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 15 毫秒
1.
The rare-earth element （REE） concentrations of representative granite samples from the southeast of the Obudu Plateau, Nigeria, were analyzed with an attempt to determine the signatures of their source, evolutionary history and tectonic setting. Results indicated that the granites have high absolute REE concentrations （190×10^-6-1191×10^-6; av.=549×10^-6） with the chondrite-normalized REE patterns characterized by steep negative slopes and prominent to slight or no negative Eu anomalies. All the samples are also characterized by high and variable concentrations of the LREE （151×10^-6-1169×10^-6; av.= 466×10^-6）, while the HREE show low abundance （4×10^-6-107×10^-6; av.=28×10^-6）. These are consistent with the variable levels of REE fractionation, and differentiation of the granites. This is further supported by the range of REE contents, the chondrite-normalized patterns and the ratios of LaN/YbN （2.30-343.37）, CeN/YbN （5.94-716.87）, LaN/SmN （3.14-11.68） and TbN/YbN （0.58-1.65）. The general parallelism of the REE patterns, suggest that all the granites were comagmatic in origin, while the high Eu/Eu＊ ratios （0.085-2.807; av.=0.9398） indicate high fo2 at the source. Similarly, irregular variations in LaN/YbN, CeN/YbN and Eu/Eu＊ ratios and REE abundances among the samples suggest behaviors that are related to mantle and crustal sources.  相似文献   

2.
The Karbi Anglong hills (erstwhile Mikir hills) in northeast India are detached and separated from the Meghalaya plateau by a NW-SE trending Kopili rift. The Karbi Anglong hills granitoids (KAHG) and its granite gneissic variants belong to Cambrian plutons formed during Pan-African orogenic cycle, which commonly intrude the basement granite gneisses and Shillong Group metasediments. The KAHG can be broadly classified into three major granitoid facies viz., coarse grained porphyritic granitoid, medium grained massive non-porphyritic granitoid, and granite gneiss, which share a common mineral assemblage of plagioclase-K-feldspar-quartz-biotite±hornblende-apatite-titanite-zircon-magnetite but differ greatly in mineral proportion and texture. Modal mineralogy of KAHG, granite gneiss and basement granite gneiss largely represents monzogranite and syenogranite. The magnetic susceptibility (MS) of the KAHG, granite gneiss and basement granite gneiss varies widely between 0.11×10-3 and 43.144×10-3 SI units, corresponding to ilmenite series (<3×10-3 SI; reduced type) and magnetite series (>3×10-3 SI; oxidized type) of granitoids respectively. The observed MS variations are most likely intrinsic to heterogeneous source regions, modal variations of orthomagnetic and ferromagnetic minerals, and tectonothermal and deformational processes that acted upon these rocks. The primary and re-equilibrated compositions of biotites from the KAHG, granite gneiss and basement granite gneiss suggest calcalkaline, metaluminous (I-type) nature of felsic host magma formed in a subduction or post-collisional to peraluminous (S-type) host magma originated in syn-collisional tectonic settings, which were evolved and stabilized between FMQ and NNO buffers typically corresponding to reducing and oxidising magma environments respectively.  相似文献   

3.
Granitoids and Their Magnetic Susceptibility in South Korea   总被引：1，自引：0，他引：1  
Abstract: Magnetic susceptibility (MS) measurements were carried out for 1,120 samples in the Middle Proterozoic to Early Tertiary granitoids so far recognized in South Korea, and the lateral and spatial variation of their magnetic susceptibility, i.e., content of magnetite, is studied. The Middle Proterozoic two mica granitoids related to cassiterite (Sn) deposits in northeastern part of the Sobaegsan Massif show very low MS (less than 0.3 A‐ 10‐3 SI unit), and the Permo‐Triassic tonalitic to granodioritic and monzonitic rocks which are barren in mineralization, distributed in the middle part of South Korea also show low MS (less than 1 A‐ 10‐3 SI unit). On the contrary the Late Triassic to Jurassic granitoids (= Daebo granitoids) which were evolved from tonalite through granodiorite to granite, and are most widely distributed in South Korea, show a wide variation on MS. Particularly in the Andong, Igsan, Gimcheon and Geochang areas, the granitoids which are barren in mineralization, are characterized by high MS (more than 10 A‐ 10‐3 SI unit). The Chuncheon, Jecheon, Namyang and Geumsan plutons related to molybdenite (Mo) and/or wolframite or scheelite (W), and fluorite (F) mineralizations show a little high MS (more than 3 A‐ 10‐3 SI unit). However, more than 60% of the Daebo granitoids show low MS (less than 3 A‐ 10‐3 SI unit) and the rest show a little high MS (more than 3 A‐ 10‐3 SI unit). Heterogeneous distribution of magnetite content in the Daebo granitoids is considered to reflect heterogeneity of redox state of the source materials for these granitoids. The Cretaceous to Early Tertiary granitoids (= Bulgugsa granitoids) in the Gyeongsang Basin had been generally evolved in the order of tonalite, diorite, granodiorite, granite and alkali‐feldspar granites, which are closely related to base metal ore deposits, and mostly show higher MS (more than 3 A‐ 10‐3 SI unit) than other granitoids mentioned above, although some exceptions are recognized in highly evolved alkali‐feldspar granites (SiO2 > 76%). In contrast, as most of the highly oxidized or evolved Cretaceous granitoids distributed in areas other than the Gyeongsang Basin show lower MS than those of the Gyeongsang Basin, and appear to be magnetite free, ilmenite‐series granites, but they might be hematite bearing magnetite‐series granitoids. Highly oxidized nature of the Bulgugsa granitoids may be due to high Fe2O3/FeO ratio of the source materials and also high level intrusion style of the granitic magma activities. Most of the granitic rocks of the Middle Proterozoic, Permo‐Triassic and more than 60% of the Late Triassic to Jurassic (Daebo granitoids) belong to ilmenite–series, however less than 40% of the Daebo granitoids and most Cretaceous ones are magnetite–series. Thus, the granitic magma intruded in Korean Peninsula became oxidized while the intrusive ages become younger.  相似文献   

