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1.
来自马里亚纳岛弧南部前弧的方辉橄榄岩是一种高度亏损的橄榄岩, 其中含少量的单斜辉石(0.7 vol%). 在方辉橄榄岩中发现两种类型的角闪石: 镁角闪石与单斜辉石密切伴生, 具有较高的Al2O3含量(>7%)和较低的Mg#; 透闪石在斜方辉石周围出现, 具有较低的Al2O3含量(<2%)和较高的Mg#. 单斜辉石和镁角闪石的原始地幔标准化的REE分配模式显示HREE相对LREE的富集, 且镁角闪石的微量元素含量普遍高于单斜辉石. 透闪石的微量元素含量远低于单斜辉石. 与深海橄榄岩中的单斜辉石相比, 马里亚纳岛弧南部前弧的方辉橄榄岩中的单斜辉石出现大量微量元素的相对富集, 只有Ti和HREE等少量元素除外. 岩石特征和单斜辉石、角闪石微量元素特征暗示这些方辉橄榄岩经历了2个阶段的交代作用. 含水熔体的渗透交代引起了Al, Ca, Fe, Mg, Na和大量微量元素的迁移. 在含水熔体中, LILE和LREE相对于HREE和Ti更活跃, 并且大部分微量元素在含水熔体中的活动性显然受到温度和压力的影响.  相似文献   

2.
The Chilas Complex is a major lower crustal component of the Cretaceous Kohistan island arc and one of the largest exposed slices of arc magma chamber in the world. Covering more than 8000 km2, it reaches a current tectonic width of around 40 km. It was emplaced at 85 Ma during rifting of the arc soon after the collision of the arc with the Karakoram plate. Over 85% of the Complex comprises homogeneous, olivine‐free gabbronorite and subordinate orthopyroxene–quartz diorite association (MGNA), which contains bodies of up to 30 km2 of ultramafic–mafic–anorthositic association (UMAA) rocks. Primary cumulate textures, igneous layering, and sedimentary structures are well preserved in layered parts of the UMAA in spite of pervasive granulite facies metamorphism. Mineral analyses show that the UMAA is characterized by more magnesian and more aluminous pyroxene and more calcic plagioclase than those in the MGNA. High modal abundances of orthopyroxene, magnetite and ilmenite (in MGNA), general Mg–Fe–Al spatial variations, and an MFA plot of whole‐rock analyses suggest a calc‐alkaline origin for the Complex. Projection of the pyroxene compositions on the Wo–En–Fs face is akin to those of pyroxenes from island arcs gabbros. The presence of highly calcic plagioclase and hornblende in UMAA is indicative of hydrous parental arc magma. The complex may be a product of two‐stage partial melting of a rising mantle diaper. The MGNA rocks represent the earlier phase melting, whereas the UMAA magma resulted from the melting of the same source depleted by the extraction of the earlier melt phase. Some of the massive peridotites in the UMAA may either be cumulates or represent metasomatized and remobilized upper mantle. The Chilas Complex shows similarities with many other (supra)subduction‐related mafic–ultramafic complexes worldwide.  相似文献   

3.
Boninite is an unusual, plagioclase-free magnesian andesite, occurring as vesicular pillow lavas and hyaloclastites, accompanied by andesites and dacites in Chichi-jima, Bonin Islands. The Bonin Islands belong to the Izu-Mariana arc and consist of dominant volcanic rocks and subordinate sedimentary rocks of late Oligocene-early Miocene age. The chemistry of boninite is characterized by high contents of MgO. Cr and Ni similar to primitive basalts, but apparently in ill accord with its relatively high SiO2 content of ? 55%. The relation of SiO2 to total FeO/MgO ratio indicates that boninite belongs to the cale-alkalic rock suite. The mineralogy of boninite consists of olivine (Fo87-90), orthopyroxene (En87-90), clinopyroxene (Wo38-35En37-44Fs25-21), hydrous glass and Cr-spinel, Experimental studies show that the magma of boninite composition could be in equilibrium with upper mantle peridotite at pressures less than 17 kb and temperatures of 1200–1050°C under high PH2O. It is suggested that boninite is a sea-floor quenched product (900°C) of a direct partial melt of the upper mantle. Related andesites and dacites are considered to be probably fractional crystallization products from the same magma.  相似文献   

