首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 609 毫秒
1.
《Gondwana Research》2002,5(2):401-408
In this paper, field evidence documenting the polydeformed nature of banded gneiss that comprises a part of the Godhra Granite and Gneiss in the southern portion of Aravalli Mountain Belt (AMB), India, is presented. The structural geometry involving an episode of recumbent-reclined folding in the gneiss lying in the vicinity of Devgadh Bariya town is worked out. The banded gneiss occurs as enclaves in the granite. Therefore, it is suggested that the banded gneiss of the region is older than Godhra Granite. Microstructures preserved in the granitic rocks are documented and it is suggested that the granitic rocks underwent deformation and strain during their evolutionary history. Variation in the mesoscopic scale fabric of the gneiss and granite along a south-to-north traverse within the study area is documented, and different possibilities for evolution of Godhra Granite are discussed.  相似文献   

2.
Quartz feldspar augen gneisses, quartz augen schists and trondhjemites outcrop at Nooldoonooldoona Waterhole in the southwestern corner of the Proterozoic Mt Painter Inlier, northern Flinders Ranges, South Australia. These rocks were previously interpreted as having different origins and ages. However, we argue that all rock types were the result of deformation and strong metasomatic alteration of one common precursor: the Mt Neill Granite. Our conclusion is based on field observations that show that the different lithologies grade into each other and that intrusive contacts are lacking. Whole rock major and trace element analyses also point to a common protolith. Finally, Pb/Pb dating of magmatic zircons gave the same ca 1576 Ma age for the different rock types. Our findings necessitate a re evaluation of the published regional geology and lithostratigraphy of the Mt Painter Inlier. They also indicate that extreme care should be taken in the classification and genetic interpretation of rocks that have experienced extensive metasomatic alteration, which is common in many high grade terrains in Australia.  相似文献   

3.
The Basement Complex in Western Nigeria in general, and in the Ibadan area in particular, is composed primarily of a banded gneiss in which hornblende-biotite rich bands alternate with quartz-oligoclase rich bands. The banded gneiss, which originated as part of a sedimentary sequence, contains large lenses of granite gneiss and thin intercolated layers of quartzite and amphibolite.Two distinct major structural events can be clearly identified in the early geological history of the Ibadan area. Detailed field studies suggest that the formation of the Ibadan Granite Gneiss, which had yielded an Eburnean Rb-Sr isochron age, was associated with the later of these events. Five phases of dyke or vein formation, two of which pre-date the formation of the Granite Gneiss, have also been identified giving an overall sequence of geological events the first of which may correspond to the beginning of the Liberian orogeny, around 3000 m.y. ago, and the last of which reflects the waning of the Pan-African thermo-tectonic event about 500 m.y. ago.  相似文献   

