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1.
南海新生代玄武岩中单斜辉石地球化学特征
及其地质意义 总被引:2,自引:0,他引:2
南海新生代玄武岩中发育有大量的单斜辉石斑晶,与橄榄石、角闪石等斑晶共存,多数被熔蚀,呈浑圆状,部分发育有很窄的反应边。研究表明单斜辉石具有巨晶的特征,是在高压下在与寄主岩同源的岩浆中形成,没有经历长时间的沉淀生长而直接向上运移被带到地表,因而具有巨晶的主量及微量元素的特征却不具有巨晶的形态。单斜辉石稀土元素含量不高,HREE及LREE亏损,而MREE富集;大离子亲石元素Rb、Ba、Sr均出现明显的亏损,高场强元素Nb、Zr出现亏损而Hf略富集。Nb、Ta与Zr、Hf分馏明显。Th较为富集,而Pb强烈亏损,U的变化范围较大。研究发现南海新生代玄武岩中的单斜辉石的来源较为简单,为地幔柱的直接产物,并没有受到洋中脊—地幔柱相互作用的影响,由于岩浆上升速度较快压力迅速下降,橄榄石大量晶出,引起了岩浆成分的变化,致使单斜辉石与寄主岩成分和结构上没有达到平衡。 相似文献
2.
《Journal of Structural Geology》1986,8(7):781-798
Paleozoic to Oligocene metasedimentary rocks present in the Alpi Apuane region of the Northern Apennines, Italy, have been sequentially deformed during a Tertiary progressive deformation. In an attempt to date the individual deformation episodes, over 50 conventional K-Ar and 1140 Ar/39Ar incremental gas release analyses have been carried out on fine grained white micas separated from samples whose structural settings were well known. Mineralogy, X-ray diffractometry, and thin-section analyses indicate that the constituent muscovite and phengite formed under metamorphic conditions of 3–4 kbars and 300–400°C during all deformational phases. Pre-existing micas were variably crenulated during each subsequent deformational phase. Both K-Ar and 40Ar/39Ar analyses were carried out on 0.6-2μm, 2–6 μm and 6–20 μm size separates of the phengitic white mica. Although the K-Ar apparent ages range from 11 to 27 Ma and are consistent with available stratigraphic constraints, the 40Ar/39Ar age spectra display variable internal discordancy. These isotopic data indicate that: (1) both the K-Ar and 40Ar/39Ar total-gas ages decrease as the degree of crenulation increases; (2) the K-Ar and 40Ar/39Ar total-gas ages decrease as grain size decreases; (3) for each sample, characteristics of the 40Ar/39Ar age spectra depend upon grain size, with fine sizes yielding discordant patterns which systematically increase in apparent age from low to high temperature and (4) phengitic micas associated with earliest structures yield generally older ages than micas associated with later structures.The isotopic results are interpreted to indicate that the major deformation phase (D1) occurred at approximately 27 Ma with subsequent pulses ending by c. 10 Ma. These results may be combined with finite strain data to suggest that the region was deformed at strain rates between 10−15 and 10−14 s−1. A 27 Ma age indicates Mid-Oligocene initiation of plate tectonic activity in the Western Mediterranean and concomitant deformation in the Northern Apennines. 相似文献
3.
A regional melange zone, 150 km long and 30 km wide, forms the southern boundary and structural capping to a high-pressure blueschist belt in northern New Caledonia. The disrupted country rocks in the melange zone are Mesozoic metagrey-wackes and Eocene chert-limestone sequences which have been penetrated from below by tectonically-injected ophiolite slivers containing metamorphosed serpentinite, gabbro, dolerite, basalt, tuff, chert and shale. An ocean crust origin for these rocks is indicated by chemical, mineralogical and radiometric data from coastal outcrops at Anse Ponandou on the northeast coast. The age (41 m.y.), metamorphic environment (350 ° C at 7 kb), and mineral association (acmitic jadeite-riebeckite-pyropic spessartine-pistacitic epidote-lawsonite-high Si phengite) are significantly different from those of the adjacent regional high-pressure schist belt, indicating a separate structural site for blueschist metamorphism of buried ophiolitic ocean crust during early Tertiary orogenesis. 相似文献
4.
