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1.
The petrography and mineral composition of a mantle-derived garnet peridotite xenolith from the V. Grib kimberlite pipe (Arkhangelsk Diamond Province, Russia) was studied. Based on petrographic characteristics, the peridotite xenolith reflects a sheared peridotite. The sheared peridotite experienced a complex evolution with formation of three main mineral assemblages: (1) a relict harzburgite assemblage consist of olivine and orthopyroxene porphyroclasts and cores of garnet grains (Gar1) with sinusoidal rare earth elements (REE) chondrite C1 normalized patterns; (2) a neoblastic olivine and orthopyroxene assemblage; (3) the last assemblage associated with the formation of clinopyroxene and garnet marginal zones (Gar2). Major and trace element compositions of olivine, orthopyroxene, clinopyroxene and garnet indicate that both the neoblast and clinopyroxene-Gar2 mineral assemblages were in equilibrium with a high Fe-Ti carbonate-silicate metasomatic agent. The nature of the metasomatic agent was estimated based on high field strength elements (HFSE) composition of olivine neoblasts, the garnet-clinopyroxene equilibrium condition and calculated by REE-composition of Gar2 and clinopyroxene. All these evidences indicate that the agent was a high temperature carbonate-silicate melt that is geochemically linked to the formation of the protokimberlite melt.  相似文献   

2.
One mantle xenolith from a basanite host of the Mt. Melbourne Volcanic Field (Ross Sea Rift) is extraordinary in containing veins filled with leucite, plagioclase, clinopyroxene, nepheline, Mg-ilmenite, apatite, titaniferous mica, and the rare mineral zirconolite. These veins show extensive reaction with the dunitic or lherzolitic host (olivine+spinel+orthopyroxene+clinopyroxene). The reaction areas contain skeletal olivine and diopside crystals, plagioclase, phlogopite, aluminous spinel and ilmenite in a fine grained groundmass of aluminous spinel, clinopyroxene, olivine, plagioclase and interstitial leucite. The vein composition estimated from modal abundances and microprobe analyses is a mafic leucite-phonolite with high amounts of K, Al, Ti, Zr and Nb but low volatile contents. The melt is unrelated to the host basanite and was probably derived by smallscale melting of incompatible element-enriched phlogopite-bearing mantle material and must have lost most of its volatile content during migration, crystallization and reaction with the host dunite. While the veins are completely undeformed the dunitic host shows slight deformation. Vein minerals crystallized at high temperatures above 1000°C and pressures below 5 kbar according to the phase assemblage including leucite, nepheline and K-feldspar. Spinel/olivine geothermometry yielded 800–920°C for the re-equilibration of the host peridotite. Thus the xenolith must have been at shallow depth prior to and during the late veining event. Mantle material at shallow depths is consistent with rifting and the regional extreme displacement at the transition from the rifted Victoria Land Basin in the Ross Sea to the uplifted Trans-Antarctic Mountains.  相似文献   

3.
GREEN  D. H. 《Journal of Petrology》1964,5(1):134-188
The Lizard peridotite produced a high-temperature metamorphicaureole during diapirie emplacement in a period of amphibolitefacies regional metamorphism. The peridotite preserves a coarse-grainedprimary core within a cataclastic, finely foliated and recrystallizedmarginal shell. Chemical analyses demonstrate the constancyof rock composition in the primary and recrystallized mineralassemblages. The primary mineral assemblage of the peridotiteconsists of olivine (Fo89), aluminous enstatite, aluminous diopside,and green aluminous spinel. In contrast the mineral assemblageon recrystallization is olivine, enstatite (normal Al2O3 content),diopside (normal Al2O3 content), plagioclase, and brown chromite.A third assemblage of olivine, pargasite, and brown chromianspinel is developed locally from the plagioclase-bearing assemblage,particularly in contact areas. Major and trace element analyses of twenty minerals and eightrocks are given. These are compared in detail with peridotitesand minerals occurring as accumulates from basaltic magma, asperidotite nodules in basalts and as ’intrusive' peridotitesin orogenic areas. It is concluded that the primary mineralsof the Lizard peridotite have not crystallized and accumulatedfrom a basaltic magma but have crystallized in a similar environmentto that of peridotite nodules in basalts. It is further concludedthat the cause of the differences between the primary and recrystallizedassemblages of the peridotite is primarily a difference in loadpressure at crystallization. In particular the high aluminacontent of both enstatite and diopside and the coexistence ofpyroxenes+ aluminous spinel instead of olivine+anorthite areconsidered to be a direct consequence of the high load pressureat the initial crystallization of the peridotite.  相似文献   

