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1.
北苏鲁荣成地区超高压变质带大理岩中的退变质石榴辉石岩包裹体具有“核-幔-边”结构,核部主要由石榴子石与单斜辉石组成,幔部含有由细小角闪石与绿帘石组成的后成合晶以及颗粒较大的角闪石与单斜辉石,通过详细的岩相学分析以及矿物成分分析,认为这些后成合晶是由石榴子石、单斜辉石以及来自围岩的流体共同反应而产生,大颗粒的角闪石主要是由辉石转变而来的,在幔部这个转变并不彻底,仍有一些残余辉石颗粒。边部主要由角闪石和绿帘石组成。该石榴辉石岩曾经历榴辉岩相超高压变质阶段。没有柯石英超高压代表性矿物（采样处的其他类型岩石都含有柯石英）的原因是石榴辉石岩的原岩为超基性岩。  相似文献   

2.
Mushroom‐ and atoll‐shaped garnet crystals were found in high‐pressure quartz‐rich pelitic layers from the Monte Mucrone area (Western Alps, Italy). These garnet crystals are characterized by a peninsula‐shaped core surrounded by a partially crystallized, dodecahedral external rim. Textural observations and thermodynamic modelling point towards growth of the atoll garnet from the Monte Mucrone area during two distinct orogenic cycles. The core region and the inner part of the ring forming the edge of the atoll grew under Barrovian metamorphic conditions of likely Hercynian age, while the outer rim of the atoll structure developed under Alpine high‐pressure conditions. Electron backscatter diffraction analyses indicate that the atoll‐shaped structure has one single crystallographic orientation, despite its complex compositional zoning. Thermodynamic modelling reveals marked changes in equilibrium assemblage leading to changes in stoichiometry of the garnet‐forming reaction, which in turn explain the characteristic garnet morphology. Small amounts of quartz are consumed during the earlier stage of garnet growth history, whereas the production of garnet requires a much larger amount of quartz during the final stage of its growth. This leads to a change from initial poikiloblastic to non‐poikiloblastic textures. This change is responsible for the formation of atoll‐shaped garnet. Finally, garnet in intercalated mica‐rich layers forms idiomorphic crystals, continuous from the centre to rim. This study highlights the importance of the difference between the local matrix composition and the aggregate composition of the reactants needed for the garnet‐forming reaction. Finally, we show that interaction between matrix and reaction stoichiometry can lead to porphyroblast precipitation inside the already grown porphyroblast.  相似文献   

3.
We report the first finding of diamond in crustal rocks from the Tromsø Nappe of the North Norwegian Caledonides. Diamond occurs in situ as inclusions in garnet from gneiss at Tønsvika near Tromsø. The rock is composed essentially of garnet, biotite, white mica, quartz and plagioclase, minor constituents include kyanite, zoisite, rutile, tourmaline, amphibole, zircon, apatite and carbonates (magnesite, dolomite, calcite). The microdiamond, identified by micro‐Raman spectroscopy, is cuboidal to octahedral in shape and ranges from 5 to 50 μm in diameter. The diamond occurs as single grains and as composite diamond + carbonate inclusions. Diamond vibration bands show a downshift from 1 332 to 1 325 cm?1, the majority of Raman peaks are centred between 1 332 and 1 330 cm?1 and all peaks exhibit a full width at half maximum between 3 and 5 cm?1. Several spectra show Raman bands typical for disordered and ordered graphite (sp2‐bonded carbon) indicating partial transformation of diamond to graphite. The calculated peak P–T conditions for the diamond‐bearing sample are 3.5 ± 0.5 GPa and 770 ± 50 °C. Metamorphic diamond found in situ in crustal rocks of the Tromsø Nappe thus provides unequivocal evidence for ultrahigh pressure metamorphism in this allochthonous unit of the Scandinavian Caledonides. Deep continental subduction, most probably in the Late Ordovician and shortly before or during the initial collision between Baltica and Laurentia, was required to stabilize the diamond at UHP conditions.  相似文献   