石榴石辉石岩的单斜辉石中韭闪石和钛铁矿出溶结构及地质意义 总被引:10,自引:1,他引:10
本文首次报道了在苏鲁超高压变质带胡家岭地区石榴石辉石岩中发现的韭闪石出溶结构。该石榴石辉石岩是由石榴子石、单斜辉石以及镁铝尖晶石、钛铁矿和磁铁矿组成。在石榴子石巨斑晶中有单斜辉石包体,单斜辉石包体中又有石榴子石包体以及大量韭闪石出溶片晶和钛铁矿( 磁铁矿)出溶棒。单斜辉石中韭闪石出溶片晶呈平行条带状,其宽度在1~10μm之间。透射电子显微镜(TEM)观察韭闪石的(010)面平行于主晶透辉石的(010)面,且两者的界面在纳米尺度上是连续过渡的,证明韭闪石是出溶形成的。大量钛铁矿出溶棒的定向排列也显示出它们是自然出溶形成的。从单斜辉石出溶的矿物组合推测其原始单斜辉石成分中应富含H2O、Na和Ti,属高压型单斜辉石。该石榴石辉石岩经历了超高压变质作用后,在抬升减压过程中形成这些出溶结构。大量钛铁矿出溶棒均斜切韭闪石出溶片晶,表明该超高压岩石在减压过程中至少经历了两期以上导致温压明显变化的地质事件。 相似文献
5.
A clinopyroxene suite from Somma-Vesuvius clinopyroxenite cjectaand mcgacrysts has been investigated by single crystal X-raydiffraction combined with electron probe microanalysis. Theaim of the study was to characterize the intracrystalline configurationof the clinopyroxenes and delineate their possible parentalliquid(s). Vesuvius megacrysts and nodule clinopyroxenes show the initialsignature of the leucitite (sensu lato) magmawhile Somma clinopyroxenes are characteristic of leucite phonolitemagma. The crystal chemical variations demonstrate that thespecimens investigated had originated by shallow-level crystallizationof leucitite and its derivatives, under reducing conditions.T-site (Si + Al) deficiency, typical of clinopyroxene from lamproitesis reported in some specimens from the Roman Volcanic Regionfor the first time. Intracrystalline Mg-Fe2+ ordering in M1-M2 appears to be configuration-dependent,yielding the largest KD variation in the clinopyroxene fromphonolite. This is contrary to expectation. 相似文献
6.
柴达木盆地北缘侏罗纪古生态特征及其古地理意义 总被引:1,自引:1,他引:0
根据多门类化石古生态和岩相、沉积构造等资料,分析了柴达木盆地北缘侏罗纪的古气候和沉积环境。介形类、轮藻、叶肢介、双壳类和植物等化石在地层中的产出特征反映了当时以湖沼及滨浅湖为主的沉积环境,广泛分布的植物及孢粉化石组合面貌的变化揭示出盆地北缘早、中侏罗世为热带-亚热带温湿气候,早侏罗世晚期和中侏罗世晚期古气候两度明显干热化。陆生植物与湖沼相动物化石的交替出现,反映了盆地北缘侏罗纪湖泊、沼泽与低山相间分布的古地理面貌。早侏罗世湖泊多期发育但规模较小,中侏罗世中晚期湖泊规模最大。 相似文献
7.
