Aragonite and omphacite-bearing metapelite from Besshi region, Sambagawa belt in central Shikoku, Japan and its implication

Aragonite and omphacite-bearing metapelite occurs in the albite–biotite zone of the Togu (Tohgu) area, Besshi region, Sambagawa metamorphic belt, central Shikoku, Japan. This metapelite consists of alternating graphite-rich and graphite-poor layers that contain garnet, phengite, chlorite, epidote, titanite, calcite, albite, and quartz. A graphite-poor layer contains a 1.5-cm ivory-colored lens that mainly consists of phengite, calcite, albite, and garnet. Aragonite, omphacite, and paragonite occur as inclusions in the garnet of the ivory lens. The aragonite has a composition that is close to the CaCO₃ end-member: the FeCO₃ and MnCO₃ components are both less than 0.3 mol% and the SrCO₃ component is about 1 mol%. The aragonite + omphacite + quartz assemblage in garnet indicates equilibrium conditions of P  > 1.1–1.3 GPa and T  = 430–550°C. Quartz grains sealed in garnet of the aragonite and omphacite-bearing sample and other metapelites in the Togu area preserve a high residual pressure that is equivalent to the Sambagawa eclogite samples. These facts suggest that: (i) the Togu area experienced eclogite facies metamorphism; and (ii) thus, eclogite facies metamorphism covered the Sambagawa belt more extensively than previously recognized.     

Prograde eclogites from the Tonaru epidote amphibolite mass in the Sambagawa Metamorphic Belt, central Shikoku, southwest Japan

Abstract   Prograde eclogites occur in the Tonaru epidote amphibolite mass in the Sambagawa Metamorphic Belt of central Shikoku. The Tonaru mass is considered to be a metamorphosed layered gabbro, and occurs as a large tectonic block (approximately 6.5 km × 1 km) in a high-grade portion of the Sambagawa schists. The Tonaru mass experienced high- P /low- T prograde metamorphism from the epidote-blueschist facies to the eclogite facies prior to its emplacement into the Sambagawa schists. The estimated P – T conditions are T  = 300–450°C and P  = 0.7–1.1 GPa for the epidote-blueschist facies, and the peak P – T conditions for the eclogite facies are T  = 700–730°C and P  ≥ 1.5 GPa. Following the eclogite facies metamorphism, the Tonaru mass was retrograded to the epidote amphibolite facies. It subsequently underwent additional prograde Sambagawa metamorphism, together with the surrounding Sambagawa schists, until the conditions of the oligoclase–biotite zone were reached. The high- P /low- T prograde metamorphism of the eclogite facies in the Tonaru mass and other tectonic blocks show similar steep d P /d T geothermal gradients despite their diverse peak P – T conditions, suggesting that these tectonic blocks reached different depths in the subduction zone. The individual rocks in each metamorphic zone of the Sambagawa schists also recorded steep d P /d T geothermal gradients during the early stages of the Sambagawa prograde metamorphism, and these gradients are similar to those of the eclogite-bearing tectonic blocks. Therefore, the eclogite-bearing tectonic blocks reached greater depths in the subduction zone than the Sambagawa schists. All the tectonic blocks were ultimately emplaced into the hanging wall side of the later-subducted Sambagawa high-grade schists during their exhumation.     

Petrology and retrograde P-T path for eclogites of the Maksyutov Complex, Southern Ural Mountains, Russia

