南苏鲁芝麻房石榴石橄榄岩中橄榄石的“C”类组构及其形成条件探讨

上地幔地震的各向异性主要归因于橄榄石的优选方位,不同的橄榄石优选方位模式可以作为上地幔不同动力学作用的指示剂。不同应力和含水量条件下的高温变形实验已经确定出五类橄榄石组构模式(“A”型、“B”型、“C”型、“D”型和“E”型)。本文运用电子背散射(EBSD)技术对来自苏鲁超高压变质带南部的芝麻房石榴石橄榄岩的橄榄石进行了优选方位测定,不同变形程度的橄榄石均显示了[100]轴近垂直于面理和[001]轴近平行于线理的特征,为“C”类组构模式,可见组构类型与变形程度没有关系,并且橄榄石组构所显示的NW向SE的剪切指向,与围岩-正、副片麻岩中形成于折返过程的石英优选方位所显示的SE向NW的剪切指向完全不同,说明芝麻房石榴石橄榄岩中橄榄石的“C”类组构是折返前形成的。结合橄榄石结构水的测量和已有的芝麻房石榴石橄榄岩形成的温压条件,推测该组构形成于含水俯冲带中,认为芝麻房石榴石橄榄岩的原岩来自于高含水的上部地幔楔碎块,与俯冲的陆壳物质一起经历了超高压变质作用并最终折返至地表。     

Deformation microstructures of olivine and chlorite in chlorite peridotites from Almklovdalen in the Western Gneiss Region,southwest Norway,and implications for seismic anisotropy

Chlorite peridotites from Almklovdalen in southwest Norway were studied to understand the deformation processes and seismic anisotropy in the upper mantle. The lattice preferred orientation (LPO) of olivine and chlorite was determined using electron backscattered diffraction (EBSD)/scanning electron microscopy. A sample with abundant garnet showed [100] axes of olivine aligned sub-parallel to lineation, and [010] axes aligned subnormal to foliation: A-type LPO. Samples rich in chlorite showed different olivine LPOs. Two samples showed [001] axes aligned sub-parallel to lineation, and [010] axes aligned subnormal to foliation: B-type LPO. Two other samples showed [100] axes aligned sub-parallel to lineation, and [001] axes aligned subnormal to foliation: E-type LPO. Chlorite showed a strong LPO characterized by [001] axes aligned subnormal to foliation with a weak girdle subnormal to lineation. Fourier transform infrared (FTIR) spectroscopy of the specimens revealed that the olivines with A-type LPO contain a small amount (170 ppm H/Si) of water. In contrast, the olivines with B-type LPOs contain a large amount (340 ppm H/Si) of water.

The seismic anisotropy of the olivine and chlorite was calculated. Olivine showed Vp anisotropy of up to 3.8% and a maximum Vs anisotropy of up to 2.7%. However, the chlorite showed a much stronger Vp anisotropy, up to 21.1%, and a maximum Vs anisotropy of up to 31.7%. A sample with a mixture of 25% of olivine and 75% of chlorite can produce a Vp anisotropy of 14.2% and a maximum Vs anisotropy of 22.9%. Because chlorite has a wide stability field at high pressure and high temperature in the subduction zone, the strong LPO of chlorite can be a source of the observed trench-normal or trench-parallel seismic anisotropy in the mantle wedge as well as in subducting slabs depending on the dipping angle of slab in a subduction zone where chlorite is stable.     

苏鲁-大别山超高压变质带迟家店和碧溪岭石榴二辉橄榄岩橄榄石组构

大别山碧溪岭和山东荣成分别发育壳源和幔源的石榴二辉橄榄岩。它们在扬子板块向华北板块的俯冲过程中与俯冲板片一起经历了超高压变质作用。这两种原岩来源不同的石榴橄榄岩中的橄榄石具有相似的结晶学优选方位(LPO):[100]轴主板密垂直于面理面,[010]轴主极密平行于线理,这种橄榄石结晶学优选方位明显不同于高温地幔橄榄岩包体中的橄榄石组构,也不同于最近在超高压变质地体中发现的水导致的橄榄石组构。我们认为碧溪岭和荣成石榴二辉橄榄岩中的这种橄榄石组构可能是在相对较干的超高压变质条件下形成的。本研究为大陆深俯冲超高压变质石榴橄榄岩橄榄石组构特征提供了第一手资料。     

