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1.
Summary The Palim granite, hosted by the metasedimentary country rocks in the Bastar tin province, is a heterogeneous pluton that comprises hornblende granite, biotite granite and two-mica granite. Spherical inhomogeneous surmicaceous enclaves occur within the granites with coarse grained cores of muscovite mantled by finer muscovite-quartz-biotite (± sillimanite) rims. Geochemical features imply that the granites are highly evolved and geochemically distinct. Petrographic and geochemical considerations point towards a transition from metaluminous I-type hornblende-bearing granite in the south to peraluminous volatile-enriched S-type like lithologies (biotite and two-mica granites) towards north. Modeling of highly incompatible elements such as Nb and Cs, implies 31 to 33% assimilated fractional crystallization of a melt with an initial composition close to that of the hornblende granite to form the two-mica granite. Hornblende geobarometry, plagioclase-hornblende thermometry (in hornblende granite) and phengite barometry (in two-mica granite), yield P-T estimates of 5–7 kb/725°–760 °C, and 6 kb/700 °C, respectively. The study further implies that a genetic link exists between granite magmatism and the formation of tin pegmatites in the region. The preponderance of peripheral pegmatites to the north-east of the Palim granite is regarded a result of outward crystal-melt fractionation and tectonic tilting of the pluton. Received October 21, 1999; revised version accepted December 12, 2000  相似文献   

2.
The Durulgui granite?pegmatite system unites the Dedova Gora granite massif and pegmatite field with the Chalotskoe beryl deposit. New geochronological data on micas from porphyric biotite granites, fine-grained biotite granites, two-mica granites, and Be-bearing pegmatites are discussed. The plateau age of 128.5(±1.5)–131.2(±1.5) should be considered as indicating the formation time of the granite?pegmatite system as a whole. The age of the system implies the possibility of its formation owing to several magmatic pulses. This assumption concerns porphyric and fine-grained biotite granites and two-mica and muscovite granites, the contact between which is locally sharp. At the same time, the succession “two-mica granites → muscovite granites → granite?pegmatites → microcline pegmatites → microcline?albite pegmatites → albite pegmatites” demonstrates gradual facies transitions between rocks, which indicates their emplacement during a single magmatic pulse.  相似文献   

3.
西昆仑大红柳滩地区相继发现了众多伟晶岩型锂铍矿床,已成为我国新的锂资源基地。目前关于这些锂铍花岗伟晶岩的成因多强调其源于地壳深熔形成的二云母二长花岗岩的结晶分异,但研究区出露的同时代的黑云母花岗岩与成矿的关系没有被讨论和关注。为了探讨黑云母花岗岩与成矿的关系,作者对龙门山矿区黑云母花岗岩、二云母二长花岗岩、花岗伟晶岩以及与成矿相关的细晶花岗岩开展了详细的地球化学及年代学研究。结果显示：1)黑云母花岗岩与二云母二长花岗岩具相似的地球化学特征,富集Rb、La和Nd,亏损Ba、Nb、Sr、P和Ti元素,均表现出S型花岗岩的特征;2)从黑云母花岗岩→二云母二长花岗岩→细晶花岗岩,表现出连续分异演化的特征;3)黑云母花岗岩的锆石LA-ICP-MS U-Pb年龄为216.8±0.85Ma,二云母二长花岗岩的锆石SIMS U-Pb年龄为216.0±1.5Ma,细晶花岗岩的锆石LA-ICP-MS U-Pb年龄为209.5±1.2Ma,花岗伟晶岩的锡石LA-MC-ICP-MS U-Pb年龄为211.3±5.0Ma,这意味着从黑云母花岗岩到二云母二长花岗岩与细晶花岗岩的形成时间是连续的并且是接近的。基于此...  相似文献   

