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1.
山东西部莱芜－淄博地区的中生代雪野碳酸岩中发育有大量罕见的反环带云母，云母的电子探针分析表明，云母斑晶的核心为黑云母，过渡带和边缘是金云母，基质中云母为金云母，核心黑云母与过渡带的金云母界限清晰，成分突变以及核心黑云母的溶蚀结构表明，核心黑云母和过渡带与边缘带金云母不是同一岩浆体系结晶的产物，核心黑云母可能是碳酸岩岩浆捕获的外来黑云母，过渡带与边缘带金云母同基质中金云母具有相似的化学成分，从过渡带到边缘带，金云母的Mg(Mg Fe)逐渐降低，反映了碳酸岩岩浆的不断结晶演化过程，TiO2的不断降低，一方面是由于岩浆的分异结晶，另一方面则是岩浆上升期间去气作用导致岩浆内CO2／H2O值降低所致。云母斑晶从过渡带向边缘带Al2O3含量逐渐减少，表明碳酸岩浆自过渡带云母结晶后没有富Al的圈岩物质加入。  相似文献   

2.
Three distinct alkaline magmas, represented by shonkinite, lamprophyre and alkali basalt dykes, characterize a significant magmatic expression of rift-related mantle-derived igneous activity in the Mesoproterozoic Prakasam Alkaline Province, SE India. In the present study we have estimated emplacement velocities (ascent rates) for these three varied alkaline magmas and compared with other silicate magmas to explore composition control on the ascent rates. The alkaline dykes have variable widths and lengths with none of the dykes wider than 1 m. The shonkinites are fine- to medium-grained rocks with clinopyroxene, phologopite, amphibole, K-feldspar perthite and nepheline as essential minerals. They exhibit equigranular hypidiomorphic to foliated textures. Lamprophyres and alkali basalts characteristically show porphyritic textures. Olivine, clinopyroxene, amphibole and biotite are distinct phenocrysts in lamprophyres whereas olivine, clinopyroxene and plagioclase form the phenocrystic mineralogy in the alkali basalts. The calculated densities [2.54–2.71 g/cc for shonkinite; 2.61–2.78 g/cc for lamprophyre; 2.66–2.74 g/cc for alkali basalt] and viscosities [3.11–3.39 Pa s for shonkinite; 3.01–3.28 Pa s for lamprophyre; 2.72–3.09 Pa s for alkali basalt] are utilized to compute velocities (ascent rates) of the three alkaline magmas. Since the lamprophyres and alkali basalts are crystal-laden, we have also calculated effective viscosities to infer crystal control on the velocities. Twenty percent of crystals in the magma increase the viscosity by 2.7 times consequently decrease ascent rate by 2.7 times compared to the crystal-free magmas. The computed ascent rates range from 0.11–2.13 m/sec, 0.23–2.77 m/sec and 1.16–2.89 m/sec for shonkinite, lamprophyre and alkali basalt magmas respectively. Ascent rates increase with the width of the dykes and density difference, and decrease with magma viscosity and proportion of crystals. If a constant width of 1 m is assumed in the magma-filled dyke propagation model, then the sequence of emplacement velocities in the decreasing order is alkaline magmas (4.68–15.31 m/sec) > ultramafic-mafic magmas (3.81–4.30 m/sec) > intermediate-felsic magmas (1.76–2.56 m/sec). We propose that SiO2 content in the terrestrial magmas can be modeled as a semi-quantitative “geospeedometer” of the magma ascent rates.  相似文献   

3.
西南极菲尔德斯半岛第三纪火山岩的岩石学和岩石化学特征表明,它们基本属于钙碱性火山岩系列,是岛弧火山作用的产物。该岩石组合中,随岩石中SiO_2含量的增加,斜长石斑晶数量减少,微量元素Cr、V丰度降低,Sr、Ba丰度下降,这些揭示了岩浆中斜长石和单斜辉石的分离结晶作用。稀土元素的系统变化也证明了这一点。主元素和微量元素的定量计算所验证了岩浆的分离结晶作用演化过程。  相似文献   