西藏南部若干变质橄榄岩体橄榄石的组构特征

藏南变质橄榄岩的橄榄石组构可划分为三个主要类型:[100]、[010]和[001]组构。橄榄石[100]组构反映洋壳下部—上地幔上部的高温粘性流变和高温塑性流变,橄榄石[010]和[001]组构基本上是构造侵位阶段陆壳中低温塑性流变结果。变质橄榄岩中橄榄石组构转化具有明显的规律性,在岩体变形过程中,总的显示由高温的[100]组构逐渐转化为低温的[001]、[010]组构。高温塑性流变组构影响和制约低温塑性流变组构的发育,叠加变形有利于橄榄石[100]极密的加强和[100]组构向[010]、[001]组构的转化。     

Deformation microstructures of olivine in peridotite from Spitsbergen, Svalbard and implications for seismic anisotropy

To understand the deformation mechanism and seismic anisotropy in the uppermost mantle beneath Spitsbergen, Svalbard, in the Arctic, the deformation microstructures of olivine in the peridotite of Spitsbergen were studied. Seismic anisotropy in the upper mantle can be explained mainly by the lattice-preferred orientation (LPO) of olivine. The LPOs of the olivine in the peridotites were determined using electron backscattered diffraction patterns. Eight specimens out of 10 showed that the [100] axis of the olivine was aligned subparallel to the lineation and that the (010) plane was subparallel to the foliation, showing a type A LPO. In the other two specimens the [100] axis of olivine was aligned subparallel to the lineation and both the [010] and [001] axes were distributed in a girdle nearly perpendicular to the lineation, showing a type D LPO. The dislocation density of the olivine in the samples showing a type D LPO was higher than that in the samples showing a type A LPO. The result of an Fourier transformation infrared study showed that both the types A and D samples were dry. These observations were in good agreement with a previous experimental study ( Tectonophysics , 421 , 2006, 1 ): samples showing a type D LPO for olivine were observed at a high stress condition and samples showing both types A and D LPO were deformed under dry condition. Observations of both strong LPOs and dislocations of olivine indicate that the peridotites studied were deformed by dislocation creep. The seismic anisotropy calculated from the LPOs of the olivine could be used to explain the seismic anisotropy of P - and S -waves in the lithospheric mantle beneath Spitsbergen, Svalbard.     

The Role of Pre-existing Mechanical Anisotropy on Shear Zone Development within Oceanic Mantle Lithosphere: an Example from the Oman Ophiolite

Structural and fabric analysis of the well-exposed Hilti mantlesection, Oman ophiolite, suggests that shear zone development,which may have resulted from oceanic plate fragmentation, wasinfluenced by pre-existing mantle fabric present at the paleo-ridge.Detailed structural mapping in the mantle section revealed agently undulating structure with an east–west flow direction.A NW–SE strike-slip shear zone cuts across this horizontalstructure. The crystal preferred orientation (CPO) of olivinewithin the foliation is dominated by (010) axial patterns ratherthan more commonly observed (010)[100] patterns, suggestingthat the horizontal flow close to the Moho involved non-coaxialflow. Olivine CPO within the shear zone formed at low temperatureis characterized by (001)[100] patterns and a sinistral senseof shear. The olivine CPO becomes weaker with progressive mylonitizationand accompanying grain size reduction, and ultimately developsinto an ultra-mylonite with a random CPO pattern. The olivine[010]-axis is consistently sub-vertical, even where the horizontalfoliation has been rotated to a sub-vertical orientation withinthe shear zone. These observations suggest that the primarymechanical anisotropy (mantle fabric) has been readily transformedinto a secondary structure (shear zone) with minimum modification.This occurred as a result of a change of the olivine slip systemsduring oceanic detachment and related tectonics during cooling.We propose that primary olivine CPO fabrics may play a significantrole in the subsequent structural development of the mantle.Thus, the structural behavior of oceanic mantle lithosphereduring subduction and obduction may be strongly influenced byinitial mechanical anisotropy developed at an oceanic spreadingcenter. KEY WORDS: mantle lithosphere; anisotropy; shear zone; olivine CPO; Oman ophiolite     

Iron Isotopic Compositions of Forearc Mantle Peridotites Constrained by Ophiolites from the North Qilian Orogen,Northern Tibet

