首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 199 毫秒
1.
Multiscale structural analysis and petrological modelling were used to establish the pressure‐peak mineral assemblages and pressure–temperature (P–T) conditions recorded in the rodingites of the upper Valtournanche portion of the oceanic Zermatt‐Saas Zone (ZSZ; Western Alps, northwestern Italy) during Alpine subduction. Rodingites occur in the form of deformed dykes and boudins within the hosting serpentinites. A field structural analysis showed that rodingites and serpentinites record four ductile deformation stages (D1–D4) during the Alpine cycle, with the first three stages associated with new foliations. The most pervasive fabric is S2 that is marked by mineral assemblages in serpentinite indicating pressure‐peak conditions, involving mostly serpentine, clinopyroxene, olivine, Ti‐clinohumite and chlorite. Three rodingite types can be defined: epidote‐bearing, garnet–chlorite–clinopyroxene‐bearing and vesuvianite‐bearing rodingite. In these, the pressure‐peak assemblages coeval with S2 development involve: (i) epidoteII + clinopyroxeneII + Mg‐chloriteII + garnetII ± rutile ± tremoliteI in the epidote‐bearing rodingite; (ii) Mg‐chloriteII + garnetII clinopyroxeneII ± vesuvianiteII ± ilmenite in the garnet–chlorite–clinopyroxene‐bearing rodingite; (iii) vesuvianiteII + Mg‐chloriteII + clinopyroxeneII + garnetII ± rutile ± epidote in vesuvianite‐bearing rodingite. Despite the pervasive structural reworking of the rodingites during Alpine subduction, the mineral relicts of the pre‐Alpine ocean floor history have been preserved and consist of clinopyroxene porphyroclasts (probable igneous relicts from gabbro dykes) and Cr‐rich garnet and vesuvianite (relicts of ocean floor metasomatism). Petrological modelling using thermocalc in the NCFMASHTO system was used to constrain the P–T conditions of the S2 mineral assemblages. The inferred values of 2.3–2.8 GPa and 580–660 °C are consistent with those obtained for syn‐S2 assemblages in the surrounding serpentinites. Multiscale structural analysis indicates that some ocean floor minerals remained stable under eclogite facies conditions suggesting that minerals such as vesuvianite, which is generally regarded as a low‐P phase, could also be stable in favourable chemical systems under high‐P/ultra‐high‐pressure (HP/UHP) conditions. Finally, the reconstructed P–T–d–t path indicates that the P/T ratio characterizing the D2 stage is consistent with cold subduction as estimated in this part of the Alps. The estimated pressure‐peak values are higher than those previously reported in this part of ZSZ, suggesting that the UHP units are larger and/or more abundant than those previously suggested.  相似文献   