4.
There are several pre-orogenic Neoproterozoic granitoid and metavolcanic rocks in the Lufilian–Zambezi belt in Zambia and Zimbabwe that are interpreted to have been emplaced in a continental-rift setting that is linked to the break-up of the Rodinia supercontinent. However, no geochemical data were previously available for these rocks in the Zambian part of the belt to support this model. We conducted petrographic and whole-rock chemical analyses of the Neoproterozoic Nchanga Granite, Lusaka Granite, Ngoma Gneiss and felsic metavolcanic rocks from the Lufilian–Zambezi belt in Zambian, in order to evaluate their chemical characteristics and tectonic settings. Other magmatic rocks of importance for understanding the evolution of the belt in Zambia, included in this study, are the Mesoproterozoic Munali Hills Granite and associated amphibolites and the Mpande Gneiss. The Neoproterozoic rocks have monzogranitic compositions, aluminum-saturation indices (ASI) < 1.1, and high contents of high field strength elements (HFSE) and rare earth elements (REE). The chondrite-normalised spider diagrams are similar to those of A-type granites from the Lachlan fold belt and show negative Sr, P, and Ti anomalies. On various tectonic discrimination diagrams the Neoproterozoic rocks plot mainly in A-type granite fields. These petrographic and trace element compositions indicate that these rocks are A-type felsic rocks, but they do not have features of granites and rhyolites emplaced in true continental-rift settings, as previously suggested. On the basis of the A-type features and independent regional geological and geochronological data, we suggest that the Neoproterozoic granitoid and felsic metavolcanic rocks were emplaced during the earliest extensional stages of continental rifting in the Lufilian–Zambezi belt. The apparent continental-arc like chemistry of the granitoid and felsic metavolcanic rocks is thus inferred to be inherited from calcalkaline sources. The Mesoproterozoic Munali Hills Granite and Mpande Gneiss have trace element features e.g., Nb–Ta depletions, which indicate that that these gneisses were emplaced in a convergent-margin setting. The MORB-normalised spider diagram of co-magmatic amphibolites exhibit a fractionated LILE/HFSE pattern recognized in subduction zones. This inference is consistent with remnants of ocean crust, juvenile Island arcs and ophiolites elsewhere in the Mesoproterozoic Irumide belt in Zambia and Zimbabwe. In addition, we report the first U–Pb zircon age of 1090.1 ± 1.3 Ma for the Munali Hills Granite. The age for the Munali Hills Granite provides new constraints on correlation and tectono-thermal activity in the Lufilian–Zambezi belt. The age of the Munali Hills Granite indicates that some supracrustal rocks in the Zambezi belt of Zambia, which were previously thought to be Neoproterozoic and correlated with the Katanga Supergroup in the Lufilian belt, are Mesoproterozoic or older. Consequently, previous regional lithostratigraphic correlations in the Lufilian–Zambezi belt would require revision.  相似文献   

5.
The earliest decipherable record of the Dharwar tectonic province is left in the 3.3 Ga old gneissic pebbles in some conglomerates of the Dharwar Group, in addition to the 3.3–3.4 Ga old gneisses in some areas. A sialic crust as the basement for Dharwar sedimentation is also indicated by the presence of quartz schists and quartzites throughout the Dharwar succession. Clean quartzites and orthoquartzite-carbonate association in the lower part of the Dharwar sequence point to relatively stable platform and shelf conditions. This is succeeded by sedimentation in a rapidly subsiding trough as indicated by the turbidite-volcanic rock association. Although conglomerates in some places point to an erosional surface at the contact between the gneisses and the Dharwar supracrustal rocks, extensive remobilization of the basement during the deformation of the cover rocks has largely blurred this interface. This has also resulted in accordant style and sequence of structures in the basement and cover rocks in a major part of the Dharwar tectonic province. Isoclinal folds with attendant axial planar schistosity, coaxial open folds, followed in turn by non-coaxial upright folds on axial planes striking nearly N-S, are decipherable both in the “basement” gneisses and the schistose cover rocks. The imprint of this sequence of superposed deformation is registered in some of the charnockitic terranes also, particularly in the Biligirirangan Hills, Shivasamudram and Arakalgud areas. The Closepet Granite, with alignment of feldspar megacrysts parallel to the axial planes of the latest folds in the adjacent schistose rocks, together with discrete veins of Closepet Granite affinity emplaced parallel to the axial planes of late folds in the Peninsular Gneiss enclaves, suggest that this granite is late-tectonic with reference to the last deformation in the Dharwar tectonic province. Enclaves of tonalite and migmatized amphibolite a few metres across, with a fabric athwart to and overprinted by the earliest structures traceable in the supracrustal rocks as well as in a major part of the Peninsular Gneiss, point to at least one deformation, an episode of migmatization and one metamorphic event preceding the first folding in the Dharwar sequence. This record of pre-Dharwar deformation and metamorphism is corroborated also by the pebbles of gneisses and schists in the conglomerates of the Dharwar Group. Volcanic rocks within the Dharwar succession as well as some of the components of the Peninsular Gneiss give ages of about 3.0 Ga. A still younger age of about 2.6 Ga is recorded in some volcanic rocks of the Dharwar sequence, a part of the Peninsular Gneiss, Closepet Granite and some charnockites. These, together with the 3.3 Ga old gneisses and 3.4 Ga old ages of zircons in some charnockites, furnish evidence for three major thermal events during the 700 million year history of the Archaean Dharwar tectonic province.  相似文献   