4.
Two mineralogically different rare metal granites located in two distinct terranes from the Tuareg area are compared: the Tin-Amzi granite in the north of the Laouni Terrane and the Ebelekan granite in the Assodé–Issalane Terrane.The Tin-Amzi granite is enclosed within Eburnean granulitic gneisses, and consists of albite, quartz, protolithionite, K-feldspar and topaz granite (PG). The accessory minerals include columbite tantalite, U- and Hf-rich zircon, Th-uraninite, wolframoixiolite and wolframite. This facies is characterised by a mineralogical evolution from the bottom to the top underlined by a strong resorption of K-feldspar and albite and the crystalliK-feldspar of more abundant topaz and protolithionite II which is further altered in muscovite and Mn-siderite. It is underlain by an albite, K-feldspar, F-rich topaz, quartz and muscovite granite (MG), with W–Nb–Ta oxides, wolframite, Nb-rutile, zircon and scarce uranothorite as accessories.The Ebelekan granite intrudes into a coarse-grained biotite granite enclosed within upper amphibolite-facies metasediments. It comprises a zinnwaldite, albite, topaz porphyritic granite (ZG) with “snow ball” quartz and K-feldspar. The accessories are zircon, monazite, uranothorite, Ta bearing cassiterite, columbite tantalite and wodginite. It is capped by a banded aplite-pegmatite (AP).The geochemistry of Tin-Amzi and Ebelekan granites is nearly comparable. Both are peraluminous (A/CNK=1.10–1.29; ASI=1.17–1.31), sodolithic and fluorine rich with high SiO2, Al2O3, Na2O+K2O, Rb, Ga, Li, Ta, Nb, Sn and low FeO, MgO, TiO2, Ba, Sr, Y, Zr and REE contents. These rare metal Ta bearing granites belong to the P-poor subclass, relating to their P2O5 content ( 0.03–0.15 wt.%). Nevertheless, they are distinguished by their concentration of W, Sn and Ta. The Tin-Amzi granite is W–Ta bearing with high W/Sn ratio whereas the Ebelekan granite is Ta–Sn bearing with insignificant W content.At Tin-Amzi the W–Nb–Ta minerals define a sequence formed by W-columbite tantalite followed by wolframoixiolite and finally wolframite showing the effect of hydrothermal overprinting with an extreme W enrichment of the fluids. At Ebelekan, the Sn–Nb–Ta oxides follow a Mn sequence: manganocolumbite→manganotantalite→wodginite+titanowodginite→cassiterite that represents a trend of primary crystallisation resulting from progressive substitution Fe→Mn and Nb→Ta during the magmatic fractionation.  相似文献   

5.
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   

6.
In the Segura area, Variscan S-type granites, aplite veins and lepidolite-subtype granitic aplite-pegmatite veins intruded the Cambrian schist-metagraywacke complex. The granites are syn D3. Aplite veins also intruded the granites. Two-mica granite and muscovite granite have similar ages of 311.0 ± 0.5 Ma and 312.9 ± 2.0 Ma but are not genetically related, as indicated by their geochemical characteristics and (87Sr/86Sr)311 values. They correspond to distinct pulses of magma derived by partial melting of heterogeneous metapelitic rocks. Major and trace elements suggest fractionation trends for: (a) muscovite granite and aplite veins; (b) two-mica granite and lepidolite-subtype aplite-pegmatite veins, but with a gap in most of these trends. Least square analysis for major elements, and modeling of trace elements, indicate that the aplite veins were derived from the muscovite granite magma by fractional crystallization of quartz, plagioclase, K-feldspar and ilmenite. This is supported by the similar (87Sr/86Sr)311 and δ18O values and the behavior of P2O5 in K-feldspar and albite. The decrease in (87Sr/86Sr)311 and strong increase (1.6‰) in δ18O from two-mica granite to lepidolite-subtype aplite-pegmatite veins, and the behaviors of Ca, Mn and F of hydroxylapatite indicate that these veins are not related to the two-mica granite.  相似文献   