2.
The analysis of early stage rodingite from the ultramafic rocks of the Xialu Massif in the Xigaze Ophiolite, Tibet, in China shows that the rodingitization involved continuous changes in fluid composition during different stages of subduction. The early stage prehnite-bearing rodingite was produced at low pressures and temperatures along extensional fractures. Samples of rodingite were collected along a profile from the center to the margin of a rodingitized intrusive igneous rock (~10 m × 30 m), and they record wide variations in bulk composition, mineralogy, and texture. The mineral assemblages, from center to margin, vary from (1) relics of primary clinopyroxene (Cpxr) and primary amphibole (Ampr) + newly formed late amphibole (Act) + primary plagioclase (Plr) + clinozoisite + prehnite + albite + chlorite + titanite + ilmenite (R1 rodingite), through (2) relics of primary clinopyroxene (Cpxr) + newly formed late clinopyroxene (Cpxn) + primary and late amphiboles (Ampr + Act) + clinozoisite + prehnite + albite + chlorite + titanite (R2 rodingite), to (3) newly formed late clinopyroxene (Cpxn) and amphibole (Act) + clinozoisite + prehnite + albite + chlorite + titanite (R3 rodingite). As a result of the metasomatic process of rodingitization, the content of CaO in the whole rock chemical composition from R1 to R3 increases, SiO2 decreases, and Na2O + K2O is almost completely removed. Mass-balance diagrams show enrichments in large ion lithophile elements such as Rb, Cs, Ba, and Pb as well as Ni during rodingitization. The central part of the rodingitized intrusion (R1 rodingite) was only slightly affected by metasomatism. On the other hand, the contents of the rare earth elements (REEs), high field strength elements (HFSEs; e.g. Zr, Nb, Ta, Hf, and Y), and some highly compatible elements such as Cr and Sc decreased slightly during rodingitization. Thermodynamic modeling based on equilibrium mineral assemblages indicates that the rodingite of the Xialu Massif formed in an H2O-saturated, CO2-rich environment. The estimated conditions of metamorphism were ~281–323 °C and 0.4–3.9 kbar, representing the subgreenschist facies. In this environment, REEs and HFSEs were soluble in the fluids and partly removed. Moreover, these prehnite rodingites formed in a progressively reducing and less alkaline environment, as indicated by decreases in f(O2) and bulk-rock Fe3+/Fe2+ ratios, and the records of fluid ΔpH from the center to the margin of the studied rodingitized intrusion.  相似文献   

3.
《地学前缘(英文版)》2020,11(6):2103-2125
The investigated mantle bodies from the External Ligurians (Groppo di Gorro and Mt. Rocchetta) show evidences of a complex evolution determined by an early high temperature metasomatism, due to percolating melts of asthenospheric origin, and a later metasomatism at relatively high temperature by hydrothermal fluids, with formation of rodingites. At Groppo di Gorro, the serpentinization and chloritization processes obliterated totally the pyroxenite protolith, whereas at Mt. Rocchetta relics of peridotite and pyroxenite protoliths were preserved from serpentinization. The rodingite parageneses consist of diopside ​+ ​vesuvianite ​+ ​garnet ​+ ​calcite ​+ ​chlorite at Groppo di Gorro and garnet ​+ ​diopside ​+ ​serpentine ​± ​vesuvianite ​± ​prehnite ​± ​chlorite ​± ​pumpellyite at Mt. Rocchetta. Fluid inclusion measurements show that rodingitization occurred at relatively high temperatures (264–334 ​°C at 500 ​bar and 300–380 ​°C at 1 ​kbar). Garnet, the first phase of rodingite to form, consists of abundant hydrogarnet component at Groppo di Gorro, whereas it is mainly composed of grossular and andradite at Mt. Rocchetta. The last stage of rodingitization is characterized by the vesuvianite formation. Hydrogarnet nucleation requires high Ca and low silica fluids, whereas the formation of vesuvianite does not need CO2-poor fluids. The formation of calcite at Groppo di Gorro points to mildly oxidizing conditions compatible with hydrothermal fluids; the presence of andradite associated with serpentine and magnetite at Mt. Rocchetta suggests Fe3+-bearing fluids with fO2 slightly higher than iron-magnetite buffer. We propose that the formation of the studied rodingite could be related to different pulses of hydrothermal fluids mainly occurring in an ocean-continent transitional setting and, locally, in an accretionary prism associated with intra-oceanic subduction.  相似文献   

4.
OCCURRENCE OF HIGH-PRESSURE RODINGITES IN THE OPHIOLITES OF INDUS SUTURE ZONE, EASTERN LADAKH,THE HIMALAYA:TECTONIC AND METALLOGENIC SIGNIFICANCE  相似文献   