6.
The Charleston Granite from the Gawler Craton, South Australia, has been dated by the ion‐microprobe U‐Pb zircon method at 1585 ± 5 Ma (2σ). This confirms previous interpretations of population‐style U‐Pb zircon analyses which record a slightly older age due to the presence of inherited zircon. Inherited cores are present in many zircon crystals, and while the age of some cores can not be accurately determined due to extreme loss of radiogenic Pb, others have ages of ～ 1780, ～ 1970, and > 3150 Ma. These cores record a diverse crustal heritage for the Charleston Granite and indicate that ancient crustal material (> 3150 Ma) is present at depth in the Gawler Craton. This is also suggested by available Nd isotopic data for both the Charleston Granite and other Gawler Craton Archaean rocks. The Rb‐Sr and K‐Ar biotite ages from the Charleston Granite of 1560 to 1570 Ma are close to the U‐Pb zircon crystallization age and suggest that the granite has not experienced sustained thermal disturbance (> 250° C) since emplacement and cooling. However, a much younger Rb‐Sr total‐rock age of 1443 ± 26 Ma probably reflects low‐temperature disturbance to the Sr isotope system in feldspar.  相似文献   

7.
The Koperberg Suite comprises some 1700 small bodies of intrusiverocks largely composed of andesine anorthosite, biotite diorite,and leuconorite, norite and melanorite-hypersthenite; 30 mineshave been established in the O'okiep District in the cupriferousrocks of this anorthosite-charnockite kindred. The suite isintrusive into a sequence of granite gneiss and metavolcanicand metasedimentary rocks, and intrusive granite, that wereelevated to the granulite fades of regional metamorphism.TheSm-Nd model ages for the country rocks and the Koperberg Suiteare all 1700 Ma (TCHUR) and 2000 Ma (TDM) supporting a majorcrustforming event in this portion of Namaqualand at the endof Lower Proterozoic times. The granulite fades metamorphismin the O'okiep District is recorded by a Rb-Sr isochron ageof 1223 48 Ma on the Nababeep Granite Gneiss, and by (1197 15)-Ma-old inherited cores of zircons in the Koperberg Suite.The time of intrusion of the Concordia and Rietberg Granitesis believed to be reflected by their Rb-Sr whole-rock age of1105 24 Ma. The mean U-Pb age of 1029 10 Ma on individualzircon grains and zircon rims from the Koperberg Suite recordsthe time of its intrusion, and this is supported by the Sm-Ndwhole-rock age of 1022 42 Ma for the suite. Subsequent coolingand reheating events are recorded by the Ar-Ar ages of 800–850Ma for the Koperberg Suite, and of 500–550 Ma for thesuite and certain country rocks, respectively.An Nd value of-7,and its volume and composition, suggest a crustal-melt sourcefor the intrusive Concordia Granite. Moreover, the age-correctedhigh lSr (07061-07272) and low Nd (-9), and the high µ2(101), that characterize the Koperberg Suite also imply a crustalsource, and a model is presented for the generation of the majorpart of the suite by partial melting of granulites of overallintermediate (diorite) composition in the lower crust. Corresponding author  相似文献   

8.
Rubidium‐strontium and strontium isotope data for eight whole‐rock samples of granite varieties from the Encounter Bay area, South Australia, yield an isochron age of 487 ± 37 m.y. Two specimens of albitised granite, formed as a result of late‐stage metasomatic alteration of original megacrystic granite, conform to this isochron. These data support a genetic relation between granites and late‐stage metasomatic alteration as suspected from field, petrographical and geochemical studies. Eight samples from contiguous Kanmantoo Group metasedimentary rocks have an isochron age of 487 ± 60 m.y. Thus this metamorphic event is coincident with emplacement of the Encounter Bay Granite.