4.
ABSTRACT

Polycrystalline microdiamonds are rare in ultrahigh-pressure (UHP) rocks worldwide. Among samples collected at Erzgebirge, Germany, we found abundant polycrystalline microdiamonds as inclusions in zircons from a quartzofeldspathic rock. To illuminate their origin and forming age, we investigated morphologies and Raman spectra of 52 microdiamond inclusions, and dated the zircon host. The zircons have low Th/U values (0.03–0.07) and a concordia U/Pb age of 335.8 ± 1.9 Ma. Polycrystalline diamond (10–40 µm) consists of many fine-grained crystals (1.5–3 µm) with different orientations; discrete single diamonds (2–20 µm) are rare. All measured Raman spectra show an intense diamond band at 1332–1328 cm?1 and have a negative correlation with full width at half maximum (FWHM) of 5.8–11.3 cm?1. These data combined with previously reported diamond band data (1331–1337 cm?1) are compatible with those of diamond inclusions in various host minerals from other UHP terranes, but are different from those of ureilite diamonds. The Erzgebirge microdiamonds in zircon do not display visible disordered sp3-carbon, but show downshifting of the Raman band from the ideal value (1332 cm?1), and have a broader diamond band (FWHM >3 cm?1) than those of well-ordered diamonds. These features may reflect imperfect ordering due to rapid nucleation/crystallization during UHP metamorphism and rapid exhumation of the UHP terrane. Graphite inclusions in zircon show a typical G-band at 1587 cm?1. Our study together with previously reported C-isotopic compositions (δ13C, ?17 to ?27‰) of diamond and occurrences of fluid/melt inclusions in diamond and garnet indicates that Erzgebirge microdiamonds are metamorphic, have an organic carbon source, and crystallized from aqueous fluids. Limited long-range ordering suggested by the Raman spectra is a function of the P–T time of crystallization and subsequent thermal annealing on decompression. Combined with regional geology, our work further constrains the tectonic evolution of the Erzgebirge terrane.  相似文献   

5.
We analysed isotopic compositions of metamorphic microdiamond secondary ion mass spectrometry. Typical microdiamonds in this dolomite marble show star-shaped morphologies (S-type) consisting of single-crystal cores and polycrystalline rims. Four S-type microdiamonds and two R-type microdiamonds (single crystals with rugged surfaces) were analysed using a 5 μm diameter ion beam. S-type microdiamonds have heterogeneous carbon isotopic compositions even in a single grain. Analysis of a typical S-type microdiamond (no. xx01-1-13) revealed clear difference in δ13C between core and rim. The rim shows lighter isotopic compositions ranging from??17.2‰ to??26.9‰, whereas the core is much heavier, with δ13C ranging from??9.3‰ to??13.0‰. The δ13C values of R-type microdiamonds fall into narrow ranges from??8.3‰ to??14.9‰ for no. xx01-1-10 and from??8.3‰ to??15.3‰ for no. xx01-1-16. These δ13C values are similar to those of the S-type microdiamond cores. The R-type probably formed at the same stage as the core of the S-type, whereas rim growth at a second stage did not occur or occurred very weakly in R-type microdiamonds. These carbon isotopic data support the two-stage growth of microdiamonds in the Kokchetav ultrahigh-pressure host rock. To explain the second stage growth of S-type microdiamonds, we postulate a simple fluid infiltration of light carbon from neighbouring gneisses into the dolomite marble.  相似文献   

6.
Granulite facies pargasite orthogneiss is partially to completely reacted to garnet granulite either side of narrow (<20 mm) felsic dykes, in Fiordland, New Zealand, forming ~10–80 mm wide garnet reaction zones. The metamorphic reaction changed the abundance of minerals, and their shape and grain size distribution. The extent of reaction and annealing (temperature‐related coarsening and nucleation) is greatest close to the dykes, whereas further away the reaction is incomplete. As a consequence, grain size and the abundance of garnet decreases away from the felsic dykes over a few centimetres. The aspect ratios of clusters of S1 pyroxene and pargasite in the orthogneiss, which are variably reacted to post‐S1 garnet, decrease from high in the host, to near equidimensional close to the dyke. Post‐reaction deformation localized in the fine‐grained partially reacted areas. This produced a pattern of ‘paired’ shear zones located at the outer parts of the garnet reaction zone. Our study shows that grain size sensitive deformation occurs where the grain size is sufficiently reduced by metamorphic reaction. The weakening of the rock due to the change in grain size distribution outweighs the addition of nominally stronger garnet to the assemblage.  相似文献   