Previous studies on iron isotope compositions of subduction zone magmas have revealed significant and complex variations that have great bearings on petrogenetic processes in the mantle wedge, e.g., partial melting, fluid metasomatism and redox state. However, interpretations for the fractionations are highly debatable and lack direct constraints from mantle wedge peridotites. This study presents iron isotope compositions for whole rocks and mineral separates in fresh forearc peridotites from the Yushigou ophiolite, North Qilian orogen in northern Tibet. Major and trace element compositions of whole rock and mineral indicate that the peridotites are highly depleted forearc peridotites overprinted by melt metasomatism, in contrast to the long‐holding opinion that the peridotites are derived from mid‐oceanic ridges. The minerals fall on a line with a slope of ～1 on the plot of δ⁵⁶Fe vs. δ⁵⁶Fe, indicating isotope equilibrium between minerals. δ⁵⁶Fe fractionation between olivine and orthopyroxene is within the range of 0～0.05, while fractionation between olivine and spinel is about 0.05～0.10. The fractionation trend between olivine and spinel is opposite to previous theoretical and experimental constraints, which may be due to substantial Cr substitution into the spinel. This indicates that negative correlations between spinel Cr#, fO₂ and spinel δ⁵⁶Fe in previous studies are probably a reflection of gradual Cr enrichment in spinel during melt extraction, and spinel δ⁵⁶Fe values are not a proxy for oxygen fugacity. Whole rock δ⁵⁶Fe values are well correlated with mineral δ⁵⁶Fe values, varying from overlapping with depleted mantle to slightly lower than depleted mantle. Therefore, variations in iron isotope compositions of subduction zone magmas are probably due to combined effect of source heterogeneity and partial melting fractionation.     

Lattice-preferred orientation of olivine found in diamond-bearing garnet peridotites in Finsch,South Africa and implications for seismic anisotropy

Seismic anisotropy in the upper mantle provides important constraints on mantle dynamics, continental evolution and global tectonics and is believed to be produced by the flow-induced lattice-preferred orientation (LPO) of olivine. Recent experimental studies at high pressure and temperature have suggested that the LPO of olivine is affected by pressure in addition to water and stress. However, there has been no report yet for the pressure-induced LPO of natural olivine because samples from the deep upper mantle are rare and often unsuitable for study due to ambiguous foliation and lineation. Here we show evidence of the pressure-induced LPO of natural olivine in diamond-bearing garnet peridotites from Finsch, South Africa. We found that the [010] axes of olivine are aligned subnormal to foliation and that the [001] axes are aligned subparallel to lineation, which is known as B-type LPO of olivine. The equilibrium pressure of the samples, as estimated using geobarometer, was greater than 4 GPa, indicating that the samples originated from a depth greater than ∼120 km. In addition, FTIR spectroscopy of the olivine showed that the samples are dry, with a water content of less than 90 ± 20 ppm H/Si (5.5 ± 1.2 ppm wt. H₂O). These data suggest that the samples are the first natural examples of olivine displaying B-type LPOs produced due to high pressure under dry condition. Our data indicate that the trench-parallel seismic anisotropy observed in many subduction zones in and below subducting slabs at depths greater than ∼90 km under dry condition may be attributed to the pressure-induced olivine fabrics (B-type LPO) and may be interpreted as the entrainment of the sub-lithospheric mantle in the direction of subduction rather than anomalous trench-parallel flow.     

Strain coupling between upper mantle and lower crust: natural example from the Běstvina granulite body, Bohemian Massif

