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1.
Two distinct series of Variscan granitic rocks have been distinguished in the Gravanho-Gouveia area of Portugal, based on field work, variation diagrams for major and trace elements, rare earth patterns and δ18O versus total FeO diagram of rocks, anorthite content of plagioclase, BaO and P2O5 contents of feldspars and AlVI versus Fe2+ diagram for magmatic muscovite. One series consists of a late-orogenic porphyritic biotite > muscovite granite (G1), less evolved beryl-columbite pegmatites and more evolved beryl-columbite pegmatites showing gradational contacts. The other series consists of post-orogenic porphyritic muscovite > biotite granodiorite to granite (G2), slightly porphyritic muscovite > biotite granite (G3) and lepidolite pegmatites. In each series, pegmatites are derived from the parent granite magma by fractional crystallization of quartz, plagioclase, K-feldspar, biotite and ilmenite. Some metasomatic effects occur like muscovite replacing feldspars, chlorite in pegmatites of the first series and a late muscovite in pegmatites of the second series, probably due to hydrothermal fluids. The lepidolite pegmatites contain cassiterite and two generations of rutile. The first magmatic generation consists of homogeneous crystals and the second generation occurs as heterogeneous zoned crystals derived from hydrothermal fluids. The beryl-columbite pegmatites and lepidolite pegmatites also contain the first magmatic generation and the late hydrothermal generation of zoned columbite-group minerals. More evolved beryl-columbite pegmatites were converted into episyenite by intense hydrothermal alteration and regional circulation of fluids in the granitic rocks.  相似文献   

2.
Mesozoic granitic intrusions are widely distributed in the Nanling region,South China.Yanshanian granites are closely connected with the formation of tungsten deposits.The Xihuashan granite is a typica...  相似文献   

3.
丹池成矿带是我国重要锡多金属成矿带,过去对丹池成矿带成岩成矿研究主要集中在大厂矿田及五圩矿田,而对丹池成矿带北部的芒场矿田岩浆作用时代、源区特征及其成矿性缺少分析。芒场矿田岩浆活动强烈,发育隐伏斑状花岗岩及有关的花岗斑岩脉和隐伏细粒花岗斑岩及有关的白云母花岗斑岩脉。本文分析花岗斑岩脉U-Pb年龄及花岗斑岩脉和白云母花岗斑岩脉主、微量元素组成,以探讨岩浆活动时代、岩浆源区特征及其成矿潜力。芒场矿田花岗斑岩脉锆石U-Pb年龄为89.1±0.9Ma (MSWD=0.9),和丹池成矿带内大厂矿田岩浆活动时代相近,表明丹池成矿带岩浆活动时代都发生于90Ma左右。白云母花岗斑岩脉具高A/CNK比值(2.69～2.88),含高铝硅酸盐矿物白云母及在Th-Rb图上沿S型花岗岩趋势线分布,和大厂矿田S型黑云母花岗岩的特征基本一致,表明其主要为S型花岗岩。花岗斑岩脉形成时代晚于白云母花岗斑岩脉,但其具有更低的SiO_2含量和更高的MgO、Fe_2O_3~T、CaO和TiO_2含量,且在SiO_2与TiO_2、Fe_2O_3~T、Al_2O_3和P_2O_5关系图中分布于不同区域,没有线性变化关系,显示两者不是同一岩浆结晶分异演化形成的,而为不同沉积变质岩部分熔融形成的。白云母花岗斑岩脉富Al_2O_3、K_2O、Rb、Cs、Sn、W、Nb和Ta,在Rb/Ba-Rb/Sr图上位于富粘土源区,为强风化作用形成的富粘土质富稀有金属源区部分熔融形成的产物。华南西部基底发育经强风化作用形成的富粘土质富稀有金属元素沉积变质源区,为华南西南缘大规模锡矿床的形成提供了物质基础。  相似文献   