7.
The origin of fine‐grained dolomite in peritidal rocks has been the subject of much debate recently and evidence is presented here for a microbial origin of this dolomite type in the Norian Dolomia Principale of northern Calabria (southern Italy). Microbial carbonates there consist of stromatolites, thrombolites, and aphanitic dolomites. High‐relief thrombolites and stromatolites characterize sub‐tidal facies, and low‐relief and planar stromatolites, with local oncoids, typify the inter‐supratidal facies. Skeletal remains are very rare in the latter, whereas a relatively rich biota of skeletal cyanophycea, red algae and foraminifera is present in the sub‐tidal facies. Some 75% of the succession consists of fabric‐preserving dolomite, especially within the microbial facies, whereas the rest is composed of coarse dolomite with little fabric preservation. Three end‐members of dolomite replacement fabric are distinguished: type 1 and type 2, fabric retentive, with crystal size <5 and 5–60 μm, respectively; and type 3, fabric destructive, with larger crystals, from 60 to several hundred microns. In addition, there are dolomite cements, precipitated in the central parts of primary cavities during later diagenesis. Microbialite textures in stromatolites are generally composed of thin, dark micritic laminae of type 1 dolomite, alternating with thicker lighter‐coloured laminae of the coarser type 2 dolomite. Thrombolites are composed of dark, micritic clotted fabrics with peloids, composed of type 1 dolomite, surrounded by coarser type 2 dolomite. Marine fibrous cement crusts are also present, now composed of type 2 dolomite. Scanning electron microscope observations of the organic‐rich micritic laminae and clots of the inter‐supratidal microbialites reveal the presence of spherical structures which are interpreted as mineralized bacterial remains. These probably derived from the fossilization of micron‐sized coccoid bacteria and spheroidal–ovoidal nanometre‐scale dwarf‐type bacterial forms. Furthermore, there are traces of degraded organic matter, probably also of bacterial origin. The microbial dolomites were precipitated in a hypersaline environment, most likely through evaporative dolomitization, as suggested by the excess Ca in the dolomites, the small crystal size, and the positive δ18O values. The occurrence of fossilized bacteria and organic matter in the fabric‐preserving dolomite of the microbialites could indicate an involvement of bacteria and organic matter degradation in the precipitation of syn‐sedimentary dolomite.  相似文献   

8.
Evidence for ultrahigh‐pressure metamorphism (UHPM) in the Rhodope metamorphic complex comes from occurrence of diamond in pelitic gneisses, variably overprinted by granulite facies metamorphism, known from several areas of the Rhodopes. However, tectonic setting and timing of UHPM are not interpreted unanimously. Linking age to a metamorphic stage is a prerequisite for reconstruction of these processes. Here, we use monazite in diamond‐bearing gneiss from Chepelare (Bulgaria) to date the diamond‐forming UHPM event in the Central Rhodopes. The diamond‐bearing gneiss comes from a strongly deformed, lithologically heterogeneous zone (Chepelare Mélange) sandwiched between two migmatized orthogneiss units, known as Arda‐I and Arda‐II. Diamond, identified by Raman micro‐spectroscopy, shows the characteristic band mostly centred between 1332 and 1330 cm?1. The microdiamond occurs as single grains or polyphase diamond + carbonate inclusions, rarely with CO2. Thermodynamic modelling shows that garnet was stable at UHP conditions of 3.5–4.6 GPa and 700–800 °C, in the stability field of diamond, and was re‐equilibrated at granulite facies/partial melting conditions of 0.8–1.2 GPa and 750–800 °C. The texture of monazite shows older central parts and extensive younger domains which formed due to metasomatic replacement in solid residue and/or overgrowth in melt domains. The monazite core compositions, with distinctly lower Y, Th and U contents, suggest its formation in equilibrium with garnet. The U–Th–Pb dating of monazite using electron microprobe analysis yielded a c. 200 Ma age for the older cores with low Th, Y, U and high La/Nd ratio, and a c. 160 Ma age for the dominant younger monazite enriched in Th, Y, U and HREE. The older age of c. 200 Ma is interpreted as the timing of UHPM, whereas the younger age of c. 160 Ma as granulite facies/partial melting overprint. Our results suggest that UHPM occurred in Late Triassic to Early Jurassic time, in the framework of collision and subduction of continental crust after the closure of Paleotethys.  相似文献   