4.
There are 61 major peraluminous granitic bodies in Tibet (TPGs) along the south of the Bangong Co-Gêrzê-Amdo-Nujiang suture, whose lithology includes tourmaline granite, muscovite granite and two-mica granite. The TPGs have SiO2 = 65.7%−79.52%, K2O + Na2O = 2.20%−12.51%, K2O/Na2O = 0.49−1.04 and A/CNK = 1.04−1.38. Al2O3 gradually decreases and the other oxides disperse with the increase in SiO2. The rock series is mainly calc-alk series with high potassium. It has typical characteristics of strongly peraluminous granite. Based on the aluminum saturation index and QAP plots, the peraluminous granite plot is mostly within the continental collision granite (CCG) field, indicating that the peraluminous granites in Tibet formed in a continental collisional setting. Ab-Or-Q-H2O phase diagram indicates the pressure of 0.5 × 108−2 × 108 Pa in TPGs, from which it can be deduced that the forming temperature was under 700°C. The TPGs mainly occurred at the collision stage between two continental crust plates, and the original magma is rooted in the remelting from the upper crust. It is the S-type granite in petrogenesis. The South Gandise belt and the Lhagoi Kangri belt have similar characteristics, suggesting that the two belts have the same magma source and the same tectonic setting. Translated from Acta Geologica Sinica, 2006, 80(9): 1329–1341 [译自: 地质学报]  相似文献   

5.
Bulk δ 34Srock values, sulfur contents, and magnetic susceptibility were determined for 12 gold-related granitoid intrusions in southwestern New Brunswick, the Canadian Appalachians. The sulfur isotope compositions of sulfide minerals in some of the granitoid samples were also analyzed. This new dataset was used to characterize two distinctive groups of granitoids: (1) a Late Devonian granitic series (GS) and (2) a Late Silurian to Early Devonian granodioritic to monzogranitic series (GMS). The GS rocks have a large range in δ 34S values of −7.1‰ to +13‰ with an average of 2.2 ± 5.0‰ (1σ), low bulk-S contents (33 to 7,710 ppm) and low magnetic susceptibility values (<10−4 SI), consistent with reduced ilmenite-series granites. The GMS rocks have a relatively narrower variation in δ 34S values of −4.4‰ to +7.3‰ with an average 1.2 ± 2.9‰ but with larger ranges in bulk-S contents (45 to 11,100 ppm) and high magnetic susceptibility values (>10−3 SI), indicative of oxidized magnetite-series granites. The exceptions for the GMS rocks are the Lake George granodiorite and Tower Hill granite that display reduced characteristics, which may have resulted from interaction of the magmas forming these intrusions with graphite- or organic carbon-bearing sedimentary rocks. The bulk δ 34S values and S contents of the GMS rocks are interpreted in terms of selective assimilation–fractional crystallization (SAFC) processes. Degassing processes may account for the δ 34S values and S contents of some GS rocks. The characteristics of our sulfur isotope and abundance data suggest that mineralizing components S and Au in intrusion-related gold systems are dominantly derived from magmatic sources, although minor contaminants derived from country rocks are evident. In addition, the molar sulfate to sulfide ratio in a granitic rock sample can be calculated from the δ 34Srock value of the whole-rock sample and the δ 34Ssulfide (or δ 34Ssulfate) value of sulfide and/or sulfate mineral in the sample on the basis of S-isotope fractionation and mass balance under the condition of magmatic equilibrium. This may be used to predict the speciation of sulfur in granitic rocks, which can be a potential exploration tool for intrusion-related gold systems.  相似文献   