4.
Composite dykes consisting of leucominette and dacite as wellas discrete dykes and flows of minette and lamproite composition,occur in the Veliki Majdan area, western Serbia. This area ispart of the Serbian Tertiary magmatic province, which consistsof numerous small occurrences of ultrapotassic igneous rocks.The composite dykes have leucominette margins (up to 150 cmthick) enclosing a central part of dacite up to 100 m in width.Between these two lithologies, a decimetre-sized transitionzone may occur. Petrography, mineral chemistry and bulk-rockgeochemistry, including Sr, Nd and Pb isotopes, provide evidencethat the minettes and leucominettes formed by hybridizationbetween a felsic magma similar in composition to dacite anda mantle-derived lamproitic magma. The leucominettes and minettescontain all phenocryst types (biotite, plagioclase, quartz)present in the dacites, but in partly resorbed and reacted form.The mica displays a great diversity of resorption textures asa result of partial dissolution, incipient melting and phlogopitization,suggesting superheating of the felsic melt during hybridization;the mineral modes and mineral compositions of the leucominettesand minettes resemble those in the lamproites. A model for themodification of lamproite melt towards minette is presentedin which minette is formed by mixing of lamproite and <30%felsic magma. The lack of any significant correlation betweenPb isotopic ratios and some of the ‘mixing-indices’(SiO2, Zr, Zr/Nb, 143Nd/144Ndi) recognized in the hybridizationmodel for the Veliki Majdan dykes may be a result of similarityof the Pb-isotopic signature in the two end-members. Highlyphlogopitized biotite xenocrysts in the minettes are ascribedto the retention of volatile components after magma mixing andcrystallization of a new generation of phlogopite from the hybridizedmagma. The magma-mixing model explains the reverse zoning andresorption features of phlogopite macrocrysts commonly recognizedin calcalkaline lamprophyres elsewhere. Therefore, this mixingmechanism may be globally applicable for the origin of minettesassociated with calcalkaline granitic plutonism in post-orogenicsettings. KEY WORDS: Serbia; lamproites; micas; phlogopitization; calcalkaline lamprophyres; superheating; magma mixing  相似文献   

5.
One of the most significant mafic intrusive events in the Zimbabwe Craton was the emplacement of the Great Dyke layered ultramafic-mafic complex and its two parallel ‘satellite’ dykes at the end of the Archæan (∼2.6 Ga). The two satellite dykes, the East Dyke and the West (Umvimeela) Dyke, were far less affected by crystal accumulation and layering processes than the Great Dyke, and therefore may provide a clearer picture of parental magma compositions of the Great Dyke event. The geochemical character of this major episode of mafic magmatism is markedly different to that of more typical continental tholeiites, such as the ∼1.9 Ga Mashonaland sills, and reflects significant contrasts in petrogenetic influences. Despite its mid-continental setting, the parental magma of the satellite dykes had geochemical characteristics more often associated with magmas generated at destructive plate margins, suggesting that the nature of this magma was perhaps more similar to high Mg andesitic, rather than continental tholeiitic magmatism. Fine-grained samples with ∼14% MgO perhaps most closely approximate to the parental magma composition. Certain major and trace element concentrations (high MgO, SiO2, LREE and LILE, and low Nb, Ta and Ti) and the lack of an Fe enrichment trend, suggest that the mafic magma had either suffered severe crustal contamination or had been derived from a metasomatised harzburgitic mantle source.  相似文献   

6.
The diamondiferous Ellendale 9 (E9) pipe is a funnel-shaped maar-diatreme volcano consisting of inward-dipping tuff sequences intruded by lamproite plugs and dykes. The host rocks for the E9 pipe are Permian sandstones. The multiple lithological contacts exposed within the mined maar volcano provide a natural laboratory in which to study the effect of volcanic processes on U–Th–Pb–He systematics. Zircon from the regional sandstone and E9 lamproite display a bimodal distribution of ages on (U–Th)/He–U/Pb plots. The zircon U/Pb ages for the E9 pipe (n?=?52) range from 440 to 2,725 Ma, while the cluster of (U–Th)/He ages for the lamproite dyke zircon indicate that dyke emplacement occurred at 20.6?±?2.8 Ma, concordant with a maximum emplacement age of about ≤22 Ma from phlogopite 40Ar/39Ar. These ages indicate a xenocrystic origin for the zircon entrained in the E9 dyke. The U/Pb ages of detrital zircon from the regional sandstone host (373–3,248 Ma; n?=?41) are indistinguishable from those of the lamproite zircon xenocrysts, whereas the detrital zircon in the host sandstone yield (U–Th)/He ages from 260 to 1,500 Ma. A thermochronology traverse across the E9 lamproite dyke reveals that the zircon (U–Th)/He ages in the host sandstone have not been significantly thermally reset during dyke emplacement, even at the contact. The capability of the zircon (U–Th)/He method to distinguish deep, mantle source lithologies from upper crustal source lithologies could be used in geochemical exploration for diamonds. Pre-screening of detrital samples using etching and helium assay methods will improve the efficiency and decrease the cost of greenfields exploration.  相似文献   