4.
Summary We present a detailed textural and compositional study of two orthopyroxene-rich olivine websterites. One occurs as a vein in a harzburgite xenolith and the other is an individual xenolith, both found at Szentbékkálla in the Bakony–Balaton Highland Volcanic Field (central Pannonian Basin, western Hungary). The textural features of these orthopyroxene-rich rocks suggest that they crystallized from silicate melts to form veins in peridotite mantle rock. Their geochemical features, such as the presence of Al2O3-poor orthopyroxenes, Cr-rich spinels, and clinopyroxenes with U-shaped chondrite-normalized REE-patterns, indicate that the vein material formed from Mg-rich silicic (boninitic) melts at mantle depths. The olivine fabric investigation of both the veins and the wall-rock suggest that the development of the veins was followed by subsequent recrystallization during the Cenozoic evolution of the Carpathian–Pannonian region.  相似文献   

5.
More than twenty mafic-ultramafic complexes, which host several mediumor large-sized Cu−Ni deposits, occur along the Huangshan-Jingerquan (HJ) belt in East Xinjiang. Rock types in these complexes are predominated by peridotite, pyroxene peridotite, olivine pyroxenite, gabbronorite, orthopyroxene gabbro, troctolite, gabbro and diorite. The ultramafic rocks are relatively Fe-enriched and are characterized by an assemblage of olivine+orthopyroxene+clinopyroxene+hornblende±plagioclase without obvious metamorphic textures. Chemically, these complexes are relatively Fe-enriched and show a tholeiitic trend of evolution. The complexes in this belt are intruded under the extensional environment in a Mid-Carboniferous back-arc basin. They can be considered as a new type of mafic-ultramafic complexes in orogenic belts, as designated by the name of the East-Xinjiang-type complexes. This project was financially supported by the National Natural Science Foundation of China.  相似文献   

6.
裴进云 《地质与勘探》2022,58(5):954-964
山西省岩浆热液型矿床主要形成于中生代,与燕山期岩浆活动关系密切,在东部灵丘地区发育多个与次火山岩有关的铜铅锌银矿,近年来在吕梁山地区离石断裂东部也发现了多处钨铜多金属矿化,其中秦家崖钨铜矿床是工作程度最高、规模最大的矿床。本文在野外工作的基础上,对矿床地质、矿物组合、物化探异常体特征与成矿规律进行分析,研究认为:(1)钨矿体主要赋存于NE走向F_(1)断层破碎带中,矿体类型以石英脉型为主,另有少量蚀变岩型,矿物组合为黑钨矿+白钨矿+黄铜矿+黄铁矿,热液蚀变以硅化与绢云母化为主;(2)断裂破碎带中发育多条与矿体关系密切的长英质岩脉,激电测深剖面指示深部(~1000 m)存在隐伏的岩体;(3)秦家崖一带已发现的钨铜矿脉仍处于浅部细脉带,物探剖面揭示矿脉走向与延深位置均有高阻、中低极化的物探异常,指示矿体延深稳定,化探剖面与钻孔岩心元素组成证实矿体附近Cu、W异常明显,同时伴有Au、Mo异常,可作为找矿指标。综合分析认为,秦家崖钨铜矿床是与花岗岩类侵入有关的中、高温热液脉型矿床,进一步进行深边部勘查工作有望在研究区实现钨、铜、钼的找矿突破。  相似文献   