6.
The Planalto Suite is located in the Canaã dos Carajás subdomain of the Carajás Province in the southeastern part of the Amazonian Craton. The suite is of Neoarchean age (∼2.73 Ga), ferroan character, and A-type affinity. Magnetic petrology studies allowed for the distinction of two groups: (1) ilmenite granites showing low magnetic susceptibility (MS) values between 0.6247×10−3 and 0.0102 × 10−3 SI and (2) magnetite-ilmenite-bearing granites with comparatively higher but still moderate MS values between 15.700×10−3 and 0.8036 × 10−3 SI. Textural evidence indicates that amphibole, ilmenite, titanite, and, in the rocks of Group 2, magnetite also formed during magmatic crystallization. However, compositional zoning suggests that titanite was partially re-equilibrated by subsolidus processes. The amphibole varies from potassian-hastingsite to chloro-potassian-hastingsite and shows Fe/(Fe + Mg) > 0.8. Biotite also shows high Fe/(Fe + Mg) ratios and is classified as annite. Plagioclase porphyroclasts are oligoclase (An25-10), and the grains of the recrystallized matrix show a similar composition or are albitic (An9-2). The dominant Group 1 granites of the Planalto Suite were formed under reduced conditions below the FMQ buffer. The Group 2 granites crystallized under more oxidizing conditions on or slightly above the FMQ buffer. Pressures of 900–700 MPa for the origin and of 500–300 MPa for the emplacement were estimated for the Planalto magmas. Geothermometers suggest initial crystallization temperatures between 900 °C and 830 °C, and the water content in the magma is estimated to be higher than 4 wt%. The Neoarchean Planalto Suite and the Estrela Granite of the Carajás Province reveal strong mineralogical analogies, and their amphibole and biotite compositions have high total Al contents. The latter characteristic is also observed in the same minerals of the Neoarchean Matok Pluton of the Limpopo Belt but not in those of the Proterozoic rapakivi A-type granites. On the other hand, in terms of the degree of magma oxidation, the Planalto and Estrela granites approach the reduced Mesoproterozoic rapakivi granites and the reduced to moderately oxidized Paleoproterozoic granites of the Velho Guilherme and Serra dos Carajás Suites, respectively, and differ from the oxidized granites (Jamon Suite) of the Carajás Province as well as those of Matok pluton. The high total Al content of amphibole and mica could be caused by crystallization at high pressures that, in turn, can be a reflex of the association of the studied granites and Matok with charnockitic rocks.  相似文献   

7.
Chemical composition of rock-forming minerals in Appalachian Siluro-Devonian granitoid intrusions, southwestern New Brunswick, was systematically determined by electron microprobe. The mineral chemical data together with petrographic examination was used to test magmatic equilibration and to constrain crystallization conditions, volatile exsolution, and fluorine-chlorine activity of fluids associated with these intrusions. Mineralogical distinction between Late Silurian to Early Devonian granodioritic to monzogranitic series (GMS) and Late Devonian granitic series (GS) rocks is evident, although both are subsolvus I-type to evolved I-type granitoids. Oxidized to reduced GMS rocks consist of quartz, plagioclase (An>10), K-feldspar, biotite, apatite, titanite, zircon, monazite, ± hornblende, ± pyroxene, ± magnetite, ± ilmenite, and ± sulfide. GS rocks comprise quartz, K-feldspar, plagioclase (An<10), mica group minerals, zircon, monazite, apatite, sulfide, ± ilmente, ± magnetite, ± topaz, ± columbite, and ± xenotime. Inter-intrusion and intra-intrusion variations in mineral chemistry are interpreted to reflect petrogenetic processes (e.g., assimilation and fractional crystallization) during granitoid evolution. Although magmatic equilibration among rock-forming minerals are disturbed by subsolidus hydrothermal processes, GMS rocks appear to have higher magmatic temperatures, variable levels of emplacement, a range of (i.e., reduced intrusions 10−16.7∼10−13.4 and oxidized intrusions 10−14.0∼10−10.5 bars), and relatively low f HF/f HCl ratios (10−3.0∼10−1.0) in exsolved fluids, compared to GS rocks. Reduced GMS intrusions bear higher gold potential and thus may be prospective targets for intrusion-related gold systems. Electronic Supplementary Material Supplementary material is available for this article at  相似文献   

8.
A gabbro-diorite plutonic complex from the Southeast Obudu Plateau, representing limited volumes of magma, was studied for its trace and rare-earth element characteristics, in an attempt to document its genetic and geodynamic history. Geochemical studies indicate that the gabbro samples are characterized by variable concentrations and low averages of such index elements as Cr (40×10-6–200×10-6; av. 80×10-6), Ni (40×10-6–170×10-6; 53.33×10-6) and Zr (110×10-6–240×10-6; 116.67×10-6); variable and high average...  相似文献   