9.
华北秦皇岛地区的中奥陶统马家沟组以广泛发育海相白云质灰岩、灰质白云岩和白云岩为特征。在石门寨奥陶系亮甲山剖面,马家沟组自下而上识别出四类碳酸盐岩:含白云石泥晶灰岩（类型I）、细-粉晶白云岩（类型II）、“麦粒状”细-粉晶白云岩（类型III）和钙质泥晶白云岩（类型IV）。类型I主要由泥晶方解石构成,含三叶虫和介形类生物碎片,少量自形的粉晶白云石呈“漂浮状”分布于压溶缝合线内,基质中少见。岩相学和地球化学特征表明此类白云石形成于埋藏成岩期压溶作用之后,压溶缝为云化流体提供通道,压溶缝内泥质组分的成岩转化可能为白云化作用提供了部分镁离子来源;类型II白云岩主要由自形、半自形不等粒粉晶-细晶白云石构成,白云石普遍具有“雾心亮边”,在背散射和阴极发光照片中白云石可见清晰的多圈亮、暗相间环带。环带和带间主量元素的差异表明白云石经历了埋藏成岩期多期成岩流体的改造;类型III白云岩中白云石呈单向延伸的“米粒”或“麦粒”状,粉晶为主,晶体长轴方向具有垂直结晶轴c的特点,白云石具富铁、贫锰、锶的特点,长、短对角线上钙、镁离子的微小差异以及阴极发光特征表明此类白云石也经历了埋藏成岩期的改造,成岩流体使白云石发生微溶作用可能是导致白云石晶体单向延伸且光学性质固定取向的主要原因;类型IV为钙质泥晶球粒白云岩,含石膏假晶,白云石多为微晶和微亮晶,球粒也多由微晶白云石构成,溶孔发育,但全被亮晶贫铁方解石充填,此类岩石的白云化作用发生得很早,可能形成于潮上带澙湖或潮坪环境。综上所述,研究区马家沟组碳酸盐岩具有经历了不同类型及多期白云化作用的特点。自剖面底部向顶部,白云石的有序度由0.8降至0.47,而去云化作用则呈现逐渐增强的趋势。  相似文献   

10.
Ultra‐high pressure metamorphic rocks have been found worldwide. The volume and areal extent of an exhumed UHPM domain are important for understanding the geodynamic mechanisms responsible for the high pressure and relatively medium temperature conditions needed for their creation. We report here Raman microspectroscopical data that prove the existence of microdiamonds at the Svartberget Fe‐Ti type peridotite locality in the Western Gneiss Region of Norway. Raman microscopy of two carbon microinclusions belonging to polyphase inclusion assemblages included in garnets from a garnet‐phlogopite websterite vein yielded a sharp, narrow, intense peak at 1332 cm?1, characteristic of diamond. The diamond is associated with polyphase solid inclusions possibly originating from supercritical, dense, H‐C‐N‐O‐F‐P‐S‐Cl fluids. Lithological, textural and geochronological evidence points towards a Caledonian origin of the trapped fluid and subsequent diamond formation.  相似文献   

11.
Abstract Widespread ultra-high-P assemblages including coesite, quartz pseudomorphs after coesite, aragonite, and calcite pseudomorphs after aragonite in marble, gneiss and phengite schist are present in the Dabie Mountains eclogite terrane. These assemblages indicate that the ultra-high-P metamorphic event occurred on a regional scale during Triassic collision between the Sino-Korean and Yangtze cratons. Marble in the Dabie Mountains is interlayered with coesite-bearing eclogite and gneiss and as blocks of various size within gneiss. Discontinuous boudins of eclogite occur within marble layers. Marble contains an ultra-high-P assemblage of calcite/aragonite, dolomite, clinopyroxene, garnet, phengite, epidote, rutile and quartz/coesite. Coesite, quartz pseudomorphs after coesite, aragonite and calcite pseudomorphs after aragonite occur as fine-grained inclusions in garnet and omphacite. Phengites contain about 3.6 Si atoms per formula unit (based on 11 oxygens). Similar to the coesite-bearing eclogite, marble exhibits retrograde recrystallization under amphibolite–greenschist facies conditions generated during uplift of the ultra-high-P metamorphic terrane. Retrograde minerals are fine grained and replace coarse-grained peak metamorphic phases. The most typical replacements are: symplectic pargasitic hornblende + epidote after garnet, diopside + plagioclase (An18) after omphacite, and fibrous phlogopite after phengite. Ferroan pargasite + plagioclase, and actinolite formed along grain boundaries between garnet and calcite, and calcite and quartz, respectively. The estimated peak P–T conditions for marble are comparable to those for eclogite: garnet–clinopyroxene geothermometry yields temperatures of 630–760°C; the garnet–phengite thermometer gives somewhat lower temperatures. The minimum pressure of peak metamorphism is 27 kbar based on the occurrence of coesite. Such estimates of ultra-high-P conditions are consistent with the coexistence of grossular-rich garnet + rutile, and the high jadeite content of omphacite in marble. The fluid for the peak metamorphism was calculated to have a very low XCO2 (<0.03). The P–T conditions for retrograde metamorphism were estimated to be 475–550°C at <7 kbar.  相似文献   

