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1.
The Susunai Complex of southeast Sakhalin represents a subduction-related accretionary complex of pelitic and basic rocks. Two stages of metamorphism are recognized: (1) a local, low- P / T event characterized by Si-poor calcic amphiboles; (2) a regional, high- P / T event characterized by pumpellyite, actinolite, epidote, sodic amphibole, sodic pyroxene, stilpnomelane and aragonite. The major mineral assemblages of the high- P / T Susunai metabasites contain pumpellyite+epidote+actinolite+chlorite, epidote+actinolite+chlorite, epidote+Na-amphibole+Na-pyroxene+chlorite+haematite. The Na- amphibole is commonly magnesioriebeckite. The Na-pyroxene is jadeite-poor aegirine to aegirine-augite. Application of empirically and experimentally based thermobarometers suggests peak conditions of T =250–300 °C, P= 4.7–6 kbar. Textural relationships in Susunai metabasite samples and a petrogenetic grid calculated for the Fe 3+-rich basaltic system suggest that pressure and temperature increased during prograde metamorphism. 相似文献
2.
Within the 1.16 Ga old Ilímaussaq intrusion, up to 700 m large autoliths occur in one stratigraphic unit of the layered floor series of agpaitic nepheline syenites (kakortokites). These autoliths consist of two different rock types: augite syenite and naujaite (agpaitic nepheline syenite). All three rock types show a number of alteration features related to the entrapment of the autoliths in the kakortokite magma caused by the interaction with a fluid phase. In the kakortokites, the oxidation of primary arfvedsonite to aegirine and fluorite is restricted to the close proximity to the autoliths. Close to the surrounding kakortokite, the primary mafic phases of the augite syenites (augite, fayalite, Fe–Ti oxides) are completely replaced by arfvedsonite, aenigmatite, biotite, aegirine and fluorite. The decomposition of primary hastingsite to spectacular aegirine–augite–nepheline–aenigmatite symplectites can be observed up to several meters inside the autoliths. Additionally, fluorite formed at grain boundaries of primary nepheline. In the naujaite autoliths, primary arfvedsonite is replaced by aegirine–biotite intergrowths and abundant aenigmatite is occasionally replaced by Ti-rich aegirine and Fe–Ti oxides. The mineral reactions in the autoliths are used to decipher details of the late to post-magmatic processes in a peralkaline syenitic intrusion. Mineral equilibria record an evolution governed by falling temperature (620 to ca. 500 °C) and increasing relative oxygen fugacity from FMQ + 1 to above FMQ + 4. Quantification of the observed mineral reactions reveals the infiltration of the autoliths with an oxidizing fluid phase rich in Na and F and minor addition of K. Volatiles (H and F) and in some cases also Fe, Ti and Ca (± Mg) released from primary autolith phases were mainly just relocated within the autoliths. 相似文献
3.
碱性花岗岩一般以含钠质镁铁硅酸盐矿物为特征,但光头山碱性花岗岩中除钠铁闪石和霓辉石外,还有钢铁非石。钠铁非石是一种三斜晶系的富钛、铁、铀的单链非辉石类硅酸盐矿物,通常和钠质辉石共生。和钠质角门石、钠质辉石一样,钠铁非石也是一种碱性指示矿物,只是比较少见。过碱性岩浆岩中是否出现钢铁非石,主要受岩浆的温度、氧逸度和成分的控制。钠铁非石形成于过碱性岩浆演化的晚期阶段。 相似文献
4.