Abstract The Maksyutov Complex, situated in the southern Ural Mountains of Russia, is the first location where quartz aggregates within garnets exhibiting radial fractures were identified as coesite pseudomorphs (Chesnokov & Popov 1965). The complex consists of two tectonic units: a structurally lower eclogite-bearing schist unit and an overlying meta-ophiolite unit. Both units show evidence for multiple stages of metamorphism and deformation. The high-pressure metamorphism of the eclogite-bearing schist unit, discussed in this report, is suspected to be related to a collision between the Russian platform and a fragment of the Siberian continent during the early Cambrian. At least three stages of metamorphism (M_1-3) and two stages of deformation (S₁ and S₂) were observed in thin sections: M₁) garnet (Alm_55-60, Prp_22-28, Grs_16-20) + omphacite (Jd_46-56) + phengite (Si ≅ 3.5) + rutile; M₂) garnet + glaucophane ± lawsonite + white mica; and M₃) epidote + chlorite ± albite ± actinolite + white mica. Observed mineral parageneses define a retrograde P-T path for the eclogite. Mineral assemblages within the most representative eclogite from the lower unit of the Maksyutov Complex indicate minimum peak pressures of 15 kbar at temperatures of approximately 600°C. If the presence of coesite pseudomorph is confirmed, the peak ultrahigh-pressure metamorphism may be as high as 27 kbar at 615°C.     

秦岭商丹糜棱岩带构造变形环境的显微构造标志

商丹糜棱岩带不同区段石英和长石的显微构造及石英组构特征表明,商丹糜棱岩带自西向东构造变形环境显示从低绿片岩相至中- 高绿片岩相至高绿片岩相—低角闪岩相的变化规律。低绿片岩相变形环境下,石英多为Ⅰ型条带,长石主要显示脆性破裂特征,石英c 轴组构呈单一环带型式。中- 高绿片岩相变形环境下,石英主要为Ⅱ型石英多晶条带,斜长石主要处于脆性碎裂流动状态,钾长石开始向韧性转化,石英c 轴组构呈绕y 轴分布的点极密型式。高绿片岩相—低角闪岩相变形环境下,石英普遍呈现光性均匀并有120°三连点的动态重结晶和Ⅳ型条带,斜长石开始显示脆- 韧性过渡状态的变形特点,钾长石显示明显的韧性变形特点,石英c 轴组构呈Ⅰ型交叉环带型式     

Paleostress orientation from 3-D orientation distribution of microcracks in quartz from the Cretaceous granodiorite core samples drilled through the Nojima Fault, south-west Japan

Abstract The orientations of both healed extension microcracks and microcracks in quartz grains sealed mostly by carbonate minerals were measured from Cretaceous granodiorite core samples drilled along the Nojima Fault, southwest Japan. The preferred orientations of both healed and sealed microcracks consist of approximately three orthogonal sets, (components) A, B and C, in which A strikes NS–NW-SE and dips vertically, B strikes EW–NE-SW and dips vertically, and C is subhorizontal. Both the healed and sealed microcracks were possibly formed by hydraulic fracturing, and the successive release of tensile stress due to pore fluid overpressure in the principal stress directions could have caused this microcracking in mutually orthogonal directions. The quartz grains are also very moderately plastically deformed, which is indicated by the occurrence of kink bands and undulose extinction. The association of healed microcracks and kink bands in the quartz suggests that these microstructures formed under subgreenschist facies conditions (≈300°C) during hydrothermal activity that could have occurred immediately after the emplacement of granodiorite during the Late Cretaceous period. Based on both the preferred orientation of microcracks, and c-axis fabrics of kinked and unkinked grains (so called kink method), it is inferred that the σ₁-and σ₃-axis were oriented horizontally in NS–NW-SE and EW–NE-SW directions, respectively. The inferred paleostress field does not conform to the east–west-trending compression during the Quaternary period, but to the activation of EW–NE-SW-trending, left–lateral strike–slip faults during the Late Cretaceous period in southwest Japan.     

Inclusion-trail geometry of albite porphyroblasts in a fold structure in the Sambagawa Belt, central Shikoku, Japan

The orientation of straight inclusion trails within albite porphyroblasts from basic schists has been measured around a north-closure fold, in the Besshi district of the Sambagawa Belt, central Shikoku, Japan. The porphyroblasts are aligned with their longest dimension parallel to both the subhorizontal, east–west-directed mineral lineation and to the fold axis. There is a systematic variation in inclusion-trail geometry between the upper (northern) and lower (southern) fold limbs. The shear sense deduced from quartz c-axis fabrics is top-to-the-west in the upper limb and top-to-the-east in the lower limb. Based on observed variations in porphyroblast inclusion trails, the structural history can be modelled as follows: (i) shear flow caused east–west stretching and folding of the metamorphic zonation; (ii) east–west ductile shear resulted in opposing senses of shear in the upper and lower limbs as the eclogite body situated in the core of the fold was extruded to the east.     