7.
The Bafoussam area in western Cameroon is part of the Central African Orogenic Belt. It is dominated by granitoids which belong to the Pan-African syn- to post-collisional post-650 Ma group. Syenogranites are predominant, but alkali-feldspar granite, monzogranite, quartz-monzonite and quartz-monzodiorite occur as well. Four granitoid suites, biotite granitoids and deformed biotite granitoids with amphibole, megafeldspar granitoids with megacrysts and two-mica granitoids with primary muscovite and igneous garnet are distinguished. The granites can be assigned to high-K calc-alkalic to shoshonitic series. The partly shoshonitic biotite granitoids are metaluminous to weakly peraluminous and can be labelled as a highly fractionated I-type suite. The megafeldspar granitoids are weakly peraluminous with I-type character whereas the two-mica granitoids are weakly to strongly peraluminous and belong to an S-type suite. Emplacement ages at 558–564 Ma for the two-mica granitoids have been dated from monazite by the EMP Th–U–Pb method.The REE in the biotite granitoids are moderately fractionated with (La/Lu)N = 23–38. Enrichment of Nb and Ta varies by one order of magnitude. The megafeldspar granitoids show homogeneous and strongly fractionated REE patterns with (La/Lu)N = 27–42. The primitive mantle-normalized element patterns are homogeneous with marked negative Ba, Nb, Ta, Sr, Eu and Ti anomalies. The two-mica granitoids are characterized by low to moderate total REE contents with strongly fractionated REE expressed by (La/Lu)N ranging from 7 to 59. The negative Nb and Ta anomalies are less significant. Nd and Sr whole-rock isotope data confirm different sources for the granitoid suites. The source of the I-type biotite granitoids was probably a juvenile mantle which has been variably metasomatized. The source of the I-type megafeldspar granitoids is characterized by juvenile mantle and lower crust components. Anatectic melts of the upper continental crust with variable contribution of lower continental crust or mantle melts can explain the heterogeneous isotopic signatures of the S-type two-mica granitoids. It is suggested that the melting of these sources was successively initiated by the rising isotherms during a syn- to post-collisional setting which followed a subduction.  相似文献   

8.
The tin‐ and tantalum‐bearing pegmatites of the Bynoe area are located in the western Pine Creek Geosyncline. They are emplaced within psammopelitic rocks in the contact aureole of the Two Sisters Granite. The latter is a Palaeoproterozoic, fractionated, granite with S‐type characteristics and comprises a syn‐ to late‐orogenic, variably foliated, medium‐grained biotite granite and a post‐orogenic, coarse‐grained biotite‐muscovite granite. The pegmatites comprise a border zone of fine grained muscovite + quartz followed inward by a wall zone of coarse grained muscovite + quartz which is in turn followed by an intermediate zone of quartz + feldspar + muscovite. A core zone of massive quartz is present in some occurrences. Feldspars in the intermediate zone are almost completely altered to kaolinite. This zone contains the bulk of cassiterite, tantalite and columbite mineralization. Fluid inclusions in pegmatitic quartz indicate that early Type A (CO2 + H2O ± CH4) inclusions were trapped at the H2O‐CO2 solvus at P～100 MPa, T～300°C (range 240–328°C) and salinity ～6 wt% eq NaCl. Pressure‐salinity corrected temperatures on Type B (H2O + ～20% vapour), C (H2O + < 15% vapour) and D (H2O + halite + vapour) inclusions also fall within the range of Type A inclusions. Oxygen and hydrogen isotope data show that kaolin was either formed in isotopic equilibrium with meteoric waters or subsequent to its formation, from hydrothermal fluid, underwent isotopic exchange with meteoric waters. Fluid inclusion waters from core zone quartz show enrichment in deuterium suggesting metamorphic influence. Isotope values on muscovite are consistent with a magmatic origin. It is suggested that the pegmatites were derived from the post‐orogenic phase of the Two Sisters Granite. Precipitation of cassiterite took place at about 300°C from an aqueous fluid largely as a result of increase in pH due to feldspar alteration.  相似文献   

9.
通过显微镜下观察和电子探针成分分析,发现赣南富城强过铝质花岗岩中存在3种类型白云母,即原生白云母、交代型白云母和次生白云母,其平均晶体化学式分别为:K1.62Na0.06Fe0.32Mg0.39Ti0.02Al4.89Si6.54O10(OH)4(原生白云母)、K1.55Na0.07Fe0.43Mg0.24Ti0.03Al4.96Si6.50O10(OH)4(交代型白云母)、K1.51Na0.07Fe0.27Mg0.21Ti0.00Al4.98Si6.65O10(OH)4(次生白云母)。根据富城花岗岩主要造岩矿物的结晶顺序(斜长石→钾长石→黑云母→白云母→石英),结合白云母、黑云母稳定曲线及合成花岗岩初融曲线对比分析,富城强过铝质花岗岩中交代型白云母是在花岗岩结晶过程中交代较早晶出的黑云母形成的,其形成温度高于花岗岩熔体的固相线温度(～650℃),故应归属于原生白云母。本文提出根据岩石学宏观特征、岩石化学特征及岩相学微观特征区分花岗岩中原生白云母与次生白云母的综合鉴别方法。  相似文献   