Acidithiobacillus ferrooxidans are commonly present in acid mine drainage (AMD). A. ferrooxidans derive metabolic energy from oxidation of Fe 2+ present in natural acid solutions and also may be able to utilize Fe 2+ released by dissolution of silicate minerals during acid neutralization reactions. Natural and synthetic fayalites were reacted in solutions with initial pH values of 2.0, 3.0 and 4.0 in the presence of A. ferrooxidans and in abiotic solutions in order to determine whether these chemolithotrophic bacteria can be sustained by acid-promoted fayalite dissolution and to measure the impact of their metabolism on acid neutralization rates. The production of almost the maximum Fe 3+ from the available Fe in solution in microbial experiments (compared to no production of Fe 3+ in abiotic controls) confirms A. ferrooxidans metabolism. Furthermore, cell division was detected and the total cell numbers increased over the duration of experiments. Thus, over the pH range 2–4, fayalite dissolution can sustain growth of A. ferrooxidans. However, ferric iron released by A. ferrooxidans metabolism dramatically inhibited dissolution rates by 50–98% compared to the abiotic controls. Two sets of abiotic experiments were conducted to determine why microbial iron oxidation suppressed fayalite dissolution. Firstly, fayalite was dissolved at pH 2 in fully oxygenated and anoxic solutions. No significant difference was observed between rates in these experiments, as expected, due to extremely slow inorganic ferrous iron oxidation rates at pH 2. Experiments were also carried out to determine the effects of the concentrations of Fe2+, Mg2+ and Fe3+ on fayalite dissolution. Neither Fe2+ nor Mg2+ had an effect on the dissolution reaction. However, Fe3+, in the solution, inhibited both silica and iron release in the control, very similar to the biologically mediated fayalite dissolution reaction. Because ferric iron produced in microbial experiments was partitioned into nanocrystalline goethite (with very low Si) that was loosely associated with fayalite surfaces or coated the A. ferrooxidans cells, the decreased rates of accumulation of Fe and Si in solution cannot be attributed to diffusion inhibition by goethite or to precipitation of Fe–Si-rich minerals. The magnitude of the effect of Fe3+ addition (or enzymatic iron oxidation) on fayalite dissolution rates, especially at low extents of fayalite reaction, is most consistent with suppression of dissolution by interaction between Fe3+ and surface sites. These results suggest that microorganisms can significantly reduce the rate at which silicate hydrolysis reactions can neutralize acidic solutions in the environment. 相似文献
5.
碳酸盐岩的Fe/Mn元素比值,作为一项新的地球化学指标,可以用于恢复海洋的氧化还原状态.在氧化条件下,Fe 3+和Mn 4+均不可溶,因此氧化海水中的溶解Fe和Mn的含量均很低.Fe 3+和Mn 4+在还原条件下可以被细菌还原为可溶的Fe 2+和Mn 2+,而氧化还原电位的计算表明,Mn 4+的还原要早于Fe 3+的还原,因此细菌的Mn还原过程发生在沉积物的更浅层.可溶的Fe 2+和Mn 2+向上扩散到海水中,替代碳酸盐岩晶格里的Ca 2+,因此碳酸盐岩晶格中的Fe 2+和Mn 2+的含量受控于来自沉积物孔隙水的扩散,而后者又与水岩界面的氧化还原状态相关.因此可以预测,随着海水变得逐渐缺氧,碳酸盐岩中的Fe/Mn比值会逐渐增高.为了验证这一假说,我们分析了中元古代高于庄组白云岩的Fe/Mn比值.研究发现,几乎所有的样品的Fe/Mn比值介于20～30之间,显著高于泥盆纪末期深水碳酸盐岩和浅水台地碳酸盐岩的Fe/Mn比值.高于庄组碳酸盐岩高的Fe/Mn比值一方面可能指示了中元古代低的大气氧气浓度和海洋的广泛缺氧,也可能反映了白云岩形成于缺氧的沉积物空隙水里. 相似文献
6.