Phase relations and P–T conditions of the Ryoke pelitic and psammitic metamorphic rocks in the Ina district, Central Japan

The Ina district of the Ryoke Belt is divided into two mineral zones, based on the mineral parageneses of the pelitic and psammitic rocks at the peak metamorphism. A biotite–muscovite zone (quartz + plagioclase + biotite + muscovite with or without K-feldspar) constitutes the northwestern part, and a biotite–cordierite–K-feldspar zone (quartz + plagioclase + biotite + cordierite + K-feldspar) comprises the central to southern and eastern parts. The isograd reaction between two mineral zones is defined by a divariant reaction: Mg-rich biotite + muscovite + quartz = Fe-rich biotite + cordierite + K-feldspar + H₂O (1), which, in the K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O (KFMASH) system, occurs at ∼ 590 °C at 0.2 GPa and 660 °C at 0.4 GPa. Fibrolite accompanied by andalusite porphyroblasts in aluminous pelitic rocks of the biotite–muscovite zone and the low-grade part of the biotite–cordierite–K-feldspar zone, suggests that sillimanite was the stable aluminosilicate at the peak metamorphic condition throughout the area. In the high-grade part of the biotite–cordierite–K-feldspar zone, fibrolite mostly occurs as inclusions in cordierite or in plagioclase. The phase relations and the compositional zoning of plagioclase in relation to fibrolite inclusions suggest that fibrolite was formed under relatively high-pressure conditions, and that partial melting took place.     

Structural position of the Seba eclogite unit in the Sambagawa Belt: Supporting evidence for an eclogite nappe

Abstract   Eclogite-bearing units in the Sambagawa Metamorphic Belt have long been considered tectonic blocks that have disparate tectonic and metamorphic histories that are distinct from each other and from the major non-eclogitic Sambagawa schists. However, recent studies have shown that eclogite facies metamorphism of the Seba eclogite unit is related to the subduction event that caused the metamorphism of the non-eclogitic Sambagawa schist. New structural data further show that the Seba eclogite unit, which appears to be isolated from the other eclogite units, is in fact in structural continuity with them, occupying the highest structural levels in the Sambagawa Belt. This suggests that eclogitic metamorphism of the other eclogite units is also related to the Sambagawa subduction event. It is, therefore, possible that all eclogite units in the Sambagawa Belt constitute a single coherent unit, the eclogite nappe, members of which underwent the same eclogitic metamorphism related to the Sambagawa subduction event.     

组构对花岗片麻岩高温流变影响的实验研究

深部岩石先存的变形组构对流变特性影响的实验研究是新的研究热点之一,然而目前相关的实验研究非常有限.本文利用3 GPa固体介质熔融盐三轴高温高压容器,选择华北克拉通北部辽东拆离断层带中具有变形组构的花岗片麻岩样品,在温度600~840℃、围压800~1200 MPa、应变速率1×10^-4~2.5×10^-6/S条件下,对不同组构方向的样品(实验压缩方向分别垂直和平行花岗片麻岩的面理)开展高温高压流变实验.实验结果表明,在相同的应变速率和温度条件下,垂直面理的岩石强度比平行面理的岩石强度要高.两组实验样品在600~700℃时,应力指数平均值为6.5,为半脆性流变;在800~840℃时,应力指数平均值为2,垂直面理样品的激活能为Q=380 kJ/mol,平行面理样品的激活能为Q=246.4 kJ/mol,以塑性变形为主,局部存在黑云母和角闪石的脱水熔融.微观结构研究表明,垂直面理的样品,在变形过程中形成了新的变形条带,把原有的面理破坏改造;而平行面理的样品,在实验变形过程中新的变形带主体继承了原有组构.EBSD分析显示花岗片麻岩原岩中石英轴极密区位于Z轴附近,为底面滑移;压缩方向垂直面理的样品,石英组构轴极密区位于X轴附近,为柱面滑移;压缩方向平行面理的样品,石英组构轴极密区位于Z轴附近,伴有少量的Y轴极密,底面滑移和柱面滑移.这表明垂直面理的样品中石英变形改造比平行面理的样品更彻底,这与微观结构分析结果一致.显然实验样品的非均匀组构对样品强度和石英轴定向等具有显著影响,但对样品的脆塑性转化和塑性变形机制没有实质影响,这对理解地壳深部普遍存在的形态各向异性岩石流变具有重要参考价值.     

