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1.
B. DEBRET C. NICOLLET M. ANDREANI S. SCHWARTZ M. GODARD 《Journal of Metamorphic Geology》2013,31(2):165-186
The Lanzo peridotite massif is a fragment of oceanic lithosphere generated in an ocean–continent transition context and eclogitized during alpine collision. Despite the subduction history, the massif has preserved its sedimentary oceanic cover, suggesting that it may have preserved its oceanic structure. It is an exceptional case for studying the evolution of a fragment of the lithosphere from its oceanization to its subduction and then exhumation. We present a field and petrological study retracing the different serpentinization episodes and their impact on the massif structure. The Lanzo massif is composed of slightly serpentinized peridotites (<20% serpentinization) surrounded by an envelope of foliated serpentinites (100% serpentinization) bordered by oceanic metabasalts and metasedimentary rocks. The limit between peridotites and serpentinites defines the front of serpentinization. This limit is sharp: it is marked by the presence of massive serpentinites (80% serpentinization) and, locally, by dykes of metagabbros and mylonitic gabbros. The deformation of these gabbros is contemporaneous with the emplacement of the magma. The presence of early lizardite in the peridotites testifies that serpentinization began during the oceanization, which is confirmed by the presence of meta‐ophicarbonates bordering the foliated serpentinite envelope. Two additional generations of serpentine occur in the ultramafic rocks. The first is a prograde antigorite that partially replaced the lizardite and the relict primary minerals of the peridotite during subduction, indicating that serpentinization is an active process at the ridge and in the subduction zone. Locally, this episode is followed by the deserpentinization of antigorite at peak P–T (estimated in eclogitized metagabbros at 2–2.5 GPa and 550–620 °C): it is marked by the crystallization of secondary olivine associated with chlorite and/or antigorite and of clinopyroxene, amphibole and chlorite assemblages. A second antigorite formed during exhumation partially to completely obliterating previous textures in the massive and foliated serpentinites. Serpentinites are an important component of the oceanic lithosphere generated in slow to ultraslow spreading settings, and in these settings, there is a serpentinization gradient with depth in the upper mantle. The seismic Moho limit could correspond to a serpentinization front affecting the mantle. This partially serpentinized zone constitutes a less competent level where, during subduction and exhumation, deformation and fluid circulation are localized. In this zone, the reaction kinetics are increased and the later steps of serpentinization obliterate the evidence of this progressive zone of serpentinization. In the Lanzo massif, this zone fully recrystallized into serpentinite during alpine subduction and collision. Thus, the serpentinite envelope represents the oceanic crust as defined by geophysicists, and the sharp front of serpentinization corresponds to an eclogitized seismic palaeo‐Moho. 相似文献
2.
冀北发现具鬣刺结构的超基性岩 总被引：1，自引：0，他引：1
冀北具鬣刺结构的超基性岩呈透镜状，产于华北克拉通北缘的早元古宙红旗营子群黑云斜长片麻岩之中。富MgO，贫CaO，A12O3和FeO*＝0．86，TiO2介于0．01％-0．02％之间，与SSZ型蛇绿岩中相应岩石的TiO2含量相当。原始地幔标准化的过渡元素配分型式表现为不对称的“W”型，在TiO2和Cu处形成两个明显的负异常“谷”。据此地球化学特征意味着它们可能来自于消减带之下上地幔，为上地幔高度部分熔融的残余物。岩石中鬣刺结构可能是叶蛇纹石高压分解的结果，表明研究区某些变质橄榄岩岩块曾经受过俯冲—消减作用。 相似文献
3.