4.
阳储岭斑岩型W-Mo矿床位于江南造山带中部,是华南地区最早发现的斑岩型钨钼矿床。已探明WO3资源储量6.13万吨(平均品位0.2%),Mo资源储量1.69万吨(平均品位0.03%～0.06%),其成矿作用与中生代花岗质岩浆活动密切相关。区内发育早期花岗闪长岩和晚期二长花岗斑岩,钨矿体以细脉状和浸染状产于二长花岗斑岩体内,而花岗闪长岩内未见矿化。两期岩浆活动与钨成矿的关系尚不明确,制约其含矿差异性的因素尚不清楚。本文以花岗闪长岩和二长花岗斑岩中的黑云母为研究对象,对比研究两类岩浆结晶分异程度、氧逸度、岩浆流体卤素浓度,探讨其对钨成矿的制约。黑云母主量元素分析结果显示,阳储岭两类岩浆岩均为壳源,但显著不同于S型花岗岩的Mg/(Fe+Mg)和AlVI值,指示其具有I型花岗岩的特征。黑云母的微量元素信息显示,相对于花岗闪长岩中的黑云母(类型一),二长花岗斑岩中的黑云母(类型二)显示低的K/Rb、Nb/Ta比值,高的Rb、Cs、Nb和Ta含量,表明其分异程度较花岗闪长岩更高,更有利于钨的富集。两类黑云母所指示的岩浆氧逸度均在NNO缓冲线附近,表明其母岩...  相似文献   

5.
The Sisson Brook W–Mo–Cu deposit was formed by hydrothermal fluids likely related to the Nashwaak Granites (muscovite–biotite granite, Group I; and biotite granite, Group II) and related dykes (biotite granitic dykes, Group III; and a feldspar–biotite–quartz porphyry dyke, Group IV). Chemical data obtained using EPMA and LA-ICP-MS data of primary magmatic biotites were used to investigate magmatic processes and associated hydrothermal fluids.Trace element features of biotite in the Group I two-mica granite suggest other magmatic processes along with a simple fractional crystallization. The K/Rb ratios and compatible elements (Cr, Ti, Co, V, and Ba) in biotite from Groups II, III, and IV decrease, whereas incompatible elements including Ta, Tl, Ga, Cs, Li, and Sn increase with magma fractionation. No correlation of Cu, W and Mo with K/Rb ratios is evident, suggesting that partitioning of Cu, W, and Mo into biotite may not be entirely controlled by magma fractionation.Halogen fugacity of the parental magma of the Nashwaak Granites and related dykes, calculated from zircon saturation temperature shows that Group I has high fHF/fCl ratios (broadly higher than 0), similar to the plutons at the Henderson porphyry Mo deposit. The fHF/fCl ratios of the other groups are generally lower than 0, comparable to the Santa Rita porphyry Cu deposit. The fH2O/fHCl and fH2O/fHF ratios inferred from biotite in the Nashwaak Granites and related dykes range from 3 to 5 and from 4 to 5, respectively. The inferred oxygen fugacity shows that the dyke phases (Groups III and IV) have the oxygen fugacity around the nickel–nickel oxide buffer. The plutonic phases (Groups I and II) have the oxygen fugacity around the quartz–fayalite–magnetite (QFM) buffer at high temperatures and oxidized to nickel–nickel oxide buffer at lower temperatures. This oxidation process in the plutonic phases (Groups I and II) could be caused by H2 release at or near H2O vapor saturation at high H2O/Fe2 +. The magma associated with the biotite dykes (Group III) is more likely the source of the hydrothermal fluids at the Sisson Brook deposit since it has the highest differentiation degree and seems to have formed in an oxidized setting, necessary for Mo to concentrate in the late stage fluids.  相似文献   