5.
Petrology of rodingite derived from eclogite in western Tianshan, China   总被引：8，自引：0，他引：8  
This study reports a new rodingite type which was derived from eclogite enclosed in the ultramafic rocks of Changawuzi ophiolites, western Tianshan, China. Based on petrographical investigations, rodingite, partial rodingitized rock and completely rodingitized rock are characterized in this paper. These rocks show a continuous variation in their bulk compositions, mineralogy and their textural properties from eclogite to rodingite. The completely rodingitized rocks can be further divided into prehnite rodingites, hydrogrossular‐diopside rodingite and vesuvianite rodingites on the basis of the mineral assemblage and textural character. The chemical potential path of μ(SiO2)–μ(CaO/MgO) can be used to constrain the evolution of two stages of rodingitization. The first rodingitization possibly started under conditions of 410–430 °C and 7–9 kbar at upper greenschist facies, and resulted from a secondary serpentinization during exhumation of the subducted slab. A second and pervasive rodingitization took place under conditions of 250–350 °C and 2–10 kbar from greenschist to subgreenschist facies. The P–T path presented shows a retrograde evolution from eclogite to rodingite. We conclude that the process of rodingitization may also take place under subduction zone conditions in addition to its more common occurrence under ocean‐floor metamorphic conditions.  相似文献   

6.
The whole-rock chemistry of eclogites, partially amphibolitized eclogites, and dyke amphibolites from the metamorphic Kechros complex in the eastern Rhodope Mountains preserves evidence of the geodynamic framework for the origin of their protoliths. Major and trace-element concentrations define two distinct protolith groups for the eclogites. The low-Fe–Ti (LFT) eclogites have low-TiO2 content (<0.67 wt%), negative high field strength element anomalies, and variable enrichments in large ion lithophile elements (LILE). The rare earth element (REE) patterns are characterized by strong light-REE (LREE) enrichment and heavy-REE (HREE) depletion. The high-Fe–Ti (HFT) eclogites have small to moderate LILE enrichment and lack Nb anomalies. The REE patterns of the HFT eclogites are characterized by LREE depletion and relatively flat MREE–HREE patterns. The rock compositions and petrographic features of the LFT eclogites resemble gabbros formed in a continental rift environment with minor to moderate contamination of a mantle-derived mafic magma by continental crust, whereas the HFT eclogites resemble mafic rocks formed in extensional oceanic environments. We interpret the HFT suite to represent a later stage in an evolution from continental rift to open ocean, following the origin of the LFT suite. Dyke amphibolite compositions, except for probable SiO2 loss associated with metamorphic dehydration reactions, appear to represent liquid compositions quenched in conduits through the lower crust. MELTS modeling shows that dyke amphibolite compositions can be related to each other by fractional crystallization under strongly oxidizing conditions at ~0.5 GPa pressure, and all can be derived from a low-degree melt of modified fertile peridotite from around 1.7 GPa. Cumulates crystallized from the parental liquids of the amphibolites under oxidizing conditions may have yielded the protoliths of the HFT suite.  相似文献   

7.
In this study we analyzed the chemical composition of hydrothermally altered dacite and basalt from the Kuroko mining area, northeastern Honshu, Japan, by REE (rare earth element). Features of rare earth element analyses include: (1) altered footwall dacite exhibits a negative Eu anomaly compared with fresh dacite, suggesting preferential removal of Eu2+ from the altered dacite via hydrothermal solutions, (2) altered hangingwall dacite and basalt and dacite and basalt adjacent to ore deposits exhibit positive Eu anomalies compared with fresh dacite and basalt, suggesting addition of Eu2+ from hydrothermal solutions, (3) LREE ratio (∑LREE/∑REE) from altered dacite of chlorite–sericite zone and K-feldspar zone show a negative relationship with δ18O, and La/Sm ratios show a positive correlation with the K2O index. These trends indicate the addition of light rare earth elements such as La to the altered dacite from hydrothermal solution and/or leaching of heavy rare earth elements such as Sm and Yb, (4) Principal component analysis (PCA) indicates that light rare earth elements enrichment is related to the formation of sericite zone near the Kuroko deposits but not to the formations of chlorite and K-feldspar zones, and (5) The correlations among REE features (LREE ratio, MREE ratio, HREE ratio, Eu/Eu?), δ18O and K2O index are not found for montmorillonite zone, mixed layer clay mineral zone and mordenite zone. Therefore, it is inferred that sericite, chlorite and K-feldspar alterations are related to the Kuroko and vein-type mineralization, but montmorillonite and mordenite alterations are not related to the mineralizations, and probably they formed at the post-mineralization stage.  相似文献   