7.
塔里木盆地南北缘新生代火山岩成因及其地质意义   总被引：29，自引：3，他引：29  
塔里木盆地南北缘几个地方的新生代火山岩类具有贫硅富碱、特别是富钾的特点 ,均属于碱性火成岩系列。大部分样品SiO2 质量分数低于 5 3% ,全碱质量分数大于 3%。按照全碱 SiO2 的关系 ,它们可以划分为 3种类型 :钾质碧玄岩钾质粗面玄武岩钾质碱性橄榄玄武岩类、钾质橄榄安粗岩粗面岩类和钾质粗面玄武岩橄榄玄武粗安岩安粗岩类。岩石普遍含有橄榄石、单斜辉石、斜长石与黑云母等矿物斑晶 ,部分火山岩中见有辉石、金云母、角闪石高压巨晶 ,岩石地球化学特征与幔源捕虏体和高压巨晶的产出表明它们具有原生岩浆的性质。岩石地球化学参数的变异趋势暗示岩浆起源于深部流体强烈活动的地幔环境 ,而不是受控于分离结晶作用。岩浆活动受控于印度 -亚洲大陆碰撞构造体制派生的大型走滑体制 ,与大陆裂谷、地幔柱和陆内俯冲作用无关。  相似文献   

8.
应用陆地卫星 TM 资料对南盘江地区进行了烃类微渗漏蚀变信息检测研究。岩石土壤地球化学特征分析表明,该区不同程度地存在着“褪红”、“粘土矿化”、“碳酸盐矿化”等烃类蚀变异常现象。选择 FeO/Fe_2O_3作为褪红的成分因子;(K_2O+Na_2O+Al_2O_3)、(K_2O+Na_2O)/Al_2O_3、(K_2O+Na_2O+Al_2O_3)/S∶O_2和 K_2O/Na_2O 等作为粘土矿化的成分因子;CaO+MgO 和 CaO/MgO 作为碳酸盐矿化的成分因子。利用 TM1/TM3、TM1/TM4识别褪红蚀变信息,TM5/TM7、TM5/TM4用干识别粘土矿化和碳酸盐矿化蚀变信息。建立了地球化学—光谱—遥感图像三位一体的遥感找油气模式,根据油气遥感色调异常对南盘江地区进行了评价和分类。  相似文献   