10.
A magmatic gap from 1.82 to 1.76 b.y. in the Lake Superior region represents the transition from synorogenic calc-alkaline igneous activity of the Penokean Orogeny to anorogenic potassic granophyric granite and ignimbrite. This paper deals with the petrogenetic evolution of 1.76 b.y. granites which represent a major change in source material and conceivably tectonic setting. Although perhaps related to a termination of the Penokean Orogeny by melting of a tectonically thickened crust during collision, these post-Penokean granites may represent the initial appearance of anorogenic, potentially rift-related igneous activity that was widespread throughout North America during late Precambrian time.These post-Penokean granites are too iron-rich and Al-poor to be considered calc-alkaline, a compositional feature shared with most anorogenic igneous activity of continental regions. Within this suite in central and northern Wisconsin, regional differences in composition indicate at least two different granite magma types: one a metaluminous suite of biotite and biotite-hornblende granite and a peraluminous suite of two-mica granite. The systematic compositional differences (Al, Fe/Mg, Ba/Sr, REE) in the two magma suites are likely the result of small differences in residue mineralogy and/or source composition. In general, the degree of fusion was small (10%) and probably of relatively young Penokean material. Both suites have a range of composition due to feldspar dominated fractional crystallization. Removal of the accessory minerals apatite, zircon, and allanite resulted in the REE depletion with differentiation of the two-mica granites.The granites intruded into the upper levels of the crust, and the appearance of primary celadonitic muscovite and subsolvus alkali feldspars (silicic members only) in the two mica granites indicate crystallization at depths of 10–11 km. The biotite granites contain both hypersolvus and subsolvus members and are intruded at depths less than 6 km with the more shallow members generating major volumes of ignimbrite. As a marked departure from the characteristics of most anorogenic granites, these melts crystallized at fairly oxidizing conditions (higher for the two-mica suite) as reflected in the composition of biotite, predominance of magnetite over ilmenite, and early appearance of the Fe-Ti oxides in the crystallization sequence.  相似文献   

11.
Improved precision of radiometric dating of ore deposits can provide information about the thermal history of hydrothermal circulation in cooling plutons. In Jales a Hercynian porphyritic two-mica granite and pre-Ordovician mica schists are cut and intensely altered by the Campo gold-quartz vein. The unaltered granite must be younger than 320 ± 6 Ma, and gives mica Rb-Sr ages of 308.5 ± 2.4 (1) Ma (muscovite) and 294.5 ± 1.1 Ma (biotite). Alteration muscovites from the granite give a weighted mean Rb-Sr age of 308.1 ± 1.5 Ma, and a mean 39Ar-40Ar age of 300.7 ± 2.8 Ma. Alteration muscovites from the mica schists give similar 39Ar-40Ar ages, averaging 303.0 ± 2.8 Ma. The results suggest that circulation of the Campo mineralising fluids took place no more than 2–4 Ma after the granite cooled through the muscovite Rb-Sr closure temperature, about 500 °C, and that subsequent cooling to biotite closure at about 300 °C took place at less than 14°C/Ma. The mean cooling rate following emplacement was 15 to 25 °C/Ma. The most detailed comparable published data, for the Cornubian ore field, imply much faster cooling rates.  相似文献   

12.
江西大吉山钨矿成矿年代学研究   总被引：25，自引：0，他引：25  
大吉山钨矿床是一个大型的石英脉型钨矿床。矿床在空间上与燕山早期复式花岗岩相伴。利用云母单矿物的K Ar同位素年龄测试方法,测定了矿床有关的花岗岩及矿脉的同位素年龄。结果表明,黑云母花岗岩中黑云母K Ar年龄为(160 3±3 03)Ma和(164 2±3 5)Ma,二云母花岗岩中白云母K Ar年龄为(160 6±2 8)Ma,含钨石英脉中白云母K Ar年龄为(152 6±2 35)Ma和(158 1±2 8)Ma。钨成矿与花岗岩的成岩基本不存在时差,推测含钨石英脉的形成可能与岩浆分异出的富含矿化剂和成矿元素的流体有关。  相似文献   