4.
Pumiceous pyroclastic products are present as flows and falls at several stages of the evolution of the southern Guadeloupe volcanic island. An understanding of this volcanism had to rely on detailed petrochemical data of these products to complement similar data for effusive rocks so as to yield complete stratigraphical coverage. On the other hand most pumiceous rocks are more or less conspicuousily banded suggesting that mixing phenomena occurred to different degrees in their genesis. Three major classes of pumiceous products are found: (1) the Axial Chain deposits (2.0–0.5 My) are characterized by An90, 75-55 + En55 + Wo42En37 + Usp35-37 ± Fo68 ± Hble, SiO2 60%, SiO2/Th 35.6, and La/Th 3.9. Banded samples have components that differ in evolution indices by about 50%; (2) the Bouillante Chaine pyroclastics (0.3–0.1 My) consist of scattered deposits with variable mineralogical and geochemical compositions that seem to have erupted from a number of small eruptive centers. Qz-dacitic pumice is common with An90, 70-45 + En66-56 + Usp32-38 + Ilm94 + Hble ± Wo40-42En40-42, SiO2 62%, SiO2/Th 22.9, and La/Th >4. Mixed pumice samples have highly contrasted evolution indices differing by up to 120%; (3) the Pintade pumice flows and falls correspond to the major pyroclastic event (approx. 10 km3) in the southern Basse Terre area. They are characterized by An85-70 + En66-56 + Usp32-37 ± Wo42En42, SiO2/Th 18.7-22?6, and La/Th 3.0-4.0. Banded pumice lumps are scarce and show slight compositional contrasts; differences in evolution indices do not exceed 38%. Axial and Bouillante chain pyroclastics and Grande Découverte volcano pumice respectively, form two different families in trace element plots. Minor elements in pyroxenes also are distinctive. These trends are similar to those obtained for effusive rocks and define comagmatic series. Major and trace element data for the separated components of inhomogeneous pumice in each formation always plot in the corresponding series. These chemical discriminants can be used to attribute samples of unknown provenance to a given volcanic ensemble. An inverse relationship between differences in evolution indices in inhomegeneous pumice and the volume of any single eruptive sequence is noted. This is an indication that pumiceous pyroclastic rocks were erupted from a zoned magma chamber. We favor an interpretation where zonation is produced by influx of less envolved magma in superficial differentiated chambers which is a direct cause for eruption.  相似文献   