7.
Gold-bearing veins within the Liese zone of the Pogo deposit display a two-stage evolutionary history that records temporal variation in kinematics, fluid chemistry and temperature. Several stacked shallow northwest-dipping shear veins are developed at Pogo, and collectively comprise the Liese Zone. Veins consist of: (1) early, narrow biotite-bearing shear veins; (2) white quartz veins with pyrite-arsenopyrite bands, referred to as main stage quartz veins, that have sericite-Fe-Mg carbonate alteration envelopes and which exploit the early shear veins; and (3) extension veins that form as steeper offshoots from the main stage veins. The presence and orientation of oblique fabrics developed in the older biotite-bearing shear veins are indicative of top-to-the-south displacement under ductile to semi-brittle conditions at higher temperatures. In contrast, the orientation of the extension veins and local sigmoidal shapes indicate a component of top-to-the-northwest normal displacement on the main stage veins in their present orientation, and brittle to semi-brittle conditions of formation. Dolomite-sericite alteration surrounding main stage veins may represent late to post-mineral hydrothermal fluid exploitation of vein margins during ongoing normal displacement along vein systems. All types of veining overprint 107–106 Ma, post-metamorphic granitic dykes. Molybdenite in main stage quartz assemblages has returned Re-Os ages of 104.2±1.1 Ma, significantly older than 96 to 91 Ma 40Ar/39Ar ages obtained from vein alteration assemblages that may reflect thermal resetting during post-mineral fault related hydrothermal activity, magmatism and/or retrograde cooling of the lithologic sequence. Unlike typical mesothermal shear vein hosted gold systems, Pogo is temporally and tectonically separated from metamorphic deformation events, and has a comparable kinematic and geometric architecture to Cretaceous plutonic gold deposits in the region. We interpret the deposit to have formed during a regional Cretaceous extensional event during multi-stage exploitation of extensional fault surfaces by hydrothermal fluid from a cooling magmatic source.Editorial handling: S.G. Hagemann  相似文献   

8.
In Central Dronning Maud Land, East Antarctica, rare metre-sizedlenses of spinel peridotite are enclosed in high-grade metamorphicrocks. The rocks experienced a medium-P granulite-facies metamorphismat 575 Ma and a low-P amphibolite-facies overprint at 530 Ma.The latter is probably related to extensive granitoid magmatismbetween 530 and 500 Ma, which produced large volumes (abouthalf of the outcrops today) of granitic to syenitic rocks aswell as abundant K-feldspar–quartz pegmatites. One ofthe spinel peridotite lenses in the Schirmacher Oasis of CentralDronning Maud Land is crosscut by several small (up to 10 cmwide) veins with a characteristic zoned sequence of mineralassemblages, which was formed by reaction of a hydrous, SiO2-saturatedfluid or pegmatitic melt with the peridotite. The zoned sequenceconsists of the following mineral assemblages (from the centreof the vein towards the outer margin): zone 0, plagioclase +quartz; zone 1, green biotite intergrown with zircon + clinoamphibole;zone 2, cummingtonite + dark brown biotite intergrown with rutile+ clinoamphibole; zone 3, cummingtonite + light brown biotite+ spinel; zone 4, olivine + orthopyroxene + spinel ±clinopyroxene (unaltered peridotite). This sequence was investigatedwith respect to its conditions of formation, modal mineralogy,mineral chemistry, fluid inclusions, and oxygen and hydrogenisotope compositions of selected minerals. Based on the stabilityof cummingtonite and on equilibrium calculations in the MgO–SiO2–H2Osystem and on quartz–biotite oxygen isotope thermometry,the reaction vein formed at 650°C, which is in accord withtypical pegmatite crystallization temperatures. The pegmatiteof zone 0 is interpreted to have formed in an open fissure whereas,on textural grounds, zone 3 replaces former peridotite. On thebasis of mass balance constraints, the boundary between zones1 and 2 is interpreted to approximately represent the formerboundary between peridotite and the open fissure before reaction.Oxygen isotope systematics show that the infiltrating fluidhad an isotopic composition of 9–10 SMOW. All mineralsof the reaction vein with the exception of the inherited spineland olivine in the adjacent peridotite are in equilibrium withsuch a fluid. Spinel in the peridotite is depleted in 18O comparedwith coexisting olivine, which suggests isotopic disequilibrium.Spinel in zone 3 has a distinctly different isotopic compositioncompared with that in the peridotite, apparently approachingbut not reaching equilibrium. The combination of mineral chemistryand mass balance constraints of the modal mineralogy constrainsthe volume change during metasomatism and the direction of elementaldiffusion. It is indicated that Mg, Cr and Ni always diffusedtowards the vein, whereas Si, Al, K, Na, H2O and possibly Fediffused into the peridotite. KEY WORDS: peridotite; metasomatism; pegmatite; diffusion; reaction  相似文献   