Strain patterns within mantle rocks and surrounding coarse‐grained felsic granulites from the Kutná Hora Crystalline Complex in the Variscan Bohemian Massif have been studied in order to assess their strain coupling. The studied rock association occurs within low‐strain domains surrounded by fine‐grained granulite and migmatite. The Doubrava peridotite contains closely spaced and steeply dipping layers of garnet clinopyroxenite, which are parallel to the NE–SW‐striking, high‐temperature foliation in nearby granulites, while the Úhrov peridotite lacks such layering. The Spa?ice eclogite is not associated with peridotite and shows upright folds of alternating coarse‐ and fine‐grained varieties bearing NE–SW‐striking axial planes. Electron back‐scattered diffraction measurements revealed full strain coupling between clinopyroxenites and coarse‐grained granulites in the S₁ fabric that is superposed on the S₀ fabric preserved in peridotites. The B‐type olivine lattice preferred orientation (LPO) characterizes the S₀ fabric in peridotites and its reworking is strongly controlled by the presence of macroscopic clinopyroxenite layering. The S₁ in clinopyroxenites and coarse‐grained granulites is associated with the LS‐type clinopyroxene LPO and prism <c> slip in quartz respectively. While the S₁ fabric in these rock types is accompanied invariably by a sub‐vertical stretching lineation, the S₁ fabric developed in reworked Úhrov peridotite is associated with strongly planar axial (010) type of olivine LPO. The peridotites with the S₀ fabric are interpreted to be relicts of a fore‐arc mantle wedge hydrated to a various extent above the Saxothuringian subduction zone. The prograde metamorphism recorded in peridotites and eclogites occurred presumably during mantle wedge flow and was reaching UHP conditions. Strain coupling in the S₁ fabric between clinopyroxenites and granulites at Doubrava and upright folding of eclogites at Spa?ice document a link between tectonic and magmatic processes during orogenic thickening, coeval with intrusions of the arc‐related calcalkaline suites of the Central Bohemian Plutonic Complex (c. 360–345 Ma). Juxtaposition of peridotites and granulites could be explained by a rheological heterogeneity connected to the development of clinopyroxenite layering in the upper mantle and a previously published model of a lithospheric‐scale transpressional arc system. It invokes vertical shearing along NE–SW trending, sub‐vertical foliations in the upper mantle that could have led to an emplacement of mantle bodies into the granulitized, orogenic root in the sub‐arc region. Clearly, such a transpressional arc system could represent an important pathway for an emplacement of deep‐seated rocks in the orogenic lower crust.     

藏南超基性岩的塑性流变

随着深部构造研究的不断深入,岩石流变作用日益受到人们重视,普遍认为塑性流变是地壳深部构造形成的主要机制。超基性岩的流变作用目前已成为研究上地幔流变、岩石圈板块动力学和热对流,甚至震源机制的重要内容。 本文拟通过对构成雅鲁藏布江蛇绿岩套底部的藏南超基性岩的研究,重点划分塑性流变的结构类型,探讨不同结构类型中橄榄石的组构特征及实际存在的滑动系,用不同方法计算超基性岩形成和侵位过程中的流动应力值,并认为由于橄榄石塑性流变而获得的晶格方位排列是造成地震波速度不连续性的主因,岩石的塑性流动有可能是深部能量释放和诱发     

Textures in deforming forsterite aggregates up to 8 GPa and 1673 K

We report results from axisymmetric deformation experiments carried out on forsterite aggregates in the deformation-DIA apparatus, at upper mantle pressures and temperatures (3.1–8.1 GPa, 1373–1673 K). We quantified the resulting lattice preferred orientations (LPO) and compare experimental observations with results from micromechanical modeling (viscoplastic second-order self-consistent model—SO). Up to 6 GPa (~185-km depth in the Earth), we observe a marked LPO consistent with a dominant slip in the (010) plane with one observation of a dominant [100] direction, suggesting that [100](010) slip system was strongly activated. At higher pressures (deeper depth), the LPO becomes less marked and more complex with no evidence of a dominant slip system, which we attribute to the activation of several concurrent slip systems. These results are consistent with the pressure-induced transition in the dominant slip system previously reported for olivine and forsterite. They are also consistent with the decrease in the seismic anisotropy amplitude observed in the Earth’s mantle at depth greater than ~200 km.     

Olivine [100] normal to foliation: lattice preferred orientation in prograde garnet peridotite formed at high H<Subscript>2</Subscript>O activity,Cima di Gagnone (Central Alps)

Automated electron backscattered diffraction (EBSD) was applied using a scanning electron microscope to obtain lattice preferred orientation (LPO) data for olivine in garnet peridotites of the Central Alps. As a reference frame, the LPOs of enstatite were also investigated. In the garnet peridotite at Cima di Gagnone (CDG), a weak foliation carrying a distinct lineation is present. The lineation is characterized by elongated enstatite, olivine and poikiloblastic garnet. Olivine shows a very unusual LPO with [100] normal to foliation and [001] parallel to lineation. Achsenverteilungsanalyse (AVA) maps demonstrate that [001] of olivine grains corresponds quite well to their maximum length axes which are preferentially parallel to the lineation. Numerous planar hydrous defects within (001) planes of olivine are marked by palisades of ilmenite rods and show a preferred orientation normal to lineation. Calculated P-wave velocities for CDG are fastest (8.32 km s^у) normal to foliation with a relatively low anisotropy (2.9%). Compared to mantle peridotites with the usual (010)[100] LPO where the fastest Vp direction is towards the lineation, the relationship between flow geometry and seismic anisotropy is significantly different at CDG. Several mechanisms for the formation of the LPO type at CDG are considered, with glide possible on (100)[001] of olivine. On the basis of field data as well as petrographic and petrologic evidence, it has been demonstrated that the CDG garnet peridotite formed by prograde metamorphism from a hydrous protolith at pressures and temperatures of about 3.0 GPa and 750 °C, respectively. The CDG LPO is interpreted to have formed during hydrous subduction zone metamorphism. The same interpretation may hold for the previously investigated olivine LPO at Alpe Arami, which is similar to that at the nearby CDG. The observed anomalous LPO is no proof for ultradeep (>3.0 GPa) conditions.     