Rendeng Shi William L. Griffin Suzanne Y. O'ReillyQishuai Huang Xiaoran ZhangDeliang Liu Xiachen ZhiQiongxia Xia Lin Ding 《Gondwana Research》2012,21(1):194-206
Podiform chromite deposits occur in the mantle sequences of many ophiolites that were formed in supra-subduction zone (SSZ) settings. We have measured the Re-Os isotopic compositions of the major chromite deposits and associated mantle peridotites of the Dongqiao Ophiolite in the Bangong-Nujiang suture, Tibet, to investigate the petrogenesis of these rocks and their genetic relationships.The 187Os/188Os ratios of the chromite separates define a narrow range from 0.12318 to 0.12354, less variable than those of the associated peridotites. Previously-reported 187Os/188Os ratios of the Os-rich alloys enclosed in the chromitites define two clusters: 0.12645 ± 0.00004 (2 s; n = 145) and 0.12003 to 0.12194. The ultra-depleted dunites have much lower 187Os/188Os (0.11754, 0.11815), and the harzburgites show a wider range from 0.12107 to 0.12612. The average isotopic composition of the chromitites (187Os/188Os: 0.12337 ± 0.00001) is low compared with the carbonaceous chondrite value (187Os/188Os: 0.1260 ± 0.0013) and lower than the average value measured for podiform chromitites worldwide (0.12809 ± 0.00085). In contrast, the basalts have higher 187Os/188Os, ranging from 0.20414 to 0.38067, while the plagioclase-bearing harzburgite and cumulates show intermediate values of 187Os/188Os (0.12979 ~ 0.14206). Correspondingly, the basalts have the highest 187Re/188Os ratios, up to 45.4 ± 3.2, and the chromites have the lowest 187Re/188Os ratios, down to 0.00113 ± 0.00008. We suggest that melts/fluids, derived from the subducting slab, triggered partial melting in the overlying mantle wedge and added significant amounts of radiogenic Os to the peridotites. Mass-balance calculations indicate that a melt/mantle ratio of approximately 15:1 (melt: 187Re/188Os: 45.4, 187Os/188Os: 0.34484; mantle peridotite: 187Re/188Os: 0.0029, 187Os/188Os: 0.11754) is necessary to increase the Os isotopic composition of the chromitite deposits to its observed average value. This value implies a surprisingly low average melt/mantle ratio during the formation of the chromitite deposits. The percolating melts probably were of variable isotopic composition. However, in the chromitite pods the Os from many melts was pooled and homogenized, which is why the chromitite deposits show such a small variation in their Os isotopic composition. The results of this study suggest that the 187Os/188Os ratios of chromitites may not be representative of the DMM, but only reflect an upper limit. Importantly, the Os-isotope compositions of chromitites strongly suggest that such deposits can be formed by melt/mantle mixing processes. 相似文献
8.
Coexisting amphiboles in an eclogite from the Western Alps: new constraints on the miscibility gap between sodic and calcic amphiboles 总被引:1,自引:0,他引:1
Abstract Crystal-chemical relationships between coexisting sodic and calcic amphiboles have been studied in eclogitic metagabbros from the Aosta Valley, Western Alps. Textural analysis gives evidence of three successive high-pressure parageneses:
1. Pre-kinematic high-grade blueschist assemblages, preserved as polymineralic inclusions in garnet cores and made of glaucophane and actinolite (stage A).
2. Synkinematic eclogite assemblages, composed of garnet + omphacite + glaucophane ± actinolite ± white mica ° Clinozoisite + quartz + rutile (stage B).
3. Post-kinematic epitactic overgrowths of barroisitic amphibole on glaucophane and actinolite (stage C).
P–T conditions of the eclogitic metamorphism have been estimated at around 500–550°C, 16 kbar.
Glaucophane and actinolite coexist as discrete grains in stage A and B assemblages. This texture and the chemistry of the amphiboles unambiguously denotes the existence of a miscibility gap between sodic and calcic amphiboles (from NaM4 = 0.80 in actinolite to NaM4 = 1.70 in glaucophane at T = 500–550°C). A comparison with published analyses allows a new solvus along the glaucophane–actinolite join to be drawn.
The later barroisitic amphibole (stage C) exhibits strong chemical zonation indicating disequilibrium growth. This amphibole cannot either be used to define a miscibility gap with glaucophane or actinolite or be considered as an intermediate stage between these two end-members. 相似文献
1. Pre-kinematic high-grade blueschist assemblages, preserved as polymineralic inclusions in garnet cores and made of glaucophane and actinolite (stage A).
2. Synkinematic eclogite assemblages, composed of garnet + omphacite + glaucophane ± actinolite ± white mica ° Clinozoisite + quartz + rutile (stage B).
3. Post-kinematic epitactic overgrowths of barroisitic amphibole on glaucophane and actinolite (stage C).
P–T conditions of the eclogitic metamorphism have been estimated at around 500–550°C, 16 kbar.
Glaucophane and actinolite coexist as discrete grains in stage A and B assemblages. This texture and the chemistry of the amphiboles unambiguously denotes the existence of a miscibility gap between sodic and calcic amphiboles (from Na
The later barroisitic amphibole (stage C) exhibits strong chemical zonation indicating disequilibrium growth. This amphibole cannot either be used to define a miscibility gap with glaucophane or actinolite or be considered as an intermediate stage between these two end-members. 相似文献
9.