9.
Abstract The E-W-trending Kohistan terrane in the NW Himalaya is a sandwich of a magmatic arc between the collided Karakoram (Asian) and Indian plates. The southern part of the Kohistan arc is principally made up of amphibolites derived from volcanic and plutonic rocks of Early Cretaceous age. Gabbroic relics in the amphibolites display calc-alkaline character, and their mineralogy is similar to low-P plutonic rocks reported from modern and ancient island arcs. The largest of these relics, occurring along the southern margin of the amphibolite belt near Khwaza Khela, is subcircular in outline and is about 1 km across. It consists of cumulate gabbros and related rocks displaying a record of cooling and crustal thickening. Primary olivine and anorthite reacted to produce coronas consisting of two pyroxenes +Mg-Fe2+-Al spinel ± tschermakitic hornblende at about 800° C, 5.5–7.5 kbar. This thermotectonic event is of regional extent and may be related to the overthrusting of the Karakoram plate onto the Kohistan arc some 85 Ma ago, or even earlier. Later the gabbros were locally traversed by veins containing high-P assemblages: garnet, kyanite, zoisite, paragonite, oligoclase, calcite, scapolite and quartz ° Chlorite ° Corundum ± diopside. Formed in the range 510–600° C, and 10–12 kbar, these suggest further thickening and cooling of the crust before its uplift during the Tertiary. This paper presents microprobe data on the minerals, and discusses the tectonic implications of the coronitic and vein assemblages in the gabbros.  相似文献   

10.
Petrological investigations supported by multi‐scale structural analysis of eclogitized serpentinite in the Zermatt–Saas Zone of the Western Alps allows for the determination of mineral assemblages related to successive fabrics, upon which the P–T–d–t path of these hydrated mantle rocks can be inferred. Serpentinites of the upper Valtournanche, with lenses and dykes of metagabbro and meta‐rodingite, display an Alpine polyphase metamorphic evolution from eclogite to epidote‐amphibolite facies conditions associated with three successive foliations having different parageneses in these rocks. Serpentinite mainly consists of serpentine with minor magnetite; however, where S1 and S2 foliations are pervasive, metamorphic olivine, together with Ti‐clinohumite and clinopyroxene, are also found. The mineral assemblage associated with D1 includes serpentine1, clinopyroxene1, opaque minerals, titanite ± olivine1, Ti‐clinohumite1 and ilmenite; the D2 assemblage is the same (±chlorite) but minerals have different compositions. The assemblage associated with D3 comprises serpentine3, opaque minerals, ±chlorite3, ilmenite and amphibole3. Ti‐clinohumite is associated with veins that are older than D2 and pre‐date D3. Veins that post‐date D3 are characterized by amphibole + chlorite or by serpentine. P–T conditions for S2 parageneses evaluated using two pseudosections for different bulk compositions suggest that these rocks experienced pressures >2.5 ± 0.3 GPa at temperatures slightly higher than 600 °C. The late epidote–amphibolite facies re‐equilibration associated with D3 and D4 developed during late syn‐exhumation deformation related to folding and testifies to a small temperature decrease. These results, which were integrated in the regional framework, suggest that different portions of the Zermatt–Saas Zone registered different P–T peak conditions and underwent different exhumation paths. In addition, the inferred P–T–d–t path suggests that the Valtournanche serpentinites re‐equilibrated close to the UHP conditions registered by the Cignana meta‐cherts. These results imply that tectonic slices exhumed after UHP metamorphism might be wider than previously reported or that small‐size UHP units, tectonically sampled during the Alpine convergence, are more abundant than those that have been detected to date.  相似文献   