6.
M.E.P. Gomes  A.M.R. Neiva 《Lithos》2005,81(1-4):235-254
Deformed Hercynian peraluminous granitoids ranging from tonalite to granite crop out in the Rebordelo–Agrochão area, northern Portugal and some of them contain tonalitic and granodioritic enclaves. Variation diagrams of major and trace elements of the rocks, biotites and sphenes show fractionation trends. The most- and the least-deformed samples of granite and their biotites also define fractionation trends. There is decrease in all rare earth element (REE) contents and increase in the Eu anomaly in REE patterns from the most- to the least-deformed samples of granite. All the granitoids define a whole-rock Rb–Sr errorchron. A whole-rock Rb–Sr isochron for the least-deformed samples of granite yields an age of 357±9 Ma and an initial 87Sr/86Sr ratio of 0.7087±0.0007. Geochemical modelling suggests that the tonalitic magma evolved by AFC (fractional crystallization of magnesiohornblende, plagioclase, quartz, biotite and ilmenite, and assimilation of metasediments) to originate tonalitic and granodioritic enclaves, granodiorite and granite. δ18O values support this mechanism. The tonalite is hybrid and derived by interaction of a mantle-derived magma and crustal materials.  相似文献   

7.
冈底斯带西段那木如岩体始新世岩浆作用及构造意义   总被引：8，自引：4，他引：4  
冈底斯带西段狮泉河南部那木如岩体岩性变化较大,其中产出大量基性岩透镜体及暗色微细粒包体,空间上与花岗岩类呈渐变过渡接触关系。本文在详细野外调研的基础上,对狮泉河-札达一带那木如花岗岩及其中基性岩石进行了系统的岩石学、地球化学和同位素年代学研究。结果表明,那木如岩体主体岩性为黑云母花岗岩,其SiO2为65%~76%,全碱含量较高,花岗岩中K2O+Na2O=5.50%~8.71%,基性岩石中则4.42%~6.7%。花岗岩类稀土元素最高含量为284.8×10-6,最低只有105.4×10-6;而基性岩类最高为120.4×10-6,最低72.48×10-6。两者稀土元素分配曲线均呈右倾平缓样式,花岗岩具有不明显Eu负异常,微量元素显示出花岗岩类和基性岩类具有相似的蛛网分布样式。两者均明显富K而亏损Nb、Ti等不活泼的HFS元素,显示出明显的岩浆混合作用趋势。4件花岗岩和基性岩样品所显示的LA-ICP-MS法锆石U-Pb年龄分别为46.11±0.78Ma、45.47±0.4Ma、46.7±2.9Ma和45.4±1.4Ma,变化在45.4~46.7Ma范围内,表明始新世早期(~46Ma)区域发生了岩浆混合作用。这一时限与冈底斯中、东部岩浆作用时代(40~52Ma)非常一致,表明始新世早期整个冈底斯发生了规模巨大的岩浆事件,暗示着印度-欧亚大陆碰撞作用在东西方向上所表现出的同时性。  相似文献   

8.
This paper reports petrography, geochemistry and Rb-Sr age data on the rare metal bearing Neoarchean fertile (Nb-Ta) granite at Allapatna and elucidates its petrogenesis and role in Nb-Ta-Li-Be mineralization. The Allapatna granite (AG) intrudes the Tonalitic-Trondhjemitic - Granodioritic (TTG) Peninsular Gneiss and analysed SiO2 (72.3-75.6 wt%), K2O (4.0-5.7wt%), Na2O (3.0-4.4wt%), CaO (0.7-1wt%), MgO (0.13-0.25wt%) and K2O/Na2O (>1) indicating evolved nature. The presence of muscovite, biotite and garnet in the mode, peraluminous nature and high initial 87Sr/86Sr ratio (0.7284±0.0083) attest to their S-type characteristics. Varying Nb/Ta ratio and high Li with moderate abundance of Cs further indicate affinity to Li-Cs-Ta (LCT) type granite-pegmatite system. TheAG showing whole rock Rb-Sr isochron age of 2803± 68 Ma, is the oldest reported fertile granite in India parental to rare metal pegmatites hosting Nb-Ta, Be, and Li resources. Partial melting of a mixed source consisting of both basement TTG rocks and metapelites has generated such type of granitic magma. Fractionation of such granitic magma possibly has given rise to the rare metal (Ta-Nb-Li-Be) bearing pegmatites intruding the nearby schist belt.  相似文献   

