Compressional origin of the Aegean Orogeny,Greece

The Aegean Sea area is thought to be an actively extending back-arc region, north of the present day Hellenic volcanic arc and north-dipping subduction zone in the Eastern Mediterranean. The area shows extensive normal faulting, ductile ‘extensional’ shear zones and extensional S-C fabrics throughout the islands that have previously been related to regional Aegean extension associated with slab rollback on the Hellenic Subduction Zone. In this paper, we question this interpretation, and suggest the Cenozoic geodynamic evolution of the Aegean region is associated with a Late Cretaceous–Eocene NE-dipping subduction zone that was responsible for continent-continent collision between Eurasia and Adria-Apulia/Cyclades. Exhumation of eclogite and blueschist facies rocks in the Cyclades and kyanite-sillimanite grade gneisses in the Naxos core complex have pressures that are far greater than could be accounted for purely by lithospheric extension and isostatic uplift. We identify four stages of crustal shortening that affected the region prior to regional lithospheric extension, herein called the Aegean Orogeny. This orogeny followed a classic Wilson cycle from early ophiolite obduction (ca. 74 Ma) onto a previously passive continental margin, to attempted crustal subduction with HP eclogite and blueschist facies metamorphism (ca. 54–45 ?Ma), through crustal thickening and regional kyanite – sillimanite grade Barrovian-type metamorphism (ca. 22–14 ?Ma), to orogenic collapse (<14 ?Ma). At least three periods of ‘extensional’ fabrics relate to: (1) Exhumation of blueschists and eclogite facies rocks showing tight-isoclinal folds and top-NE, base-SW fabrics, recording return flow along a subduction channel in a compressional tectonic setting (ca. 50–35 ?Ma). (2) Extensional fabrics within the core complexes formed by exhumation of kyanite- and sillimanite gneisses showing thrust-related fabrics at the base and ‘extensional’ fabrics along the top (ca. 18.5–14 ?Ma). (3) Regional ductile-brittle ‘extensional’ fabrics and low-angle normal faulting related to the North Cycladic Detachment (NCD) and the South(West) Cycladic Detachment (WCD) during regional extension along the flanks of a major NW–SE anticlinal fold along the middle of the Cyclades. Major low-angle normal faults and ductile shear zones show symmetry about the area, with the NE chain of islands (Andros, Tinos, Mykonos, Ikaria) exposing the NE-dipping NCD with consistent top-NE ductile fabrics along 200 ?km of strike. In contrast, from the Greek mainland (Attica) along the SE chain of islands (Kea, Kythnos, Serifos) a SW-dipping low-angle normal fault and ductile shear zone, the WCD is inferred for at least 100 ?km along strike. Islands in the middle of the Cyclades show deeper structural levels including kyanite- and sillimanite-grade metamorphic core complexes (Naxos, Paros) as well as Variscan basement rocks (Naxos, Ios). The overall structure is an ~100 ?km wavelength NW–SE trending dome with low-angle extensional faults along each flank, dipping away from the anticline axis to the NE and SW. Many individual islands show post-extensional large-scale folding of the low-angle normal faults around the domes (Naxos, Paros, Ios, Sifnos) indicating a post-Miocene late phase of E–W shortening.     

Yria (western Naxos island,Greece): Sea level changes in Upper Holocene and palaeogeographical reconstruction

The Saint Georgios coastal zone, located at the W coast of Naxos, the largest island of the Central Aegean Sea, was investigated in order to determine the palaeo-geography, sea level changes and their effect to the palaeo-environment of western Naxos island and to human activity. Detailed geomorphological mapping, study of micropaleontological and sedimentological characteristics and dating analyses of the Late Holocene of St. Georgios coastal zone were conducted.

To obtain information about the Holocene stratigraphy under the recent alluvial cover, three boreholes followed the detailed geomorphological mapping. Microfaunal analysis took place and five samples of plants, shells, peat and charred material were also collected from several layers of the sedimentary sequence and were dated using AMS and conventional radiocarbon techniques providing temporal control of the sediments. The sea-land interactions during Upper Holocene, in relation to the eustatic sea level oscillations, as well as the geomorphologic observations and analysis on deposited sediments, aims to reveal the palaeo-geographic evolution of the landscape and its impact on the archaeological sites. Sea level rise along with sea-land interactions to the landscape evolution and the transgression of sea in 6144 BP have been verified.     