Deformation fabrics of the Cima di Gagnone peridotite massif, Central Alps, Switzerland: evidence of deformation at low temperatures in the presence of water

We report a new observation of the olivine B-type lattice-preferred orientation (LPO), from the garnet peridotite at Cima di Gagnone, Switzerland. The olivine B-type fabric forms at low temperatures and/or high stress in the presence of water, and is of particular interest because it may be used to explain the trench-parallel shear-wave splitting that is often observed at subduction zones. In conjunction with the olivine B-type fabric, we have found strong orthopyroxene LPO that is identical to those formed under water-free conditions. This suggests that water may not have a significant effect on orthopyroxene fabric. From the olivine microstructure, we determine that a stress of 22 ± 8 MPa was applied during the deformation event that formed the olivine LPO. Using an olivine flow-law, and assuming geological strain-rates, we determine the temperature of deformation to be 800 ± 175°C. This does not preclude an ultra-deep origin for the ultramafic rocks at Cima di Gagnone, but indicates that much of the deformation recorded in the microstructure occurred at modest temperatures.Communicated by T.L. Grove     

洋壳俯冲过程中的地幔楔上升对流：来自芝麻坊石榴子石二辉橄榄岩早期变质的证据

苏鲁超高压变质地体中发现了大量包裹在超高压（UHP）变质片麻岩和混合岩中的造山带石榴橄榄岩。根据它们的野外产出特征和全岩地球化学成分,其中一部分石榴橄榄岩的原岩来自于亏损地幔,后来被卷入俯冲陆壳并经受过俯冲陆壳产生的熔/流体的交代。但是,对这些岩石早期的亏损过程尚缺乏清晰的认识。本文报道了东海芝麻坊石榴子石二辉橄榄岩早期变质演化的新证据。根据详细的变质反应结构观察和矿物成分研究,芝麻坊石榴子石二辉橄榄岩在经历高压低温俯冲带型超高压变质之前经历了至少两期变质演化。其原岩矿物组合由石榴子石变斑晶的高Ca-Cr核部及其中包裹的高Mg单斜辉石、高Al-Cr斜方辉石和高Mg-Ni橄榄石所记录;指示芝麻坊石榴子石二辉橄榄岩的原岩为高温-高压的富集石榴子石二辉橄榄岩。第二期矿物组合为包裹在低Cr变斑晶石榴子石幔部和细粒新生石榴子石核部的大量富Al铬铁矿和高Mg低Ni橄榄石以及少量高Mg斜方辉石。该期组合未发现单斜辉石,表明岩石随后被转变为高温低压的难熔尖晶石方辉橄榄岩或尖晶石纯橄岩。芝麻坊石榴子石二辉橄榄岩的早期变质演化记录了它们被卷入大陆板片俯冲带之前的地幔楔上升对流过程。笔者认为芝麻坊石榴子石二辉橄榄岩的原岩来源于早期俯冲大洋板片之上的深部高温富集地幔楔,洋壳俯冲过程中的地幔楔对流导致其上升到弧后或岛弧之下的地幔楔浅部,减压部分熔融使原本富集的石榴子石二辉橄榄岩转化为难熔的尖晶石方辉橄榄岩或尖晶石纯橄岩。     