9.
The zoned pluton from Castelo Branco consists of Variscan peraluminous S-type granitic rocks. A muscovite>biotite granite in the pluton's core is surrounded successively by biotite>muscovite granodiorite, porphyritic biotite>muscovite granodiorite grading to biotite=muscovite granite, and finally by muscovite>biotite granite. ID-TIMS U–Pb ages for zircon and monazite indicate that all phases of the pluton formed at 310 ± 1 Ma. Whole-rock analyses show slight variation in 87Sr/86Sr310 Ma between 0.708 and 0.712, Nd310 Ma values between − 1 and − 4 and δ18O values between 12.2 and 13.6. These geological, mineralogical, geochemical and isotopic data indicate a crustal origin of the suite, probably from partial melting of heterogeneous Early Paleozoic pelitic country rock. In detail there is evidence for derivation from different sources, but also fractional crystallization linking some of internal plutonic phases. Least-squares analysis of major elements and modelling of trace elements indicate that the porphyritic granodiorite and biotite=muscovite granite were derived from the granodiorite magma by fractional crystallization of plagioclase, quartz, biotite and ilmenite. By contrast variation diagrams of major and trace elements in biotite and muscovite, the behaviours of Ba in microcline and whole-rock δ18O, the REE patterns of rocks and isotopic data indicate that both muscovite-dominant granites were probably originated by two distinct pulses of granite magma.  相似文献   

10.
The Baishitouquan amazonite and topaz-bearing granite is one of the typical high-rubidium and high-fluorine granites in the eastern part of the Mid-Tianshan belt. This intrusion is in sharp contact with Mid-Proterozoic schists, gneisses and marbles, and is composed of four zones transitional from the bottom upwards: leucogranite, amazonite granite, topaz-bearing amazonite granite and topaz quartz albitite. The Baishitouquan granite contains highly ordered K-feldspar, Li-rich mica, Mn-rich garnet, α-quartz and low temperature zircon and is chemically high in Si, K, Na, Al, Li, Rb, Cs and F, and low in Ti, Fe, Ca, Mg, P, Co, Ni, Cr, V, Sr and Ba, with Na2O<K2O. Amazonite from the amazonite granite zone contains 1867 ppm Rb. The F contents of bulk rocks are 3040 and 4597 for the amazonite granite and topaz-bearing amazonite granite zones, respectively. These two zones have δ18O values of 8.97–9.85‰ (SMOW) and show flat REE distribution patterns with strong Eu depletion. K-Ar and Rb-Sr ages of this intrusion are 226. 6 Ma and 209. 6 Ma respectively, with an initial87Sr/86Sr ratio of 0.987±0.213. The Baishitouquan granite is the product of crystallization of a low temperature, and water, rubidium and fluorine-rich magma, which may have been derived from partial melting of muscovite-rich crustal rocks. Consolidation of this granite involved two contrasting and successive stages: melt crystallization and hydrothermal metasomatism and precipitation. Various geological features of this granite were formed during the transition from the magmatic to the hydrothermal stage.  相似文献   