12.
Metamorphic diamond in crustal rocks provides important information on the deep subduction of continental crust. Here, we present a new occurrence of diamond within the Seve Nappe Complex (SNC) of the Scandinavian Caledonides, on Åreskutan in Jämtland County, Sweden. Microdiamond is found in situ as single and composite (diamond+carbonate) inclusions within garnet, in kyanite‐bearing paragneisses. The rocks preserve the primary peak pressure assemblage of Ca,Mg‐rich garnet+phengite+kyanite+rutile, with polycrystalline quartz surrounded by radial cracks indicating breakdown of coesite. Calculated P–T conditions for this stage are 830–840 °C and 4.1–4.2 GPa, in the diamond stability field. The ultrahigh‐pressure (UHP) assemblage has been variably overprinted under granulite facies conditions of 850–860 °C and 1.0–1.1 GPa, leading to formation of Ca,Mg‐poor garnet+biotite+plagioclase+K‐feldspar+sillimanite+ilmenite+quartz. This overprint was the result of nearly isothermal decompression, which is corroborated by Ti‐in‐quartz thermometry. Chemical Th–U–Pb dating of monazite yields ages between 445 and 435 Ma, which are interpreted to record post‐UHP exhumation of the diamond‐bearing rocks. The new discovery of microdiamond on Åreskutan, together with other evidence of ultrahigh‐pressure metamorphism (UHPM) within gneisses, eclogites and peridotites elsewhere in the SNC, provide compelling arguments for regional (at least 200 km along strike of the unit) UHPM of substantial parts of this far‐travelled allochthon. The occurrence of UHPM in both rheologically weak (gneisses) and strong lithologies (eclogites, peridotites) speaks against the presence of large tectonic overpressure during metamorphism.  相似文献   

13.
This study presents calcite–graphite carbon isotope fractionations for 32 samples from marble in the northern Elzevir terrane of the Central Metasedimentary Belt, Grenville Province, southern Ontario, Canada. These results are compared with temperatures calculated by calcite–dolomite thermometry (15 samples), garnet–biotite thermometry (four samples) and garnet–hornblende thermometry (three samples). Δcal‐gr values vary regularly across the area from >6.5‰ in the south to 4.0‰ in the north, which corresponds to temperatures of 525 °C in the south to 650 °C in the north. Previous empirical calibration of the calcite–graphite thermometer agrees very well with calcite–dolomite, garnet–biotite and garnet–hornblende thermometry, whereas, theoretical calibrations compare less well with the independent thermometry. Isograds in marble based on the reactions rutile + calcite + quartz =titanite and tremolite + calcite + quartz = diopside, span temperatures of 525–600 °C and are consistent with calculated temperature–X(CO2) relations. Results of this study compare favourably with large‐scale regional isotherms, however, local variation is greater than that revealed by large‐scale sampling strategies. It remains unclear whether the temperature–Δcal‐gr relationship observed in natural materials below 650 °C represents equilibrium fractionations or not, but the regularity and consistency apparent in this study demonstrate its utility for thermometry in amphibolite facies marble.  相似文献   