The initial Sr87Sr86 ratio for the Encounter Bay Granite (0.719) is significantly higher than initial ratios for the Palmer (0.709) and Anabama (0.705) Granites from the same region and can be attributed to either remobilisation or incorporation of strontium from older crustal material in the intrusion. The apparent initial Sr87/Sr86 ratio for the Kanmantoo Group metasedimentary rocks (0.722) can not be distinguished from that for the Encounter Bay Granite within the analytical uncertainties. Compatability of ages and high initial Sr87Sr86 ratios suggest that the granites formed by remobilisation of associated crustal rock.  相似文献   

9.
周永胜  林强 《吉林地质》1997,16(1):33-41
根据岩石组合，分布，接触，包裹关系以及结构和构造，吉林省桦甸地区太古宙花岗质林岩为角闪英云闪长质片麻岩类和黑云母花岗闪长质片麻岩类，角闪英云闪长质片麻岩类为变形前侵入体，黑云母花岗闪长质片麻岩类为同变形侵入体，两类花岗质片麻岩的侵位是晚太古宙近东西向构造变形相伴生的岩浆活动事件。  相似文献   

10.
拉萨地块西部呈断块状沿狮泉河-申扎-嘉黎蛇绿混杂岩带附近分布的念青唐古拉岩群被认为是前寒武纪变质基底。本文对念青唐古拉岩群进行了系统的岩石学、地球化学、同位素年代学及构造地质学研究。研究结果表明片岩-片麻岩-变粒岩含十字石、石榴子石等特征变质矿物,遵循粒度分异规律,其原岩可能为来自冈瓦纳古陆核北缘中新元古代弧盆体系的活动大陆边缘浊积岩。斜长角闪岩具低硅、高铁镁、富钙的基性岩特征,其原岩为岛弧型基性火山岩。念青唐古拉岩群中的花岗伟晶岩锆石LA-ICPMS U-Pb年龄为1150±13Ma,具过铝质S型花岗岩地球化学特征,可能为中元古代(1150±13 Ma)以前就开始沉积的念青唐古拉岩群基底岩石通过部分熔融形成。与花岗伟晶岩渐变过渡接触的二云斜长片麻岩第一组变质重结晶锆石U-Pb年龄为701±15 Ma,结合十字石特征变质矿物,暗示了该地区中温高压变质作用的峰期变质,变质程度达角闪岩相;第二组热液流体锆石UPb年龄为301±8.4 Ma,可能与冈瓦纳大陆北缘古特提斯洋演化过程中的岩浆热液作用有关。  相似文献   

11.
The ca 1600–1580 Ma time interval is recognised as a significant period of magmatism, deformation and mineralisation throughout eastern Proterozoic Australia. Within the northern Yorke Peninsula in South Australia, this period was associated with the emplacement of multiple phases of the Tickera Granite, an intensely foliated quartz alkali-feldspar syenite, a leucotonalite and an alkali-feldspar granite. These granites belong to the broader Hiltaba Suite that was emplaced at shallow crustal levels throughout the Gawler Craton. Geochemical and isotopic analysis suggests these granite phases were derived from a heterogeneous source region. The syenite and alkali-feldspar granite were derived from similar source regions, likely the underlying ca 1850 Ma Donington Suite and/or the ca 1750 Ma Wallaroo Group metasediments with some contamination from an Archean basement. The leucotonalite is sourced from a similar but more mafic/lower crustal source. Phases of the Tickera Granite were emplaced synchronously with deformation that resulted in development of a prominent northeast-trending structural grain throughout the Yorke Peninsula region. This fabric is associated with composite events resulting from folding, shearing and faulting within the region. The intense deformation and intrusion of granites within this period resulted in mineralisation throughout the region, as seen in Wheal Hughes and Poona mines. The Yorke Peninsula shares a common geological history with the Curnamona Province, which was deformed during the ca 1600–1585 Ma Olarian Orogeny, and resulted in development of early isoclinal and recumbent folds overprinted by an upright fold generation, a dominant northeast-trending structural grain, mineralisation, and spatially and temporally related intrusions. This suggests correlation of parts of the Gawler Craton with the Curnamona Province, and that the Olarian Orogeny also affected the southeastern Gawler Craton.  相似文献   