9.
Co-genetic pegmatites associated with the granite of the Kawadgaon area in the Bastar craton, Central India, contain a wide range of ore minerals of Nb, Ta, Be, Sn, Zr, Ti, and REE, including columbite-tantalite, ixiolite, pseudo-ixiolite, wodginite, tapiolite, microlite, fersmite, euxenite, aeschynite, beryl, cassiterite, monazite, xenotime, zircon, ilmenite, triplite, and magnetite. There is a distinct vertical zonation between the rare metal and tin pegmatites in apical parts of the host granite. Geochemically, these are LCT-S type, beryl-columbite-phosphate pegmatites that have notably high contents of SiO2 (av. 73.80%), Rb (av. 381 ppm), and Nb (av. 132 ppm). The investigated granites probably were derived from the melting of older crustal rocks, as indicated by a high initial 87Sr/86Sr isotopic ratio, and the major-element geochemistry of the granites and pegmatites. Plots of mol. CaO/(MgO+FeOt) vs. mol. Al2O3/(MgO+FeOt) suggest that the source rock was pelitic metasediments. Based on the available data, it is postulated that the derivation of pegmatites from the parent granite occurred shortly after granite emplacement in the late Archaean-early Proterozoic (~2500 Ma). The K/Rb, Ba/Rb, and Rb/Sr ratios of the felsic bodies reveal that a substantial part of the granite formed from evolved melts, and further fractionation produced the co-genetic pegmatites and associated rare metal and rare earth deposits.  相似文献   

10.
The Mirpur granite body represents a relatively small (10 km2) pluton intruded along the northern margin of the adjacent Mt. Abu batholith (∼125 km2) in NW India. It is a visibly undeformed alkali feldspar rich pink granite; in contrast, the Mt. Abu is a composite granitoid body and variably deformed. Both are intruded by rhyolitic dykes and the terminal magmatic events in both the cases are mafic dykes. The AMS (Anisotropy of Magnetic Susceptibility) data identify the Mt. Abu with SE-dipping foliations and subvertical lineations as a single structural domain while the Mirpur granite body shows two domains characterized by predominantly E — W trend of magnetic foliation in the eastern part (domain I) and N — S orientations in the western part (domain II). The domain I shows magmatic fabrics, typical for the peraluminous granites of Malani Igneous Suite (MIS). Change in fabric orientation in the domain II has resulted from cataclasis wherein the samples show destruction of the original E — W fabric and complete transposition by N — S trends. The foliations in the Mt. Abu granites have been related to SE orientation of maximum horizontal stress. The same maximum stress direction can be inferred from dyke orientation in the Mirpur granite, which is interpreted as continuation of the tectonic imprint in this region during emplacement of both the granites. Age of the cataclastic overprint with a predominant N — S orientation is not yet constrained but corresponds with the trend of the nearby Sindreth basin within the Malani Igneous Suite. The Neoproterozoic tectonic scenario for the region has been interpreted in terms of an ongoing crustal convergence and granitic magma emplacement against the back stop offered by the rigid Delhi Fold Belt.  相似文献   

11.
The Longwangzhuang granite pluton occurs on the southern margin of the North China Craton and consists mainly of biotite syenogranite with aegirine granite being locally distributed.The granites are characterized by high silicon and alkaline contents(SiO2=72.17%-76.82%,K2O+Na2O=8.28%-10.22%,K2O/Na2O>>1),AI(agpaitic index) =0.84-0.95,DI=95-97,ASI(aluminum saturation index)=0.96-1.13,and very high Fe* number(FeO*/(FeO*+Mg)=0.90-0.99),thus the granites are assigned to the metaluminous to weakly peraluminous,alkalic to calc-alkalic ferroan A-type granites.The granites are rich in large ion lithophile elements(LILE),especially high in REE concentrations(REE+Y=854×10-6-1572×10-6);whereas the enrichment of high strength field elements(Nb,Ta,Zr,Hf) is obviously less than that of LILEs,exhibiting mild depletions on trace element spider plots;and the rocks are significantly depleted in Ba,Sr,Ti,and Pb.The low εNd(t) values(-4.5--7.2) and high model ages(2.3-2.5 Ga) of the granites as well as the low εHf(t) values(-1.11--5.26) and high Hf model ages(THf1= 2.1-2.3 Ga,THf2=2.4-2.6 Ga) of zircons from the biotite syenogranite suggest that the granites were probably derived from an enriched mantle source.The zircons from the biotite syenogranite are mainly colorless transparent crystals exhibiting well-developed oscillatory zoning on the cathodoluminescence images with a LA-ICPMS zircon U-Pb age of 1602.1±6.6 Ma(MSWD=0.48).Petrochemical,trace elements,as well as Nd and Hf isotopic compositions of the rocks demonstrate that the granites were formed in a within-plate extensional tectonic regime possibly related to the breakup of the Columbia supercontinent.The granites were most likely formed through extreme fractional crystallization of alkali basaltic magma resulted from partial melting of the mantle,which was fertilized by recycling crustal rocks triggered by the delamination of lithospheric mantle and lower crust following the ～1.8 Ga collision and amalgamation of the North China Craton which is part of the Columbia supercontinent.However,contamination of neo-Archean to Paleoproterozoic crustal rocks during the ascent and emplacement of the magma could not be excluded.Being the youngest known anorogenic magmatism on the southern margin of the North China Craton related to Columbia breakup,it might represent the break off of the North China Craton from Columbia supercontinent at the end of Paleoproterozoic.  相似文献   