An outline of the petrology, structure and age of the Pompangeo Schist Complex of central Sulawesi, Indonesia

Variably dismembered and metamorphosed accretionary complexes constitute the basement of much of the Indonesian island of Sulawesi. The most extensive of these is the Pompangeo Schist Complex, which crops out over ∼ 5000 km² in central Sulawesi, and is predominantly composed of interbanded phyllitic marble, calcareous phyllite, graphitic schist and quartzite; rocks of terrigenous to shallow marine origin. Along the eastern margin of the complex, schists are interthrust with unmetamorphosed Jurassic sandstone, which may represent parental material of the complex. The schists are unconformably overlain by pelagic sediments with an Albian–Cenomanian biostratigraphy. Synmetamorphic progressive deformation of the Pompangeo Schist Complex has resulted in repeated isoclinal folding and a strong transposition foliation striking north-northwest/south-southeast and dipping west, subparallel to the compositional banding of the complex; microstructural fabrics indicate a top-to-east sense of shear. On a regional scale the Pompangeo Schist Complex is lithostratigraphically coherent and an east-to-west metamorphic field gradient is recognizable, which, if continuous, represents a relatively low thermal gradient of ∼ 15 °C/km. K–Ar dating yielded ages of ca 111 Ma. Correlative metamorphic rocks appear to underlie the entire Neogene magmatic province, since they occur sporadically throughout western Sulawesi, including the Bantimala region of the South Arm. The Pompangeo schist metamorphism cannot be correlated with arc magmatism in western Sulawesi, which is of Neogene age. The Pompangeo and Bantimala schists, as well as other accretionary complexes in western Sulawesi, were probably generated in the same subduction system that was responsible for the extensive Mesozoic continental arc in central Kalimantan, at the eastern margin of Sundaland.     

Quartz microstructures and deformation conditions in the Hatagawa shear zone, north-eastern Japan

Abstract Meso- and microscale structural analyses were carried out in the Hatagawa shear zone, north-eastern Japan. The shear zone is a sinistral strike–slip ductile shear zone, and has a 1-km wide north-south trending region where quartz mean grain size of approximately 35 μm does not vary significantly. Two types of quartz microstructures, A and B, occur in the western and eastern part of the shear zone, respectively. The lattice preferred orientation (LPO) patterns are characterized by a type I crossed girdle in the samples with the quartz microstructure A, and a Y maximum in the samples with the quartz microstructure B. The microstructures under optical and transmission electron microscopes indicates that the quartz recrystallization occurred predominantly by progressive subgrain rotation in microstructure A, and by both progressive subgrain rotation and grain boundary migration in microstructure B. This suggests different deformation conditions in the western and eastern parts of the shear zone. Quartz LPO patterns, presence of myrmekite, feldspar compositions, and dislocation density also suggest difference in the deformation conditions. The syn-deformation temperature in the western part of the shear zone was lower than that of the eastern part of the shear zone. The stress in the western part of the shear zone was higher than that of the eastern part of the shear zone. The difference in the deformation conditions would affect not only the quartz microstructure, but also the quartz LPO patterns and the presence of myrmekite around K-feldspar porphyroclasts.     