11.
The Late Paleozoic intraplate magmatism of the Selenga-Vitim structural zone of the Baikal region (Khamar-Daban Range) produced granitoids of different geochemical types: palingenic calc-alkaline granitoids, subalkaline monzogranites, and rare-metal Li-F granitoids and their subvolcanic analogues. Subalkaline and rare-metal granitoids occur in the periphery of the Late Paleozoic magmatic zone. Rare metal granite magmatism is manifested in this region as nearly N-S trending intrusive-dike belts comprising multiphase intrusions (Kharagul, Urugudei, and Bitu-Dzhida massifs) with an exposed area of ∼10 km2 and an age of formation from 311 to 321 Ma and series of accompanying dikes. The early phases of the intrusions are made up of biotite granites usually with fluorite, which are changed during the late stage by typical topazbearing rare-metal amazonite-albite granites. In the subvolcanic facies, thicker subalkaline dikes of monzonite porphyry, granite porphyry, and elvan are changed by ongonites, topaz rhyolites, and topazites, which occasionally serve as cement in eruptive and fluid-explosive breccias. The development of multiphase intrusions from early biotite granites to late amazonite-albite granites with Li-F mica was accompanied by an increase in SiO2 and, especially, Na2O contents, whereas the level of (FeO + Fe2O3), CaO, and K2O declined. Geochemical evolution includes an increase in the same direction in the contents of F, Li, Rb, Cs, Sn, Be, Ta, and Pb and a decrease in Ba, Sr, Zn, Zr, Th, and U. Similar evolution is also characteristic of the subvolcanic rocks, which emphasizes the genetic relation of the whole intrusive-dike complex of the Khamar-Daban province. Significant differences were detected in the distribution of K, Ba, Sr, and Zr between the calc-alkaline granitoids and rare-metal Li-F granites. The continental crust-normalized patterns of the raremetal granites show positive anomalies for Li, Rb, Nb, and Pb. The rare-metal Li-F granites could not be produced by palingenesis only, and their formation required specific conditions causing extensive accumulation of characteristic trace elements. During the evolution of granite melts, Li, Rb, Ta, Nb, Sn, W, and F are extensively accumulated in late intrusive phases, which indicates an important role of the processes of magmatic and fluid-magmatic differentiation during their formation. The composition and isotope geochemical characteristics of the supposed magma source material correspond to the ancient Precambrian continental crust with a mean model age of more than 1200 Ma.  相似文献   

12.
沙麦钨矿床位于内蒙古东乌旗地区,是该区目前已探明的中型岩浆热液型钨矿床。矿体主要赋存在黑云母二长花岗岩和黑云母二长花岗斑岩中,对这两种花岗质岩石的岩相学、岩石地球化学和LA-ICP-MS锆石U-Pb年代学进行了研究。结果表明,黑云母二长花岗岩锆石U-Pb年龄为135.6±1.6 Ma和136.3±1.8 Ma,黑云母二长花岗斑岩锆石U-Pb年龄为138.6±1.1 Ma,二者侵位时间均为早白垩世。两种花岗质岩体具有富SiO2(73.73%～78.23%)、高钾钠(Na2O+K2O)(7.56%～8.89%)、贫MgO(0.09%～0.20%)、贫CaO(0.51%～0.89%)、贫TiO2(0.03%～0.12%)的特征,属于过铝质-高钾钙碱性系列;微量元素富集Rb、K、Th和U,相对亏损Sr、Ba、Nb、P和Ti元素,具有强烈的Eu负异常,具有较高的FeOT含量,较高的FeOT/MgO和FeOT/(FeOT+M...  相似文献   

13.
A small body of mafic texturally and compositionally varied igneous intrusive rocks corresponding to redwitzites occurs at Abertamy in the Western pluton of the Krušné hory/Erzgebirge granite batholith (Czech Republic). It is enclosed by porphyritic biotite granite of the older intrusive suite in the southern contact zone of the Nejdek-Eibenstock granite massif. We examined the petrology and geochemistry of the rocks and compared the data with those on redwitzites described from NE Bavaria and Western Bohemia.The redwitzites from Abertamy are coarse- to medium-grained rocks with massive textures and abundant up to 2 cm large randomly oriented biotite phenocrysts overgrowing the groundmass. They are high in MgO, Cr and Ni but have lower Rb and Li contents than the redwitzites in NE Bavaria. Compositional linear trends from redwitzites to granites at Abertamy indicate crystal fractionation and magma mixing in a magma chamber as possible mechanisms of magma differentiation. Plots of MgO versus SiO2, TiO2, Al2O3, FeO, CaO, Na2O, and K2O indicate mainly plagioclase and orthopyroxene fractionation as viable mechanisms for in situ differentiation of the redwitzites.The porphyritic biotite monzogranite enclosing the redwitzite is the typical member of the early granitic suite (Older Intrusive Complex, OIC ) with strongly developed transitional I/S-type features. The ages of zircons obtained by the single zircon Pb-evaporation method suggest that the redwitzites and granites at Abertamy originated during the same magmatic period of the Variscan plutonism at about 322 Ma.The granitic melts have been so far mainly interpreted to be formed by heat supply from a thickened crust or decompression melting accompanying exhumation and uplift of overthickened crust in the Krušné hory/Erzgebirge due to a previous collisional event at ca. 340 Ma. The presence of mafic bodies in the Western pluton of the Krušné hory/Erzgebirge batholith confirms a more significant role of mantle-derived mafic magmas in heating of the sources of granitic melts than previously considered.  相似文献   

