Metamorphic evolution and geochemistry of kyanite eclogites in central Rhodope,northern Greece

In central Rhodope of northern Greece, kyanite eclogites were discovered in the area of Thermes. They are strongly overprinted and exhibit a multi-stage development of minerals and mineral assemblages formed during successive stages of the exhumation. The initial high-pressure assemblage was garnet+omphacite+kyanite+zoisite+phengite+rutile+quartz. Corundum, Fe-Mg-spinel, sapphirine and högbomite occur as products of a first, high-temperature overprint, still at high pressures, whereas various symplectites [corundum-plagioclase (pl), spinel-pl, sapphirine-pl, clinopyroxene-pl, biotite-pl, amphibole-pl] grew during subsequent stages of the exhumation. Diablastic amphibole+plagioclase formed as end-products of the amphibolitization. According to geochemical data, the protoliths of the kyanite eclogites were basalts to basaltic andesites with “volcanic arc” affinities. For the high-pressure stage of metamorphism, minimum PT conditions were around 19 kbar, 700°C, while for the initial stages of the overprint, high-pressure granulite-facies conditions prevailed (T>800°C, at P>15 kbar). The PT conditions of the amphibolite facies were 8–11 kbar, 580–690°C. The kyanite eclogites of Thermes record the highest temperatures of metamorphism within the whole of Rhodope.     

新疆西南天山哈布腾苏一带榴辉岩的岩石学特征及变质作用P-T轨迹

西南天山哈布腾苏河沿岸的含石墨的石榴石多硅白云母石英片岩中出露一套若干大小不等的布丁状变基性岩块,产状与区域面理一致。本文对其中保存完好的榴辉岩体进行了较为细致的岩石学研究和温压演化条件计算。根据主要矿物的含量,将该套榴辉岩大致分为两类一角闪榴辉岩和钠云母榴辉岩,二者的主要矿物均为Grt＋Omp＋Na—Ca-Amp＋Pg＋Dol／Cal＋Rt±Qtz。石榴石变斑晶两阶段生长明显,从核部到边部XMn和XFe降低,XMg和XCa升高,指示了升温降压的变质过程。根据石榴石核部和边部的包体组合特征,确立了两期榴辉岩相变质作用：前一阶段经历了高压但较低温的硬柱石．硬绿泥石（仅见假象）榴辉岩相,变质温度为400～5000C,压力不低于1．8～1．9GPa,表明早期经历了快速俯冲过程;后一阶段的变质温度为570±300C,压力为2．0～2．5GPa。在退变质绿帘角闪岩相阶段,形成低压脉体（矿物组合为Ab4-Di＋Na—Ca—Amp＋Ep／Czo＋Cal）和一系列退变质反应结构．如Dol的Cal增生边．Omp的Di＋Ab后成合晶结构。利用Dol—Cal分溶温度计和Di的Jd分子含量得到该阶段的温度约500—530℃,压力小于0．9～1．1GPa,表明其退变质经历的是降温降压过程。这与利用Thermocale 3．1在NCFMASH体系下计算的PT视剖面图是一致的。     

苏鲁超高压榴辉岩的石榴石生长成分环带及变质作用P—T轨迹

在南苏鲁东海地区,部分超高压榴辉岩中的变斑晶石榴石具有复杂的生长成分环带和多期矿物包体组合,它们记录了超高压变质岩的多阶段变质演化过程,即绿帘角闪岩相进变质、柯石英榴辉岩相峰期变质、石英榴辉岩相和角闪岩相退变质作用。运用相关的地质温、压计,使用代表最高变质温度的变斑晶石榴石慢部（具最低的Fe／Mg比值）和与其平衡的绿辉石包体成分,获得了〉900℃和4．1～4．5GPa的超高压变质务件。联合其他变质阶段的温、压条件,一个顺时针的变质作用P—T轨迹得以建立。它的特征是进变质与退变质路径近于平行,早期退变质作用为降温、降压过程．榴辉岩石榴石生长成分环带的保存说明超高压变质岩在峰期变质阶段有非常短暂的停留时间,并以很快的折返速率抬升到地壳浅部,超高压变质岩折返过程中的明显降温是石榴石生长环带得以保存的另一个有利条件,     

Dolomitic marbles from the ultrahigh-pressure metamorphic Kimi complex in Rhodope, N.E. Greece