12.
The A-type Mayurbhanj Granite Pluton (3.09 Ga), occurring along the eastern margin of the Singhbhum-Orissa Craton, eastern India, represents the final phase of acid plutonism in this crustal block of Archean age. The granite shows a bimodal association with a voluminous gabbroid body, exposed mainly along its western margin, and is associated with the Singhbhum Shear zone. The granite pluton is composed mainly of a coarse ferrohastingsite–biotite granite phase, with an early fine-grained granophyric microgranitic phase and a late biotite aplogranitic phase. Petrogenetic models of partial melting, fractional crystallisation and magma mixing have been advocated for the evolution of this pluton. New data, combined with earlier information, suggest that two igneous processes were responsible for the evolution of the Mayurbhanj Granite Pluton: partial melting of the Singhbhum Granite; followed by limited amount of mixing of acid and basic magmas in an anorogenic extensional setting. The necessary heat for partial melting was provided by the voluminous basaltic magma, now represented by the gabbroid body, emplaced at a shallow crustal level and showing a bimodal association with the Mayurbhanj Granite Pluton. The Singhbhum Shear Zone provided a possible channel way for the emplacement of the basic magma during crustal extension. It is concluded that all three phases of the Mayurbhanj Granite Pluton were derived from the same parent magma, generated by batch partial melting of the Singhbhum Granite at relatively high temperatures (980 °C) and low pressures (4 to <2 kbar) under anhydrous conditions. The coarse ferrohastingsite biotite granite phase shows evidence of limited and heterogeneous assimilation of country rock metasediments. However, the early microgranite phase and late aplogranite phase have not assimilated any metasediments. Compositional irregularities observed along the western margin of the Mayurbhanj Granite Pluton in contact with the gabbro body including a continuous fractionating sequence from quartz diorite to alkali-feldspar granite in the Notopahar area. Gradational contacts between the gabbro and the Mayurbhanj Granite Pluton in the Gorumahisani area etc., may be attributed to a limited amount of mixing between the gabbroid magma and the newly generated Mayurbhanj Granite magma. The mixing was mainly of liquid–liquid diffusive type, with a subordinate amount of mixing of solid–liquid type. Although A-type granites are commonly described as having high total REE (e.g. 270–400 ppm), studies on the late aplogranite phase of the Mayurbhanj Granite show that total REE values (100 ppm) are low. This low REE abundance may be attributed to the progressive residual nature of the Singhbhum Granite source during continued partial melting, when the magmas of the microgranite and coarse granite phases had already been removed from the source region.  相似文献   

13.

The Barry Granodiorite is a weakly deformed I‐type, and the Sunset Hills Granite is a moderately deformed S‐type, granite. Both granites were passively intruded into an already foliated greywacke and volcanic sequence. Emplacement may have been facilitated by faults related to the oblique opening of the late Early Silurian Hill End Trough. The granites display a dominant foliation which formed during the late Middle Devonian and subsequently was reoriented during the Early Carboniferous. The Barry Granodiorite and Sunset Hills Granite are on the margin of north‐south ductile shear zones related to the Wyangala Batholith. These granites and the adjacent Carcoar Granodiorite have undergone reorientation during movement on ductile shear zones either due to megakinking during late‐stage north‐south shortening, or southeastward movement of the southern margin of the west‐northwest‐trending Lachlan Transverse Zone.  相似文献   