12.
锆石Ge含量和年龄对云南临沧锗矿床物质来源的约束   总被引：1，自引：0，他引：1  
云南临沧锗矿床产在以临沧花岗岩为基底的中新世断陷盆地中。已有研究表明,成矿元素Ge来自基底花岗岩,但临沧花岗岩是复式岩基,具有多期次的岩浆活动,而Ge来自哪期岩浆活动并不清楚。文章对采自晓街、临沧、勐库和勐海的7个花岗岩样品(纵贯临沧岩基)进行了LA-ICP-MS锆石原位Ge元素含量分析和U-Pb年代学研究。研究结果表明,岩基主体岩浆活动时代与前人研究成果一致,为(215.6±2.7)Ma~(236.9±3.7)Ma,同时也有少量继承锆石(303~2533 Ma);w(Ge)在主岩浆期的锆石中为0.23×10-6~4.21×10-6,平均0.53×10-6,在继承锆石中为0.34×10-6~3.43×10-6,平均1.18×10-6;而在本次研究中新发现的年龄为106.5 Ma和87.7 Ma的年轻锆石中w(Ge)高达102×10-6。因此,推测临沧锗矿床中Ge来源于晚期岩浆或热液作用。这个结果为成矿理论的深入研究提供了新依据。  相似文献   

13.
In the present study, the grain size (d) and shape of 225 magnetite grains, that crystallized at T>600°C in a syntectonic granite (Godhra Granite, India) are evaluated and implications of data to decipher deformation mechanism of magnetite are discussed. Fractal (ruler) dimension (D) analysis of magnetite grains is performed and it is demonstrated that they show fractal behaviour. Smaller magnetite grains tend to be more serrated than the larger ones, which is manifested in the higher fractal (ruler) dimension (D) of the former. Assuming a natural strain rate ranging between 10−10 s−1 and 10−14 s−1, the grain size data fall dominantly in the dislocation creep field of the existing deformation mechanism map of magnetite for 630°C. However, SEM-EBSD studies reveal that subgrains are absent in the magnetite grains and they did not undergo dislocation creep. Thus it is inferred that the shape of magnetite grains was not controlled by dislocation creep. It is concluded that the higher serration and increased fractal dimension of finer magnetite grains implies the importance of diffusion creep as an important deformation mechanism at high-T for magnetite in polymineralic rocks.  相似文献   

14.
The formation of ore-bearing granites in the Yenshanian movement in southeast China and the geochemical characteristics of some RE-bearing granites have been studied through multivariate statistical analysis and physicochemical approach. The main conclusions have Been drawn as follows: (1) The granites are believed to be products of anatexis of the crustal materials. The formation temperature of granitic magma is estimated at about 600°C and the crystallization temperatures range from 600° to 500°C. The temperature of Li-, Fand H2O-rich residual magma in the latest stage of magmatism is probably below 500°C. (2) A rock series from early lepidomelane-granite through protolithionite-and zinnwalditegranite to lithioniteor lepidolite-granite is considered as a result of actual crystallization. (3) The mineral paragenesis and fades zonation of granite plutons are mainly controlled by Ph2o, μNa2O and μK2O in the magma. (4) During the magmatic evolution the ore-forming elements (REE, Nb, Ta, etc.) are variable in geochemistry. REEs, similar to mafic components, were highly concentrated at the early stage of the magmatic evolution and deposited under favourable conditions in the zinnwaldite-and protolithionite-granites; Nb and Ta have a preference for felsic and volatile components, thus are mainly concentrated at later stages of the magmatic evolution. Nb ore deposits are formed in the lithionite and lepidolite granites, for Ta is intimately associated with Na2O, Li2O, F and H2O.  相似文献   