11.
Outside the Bergell tonalite contact aureole, ophicarbonate rocks consist of blocks of antigorite schist embedded in veins of calcite ± tremolite. An antigorite schistosity predates some of these calcite veins. Mono- and bimineralic assemblages occur in reaction zones associated with the veins. Within the aureole, the ophicarbonate veining becomes less distinct and polymineralic assemblages become more frequent. A regular sequence of isobaric univariant assemblages is found, separated by isograds corresponding to isobaric invariant assemblages. In order of increasing grade the invariant assemblages are: antigorite+diopside+olivine+tremolite+calcite antigorite+dolomite+olivine+tremolite+calcite antigorite+olivine+talc+magnesite antigorite+dolomite+olivine+tremolite+talc These assemblages match a previously derived topology in P-T-XCO2 space for the system CaO-MgO-SiO2-H2O-CO2; the field sequence can be used to adjust the relative locations of calculated invariant points with respect to temperature. Isobaric univariant and invariant assemblages are plotted along a profile map to permit direct comparison with the phase diagram.It is inferred that, during the formation of the ophicarbonate veins, calcite precipitated from fluid introduced into the serpentinite. During contact metamorphism, however, the compositions of pore fluids evolved by reaction in the ophicarbonate rocks were largely buffered by the solid phases. This control occurred on a small scale, because there are local variations in the buffering solid assemblages within a centimeter range.  相似文献   

12.
ABSTRACT

New field and petrographic data from the Birimian of the Kolia-Boboti Basin in the Kédougou-Kéniéba inlier indicate two phases of gold mineralization related to Eburnean tectono-magmatic events. Syn- to late-tectonic (D2/D3) mineralization, controlled by stockwork sulphide-bearing quartz-chlorite and quartz-carbonate veins, is associated with fluid circulation related to magmatic intrusions. V2 veins and disseminated assemblages are mainly composed of quartz + chlorite + pyrite and ± gold. Haematite and arsenopyrite are added in the sediments (pelite, greywake, sandstone, quartzite, and marble) and albite in the felsic rocks (andesite, rhyolite, granodirite, and diorite). V3 veins assemblage is composed of quartz + carbonate + pyrite + chalcopyrite and ± gold. Pyrrhotite appears in the sediments (greywacke, quartzite, marble). Sericite, tourmaline, haematite, and magnetite are common in both V2 and V3 assemblages. The first sulphide-bearing quartz-chlorite assemblage is related to the hydrothermal activity of the Eburnean D2 deformation, which was focused mostly along NW- to NNE-trending tectonic structures. The second sulphide-bearing quartz-carbonate assemblage is associated with hydrothermal activity during late Eburnean D3 deformation, mainly located in NE- to E–W-trending tectonic structures. Gold is correlated with the abundance of sulphides (pyrite, chalcopyrite, arsenopyrite), and sulphide stockworks are more abundant in the veins sub-parallel (V2b) or oblique (V2c) to the N–S- to NNE-oriented S2 foliation, as well as in the N45°–N90°-oriented V3 veins. V1 veins, which are related to D1 Eburnean tectonics, are highly deformed (folded and boudinaged) and are poor in sulphides. The host structures of mineralization (V2 and V3 veins) represent the low- and medium-stress domains resulting from the Eburnean D2 and D3 tectonic phases, respectively. The intra-crystalline deformation of the quartz grains associated with these three vein types indicates relatively low temperatures. These different features suggest that most of the mineralization was associated with sulphides formed during the D2 and D3 Eburnean tectono-magmatic events dated around 2080 ± 0.9 and 2061 ± 15 Ma, respectively.  相似文献   