14.
叶茂  赵赫  赵沔  舒珣  张若曦  杨水源 《岩石学报》2017,33(3):896-906
灵山花岗岩体在平面上为一环状分布的侵入体,中心为角闪石黑云母花岗岩,外围为黑云母花岗岩。在角闪石黑云母花岗岩中分布有大量的暗色镁铁质微粒包体。黑云母是大多数中酸性火成岩中比较重要的一种镁铁质矿物,它能很好地反映寄主岩浆的属性和成岩时的物理、化学条件,因此,本文对这两种花岗岩及镁铁质微粒包体中的黑云母开展了系统的岩相学观察和电子探针化学组成研究,探讨灵山岩体的物质来源、成岩条件和岩浆的混合作用过程。研究结果表明两种花岗岩体的黑云母具有不同化学成分,而暗色镁铁质微粒包体中黑云母的化学成分则变化较大。三种黑云母均在低氧逸度条件下晶出。两种花岗岩中的黑云母均富Fe贫Mg,属于铁质黑云母,含铁系数[(Fe~(3+)+Fe~(2+))/(Fe~(3+)+Fe~(2+)+Mg~(2+))]分别为0.65~0.70,0.72~0.78,FeOT/MgO均接近7.04。MF值[2×Mg/(Fe~(2+)+Mg+Mn)]分别为0.64~0.76和0.48~0.60,指示两种花岗岩的物质来源都是以壳源为主。镁铁质微粒包体中黑云母的MF值变化范围比较大,为0.63~1.06,为铁质黑云母到镁质黑云母,暗示包体岩浆经历过不同程度的岩浆混合作用。镁铁质微粒包体中部分黑云母与角闪石黑云母花岗岩中黑云母的结晶条件相似,而部分则有明显差异,推测是由于基性的镁铁质包体岩浆注入到酸性的花岗岩浆是一个连续多阶段的过程。  相似文献   

15.
The Quérigut mafic–felsic rock association comprisestwo main magma series. The first is felsic comprising a granodiorite–tonalite,a monzogranite and a biotite granite. The second is intermediateto ultramafic, forming small diorite and gabbro intrusions associatedwith hornblendites and olivine hornblendites. A U–Pb zirconage of 307 ± 2 Ma was obtained from the granodiorite–tonalites.Contact metamorphic minerals in the thermal aureole providea maximum emplacement pressure of between 260 and 270 MPa. Petrographiccharacteristics of the mafic and ultramafic rocks suggest crystallizationat <300 MPa, demonstrating that mantle-derived magmas ascendedto shallow levels in the Pyrenean crust during Variscan times.The ultramafic rocks are the most isotopically primitive components,with textural and geochemical features of cumulates from hydrousbasaltic magmas. None of the mafic to ultramafic rocks havedepleted mantle isotope signatures, indicating crustal contaminationor derivation from enriched mantle. Origins for the dioritesinclude accumulation from granodiorite–tonalite magma,derivatives from mafic magmas, or hybrids. The granitic rockswere formed from broadly Proterozoic meta-igneous crustal protoliths.The isotopic signatures, mineralogy and geochemistry of thegranodiorite–tonalites and monzogranites suggest crystallizationfrom different magmas with similar time-integrated Rb/Sr andSm/Nd isotope ratios, or that the granodiorite–tonalitesare cumulates from a granodioritic to monzogranitic parent.The biotite granite differs from the other felsic rocks, representinga separate magma batch. Ages for Quérigut and other Pyreneangranitoids show that post-collisional wrenching in this partof the Variscides was under way by 310 Ma. KEY WORDS: Variscan orogeny; Pyrenees; Quérigut complex; epizonal magmatism; post-thickening; mafic–felsic association  相似文献   