Pressure-Temperature-time Evolution of Metamorphic Rocks from Naxos (Cyclades,Greece): Constraints from Thermobarometry and Rb/Sr dating

The Pressure-Temperature-time paths of metapelites sampled on an east-west transect across the structural dome of Naxos (Greece) have been reconstructed on the basis of new geothermobarometric data and Rb/Sr dating, as well as previously published data. One sample from an intermediate structural level records pressure and temperature conditions of 10 kbar, 500°C, corresponding to its exhumation in a highpressure/low-temperature (HP/LT) setting. The corresponding Rb/Sr exhumation age is 29.3±1.3(2s) Ma. Toward the center of the dome, metamorphic assemblages record an increase in peak-temperature and corresponding pressure (from 500 to 700°C, and from 5 to 8 kbar), in a medium-pressure/medium-temperature (MP/MT) metamorphic field gradient. Whole-rock, muscovite and biotite on two samples from deep structural levels define ages of 5.2 and 7.0 Ma, whereas garnets fall outside the isochrons and retain earlier less radiogenic signatures. Rb/Sr data on these two samples demonstrate open system behaviour coeval with, or subsequent to MP/MT metamorphism. The interpretation of the Pressure-Temperature-time paths indicates a time span of 15-20 Ma for dome formation. It also suggests that the HP/LT to MP/MT transition is due to i) heating of deeper parts of the dome through magma injection or ii) either homogeneous (75 %) or localized thinning during dome formation.     

The tectonic significance of a porphyroblastic blueschist facies overprint during Alpine orogenesis: Sifnos, Aegean Sea, Greece

The MacArgon program^∗ has been used to model published apparent age spectra for white micas from the island of Sifnos, Cyclades, Greece. These micas formed during a period of porphyroblastic mineral growth (M₂) in the epidote-blueschist facies, at > ~ 460 °C and < ~ 14 kbar. M₂ marked the onset of a major period of deformation (D₃) during which kilometre-scale ductile shear zones formed and widespread recumbent folding took place. The modelling experiments suggest that flat apparent age spectra observed in the eclogite-blueschist domain (EBD) can only be obtained if the rocks cooled rapidly below ~ 350 °C after M₂ at ~ 42 Ma. Cooling rates must be > 50 °C/m.y., and since M₂ is followed by D₃ we infer that these are the cooling rates during D₃. Modelling experiments for micas from the greenschist domain (GSD) structurally underneath the EBD suggest that the GSD cooled rapidly, but at ~ 32 Ma. One tectonic model that might explain the rapidity of cooling rates inferred for the EBD model supposes that ambient temperatures were at ~ 500 °C when M₂ took place, but thereafter the high pressure rocks were thrust to the north and rapidly cooled as the result of their juxtaposition against cooler, shallower levels of the crust.     

Origin and metamorphism of ultrabasic rocks associated with a subducted continental margin, Naxos (Cyclades, Greece)

Meta-peridotites outcropping at different structural levels within the Alpine metamorphic complex of the Cycladic island of Naxos were studied to re-examine their metamorphic evolution and possible tectonic mechanisms for emplacement of mantle material into the continental crust. The continental margin section exposed on Naxos, consisting of pre-Alpine basement and c. 7 km thick Mesozoic platform cover, has undergone intense metamorphism of Alpine age, comprising an Eocene (M1) blueschist event strongly overprinted by a Miocene Barrovian-type event (M2). Structural concordance with the country rocks and metasomatic zonation at the contact with the felsic host rocks indicate that the meta-peridotites have experienced the M2 metamorphism. This conclusion is supported by the similarity between metamorphic temperatures of the ultrabasic rocks and those of the host rocks. Maximum temperatures of 730–760 °C were calculated for the upper-amphibolite facies meta-peridotites (Fo–En–Hbl–Chl–Spl), associated with sillimanite gneisses and migmatites. Relict phases in ultrabasics of different structural levels indicate two distinct pre-M2 histories: whereas the cover-associated horizons have been affected by low-grade serpentinization prior to metamorphism, the basement- associated meta-peridotites show no signs of serpentinization and instead preserve some of their original mantle assemblage. The geochemical affinities of the two groups are also different. The basement-associated meta-peridotites retain their original composition indicating derivation by fractional partial melting of primitive lherzolite, whereas serpentinization has led to almost complete Ca-loss in the second group. The cover-associated ultrabasics are interpreted as remnants of an ophiolite sequence obducted on the adjacent continental shelf early in the Alpine orogenesis. In contrast, the basement-associated meta-peridotites were tectonically interleaved with the Naxos section at great depth during the Alpine collision and high P/T metamorphism. Their emplacement at the base of the orogenic wedge is inferred to have involved isobaric cooling from temperatures of c. 1050 °C within the spinel lherzolite field to eclogite facies temperatures of c. 600 °C.     