9.
This paper investigates the origin of low-δ18O quartz porphyry dykes associated with the 144–133 Ma Koegel Fontein Igneous Complex, which was intruded during the initial phase of breakup of Africa and South America. The 25-km diameter Rietpoort Granite is the largest and youngest phase of activity, and is roofed by a 10-km diameter pendant of gneiss. Quartz porphyry (QP) dykes, up to 15 m in width, strike NW–SE across the complex. The QP dykes that intruded outside the granite have similar quartz phenocryst δ18O values (average 8.0‰, ± 0.7, n?=?33) to the granite (average 8.3?±?1.0, n?=?7). The QP dykes that intruded the roof pendant have quartz phenocrysts with more variable δ18O values (average 1.6‰, ± 2.1, n?=?55). In some cases quartz phenocrysts have δ18O values as low as ? 2.5‰. The variation in δ18O value within the quartz crystal population of individual dykes is small relative to the overall range, and core and rim material from individual quartz phenocrysts in three samples are identical within error. There is no evidence that quartz phenocryst δ18O values have been affected by fluid–rock interaction. Based on a ?quartz?magma value of 0.6‰, magma δ18O values must have been as low as ? 3.1‰. Samples collected along the length of the two main QP dykes that traverse the roof pendant have quartz phenocryst δ18O values that range from +?1.1 to +?4.6‰, and ? 2.3 to +?5.6‰, respectively. These δ18O values correlate negatively (r = ? 0.96) with initial 87Sr/86Sr, which can be explained by the event that lowered δ18O values of the source being older than the dykes. We suggest that the QP dykes were fed by magma produced by partial melting of gneiss, which had been variably altered at high temperature by 18O-depleted meteoric water during global glaciation at ~?550 Ma. The early melts had variable δ18O value but as melt pockets interconnected during melting, the δ18O values approached that of average gneiss. Variable quartz phenocryst δ18O values in the same dyke can be explained by vertical emplacement, at variable rates of ascent along the dyke. The lateral variation in quartz, and hence magma δ18O value at a particular point along a single dyke would depend on the rate of ascent of magma at that point along the dyke, and the ‘age’ of the particular magma batch.  相似文献   

10.
The Younger Giant Dyke Complex consists of a set of massivecoalescing dykes of Proterozoic age (c. 1170 Ma), resultingfrom intrusion of a suite of transitional olivine basaltic/hawaiiticmagmas in a continental rift setting. The suite, compositionallyrelated by low pressure (< 10 kb) olivine-plagioclase fractionation,is believed to have had a deeper level evolution dominated bypyroxene and possibly garnet fractionation. Slow cooling insitu of the interior parts of the dyke complex produced cumuliticsuites. Those exposed range from gabbroic to syenitic; residualbodies of riebeckite granite and, very subordinate, feldspathoidalsyenite were also generated. The basic magmas had notably lowfO2 values, leading to delayed magnetite and clinopyroxene precipitation,relatively iron-rich differentiates and some residual liquidsof pantelleritic composition. The basic magmas had high F/Clvalues and are inferred to have had low H2O contents. They werealso characterized by relatively high K/Rb and low 87Sr/86Srvalues; these characteristics imply a mantle source with highF/Cl but depleted in Rb relative to K and Sr. Basaltic magmasresponsible for (a) the preceding Older Giant Dyke Complex and(b) a suite of anorthositic xenoliths within the Younger GiantDyke Complex, are inferred to have been derived from separateprimary magma batches independent of those that yielded theYounger Giant Dyke Complex. The giant dykes are the highest-levelrepresentatives of a larger basic complex responsible for theextensive linear gravity ‘high’ in the Tugtutôq-Narssaqarea.  相似文献   

11.
Olav Eklund  Alexey Shebanov 《Lithos》2005,80(1-4):229-247
The Åva ring complex is one of four Paleoproterozoic postcollisional shoshonitic ring complexes in southwestern Finland. It is composed of ring dykes of K-feldspar megacryst-bearing granite, mingled in places with a shoshonitic monzonite, and lamprophyre dykes crosscutting all the rocks in a radial pattern. A survey was undertaken to trace the magma chamber beneath the ring complex to date it and measure some intensive parameters to clarify the crystallisation conditions at depth before the granite was emplaced in the upper crust. Mineral separates were extracted from the core zones of K-feldspar megacrysts in the granite, heavy mineral fractions (including zircons) from these separates were used for P-T assessment and age determinations, and the results were compared to data obtained from bulk rock samples. It appears that magma differentiation took place in a midcrustal magma chamber (at 4 to 7 kbar) possibly 30 Ma before the emplacement of the ring complex in the upper crust (deep assemblage 1790 Ma, shallow assemblage 1760 Ma). Relatively high activity of the alkalies and a low oxygen fugacity characterised the midcrustal chamber. The juvenile Svecofennian crust was invaded by shoshonitic magmas from an enriched lithospheric mantle over a long period of time. Some of these magmas were stored and differentiated in the middle crust before transportation to the upper crust. The results also show that coarse-grained granites may provide evidence for several magmatic evolutionary episodes, e.g., differentiation and crystallisation in different environments prior to final emplacement.  相似文献   