14.
"片麻岩穹窿"是指中下地壳热动力过程产生的与岩浆作用(或混合岩化作用)密切相关的穹状构造,是折返造山的产物.片麻岩穹窿的形成经历了从垂直上升的地壳流导致的岩浆上涌的挤压收缩到岩体侵位的顶部伸展机制的转化过程,这一过程有利于富含锂-铯-钽型(LCT)型伟晶岩的生成和锂族元素的富集.研究表明,位于青藏高原北部的中国松潘-甘孜-甜水海印支造山带是中国大型"伟晶岩型"锂矿资源赋存的基地,松潘-甘孜东南部的超大型甲基卡型伟晶岩型锂矿带,产于具有巴罗式"低/中压-高温"变质组合的三叠纪复理石围岩中,早中生代花岗岩以及衍生的大量含锂稀土矿物的伟晶岩脉侵位有成因关系.研究认为,探究片麻岩穹窿的形成过程和构造成因机制;识别花岗岩-含矿伟晶岩的地球化学属性,揭示花岗岩浆分异作用与含矿伟晶岩相演变的成因联系,以及锂元素迁移、富集熔浆的过程;圈定三叠纪地层中巴罗式变质相带的展布,探明富锂伟晶岩矿带赋存的有利变质相带及形成的P-T条件;揭示"变形-变质-岩浆深熔-成矿"的时空耦合、制约与相互作用,再造造山过程中锂资源富集和保存的规律,以及建立成矿动力学模式;是揭示片麻岩穹窿与伟晶岩型锂矿的成矿规律的重要科学途径.  相似文献   

15.
中条山西南段基底岩系的地质特征   总被引：9，自引：1，他引：9  
山西中条山西南段基底岩系的基本构成是：涑水岩群、西姚片麻岩、解州片麻岩和五老峰花岗岩。涑水岩群残存于西姚片麻岩之中，是由斜长角闪岩、磁铁石英岩、黑云变粒岩、钙质片岩和白云质大理岩组成。斜长角闪岩为区内最早的基性火山沉积，是铜矿形成的矿源层，吕梁(中条)期造山运动为铜矿的再次富集起到关键性作用。区内主体片麻岩为西姚片麻岩(TTG)，发生岩浆垫托作用的时间为太古宙末。基底岩系经历了三次构造变形，相应划分为五台期构造变形、吕梁(中条)期构造变形和吕梁(中条)期后构造变形。  相似文献   

16.
A Lower Ordovician age for the Oughterard Granite is established by Rb–Sr dating of magmatic white micas from pegmatites cutting two of the satellite bodies found 12–15 km west of the main granitic intrusions. These micas give a minimum age of c. 473 Ma for the emplacement of the satellite bodies, and for the completion of the D3 deformation in the Dalradian host rocks. The main granite intrusions are post-D4 in age, but some of the small outlying sheet-like intrusions are either deformed by D4 folds, or were present during this deformation and were responsible for the development of locally disharmonic D4 minor folds. The correlation of the smaller granite bodies with the main bodies of Oughterard Granite is supported by their similar field appearance, petrography and trace element geochemistry. In common with other late orogenic granites, such as the 470 Ma Aberdeen Granite in NE Scotland, the Oughterard Granite is thought to have been part of a discontinuous magmatic arc, which formed along the southern margin of the Dalradian outcrop in Connemara and in NE Scotland in the early Ordovician. Members of the Oughterard Granite suite were emplaced during the final stages of the Grampian orogeny, from post-D3 to post-D4, while the country rocks were still at an elevated temperature (>500°C) following the peak of the regional metamorphism. The new minimum age of intrusion of c. 473 Ma, together with existing radiometric data, confirms that emplacement of the metagabbro and gneiss complex in southern Connemara, followed by the D3 structural and metamorphic events, all took place during a period of less than 20 Ma. © 1997 John Wiley & Sons, Ltd.  相似文献   

17.