5.
The products emitted by Mount Etna in the 1977–1983 period are porphyric, sodic trachybasalts («etnaïtes») that, though substantially higher in K2O, resemble most of the historic lavas.The 1983 flows, for instance, are composed of 50–55% glassy groundmass and abundant phenocrysts or microphenocrysts: plagioclase (25–30%), clinopyroxene (12–15%), olivine (3–5%), titanomagnetite (2%). Scarce olivine Fo 83-78 and diopsidic pyroxene are present in the cores of some phenocrysts. Outer zones and/or more numerous crystals are olivine Fo 75-68, salite En 43-37 Fs 10-15, plagioclase An 83-55 an Al-Mg rich magnetite Usp 43-33. The chemical composition of glass inclusions interpreted as trapped liquids suggests that olivine and magnetite crystallize first and are followed by pyroxene and plagioclase. The early appearance of olivine has been determined by optical thermometry to about 1170°C, soon followed by plagioclase and pyroxene (1160–1140°C). Pre-eruptive crystallization further progresses until 1073°C which is the field-measured temperature at the lava vent. Then, the appearance and composition of microlites (and outer rims of phenocrysts) depend upon the cooling rate of the samples (water quenching or natural cooling). Electron microprobe analyses of chlorine and sulphur have been performed on crystal trapped and groundmass glasses. Sulphur ranges from an initial content of 1500 ppm to 400 ppm in residual glass, leading to an average sulphur emission of 1500 tons/day (3000 t/d SO2), as estimated from the volume of erupted lava.The most striking chemical feature of the 1977–1983 lavas lies in the abnormal potassium behaviour, especially in the earliest flows of the 1978–1979 fissure eruptions. Although these lavas are amongst the most femic of the period, they are relatively high in K2 O, leading to an aberrant K/Na ratio (i.e. increasing with increasing basicity). Furthermore, a less pronounced but steady increase of K2 O/Na2 O is evidenced in all the lavas of the past 12 years. This ratio ranges from 0.42 in the 1971 lavas to 0.51 in the 1983 ones, for about the same differentiation index. Such an abnormal potassium behaviour first appeared during the 1974 eruption on the west slope of the volcano. It is unknown, however, in any other historic lava, nor in any lava of entire Etna so far analyzed.
Resume Les produits rejetés par l'Etna de 1977 à 1983 sont des trachybasaltes porphyriques («etnaïtes») semblables à ceux qui caractérisent la majorité des laves de ce volcan, mais avec une teneur en K plus élevée.Les coulées de 1983, par exemple, sont formées de 50–55% de mésostase et par de nombreux phéno et microphénocristaux de plagioclase (25–30%), clinopyroxène (12–15%), olivine (3–5%), titanomagnétite (2%). De l'olivine Fo 83-78 et un peu de diopside-salite cristallisent d'abord, passant rapidement à une olivine Fo 78-68, une salite (Wo 47,5 — En 43/37 — Fs 10/15), du plagioclase An 83-55, une titanomagnétite Usp 43-33 riche en Al et Mg. Le domaine de température de cristallisation commençante des phénocristaux établi par thermométrie optique est compris entre 1170° et 1140°C. La cristallisation intratellurique progresse ensuite jusqu'à 1073°C, qui représente la température d'émission de la lave mesurée sur le terrain. La nature et la quantité des microlites, la composition des stades terminaux de cristallisation des phénocristaux et celle du verre résiduel de la mésostase dépendent du mode de refroidissement de la lave: trempe à l'eau, à l'air, ou refroidissement lent naturel. La quantité de soufre dans les verres inclus et la mésostase varie de 1500 à 400 ppm, celle du chlore de 1700 à 2800 ppm.La composition chimique est celle d'un trachybasalte sodique (hawaiite basaltique). Le caractère le plus marquant réside dans le comportement anormal du potassium, en particulier dans les laves du début des éruptions de 1978 et 1979. Alors que ces produits sont parmi les plus basiques de la période considérée, leur teneur en K est relativement élevée, ce qui conduit à un rapport K/Na aberrant (augmentant avec la basicité de la roche). En dehors de ces cas extrêmes, on note une augmentation régulière, quoique plus faible, de K/Na dans tous les produits des 12 dernières années. Ce comportement anormal du potassium était déjà apparu dans les produits de l'éruption excentrique de 1974 sur le versant ouest. Cependant, on ne trouve aucune variation aberrante du potassium dans les autres laves historiques, ni même dans aucun des produits connus de l'Etna.
  相似文献   

6.
The concept that the plutonism of the lunar highlands and the mare-type volcanism are two separate problems in both time (> 4.4 AE versus < 3.95 AE) and space is seriously questioned by the discovery of a 4.23-AE low-Ti mare basalt from Fra Mauro Formation.Apollo 14 breccia 14305 contains a clast (,122) which is an olivine gabbronorite that is texturally and mineralogically similar to several Apollo 12 basalts (e.g., 12005, 12035, 12040). It consists of cumulus olivine (40 modal %; Fo 62–70) and Ti-chromite (2.5 modal %); post-cumulus phases include low-Ca pyroxene (29 modal %; Wo 7–13 En 68–75), augite (10 modal %; Wo 31–40 En 47–50), plagioclase (15 modal %, An 82–93), and ilmenite (4 modal %, 5–7 MgO). The TiO2 content of this rock = 4.3%; CaO/Al2O3 ? 1.0, CaO = 5.1%; MgO/FeO ? 1.0, MgO = 21.9%. The REE pattern, normalized to chondritic abundances, is approximately 30 × Ch and “hump-shaped” with a pronounced Eu depletion and a non-KREEPy signature. A four-point Rb-Sr isochron reveals an age of 4.23 ± 0.05 AE. The sample has a low initial 87Sr/86Sr= 0.69911 ± 3.The data presented here show that non-KREEPy, mare-type volcanism commenced at least as early as 4.2 AE in the Fra Mauro region and probably across much of the lunar surface. Massive bombardment during the “terminal cataclysm” and the subsequent veneer of younger mare basalts has obliturated most of the evidence for these ancient volcanic events. These old, mare-type volcanics may be related to basin-forming events such as made Procellarum (i.e., impact-triggered igneous activity).  相似文献   

7.
本文提出冀东太古宙麻粒岩可划分为无角闪石或黑云母的辉石亚相和含角闪石或黑云母的角闪石亚相,基于平衡的矿物组合估计的压力和温度,辉石亚相和角闪石亚相分别为800—850℃,9—11kbar,700—750℃,7—9kbar。由麻粒岩亚相推导的冀东下地壳由二层构成,下部层(30—37公里)为辉石亚相,上部层(23—30公里)为角闪石亚相,它与地球物理资料相符合  相似文献   