15.
The Precambrian Chhotanagpur granite gneiss complex (CGGC) terrain covers more than 80,000 sq km area, and is dominated by granitoid gneisses and migmatites. Recent geochronological data indicate that the CGGC terrain has witnessed five tectonomagmatic thermal events at: (i) 2.5–2.4 Ga, (ii) 2.2–2.0 Ga, (iii) 1.6–1.4 Ga, (iv) 1.2–1.0 Ga, and (v) 0.9–0.8 Ga. Of these, the third and the fourth events are widespread. The whole-rock Rb-Sr isotopic analysis of twenty granite samples from the CGGC of Raikera-Kunkuri region, Jashpur district, Chhattisgarh, Central India, yields two distinct isochrons. The eleven samples of grey granites define an isochron age of 1005±51 Ma with moderate initial 87Sr/86Sr ratio of 0.7047±0.0065, which corresponds to the fourth tectonomagmatic event. On the other hand, the nine samples of pink granites indicate younger isochron age of 815±47 Ma with a higher initial 87Sr/86Sr ratio of 0.7539±0.0066 that matches with the fifth phase of the thermal event. The data suggest emplacement of large bodies of grey granite at ∼1005 Ma that evolved possibly from precursors of tonalitic-granodioritic composition. Furthermore, the younger age (∼815 Ma) suggests the age of metasomatism, involving isotopic resetting, that resulted in genesis of pink granite bodies of limited areal extent. By analogy, the age of metasomatism (∼815 Ma) may also be taken to represent the age of Y-mineralisation in the Raikera-Kunkuri region of the CGGC terrain.  相似文献   

16.
锡矿往往与长英质岩浆岩伴生,然而锡矿形成的热能源区尚不清楚,其可能与地幔物质相关。我国云南锡矿带中出露的中-酸性岩石及碱性岩杂岩体为研究锡矿及其周围岩浆成因提供了良好的物质条件。本文报道了云南个旧地区代表性的花岗岩、辉长-闪长岩和碱性岩类新的全岩地球化学、锆石U-Pb年代学和Hf同位素数据。LA-ICP-MS锆石U-Pb定年表明上述岩石分别形成于81.43±0.46Ma(82.89±0.58M)、81.35±0.22Ma和80.35±0.72Ma,指示它们为晚白垩世近同期岩浆活动的产物。其中闪长岩、碱性岩和花岗岩中锆石的Hf同位素组成不均一,ε_(Hf)(t)分别为-4.2~+0.8、-7.5~-1.9和-8.4~+0.4。尽管这些岩体的侵入时代一致,但它们的地球化学特征和同位素特征存在差异,表明这三类火成岩来自不同的岩浆源区,三者不是同一母岩浆相互演化的关系。个旧杂岩体中花岗岩为弱过铝质岩石,SiO_2与P_2O_5含量呈负相关的关系,排除S型花岗岩的可能。亏损Zr、Nb、Sr、Eu等大离子亲石元素的特征可能为锆石、磷灰石、长石类造岩矿物分离结晶作用的结果;Zr、Nb、Ce和Y总量较低,低的FeOT/MgO比值和低的锆石饱和温度表明,指示出个旧地区的花岗岩应为高分异I型花岗质岩石而非A型花岗岩。个旧地区形成于晚白垩世时期的中基性、碱性岩石可能为不同的幔源岩浆近同时侵入的产物,底侵的幔源熔体带来热量诱发中、下地壳岩石发生部分熔融形成含矿的花岗岩,幔源岩浆对于成矿至少在能量也可能在成矿物质上有重要的贡献。  相似文献   