Tadao Nishiyama Hibiki Eguchi Dai Shiosaki Akira Yoshiasa Nobutatsu Mochizuki Yasushi Mori Kazumasa Sugiyama Hiroshi Arima Kunio Yubuta 《Island Arc》2021,30(1):e12382
Spinifex-like textured metaperidotites from the Higo Metamorphic Rocks (HMR), west-central Kyushu, Japan, may be formed by high-pressure dehydration of antigorite, and may indicate deep subduction of serpentinite reaching a pressure–temperature condition of 1.6 GPa and 740–750 °C. Three rock types have been identified based on mineral assemblage and rock texture: Type I (L) consisting of medium-grained (1–5 cm long) olivine + enstatite + chromite ±tremolite with secondary talc and anthophyllite that occurs in low-grade metamorphic rocks of the biotite zone, Type I (H) of coarse-grained (up to 10 cm long) olivine + enstatite (with clinoenstatite lamella) + chromite ±tremolite with secondary talc that occurs in high-grade metamorphic rocks of the garnet-cordierite zone, and Type II composed of Al-spinel + chlorite + olivine + apatite + ilmenite with minor sodic gedrite in the garnet-cordierite zone together with Type I (H). Olivines in all rock types are mostly serpentinized during exhumation. The chromite-olivine thermometer gives 560–690 °C for Type I (L) rocks, and the spinel-olivine thermometer gives 610–740 °C for Type II rocks. The peak metamorphic pressure will be higher than 1.6 GPa based on the location of the experimentally determined invariant point (P = 1.6 GPa and T = 670 °C) of antigorite + forsterite + enstatite + talc + H2O. This estimate is consistent with the occurrence of chlorite in Type II rocks, which is stable up to 890 °C at 2.0 GPa. The spinifex-like textured metaperidotites occur as small bodies in the low P/T type gneisses, implying tectonic juxtaposition of them probably during exhumation of the HMR. Recent findings of medium pressure (0.9–1.2 GPa) granulites and gneisses from the HMR may indicate that the HMR has a deep root into the wedge mantle from which the spinifex-like textured metaperidotites have derived. 相似文献
4.
Jasmine Rita Petriglieri Christine Laporte-Magoni Peggy Gunkel-Grillon Mario Tribaudino Danilo Bersani Orietta Sala Monika Le Mestre Ruggero Vigliaturo Nicola Bursi Gandolfi Emma Salvioli-Mariani 《地学前缘(英文版)》2020,11(1):189-202
Covered by ultrabasic units for more than a third of its surface,the New Caledonia(South West Pacific)is one of the largest world producers of Ni-ore from lateritic deposits.Almost all outcrops of geological units and open mines contain serpentine and amphibole,also as asbestos varieties.In this geological context,in which weathering processes had a great contribution in the production and dispersion of mineral fibres into the environment,the development of a routinely analytical strategy,able to discriminate an asbestiform fibre from a non-harmful particle,is a pivotal requisite.However,the acquisition of all these parameters is necessary for determining the risk associated to fibres exposition.A multidisciplinary routinely approach,based on the use of complementary simply-to-use but reliable analytical methods is the only possible strategy.In addition,the instrumental apparatus must be easily transportable on the field,directly on the mining site.The employment of specialized tools such as Polarized Light Microscopy associated to Dispersion Staining method(PLM/DS)and portable Raman spectroscopy for identification of environmental asbestos,are proved extremely effective in the improvement of the performance and rapidity of data acquisition and interpretation.Both PLM/DS and handheld Raman devices confirmed to be discriminant in the detection and characterization of asbestos fibres for both serpentine and amphibole.Furthermore,these techniques proved extremely effective even in the presence of strongly fibrous and altered samples. 相似文献
5.
Metasomatism of continental crust during subduction: the UHP whiteschists from the Southern Dora-Maira Massif (Italian Western Alps) 总被引：1，自引：0，他引：1
S. FERRANDO M. L. FREZZOTTI M. PETRELLI R. COMPAGNONI 《Journal of Metamorphic Geology》2009,27(9):739-756
The ultrahigh‐pressure pyrope whiteschists from the Brossasco‐Isasca Unit of the Southern Dora‐Maira Massif represent metasomatic rocks originated at the expense of post‐Variscan granitoids by the influx of fluids along shear zones. In this study, geochemical, petrological and fluid‐inclusion data, correlated with different generations of pyrope‐rich garnet (from medium, to very‐coarse‐grained in size) allow constraints to be placed on the relative timing of metasomatism and sources of the metasomatic fluid. Geochemical investigations reveal that whiteschists are strongly enriched in Mg and depleted in Na, K, Ca and LILE (Cs, Pb, Rb, Sr, Ba) with respect to the metagranite. Three generations of pyrope, with different composition and mineral inclusions, have been distinguished: (i) the prograde Prp I, which constitutes the large core of megablasts and the small core of porphyroblasts; (ii) the peak Prp II, which constitutes the inner rim of megablasts and porphyroblasts and the core of small neoblasts; and (iii) the early retrograde Prp III, which locally constitutes an outer rim. Two generations of fluid inclusions have been recognized: (i) primary fluid inclusions in prograde kyanite that represent a NaCl‐MgCl2‐rich brine (6–28 wt% NaCleq with Si and Al as other dissolved cations) trapped during growth of Prp I (type‐I