8.
Eclogite facies metarodingites occur as deformed dykes in serpentinites of the Zermatt‐Saas ophiolite (Western Alps). They formed during the subduction of the Tethys oceanic lithosphere in the Early Tertiary. The metarodingites developed as a consequence of serpentinization of the oceanic mantle. Three major types of metarodingites (R1, R2 & R3) can be distinguished on the basis of their mineralogical composition. All metarodingites contain vesuvianite, chlorite and hydrogrossular in high modal amounts. In addition they contain: R1 – diopside, tremolite, clinozoisite, calcite; R2 – hydroandradite, diopside, epidote, calcite; and R3 – hydroandradite. Both garnets contain a small but persistent amount of hydrogarnet component. The different metarodingites reflect different original dyke rocks in the mantle. In each group of metarodingite, textural relations suggest that reactions adjusted the assemblages along the P–T path travelled by the ophiolite during subduction and exhumation. Reactions and phase relations derived from local textures in metarodingite can be modelled in the eight‐component system: SiO2‐Al2O3‐Fe2O3‐FeO‐MgO‐CaO‐CO2‐H2O. This permits the analysis of redox reactions in the presence of andradite garnet and epidote in many of the rocks. Within this system, the phase relations in eclogite facies metarodingites have been explored in terms of T–XCO2, T–μ(SiO2), μ(Cal)–μ(SiO2) and P–T sections. It was found that rodingite assemblages are characterized by low μ(SiO2) and low XCO2 conditions. The low SiO2 potential is externally imposed onto the rodingites by the large volume of antigorite‐forsterite serpentinites enclosing them. Moreover, μ(SiO2) decreases consistently from metarodingite R1 to R3. The low μ(SiO2) enforced by the serpentinites favours the formation of hydrogarnet and vesuvianite. Rodingite formation is commonly associated with hydrothermal alteration of oceanic lithosphere at the ocean floor, in particular to ocean floor serpentinization. Our analysis, however, suggests that the metarodingite assemblages may have formed at high‐pressure conditions in the subduction zone as a result of serpentinization of oceanic mantle by subduction zone fluids.  相似文献   

9.
Garnetite xenoliths from ultramafic diatremes in northeasternArizona provide insights into hydration and metasomatism inthe mantle. The garnetites contain more than 95% garnet, someof which has complex compositional zonation related to growthin fractures within grains. Accessory minerals include rutile,ilmenite, chlorite, clinopyroxene, and zircon. Zircon grainsin one rock were analyzed in situ to determine U–Pb agesand Hf isotopic compositions. Most U–Pb analyses ploton or near concordia in the range 60–85 Ma but a few arediscordant. The range in 176Hf/177Hf is about 0·2818–0·2828,with grains zoned to more radiogenic Hf from interiors to rims.The garnetite protolith contained zircons at least 1·8Ga in age, and garnet and additional zircon crystallized episodicallyduring the interval 85–60 Ma. The garnetites are interpretedas mantle analogues of rodingites, formed in metasomatic reactionzones caused by water–rock interactions in Proterozoicmantle during late Cretaceous and Cenozoic subduction of theFarallon plate. Associated eclogite xenoliths may have beenparts of these same reaction zones. The rodingite hypothesisrequires serpentinization in the mantle wedge 700 km from thetrench, beginning 5–10 Myr before tectonism related tolow-angle subduction. KEY WORDS: garnetite; Lu–Hf, mantle; rodingite; metasomatism  相似文献   