14.
In the meta-ophiolitic belt of Chinese western Tianshan, marble (5–50 cm thick) is found interlayered with pelitic schist. The marble is mainly composed of calcite (>90% in volume) and accessory phases include omphacite, quartz, dolomite, albite, phengite, clinozoisite and titanite with or without rutile core. This is the first omphacite (Jd35–50) reported from marble of Chinese western Tianshan. It mainly occurs in the calcite matrix, rarely as inclusion in albite. The presence of omphacite suggests that the layered marble was subjected to eclogite-facies metamorphism, consistent with the occurrence of high-Si phengite (Si a.p.f.u. up to 3.7) and aragonite relic in albite. The associated pelitic schist consists of quartz, white mica (phengite + paragonite), garnet, albite, amphibole (barroisite ± glaucophane) and rutile/titanite, as well as minor amounts of dolomite, tourmaline and graphite. Coesite is optically recognized within porphyroblastic pelitic garnet and is further confirmed via Raman spectroscopy. Thermodynamic models support the UHP metamorphism of calcite marble, similar to the associated pelitic schist. Shared UHP-LT history of calcareous and pelitic rocks in Chinese western Tianshan suggests that the supracrustal carbon-rich sediments have been carried to depths of >90 km during fast subduction and thus are potential sources for carbon recycled into arc crust.  相似文献   

15.
Exsolution lamellae of pyroxene in garnet (grt), coesite in titanite and omphacite from UHPM terranes are widely accepted as products of decompression. However, interpretation of oriented lamellae of phyllosilicates, framework silicates and oxides as a product of decompression of pyroxene is very often under debate. Results are presented here of FIB‐TEM, FEG‐EMP and synchrotron‐assisted infrared (IR) spectroscopy studies of phlogopite (Phlog) and phlogopite + quartz (Qtz) lamellae in diamond‐bearing clinopyroxene (Cpx) from ultra‐high pressure (UHP) marble. These techniques allowed collection of three‐dimensional information from the grain boundaries of both the single (phlogopite), two‐phase lamellae (phlogopite + quartz), and fluid inclusions inside of diamond included in K‐rich Cpx and understanding their relationships and mechanisms of formation. The Cpx grains contain in their cores lamellae‐I, which are represented by topotactically oriented extremely thin lamellae of phlogopite (that generally are two units cell wide but locally can be seen to be somewhat broader) and microdiamond. The core composition is: (Ca0.94K0.04Na0.02) (Al0.06Fe0.08Mg0.88) (Si1.98Al0.02)O6.00. Fluid inclusions rich in K and Si are recognized in the core of the Cpx, having no visible connections to the lamellae‐I. Lamellar‐II inclusions consist of micron‐size single laths of phlogopite and lens‐like quartz or slightly elongated phlogopite + quartz intergrowths; all are situated in the rim zone of the Cpx. The composition of the rim is (Ca0.95Fe0.03Na0.02) (Al0.05Fe0.05Mg0.90)Si2O6, and the rim contains more Ca, Mg then the core, with no K there. Such chemical tests support our microstructural observations and conclusion that the phlogopite lamellae‐I are exsolved from the K‐rich Cpx‐precursor during decompression. It is assumed that Cpx‐precursor was also enriched in H2O, because diamond included in the core of this Cpx contains fluid inclusions. The synchrotron IR spectra of such diamond record the presence of OH? stretching and H2O bending motion regions. Lamellar‐II inclusions are interpreted as forming partly because of modification of the lamellae‐I in the presence of fluid enriched in K, Fe and Si during deformation of the host diopside; the latter is probably related to the shallower stage of exhumation of the UHP marble. This study emphasizes that in each case to understand the mechanism of lamellar inclusion formation more detailed studies are needed combining both compositional, structural and three‐dimensional textural features of lamellar inclusions and their host.  相似文献   