Summary Dolomitic marbles from the Organi and Pandrosos areas of the ultrahigh-pressure (UHP) metamorphic Kimi complex in East Rhodope, N.E. Greece have the mineral assemblage: Cal + Dol + Ol + Phl ± Di ± Hbl ± Spl ± Ti–Chu + retrograde Srp and Chl. Several generations of calcite and dolomite with variable composition and texture represent different stages of the P–T evolution: The first stage is represented by matrix dolomite ( = 0.48) and relic domains of homogenous composition in matrix calcite ( = 0.11–0.13); the second stage is evident from precipitation of lath-shaped and vermicular dolomite in matrix calcite. The third stage is represented by veinlets of almost pure CaCO₃ and domainal replacement of prior calcite by nearly pure CaCO₃ + Ca-rich dolomite ( = 0.34–0.43). Matrix dolomite adjacent to CaCO₃ veinlets also becomes Ca-rich ( = 0.42). In fact, Ca-rich dolomites with in the range of 0.40–0.34 are reported for the first time from metamorphic marbles. Coexisting Ca-rich dolomite and Mg-poor calcite cannot be explained by the calcite-dolomite miscibility gap. This assemblage rather suggests that Mg-poor calcite was aragonite originally, which formed together with Ca-rich dolomite according to the reaction Mg–Cal → Arg + Dol (1) at ultrahigh pressures and temperatures above at least 850 °C, when dolomite becomes disordered and incorporates more Ca than coexisting aragonite does in terms of Mg. The simplest explanation of these observations probably is to suggest two metamorphic events: The first one represented by relic matrix carbonates at relatively low to moderate pressures and temperatures of ca. 750 °C, and the second one limited by the minimum temperatures for dolomite disorder (ca. 850 °C) and in the aragonite + dolomite stability field, i.e. at a minimum pressure of 3 GPa and, if the presence of diamond-bearing metapelites nearby is considered, at conditions of at least 850 °C and 4.3 GPa in the diamond stability field. As there is hardly any back-reaction of Ca-rich dolomite + Mg-poor calcite to Mg-rich calcite, peak temperatures remained below the reaction (1) and the exhumation path probably crossed the aragonite-calcite transition at much lower than peak temperature. Cooling and decompression must have both occurred extremely fast in order for the μm-sized Ca-rich dolomite textures to be preserved. An alternative explanation of the formation of “UHP”-textures and compositions is by a fluid influx that not only caused serpentinisation and chloritisation of silicates but also Mg-leaching from carbonates, particularly from Mg-rich calcite and its fine grained dolomite-precipitates, thus transforming them into Mg-poor calcite + Ca-rich dolomite.     

Characterisation of igneous terranes by zircon dating: implications for UHP occurrences and suture identification in the Central Rhodope, northern Greece

The occurrence of ultra high pressure (UHP) and high pressure (HP) relicts associated with oceanic material suggests the presence of a suture zone within the Rhodope Massif. Characterisation of the accreted igneous terranes and their relationship with the UHP/eclogite occurrences provide new constraints on the location of this suture. Single-zircon evaporation and sensitive high-resolution ion microprobe dating of orthogneiss protoliths define two groups of intrusion ages: Permo-Carboniferous and Late Jurassic–Early Cretaceous. Structurally, the Late Jurassic gneissic complex overthrust a unit with Permo-Carboniferous orthogneisses. A “melange zone” marked by mylonites, eclogites, amphibolites, and UHP micaschists separates these two units. We interpret these observations in terms of two distinct igneous terranes, the Thracia (Permo-Carboniferous) and Rhodope (Late Jurassic) terranes, separated by the Nestos suture, and assembled during the closure of an oceanic basin of the Tethys. Geochemically, the Late-Jurassic rocks are akin to subduction magmatism, possibly the same subduction that caused the UHP metamorphism of metasediments within the “melange zone”. Observed UHP–HP relicts are restricted to the tectonic contact zone, suggesting that a single subduction/collison event can explain the occurrences of UHP relicts and eclogites in the Central Rhodope, and that subducted rocks are exhumed only within the Nestos suture.     