12.
Continental tholeiites have higher SiO2, K2O and light rare earth element contents and more evolved isotopic characteristics than their oceanic counterparts. These differences can be explained if the compositions of the parent magmas to both types of tholeiites are similar but if continental magmas assimilate significant amounts of continental crust en route to the surface. Although there is little doubt that most continental tholeiites have assimilated crustal material, the lcoation and mechanism of assimilation remain uncertain. Longhi (1981) has argued that magmas derived directly from the mantle should crystallize little orthopyroxene. The abundance of orthopyroxene in most continental layered intrusions suggests that they have crystallized from magmas which have assimilated continental crust. Since orthopyroxene is an early crystallizing phase in layered intrusions, this assimilation must occur early, before the magma enters the chamber. Assimilation can occur at the margins of the dykes which feed magma chambers, depending on the nature of the flow. If the flow is turbulent the high temperatures at the centre of the dyke will extend to the margins and the magma will erode the dyke walls. If the flow is laminar, a conductive profile develops at the margin and the flowing magma chills against the walls, protecting them from thermal erosion. The nature of flow in a dyke can be predicted from the Reynolds number, the criteria for turbulence. Reynolds number calculations suggest that the flow of primitive magmas in continental dykes will be fully turbulent and, if this is the case, assimilation of low melting point components in the walls of the dyke is inevitable. It is therefore suggested that many of the geochemical characteristics of continental tholeiites result from melting at the walls of dykes as primitive magmas ascend through the crust.  相似文献   

13.
张臣  侯贵廷 《地质论评》1994,40(3):245-251
华北克拉通区内的吕梁-晋北地区广泛发育晚前寒武纪镁铁质岩墙群。该地区近EW向和NW-NNW向岩墙的磁组构测量结果表明,岩墙群磁各向异性与岩墙侵位的关系密切,磁化率的长轴能指示其岩浆侵位的流向。该区的岩墙群的岩浆源位于东侧的燕辽拗拉槽处,岩浆沿岩墙走向以一定仰角由ESE(或E)朝NW-NNW(或W)向流动,具板内裂谷模式。  相似文献   

14.
Field data on Cenozoic faults and the McMurdo dyke arrays in the Reeves Glacier–Mawson Glacier area, Victoria Land, Antarctica, allow us to support noncoaxial transtensional tectonics along the N–S-trending western shoulder of the Ross Sea. Dyke injection within a crustal-scale right-lateral strike-slip shear zone is testified by magma filled, tension gash-like arrangements within some master fault zones, and by the left-stepping arrangements of dykes in the intrafault zones. The noncoaxiality of deformation is shown by the re-activation of many dyke walls as right-lateral strike-slip faults. This suggests an increase in the strike-slip component over time along the western shoulder of the Ross Sea. Our data support the relevance of transtensional to strike-slip tectonics for triggering melting and controlling the geometry and modes of magma emplacement.  相似文献   

15.
白云鄂博REE-Nb-Fe矿床的富钠岩石类型析及成因分   总被引：4，自引：1，他引：4  
白云鄂博稀土-铌-铁矿是世界最大的稀土多金属矿床之一,主要产于中元古界富钠岩石、白云岩和富钾板岩中。富钠岩石与含矿碳酸岩、富钾板岩一起在白云鄂博广泛分布,特别是在主矿和东矿南侧分布广泛,也产在含矿围岩和底盘围岩中。富钠岩石包括富钠长石岩、含钠闪石钠长岩脉、钠闪石岩脉及钠闪石碳酸岩脉和钠闪石化蚀变白云岩。富钠岩石在一个地质单元内出现,但成分略有不同,其岩石化学分析表现出富钠特征,大部分样品的w(Na2O)在6%以上,w(Na2O+K2O)>8%,w(Na2O)>w(K2O)。富钠长石岩的里特曼指数δ为2.96~8.56,n(A l)/n(K+Na)>1,n(A l)/n(K+Na+Ca)<1,表明为弱碱性偏铝质岩石。富钠岩石以富集地壳不相容元素、亏损地幔元素为特征,其稀土总量低于矿化白云岩,但轻稀土元素高度富集,具弱负铕异常,与含矿白云质火山岩一致。根据岩石产状、组构及化学成分分析,富钠长石岩属于富钠火山岩,而其他含钠闪石岩脉则属于岩浆分异侵入的岩脉与岩浆热液充填岩脉。钠化交代以霓石化、钠铁闪石化和钠长石化出现为特征。另外,富钠流体与白云岩反应形成了钠铁闪石化白云岩,强烈的钠化蚀变作用主要出现在稀土强烈富集的主矿和东矿。比较蚀变与未蚀变岩石的化学成分,表明蚀变作用引起稀土的重新分配和弱负铕异常。钠铁闪石化白云岩的w(S iO2)、w(TiO2)、w(K2O)和w(Na2O)升高,w(P2O5)降低。钠化蚀变前的碳酸岩中稀土元素已经富集,热液蚀变引起稀土元素的活化和重新分配,但是没有提供新的稀土来源。  相似文献   