10.
The late Cretaceous-lower Tertiary hydrothermal alteration of serpentinized peridotite in the Semail ophiolite has formed two distinct types of listwaenite. Type I is characterized by the presence of calcite (Type IA) or dolomite (Type IB)+fuchsite±spinel. Type II is dominated by silicate minerals (quartz, chlorite, fuchsite)±calcite+dolomite±magnetite±apatite±plagioclase. Most listwaenites occur as veins along thrust fault zones within the ophiolite mélange. High Cr and Ni contents, abundant occurrence of Cr-spinel within a matrix of red-brown ferruginous carbonates within a micro-vein network of goethite, and the relics of mesh texture indicate an ultramafic protolith. Type I and II listwaenites represent different stages of hydrothermal alteration. The mineralogical and chemical distinctions of both types are the response to the extent of the reactions between the protoliths and the solutions leading to different stages of metasomatic replacement. The hydrothermal fluids involved in the formation of Type I listwaenite were enriched in Ca, Mg, and CO2, whereas Type II listwaenite bodies were formed from a hydrothermal fluid enriched in SiO2. REE and trace elements in both listwaenite types were extracted in part from adjoining peridotite. No Au anomaly in the study areas has been detected.  相似文献   

11.
The rodingites of the Karabash Massif are distinguished by the presence of native cupriferous gold. This zonal hydrothermal-metasomatic complex was formed in three stages. The inner zone of rodingite proper is made up of chlorite-andradite-diopside rocks of stage 1, which are cut by diopside veinlets of stage 2 and calcite veinlets of stage 3. The intermediate zone consists of chloritolites, which give way to the antigorite and chrysotile-lizardite serpentinites of the outer zone. Thermometric and cryometric studies and gas chromatography showed that the gold-bearing rodingites of stages 1 and 2 were formed at t = 420–470°C, P = 2–3 kbar, and \(X_{CO_2 } \) = 0.001–0.02, i.e., under conditions typical of rodingite formation. The final stage was accompanied by a decrease in P-T parameters (0.5–1.0 kbar and 230–310°C) and an increase in \(X_{CO_2 } \) up to 0.04. The rodingite-forming fluid was extremely rich in water (\(X_{H_2 O} \) = 0.942–0.981) and contained hydrogen as the major gas component (\(X_{H_2 } \) = 0.012–0.023); its composition was essentially chloride-magnesium with minor amounts of CaCl2 and FeCl2 and a low salinity of 2.6–8.0 wt % NaCl equiv. The rodingite minerals showed the following isotopic characteristics (‰): δ18O from 5.5 to 6.6 and δD from 42.8 to ?44.3 for chlorite, δ180 from 2.0 to 3.8 for andradite, δ18O from 6.0 to 6.6 for diopside, and δ18O from 10.6 to 11.4 and δ13C from 0.1 to ?1.8 for calcite. The chloritolite is characterized by δ18O from 5.9 to 6.6 and δD from ?49.8 to ?64.4; the antigorite serpentinite shows δ18O=6.5 and δD=?65.2; and the antigoritized chrysotile-lizardite serpentinite shows δ18O from 6.8 to 6.9 and δD from ?127 to ?128. The calculated isotopic composition of fluid in equilibrium with various rocks suggested its metamorphic origin. It was formed from the water released during dehydration of oceanic serpentinites, from the components of ultrabasic and basic magmatic rocks, and, at the final stage, from marine carbon.  相似文献   

12.
A stratigraphically coherent blueschist terrane near Aksu in northwestern China is unconformably overlain by unmetamorphosed sedimentary rocks of Sinian age (～600 to 800 Ma). The pre-Sinian metamorphic rocks, termed the Aksu Group, were derived from shales, sandstones, basaltic volcanic rocks, and minor cherty sediments. They have undergone multi-stage deformation and transitional blueschist/greenschist-facies metamorphism, and consist of strongly foliated chlorite-stilpnomelane-graphite schist, stilpnomelane-phengite psammitic schist, greenschist, blueschist, and minor quartzite, metachert, and meta-ironstone. Metamorphic minerals of basaltic blueschists include crossitic amphibole, epidote, chlorite, albite, quartz, and actinolite. Mineral parageneses and compositions of sodic amphibole suggest blueschist facies recrystallization at about 4 to 6 kbar and 300 to 400° C. Many thin diabasic dikes cut the Aksu Group; they are characterized by high alkali, TiO2, and P2O5 contents and possess geochemical characteristics of within-plate basalts; some of these diabasic rocks contain sodic clinopyroxene and amphibole as primary phases and have minor pumpellyite, albite, epidote, chlorite, and calcite as the prehnite/pumpellyite-facies metamorphic assemblage. This prehnite/pumpellyite-facies overprint did not affect the host rocks of the blueschist-facies lithologies.