13.
西藏罗布莎地幔橄榄岩变形显微构造特征及其地质意义   总被引：1，自引：0，他引：1  
徐梦婧  金振民 《地质通报》2010,29(12):1795-1803
地幔橄榄岩是罗布莎蛇绿岩的主要组成成分之一,通过显微构造特征可对其变形特征进行分析,确定流变学参数,探讨地幔橄榄岩的变形历史。以蛇纹石化程度较低的二辉橄榄岩和方辉橄榄岩为研究对象,橄榄石位错特征研究显示,本区橄榄石主要发育了低温常见的直线型自由位错,局部可见位错弓弯、位错环、位错壁等高温位错,反映罗布莎地幔橄榄岩变形以低温塑性流变为主,局部经历了高温塑性流变,主导变形机制为位错蠕变。橄榄石自由位错统计结果表明,二辉橄榄岩中的橄榄石自由位错密度为4.422×107/cm2,方辉橄榄岩中的橄榄石为9.137×107/cm2,变形过程中所受差异应力分别为65MPa和93MPa。橄榄石和斜方辉石显微组构测量采用了电子背散射衍射技术(EBSD),分析结果表明,橄榄石均发育A型组构,为浅部地幔常见的组构类型,该结果与金刚石、柯石英等超高压矿物所指示的形成深度不一致。  相似文献   

14.
A major arc batholith, the Western Fiordland Orthogneiss (WFO) in Fiordland, New Zealand, exhibits irregular, spatially restricted centimetre-scale recrystallization from two-pyroxene hornblende granulite to garnet granulite flanking felsic dykes. At Lake Grave, northern Fiordland, the composition and texture of narrow (<10–20 mm across) felsic dykes that cut the orthogneiss are consistent with an igneous origin and injection of melt to form orthogneiss migmatite. New U–Pb geochronology suggests that the injection of dykes and migmatization occurred at c . 115 Ma, during the later stages of arc magmatism. Recrystallization to garnet granulite is promoted by volatile extraction from the host two-pyroxene hornblende granulite via adjacent dykes and the patchy development of garnet granulite is left as a marker adjacent to the melt migration path. New mineral equilibria modelling suggests that a two-pyroxene hornblende assemblage is stable at <11 kbar, whereas a garnet granulite assemblage is stable at >12 kbar, suggesting that garnet granulite may have formed with <5 km crustal loading of the batholith. Although the garnet granulite assemblages signify that the WFO experienced high- P conditions, the very local nature of these textures indicates widespread metastability (>90%) of the two-pyroxene hornblende granulite assemblages. These results indicate the strongly metastable nature of assemblages in mafic lower arc crust during deep burial and demonstrate that the degree of reaction in the case of Fiordland is related to interaction with migrating melts.  相似文献   

15.
Metasomatic tremolite-rich mylonites are widespread in imbricate thrust slices of ultramafic rocks of the ophiolitic Ingalls Complex in Washington State. Protoliths for these amphibolite-facies mylonites were peridotite and serpentinite. Abundant syntectonic tremolite veins in the ultramafites record narrowly channelized flow of infiltrating fluids, whereas metasomatic mylonite zones record more pervasive flow. Fluids were probably released mainly by prograde devolatization reactions within serpentinite and mafic ophiolitic rocks that experienced earlier hydrothermal metamorphism.Olivine apparently deformed by dislocation creep in the mylonites. In the tremolite-rich rocks, locally preserved amphibole porphyroclasts deformed mainly by microfracturing. Acicular tremolites, which dominate the mylonites, form syntectonic overgrowths on porphyroclasts and probably record diffusive mass transfer which may have accompanied cataclasis. Acicular tremolites subsequently were folded and define both post-crystalline crenulations and polygonal arcs.Fluid flow, deformation and metamorphism were apparently complexly interrelated in the imbricate zone. Thrusts juxtaposed contrasting rock types that were sources and sinks for fluids, and shear zones focused fluid flow. Metamorphism probably facilitated deformation through the release of fluids during dehydration reactions. High fluid pressure may have led to hydraulic fracturing and may have controlled strain softening in the tremolitic mylonite zones as it favored microcracking and diffusive mass transfer over dislocation creep. Infiltrating metasomatic fluids probably play an important role in the evolution of shear zones in many ultramafic bodies during medium-grade metamorphism.  相似文献   