K. P. Sarma M. Venkateshwarlu S. K. Patil J. J. Laskar Niva Rani Devi J. Mallikharjuna Rao 《Journal of the Geological Society of India》2014,83(2):147-155
The basement gneiss of the Shillong plateau and the overlying rocks of the Shillong basin have been dissected by a number of mafic dyke swarms represented by older Proterozoic dolerite dykes and sills named as Khasi greenstone and the younger set of Cretaceous dykes. The older dolerites dykes of Shillong basin are partly metamorphosed and have undergone low-grade metamorphism compared to the fresh unmetamorphosed Cretaceous dykes dominantly exposed in the BGC of West Garo hills region. The Khasi greenstones are tholeiitic in composition and range from basalt to basaltic andesite. Most of the metamorphosed mafic dykes indicate continental nature with some amount of overlapping oceanic tectonic setting. Palaeomagnetic study of the metadolerite dykes show a direction of magnetization of Dm=29, Im=38 (α95 = 28.84; k = 48.33; N = 2) with a palaeolatitude of 21.3° N to the Indian sub-continent that clearly support the Proterozoic dyke/dyke swarm emplacement in the region. The magnetic carrier as inferred from IRM studies is magnetite in the range of psuedosingle to multi domain (MD) states with minor contribution from hematite. 相似文献
10.
准噶尔北部蛇绿岩形成时限新证据及其东、西准噶尔蛇绿岩的对比研究 总被引:17,自引:13,他引:17
准噶尔北部出露有塔尔巴哈台库吉拜蛇绿岩、洪古勒楞-和布克赛尔蛇绿岩及扎河坝-阿尔曼太蛇绿岩。洪古勒楞蛇绿岩中的堆晶辉长岩样品进行SHRIMP U-Pb定年,结果为472±8.4Ma(MSWD=1.4),限定洪古勒楞-和布克赛尔蛇绿岩形成于早奥陶世。对扎河坝蛇绿岩中斜长花岗岩采用SHRIMP U-Pb方法测定年龄为495.9±5.5Ma(MSWD=2.7),证实扎河坝蛇绿岩形成于晚寒武世-早奥陶世。塔尔巴哈台蛇绿岩、和布克赛尔-洪古勒楞蛇绿岩以及扎河坝-阿尔曼太蛇绿岩均在早奥陶世已经形成,大部分以断层侵位的形式侵位于泥盆纪-石炭系火山-沉积地层中。准噶尔北部东西三段蛇绿岩在形成时代、区域地质以及地球物理特征等方面具有可对比性,认为它们构成一条贯穿东、西准噶尔的蛇绿岩带,这为新疆北部及邻区古生代构造格局的重建以及区域构造的对比连接提供了重要信息。 相似文献
11.
《Gondwana Research》2014,25(2):842-858
The northern margin of the Alxa block (NMAB), located in the southernmost part of the Altaids, is important for understanding the tectonic processes associated with the closure of the Paleo-Asian ocean. In this study, we report results from our studies on two ophiolitic belts (the Enger Us and Quagan Qulu ophiolitic belts) to constrain the tectonic evolution of the Altaids. The tectonic blocks in the Enger Us ophiolite are mainly composed of ultramafic and mafic rocks, with a matrix comprising highly deformed Carboniferous clastic rocks and tuffs. Zircons from a pillow lava sample yielded SHRIMP zircon U–Pb age of 302 ± 14 Ma. Massive and pillow basalts in the Enger Us ophiolite exhibit N-MORB geochemical affinities, displaying high TiO2 and low K2O contents with tholeiitic signatures. They are characterized by depletion of light rare earth elements (LREEs) without fractionation of high field strength elements (HFSEs) and negative Nb–Ta anomalies. It is inferred that the magmas of these rocks were derived from a depleted mantle source in a mid-ocean ridge setting. The Quagan Qulu ophiolite is composed of tectonic blocks, including ultramafic, gabbros and siliceous rocks, and matrix, including deformed clastic rocks and limestones. Zircons in a gabbro sample from the Quagan Qulu ophiolite yielded SHRIMP zircon U–Pb age of 275 ± 3 Ma. The gabbros show high MgO contents, compatible elements (Ni, Co, Sc, and V), and Al2O3/TiO2 ratios, but low TiO2 and SiO2 contents. They are enriched in large-ion lithophile elements (LILEs) and depleted in LREEs and HFSEs, indicating that they were derived from an extremely depleted mantle source which was infiltrated by a subduction-derived fluid or melt. Our geochemical data suggest that gabbros in the Quagan Qulu ophiolite were formed in a back-arc basin setting. A synthesis of evidence from geochemistry, regional geology, and paleobiogeography support the notion that the Enger Us ophiolitic belt represents the major suture of the Paleo-Asian Ocean in the NMAB and the Quagan Qulu ophiolitic belt represents a back-arc basin. These two ophiolitic belts, together with the Zongnaishan–Shalazhashan arc have been suggested to be a late Paleozoic ocean-arc–back-arc basin system in the southernmost part of the Altaids. The geochronological data suggest that the subduction process occurred even in the early Permian, indicating that the final closure of the Paleo-Asian Ocean might have taken place later than the early Permian. 相似文献
12.