16.
华北克拉通1.75Ga基性岩墙群特征及其研究进展   总被引：2，自引：1，他引：2  
基性岩墙群是地壳伸展背景下,来自地幔的基性岩浆侵入体。华北克拉通同世界上其它克拉通一样,广泛发育前寒武纪基性岩墙群。它们在不同时代均有产生,其中1.75Ga前后的规模最大,分布范围最广,几乎遍布整个克拉通,对其进行深入研究,可以揭示华北克拉通该期构造演化过程。华北克拉通1.75Ga前后的岩墙几何形态多变,直立或近直立,走向主要为NNW向和近EW向。岩石以拉斑玄武质岩类占绝对优势(>80%),主要造岩矿物为单斜辉石和斜长石。根据岩墙走向、岩浆分异程度和岩石地球化学特征可将其分五组:低分异LT组、低分异HT组、高分异NW组、高分异EW组,以及具明显差异的高铁系列。同位素和微量元素研究显示,岩浆源区主要与富集Ⅰ型地幔(EMⅠ)、弱亏损的常规地幔(DM-PREMA)以及陆下岩石圈地幔有关。目前对华北克拉通1.75Ga基性岩墙群产出的构造环境在认识上有分歧,其中地幔柱观点和碰撞后伸展观点最为人们所关注。  相似文献   

17.
对近几十年来岩墙群在构造环境、侵位模式、形成机制、岩相学、地球化学、同位素地质年代学、几何学和比较行星学等方面的研究成果进行了总结,为岩墙群的深入研究提供重要信息。岩墙群主要与伸展构造环境有关,是岩浆侵位事件(如地幔柱)造成上覆地壳形成裂隙系统,岩浆随后灌入而形成的。岩墙群的侵位模式主要有垂向和侧向两种,也可能是两种模式共同作用的结果。岩墙群的岩性多样,但主要是超基性-基性岩,说明其与深部岩浆作用密切相关。对岩墙群的地球化学研究可揭示岩浆事件的构造环境、岩浆演化过程以及源区特征。放射性岩墙群的几何学特征有重要的指示意义,其收敛中心被认为是地幔柱的中心位置。岩墙群是古陆重建十分重要的"契合点",对古陆重建研究十分有帮助。类地行星(金星)巨型放射状系统很可能是岩墙群系统,暗示了地球上也存在过这种完整的巨型放射状岩墙群系统。岩墙群对于古陆重建、地幔柱中心的指示、地幔源区示踪、火山机构以及区域岩浆演化的研究都有十分重要的意义,并将对地球科学的发展产生重要影响。  相似文献   

18.
Field and petrographic studies are carried out to characterize the interactions of mafic and felsic magmas from Pithora region of the northeastern part of the Bastar Craton. The MMEs, syn-plutonic mafic dykes, cuspate contacts, magmatic flow textures, mingling and hybridization suggest the coeval emplacement of end member magmas. Petrographic evidences such as disequilibrium assemblages, resorption textures, quartz ocelli, rapakivi and poikilitic textures suggest magma mingling and mixing phenomena. Such features of mingling and mixing of the felsic and mafic magma manifest the magma chamber processes. Introduction of mafic magmas into the felsic magmas before initiation of crystallization of the latter, results in hybrid magmas under the influence of thermal and chemical exchange. The mechanical exchange occurs between the coexisting magmas due to viscosity contrast, if the mafic magma enters slightly later into the magma chamber, then the felsic magma starts to crystallize. Blobs of mafic magma form as MMEs in the felsic magma and they scatter throughout the pluton due to convection. At a later stage, if mafic magma enters the system after partial crystallization of felsic phase, mechanical interaction between the magmas leads to the formation of fragmented dyke or syn-plutonic mafic dyke. All these features are well-documented in the study area. Field and petrographic evidences suggest that the textural variations from Pithora region of Bastar Craton are the outcome of magma mingling, mixing and hybridization processes.  相似文献   