The nature and distribution of fluids during amphibolite facies metamorphism, Naxos (Greece)

On the basis of fluid inclusion evidence, pervasive influx of deep-seated CO₂-rich fluids has been invoked to account for mid- to upper amphibolite facies (M2_B) metamorphism on the island of Naxos (Cyclades, Greece). In this paper, mineral devolatilization and melt equilibria are used to constrain the composition of both syn- and post-peak-M2_B fluids in the deepest exposed levels of the metamorphic complex. The results indicate that peak-M2_B fluids were spatially and compositionally heterogeneous throughout the high-grade core of the complex, whereas post-peak-M2_B fluids were generally water-rich. The observed heterogeneities in syn-M2_B fluid composition are inconsistent with pervasive CO₂-flushing models invoked by previous workers on the basis of fluid inclusion evidence. It is likely that few CO₂-rich fluid inclusions on Naxos preserve fluids trapped under peak metamorphic conditions. It is suggested that many of these inclusions have behaved as chemically open systems during the intense deformation that accompanied the uplift of the metamorphic complex. A similar process may explain the occurrence of some CO₂-rich fluid inclusions in granulite facies rocks.     

Geochemistry and palaeotectonic setting of amphibolites from the Western Tatra Mountains,southern Poland

Amphibolites from the crystalline basement of the Western Tatra Mountains, which are found as small lenses within migmatitic gneisses and mica schists, were formed during pre‐ or early Variscan amphibolite‐facies metamorphic events, and subsequently intruded by the post‐metamorphic Variscan Tatra Granite. The amphibolites occur in both the upper and lower metamorphic complexes, which are separated by a major subhorizontal shear zone in the Western Tatra Mountains. The amphibolites can be divided into three types: massive, striped and garnetiferous. The striped and massive amphibolites, concordant with a dominant S₁ foliation, and the garnet amphibolites, which cross‐cut the S₁ banding in the gneisses, were all originally intrusive dolerites. The striped amphibolites (consisting primarily of hornblende, andesine and quartz), and later, cross‐cutting garnet‐hornblende‐andesine‐quartz‐bearing amphibolites, predominate in the lower part of the dominantly migmatitic upper complex, and are exposed mainly on the ridges. The massive amphibolites, which contain a similar mineral assemblage, mainly occur in the usual unmigmatized lower structural unit. Chemical studies indicate that three amphibolite suites are present, which probably originated as a series of enriched tholeiites, similar to more recent plume‐influenced magmas, which were derived by partial melting of a spinel lherzolite with primitive mantle composition and compositionally slightly modified by crustal contamination. The amphibolites were intruded as dolerites into clastic sediments which had accumulated in an extensional basin floored by attenuated continental crust, a situation similar to that of amphibolites found in metamorphic complexes within the Variscan belt, e.g. in the Orlica–Snieznik area of the Sudetes, where amphibolites chemically similar to those in the Western Tatra also occur. Copyright © 2000 John Wiley & Sons, Ltd.     