16.
The conglomerates of the Solund Devonian basin of SW-Norway contain numerous (locally up to 20 vol.%) peridotitic clasts with concentric mm- to 10-cm thick zones of varying red to black color. The peridotite clasts show a clear, alteration-related textural evolution. The least-altered rocks are partly serpentinized peridotites, showing a typical mesh texture with veins of serpentine, magnesite and Ni-rich magnetite surrounding olivine (Fo91) relicts and its Mg-depleted, clay-like alteration product (deweylite assemblage). In the more advanced ophicarbonate stage, the mesh cells contain calcite, silica and are surrounded by talc. In the final stage, quartz, calcite, and hematite dominate the mineralogy and occur together with minor amounts of chromite, talc, Cr-chlorite, and Cr-hydroandradite. In tandem with this textural evolution is a decrease in MgO from 40 to 2.5 wt% and a CaO increase from 1 to 35 wt%. All peridotite clasts are characterized by high Cr and Ni concentrations. The chemistry and the textural evolution show that the clasts formed by an extreme Mg-mobilization from the peridotite, with development of secondary porosity and subsequent precipitation of calcite. MgO removed from the clasts after burial is in part consumed by replacement reactions in the sediment matrix around the clasts where Mg-free minerals (e.g., almandine) are replaced by Mg-bearing minerals (e.g., talc). Calculated apparent 87Sr/86Sr ratios of the clasts at 385 Ma (0.7124-0.7139), corresponding to the inferred age of sediment deposition and incipient clast alteration, indicate interaction with diagenetic basinal fluids. We explain the reaction history as a three stage process involving (a) partial serpentinization of olivine in an oceanic environment (b) breakdown of olivine relicts to the deweylite assemblage resulting in mobilization of MgO under (near-) surface conditions in a tropical Devonian climate and (c) further Mg-mobilization and replacement of the deweylite assemblage by calcite and quartz after diagenesis. Sedimentary basins with abundant weathered peridotite represent potential sites for a permanent CO2 storage by formation of calcite in a low-temperature environment.  相似文献   

17.
The Sierra Ballena Shear Zone (SBSZ) is part of a high-strain transcurrent system that divides the Neoproterozoic Dom Feliciano Belt of South America into two different domains. The basement on both sides of the SBSZ shows a deformation stage preceding that of the transcurrent deformation recognized as a high temperature mylonitic foliation associated with migmatization. Grain boundary migration and fluid-assisted grain boundary diffusion enhanced by partial melting were the main deformation mechanisms associated with this foliation. Age estimate of this episode is >658 Ma. The second stage corresponds to the start of transpressional deformation and the nucleation and development of the SBSZ. During this stage, pure shear dominates the deformation, and is characterized by the development of conjugate dextral and sinistral shear zones and the emplacement of syntectonic granites. This event dates to 658–600 Ma based on the age of these intrusions. The third stage was a second transpressional event at about 586 to <560 Ma that was associated with the emplacement of porphyry dikes and granites that show evidence of flattening. Deformation in the SBSZ took place, during the late stages, under regional low-grade conditions, as indicated by the metamorphic paragenesis in the supracrustals of the country rocks. Granitic mylonites show plastic deformation of quartz and brittle behavior of feldspar. A transition from magmatic to solid-state microstructures is also frequently observed in syntectonic granites. Mylonitic porphyries and quartz mylonites resulted from the deformation of alkaline porphyries and quartz veins emplaced in the shear zone. Quartz veins reflect the release of silica associated with the breakdown of feldspar to white mica during the evolution of the granitic mylonites to phyllonites, which resulted in shear zone weakening. Quartz microstructures characteristic of the transition between regime 2 and regime 3, grain boundary migration and incipient recrystallization in feldspar indicate deformation under lower amphibolite to upper greenschist conditions (550–400°C). On the other hand, the mylonitic porphyries display evidence of feldspar recrystallization suggesting magmatic or high-T solid-state deformation during cooling of the dikes.  相似文献   