16.
侵位于狮泉河结合带南缘和结合带南侧古生代地层的晚白垩世侵入岩中,解体出一套中性侵入岩,呈岩珠、岩枝状产出,主要岩石类型为石英闪长岩、英云闪长岩和花岗闪长岩,其中石英闪长岩和英云闪长岩的LA-ICP-MS锆石U-Pb年龄为157.3±1.5 Ma(MSWD=0.96)和159.3±3.7 Ma(MSWD=2.30),指示岩浆活动时间为晚侏罗世牛津期。该岩体Si O2含量在59.65%~64.57%之间,K2O为1.95%~2.69%之间,Na2O为1.97%~3.42%,K2O/Na2O在0.74~1.29之间,P2O5为0.085%~0.139%之间,Al2O3为13.63%~15.68%之间,A/CNK为0.89~1.07,属于准铝质—弱过铝质花岗岩。岩石稀土元素总量(∑REE)为107.37×10-6~148.33×10-6,LREE/HREE在6.77~9.67之间,轻重稀土分异程度强,δEu为0.63~0.84,铕略具负异常;在原始地幔标准化蛛网图上,富Rb、Th等大离子亲石元素(LILE)而亏损Nb、Ta、Ti和P等高场强元素(HFSE),具有岛弧钙碱性岩浆的地球化学性质。从岩石学、岩石地球化学特征研究表明,该时期岩石显示同源岩浆演化的特征,具有典型I型花岗岩特征,形成于板块俯冲下的岛弧环境。表明狮泉河一带在晚侏罗世已经存在俯冲型花岗岩,研究认为该时期岩浆活动很可能是狮泉河洋盆北缘(北亚带)持续扩张时,向南缘(南亚带)及中冈底斯带北缘俯冲的产物。  相似文献   

17.
Summary The F-rich Hongshan pluton in the eastern Nanling Range, southern China, is a topaz-bearing albite leucogranite. It is distinctive from other topaz-bearing felsic rocks in South China with respect to age, size, geochemical evolution and topaz mode and morphology. The Hongshan granites are highly peraluminous and characterized by high K2O/Na2O, Si, Rb, Cs, Nb, Ta and F, and low Ca, Ba, Sr, Zr, Hf, P, K/Rb, Zr/Hf and Eu/Eu*. The granites show significant trace-element variations with magma evolution, with increasing Rb, Cs, Nb, Ta, Sn, W and decreasing Sr, Ba, Zr, Hf, Y, REE, Pb, Th, K/Rb, Zr/Hf, Th/U and Eu/Eu*. These changes dominantly reflect fractional crystallization of plagioclase, biotite and accessory minerals such as zircon and monazite. The granites also exhibit a decrease in ɛNd(t = 225 Ma) from −7.9 to −11.7 with magma evolution. Modeling shows that the Nd isotopic variation could result from assimilation of the Taoxi Group wall rocks during fractional crystallization. The Hongshan pluton also shows spatial geochemical variations; the most evolved parts are located in the southeastern part of the pluton, which would be the most likely target area for rare-metal mineralization commonly associated with other topaz-bearing granites. Zircon grains from two rock types in the Hongshan body were analyzed in situ for U–Pb ages and Hf isotopic values. The concordant zircon grains mostly range from 218 to 230 Ma with an average of 224.6 ± 2.3 Ma (Indosinian). Some zircons with different internal structures and Hf isotope compositions, as well as monazite fragments, yield U–Pb ages of ca. 280 to 240 Ma, suggesting older thermal events in the studied area. The ɛHf(t) of these older zircons is strongly negative (−12.3), implying a crustal source with a Paleoproterozoic model age, similar to that for the Proterozoic Zhoutan Group. The main (∼225 Ma) zircon population exhibits less negative ɛHf(t) (−3.0 to −7.6) and Mesoproterozoic model ages, suggesting that the original magma of the Hongshan granite was generated from deeper Mesoproterozoic crust.  相似文献   