鲁西下常庄中三叠世磁铁角闪岩的岩石地球化学和Sr-Nd同位素:对岩石成因和构造背景的制约

下常庄中三叠世(242Ma)磁铁角闪岩的岩相学、岩石地球化学和Sr-Nd同位素的研究结果表明,下常庄磁铁角闪岩主要由角闪石(40%~45%)、磁铁矿(35%~45%)和斜长石(10%~15%)及少量的单斜辉石(1%~3%)组成;具低的SiO_2(24.25%~38.40%)、K_2O(0.02%~1.09%)和Na_2O(0.16%~1.68%)含量,较高的MgO(6.86%~11.81%)、Fe_2O_3~T(21.69%~36.62%)和TiO_2(7.37%~13.46%)含量;轻稀土元素(LREE)相对富集、重稀土元素(HREE)相对亏损(∑LREE/∑HREE=7.09~8.97、(La/Yb)_N=10.67~16.62),无明显的Eu异常(δEu=0.83~1.01);富集大离子亲石元素(Rb、Ba、Sr),高场强元素(Nb、Ta、Zr、Hf)不亏损,具Pb和Ti的正异常;全岩~(87)Sr/~(86)Sr初始比值介于0.702 645~0.706 534之间,ε_(Nd)(t)值变化于-7.51~-12.59之间,T_(DM1)=2 374~2 413 Ma。上述结果暗示,下常庄磁铁角闪岩的原岩可能为角闪辉长岩,起源于受下地壳物质混染的华北地块富集岩石圈地幔的部分熔融。下常庄中三叠世磁铁角闪岩原岩的形成时代与大别—苏鲁高压-超高压变质的峰期时间一致,暗示它们可能形成于中生代早期洋壳俯冲-消减向陆-陆碰撞造山转换的构造背景。     

A statistical approach in investigating the hydrogeological significance of remotely sensed lineaments in the crystalline mountainous terrain of the island of Naxos, Greece

A map indicating zones related to groundwater on the mountainous terrain of the island of Naxos, Greece, was produced, using statistics, remote sensing and geographic information system (GIS) techniques. Naxos mainly consists of polydeformed and polymetamorphosed crystalline formations where groundwater is restricted to secondary porosity; its movement is erratic and occurs along lithological contacts, solution openings, faults and fractures. As in the most central Aegean islands, water in Naxos is a scarce commodity. Many dry holes have been drilled. It is known that in areas of such geology, linear features may play a significant role in their hydrogeological regime. Various lineaments’ directional properties were calculated and statistically tested against collected spring data using GIS techniques in an attempt to evaluate the hydrogeological significance of remotely sensed lineaments. Based on the results achieved, a map was prepared to contribute to conventional ground surveys in the selection of drilling sites. The reliability of the map was tested with existing borehole data. The results obtained encourage the use of statistical analysis on remotely sensed lineaments for groundwater targeting studies in crystalline mountainous areas.     

祁连山西段古元古代北大河岩群中斜长角闪岩的地球化学特征及构造背景

以大陆碎块形式残存于北祁连造山带内部的古元古代北大河岩群早期发育有大量的斜长角闪岩,其原岩为玄武岩,Alk-∑FeO-MgO图解、REE特征、里特曼组合指数等均显示斜长角闪岩属钙碱性系列范围,个别显示弱碱性,斜长角闪岩具有较高的Al2O3、∑FeO和低的Mg#,这些特点反映其成分演化程度较高,表明火山岩的形成与古老地壳的活化有关,有较强分异的稀土元素分配模式,在Ti-Zr-Y图、Ni-FeO/MgO图解和Ta/Yb-Th/Yb图解上斜长角闪岩均显示火山弧或活动大陆边缘的特点,在玄武岩的地球化学形式图、稀土元素整体特征与钙碱性岛弧玄武岩基本相同;87Sr/86Sr=0.71617～0.72033,143Nd/144Nd=0.512414～0.512495,εNd=-2.79～-4.37.显示出壳源性质或者是受到大陆下地壳的混染作用而形成.因此推测斜长角闪岩的原岩为不同程度部分熔融的岩浆弧作用的产物,说明在古元古代华北克拉通和柴达木克拉通之间存在古洋盆的俯冲消减.     

陈蔡岩群下河图斜长角闪岩年代学、地球化学特征及其构造意义

浙江诸暨市下河图村的陈蔡岩群中出露一套斜长角闪岩,空间上与块状大理岩相伴产出。地球化学研究表明,下河图斜长角闪岩SiO_2含量为43.22%~46.56%,MgO为3.23%~7.87%,TiO_2为1.90%~2.98%,与碱性洋岛玄武岩特征类似。稀土元素总量为114.47×10~(-6)~192.39×10~(-6),(La/Yb)N为5.93~12.13,稀土元素球粒陨石标准化配分模式为轻稀土元素富集的右倾型;原始地幔标准化微量元素蛛网图表现出向上隆起的富集形态,微量元素特征表明其可能形成于洋岛构造环境,陆壳物质对其混染的可能性较小,岩石成分主要受熔融源区控制。推测下河图斜长角闪岩原岩很可能形成于靠近消减带的海山环境,来源于洋壳俯冲过程中增生的海山碎片,陈蔡岩群很可能为一套俯冲增生杂岩。LA-ICP-MS锆石U-Pb法获得斜长角闪岩变质年龄为420.6±1.8 Ma,可能代表了扬子和华夏两大地块碰撞拼合的时代。     