18.
The oceanic serpentinization of peridotites and the influenceof such an alteration on element cycling during their subductiondewatering are here investigated in a mantle slice (Erro–Tobbioperidotite), first exposed to oceanic serpentinization and laterinvolved in alpine subduction, partial dewatering and formationof a high-pressure olivine + titanian-clinohumite + diopside+ antigorite assemblage in the peridotites and in veins. Previouswork indicates that high-pressure veins include primary brines,representing a residue after crystallization of the vein assemblageand containing recycled oceanic Cl and alkalis. To reconstructthe main changes during oceanic peridotite serpentinizationand subsequent subduction, we analysed samples in profiles fromserpentinized oceanic peridotites to high-pressure serpentinites,and from high-pressure ultramafites to veins. Here we presentresults indicating that the main features of the oceanic serpentinizationare immobility of rare earth elements (REE), considerable waterincrease, local CaO decrease and uptake of trace amounts ofSr, probably as a function of the intensity of alteration. Srentered fine-grained Ca phases associated with serpentine andchlorite. Trace-element analyses of mantle clinopyroxenes andhigh-pressure diopsides (in country ultramafites and veins),highlight the close similarity in the REE compositions of thevarious clinopyroxenes, thereby indicating rock control on thevein fluids and lack of exotic components carried by externallyderived fluids. Presence of appreciable Sr contents in vein-formingdiopside indicates cycling of oceanic Sr in the high-pressurefluid. This, together with the recognition of pre-subductionCl and alkalis in the vein fluid, indicates closed-system behaviourduring eclogitization and internal cycling of exogenic components.Diopside and Ti-clinohumite are the high-pressure minerals actingas repositories for REE and Sr, and for high field strengthelements (HFSE), respectively. The aqueous fluid equilibratedwith such an assemblage is enriched in Cl and alkaline elementsbut strongly depleted in REE and HFSE (less than chondrite abundances).Sr is low [(0·2–1·6) x chondrites], althoughselectively enriched relative to light REE. KEY WORDS: eclogite facies; fluid; trace elements; serpentinite; subduction  相似文献   

19.
安徽省枞阳县柳峰山地区金铜矿点地质特征及找矿方向   总被引：1，自引：0，他引：1  
安徽省枞阳县柳峰山地区金铜矿是热液充填形成的脉状矿床.矿体严格受断裂带控制、矿脉的力学性质以扭压性为主、矿脉铜金矿与岩脉关系极为密切、主要的铜金矿脉均有岩脉相伴.火山口的外围断裂构造中,岩脉的发育的地区,断裂构造的交汇部位是今后找矿的重点部位.  相似文献   

20.
韦博  金振民  章军锋 《地球科学》2013,38(5):983-994
对超高压变质带中橄榄岩变形显微构造的研究, 有助于了解板块边界构造环境中地幔物质的流变性质和变形机制, 进而探讨其在深俯冲/折返过程中的地球动力学过程的作用.采用光学显微镜、电子探针、红外光谱、电子背散射衍射(EBSD)、位错氧化缀饰等多种方法系统研究了来自中国大别碧溪岭的石榴异剥橄榄岩中的变形显微构造.研究结果表明: (1)碧溪岭石榴异剥橄榄岩发育较好的形状优选方位, 但只有单斜辉石显示了强晶格优选方位, 而橄榄石晶格优选方位很弱, 与常见上地幔橄榄岩中单斜辉石组构弱而橄榄石组构强的特点差异显著, 反映了单斜辉石经历位错蠕变而橄榄石经历位错调节的颗粒边界滑移变形; (2)碧溪岭异剥橄榄岩中单斜辉石和橄榄石均含有一定量的结构水, 其中单斜辉石含水量124×10-6~274×10-6, 橄榄石含水量38×10-6~80×10-6, 高于常见造山带橄榄岩中各矿物的含水量, 可能反映了壳源物质混染引起的高含水量变形环境; (3)橄榄石中发育显著位错显微构造, 根据位错显微构造计算的变形差异应力为230~600 MPa, 高于正常上地幔稳态流变应力, 反映了俯冲带中的相对低温变形环境.综合分析研究表明, 超高压变质带中的高压、低温、高差异应力和高结构水含量是形成碧溪岭相对独特的橄榄石、单斜辉石变形显微构造的原因.   相似文献   

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