The Hohonu Dyke Swarm and French Creek Granite represent contemporaneous and cogenetic alkaline magmatism generated during crustal extension in the Western Province of New Zealand. The age of 82 Ma for French Creek Granite coincides with the oldest oceanic crust in the Tasman Sea and suggests emplacement during the separation of New Zealand and Australia. The French Creek Granite is a composite A‐type granitoid, dominated by a subsolvus biotite syenogranite with high silica, low CaO, MgO, Cr, Ni, V and Sr and elevated high‐field‐strength elements (Zr, Nb, Ga, Y). Subordinate varieties of French Creek Granite include a hypersolvus alkali amphibole monzogranite and a quartz‐alkali feldspar syenite. Spatially associated rhyolitic dykes are considered to represent hypabyssal equivalents of French Creek Granite. The Hohonu Dyke Swarm represents mafic magmatism which preceded, overlapped with, and followed emplacement of French Creek Granite. Lamprophyric and doleritic varieties dominate the swarm, with rare phonolite dykes also present. Geochemical compositions of French Creek Granite indicate it is an A1‐subtype granitoid and suggest derivation by fractionation of a mantle‐derived melt with oceanic island basalt ‐ like characteristics. The hypothesis that the French Creek Granite represents fractionation of a Hohonu Dyke Swarm composition, or a mantle melt derived from the same source, is tested. Major‐ and trace‐element data are compatible with derivation of the French Creek Granite by fractionation of amphibole, clinopyroxene and plagioclase from mafic magmas, followed by fractionation of alkali and plagioclase feldspar at more felsic compositions. Although some variants of the French Creek Granite have Sr and Nd isotopic compositions overlapping those of the Hohonu Dyke Swarm, most of the French Creek Granite is more radiogenic than the Hohonu Dyke Swarm, indicating the involvement of a radiogenic crustal component. Assimilation‐fractional crystallisation modelling suggests isotopic compositions of French Creek Granite are consistent with extreme fractionation of Hohonu Dyke Swarm magmas with minor assimilation of the Greenland Group metasediments.  相似文献   

18.
沈少雄 《云南地质》2005,24(4):434-441
九道湾花岗岩体为复式岩体,由大小6个侵入体组成。按形成的先后顺序可划分为三个单元,三者均由“S”型花岗岩组成。从早期至晚期,三者岩石成分从较基性向较酸性变化,岩石结构由细粒向粗粒含斑演化,变化的有序性和单向性明显,且在空间上紧密共生,形成时间及成分、结构变化上表现出清楚的亲缘和演化关系,说明它们是同一岩浆热事件的产物,故将其归并为一超单元,称之为九道湾超单元。罗台旧单元罗台旧岩体花岗岩Rb-Sr年龄值为730Ma,应为晋宁晚期产物。  相似文献   

19.
Despite a wide latitude for interpretation of previous Rb–Sr isotopic data on the Oughterard Granite the age of this intrusion has been regarded as a critical time-marker in resolving the Caledonian evolution of Connemara. New isotopic data suggest that the age of the intrusion be revised from c. 460 Ma to c. 400 Ma, thus making the Oughterard Granite one among the many Newer Caledonian Granites in Ireland. The preferred age is 407 ± 23 Ma, and the initial 87 Sr/86Sr ratio is 0·7076 ± 1. Heterogeneity within the granite is demonstrated, which explains the difficulty in obtaining reliable isotopic ages from this intrusion.  相似文献   

20.
The Wangrah Suite granites (Lachlan Fold Belt, Australia) reflect different stages of differentiation in the magmatic history of an A-type plutonic suite. In this study we use experimentally determined phase equilibria of four natural A-type granitic compositions of the Wangrah Suite to constrain phases and phase compositions involved in fractionation processes. Each composition represents a distinct granite intrusion in the Wangrah Suite. The intrusions are the Danswell Creek (DCG), Wangrah (WG), Eastwood (EG) and Dunskeig Granite (DG), ordered from “most mafic” to “most felsic” by increasing SiO2 and decreasing FeOtotal.

Experimental investigation show that the initial water content in melts from DCG is between 2–3 wt. % H2O. If the DCG is viewed as the parental magma for the Wangrah Suite, then (1) fractionation of magnetite, orthopyroxene and plagioclase ( 20 wt. %) of the DCG composition, leads to compositions similar to that of the EG; (2) further fractionation of plagioclase, quartz, K-feldspar and biotite ( 40 wt. %) from the EG composition, leads to the DG composition. These fractionation steps can occur nearly isobarically and are confirmed by bulk rock Ba, Sr, Rb and Zr concentrations.

In contrast, the generation of the most abundant WG composition cannot be explained by fractional crystallisation from the DCG at isobaric conditions because of the high K2O content of this granite. Magma Mixing could be the process to explain the chemical distinctiveness of the Wangrah Granite from all the other granites of the Wangrah Suite.  相似文献   


设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号