P. N. Gramenitskaya E. N. Gramenitskii O. V. Kononov 《Moscow University Geology Bulletin》2013,68(4):242-252
Based on the results of microprobe analyses, six groups of calcium amphiboles were distinguished for the Tyrnyauz deposit. Group I (mainly edenite) includes amphiboles of hornfels and periskarn rocks. Groups II-VI (actinolite-hastingsite series) are related to post-skarn metasomatic rocks of the productive stage after hornfels (II), the tonalite and plagiogranite complex (III), ultrabasic rocks (IV), pyroxene-plagioclase periskarn rocks (V), and skarn (VI). Group IV is represented by actinolite; group VI, by hastingsite; groups II, III, and V, by hornblende varieties. 相似文献
13.
《Chemie der Erde / Geochemistry》2023,83(1):125942
Anomalous high-conductivity layers are typical of subduction zones, the largest recycling systems of the Earth. Understanding the mineral physics underlying the high conductivity of rocks has paramount implications for several planetary-scale processes, including global water cycling, earthquake activity, and arc magmatism.Here, by using in situ polarized Raman spectroscopy, we provide a direct proof for the development of anisotropic electron-phonon excitations (polarons) and delocalized H+ in riebeckite, a Fe-bearing sodic amphibole typical of blue-schist metamorphic facies. The activation of polarons starts at 500 K (227 °C) and is complete at 650 K (377 °C) under both reducing and oxidizing conditions. At higher temperatures external oxygen triggers the expulsion of H+ and e− from the crystal bulk. The temperature range observed for the development of charge carriers is in excellent agreement with the conductivity trends measured for riebeckite in previous studies, and nicely fits the temperatures for the development of high-conductivity layers in warm and cold subduction zones.The study directly demonstrates the activation of polarons at temperatures characteristic of convergent plate margins provides the atomic-scale picture whose macroscopic-scale expression is the anomalous conductivity measured in subduction zones. 相似文献
14.
15.
Clinopyroxenes as geobarometric indicators in mafic and ultramafic rocks from Canary Islands 总被引:1,自引:0,他引:1
This paper describes the use of pyroxene compounds of clinopiroxene in mafic and ultramafic rocks (inclusions and layered outcrops) in the Canary Islands as geobarometric indicators of the depth of formation of different rock types. The upper mantle origin of peridotite and gabbro inclusions was confirmed, while a crustal character is proposed for the layered complexes. Finally, a scheme of Canary substrate is outlined. 相似文献
16.
Amphibole-bearing gneiss fragments are common in the impact breccias of the Xiuyan crater, China. Three kinds of amphibole-bearing gneiss fragments with different shock-metamorphic levels have been identified. Shock-metamorphic features of amphiboles in these gneisses were investigated in situ by optical microscope, electron microprobe, Raman spectroscopy, and X-ray diffraction. Amphiboles in the weakly shocked gneiss (shock pressure less than 10 GPa) basically remain intact. Amphiboles in the moderately shocked gneiss (shock pressure range between 35 and 45 GPa) show strong deformation, reduced optical interference color, and partial loss of OH?. In the strongly shocked gneiss (shock pressure above 50 GPa), amphiboles are completely melted and dendritic pyroxenes crystallize from the melt. The formation of dendritic pyroxenes shows nearly complete loss of water in the amphibole melt at shock-induced high temperature above 1,500 °C. The occurrence of both diopside and pigeonite dendrites crystallized in the same amphibole melt shows inhomogenous melt composition and rapid cooling of the melt. 相似文献
17.