13.
It remains poorly constrained whether remobilization of Sn from granites and prograde skarns plays an essential role in forming economic (skarn-type) tin mineralization. Using both electron probe microanalysis and laser ablation–inductively coupled plasma–mass spectrometry methods, in-situ Sn contents, as well as major elements, were analyzed for numerous silicates and magnetite from fresh granite, altered granite, and skarn at the large Furong Sn deposit (530,000 t Sn @ 0.8% Sn) in the Nanling Range, South China. Hornblende and biotite in fresh granite are the main Sn-bearing phases (Sn = 44–321 ppm), while plagioclase and K-feldspar are poor in Sn (< 5 ppm). In altered granite, tin is hosted mainly by hydrothermal muscovite (299–583 ppm) replacing plagioclase, but rarely by chlorite (mostly <10 ppm) replacing hornblende and biotite. In contrast, most silicates (garnet, diopside, vesuvianite, pargasite and epidote) and magnetite from tin skarn are Sn-rich (47–44,241 ppm), except for Sn-poor phlogopite and scapolite (< 10 ppm). In particular, garnet, pargasite, and epidote reach tin concentrations in the percent range. Tin generally enters the stannous silicates and magnetite through substitutions for octahedral Alvi and Fe3+. Comparisons of Sn contents between magmatic and hydrothermal minerals in granite, prograde and retrograde minerals related to tin skarn indicate that remobilization of Sn from granite and prograde skarn is not a pre-requisite to form tin mineralization.  相似文献   

14.
Petrochemical and Rb-Sr, K-Ar and Sm-Nd isotopic data presented for the Mitterteich granite provide information on whole rock and mineral compositional characteristics, intrusion and cooling history, and protolith nature and put further constraints on the Variscan magmatic evolution in north-east Bavaria.The compositional characteristics classify the Mitterteich granite as a peraluminous (monzo-)granite (SiO2 67.3–73.5 wt.% ). Values for K2O/Na2O (> 1.2 and Al2O3/(CaO + N2O + K2O) (>1.1) are in the range of S-type granites. The rare earth elements show fractionated chondrite-normalized patterns (La N /Yb N =24–19) with negative Eu anomalies (Eu N /Eu N *=0.35–0.19). The micas have restricted ranges of major element composition, but reveal notable variations in trace element concentrations. Different biotite fractions of single specimens show a trend to lower concentrations of compatible elements in the finer fraction which can be explained as a result of asynchronous growth during the fractionation process. The PT conditions of crystallization of the magma based on muscovite and biotite is 600–640°C at 3 kbar. Regression of the whole rock samples gives an isochron corresponding to a 87Rb-87Sr age of 310 ± 7 Ma, initial 87Sr/86Sr of 0.7104±0.0010 (2 errors) and MSWD =0.03. Muscovite and biotite yield concordant K-Ar ages between 310 and 308 Ma, indicating a fast cooling rate of the granite intrusion. Nd T310values average –4.2±1.0. Nd model ages of 1.4 Ga suggest a source region of mid-Proterozoic age.The Rb-Sr isochron age and initial Sr ratio of the Mitterteich granite are indistinguishable from those of the adjacent Falkenberg granite, establishing a genetic link. However, the K-Ar mica ages suggest that the Mitterteich granite must have undergone a faster uplift or cooling history than Falkenberg. Confronted with the geochronological record of granite emplacement in north-east Bavaria, the new results substantiate the view of three key periods of magmatic activity around 330–325, 315–305 and 290 Ma.  相似文献   