When Pseudomonas aeruginosa PAO1 biofilms (attached to Sepharose surfaces) were subjected to dissolved Fe 3+, most Fe was removed from solution within 25 h by surface complexation with negatively charged functional groups on the bacterial cell wall via a nucleation and mineralization process. Chemical formation of Fe-(hydr)oxides was partially responsible for dissolved Fe removal, which stemmed from a pH increase, facilitated by microbial activity. PAO1 used Fe 3+ as an electron acceptor producing Fe 2+ under localized anaerobic conditions over the first 50 h. The high ratio of Fe 2+ to total Fe in solution produced a high proportion of Fe(II) in Fe precipitates; however, as the formation of Fe-(hydr)oxides started after 50 h, the Fe 2+ content in solution began to diminish. Biofilms can so influence the local chemical conditions and metal speciation that the bulk solution phase is also affected, thereby mediating a wide-range (bio)geochemical cycling of iron. Long-term survival of natural biofilms, even under strict oligotrophic conditions, could have a broad lasting effect on the bulk geochemical environment. 相似文献
7.
Mineral chemistry, textures and geochemistry of syenite autoliths from Kilombe volcano indicate that they crystallized in the upper parts of a magma chamber from peralkaline trachytic magmas that compositionally straddle the alkali feldspar join in the “residuum system” (ne = 0–1.03; qz = 0–0.77). Mineral reaction and/or overgrowth processes were responsible for the replacement of (i) Mg–hedenbergite by aegirine–augite, Ca–aegirine and/or aegirine, (ii) fayalite by amphiboles, and (iii) magnetite by aenigmatite. Ti–magnetite in silica-saturated syenites generally shows ilmenite exsolution, partly promoted by circulating fluids. By contrast, the Fe–Ti oxides in the silica-undersaturated (sodalite-bearing) syenites show no signs of deuteric alteration. These syenites were ejected shortly after completion of crystallization. Ilmenite–magnetite equilibria indicate fO2 between − 19.5 and − 23.1 log units (T 679–578 °C), slightly below the FMQ buffer. The subsequent crystallization of aenigmatite and Na-rich pyroxenes suggests an increase in the oxidation state of the late-magmatic liquids and implies the influence of post-magmatic fluids. Irrespective of silica saturation, the syenites can be divided into (1) “normal” syenites, characterized by Ce/Ce ratios between 0.83 and 0.99 and (2) Ce-anomalous syenites, showing distinct negative Ce-anomalies (Ce/Ce 0.77–0.24). “Normal” silica-saturated syenites evolved towards pantelleritic trachyte. The Ce-anomalous syenites are relatively depleted in Zr, Hf, Th, Nb and Ta but, with the exception of Ce, are significantly enriched in REE. The silica-saturated syenites contain REE–fluorcarbonates (synchysite-bastnaesite series) with negative Ce-anomalies (Ce/Ce 0.4–0.8, mean 0.6), corroded monazite group minerals with LREE-rich patches, and hydrated, Fe- and P-rich phyllosilicates. Each of these is inferred to be of non-magmatic origin. Fractures in feldspars and pyroxenes contain Pb-, REE- and Mn-rich cryptocrystalline or amorphous material. The monazite minerals are characterized by the most prominent negative Ce-anomalies (Ce/Cemean = 0.5), and in the most altered and Ca-rich areas (depleted in REE), Ce/Ce is less than 0.2. It is inferred that carbonatitic fluids rich in F, Na and lanthanides but depleted in Ce by fractional crystallization of cerian pyrochlore, percolated into the subvolcanic system and interacted with the syenites at the thermal boundary layers of the magma chamber, during and shortly after their crystallization. Chevkinite–(Ce), pyrochlore, monazite and synchysite-bastnaesite, occurring as accessory minerals, have been found for the first time at Kilombe together with eudialyte, nacareniobsite–(Ce) and thorite. These latter represent new mineral occurrences in Kenya. 相似文献
8.
Multiple regression analysis of a compilation of the Fe 2+–Mg distribution between garnet and hornblende from experimental runs on basaltic to intermediate compositions ( n=22) and coexisting garnet–clinopyroxene–hornblende from natural (intermediate to basaltic) rocks ( n=43) has been performed to define ln KD(Fe2+/Mg)Grt–Hbl as a function of temperature and garnet composition. The regression of data covering a large span in pressure (5–16 kbar), temperature (515–1025°C) and composition yields the ln KD(Fe2+/Mg)Grt–Hbl– P– T compositional relationship ( r2=0.93): where Application of this expression to natural garnet–hornblende pairs in intermediate to basaltic and semipelitic rock types from various settings gives temperatures that are consistent with other methods. 相似文献
9.