K-Ar and Rb-Sr ages of phengite and whole rocks from pelitic schists are ～690 to 728 Ma, and a 40Ar/39Ar age of crossite from the blueschist is 754 Ma. The basal conglomerate of the overlying Sinian to Eocambrian sedimentary succession contains clasts of both the blueschist and cross-cutting dike rocks, clearly demonstrating that conditions required for blueschist-facies metamorphism were attained and ceased at least 700 Ma. The northward-increasing metamorphic grade of the small blueschist terrane may reflect northward subduction of an accretionary complex beyond the northern edge of the Tarim craton. Abundant subparallel diabasic dikes indicate a subsequent period of Pre-Sinian rifting and diabasic intrusion along the northern margin of Tarim; a Sinian siliciclastic and carbonate sequence was deposited unconformably atop the Aksu Group and associated diabase dikes.  相似文献   

13.
Hydrogrossular replacement of plagioclase in basaltic rocks enclosed in serpentinite, and relationships between hydrogrossular, pumpellyite, vesuvianite are described. Rogingitic rocks are dominated by mixed layer chlorite-smectite, chlorite, pumpellyite, hydrogrossular and vesuvianite. In these rocks pumpellyite attains maximal Mg contents, and chemical analysis shows extreme removal of Na2O, K2O, TiO2 and SiO2 from the basalts, and increase in the Fe3+/Fe2+ ratio. Leaching during serpentinization by extremely alkaline solutions dominated by Ca-Mg(OH) may explain removal of components, but the oxidation may suggest that at an earlier stage dykes may have acted as input aquifers.  相似文献   

14.
通过对澜沧江杂岩带小黑江-上允地区蓝片岩的岩相学、地球化学、成因矿物学以及相平衡模拟的综合研究,阐述蓝片岩的原岩以及变质演化过程。地球化学分析结果显示,蓝片岩具有一致的稀土元素配分模式,具弱Eu正或负异常,稀土元素和微量元素特征与OIB相似,其原岩可能为OIB型玄武岩。详细矿物学研究表明,本区蓝片岩记录了俯冲峰期蓝片岩相变质和峰期后绿片岩相变质两个变质阶段,其矿物组合分别为蓝闪石+钠长石+多硅白云母+绿泥石+绿帘石和蓝闪石+钠长石±阳起石+绿泥石+绿帘石。通过Na_2O-Ca O-Fe O-MgO-Al_2O_3-SiO_2-H_2O-O体系相平衡计算,得到两个阶段的压力范围分别约为0.95 GPa和0.40 GPa。  相似文献   