17.
Disturbance of the zircon U-Pb isotopic system has been investigated extensively, but mostly in lab, in the last decades. Here, we reported a field-based study on intensive sericitization, K-feldsparthization and the impacts of mylonitization on zircons from the Fangcheng syenites. The Fangcheng syenites occur in the eastern part of the Qinling orogen and consist mainly of aegirine-augite syenite, aegirine nepheline syenite, biotite syenite and hornblende nepheline syenite. Zircons from the slightly sericitized aegirine augite syenite are colorless, transparent crystals and exhibit well-developed oscillatory and sector zoning on the cathodoluminescence (CL) images which are typical of magmatic zircons from alkaline rocks. Zircon U-Pb determinations by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) showed that the syenite was formed in Neoproterozoic time, the weighted average of 206Pb/238U ages is 844.3±1.6 Ma (MSWD=0.86). In contrast, the hydrothermally altered zircons (hydrothermal zircon) from the intensively sericitized, K-feldsparthized, and weakly mylonitized aegirine augite syenite are conglomerates, yellowish to brown in color, generally translucent and internally textureless. The CL and backscatter electron (BSE) images of hydrothermal zircons exhibit fractured, textureless or mosaic textures, and occasionally show “sponge texture” with the veinlets and inclusions of K-feldspar; however, relicts of magmatic oscillatory zoning can still be discerned locally in individual grains. LA-ICPMS analyses of the hydrothermal zircons demonstrated that the zircons are chemically inhomogeneous, with enhanced and widely varied Pb, U, and Th contents. The U and Th contents of the hydrothermal zircons are estimated to be 32×10−6−1550×10−6 and 188×10−6−4059×10−6, respectively, with Th/U ratios within the range of 0.7–44.9. 206Pb/238U apparent ages of the hydrothermal zircons are negatively correlated with the contents of U, and radiogenic and common Pb. As the U and Th concentrations of the magmatic zircons are rather low, the α-decay doses (3.65×1014−2.04×1015 α-decay events/mg) are much lower than those at the first percolation point (3.5× 1015 a-decay events/mg), thus, Pb mobility resultant from diffusion could be safely neglected. Disturbance of the U-Pb isotopic system of zircons is most likely to be attributed to the intensive sericitization and K-feldsparthization coupled with mylonitization, the hydrothermal fluids reacted with zircons along the rims and fractures of the distorted zircon crystals, giving rise to the chemically inhomogeneous hydrothermal zircons through a series of complicated mechanisms such as dissolution-reprecipitation. The significance of the U-Pb age (the lower intercept age on the discordia U-Pb plot) of hydrothermal zircons is uncertain. We argued that interpretations of the hydrothermal zircon data as the age of hydrothermal events or hydrothermal ore-forming processes are questionable and hence cautions must be taken.  相似文献   

18.
This study shows that the intrusive rocks distributed in the Aoyiqieke-Tamuqi area on the southern margin of the Tarim Block are composed of gabbro, diorite, granodiorite and granite, which constitute regionally a nearly EW-trending tectono-magmatic belt. Petrochemically the diorite, granodiorite and granite belong to the calc-alkaline, high-K series, with Na2O/K2O ratios varying between 0.83 and 2.63. M/F ratios in the diorite are within the range of 0.44–0.70 and those of the granodiorite ( granite) are 0.45–0.87. Petrochemistry data show that the intrusive complexes are of the I type and their ΣREE is slightly variable, within the range of 178.31–229.01 × 10−6. The LREE/HREE ratios of the diorite and granite are 3.78–5.13 and 6.69–7.66, respectively. The plutons usually show moderate negative Eu anomalies with δEu values ranging from 0.53 to 0.82, showing almost no difference among different rocks. The (La/Yb)N values of diorite and granite are 12.39−14.86 and 22.07−26.03, respectively. The diorite and granite possess very similar REE distribution patterns, indicating that they were both derived from the same source. As for their trace element ratios, the diorite has higher Nb/Ta ratios than the granite, which are 15.73−17.16 and 12.03−15.01, respectively. It can be seen that the Nb/Ta ratios of the diorite are much closer to the average mantle (17.5). Their Zr/Hf ratios are very close to each other, within the range of 29−34. Th/Y ratios in the diorite are 0.42−0.80 (all less than unity) while those of the granite are 1.02−2.04. Some difference is also noticed in Ti/V between the diorite and the granite (52.6−54.2 for the former and 52.6−54.2 for the latter). As compared with ocean ridge granites, both diorite and granite are characterized by remarkable LILE enrichment, as well as by moderate negative Ba and postive Ce anomalies. The contents of Nb and Ta in the diorite and granite are equivalent to those of the ocean ridge granites, but the contents of Zr, Hf, Sm, Y, and Yb are all lower than those of the ocean ridge granites, indicating that these granites are similar to the island-arc granites of Chile. From their geochemical characteristics, it is considered that the intrusive rocks in the area studied were formed in an island-arc environment at the continental margin.  相似文献   