Aenigmatite, sodic pyroxene and arfvedsonite occur as interstitial minerals in metaluminous to weakly peralkaline syenite patches in alkali dolerite, Morotu, Sakhalin. Aenigmatite is zoned from Ca, Al, Fe 3+-rich cores to Ti, Na, Mn, Si-rich rims reflecting the main substitutions Fe 2+Ti 4+Fe 3+, NaSiCaAl and Mn 2+Fe 2+. Aenigmatite replaces aegirine and ilmenite supporting the existence of a no-oxide field in
— T space. In one case aenigmatite has apparently formed by reaction between ilmenite and arfvedsonite. Titanian aegirine (up to 3.0 wt% TiO 2) and Fe-chlorite may replace aenigmatite. Sodic pyroxene occurs as zoned crystals with cores of aegirine-augite rimmed by aegirine and in turn by pale green aegirine containing 93 mol% NaFe 3+Si 2O 6. Additional substitution of the type NaAlCaFe 2+ is indicated by significant amounts (up to 6 mol%) of NaAlSi 2O 6. Arfvedsonite is zoned with rims enriched in Na, Fe and depleted in Ca which parallels the variation of these elements in the sodic pyroxenes.The high peralkalinity of the residual liquid from which the mafic phases formed resulted from the early crystallization of microperthite (which makes up the bulk of the syenites) leading to an increase in the Na 2O/(Na 2O+K 2O) and (Na 2O+K 2O)/Al 2O 3 ratios of the remaining interstitial liquid which is also enriched in Ti, Fe, and Mn. Bulk composition of the melt,
, temperature and volatile content were all important variables in determining the composition and stability of the peralkaline silicates.
in the residual liquid appears to have been buffered by arfvedsonite-aegirine and later by the arfvedsonite-aenigmatite and aenigmatite-aegirine equilibria under
conditions of a no-oxide field. An increase in
, above that of the alkali buffer reactions, is inferred by an increase of Ti and Mn in aenigmatite rims. The latest postmagmatic vapour crystallization stage of the syenites is marked by extremely low
which may have been facilitated by exsolution of a gas phase. Low
is supported by the replacement of aenigmatite by titanian aegirine, and the formation of rare Ti-rich garnet with a very low (Ti 4++Fe 3+)/(Ti+Fe) ratio of 0.51, associated with leucoxene alteration of ilmenite. 相似文献
10.
Cordierite samples from pegmatites and metamorphic rocks have been analysed for major [electron microprobe analysis (EMPA)] and trace elements [inductively coupled plasma mass spectrometry (ICP-MS), secondary ion mass spectrometry analyses (SIMS)] as well as for H 2O and CO 2 (coulometric titration), and the results evaluated in conjunction with published data in order to determine which exchange mechanisms are significant. Apart from the homovalent substitutions FeMg −1 and MnMg −1 on the octahedral site, some minor KNa −1 on the Ch0 channel site, and Fe 3+Al −1 on the T 11 tetrahedral site, the three most important substitution mechanisms are those for the incorporation of Li on the octahedral sites (NaLi□ −1Mg −1), and of Be and other divalent cations on the tetrahedral T 11 site (NaBe□ −1Al −1 and Na(Mg,Fe 2+)□ −1Al −1). The dominant role of the last vector is clearly demonstrated. We propose a new generalized formula for cordierite: Ch(Na,K) 0–1 VI(Mg,Fe 2+,Mn,Li) 2 IVSi 5 IVAl 3 IV(Al, Be, Mg, Fe 2+, Fe 3+)O 18 * xCh(H 2O, CO 2…). Our results show that the population of (Mg, Fe 2+) on the T 11-site is limited to about 0.08 a.p.f.u. Other exchange mechanisms that were encountered in experiments operate only under P– T conditions or in bulk compositions that are rarely realized in nature. Routine analyses by electron microprobe in which Li and Be are not determined can be plotted as (Mg+Fe+Mn) versus (Si+Al) to assess whether significant amounts of Li and Be could be present. These amounts can be calculated as Li (a.p.f.u.)=Al+Na–4 and Be (a.p.f.u.)=10–2Al–M 2+–Na. 相似文献
11.