fluid); (ii) primary multiphase‐solid inclusions in Prp II that are remnants of an alumino‐silicate aqueous solution, containing Mg, Fe, alkalies, Ca and subordinate P, Cl, S, CO32‐, LILE (Pb, Cs, Sr, Rb, K, LREE, Ba), U and Th (type‐II fluid), at the peak pressure stage. We propose a model that illustrates the prograde metasomatic and metamorphic evolution of the whiteschists and that could also explain the genesis of other Mg‐rich, alkali‐poor schists of the Alps. During Alpine metamorphism, the post‐Variscan metagranite of the Brossasco‐Isasca Unit experienced a prograde metamorphism at HP conditions (stage A: ～1.6 GPa and ≤ 600 °C), as indicated by the growth of an almandine‐rich garnet in some xenoliths. At stage B (1.7–2.1 GPa and 560–590 °C), the influx of external fluids, originated from antigorite breakdown in subducting oceanic serpentinites, promoted the increase in Mg and the decrease of alkalies and Ca in the orthogneiss toward a whiteschist composition. During stage C (2.1 < P < 2.8 GPa and 590 < T < 650 °C), the metasomatic fluid influx coupled with internal dehydration reactions involving Mg‐chlorite promoted the growth of Prp I in the presence of the type‐I MgCl2‐brine. At the metamorphic peak (stage D: 4.0–4.3 GPa and 730 °C), Prp II growth occurred in the presence of a type–II alumino‐silicate aqueous solution, mostly generated by internal dehydration reactions involving phlogopite and talc. The contribution of metasomatic external brines at the metamorphic climax appears negligible. This fluid, showing enrichment in LILE and depletion in HFSE, could represent a metasomatic agent for the supra‐subduction mantle wedge. 相似文献
6.
The fate of B, Cl and Li in the subducted oceanic mantle and in the antigorite breakdown fluids 总被引：2，自引：0，他引：2
Marco Scambelluri Othmar Müntener Thomas T Pettke 《Earth and Planetary Science Letters》2004,222(1):217-234
We present an inventory of B, Cl and Li concentrations in (a) key minerals from a set of ultramafic samples featuring the main evolutionary stages encountered by the subducted oceanic mantle, and in (b) fluid inclusions produced during high-pressure breakdown of antigorite serpentinite. Samples correspond to (i) nonsubducted serpentinites (Northern Apennine and Alpine ophiolites), (ii) high-pressure olivine-bearing antigorite serpentinites (Western Alps and Betic Cordillera), (iii) high-pressure olivine-orthopyroxene rocks recording the subduction breakdown of antigorite serpentinites (Betic Cordillera). Two main dehydration episodes are recorded by the sample suite: partial serpentinite dewatering during formation of metamorphic olivine, followed by full breakdown of antigorite serpentine to olivine+orthopyroxene+fluid. Ion probe and laser ablation ICP-MS (LA ICP-MS) analyses of Cl, B and Li in the rock-forming minerals indicate that the hydrous mantle is an important carrier of light elements. The estimated bulk-rock B and Cl concentrations progressively decrease from oceanic serpentinites (46.7 ppm B and 729 ppm Cl) to antigorite serpentinites (20 ppm B and 221 ppm Cl) to olivine-orthopyroxene rocks (9.4 ppm B and 45 ppm Cl). This suggests release of oceanic Cl and B in subduction fluids, apparently without inputs from external sources. Lithium is less abundant in oceanic serpentinites (1.3 ppm) and the initial concentrations are still preserved in high-pressure antigorite serpentinites. Higher Li contents in olivine, Ti-clinohumite of the olivine-orthopyroxene rocks (4.9 ppm bulk rock Li), as well as in the coexisting fluid inclusions, suggest that their budget may not be uniquely related to recycling of oceanic Li, but may require input from external sources.Laser ablation ICP-MS analyses of fluid inclusions in the olivine-orthopyroxene rocks enabled an estimate of the Li and B concentrations in the antigorite breakdown fluid. The inclusion compositions were quantified using a range of salinity values (0.4-2 wt.% NaClequiv) as internal standards, yielding maximum average fluid/rockDB∼5 and fluid/rockDLi∼3.5. We also performed model calculations to estimate the B and Cl loss during the two dehydration episodes of serpentinite subduction. The first event is characterized by high fluid/rock partition coefficients for Cl (∼100) and B (∼60) and by formation of a fluid with salinity of 4-8 wt.% NaClequiv. The antigorite breakdown produces less saline fluids (0.4-2 wt.% NaClequiv) and is characterized by lower partition coefficients for Cl (25-60) and B (12-30). Our calculations indicate that the salinity of the subduction fluids decreases with increasing depths. fluid/rockDB/fluid/rockDCl<1 (∼0.5) indicates that Cl preferentially partitions into the evolved fluids relative to B and that the B/Cl of fluids progressively increases with increasing depths and temperatures.Despite light element release in fluids, appreciable B, Cl and Li are still retained in chlorite, olivine and Ti-clinohumite beyond the antigorite stability field. This permits a bulk storage of about 10 ppm B, 45 ppm Cl and 5 ppm Li, i.e., concentrations much higher than in mantle reservoirs. Chlorite is the Cl repository and its stability controls the Cl and H2O budget beyond the antigorite stability; B and Li are bound in olivine and clinohumite. The subducted oceanic mantle thus retains light elements beyond the depths of arc magma sources, potentially introducing anomalies in the upper mantle. 相似文献
7.