皖南上溪群出露于“江南古陆”东北部,自下而上分为樟前组、板桥组、木坑组/环沙组和牛屋组,前人根据其中岩体的同位素年龄推定其时代为中元古代。在 20世纪 80年代后期开展的江南古陆是古陆还是造山带的大讨论中,上溪群的时代问题成了一个关键问题,而且也是了解造山带形成时代的关键问题。作者新近在皖南歙县附近的上溪群中发现了早古生代的放射虫化石和古蛋类化石。这是一个新的重要发现,它证明宿主地层上溪群,或至少其中的牛屋组,应是早寒武世沉积,其古地理环境应是古海洋,而不是古陆。其隆起和变质作用则更晚。该发现对于在研究区乃至整个华南地区的古地理学研究和能源矿产(特别是石油和天然气)的预测和勘探都是有重要意义的。 相似文献
18.
Preeti Singh Naresch C. Pant Ashima Saikia Amitava Kundu 《International Journal of Earth Sciences》2013,102(8):2137-2152
Amphiboles represent a crucial phase of the ultra-high-pressure (UHP) metamorphic rocks as their solid solution behavior reflects both bulk compositional and P–T changes. Three different types of amphibole have been reported from the UHP metamafic rocks of the Tso Morari Crystalline Complex, NW Himalayas: Na-rich (glaucophane); Na–Ca-rich (barroisite, taramite, winchite) and Ca-rich (tremolite, magnesio-hornblende, pargasite). The Na-amphibole is presented as a core of the zoned amphibole with Na–Ca-rich rim; Na–Ca-amphibole is presented as inclusion in garnets as well as in matrix, and Ca-amphibole is generally found in the matrix. The Na–Ca-amphibole is observed at two different stages of metamorphism. The first is pre-UHP, and the second is post-garnet–omphacite assemblage though with a significant difference in composition. The pressure–temperature estimations of the formation of these two sets of Na–Ca-amphiboles corroborate their textural associations. Ca-rich amphiboles are generally present in the matrix either as symplectite with plagioclase or as a pseudomorph after garnet along with other secondary minerals like chlorite and biotite. Two different types of zoning have been observed in the amphibole grains: (1) core is Na-rich followed by Na–Ca rim and (2) core of Na–Ca-amphibole is followed by Ca-rich rim. The pre-UHP (or the prograde P–T path) and post-UHP stages (or the retrograde P–T path) of Tso Morari eclogites are defined by characteristic amphibole compositions, viz. Na/Na–Ca-amphibole, Na–Ca-amphibole and Ca-amphibole and thus indicate their utility in inferring crustal evolution of this UHP terrain. 相似文献
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自四川盆地西北部(川西北)泥盆系观雾山组油气勘探取得重要进展以来,观雾山组的沉积储层研究受到了广泛重视。然而,由于观雾山组地层划分方案的变动和地层的穿时性,导致目前对观雾山组的时代分布和充填过程认识不清,从而影响了沉积古地理格局的恢复。本文以四川广元西北乡、潜溪河剖面泥盆系观雾山组牙形石生物地层研究为基础,结合区域古生物地层资料,认为江油马角坝以北的观雾山组绝大多数沉积于晚泥盆世早—中弗拉期,且地层沉积时间自南西向北东逐渐变晚。龙门山北段观雾山组具有自西向东、自南向北地层厚度减薄和地层时代变新的特征,表明来自扬子地台西部的古特提斯洋海水向东侵入龙门山地区,因地形差异和渐进式海侵形成了自南西向北东的超覆沉积。由于岩相分异,超覆沉积形成的观雾山组与金宝石组在区域上既表现为上下叠置关系,也表现为同期异相的相变关系。该研究不仅对约束川西北地区观雾山组地层时代提供了可靠的古生物证据,而且对认识龙门山地区泥盆系的充填过程、沉积古地理格局均具有重要意义。 相似文献