15.
The rodingite belt in the Karabash massif situated 40 km north of Miass is continuously traced for 2.5 km along its central part. Rodingites bear up to 1% sulfide disseminations and gold particles with high Cu content (up to 40 wt %) throughout the belt. The central part of the rodingite belt is especially rich in gold, which was mined at the Zolotaya Gora (Gold Mountain) deposit. The Au-bearing rodingite belt is zonal and was formed during three stages. The inner zone is composed of chlorite-andradite-diopside rock of the first stage, which is crosscut by diopside veinlets of the second stage and calcite veinlets of the third stage. The intermediate zone consists of fine-grained chloritolite of the first stage and coarse-grained chlorite veinlets of the second stage. The outer zone of the metasomatic column is occupied by antigorite and chrysotile-lizardite serpentinites. No relict rocks or minerals of the replaced protolith have been established except sporadic Cr-spinel grains. Native gold was being deposited during all periods of rodingite formation. In terms of the currently adopted concept of evolution of the Ural Foldbelt, the Sm-Nd isochron age of rodingite estimated at 369.4 ± 8.8 Ma corresponds to the period of collisional compression of Silurian-Devonian oceanic and island-arc complexes and upward pushing out of a block of the melanocratic basement underlying these complexes. A proposed model of rodingite formation is based on ore mineralogy, REE geochemistry, and thermobarogeochemical and isotopic study of minerals. It is suggested that in contrast to the barren bimetasomatic rodingite replacing dikes, the studied rodingite are considered to be fissure veins accompanied by metasomatic alteration of host serpentinite. The estimation of initial isotopic composition of fluid components indicates that the ore-bearing fluid is of metamorphic origin (δDfl = ?4 to ?13‰ and δ18Ofl = 5.9 to 8.3‰). The fluid was formed as a product of dehydration of oceanic serpentinite at the base of melanocratic rocks and related gabbroids that moved out to the surface. These rocks were a source of gold and other components (Ca, Al, Ti, Cu, Ni, REE, P, etc.).  相似文献   

16.
H. Austrheim  T. Prestvik   《Lithos》2008,104(1-4):177-198
  相似文献   

17.
The Eocene dyke swarm with east-west general trend intrudes the Cretaceous sedimentary rocks in ～25 km north of the Khur city (Central Iran). Some of the studied dykes can be followed for over 7 km, but the majority of exposures in the area are less than 5 km long. The dykes commonly exhibit a chilled contact with the wall rocks. These dykes are trachybasalt and basalt in composition. The trachybasalt dykes are much more abundant. The basaltic dykes cross cut the trachybasalt dykes in some locations, indicating that trachybasalt dykes are older than the basaltic ones. Primary igneous minerals of the basaltic dykes are olivine (chrysolite), clinopyroxene (diopside, augite), plagioclase (labradorite), sanidine, magnetite, orthopyroxene (enstatite), spinel and phlogopite, and secondary minerals are zeolite (natrolite and mesolite), chlorite (diabantite), calcite and serpentine. The trachybasalt dykes are composed of clinopyroxene (diopside), plagioclase (labradorite), sanidine, mica (biotite and phlogopite), amphibole (magnesio-hastingsite) and magnetite as primary minerals, and chlorite and calcite as secondary ones. Whole rocks geochemical data of the studied dykes indicate their basic and calc-alkaline nature and suggest that these two set of dykes were derived from the same parental magma. The chondrite-normalized REE patterns and the primitive mantle-normalized multi-elemental diagram of the Khur dykes show enrichment of light rare earth elements (LREE) relative to heavy rare earth elements (HREE), and negative anomalies of high field strength elements (HFSE) (e.g. Ti, Nb and Ta). These rocks show enrichment of the large ion lithophile elements (LILE) (e.g. Cs, Ba, Th and U) and depletion of the HREE and Y relative to MREE, Zr and Hf. In the chondrite-normalized REE diagram, the basalts show elevated REE abundances relative to the trachybasalt samples. Geochemical analyses of the studied samples suggest a spinel lherzolite from the mantle as the source rock and confirm the role of subduction in their generation. The chemical characteristics of the Khur dykes resemble those of continental arc rocks, and they were possibly formed by subduction of the Central-East Iranian microcontinent (CEIM) confining oceanic crust and decompression melting of a lithospheric subcontinental mantle spinel lherzolite enriched by subduction.  相似文献   