15.
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   

16.
杨长青  巴燕  焦静华  姚帅 《地质论评》2024,70(2):2024020033-2024020033
高分异花岗岩因其特殊的成矿专属性而受到广泛关注。谢通门县梅巴切勤复式岩体出露于冈底斯成矿带,由黑云母正长花岗岩、二云母正长花岗岩和白云母正长花岗岩构成,钨锡矿体处于白云母正长花岗岩内部或外接触带。在详细地质研究的基础上,用LA-ICP-MS方法获得了129.7±0.9Ma（黑云母正长花岗岩）、128.4±1.6Ma（二云母正长花岗岩）与129.5±0.5Ma（白云母正长花岗岩）的206Pb/238U加权平均年龄。花岗岩具有高SiO2、K2O、K2O+Na2O,低Al2O3、CaO、MgO的特点,相对富集Zr、Nb、Ce、Y、Hf等元素,亏损Ti、Ba、Sr、P等元素,具有较高的10000Ga/Al、全岩Zr饱和温度和明显的Eu负异常,显示其为高分异A型花岗岩,形成于碰撞后的伸展环境。白云母正长花岗岩是分异演化的最终产物,为稀有金属花岗岩,存在较明显的稀土元素四分组效应,其更为强烈的熔体-流体作用造成W、Sn、Nb、Ta等稀有金属进一步富集,碰撞后的伸展环境以及热扰动在提供通道和热源的同时,也延长了岩浆分异演化时间,有利于成矿物质在岩浆演化的晚期阶段富集和品位高、规模大的稀有金属矿床的形成。梅巴切勤地区良好的成矿地质条件预示着其具有形成大-超大型矿的潜力,该研究对于冈底斯成矿带W、Sn、Nb、Ta等稀有金属找矿有着重要的引导和参考意义。  相似文献   

17.
西藏荣玛乡冈塘错花岗岩岩体分布于南羌塘地块中,由中粗粒巨斑状黑云母二长花岗岩和中细粒含电气石二云母二长花岗岩组成。中粗粒花岗岩的锆石测年结果为214.4±4.0Ma,中细粒花岗岩锆石测年结果为222.2±6.6Ma,均为晚三叠世。中粗粒花岗岩A/CNK为1.00~1.09,刚玉标准分子的百分含量均大于1%(1.84~3.04%),Al2O3的含量均大于14%,属于含白云母过铝质花岗岩。中细粒花岗岩的A/CNK1.1,刚玉标准分子的百分含量均大于1%(2.40~3.42%),Al2O3的含量均大于14%,属于强过铝质S型花岗岩。虽主量元素特征相近,但两类花岗岩的微量元素特征差异较大。根据锆石饱和温度计得出中粗粒花岗岩为高温类型的花岗岩,中细粒花岗岩为低温类型的花岗岩。结合区域地质背景推断中细粒花岗岩形成可能与三叠纪俯冲碰撞作用有关,而中粗粒花岗岩为碰撞后拆沉岩浆底侵的结果。冈塘错岩体中两种成因类型不同的花岗岩的形成时代对古特提斯洋的演化以及缝合带增生杂岩的形成具有时代约束意义。  相似文献   

18.
Phase relations of F-rich Kymi equigranular topaz granite have been investigated experimentally at 100–500 MPa as a function of water activity and F content. Fluorite and topaz can crystallize as liquidus phases in F-rich peraluminous systems, but the F content of the melt should exceed 2.5–3.0 wt% for the crystallization of topaz. In peraluminous F-bearing melts containing more than 1 wt% F, topaz and muscovite are expected to be the first F-bearing phases to crystallize at high pressure, whereas fluorite and topaz should crystallize first at low pressure. Following reaction models the saturation of fluorite and topaz: CaAl2Si2O8 (plagioclase) + 2[AlF3]melt = CaF2 (fluorite) + 2Al2SiO4F2 (topaz). The obtained partition coefficient for F between biotite and glass D(F)Bt/glass ranges from 1.89 to 0.80 (average value 1.29) and can be used as an empirical fluormeter to determine the F content of coexisting melts. Combined petrological and experimental studies of the Kymi equigranular topaz granite indicate that plagioclase was the liquidus phase at nearly water-saturated (fluid-saturated) conditions and that the F content of the melt was at least 2 wt%. The mean F content of natural biotite (3.92 wt%) suggests that the late-stage crystallization of biotite occurred in melts containing about 3 wt% F. The early crystallization of biotite and the presence of muscovite in our experiments at 200 MPa contrasts with the late-stage crystallization of biotite and the absence of muscovite in the natural assemblage, indicating that crystallization pressure may have been lower than 200 MPa for the granite. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.  相似文献   