桐柏山地区低温榴辉岩变质作用的P-T轨迹

桐柏山地区发育南、北两条低温高压榴辉岩带,中间被桐柏山高级变质杂岩所分隔。本文对两条带内的4个保存完好的榴辉岩体及其退变产物进行了详细的岩石学研究和P—T条件估算。结果表明,北带榴辉岩的形成温压条件较高,为530～6100C、1．7～2．0GPa,南带榴辉岩较低,为470～5200C、1．3～1．7GPa,说明两条榴辉岩带不属于同一个高压岩片。实际上,二者分别对应于西大别地区的浒湾榴辉岩带（Ⅰ带）和七角山榴辉岩带（Ⅴ带）。电子探针分析发现,所有榴辉岩中的石榴石都展示了进变质环带,而退变榴辉岩的成分环带更为复杂,即保留了早期的进变环带,也显示了晚期的低温取代以及后期相对高温条件下的再生长,由此为榴辉岩的变质作用推导出一个“发卡式”演化轨迹。这种轨迹表明,榴辉岩先是在降温条件下缓慢向上回返的,这与目前已建立的多数高压、超高压岩石的快速抬升模式有所差异,而后在其进入到中地壳后又在相对较高的地温梯度下遭受退变质作用的改造。     

Chemical composition of pargasite and hornblende in low to High grade metamorphic rocks of the Rhodope zone,Xanthi, Greece

Summary The chemistry of amphiboles from schists, quartzofeldspathic gneisses and migmatites ranging in metamorphic grade from greenschist to amphibolite facies has been determined by electron microprobe. Intercalated amphibolites suggest that some of the rocks retrograded from the eclogite stability field; others were never metamorphosed above greenschist facies. Rocks which contain other mineralogical evidence for an original high pressure assemblage have amphiboles with high Na/K, low Fe and relatively low Ti. Other high-grade rocks contain amphibole of broadly pargasitic composition. The pargasites from more Ca-rich bulk compositions have less substitution of Na for Ca in the M4 site than do those from Ca-poor bulk compositions. A lower grade assemblage of amphiboles ranges from hornblende through actinolitic hornblende to actinolite; this is retrograde in the gneisses and migmatites, but may be prograde in the schists. In contrast, the high-grade assemblage shows almost constant high K with variable Na and quite different trends for edenite-type substitution (^IVAl variation with A site occupancy) and variation of^IVAl with Fe/(Fe + Mg) and with Ti.

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Chemische Zusammensetzung von Pargasit und Hornblende in niedrig- bis hoch-gradig metamorphen Gesteinen der Rhodope-Zone, Xanthi, Griechenland

Zusammenfassung Die Chemische Zusammensetzung von Amphibolen aus Schiefern, Quarz-Feldspat-Gneisen, und Migmatiten, deren metamorpher Grad von Grünschiefer- bis zur Amphibolitfazies reicht, wurde mit der Elektronenstrahl-Mikrosonde bestimmt. Zwischengeschaltete Amphibolite legen die Vermutung nahe, daß einige der Gesteine durch retrograde Metamorphose aus dem Eklogit-Stabilitatsfeld hervorgegangen sind; andere jedoch hatten niemals in ihrer Entwicklung einen höheren Grad als den der Grünschie-ferfazies erreicht. Gesteine, die andere mineralogische Hinweise für eine ursprungliche Hochdruck-Paragenese führen, enthalten Amphibole mit hohem Na/K, niedrigen Eisen und relativ niedrigen Ti. Andere high-grade Gesteine enthalten Amphibol von pargasitischer Zusammensetzung. Die Pargasite aus mehr Kalzium-reichen Gesteinen zeigen geringere Substitution von Natrium für Kalzium an den M4 Plätzen als jene aus Kalzium-armen Gesteinen. Eine niedriger-gradige Paragenese von Amphibolen umfaßt Zusammensetzungen von Hornblende über aktinolitische Hornblende bis zu Aktinolit; diese ist in den Gneisen und Migmatiten retrograd, durfte in den Schiefern jedoch prograd sein. Im Gegensatz dazu zeigt die hochgradige Paragenese fast durchwegs konstant hohe Kalium-Gehalte mit variablem Natrium und einen anderen Trend für Edenit-artige Substitution (^IVAl Variation mit Besetzung der A Plätze) und eine Variation von^IVAl mit Fe/(Fe + Mg) und mit Ti.

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With 4 Figures     

Metamorphic kyanite eclogites in the lufilian arc of Zambia

Eclogites formed by the Lufilian (post-Katanga) metamorphism of gabbros intruded into rocks of the Katanga System (Upper Proterozoic) occur in the Central Province of Zambia. Typical constituents of these rocks are omphacite, almandine garnet with significant contents of the pyrope and grossular components, kyanite and rutile. Eclogites from some localities display well preserved, relict ophitic texture inherited from the original gabbro. Hornblende-scapolite metagabbro and coronite metagabbro often accompany the eclogites, and metalherzolite and serpentinite occur at a few localities. The Zambian eclogites are broadly associated with a variety of relatively high-pressure, kyanite-bearing mineral assemblages some of which are possibly in the eclogite facies. It is suggested that, in addition to temperature and load pressure, fluid pressure, fluid composition (H₂O, CO₂, Cl, SO₃) and permeability of the gabbro due to deformation were important factors for the transformation of gabbro to eclogite in the environment of the Lufilian Arc.     