Ionprobe U-Pb zircon ages from the high-pressure/low-temperature mélange of Syros, Greece: age diversity and the importance of pre-Eocene subduction

The Cycladic blueschist belt in the central Aegean Sea has experienced high‐pressure (HP) metamorphism during collisional processes between the Apulian microplate and Eurasia. The general geological and tectonometamorphic framework is well documented, but one aspect which is yet not sufficiently explored is the importance of HP mélanges which occur within volcano‐sedimentary successions. Unresolved issues concern the range in magmatic and metamorphic ages recorded by mélange blocks and the significance of eventual pre‐Eocene HP metamorphism. These aspects are here addressed in a U‐Pb zircon study focusing on the block–matrix association exposed on the island of Syros. Two gneisses from a tectonic slab of this mélange, consisting of an interlayered felsic gneiss‐glaucophanite sequence, yielded zircon ²⁰⁶Pb/²³⁸U ages of 240.1 ± 4.1 and 245.3 ± 4.9 Ma, respectively, similar to Triassic ages determined on zircon in meta‐volcanic rocks from structurally coherent sequences elsewhere in the Cyclades. This strongly suggests that parts of these successions have been incorporated in the mélanges and provides the first geochronological evidence that the provenance of mélange blocks/slabs is neither restricted to a single source nor confined to fragments of oceanic lithosphere. Zircon from a jadeitite and associated alteration zones (omphacitite, glaucophanite and chlorite‐actinolite rock) all yielded identical ²⁰⁶Pb/²³⁸U ages of c. 80 Ma. Similar Cretaceous U‐Pb zircon ages previously reported for mélange blocks have been interpreted by different authors to reflect magmatic or metamorphic ages. The present study adds a further argument in favour of the view that zircon formed newly in some rock types at c. 80 Ma, due to hydrothermal or metasomatic processes in a subduction zone environment, and supports the interpretation that the Cycladic blueschist belt records both Cretaceous and Eocene HP episodes and not only a single Tertiary HP event.     

Pressure–temperature evolution of blueschist facies rocks from Sifnos, Greece, and implications for the exhumation of high-pressure rocks in the Central Aegean

The blueschist and greenschist units on the island of Sifnos, Cyclades were affected by Eocene high‐pressure (HP) metamorphism. Using conventional geothermobarometry, the HP peak metamorphic stage was determined at 550–600 °C and 20 kbar, close to the blueschist and the eclogite facies transition. The retrograde P–T paths are inferred with phase diagrams. Pseudosections based on a quantitative petrogenetic grid in the model system Na₂O–CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O reveal coeval decompression and cooling for both the blueschist and the greenschist unit. The conditions of the metamorphic peak and those of the retrograde stages conform to a similar metamorphic gradient of 10–12 °C km^?1 for both units. The retrograde overprint can be assigned to low‐pressure blueschist to HP greenschist facies conditions. This result cannot be reconciled with the (prograde) Barrovian‐type event, which affected parts of the Cyclades during the Oligocene to Miocene. Instead, the retrograde overprint is interpreted in terms of exhumation, directly after the HP stage, without a separate metamorphic event. Constraints on the exhumation mechanism are given by decompression‐cooling paths, which can be explained by exhumation in a fore‐arc setting during on‐going subduction and associated crustal shortening. Back‐arc extension is only responsible for the final stage of exhumation of the HP units.     