16.
A new discovery of lawsonite eclogite is presented from the Lancône glaucophanites within the Schistes Lustrés nappe at Défilé du Lancône in Alpine Corsica. The fine‐grained eclogitized pillow lava and inter‐pillow matrix are extremely fresh, showing very little evidence of retrograde alteration. Peak assemblages in both the massive pillows and weakly foliated inter‐pillow matrix consist of zoned idiomorphic Mg‐poor (<0.8 wt% MgO) garnet + omphacite + lawsonite + chlorite + titanite. A local overprint by the lower grade assemblage glaucophane + albite with partial resorption of omphacite and garnet is locally observed. Garnet porphyroblasts in the massive pillows are Mn rich, and show a regular prograde growth‐type zoning with a Mn‐rich core. In the inter‐pillow matrix garnet is less manganiferous, and shows a mutual variation in Ca and Fe with Fe enrichment toward the rim. Some garnet from this rock type shows complex zoning patterns indicating a coalescence of several smaller crystallites. Matrix omphacite in both rock types is zoned with a rimward increase in XJd, locally with cores of relict augite. Numerous inclusions of clinopyroxene, lawsonite, chlorite and titanite are encapsulated within garnet in both rock types, and albite, quartz and hornblende are also found included in garnet from the inter‐pillow matrix. Inclusions of clinopyroxene commonly have augitic cores and omphacitic rims. The inter‐pillow matrix contains cross‐cutting omphacite‐rich veinlets with zoned omphacite, Si‐rich phengite (Si = 3.54 apfu), ferroglaucophane, actinolite and hematite. These veinlets are seen fracturing idiomorphic garnet, apparently without any secondary effects. Pseudosections of matrix compositions for the massive pillows, the inter‐pillow matrix and the cross‐cutting veinlets indicate similar P–T conditions with maximum pressures of 1.9–2.6 GPa at temperatures of 335–420 °C. The inclusion suite found in garnet from the inter‐pillow matrix apparently formed at pressures below 0.6–0.7 GPa. Retrogression during initial decompression of the studied rocks is only very local. Late veinlets of albite + glaucophane, without breakdown of lawsonite, indicate that the rocks remained in a cold environment during exhumation, resulting in a hairpin‐shaped P–T path.  相似文献   

17.
Granulite facies gabbroic and dioritic gneisses in the Pembroke Valley, Milford Sound, New Zealand, are cut by vertical and planar garnet reaction zones in rectilinear patterns. In gabbroic gneiss, narrow dykes of anorthositic leucosome are surrounded by fine‐grained garnet granulite that replaced the host two‐pyroxene hornblende granulite at conditions of 750 °C and 14 kbar. Major and trace element whole‐rock geochemical data indicate that recrystallization was mostly isochemical. The anorthositic veins cut contacts between gabbroic gneiss and dioritic gneiss, but change in morphology at the contacts, from the anorthositic vein surrounded by a garnet granulite reaction zone in the gabbroic gneiss, to zones with a septum of coarse‐grained garnet surrounded by anorthositic leucosome in the dioritic gneiss. The dioritic gneiss also contains isolated garnet grains enclosed by leucosome, and short planar trains of garnet grains linked by leucosome. Partial melting of the dioritic gneiss, mostly controlled by hornblende breakdown at water‐undersaturated conditions, is inferred to have generated the leucosomes. The form of the leucosomes is consistent with melt segregation and transport aided by fracture propagation; limited retrogression suggests considerable melt escape. Dyking and melt escape from the dioritic gneiss are inferred to have propagated fractures into the gabbroic gneiss. The migrating melt scavenged water from the surrounding gabbroic gneiss and induced the limited replacement by garnet granulite.  相似文献   

18.
The analysis of texture, major element and oxygen isotope compositions of cloudy garnet crystals from a metapelite sampled on Ikaria Island (Greece) is used to assess the model of growth and re‐equilibration of these garnet crystals and to reconstruct the pressure–temperature–fluid history of the sample. Garnet crystals show complex textural and chemical zoning. Garnet cores (100–200 μm) are devoid of fluid inclusions. They are characterized by growth zoning demonstrated by a bell‐shaped profile of spessartine component (7–3 mol.%), an increase in grossular from 14 to 22 mol.% and δ18O values between 9.5 ± 0.3‰ and 10.4 ± 0.2‰. Garnet inner rims (90–130 μm) are fluid inclusion‐rich and show a decreasing grossular component from 22 to 5 mol.%. The trend of the spessartine component observed in the inner rim allows two domains to be distinguished. In contrast to domain I, where the spessartine content shows the same trend as in the core, the spessartine content of domain II increases outwards from 2 to 14 mol.%. The δ18O values decrease towards the margins of the crystals to a lowest value of 7.4 ± 0.2‰. The outer rims (<10 μm) are devoid of fluid inclusions and have the same chemical composition as the outermost part of domain II of the inner rim. Garnet crystals underwent a four‐stage history. Stage 1: garnet growth during the prograde path in a closed system for oxygen. Garnet cores are remnants of this growth stage. Stage 2: garnet re‐equilibration by coupled dissolution–reprecipitation at the temperature peak (630 < T < 650 °C). This causes the creation of porosity as the coupled dissolution–reprecipitation process allows chemical (Ca) and isotopic (O) exchange between garnet inner rims and the matrix. The formation of the outer rim is related to the closure of porosity. Stage 3: garnet mode decreases during the early retrograde path, but garnet is still a stable phase. The resulting garnet composition is characterized by an increasing Mn content in the inner rim’s domain II caused by intracrystalline diffusion. Stage 4: dissolution of garnet during the late retrograde path as garnet is not a stable phase anymore. This last stage forms corroded garnet. This study shows that coupled dissolution–reprecipitation is a possible re‐equilibration process for garnet in metamorphic rocks and that intra‐mineral porosity is an efficient pathway for chemical and isotopic exchange between garnet and the matrix, even for otherwise slow diffusing elements.  相似文献   