The Fe---Mg exchange reaction between coexisting garnet and phengite has been studied by reacting a natural phengite ( mg = 67) in the presence of quartz and water at pressures of 20–35 kb and temperatures of 800–1000°C. Compositions of coexisting garnet and mica indicate a linear relation between both the InK D((Fe/Mg) garnet/(Fe/Mg) phengite) and temperature, and InK D and pressure in the above P.T range. This Fe---Mg exchange reaction between garnet and phengite is shown to be dependent on the Ca-content of the garnet, and on the mg number of the bulk composition. These two composition effects have been studied by usin phengitic mica mixes with mg numbers of 23 and 46, and by using a synthetic basaltic composition. The overall results allow broad empirical calibration of separate geothermometers for pelitic and basaltic systems, respectively. However, because of non-ideality in the exchange reaction, this geothermometer should not be used in any practical application outside the composition ranges studied. Also, careful consideration of the presence of Fe 3+ in phengite must be made. If the Fe 3+ content of the natural phengite is unknown, then the temperatures obtained will be maximum temperatures only. 相似文献
12.
This paper presents new data on chromium mineralization in a fenitized xenolith in Mt. Kaskasnyunchorr in the Khibiny alkaline massif (Kola Peninsula, Russia) and summarizes data on Cr mineralogy in the Khibiny Mountains. Protolith silicates that contained Cr 3+ admixture are believed to be the source of this element in the fenite. Cr-bearing (maximum Cr 2O 3 concentrations, wt %, are in parentheses) aegirine (5.8), crichtonite-group minerals (2.1), muscovite (1.3), zirconolite (1.1), titanite (1.0), fluorine-magnesioarfvedsonite (0.8), biotite (0.8), ilmenite (0.6), and aenigmatite (0.6) occur in the fenite. The late-stage spinellides of the FeTi-chromite-CrTi-magnetite series, which are very poor in Mg and Al and which formed after Crrich aegirine and ilmenite, are the richest in Cr (up to 42% Ct 2O 3). Cr concentrations grew with time during the fenitization process. Unlike minerals in the Khibiny ultramafic rocks where Cr is associated with Mg, Al (it is isomorphic with Cr), and with Ca, chromium in the fenites is associated with Fe, Ti, and V (with which Cr 3+ is isomorphic) and with Na in silicate minerals. Cr 3+ Mobility of Cr 3+ and the unique character of chromium mineralization in the examined fenites were caused by high alkalinity of the fluid. 相似文献
13.
Sanbagawa metabasites metamorphosed at conditions near the upper limit of the pumpellytic-actinolite facies were examined in terms of phase equilibria in the five component system Al 2O 3---Fe 2O 3---CaO---MgO---FeO. The Fe 3+ content of epidote measured as X EpFe (=Fe/(Fe + Al) of epidote) in the assemblage epidote-chlorite-actinolite-pumpellyite decreases gradually towards the higher-grade, pumpellyite-free areas. The progressive change in X FeEp can be detected within one metabasite bed 200 meters thick near the upper limit of the pumpellyite-actinolite facies. The Mg---Fe 2+ substitution, as expressed by variation of Fe/(Fe + Mg) in chlorite (0.40–0.55) has little effect on the Fe 3+ + Al) ratios of epidote and pumpellyite in the above-mentioned assemblage. The lowet X FeEp in the pumpellite-bearing assemblage is 0.15 and hence the upper limit of the pumpellyite-actinolite facies is defined by the appearance of an epidote-chlorite-actinolite assemblage with X EpFc = C.15 相似文献
14.