Effect of water and stress on the lattice-preferred orientation of olivine

The influence of water and stress on the lattice-preferred orientation (LPO) of olivine aggregates was investigated through large strain, shear deformation experiments at high pressures and temperatures (P = 0.5–2.1 GPa, T = 1470–1570 K) under both water-poor and water-rich conditions. The specimens are hot-pressed synthetic olivine aggregates or single crystals of olivine. Water was supplied to the sample by decomposition of a mixture of talc and brucite. Deformation experiments were conducted up to γ (shear strain)  6 using the Griggs apparatus where water fugacity was up to  13 GPa at the pressure of 2 GPa. The water content in olivine saturated with water increases with increasing pressure and the solubility of water in olivine at P = 0.5–2 GPa was  400–1200 ppm H/Si. Several new types of LPO in olivine are found depending on water content and stress. Samples deformed in water-poor conditions show a conventional LPO of olivine where the olivine [100] axis is subparallel to the shear direction, the (010) plane subparallel to the shear plane (type-A). However, we identified three new types (type-B, C, and E) of LPO of olivine depending on the water content and stress. The type-B LPO of olivine which was found at relatively high stress and/or under moderate to high water content conditions is characterized by the olivine [001] axis subparallel to the shear direction, the (010) plane subparallel to the shear plane. The type-C LPO which was found at low stress and under water-rich conditions is characterized by the olivine [001] axis subparallel to the shear direction, the (100) plane subparallel to the shear plane. The type-E LPO which was found under low stress and moderate water content is characterized by the olivine [100] axis subparallel to the shear direction, the (001) plane subparallel to the shear plane. Observations by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) show that the dislocations in water-poor samples (type-A) are curved and both b =  [100] and b = [001] dislocations have a similar population. Numerous subgrains are seen in water-poor samples in backscattered electron images. In contrast, water-rich samples (both type-B and type-C) contain mostly b = [001] dislocations and dislocations are straight and sub-grain boundaries are rare compared to those in water-poor samples. These observations suggest that (1) dominant slip systems in olivine change with water fugacity (and stress) and (2) grain boundary migration is enhanced in the presence of water. Seismic anisotropy corresponding to the fabrics under water-rich condition is significantly different from that under water-poor condition. Consequently, the relationship between seismic anisotropy and flow geometry in water-rich regions is expected to be different from that in water-poor regions in which type-A fabric dominates (i.e., the lithosphere). A few cases are discussed including anisotropy in the subduction zone and in the deep upper mantle.     

碧溪岭石榴异剥橄榄岩的显微构造及成因机制

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.      

东秦岭松树沟超镁铁岩侵位机制及其构造演化

东秦岭松树沟蛇绿岩主要由镁铁质-超镁铁质岩石组成。镁铁质岩类的Sm-Nd全岩等时年龄为1030±46(2δ)Ma,ε_Nd(t)=+5.7±0.2,代表了蛇绿岩的形成时代。超镁铁质岩石由不同成因的橄榄质糜棱岩和中粗粒橄榄岩组成,橄榄质糜棱岩是地幔橄榄岩经历复杂变形并多次部分熔融的残余体,具LREE亏损特征,其中发育橄榄石高温位错构造和高温组构以及低温位错构造和低温组构。中粗粒橄榄岩具LREE略富集的分布特征,是地幔橄榄岩残余体再次部分熔融熔体分离结晶的产物。野外地质、地球化学、构造变形特征均表明超镁铁岩块是因洋壳俯冲而底辟侵位于上覆玄武岩中的地幔橄榄岩残余体。综合分析认为,松树沟蛇绿岩经历了古陆块裂解或洋脊扩张(1271-1440Ma)-洋壳形成(1030-1271Ma)-洋壳俯冲消减-橄榄岩块底辟侵位(983Ma)-蛇绿岩构造侵位及其后构造变形叠加改造的复杂演化过程。     

Reply to the comments of S. Karato on “Petrofabrics and seismic properties of garnet peridotites from the UHP Sulu terrane (China)” by Xu et al. [Tectonophysics 421 (2006) 111–127]

We thank Karato for his interest in our recent paper in Tectonophysics [Xu, Z.Q., Wang, Q., Ji, S.C.. Chen, J., Zeng, L.S., Yang, J.S., Chen, F.Y., Liang, F.H., Wenk, H.R., 2006. Petrofabrics and seismic properties of garnet peridotite from the UHP Sulu terrane (China): Implications for olivine deformation mechanism in a cold and dry subducting continental slab. Tectonophysics 421, 111–127]. However, Karato is incorrect to take water or water-related species (herein referred to generically as ‘water’) as an almost unique cause for any olivine fabric transition in nature no matter what geological conditions are. In this Reply, we will first give a succinct introduction to the context of the ongoing debate, and then provide some additional arguments in favour of our conclusion that “the olivine C-type fabric of the Zhimafang garnet-bearing peridotites was formed under the conditions of high pressure, low temperature and low water content during the subduction of cold and dry supracrustal rocks of the Yangtze plate”. The map of olivine slip systems as a function of differential stress and water content, drawn by Karato and coworkers, cannot be reliably extrapolated to Earth’s interior until relevant scaling laws have been established. The role of high water content on the fabric transitions is more likely to modify recrystallization kinetics than to switch the dominant slip systems.     