Structure and tectonic evolution of the Pirin-Pangaion Structural Zone (Rhodope Massif,southern Bulgaria and northern Greece)

The Pirin-Pangaion Structural Zone occupies the south-western part of the Rhodope Massif. It consists of Proterozoic amphibolite facies metamorphic rocks of the Rhodopian Supergroup, and granitoids of Hercynian, Late Cretaceous and Palaeogene age. The pre-Hercynian structure of the zone is dominated by an interference pattern of three superimposed fold generations of NE-SW and NW-SE trends. These structures are cut by Hercynian granitoids, and the entire complex is affected by late Hercynian or early Alpine conical folds. The zone was overthrusted by the Ogražden and Kroussia Units (Serbo-Macedonian ‘Massif’) along the north-east vergent Mid-Cretaceous Strimon overthrust, and by the Central Rhodope Zone of the Rhodope Massif, along the south-west vergent Meso-Rhodopean Overthrust. With this thrusting event, the Pirin-Pangaion Structural Zone was brought together with the Serbo-Macedonian ‘Massif’ and the Central Rhodope Zone to form the Late Cretaceous Morava-Rhodope Zone, which acted as a ‘plateau’ along the southern edge of the Eurasian plate. Late Cretaceous granitoid magma of crustal origin intruded this zone, whereas north of it the Srednogorie volcanic island arc was the site of igneous activity with magmas originating in the upper mantle. The West Thrace Zone developed as a Palaeocene to Oligocene depression superimposed over the older basement obliquely to the southern periphery of the Rhodope Massif. In the Late Eocene and Early Oligocene, this depression represented a volcanic island arc with mantle-derived basic to intermediate magmas; contemporaneous granitoid magmas formed through crustal melting in the thickened crust of the Rhodope Massif (Pirin and Pangaion Units included). Early Miocene thrusting was most intense in the Pangaion Unit, and was followed by Late Miocene to Quaternary extension.     

Amphibolites from Panama: anticlockwise P-T paths from a Pre-upper Cretaceous metamorphic basement in Isthmian Central America

Rocks from the metamorphic basement of the Azuero and Sona peninsulas, Panama, consist of schistose amphibolites and minor amounts of metasediment. In the Sona peninsula, strongly zoned amphiboles indicate that the amphibolites followed a progressive anticlockwsie P-T path prograde from low T /low P to medium T /high P , and are retrograded into the greenschist facies. In contrast, the amphibolites of the Azuero peninsula are affected by a low to medium T /low P metamorphism.
The metamorphic events of the Sona amphibolites occurred prior to the intra-Senonian tectonic phase which affects the Mesozoic formations along the Pacific coast of Costa Rica and Panama. The regional significance of such a basement in Isthmian Central America is discussed.     

Phase equilibria and metamorphic evolution of glaucophane‐bearing UHP eclogites from the Western Dabieshan Terrane,Central China

Glaucophane‐bearing ultrahigh pressure (UHP) eclogites from the western Dabieshan terrane consist of garnet, omphacite, glaucophane, kyanite, epidote, phengite, quartz/coesite and rutile with or without talc and paragonite. Some garnet porphyroblasts exhibit a core–mantle zoning profile with slight increase in pyrope content and minor or slight decrease in grossular and a mantle–rim zoning profile characterized by a pronounced increase in pyrope and rapid decrease in grossular. Omphacite is usually zoned with a core–rim decrease in j(o) [=Na/(Ca + Na)]. Glaucophane occurs as porphyroblasts in some samples and contains inclusions of garnet, omphacite and epidote. Pseudosections calculated in the NCKMnFMASHO system for five representative samples, combined with petrographic observations suggest that the UHP eclogites record four stages of metamorphism. (i) The prograde stage, on the basis of modelling of garnet zoning and inclusions in garnet, involves P–T vectors dominated by heating with a slight increase in pressure, suggesting an early slow subduction process, and P–T vectors dominated by a pronounced increase in pressure and slight heating, pointing to a late fast subduction process. The prograde metamorphism is predominated by dehydration of glaucophane and, to a lesser extent, chlorite, epidote and paragonite, releasing ～27 wt% water that was bound in the hydrous minerals. (ii) The peak stage is represented by garnet rim compositions with maximum pyrope and minimum grossular contents, and P–T conditions of 28.2–31.8 kbar and 605–613 °C, with the modelled peak‐stage mineral assemblage mostly involving garnet + omphacite + lawsonite + talc + phengite + coesite ± glaucophane ± kyanite. (iii) The early decompression stage is characterized by dehydration of lawsonite, releasing ～70–90 wt% water bound in the peak mineral assemblages, which results in the growth of glaucophane, j(o) decrease in omphacite and formation of epidote. And, (iv) The late retrograde stage is characterized by the mineral assemblage of hornblendic amphibole + epidote + albite/oligoclase + quartz developed in the margins or strongly foliated domains of eclogite blocks due to fluid infiltration at P–T conditions of 5–10 kbar and 500–580 °C. The proposed metamorphic stages for the UHP eclogites are consistent with the petrological observations, but considerably different from those presented in the previous studies.     