A series of granitoids from Proterozoic to Cenozoic age occurred in the western Kunlun orogenic belt, Xinjiang, northwestern China. Several intrusions such as the West Datong (Middle Caledonian age), North Kuda (Late Caledonian age) and Kuzigan, Karibasheng, Zankan (Himalayan age) plutons have shoshonitic affinity. Their rock assemblages include (quartz) monzodiorite–(quartz) monzonite–quartz syenite (Middle Caledonian) or monzonitic granite–granite (Late Caledonian) or biotite (monzonitic) granite–diopside granite–diopside syenite (Himalayan). Generally, biotite is iron–phlogopite, with some eastonite and high Mg/(Mg+Fe T) and Fe 3+/Fe 2+ ratio. Amphibole is mainly edenitic hornblende and magnesian hastingsitic hornblende, with some edenite and higher Mg/(Mg+Fe T) and Fe 3+/Fe 2+ ratio. The rocks show SiO 2 contents of 52.77–71.85% and high K 2O+Na 2O (mostly >8%, average 9.14%), K 2O/Na 2O (mostly >1, average 1.50) and Fe 2O 3/FeO (0.85–1.51, average 1.01) and low TiO 2 contents (0.15–1.12%, average 0.57%). Al 2O 3 contents (13.01–19.20%) are high but variable. The granitoids are prominently enriched in LILE, LREE and volatiles such as F. However, the studied shoshonitic granitoids among the three intrusive periods also show differences in isotopic compositions and trace element concentrations, suggesting their different geneses: the origin of the West Datong pluton is probably related to the involvement of subducted oceanic crust sediments into the mantle source; the North Kuda and Himalayan plutons could have been generated by partial melting of subducted oceanic crust sediments or metasediments of thickened continental lower crust in the process of late-orogenic slab break-off or lithospheric thinning. 相似文献
15.
Ferric iron solid solution in synthetic orthopyroxene has been studied along the joins MgSiO 3-Al 2O 3 · Fe 2O 3 and MgSiO 3-Fe 2O 3. The partially reduced synthetic orthopyroxenes showed that major incorporation of ferric iron can only occur together with a concomitant incorporation of Al. Maximum solid solution of ferric iron along the join MgSiO 3-Fe 2O 3 was found to be only 0.63 wt% Fe 2O 3 at 1000°C and 2 kb total pressure. From the observed Mössbauer parameters octahedral ferric iron can be assigned to the MI position in orthopyroxene. Incorporation of Fe 3+ and/or Al will increase the disorder of Fe 2+ and Mg between the M1 and M2 sites, which is also observed in a ferric iron-containing aluminous orthopyroxene of metamorphic origin. In the assemblage orthopyroxene + sillimanite + quartz the ferric iron content of orthopyroxene is directly related to oxygen fugacity. 相似文献
16.
目前,光谱波段比值法可以用于岩性信息提取与识别。以新疆北山坡北造山带辉长岩、橄榄岩、橄榄辉长岩以及闪长岩为研究对象,深入分析这四种岩性在VNIR（可见光-近红外）-SWIR（短波红外）谱域的光谱差异。基于辉长岩、橄榄辉长岩、橄榄岩以及闪长岩在2 300 nm附近存在-OH的吸收特征,建立了第8波段的相对吸收深度 RBD8;基于橄榄辉长岩、橄榄岩以及闪长岩在400~700 nm出现的弱吸收特征,建立比值指数 RI21来反映四种岩性含Fe 3+矿物的丰度;基于四种岩性在1 100 nm附近都存在由Fe 2+引起的宽而强的吸收谱带,建立了比值指数 RI54来估计Fe 2+含量。利用 RBD8、 RI21以及 RI54三个比值指数分别对影像进行增强处理,并对增强后的影像进行RGB彩色合成,不仅识别和提取出辉长岩、橄榄岩、橄榄辉长岩和闪长岩,还有效区分出片岩以及碳酸盐岩等岩性信息。 相似文献
17.
