g., Rb, Ba and K) and LREE, but a relatively limited range in HFSE and HREE. This suggests high mobility of LILE and LREE but immobility of HFSE and HREE during continental collision-zone metamorphism. Some eclogites have andesitic compositions with high SiO₂, alkalis, LREE, and LILE but low CaO, MgO and FeO contents. These features likely result from chemical exchange with gneisses, possibly due to the metasomatism of felsic melt produced by partial melting of the associated gneisses during the exhumation. On the other hand, some eclogites appear to have geochemical affinity to refractory rocks formed by melt extraction as evidenced by strong LREE and LILE depletion and the absence of hydrous minerals. These results provide evidence of melt-induced element mobility in the UHP metamorphic rocks. In particular, large variations in the abundance of such elements as SiO₂, LREE and LILE occur at the contact between eclogite and granitic gneiss, indicating their mobility between different slab components. Petrographic observations also show the presence of felsic veins on small scales in the UHP metamorphic rocks, demonstrating the occurrence of hydrous melt in local open-systems during the continental collision. As a whole, nevertheless, the protolith nature dictates the geochemical differences in both eclogite and granitic gneiss between the two core segments because mass transport during the subduction-zone metamorphism is principally dictated by the lithological differences at contact. The eclogite and granitic gneiss from the first core segment have high ε_Nd(t) values, whereas those from the second core segment show relatively low ε_Nd(t) values in concordance with majority of UHP metaigneous rocks outcropped along the Dabie-Sulu orogenic belt. Thus contrasting origins of bimodal igneous rocks were involved in the continental collision, demonstrating that the subducted continental crust is the magmatic product of active rifting margin during supercontinental breakup in the middle Neoproterozoic.     

Trace-element geochemistry of transform-fault serpentinite in high-pressure subduction mélanges (eastern Cuba): implications for subduction initiation

The Sierra del Convento and La Corea mélanges (eastern Cuba) are vestiges of a Cretaceous subduction channel in the Caribbean realm. Both mélanges contain blocks of oceanic crust and serpentinite subducted to high pressure within a serpentinite matrix. The bulk composition of serpentinite indicates spinel-harzburgite and -herzolite protoliths. The samples preserve fertile protolith signatures that suggest low melting degrees. High concentration of immobile elements Zr, Th, Nb, and REE contents (from ~0.1 to ~2 CI-chondrite) point to early melt–rock interaction processes before serpentinization took place. Major- and trace-element compositions suggest an oceanic fracture-zone–transform-fault setting. A mild negative Eu anomaly in most samples indicates low-temperature fluid–rock interaction as a likely consequence of seawater infiltration during oceanic serpentinization. A second, more important, serpentinization stage is related to enrichment in U, Pb, Cs, Ba, and Sr due to the infiltration of slab-derived fluids. The mineral assemblages are mainly formed by antigorite, lizardite, and chlorite, with local minor talc, tremolite, anthophyllite, dolomite, brucite, and relict orthopyroxene. The local presence of anthophyllite and the replacements of lizardite by antigorite indicate a metamorphic evolution from the cooling of peridotite/serpentinite at the oceanic context to mild heating and compression in a subduction setting. We propose that serpentinites formed at an oceanic transform-fault setting that was the locus of subduction initiation of the Proto-Caribbean basin below the Caribbean plate during early Cretaceous times. Onset of subduction at the fracture zone allowed the preservation of abyssal transform-fault serpentinites at the upper plate, whereas limited downward drag during mature subduction placed the rocks in the subduction channel where they tectonically mixed with the upward-migrating accreted block of the subducted Proto-Caribbean oceanic crust. Hence, we suggest that relatively fertile serpentinites of high-pressure mélanges were witness to the onset of subduction at an oceanic transform-fault setting.     

Fingerprints of forearc element mobility in blueschist-facies metaconglomerates,Catalina Schist,California

ABSTRACT

Metaconglomerates in the lawsonite–blueschist facies unit of the Catalina Schist (California) contain gabbroic and dioritic clasts exhibiting evidence for extensive metasomatism during high-P/T metamorphism. We performed whole-rock and in situ analyses of these metaconglomerate clasts to better constrain the composition of infiltrating fluids and to elucidate the history of chemical alteration. Petrographic evidence for this alteration includes replacements of plagioclase by phengite and sodic amphibole rims developed on igneous hornblende. These observations regarding mineral replacement are reinforced by corresponding shifts in chemical compositions. Relative to compositions of presumed protoliths, whole-rock compositions of the metaconglomerate clasts show enrichments in elements that are relatively mobile in aqueous fluids (LILE: K, Rb, Cs, and Ba; Li, B, N), and elevated δ¹⁵N, and show depletions in Ca and Sr. Electron and ion microprobe data, and analyses of mineral separates, show that phengite and sodic amphibole are enriched in LILE and Li and B, respectively, relative to the igneous phases they have replaced. Oxygen and C isotope compositions of finely disseminated calcite in the clasts, and of calcite in veins cross-cutting or mantling the clasts, are consistent with crystallization from fluids previously equilibrated with metasedimentary rocks within the same unit. The same fluids are implicated as the source for the Li, B, N, and LILE enrichments. These metaconglomerate clasts provide unique records of forearc metasomatism due to the presumed extremely low and well-constrained concentrations of fluid-mobile elements in their protoliths and the previously published, larger-scale fluid–rock context into which the observed metasomatic changes can be placed.