Tertiary granitoids of Rhodope, northern Greece: Magmatism related to extensional collapse of the Hellenic Orogen?

Many granites that occur in orogenic belts such as the Himalaya, the Hercynides or Caledonides are thought to result from anatexis of the lower parts of over-thickened thrust-stacked crustal units, just preceding termination of the orogenic cycle. Numerous Tertiary granitoids occur in the Rhodope Zone of northern Greece (and adjacent regions of Bulgaria), a belt regarded as an eastern extension of the Alpine System. These granites could also be regarded as syn- or late-tectonic crustal melts in that some have been emplaced as sheet-like bodies along major Alpine thrust planes, and show a strong marginal foliation parallel to the pervasive Alpine compressional fabrics. In detail, however, the geochemistry of the granitoids, their timing and relationships with country rocks, suggest they have been emplaced in an extensional regime that is related to the collapse of the Hellenic Orogen and, more particularly, to the crustal thinning that the Aegean region has suffered since the mid-Tertiary. Rather than being products of crustal thickening, it is suggested that their petrogenesis is related to the elevation of thermal gradients during the extensional phase, which initiated thermal breakdown of hydrous minerals in the lithosphere and lower crust that had existed metastably during compression and the subduction of cool ocean lithosphere beneath the region in the late Mesozoic-early Tertiary.     

Polyphase and anticlockwise P-T evolution for Franciscan eclogites and blueschists from Jenner, California, USA

High-grade exotic blocks in the Franciscan Complex at Jenner, California, show evidence for polydeformation/metamorphism, with eight distinct stages. Two parallel sets of mineral assemblages [(E) eclogite, and (BS) laminated blueschist] representing different bulk chemistry were identified. Stage 1, recorded by parallel aligned inclusions (S1) of crossite + omphacite + epidote + ilmenite + titanite + quartz (E), and glaucophane + actinolite + epidote + titanite (BS) in the central parts of zoned garnets, represents the epidote blueschist facies. The onset of a second stage (stage 2) is represented by a weak crenulation of S1 and growth of garnet. This stage develops a well-defined S2 foliation of orientated barroisite + epidote + titanite (E), or subcalcic actinolite + epidote + titanite (BS) at c. 90d? to S1, with syntectonic growth of garnet, defining the (albite-)epidote-amphibolite facies. A third stage, with aligned inclusions of glaucophane + (subcalcic) actinolite + phengite parallel to S2 in the outermost rims of large garnet grains, is assigned to the transitional (albite-)epidote-amphibolite/(garnet-bearing) epidote blueschist facies. The fourth stage represents the peak metamorphism, and was identified by unorientated matrix minerals in the least retrograded samples. In this stage the mineral assemblages garnet + omphacite + glaucophane + phengite (E) and garnet + winchite + phengite + epidote (BS) both represent the eclogite facies. Stage 5 is represented by the retrogression of eclogite facies assemblages to the epidote blueschist facies assemblages crossite/glaucophane + garnet + omphacite + epidote + phengite (E), and glaucophane + actinolite + epidote + phengite (BS), with the development of an S5 foliation subparallel to S2. Stage 6 represents a crenulation of S5, with the development of a well-defined S6 crenulation cleavage wrapping around relics of the eclogite facies assemblages. This crenulation cleavage is further weakly crenulated during a D7 event. Post-D7 (stage 8) is recorded by the growth of lawsonite + chlorite ± actinolite replacing garnet, and by veins of lawsonite + pumpellyite + aragonite and phengite + apatite. The different, yet coeval, mineral parageneses observed in rock types (E) and (BS) are probably due to differences in bulk chemistry. The metamorphic evolution from stage 1 to stage 8 seems to have been broadly continuous, following an anticlockwise P-Tpath: (1) epidote blueschist (garnet-free) to (2) (albite-)epidote-amphibolite to (3) transitional epidote blueschist (garnet-bearing)/(albite-)epidote-amphibolite to (4) eclogite to (5) epidote blueschist (garnet-bearing) to (6-7) epidote blueschist (garnet-free) facies to (8) lawsonite + pumpellyite + aragonite-bearing assemblages. This anticlockwise P-T path may have resulted from a decreasing geothermal gradient with time in the Mesozoic subduction zone of California at early or pre-Franciscan metamorphism.     