Micro-X-ray absorption near-edge structure (XANES) analysis was employed to determine the content of ferric iron in minerals formed in ultrahigh-pressure (UHP) eclogites. It is observed that omphacite and phengite contain significant amounts of Fe 3+/Fe tot (0.2–0.6), whereas only very low contents are present in garnet (Fe 3+/Fe tot=0.0–0.03), the latter being consistent with results from stoichiometric charge-balance calculations. Furthermore, considerable variations in the Fe 3+/Fe tot ratios of omphacite and phengite are observed depending on the textural sites and local bulk chemistry (eclogite and calc-silicate matrix) within one thin section. The oxidation state of isofacial minerals is thus likely to depend on the local fluid composition, which, in the studied case, is controlled by calcareous and meta-basic mineral compositions. These first in-situ measurements of ferric iron in an eclogite sample from the Dabie Shan, E China, are used to recalculate geothermobarometric data. Calculations demonstrate that the temperature during UHP metamorphism was as high as 780 °C, about 80–100 °C higher than previously estimated. Temperatures based on charge balance calculations often give erroneous results. Pressure estimates are in good agreement with former results and confirm metamorphism in the stability field of diamond (43.7 kbar at 750 °C). These P– T data result in a geothermal gradient of ca. 6 °C/km during UHP metamorphism in the Dabie Shan. However, accounting for ferric iron contents in geothermobarometry creates new difficulties inasmuch as calibrations of geothermometers may not be correctable for Fe 3+ and the actual effect on Mg–Fe 2+ partitioning is unknown. The present study further shows that micro-XANES is a promising technique for the in situ determination of ferric iron contents without destroying the textural context of the sample: a clear advantage compared to bulk methods. 相似文献
18.
The Wadi Haymiliyah section, in the Haylayn block (Semail ophiolite, Oman) displays an unusual plutonic sequence closely similar to those of supra-subduction zone harzburgitic ophiolites (“Troodos sub-type”). It comprizes a bottom, 1000 m-thick, coarse-grained layered gabbro unit (MLGU) overlain by a 1000 m-thick, fine-grained, laminated noritic gabbro unit (MLNGU). Taken as a whole, the mineralogical and bulk-rock trends of the Haymiliah plutonic sequence are those of arc-related calc-alkaline plutons. The MLGU layered gabbros are olivine gabbros and gabbros which differ from low-P cumulates of MORB (oceanic gabbros) by Fe 3+-rich cumulus chrome spinel [ Fe3+/( Fe3+ + Al + Cr) = 0.2-0.3], diopside (Mg# = 85–91) co-crystallized with highly calcic plagioclase (An 96-80) and intercumulus magnesian orthopyroxene (Mg# = 87-80). Plagioclase remains highly calcic at decreasing Fo content, indicating crystallization under high water pressure (> 500 bar). Despite an abrupt decrease in grain size, there exists modal and chemical gradations between MLGU and MLNGU. In the uppermost part of the MLGU, coarse-grained, gabbroic cyclic units culminate with two-pyroxene gabbros containing up to 20 wt.% cumulus Opx. These latter are interbedded over a thickness of ca. 300 m with fine-grained two-pyroxene gabbros and noritic gabbros layers which differ only by slightly higher modal opx and plagioclase contents. The bottom of the MLNGU is marked by norite layers containg up to 80 wt.% plagioclase, cumulus Ti-magnetite and abundant intercumulus Ti-pargasite. Unlike low-pressure differentiates of MORBs, the MLNGU lacks pigeonite and Fe-Ti oxide layers. Cumulus titanomagnetite appears immediately after the orthopyroxene (Mg# = 72–76) in the crystallization order of the norites. The abundance of interstitial Ti-poor pargasite increases at the top of the MLNGU which is brecciated by dioritic differentiates. Both features indicate increasing water pressure and oxygen fugacity (NNO + 2 log units) symptomatic of closed-system magmatic differentiation. Mg#'s of both pyroxenes (70–80) decrease moderately relative to the MLGU coarse-grained gabbros. This and the increase of plagioclase and orthopyroxene modal proportions produce increasing SiO 2-Al 2O 3-Na 2O and Sr contents at nearly constant FeO/MgO in bulk-rock chemistry. This feature similar to the calc-alkaline differentiation trend, is due primarily to a parental tholeiitic magma more hydrated and oxidized than MORBs (ƒ O2 = NNO instead of NNO-2 to NNO-3 log unit); this trend is fully developed in the Wadi Haymiliah section because closure of the magma reservoir in this region allowed larger extent of magmatic differentiation than in other blocks of the Semail ophiolite. The water-rich and oxidized nature of the parental magmas argues for the evolution of the Semail ophiolite in a marginal basin above a subduction zone rather than at a mature oceanic spreading center. 相似文献
19.