Abstract Serpentinite bodies in the Kurosegawa Belt are mapped along fault boundaries between the Cretaceous Sanchu Group (forearc basin-fill sediments) and the rocks of the Southern Chichibu Belt (Jurassic to Early Cretaceous accretionary prism) in the northwestern Kanto Mountains, central Japan. The serpentinites were divided into three types based on microtextures and combinations of serpentine minerals: massive, antigorite and chrysotile serpentinites. Massive serpentinite retains initial pseudomorphic textures without any deformation after serpentinization. Antigorite serpentinite exhibits shape-preferred orientation of antigorite replacing the original lizardite and/or chrysotile to form pseudomorphs. It has porphyroclasts of chromian spinel, and is characterized by ductile deformation under relatively high-pressure–temperature conditions. Chrysotile serpentinite shows evidence for overprinting of pre-existing serpentinite features under shallow, low-temperature conditions. It exhibits unidirectional development of chrysotile fibers. Foliations in antigorite and chrysotile serpentinites strike parallel to the elongate direction of the serpentinite bodies, suggesting a continuous deformation during solid-state intrusion along the fault zones after undergoing complete serpentinization at deeper levels (lower crust and upper mantle). 相似文献
8.
河南省淅川黑绿玉质地细腻,颜色浓绿,为一种新型的玉石材料。本文对黑绿玉的矿物成分作了详细的研究,结果表明:黑绿玉的主要矿物成分为α型叶蛇纹石,含量大于90％,电镜下观察呈细条板状及叶片状,相互穿插,且有较大程度的定向性,具交织结构。其它矿物主要为磁铁矿、阳起石、方解石等,含量较少,主要分布于玉石的显微剪切理中。通过X光衍射、电子探针、红外吸收光谱及热分析等,对主要的蛇纹石矿物进行了种属确定,对其结构特征、结晶程度及成因等作了一些探讨,并对玉石(全岩)作了显微硬度测试。 相似文献
9.
辽宁岫岩玉的组成和性质的研究 总被引：1，自引：0，他引：1
辽宁岫玉主要是由叶蛇纹石组成 ,其叶片大小多为 30 0nm× 4 4 0nm左右 ,沿a轴方向呈波状层结构。岫玉中的矿物生成先后次序为粗粒透闪石 细粒透闪石 滑石 叶蛇纹石。岫玉中的颜色和透明度明显受TFeO含量及Fe2 O3/FeO比值所制约 ,同时岫玉中的杂质包裹体 (透闪石、滑石、黄铁矿、磁黄铁矿 )对岫玉的颜色、透明度和其它性质 (光泽、硬度、抗压和抗折强度 )也有直接的影响。 相似文献
10.
采用电子探针、粉晶X射线衍射、化学分析和傅里叶变换红外光谱等方法对泰山玉进行了测试,系统论述了山东泰山玉的颜色分类、矿物组成、化学成分、微量元素特征、结构和构造类型及物理性质。泰山玉分为3种类型:泰山碧玉、泰山墨玉及泰山花斑玉。泰山玉主要由叶蛇纹石组成,杂质矿物有磁铁矿、碳酸盐、滑石、绿泥石、斜方辉石、云母、黄铁矿、褐铁矿、硫镍矿等。泰山玉中的矿物生成顺序为片状蛇纹石→脉状蛇纹石→滑石→脉状碳酸盐。泰山碧玉和花斑玉由Fe2+致色,而泰山墨玉由微粒磁铁矿致色。与辽宁岫玉的区别在于泰山玉中Cr、Ni元素的含量很高。 相似文献