19.
Gold mineralization at Jonnagiri, Dharwar Craton, southern India, is hosted in laminated quartz veins within sheared granodiorite that occur with other rock units, typical of Archean greenstone–granite ensembles. The proximal alteration assemblage comprises of muscovite, plagioclase, and chlorite with minor biotite (and carbonate), which is distinctive of low- to mid-greenschist facies. The laminated quartz veins that constitute the inner alteration zone, contain muscovite, chlorite, albite and calcite. Using various calibrations, chlorite compositions in the inner and proximal zones yielded comparable temperature ranges of 263 to 323 °C and 268 to 324 °C, respectively. Gold occurs in the laminated quartz veins both as free-milling native metal and enclosed within sulfides. Fluid inclusion microthermometry and Raman spectroscopy in quartz veins within the sheared granodiorite in the proximal zone and laminated auriferous quartz veins in inner zone reveal the existence of a metamorphogenic aqueous–gaseous (H2O–CO2–CH4 + salt) fluid that underwent phase separation and gave rise to gaseous (CO2–CH4), low saline (~ 5 wt.% NaCl equiv.) aqueous fluids. Quartz veins within the mylonitized granodiorites and the laminated veins show broad similarity in fluid compositions and P–T regime. Although the estimated P–T range (1.39 to 2.57 kbar at 263 to 323 °C) compare well with the published P–T values of other orogenic gold deposits in general, considerable pressure fluctuation characterize gold mineralization at Jonnagiri. Factors such as fluid phase separation and fluid–rock interaction, along with a decrease in f(O2), were collectively responsible for gold precipitation, from an initial low-saline metamorphogenic fluid. Comparison of the Jonnagiri ore fluid with other lode gold deposits in the Dharwar Craton and major granitoid-hosted gold deposits in Australia and Canada confirms that fluids of low saline aqueous–carbonic composition with metamorphic parentage played the most dominant role in the formation of the Archean lode gold systems.  相似文献   

20.
Prosperous granite (Rb-Sr 2520±25 Ma) occurs as several plutons (1–380 km2 outcrop area) in a thick succession of metamorphosed greywacke-mudstone of the Yellowknife Supergroup. The average mineral content of the Sparrow pluton (in vol.%) is quartz (32), plagioclase (31), K-feldspar (24), muscovite (9), biotite (3), and apatite (<1). Average trace-element concentrations (in ppm) are Li (140), Be (4), B (28), Zn (47), Rb (250), Sr (76), Zr (75) and Ba (360). The central portion of the pluton is slightly richer in K, Sr, and Ba than the margin. Li is concentrated in mica (Li in biotite/Li in muscovite=4.7), and Be and B in muscovite and plagioclase. Countless pegmatite dikes occur in the Sparrow pluton and in schist-hornfels to the east; the outer limit is marked by the cordierite isograd, 9 km from the granite contact. Dikes vary greatly in size (1 km to a few cm in length), in mineral content (quartz, albite, K-feldspar, muscovite, tourmaline, beryl, spodumene), in major element composition (especially the NaK ratio), and in trace-element content (Li 18–5000 ppm, Be 5–260 ppm, B 20–150 ppm). Compared with Prosperous granite, the pegmatite bodies are richer in P and Rb, and poorer in Ti, Fe, Mg, Zr, and Ba. Dikes rich in tourmaline, beryl, and spodumene occur in overlapping zones situated progressively farther from the centre of the Sparrow pluton. The composition of tourmaline is related to host rock; the highest concentrations of Fe and Zn occur in crystals from pegmetite and the highest concentrations of Mg and V occur in crystals from tourmalinized schist, while those from granite and quartz veins occupy on intermediate position. Complex compositional zoning is present in some tourmaline crystals in pegmatite. Estimates of temperature (500°–600° C) and pressure (2–4 kb) of granite emplacement, based on the distribution of andalusite and sillimanite in the contact rocks, suggest that the final stage of granite emplacement occurred at sub-solidus conditions. A vaportransport model is proposed to explain the widespread distribution of the pegmatite dikes and their extreme compositional variability. Some of the pegmatite constituents, including Li, Be, and B, were possibly derived from Yellowknife graywacke and mudstone.  相似文献   

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