19.
Nature of the crust in Maine,USA: evidence from the Sebago batholith   总被引：7，自引：0，他引：7  
 Neodymium and lead isotope and elemental data are presented for the Sebago batholith (293±2 Ma), the largest exposed granite in New England. The batholith is lithologically homogeneous, yet internally heterogeneous with respect to rare earth elements (REE) and Nd isotopic composition. Two-mica granites in the southern/central portion of the batholith (group 1) are characterized by REE patterns with uniform shapes [CeN/YbN (chondrite normalized) = 9.4–19 and Eu/Eu* (Eu anomaly) = 0.27–0.42] and ɛ Nd(t) = −3.1 to −2.1. Peripheral two-mica granites (group 2), spatially associated with stromatic and schlieric migmatites, have a wider range of total REE contents and patterns with variable shapes (CeN/YbN = 6.1–67, Eu/Eu* = 0.20–0.46) and ɛ Nd(t) = −5.6 to −2.8. The heterogeneous REE character of the group 2 granites records the effects of magmatic differentiation that involved monazite. Coarse-grained leucogranites and aplites have kinked REE patterns and low total REE, but have Nd isotope systematics similar to group 2 granites with ɛ Nd(t) = −5.5 to −4.7. Rare biotite granites have steep REE patterns (CeN/YbN = 51–61, Eu/Eu* = 0.32–0.84) and ɛ Nd(t) = −4.6 to −3.8. The two-mica granites have a restricted range in initial Pb isotopic composition (206Pb/204Pb = 18.41–18.75; 207Pb/204Pb = 15.60–15.68; 208Pb/204Pb = 38.21–38.55), requiring and old, high U/Pb (but not Th/U) source component. The Nd isotope data are consistent with magma derivation from two sources: Avalon-like crust (ɛ Nd>−3), and Central Maine Belt metasedimentary rocks (ɛ Nd<−4), without material input from the mantle. The variations in isotope systematics and REE patterns are inconsistent with models of disequilibrium melting which involved monazite. Received: 8 December 1995 / Accepted: 29 April 1996  相似文献   

20.
Fluid inclusions in granite quartz and three generations of veins indicate that three fluids have affected the Caledonian Galway Granite. These fluids were examined by petrography, microthermometry, chlorite thermometry, fluid chemistry and stable isotope studies. The earliest fluid was a H2O-CO2-NaCl fluid of moderate salinity (4–10 wt% NaCl eq.) that deposited late-magmatic molybdenite mineralised quartz veins (V1) and formed the earliest secondary inclusions in granite quartz. This fluid is more abundant in the west of the batholith, corresponding to a decrease in emplacement depth. Within veins, and to the east, this fluid was trapped homogeneously, but in granite quartz in the west it unmixed at 305–390 °C and 0.7–1.8 kbar. Homogeneous quartz δ18O across the batholith (9.5 ± 0.4‰n = 12) suggests V1 precipitation at high temperatures (perhaps 600 °C) and pressures (1–3 kbar) from magmatic fluids. Microthermometric data for V1 indicate lower temperatures, suggesting inclusion volumes re-equilibrated during cooling. The second fluid was a H2O-NaCl-KCl, low-moderate salinity (0–10 wt% NaCl eq.), moderate temperature (270–340 °C), high δD (−18 ± 2‰), low δ18O (0.5–2.0‰) fluid of meteoric origin. This fluid penetrated the batholith via quartz veins (V2) which infill faults active during post-consolidation uplift of the batholith. It forms the most common inclusion type in granite quartz throughout the batholith and is responsible for widespread retrograde alteration involving chloritization of biotite and hornblende, sericitization and saussuritization of plagioclase, and reddening of K-feldspar. The salinity was generated by fluid-rock interactions within the granite. Within granite quartz this fluid was trapped at 0.5–2.3 kbar, having become overpressured. This fluid probably infiltrated the Granite in a meteoric-convection system during cooling after intrusion, but a later age cannot be ruled out. The final fluid to enter the Granite and its host rocks was a H2O-NaCl-CaCl2-KCl fluid with variable salinity (8–28 wt% NaCl eq.), temperature (125–205 °C), δD (−17 to −45‰), δ18O (−3 to + 1.2‰), δ13CCO2 (−19 to 0‰) and δ34Ssulphate (13–23‰) that deposited veins containing quartz, fluorite, calcite, barite, galena, chalcopyrite sphalerite and pyrite (V3). Correlations of salinity, temperature, δD and δ18O are interpreted as the result of mixing of two fluid end-members, one a high-δD (−17 to −8‰), moderate-δ18O (1.2–2.5‰), high-δ13CCO2 (> −4‰), low-δ34Ssulphate (13‰), high-temperature (205–230 °C), moderate-salinity (8–12 wt% NaCl eq.) fluid, the other a low-δD (−61 to −45‰), low-δ18O (−5.4 to −3‰), low-δ13C (<−10‰), high-δ34Ssulphate (20–23‰) low-temperature (80–125 °C), high-salinity (21–28 wt% NaCl eq.) fluid. Geochronological evidence suggests V3 veins are late Triassic; the high-δD end-member is interpreted as a contemporaneous surface fluid, probably mixed meteoric water and evaporated seawater and/or dissolved evaporites, whereas the low-δD end-member is interpreted as a basinal brine derived from the adjacent Carboniferous sequence. This study demonstrates that the Galway Granite was a locus for repeated fluid events for a variety of reasons; from expulsion of magmatic fluids during the final stages of crystallisation, through a meteoric convection system, probably driven by waning magmatic heat, to much later mineralisation, concentrated in its vicinity due to thermal, tectonic and compositional properties of granite batholiths which encourage mineralisation long after magmatic heat has abated. Received: 3 April 1996 / Accepted: 5 May 1997  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