18.
An extensive complex zoned skarn is developed at the contact of a leucoadamellite intrusive at Doradilla, NW New South Wales. The skarn is a disequilibrium assemblage resulting from a progressive sequence of replacement of a carbonate precursor. Early grossular‐clinopyroxene rocks are replaced by andradite with 0.5–3.5 wt.% SnO2 clinopyroxene and quartz. Later alteration along fractures and bedding planes of the garnet‐clinopyroxene quartz assemblage has produced calcite‐malayaite (CaSn0.95Ti0.05SiO5) veins. The final replacement stage was the overprinting of the silicate phases by assemblages containing sulphides, cassiterite, magnetite, titanite, fluorite, biotite and chlorite. The tin content of garent increases with increasing andradite component suggesting replacement of Fe3+ by Sn4+. Associated clinopyroxenes contain 0.1% SnO2. The coexistence of titanite and its tin isomorph malayaite with extremely limited solid solution indicates late stage skarn temperatures of less than 400°C.  相似文献   

19.
申婷婷  张立飞  李旭平 《岩石学报》2012,28(7):2235-2249
新疆西天山长阿吾子异剥钙榴岩是目前首例发现的在俯冲带形成的原岩为榴辉岩的异剥钙榴岩,与其作用的流体为俯冲带流体。该异剥钙榴岩与大洋底变质作用形成的异剥钙榴岩相比,岩石学特征非常相似,但是元素迁移方面存在一定的差别。为了推断流体成分,我们使用了三种方法。一、测试了异剥钙榴岩化前后主要矿物相石榴石和辉石化学成分。二、在ACF图上,可以看出全岩成分从F端元向C端元演化,代表Fe减少,Ca增加的趋势,而且完全异剥钙榴岩成分落在了前人所做的异剥钙榴岩成分区间内。三、假设Al为不迁移元素,根据Grant的计算方法,做出主量元素、微量元素和REE的等浓度图。综合以上分析结果推断,俯冲带中流体的成分是复杂和变化的:部分异剥钙榴岩化阶段流体富集Ca、Si、Ti、Mn、Nb、Ta、Zr、Hf,贫乏Fe、Mg、Na、K、REE、Tb、Y,氧逸度升高,流体属于不饱和状态,因此导致不平衡结构发育;完全异剥钙榴岩阶段,流体富集Ca、Mg、LREE、Ni、Cr,贫乏Fe、Ti、Rb、Ba、MREE、HREE等元素,此时流体作用强烈,富水流体相的强烈淋滤作用,使得大部分元素发生了迁移。  相似文献   

20.
The major and trace element compositions of nine eclogites from the Dabie–Sulu ultrahigh pressure (UHP) metamorphic terrane in eastern China were determined for both whole rock and the main constituent minerals, garnet and clinopyroxene. The results indicate that the eclogite protoliths originated from a basaltic magma, which formed in a continental setting as shown by isotopic and immobile element data. Based on the garnet REE characteristics, the eclogites can be roughly divided into two groups. Group 1 has LREE enrichment with no Eu anomaly for whole rock, and smooth LREE depletion but HREE enrichment pattern for garnet, whereas group 2 shows a depletion of LREE with a pronounced positive Eu anomaly and flat HREE pattern for both whole rock and garnet. From these features, we suggest that the protoliths for group 2 are Fe–Ti–gabbros with relatively high cumulus plagioclase and Fe–Ti oxide, whereas the group 1 eclogites are probably from basalts. Therefore, the unusual garnet REE pattern observed in group 2 can be considered as an important signature for identifying gabbro protoliths for eclogites. The identification of gabbro protoliths from the eclogites in the Dabie–Sulu terrane provides evidence for Neoproterozoic rift magmatism in the northern margin of the Yangtze craton. During ultrahigh pressure metamorphism in the Dabie–Sulu terrane, LILEs (including Ba, Rb, Th, U, K) had high mobility, but REEs and HFSEs were immobile, and trace element distribution equilibrium was approached between garnet and clinopyroxene. An estimate of mass balance indicates that garnet and clinopyroxene host the majority of HREEs and Y, and clinopyroxene is a significant host for Sr, but minor and accessory minerals predominantly account for LREEs, Th, U, and Zr.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号