Fluid entrapment and reequilibration during subduction and exhumation: A case study from the high-grade Nestos shear zone, Central Rhodope, Greece

A fluid inclusion study on metamorphic minerals of successive growth stages was performed on highly deformed paragneisses from the Nestos Shear Zone at Xanthi (Central Rhodope), in which microdiamonds provide unequivocal evidence for ultrahigh-pressure (UHP) metamorphism. The correlation of fluid inclusion density isochores and fluid inclusion reequilibration textures with geothermobarometric data and the relative chronology of micro- and macro-scale deformation stages allow a better understanding of both the fluid and metamorphic evolution along the P–T–d path. Textural evidence for subduction towards the NE is recorded by the orientation of intragranular NE-oriented fluid inclusion planes and the presence of single, annular fluid inclusion decrepitation textures. These textures occur within quartz “foam” structures enclosed in an earlier generation of garnets with prolate geometries and rarely within recrystallized matrix quartz, and reequilibrated both in composition and density during later stages of exhumation. No fluid inclusions pertaining to the postulated ultrahigh-pressure stage for microdiamond-bearing garnet–kyanite–gneisses have yet been found. The prolate shape of garnets developed during the earliest stages of exhumation that is recorded structurally by (L  S) tectonites, which subsequently accommodated progressive ductile SW shearing and folding up to shallow crustal levels. The majority of matrix kyanite and a later generation of garnet were formed during SW-directed shear under plane-strain conditions. Fluid inclusions entrapped in quartz during this stage of deformation underwent density loss and transformed to almost pure CO₂ inclusions by preferential loss of H₂O. Those inclusions armoured within garnet retained their primary 3-phase H₂O–CO₂ compositions. Reequilibration of fluid inclusions in quartz aggregates is most likely the result of recrystallization along with stress-induced, preferential H₂O leakage along dislocations and planar lattice defects which results in the predominance of CO₂ inclusions with supercritical densities. Carbonic fluid inclusions from adjacent kyanite–corundum-bearing pegmatoids and, the presence of shear-plane-parallel fluid inclusion planes within late quartz boudin structures consisting of pure CO₂-fluid inclusions with negative crystal shapes, bear witness of the latest stage of deformation by NE-directed extensional shear.This study shows that the textures of early fluid inclusions that formed already during the prograde metamorphic path can be preserved and used to derive information about the kinematics of subduction that is difficult to obtain from other sources. The textures of early inclusions, together with later generations of unaltered primary and secondary inclusions in metamorphic index minerals that can be linked to specific deformation stages and even P–T conditions, are a welcome supplement for the reconstruction of a rather detailed P–T–d path.     

Archaean Kuru-Vaara eclogites in the northern Belomorian Province,Fennoscandian Shield: crustal architecture,timing, and tectonic implications

This paper addresses the relationships between relic amphibole-eclogite facies (AE) eclogites and their host units, Archaean amphibolites, enveloped by Archaean tonalite–trondhjemite–granodiorite (TTG) gneisses, in the Kuru-Vaara study area in the northern Belomorian Province. According to observational constraints, the crystallization of the relic peak omphacite + Mg-garnet ± kyanite assemblage and the subsequent replacement of omphacite by clinopyroxene–plagioclase symplectite occurred before the earliest deformational, metamorphic, and migmatization events that are recorded in the amphibolites. The amphibolites and their TTG hosts have a shared deformational and metamorphic history that is composed of the Archaean and Palaeoproterozoic periods. This history favours the conclusion that the AE metamorphism recorded in the relic eclogites within the amphibolites occurred during the Mesoarchaean to Neoarchaean periods. The deformation and metamorphism of the amphibolite facies of the second period resulted from the Lapland–Kola collisional orogeny at 1.91–1.93 Ga, which led to eclogite–high-pressure granulite (E–HPG) facies conditions in the lowermost portions of the over-thickened crust in Belomorian Province (the southwestern foreland of the Lapland–Kola collisional orogen). The Palaeoproterozoic E–HPG overprint was reported from the Palaeoproterozoic Gridino mafic dikes. Although the ages of the oldest low Th/U zircons are close to the time of the Lapland–Kola collision, the low Th/U 1.9–1.8 Ga zircons reflect a zircon response to regional fluid infiltration in the eclogites during slow exhumation following the Lapland–Kola orogeny and do not record any metamorphic event. Contrary to the Palaeoproterozoic E–HPG overprint, the areal occurrence of the 2.7–2.8 Ga AE eclogites with mid-ocean ridge basalt-like chemistry and their paragenetic link with the TTG gneisses suggest a tectonic regime that involves subduction. This research favours concepts suggesting that the modern-style plate tectonics has operated in some places, at least since the late Mesoarchaean.     