40Ar/39Ar ages of blueschist facies pelitic schists from Qingshuigou in the Northern Qilian Mountains, western China

Abstract   Pelitic schists from Qingshuigou in the Northern Qilian Mountains of China contain mainly glaucophane, garnet, white mica, clinozoisite, chlorite and piemontite. Isotopic age dating of these schists provides new constraints on the formation of the high-grade blueschists at Qingshuigou. White mica ⁴⁰Ar/³⁹Ar ages range from 442.1 to 447.5 Ma (total fusion age of single grain) and from 445.7 to 453.9 Ma (integrated age of white mica concentrates). These ages (442.1–453.9 Ma) represent the peak metamorphic ages or cooling ages of the blueschists during exhumation shortly after peak metamorphism. The ⁴⁰Ar/³⁹Ar dates in the present study are similar to ages previously reported for eclogites and blueschists in the area; this suggests that both the eclogites and pelitic sediments underwent high-grade metamorphism during the same subduction event. From this chronological evidence and the presence of well-developed Silurian remnant-sea flysch and Devonian molasse, it is concluded that the Northern Qilian Ocean had closed by the end of the Ordovician, and rapid orogenic uplift followed in the Devonian.     

Complex origin for the south-western Zamboanga metamorphic basement complex, Western Mindanao, Philippines

Abstract Amphibolites unconformably overlain by a metasedimentary sequence of quartz-muscovite-feldspar-kyanite schists, metagraywackes and epidote-bearing amphibolites occur in the northern portion of the south-western Zamboanga metamorphic basement complex, western Mindanao. These amphibolites (here identified as the Mount Dansalan amphibolites) display relict magmatic textures inherited from cumulate gabbro protoliths. Bulk-rock major and trace-element data are consistent with this hypothesis. Together with the chemistry of relict igneous clinopyroxenes, they indicate a magmatic arc-related signature for the gabbro protoliths. Geochemical data allow us to identify various sources for the associated metasediments: the gabbro themselves for the metagraywackes and a continental basement for the quartz-muscovite-feldspar-kyanite schists. Both sources contributed to the genesis of the epidote-amphibolite metasediments. The compositions of the metamorphic mineral assemblages suggest that the rocks have undergone metamorphism at temperatures ranging from 550°C to 700°C and pressures probably in the range of 5–9 kbar. ⁴⁰K–⁴⁰Ar isotopic study of amphibole separates from the Mount Dansalan samples document a metamorphic event dated at 24.6 ± 1.4, 22.2 ± 1.4 and 21.2 ± 1.2 Ma. Our results are in agreement with plate tectonic models which describe the south-western Zamboanga metamorphic basement as a continental terrane. However, its evolution was not as simple as it was usually considered. In particular the basement incorporated slivers of magmatic arc crust, which cannot be unambiguously related to any of the Tertiary arcs documented in the area.     

Paleomagnetism and tectonics of Karaginsky Island, Bering Sea

Abstract Rocks from Karaginsky accretionary prism (Karaginsky Island, Bering Sea) yield both prefolding (close to original) and postfolding magnetic vectors. The prefolding vectors suggest that the Maastrichtian–Paleocene volcanic–terrigenous sequences of Karaginsky Island formed at approximately 40°N to 50°N ( n = 45, D _G = 325, I _G = 57, K _G = 6, α_95G = 8, F _G = 15.06, D _S = 332, I _S = 63, K _S = 20, α_95S = 4.5, F _S = 0.3297, F _cr = 2.64) and were not originally part of either Eurasia ( F = 19, Δ F = 6.5) or North America ( F = 17, Δ F = 4.4). The geologic blocks rotated insignificantly counterclockwise about the horizontal plane, suggesting that the structure of Karaginsky Island arose without major strike-slip motions. Analysis of secondary magnetizations (for example, n = 28, D _G = 311, I _G = − 50, K _G = 9, α_95G = 8.7, F _G = 2.44; D _S = 293, I _S = − 41, K _S = 5, α_95S = 11, F _S = 12.04, F _cr = 2.65) reveals that the development of this framework involved at least two stages of deformation. During the second stage the sequences must have been tilted to west-northwest and northwest directions at 45–65°. This agrees with the northwest vergence of the structure of Karaginsky Island.     