The lateral boundary of a metamorphic core complex: The Moutsounas shear zone on Naxos,Cyclades, Greece

We describe the structure, microstructures, texture and paleopiezometry of quartz-rich phyllites and marbles along N-trending Moutsounas shear zone at the eastern margin of the Naxos metamorphic core complex (MCC). Fabrics consistently indicate a top-to-the-NNE non-coaxial shear and formed during the main stage of updoming and exhumation between ca. 14 and 11 Ma of the Naxos MCC. The main stage of exhumation postdates the deposition of overlying Miocene sedimentary successions and predates the overlying Upper Miocene/Pliocene conglomerates. Detailed microstructural and textural analysis reveals that the movement along the Moutsounas shear zone is associated with a retrograde greenschist to subgreenschist facies overprint of the early higher-temperature rocks. Paleopiezometry on recrystallized quartz and calcite yields differential stresses of 20–77 MPa and a strain rate of 10⁻¹⁵–10⁻¹³ s⁻¹ at 350 °C for quartz and ca. 300 °C for calcite. Chlorite geothermometry of the shear zone yields two temperature regimes, 300–360 °C, and 200–250 °C. The lower temperature group is interpreted to result from late-stage hydrothermal overprint.     

内蒙古北山地区北山岩群斜长角闪岩LA-ICP-MS锆石U-Pb定年及其地质意义

内蒙古北山地区标山一带北山岩群出露相对齐全,基于区域地质、岩石学及地球化学等特征研究,在北山岩群中解体出斜长角闪岩,其原岩为辉长岩、辉绿岩等基性岩脉。采用LA-ICP-MS方法对斜长角闪岩样品开展锆石U-Pb同位素测年分析,获得U-Pb加权平均年龄为1623±21Ma,207Pb/206Pb表面年龄为1547~1691Ma。样品中锆石多为半自形双锥柱状,粒径介于0.03~0.21mm,发育较明显的振荡环带,具有核-幔-边结构,30个锆石分析点Th/U比值为0.38~0.92,绝大多数大于0.4,表明这些锆石为岩浆成因而非变质成因锆石。因此,这些锆石的同位素年龄为斜长角闪岩的结晶年龄,从而说明北山岩群的成岩年龄应不晚于1623±21Ma。综合研究表明,斜长角闪岩形成于古元古代大陆扩张环境,这些认识对北山岩群的时代归属及其北山地区构造发展演化历史的研究均具有重要意义。     

内蒙古阿拉善地块中北部地区辉长岩岩体特征、锆石定年及其构造意义

1∶25万区域地质调查、地质剖面测制及研究发现,阿拉善左旗乌力吉地区分布有基性岩,其侵入上石炭统阿木山组中,后又被晚期酸性岩浆侵入,与其接触关系为侵入接触。研究表明,该基性岩主要由角闪辉长岩和辉长闪长岩组成,其中角闪辉长岩为主体岩性。采用LA-ICP-MS技术对其中典型的具基性岩浆特征的锆石进行了U-Th-Pb同位素测定,获得的206Pb/238U年龄加权平均值为248.0±0.9Ma,为早三叠世。结合区域构造和岩性关系,该地区辉长岩的形成年龄明显晚于区域内古亚洲洋板块向华北板块俯冲作用结束的时间,因此其并非是古亚洲洋板块向华北板块于晚古生代俯冲引起的构造岩浆旋回早期阶段的产物,而更可能与大规模的陆内裂解有关,并产生于地幔物质上涌造成的区域性大陆拉张环境中。     

The isotopic composition of zircon and garnet: A record of the metamorphic history of Naxos, Greece

Growth of zircon with respect to that of garnet has been studied using a combination of petrography, U–Pb dating and oxygen isotope analysis. The aim is to document the mechanism and pressure–temperature conditions of zircon growth during metamorphism in order to better constrain the Tertiary metamorphic history of Naxos, Greece. Two metamorphisms are recognised: (1) an Eocene Franciscan metamorphism (M1) and (2) a widespread Miocene Barrovian metamorphism (M2) that increases from greenschist facies up to partial melting. An amphibolite sample contains zircon crystals characterised by a magmatic core and two metamorphic rims, denoted as A and B, dated at 200–270, 42–69, and 14–19 Ma, respectively. The first metamorphic rim A (δ¹⁸O = 7 ± 1‰) preserves the δ¹⁸O value of the magmatic core (6.2 ± 0.8‰), whereas rim B is characterised by higher δ¹⁸O values (7.8 ± 1.8‰). These observations indicate the formation of A rims by solid-state recrystallisation in a closed system with regard to oxygen and those of B in an open system. Compositional zoning in garnet is interpreted as the result of decompressional heating. Zircon B rims and garnet rims display similar δ¹⁸O values which indicates a contemporaneous growth of garnet and zircon rims during the Miocene Barrovian event (M2). Calcic gneiss and metapelite samples contain zircon crystals with single metamorphic overgrowths aged 41–57 Ma. δ¹⁸O values measured in zircon overgrowths (11.8 ± 1.4‰) from the calcic gneiss are similar to those measured in garnet rims (11.4 ± 1.1‰) from the same rock. This suggests that garnet rims and zircon overgrowths grew during the high pressure–low temperature event in equilibrium with prograde fluids. In the metapelite sample, δ¹⁸O values are similar in garnet cores (14.8 ± 0.2‰) and in zircon metamorphic overgrowths (14.2 ± 0.5‰). As zircon overgrowths have been dated at ca. 50 Ma by U–Pb, garnet cores and zircon overgrowths are interpreted to have grown during the high pressure event.