18.
铷是一种具有广泛用途的稀有金属。湖南桂东县小江地区曾于1958年地质预查确定为砂锡矿,但未见进一步工作的报道。本次工作系统采集了矿区的细粒二云母花岗岩、黑云母斑状花岗岩、粗粒黑云母花岗岩的岩石样品。精细测试表明,三者是同期花岗岩浆多阶段演化的产物,随着演化程度的增高,岩体内的稀有金属含量逐渐增高。细粒二云母花岗岩属于重稀土富集型花岗岩,稀土配分模式表现出岩浆晚期富挥发份组成而造成的四分组效应,其基岩中的Rb2O含量超过0.1%,达到花岗岩型铷矿床的工业品位;Nb的含量达到伴生矿的要求,具有全岩矿化的特征;风化壳中Y的含量达到许多离子吸附型矿床的钇含量。Rb元素主要赋存在二云母花岗岩的钾长石和黑云母中,Y主要赋存在萤石中。按照二云母花岗岩的厚度50m计算,Rb2O储量约为16万吨。除此之外,矿区还应该注意寻找伟晶岩型稀有金属矿脉,并重视矿区风化壳发生离子吸附型重稀土矿化的可能性。  相似文献   

19.
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20.
In this paper we consider the mechanisms by which the mineralogy and composition of the margins of the Notch Peak granitic stock, Utah, were affected by calcareous argillite country rocks. The contact zone of the granite relative to the unaffected granite is enriched in: K2O from about 4 to 10 wt.%, Rb from 250 to 510 ppm, Sr from 150 to 790 ppm and Ba from 250 to 2160 ppm. Locally, some of the intrusive rocks at the contact are nearly devoid of quartz and can be classified as syenites. The initial 87Sr/86Sr ratios range from 0.7069 in the unaffected rocks to 0.7100 in the potassium-enriched samples, approaching the values of the calc-silicate country rocks of about 0.7110.Calculations show that the characteristics of the contact zone near the top of the stock are the result of a number of sequential processes. CO2-rich fluids derived from the heated calcareous argillites apparently caused a shift in the phase boundaries in the magma, enhancing accumulation of K-feldspar and high-Ca augite at the expense of other phases. The accumulation resulted in the high Ba and Sr concentrations in some samples. However, the high K2O and Rb concentrations and magmatic 18O values indicate infiltration of magmatic fluid emanating from the solidifying lower portions of the pluton subsequent to solidification of the cap. The minimum fluid-rock ratios of 4.6 by mass, calculated on the basis of K2O and Rb concentration shifts, indicate that a substantial amount of the fluid was channeled through this contact zone. The desilication of the rocks forming the syenitic samples at the contact apparently occurred when a chemical potential gradient of silica between the granite and wall-rocks was established as quartz was consumed in the wall-rocks during calc-silicate reactions. The infiltrating magmatic fluid probably acted as a medium for transport of silica across the contact and perhaps exchange of Sr between the country rocks and the intrusion where up to 30% of strontium in the granitic and syenitic samples from the contact zone was derived from the calc-silicates. The syenitic rocks cannot be explained by desilication reactions involving assimilation of the calc-silicates by the granite magma. The results of this study show that fluids interacting with the country rocks need to be considered to explain the effects of country rocks on the composition of the margins of granitic intrusions.  相似文献   

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