Chrome-rich clinopyroxene in orthopyroxenite from Maowu, Dabie Mountains, central China: A second record and its implications for petrogenesis

A second occurrence of chrome-rich clinopyroxene has been discovered as inclusions in orthopyroxene in orthopyroxenite from Maowu, the Dabie Mountains, Central China. The average formula for chrome-rich clinopyroxene can be expressed as (Na_0.39Ca_0.54)_0.93(Mg_0.57Fe²⁺_0.06Fe³⁺_0.01Cr_0.24Al_0.15)_1.03Si_2.02O₆, with a maximum amount of kosmochlor component of 28.52 mol%. The unit cell parameters obtained from a single-crystal are a  = 9.614 Å, b  = 8.800 Å, c =  5.240 Å, β = 106.59°, space group C2 / c . The indices of refraction are α = 1.697, β = 1.704, γ = 1.726. Chrome-rich clinopyroxene, which coexists with chromite, chromian rutile and chromian pyrope, crystallized at a temperature of 1025 °C and very high pressure, and therefore represents a mantle relic. Together with the appearance of low-pressure inclusion mineral assemblage and the estimation of physical–chemical conditions for matrix minerals, the Maowu eclogite–ultramafic complex is considered to be formed during ultrahigh-pressure metamorphism from the mantle-derived protolith.     

Talc-phengite-albite assemblage in piemontite-quartz schist of the Sanbagawa metamorphic belt, central Shikoku, Japan

Abstract The talc (Tlc) + phengite (Phn) + albite (Ab) assemblage is newly confirmed in MnO^total-rich (1.65 wt% in average) piemontite-quartz schists from the intermediate- and high-grade part of the Sanbagawa belt, central Shikoku, Japan. Talc is in direct contact with Phn, Ab and chlorite (Chl) with sharp boundaries, suggesting that these four phases mutually coexist. Other primary constituents of the Tlc-bearing piemontite-quartz schist are spessartine, braunite, hematite (Ht), crossite/barroisite and dolomite. Phlogopite (Phl) rarely occurs as a later stage mineral developing along the rim of Phn. The studied piemontite-quartz schist has mg# (= Mg/(Mg + Fe²⁺)) ~ 1.0, because of its high oxidation state. Schreinemakers' analysis in the KNMASH system and the mineral assemblage in the Sanbagawa belt propose a possible petrogenetic grid, in which the Tlc–Phn assemblage is stable in a P-T field surrounded by the following reactions: lower-pressure limit by Chl + Phl + quartz (Qtz) = Phn + Tlc + H₂O as proposed by previous workers; higher-pressure limit by glaucophane + Qtz = Tlc + Ab + H₂O; and higher-temperature limit by Tlc + Phn + Ab = Phl + paragonite + Qtz + H₂O. Thermodynamic calculation based on the database of Holland & Powell (1998) , however, suggests that the Tlc–Phn stability field defined by these reactions is unrealistically limited around 580–600 °C at 11.6–12.0 (± 0.7) kbar. Schreinemakers' analysis in the KNMA-Fe³⁺-SH system and the observed mineral assemblage predict that Chl + crossite = Tlc + Ab + Ht + H₂O is a preferable Tlc-forming reaction in the intermediate-grade part of the Sanbagawa belt and that excess Ab + hematite narrows the stability field of the Tlc–Phn assemblage.     

Deformation path of amygdules in basic schists in Itoshima area,northern Kyushu,Japan

A deformation path of amygdules in basic schists that underwent contact metamorphism in Itoshima area, northern Kyushu, Japan, has been revealed using shape and internal microstructure of the amygdules. The amygdules are filled mainly with plagioclase that was recrystallized during the contact metamorphism. Crystal size of the plagioclase decreases with increasing aspect ratio of the amygdules. Some amygdules with low‐aspect ratio contain plagioclase showing flat twin boundaries. This suggests that such amygdules were undeformed after peak of the contact metamorphism and preserved the strain accumulated until then, which probably includes prograde stage of the contact metamorphism and former regional Renge metamorphism. In contrast, the amygdules with high‐aspect ratio underwent superimposition of deformation taking place after peak of the contact metamorphism. The strain analysis shows the constriction types for the former amygdules while the plane‐strain types for the latter. This reveals a deformation path of the amygdules such that the constrictional strain before the peak of contact metamorphism was partially overprinted by later flattening strain, showing the apparent plane‐strain types.     