Middle-Late Alpine thermotectonic evolution of the southern Rhodope Massif,Greece

The transition from the Alpine tectonic assembly to the exhumation of the units in the Rhodope metamorphic province in northernmost Greece has been refined by ⁴⁰Ar/³⁹Ar laserprobe mica analyses. Preservation of pre-Alpine (∼ 280 Ma and 145 Ma) muscovite cooling ages at the western margin of the Rhodope indicate that subsequent events failed to reset the argon system thermally in white mica in the outcropping basement of this region. The central and eastern Rhodope are characterized by white mica cooling ages of 40–35 Ma with ages gradually decreasing to ca. 15 Ma near the eastern margin of the Strymon Valley. The Eo-Oligocene ages reflect the regional exhumation of the metamorphosed units to shallow crustal levels, with corresponding temperatures below ca. 350 °C, by 40–35 Ma. The younger cooling ages are attributed to the initiation and subsequent operation of the Strymon-Thasos detachment system since ca. 30 Ma. This study provides a crucial contribution to future regional tectonic models for the Rhodope region as it recognizes an early stage of development of the Strymon-Thasos detachment system, and has constrained the regional exhumation of the Rhodope metamorphic province since 40 Ma indicating that the regionally observed amphibolite facies metamorphism had terminated by this time.     

Middle-Late Alpine thermotectonic evolution of the southern Rhodope Massif,Greece

Abstract

The transition from the Alpine tectonic assembly to the exhumation of the units in the Rhodope metamorphic province in northernmost Greece has been refined by ⁴⁰Ar/³⁹Ar laserprobe mica analyses. Preservation of pre-Alpine (~ 280 Ma and 145 Ma) muscovite cooling ages at the western margin of the Rhodope indicate that subsequent events failed to reset the argon system thermally in white mica in the outcropping basement of this region. The central and eastern Rhodope are characterized by white mica cooling ages of 40–35 Ma with ages gradually decreasing to ca. 15 Ma near the eastern margin of the Strymon Valley. The Eo-Oligocene ages reflect the regional exhumation of the metamorphosed units to shallow crustal levels, with corresponding temperatures below ca. 350 °C, by 40–35 Ma. The younger cooling ages are attributed to the initiation and subsequent operation of the Strymon-Thasos detachment system since ca. 30 Ma. This study provides a crucial contribution to future regional tectonic models for the Rhodope region as it recognizes an early stage of development of the Strymon-Thasos detachment system, and has constrained the regional exhumation of the Rhodope metamorphic province since 40 Ma indicating that the regionally observed amphibolite facies metamorphism had terminated by this time. © 2000 Editions scientifiques et médicales Elsevier SAS     