The retrograde P-T trajectory of the eclogitic Fe-Ti-gabbros from the Ligurian Alps is constrained by the appearance of mineral parageneses post-dating the Na-clinopyroxene + garnet eclogitic assemblage and indicating the following sequence of metamorphic events: (1) amphibolitic stage— edenite/katophorite + plagioclase (An 33–43) + oxides in symplectitic aggregates; (2) glaucophanic stage— a porphyroblastic glaucophanic amphibole has overgrown the symplectite, winchite also occurs as thin rims around glaucophane and both amphiboles are, sometimes, armoured by atoll garnets; (3) albite-amphibolite stage—barroisite/katophorite + albite + epidote + oxides ± chlorite overprint the glaucophanic stage minerals; (4) greenschist stage—represented by actinolite + albite + epidote + oxide paragenesis. The metamorphic evolution is complex and the decompression path, on a P–T diagram, is significantly different from those defined in the literature for the Voltri eclogites. The main features inferred from the P–T path are the following: (1) the pressure climax does not match the thermal climax, the maximum temperature conditions are in fact achieved during the early stage of uplift; (2) a decrease in temperature, suggested by the appearance of glaucophane after the amphibolitic symplectite; (3) successive uplift, probably accompanied by an increase in temperature. The complexity of the P-T path drawn for the Voltri eclogites can be explained with a mechanism of successive underthrusts propagating from the innermost to the outermost sector of the Ligurian Alps. 相似文献
20.
The pre-Mesozoic metamorphic belt runs parallel to the Day Nui Con Voi - Red River shear zone in Vietnam to the south. The belt is mainly composed of hornblende gneisses, amphibolite lenses and mica-schists. Five hornblendes from a gneiss and an amphibolite were analyzed chemically and chronologically by Electron Probe Micro Analysis (EPMA) and 40Ar/ 39Ar methods. EPMA analyses show that hornblendes in the gneiss and the amphibolite have significant amount of edenite component and similar average composition. However, the recalculated Fe 3+ content is significantly heterogeneous in a thin section while total Fe is nearly the same among the analyses. The rim of each crystal is higher in Fe 3+/(Fe 3+ + Fe 2+) than the core. These chemical and petrological features suggest that the hornblendes have suffered significant oxidation, in particular, largely in the gneiss. 40Ar/39Ar analyses showed that the gneiss has a significant variation of plateau ages (2089±14, 1977±19 and 1873±13 Ma) among three hornblende grains, whereas the amphibolite gives the same plateau ages (2056±14 and 2044±21 Ma) for two grains. All grains of both samples have excess ages in the first few fractions at low temperatures and partial-loss ages between the excess and plateau spectra. The Ca/K ratios indicate some disturbed phases for the lower temperature spectra but the partial-loss ages are also derived from hornblende phase. These facts suggest that hornblende in the gneiss has experienced partial argon loss by oxidation and/or thermally activated argon diffusion process. However, the gneiss and the associated amphibolite have preserved the early Proterozoic tectono-metamorphic event in the hornblende crystals except for their rims, giving new evidence for the early Proterozoic event within the pre-Mesozoic metamorphic belt (northern Vietnam) south of the Red River shear zone in Indochina. 相似文献
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