STUDY ON THE SEISMOGENIC FAULT CHARACTERSTICS OF 2016 MW5.9 MENYUAN EARTHQUAKE BASED ON Sentinel -1A DATA

In this paper, we processed and analyzed the Sentinel-1A data by "two-pass" method and acquired the surface deformation fields of Menyuan earthquake. The results show the deformation occurred mainly in the south wall of fault, where uplift deformation is dominant. The uplift deformation is significantly larger than the subsidence and the maximum uplift of ascending and descending in the LOS is 6cm, 8cm respectively. Meanwhile, based on the Okada model, we use the ascending and descending passes data as constraints to invert jointly the fault distribution and source parameters through constructing fault model of different dip directions. The optimum fault parameters are:The dip is 43°, the strike is 128°with the mean rake of 85°. The maximum slip is about 0.27m. The inverted seismic moment M₀ is 1.13×10¹⁸N·m, and the moment magnitude M_W is 5.9. The SW-dipping Minyue-Damaying Fault is possibly the seismogenic fault, based on the comprehensive analysis of the focal mechanisms, aftershocks relocation results and the regional tectonic background. The focus property is dominated by thrust movement with a small amount of dextral strike-slip component. The earthquake is the result of local stress adjustment nearby the Lenglongling Fault under the background of northeastward push and growth of Tibet Plateau.     

Diamond-bearing and diamond-free metacarbonate rocks from Kumdy–Kol in the Kokchetav Massif, northern Kazakhstan

Abstract Dolomite marble from the Kumdy–Kol area of the Kokchetav Massif contains abundant microdiamond, mainly in garnet and a few in diopside. The mineral assemblage at peak metamorphic condition consists of dolomite + diopside + garnet (+ aragonite) ± diamond. Inclusions of very low MgCO₃ calcite and almost pure calcite occur in diopside and are interpreted as aragonite and/or aragonite + dolomite. Single-phase Mg–calcite in diopside with a very high MgCO₃ component (up to 21.7 mol%) was also found in diamond-free dolomitic marble, and is interpreted as a retrograde product from aragonite + dolomite to Mg–calcite. The dolomite stability constrains the maximum pressure (P) at < 7 GPa using previous experimental data, whereas the occurrence of diamond yields the minimum peak pressure–temperature (P–T) condition at 4.2 GPa and 980 °C at X co ₂ = 0.1. The highest MgCO₃ in Mg–calcite constrains the minimum P–T condition higher than 2.5 GPa and 800 °C for the exhumation stage. As these marbles were subjected to nearly identical P–T metamorphic conditions, the appearance of diamond in some carbonate rocks was explained by high X co ₂. A low X co ₂ condition refers to high oxidized conditions and diamond (and/or graphite) becomes unstable. Difference in X co ₂ for marble from the same area suggests local heterogeneity of fluid compositions during ultrahigh-pressure metamorphism.     

黄石台ORBES-81型伸缩仪观测结果的调和分析及海潮改正

本文对黄石台ORBES-81型石英伸缩仪的应变固体潮观测资料作了分析计算,其调和分析结果为:对O₁波,振幅因子0.4219,相位滞后6°36＇;对M₂波,振幅因子0.4256,相位滞后-4°42＇.同时,讨论了海潮引起的负荷潮汐应变的改正问题.模拟计算发现负荷潮汐应变与体潮应变大致为同一量级,文中列出了海潮改正后的结果,并作了讨论.最后,对黄石台伸缩仪的架设方位作了评价.