新疆西南天山哈布腾苏河高压-超高压变质带中榴辉岩和蓝片岩的岩石学及变质演化

对西南天山哈布腾苏河一带出露的典型榴辉岩和蓝片岩进行了详细的岩相学、矿物化学和温压条件综合研究。榴辉岩可分为蓝闪石榴辉岩、钠云母榴辉岩、绿帘石榴辉岩和蓝闪石榴角闪岩（退变榴辉岩）4类,蓝片岩可分为含蓝闪石石榴白云母钠长片岩、石榴白云母蓝闪片岩和石榴白云母蓝闪石英片岩3类。新鲜榴辉岩主要矿物组合为石榴石+绿辉石+钠云母+绿帘石,退变榴辉岩则为石榴石+蓝闪石+角闪石;蓝片岩主要矿物组合为石榴石+蓝闪石+多硅白云母+钠云母+钠长石+石英。榴辉岩和蓝片岩中石榴石变斑晶均保存进变质生长环带,从核部到边部X_Mn和X_Fe降低,X_Mg和X_Ca升高,指示了升温进变质的演化过程。根据榴辉岩矿物共生组合、石榴石内部包体组合分布特征及传统地质温压计估算结果,确定榴辉岩经历了4阶段的变质演化：早期硬柱石蓝片岩相进变质阶段、峰期榴辉岩相变质阶段（t=543～579℃,p=1.5～1.6 GPa）、峰后绿帘蓝片岩相退变质阶段（t=～450℃,p<1.0 GPa）和晚期蓝闪绿片岩相退变质阶段（t<400℃,p<0.5 GPa）。利用p-T视剖面图计算的榴辉岩、蓝片岩峰期变质温压条件与传统地质温压计估算结果十分相近,其中榴辉岩的峰期变质条件t=520～550℃,p=1.7～1.9 GPa;蓝片岩峰期变质条件t=520～620℃,p=1.7～2.3 GPa。本文估算的榴辉岩峰期变质压力条件与前人根据柯石英的发现而认为研究区部分榴辉岩及其围岩曾经历超高压变质作用的认识明显相悖,原因可能如下：① 后期退变质作用引起研究区榴辉岩全岩成分、矿物化学成分的调整,在采用Grt-Cpx-Phe温压计和以全岩成分为基础的相平衡模拟方法估算峰期温压条件时受到影响,从而使估算峰期压力条件普遍偏低;② 西南天山的榴辉岩可能并非全都经历了超高压变质作用,高压、超高压榴辉岩可能分别代表了不同变基性岩块在不同俯冲深度变质的产物。     

Compatible P-T conditions for eclogites and surrounding gneisses in the Kristiansund area,western Norway

Analyses of coexisting garnets, clinopyroxenes and plagioclases from eclogites and high pressure granulitic gneisses in the Kristiansund area within the west Norwegian basal gneiss region are used to establish the P-T conditions for the metamorphic peak for these rocks. Based on the distribution of Fe and Mg between coexisting garnet and clinopyroxene in both eclogite and granulites, equilibrium temperatures are estimated to 750 °±50 ° C. Pressures are derived from the absence of orthopyroxenes in the granulites, and from the assemblage clinopyroxene +plagioclase+quartz present in the gneisses. Equilibrium pressures are estimated to 18.5±3.0 kilobars, and these equilibrium conditions are thus compatible with equilibrium conditions derived for both orthopyroxene-free and most orthopyroxene-bearing country-rock eclogites from adjacent areas.     

蚌埠隆起区石榴辉石岩变质P-T轨迹的构建

蚌埠隆起区位于华北克拉通东南缘,胶—辽—吉造山带的最南端,主体由五河杂岩组成。前人对该地区的研究主要集中于同位素年代学和变质温压条件研究,其中变质P-T条件研究结果差异较大,以压力变化最为显著,对峰期变质P-T条件缺乏统一认识。本文对蚌埠隆起区石榴辉石岩进行了大量的岩相学、矿物化学成分分析,表明该岩石记录了3期变质作用,其中S-M1和S-M2的矿物组合类似为Grt+Cpx+Opx+Amp+Pl+Ilm,S-M3的矿物组合为Cpx+Amp+Pl+Grt （极边窄带）。结合变质温压条件分析和锆石U-Pb年代学分析,本次主要取得以下几点认识：1）石榴辉石岩WS047-1中记录的3期变质作用,温压条件分别为T-M1 = 616 ℃～647 ℃、P-M1 = 1.03～1.08 GPa,T-M2 = 721 ℃～837 ℃、P-M2 = 1.11～1.29 GPa和T-M3 = 531 ℃～607 ℃、P-M3 = 0.81～0.91 GPa,经历了由较高压力的角闪岩相→中-低麻粒岩相→角闪岩相的变质过程;2）据变质温压条件分析知,蚌埠隆起区具有顺时针的P-T轨迹特征,S-M1→S-M2和S-M2→S-M3分别为近等压升温和近等压降温的缓慢过程;3）石榴辉石岩锆石U-Pb年代学结果主要分为4组：1 839±13 Ma、1 925±31 Ma、2 041±55 Ma和2 762±14 Ma,其中峰值变质年代为1.93～1.84 Ga;4）结合温压条件和锆石U-Pb年代学分析结果,本文认为蚌埠隆起区的P-T轨迹与弗朗西斯科型俯冲或大陆碰撞环境的P-T轨迹较为类似,其应与1.93～1.84 Ga华北克拉通东、西陆块的碰撞拼合及胶—辽—吉造山带形成时限基本吻合。本次研究为深入理解华北克拉通的构造演化特征和蚌埠隆起区的变质作用及演化,提供了大量可靠的科学资料。