Polyphase Deformation of the Weihai-Rongcheng UHP Unit Rocks, NE Sulu： Insights into the Tectonic Evolution of the Dabie-Sulu UHP and HP Belts, China

Different scales of structural data reveal a complex deformation history of ultrahigh- pressure （UHP） rocks exposed in the Weihai-Rongcbeng area, NE Sulu （northern Jiangsu-eastern Shandong）, eastern China. Excluding pre-UHP deformations, at least five major sequential deformational stages （D1-Ds） are recognized. The first deformation （DO produced a weak foliation and lineation in massive eclogites. The foliated eclogite with a dominant foliation containing a stretching and mineral lineation was developed during the I）2 deformation. Both the D1 and D2 deformations occurred under UHP metamorphic conditions, and are well preserved in the eclogite bodies. D3 structures which developed shortly after the formation of granulite/amphibolite facies symplectites are characterized by imbricated associations marked by a regional, steeply dipping foliation, compositional layering, eclogite boudinage, isoclinal folds and reverse ductile shear zones. The D3 deformation was accompanied by decompressional partial melting. A regional, gently dipping amphibolite facies foliation and stretching lineation, low-angle detachments, and dome- and arc-shaped structures formed during the D4 deformation stage dominate to some degree the map pattern of the Weihai-Rongcbeng UHP domain. The last stage of deformation （Ds） gave rise to the final exhumation of the UHP rocks. Ds is characterized by development of brittle-dominated high-angle faulting associated with emplacement of large volmnes of undeformed granite plutons and dykes dated at 134-100 Ma. The deformational and metamorphic sequence followed by the UHP rocks in the Weihai-Rongcheng area is similar to that studied in the entire Dabie-Sulu UHP and HP metamorphic belts from microscopic to mapping scale. Based on structural data, combined with available petrographic, metamorphic and geochronological data, a speculative tectonic evolutionary model for the Dabie-Sulu UHP and IIP belts is proposed, involving continental subduction/collision between the Sino-Korean and Yangtze cratons and subsequent polyphase exhumation histories of the UHP and IIP metamorphic rocks.     

Metamorphic history of serpentinite mylonites from the Happo ultramafic complex, central Japan

Serpentinite mylonites from the Happo ultramafic complex show evidence of two stages of mylonitization at different temperature conditions. Peridotite mylonites exhibit two types of olivine – porphyroclasts and neoblasts – produced at the earlier stage. The olivine neoblasts have a stretching lineation with a fabric suggesting plastic deformation along (0 1 0) [0 0 1]. In addition to the olivine fabric, the stable association of olivine, orthopyroxene and tremolite in the peridotites that survived later serpentinization, and the Si and Na contents of tremolite, suggest that the earlier mylonitization took place at temperatures between 700 and 800 °C. Later mylonitization was associated with high‐temperature serpentinization to form serpentinite mylonites. In contrast to a common type of serpentinite in orogenic belts, the serpentinite mylonites are cohesively foliated, rich in olivine and diopside, and poor in antigorite. The diopside has low Al, Cr and Na contents typical of a retrograde origin, and the olivine has a homogeneous composition except in areas subjected to contact metamorphism at a later stage. Modal composition and mineral chemistry suggest that the serpentinite mylonites were formed by a hydration reaction of tremolite and olivine to produce diopside and antigorite under stable conditions of olivine, at temperatures between 400 and 600 °C. Later‐stage mylonitization has preferentially been superimposed on the earlier‐stage mylonite zone with a common direction of foliation. The difference in temperature between the two mylonitization stages suggests that the shear zone was episodically active during the emplacement of the Happo complex. Conditions of relatively high temperature for serpentinization at a convergent plate boundary and high permeability caused by the early mylonitization favoured the formation of the serpentinite mylonites.