8.
提供了扬子克拉通太古代基底崆岭群２０件代表性岩石样品的密度与磁性参数的测量结果．结果表明，变碎屑岩与ＴＴＧ（英云闪长－奥长花岗－花岗间长质）片麻岩（１３个样品）的磁化率与饱和剩磁普遍大于斜长角闪岩和辉长岩．前者的磁化率与饱和剩磁平均值分别为１２１３×１０－６ＳＩ与１９．９４Ａ／ｍ，而后者的则为８０２×１０－６ＳＩ与１０．７７Ａ／ｍ．变碎屑岩与ＴＴＧ片麻岩呈明显的亚铁磁性状态；斜长角闪岩和辉长岩则以顺磁性或顺磁性与亚铁磁性混合状态分布为主．热磁分析结果表明，变碎屑岩与ＴＴＧ片麻岩的剩磁载体以磁铁矿与磁赤铁矿为主；斜长角门岩和辉长岩则明显含有磁黄铁矿与次要的磁铁矿．变碎屑岩与ＴＴＧ片麻岩的磁性具有很强的非均一性．推断崆岭群中的变碎屑岩为麻粒岩相变质级，视深度相当于大陆下地壳；而斜长角闪岩的变质级明显低于变碎屑岩，可能为角门岩相，视深度相当于中地壳．变碎屑岩与ＴＴＧ片麻岩磁性的强非均一性可能与该区后期广泛发生的混合岩化作用密切相关。  相似文献   

9.
地壳主要岩石流变参数及华北地壳流变性质研究   总被引：23，自引：3，他引：23       下载免费PDF全文
周永胜  何昌荣 《地震地质》2003,25(1):109-122
岩石流变参数和变形机制是根据断层摩擦和岩石幂次流动本构关系建立岩石圈强度剖面的基础。近 30年来 ,高温高压实验取得了很大进展 ,获得了大量地壳矿物和岩石流变资料。本文系统总结了这些流变实验资料 ,并应用流变数据结合地震震源深度分布 ,对华北地壳流变性质进行了研究。结果表明 ,以花岗岩和低级变质岩为代表的上地壳为脆性破裂 ,其强度受断层摩擦约束 ,以长英质片麻岩为主的中地壳和以中性麻粒岩为主的下地壳上层处于塑性流变状态 ,由干的基性麻粒岩组成的下地壳下层处于脆性向塑性流变的过渡状态。华北地壳的这种物质组成和流变为地壳不同层次的解耦和强震孕育提供了力学条件 ,也构成了不同尺度块体的底边界  相似文献   

10.
The Dabie Complex can be divided into two metamorphic facies belts, granulite facies and amphibolite facies. Growth zoning in the inner segments of garnets is well preserved in the granulite belt. By contrast, garnets in the amphibolite belt have no composition variations in the inner segments, but show growth zoning in the outer segments. This may imply different incipient metamorphic history for the two metamorphic belts. However, both reaction textures and composition trends that reflect the decompression process are commonly in both of the two belts. Pressure decreased about 0.70 and 0. 85 GPa for the granulite and the amphibolite belts, respectively, estimated from mineral thermobarometers. The metamorphicP-T paths are characteristic of collision and subduction, implying that the Dabie Complex underwent rapid subsidence and rapid tectonic uplift. Uplift of the ultrahigh pressure eclogites in the region could also be related to the process.  相似文献   

11.
Polycrystalline quartz ribbons in high-grade metamorphic rocks from the Daqingshan region, are typi- cal microfabrics of, and provide information for, deep crust deformation and metamorphism. The quartz ribbons have straight boundaries and extend stably along gneissosity. They truncate other mineral grains in the rocks and may contain inclusions of such minerals that are lens-shaped and oriented. They frequently end into branching termination. Analysis fluid inclusions in polycrystalline quartz rib- bons reveal that the complex types of fluid inclusions are inhomogeneously distributed. They are ob- viously different from inclusions captured at granulite facies, in both fluid compositions and T-P esti- mations. Based on microfabric and fluid inclusion analysis, the polycrystalline quartz ribbons are suggested to be formed by SO2-rich fluids filling micro-fractures that are parallel to early gneissosity. The SO2 composition is derived from the deformed host rocks. The fluid phase has significant effects on the rheological characteristics, fracturing of rocks, and formation of quartz ribbons.  相似文献   

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