19.
The Trypali carbonate unit (Upper Triassic), which crops out mainly in central‐western Crete, occurs between the parautochthonous series (Plattenkalk or Talea Ori‐Ida series, e.g. metamorphic Ionian series) and the Tripolis nappe (comprising the Tripolis carbonate series and including a basal Phyllite–Quartzite unit). It consists of interbedded dolomitic layers, represented principally by algally laminated peloidal mudstones, foraminiferal, peloidal and ooidal grainstones, as well as by fine‐grained detrital carbonate layers, in which coarse baroque dolomite crystals and dolomite nodules are dispersed. Baroque dolomite is present as pseudomorphs after evaporite crystals (nodules and rosettes), which grew penecontemporaneously by displacement and/or replacement of the host sediments (sabkha diagenesis). However, portions of the evaporites show evidence of resedimentation. Pre‐existing evaporites predominantly consisted of skeletal halite crystals that formed from fragmentation of pyramidal‐shaped hoppers, as well as of anhydrite nodules and rosettes (salt crusts). All microfacies are characteristic of peritidal depositional environments, such as sabkhas, tidal flats, shallow hypersaline lagoons, tidal bars and/or tidal channels. Along most horizons, the Trypali unit is strongly brecciated. These breccias are of solution‐collapse origin, forming after the removal of evaporite beds. Evaporite‐related diagenetic fabrics show that there was extensive dissolution and replacement of pre‐existing evaporites, which resulted in solution‐collapse of the carbonate beds. Evaporite replacement fabrics, including calcitized and silicified evaporite crystals, are present in cements in the carbonate breccias. Brecciation was a multistage process; it started in the Triassic, but was most active in the Tertiary, in association with uplift and ground‐water flow (telogenetic alteration). During late diagenesis, in zones of intense evaporite leaching and brecciation, solution‐collapse breccias were transformed to rauhwackes. The Trypali carbonate breccias (Trypali unit) are lithologically and texturally similar to the Triassic solution‐collapse breccias of the Ionian zone (continental Greece). The evaporites probably represent a major diapiric injection along the base of the parautochthonous series (metamorphic Ionian series) and also along the overthrust surface separating the parautochthonous series from the Tripolis nappe (Phyllite–Quartzite and Tripolis series). The injected evaporites were subsequently transformed into solution‐collapse breccias.  相似文献   

20.
Three texturally distinct symplectites occur in mafic granofels of the Arthur River Complex at MtDaniel, Fiordland, New Zealand. These include symplectic intergrowths of clinopyroxene and kyanite, described here for the first time. Pods of mafic granofels occur within the contact aureole of the Early Cretaceous Western Fiordland Orthogneiss batholith. The pods have cores formed entirely of garnet and clinopyroxene, and rims of pseudomorphous coarse‐grained symplectic intergrowths of hornblende and clinozoisite that reflect hydration at moderate to high‐P. These hornfelsic rocks are enveloped by a hornblende–clinozoisite gneissic foliation (S1). Narrow garnet reaction zones, in which hornblende and clinozoisite are replaced by garnet–clinopyroxene assemblages, developed adjacent to fractures and veins that cut S1. Fine‐grained symplectic intergrowths of (1) clinopyroxene and kyanite and (2) clinozoisite, quartz, kyanite and plagioclase form part of the garnet reaction zones and partially replace coarse‐grained S1 hornblende and clinozoisite. The development of the garnet reaction zones and symplectites was promoted by dehydration most probably following cooling of the contact aureole. Maps of oxide weight percent and cation proportions, calculated by performing matrix corrections on maps of X‐ray intensities, are used to study the microstructure of the symplectites.  相似文献   

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