As demonstrated here for the island of Naxos, correlating the crystallisation of zircon with that of metamorphic index minerals such as garnet using stable isotope composition and U–Pb determination is a powerful tool for deciphering the mechanism of zircon growth and pin-pointing zircon crystallisation within the metamorphic history of a terrain. This approach is potentially hampered by an inability to verify the degree of textural equilibrium of zircon with other mineral phases, and the possible preservation (in metamorphic rims) of isotopic signatures from pre-existing zircon when they form by recrystallisation. Nevertheless, this study illustrates the application of this approach in providing key constraints on the timing and mechanism of growth of minerals important to understanding metamorphic petrogenesis.     

赞皇变基性岩中锆石的U-Pb定年及其地质意义

赞皇变质杂岩区位于阜平杂岩南部,地处华北克拉通中部造山带的中段,和中部带北段杂岩一样,是洞悉华北克拉通前寒武纪基底构造演化历史的一个重要窗口.研究区变基性岩可分为斜长角闪岩和角闪斜长片麻岩两种,二者均以似层状方式产于黑云斜长片麻岩或长英质片麻岩中,斜长角闪岩亦可呈透镜状,二者后期与围岩一起共同经历了高角闪岩相变质作用....     

Zr-in-rutile thermometry in blueschists from Sifnos, Greece

Zr-in-rutile thermometry on samples of blueschist from Sifnos, Greece, yields temperatures that reflect progressive crystallization of rutile from ca. 445 to 505°C with an analytical precision of + 18/−27 and ± 10°C using the electron microprobe and ± 1.5–3.5°C using the ion microprobe. Individual grains are generally homogeneous within analytical uncertainty. Different grains within a single sample record temperature differences as large 55°, although in most samples the range of temperatures is on the order of 25°. In several samples, Zr-in-rutile temperatures from grains within garnet are lower than temperatures from matrix grains, reflecting growth of rutile with increasing temperature of metamorphism. Although the specific rutile-producing reactions have not been identified, it is inferred that rutile grows from either continuous reaction involving the breakdown of lower grade phases (possibly ilmenite), or from pseudomorph reactions involving the breakdown of relic igneous precursors at blueschist-facies conditions. No systematic variation in rutile temperatures was observed across the blueschist belt of northern Sifnos, consistent with the belt having behaved as a coherent block during subduction.     

内蒙古阿拉善地块中北部地区辉长岩岩体特征、锆石定年及其构造意义

1∶25万区域地质调查、地质剖面测制及研究发现,阿拉善左旗乌力吉地区分布有基性岩,其侵入上石炭统阿木山组中,后又被晚期酸性岩浆侵入,与其接触关系为侵入接触。研究表明,该基性岩主要由角闪辉长岩和辉长闪长岩组成,其中角闪辉长岩为主体岩性。采用LA-ICP-MS技术对其中典型的具基性岩浆特征的锆石进行了U-Th-Pb同位素测定,获得的206Pb/238U年龄加权平均值为248.0±0.9Ma,为早三叠世。结合区域构造和岩性关系,该地区辉长岩的形成年龄明显晚于区域内古亚洲洋板块向华北板块俯冲作用结束的时间,因此其并非是古亚洲洋板块向华北板块于晚古生代俯冲引起的构造岩浆旋回早期阶段的产物,而更可能与大规模的陆内裂解有关,并产生于地幔物质上涌造成的区域性大陆拉张环境中。