19.
We present a first overview of the synplutonic mafic dykes (mafic injections) from the 2.56–2.52 Ga calcalkaline to potassic plutons in the Eastern Dharwar Craton (EDC). The host plutons comprise voluminous intrusive facies (dark grey clinopyroxene-amphibole rich monzodiorite and quartz monzonite, pinkish grey porphyritic monzogranite and grey granodiorite) located in the central part of individual pluton, whilst subordinate anatectic facies (light grey and pink granite) confined to the periphery. The enclaves found in the plutons include highly angular screens of xenoliths of the basement, rounded to pillowed mafic magmatic enclaves (MME) and most spectacular synplutonic mafic dykes. The similar textures of MME and adjoining synplutonic mafic dykes together with their spatial association and occasional transition of MME to dismembered synplutonic mafic dykes imply a genetic link between them. The synplutonic dykes occur in varying dimension ranging from a few centimeter width upto 200 meters width and are generally dismembered or disrupted and rarely continuous. Necking of dyke along its length and back veining of more leucocratic variant of the host is common feature. They show lobate as well as sharp contacts with chilled margins suggesting their injection during different stages of crystallization of host plutons in magma chamber. Local interaction, mixing and mingling processes are documented in all the studied crustal corridors in the EDC. The observed mixing, mingling, partial hybridization, MME and emplacement of synplutonic mafic dykes can be explained by four stage processes: (1) Mafic magma injected during very early stage of crystallization of host felsic magma, mixing of mafic and felsic host magma results in hybridization with occasional MME; (2) Mafic magma introduced slightly later, the viscosities of two magmas may be different and permit only mingling where by each component retain their identity; (3) When mafic magma injected into crystallizing granitic host magma with significant crystal content, the mafic magma is channeled into early fractures and form dismembered synplutonic mafic dykes and (4) Mafic injections enter into largely crystallized (>80% crystals) granitic host results in continuous dykes with sharp contacts. The origin of mafic magmas may be related to development of fractures to mantle depth during crystallization of host magmas which results in the decompression melting of mantle source. The resultant hot mafic melts with low viscosity rise rapidly into the crystallizing host magma chamber where they interact depending upon the crystallinity and viscosity of the host. These hot mafic injections locally cause reversal of crystallization of the felsic host and induce melting and resultant melts in turn penetrate the crystallizing mafic body as back veining. Field chronology indicates injection of mafic magmas is synchronous with emplacement of anatectic melts and slightly predates the 2.5 Ga metamorphic event which affected the whole Archaean crust. The injection of mafic magmas into the crystallizing host plutons forms the terminal Archaean magmatic event and spatially associated with reworking and cratonization of Archaean crust in the EDC.  相似文献   

20.
莫来铁尖晶岩中的莫来石是首次发现的一种火山岩型莫来石。在全岩X射线粉晶衍射图上莫来石的d值(nm)为:0.5378(5),0.3428(9),0.3385(10),0.2693 (4),0.2545(5),0.2295 (2),0.1601 (2),0.1542 (4),0.1443 (2)。电子探针分析结果,莫来石的成分(%)为:SiO_2 28.96,TiO_2 0.53,Al_2O_3 67.13,FeO 3.06,MnO 0.05.MgO 0.02,Na_2O 0.21,K_2O 0.02,Cr_2O_3 0.05, NiO 0.02。这种莫来石是从玄武岩浆演化后期产生的富Al_2O_3、FeO(Fe_2O_3)和贫SiO_2的莫来铁尖晶岩浆中直接结晶而成的。  相似文献   

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