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1.
拉萨地体东南缘始新世早期变质作用及其构造意义   总被引：1，自引：1，他引：0  
林彦蒿  张泽明  董昕 《岩石学报》2013,29(6):1962-1976
本文对位于青藏高原拉萨地体东南缘林芝杂岩中的片麻岩进行了岩石学和锆石U-Pb年代学研究.所研究的样品包括正片麻岩和副片麻岩,它们经历了中压角闪岩相变质作用.岩石地球化学分析结果表明,所研究的正片麻岩的原岩具有钙碱性岛弧岩浆岩的特征.锆石U-Pb年代学分析结果表明,副片麻岩中的碎屑锆石核部为岩浆成因,它们给出的206Pb/238U年龄范围为3012～ 522Ma,其锆石的增生边给出了～51Ma的变质年龄.在正片麻岩中,黑云母片麻岩给出了～67Ma的原岩结晶年龄和～ 55 Ma的变质年龄;石榴石角闪黑云斜长片麻岩给出了～58Ma的原岩结晶年龄和～54Ma的变质年龄.因此,所研究的林芝杂岩并不能代表拉萨地体中的前寒武纪变质基底,而是古生代的沉积岩和晚白垩纪至早新生代的岩浆岩在始新世早期变质而成.这一时期,表壳岩和侵入岩一起经历的中压角闪岩相变质作用很可能跟新特提斯洋俯冲导致的地壳增生、加厚有关.  相似文献   

2.
Zircon populations of Neoproterozoic and early Paleozoic age occur in metabasites of a high-pressure amphibolite-facies unit of the Austroalpine basement south of the Tauern Window. The host rocks for these zircons are eclogitic amphibolites of N-MORB-type character, hornblende gneisses with volcanic-arc basalt signature, and alkaline within-plate-basalt amphibolites. Bulk rock magmatic trace element patterns were preserved during amphibolite-facies high-pressure and subsequent high-temperature events, as well as a greenschist-facies overprint. Positive Ce and negative Eu anomalies and enrichment of HREE in normalized zircon REE patterns, as analysed by LA-ICP-MS, are typical for an igneous origin of these zircon suites. Zircon Y is well correlated to HREE, Ce, Th, U, Nb, and Ta and allows discrimination of compositional fields for each host rock type. Low Th/U ratios are correlated to low Y and HREE abundances in zircon from low bulk Th/U host rocks. This is likely a primary igneous characteristic that cannot be attributed to metamorphic recrystallization. Variations of zircon/host rock element ratios confirm that ionic radii and charges control abundances of many trace elements in zircon. The trace element ratios—presented as mineral/melt distribution coefficients—indicate a selectively inhibited substitution of Zr and Si by HREE and Y in zircon which crystallized from a N-MORB melt. Correlated host rock and zircon trace element concentrations indicate that the metabasite zircons are not xenocrysts but crystallized from mafic melts, represented by the actual host rocks.  相似文献   

3.
The origins of >3900 Ma detrital zircons from Western Australia are controversial, in part due to their complexity and long geologic histories. Conflicting interpretations for the genesis of these zircons propose magmatic, hydrothermal, or metamorphic origins. To test the hypothesis that these zircons preserve magmatic compositions, trace elements [rare earth elements (REE), Y, P, Th, U] were analyzed by ion microprobe from a suite of >3900 Ma zircons from Jack Hills, Western Australia, and include some of the oldest detrital zircons known (4400-4300 Ma). The same ∼20 μm domains previously characterized for U/Pb age, oxygen isotope composition (δ18O), and cathodoluminescence (CL) zoning were specifically targeted for analysis. The zircons are classified into two types based on the light-REE (LREE) composition of the domain analyzed. Zircons with Type 1 domains form the largest group (37 of 42), consisting of grains that preserve evolved REE compositions typical of igneous zircon from crustal rocks. Grains with Type 1 domains display a wide range of CL zoning patterns, yield nearly concordant U/Pb ages from 4400 to 3900 Ma, and preserve a narrow range of δ18O values from 4.7‰ to 7.3‰ that overlap or are slightly elevated relative to mantle oxygen isotope composition. Type 1 domains are interpreted to preserve magmatic compositions. Type 2 domains occur in six zircons that contain spots with enriched light-REE (LREE) compositions, here defined as having chondrite normalized values of LaN > 1 and PrN > 10. A subset of analyses in Type 2 domains appear to result from incorporation of sub-surface mineral inclusions in the analysis volume, as evidenced by positively correlated secondary ion beam intensities for LREE, P, and Y, which are anti-correlated to Si, although not all Type 2 analyses show these features. The LREE enrichment also occurs in areas with discordant U/Pb ages and/or high Th/U ratios, and is apparently associated with past or present radiation damage. The enrichment is not attributed to hydrothermal alteration, however, as oxygen isotope ratios in Type 2 domains overlap with magmatic values of Type 1 domains, and do not appear re-set as might be expected from dissolution or ion-exchange processes operating at variable temperatures. Thus, REE compositions in Type 2 domains where mineral inclusions are not suspected are best interpreted to result from localized enrichment of LREE in areas with past or present radiation damage, and with a very low fluid/rock ratio. Correlated in situ analyses allow magmatic compositions in these complex zircons to be distinguished from the effects of secondary processes. These results are additional evidence for preservation of magmatic compositions in Jack Hills zircons, and demonstrate the benefits of detailed imaging in studies of complicated detrital zircons of unknown origin. The data reported here support previous interpretations that the majority of >3900 Ma zircons from the Jack Hills have an origin in evolved granitic melts, and are evidence for the existence of continental crust very early in Earth’s history.  相似文献   

4.
A comprehensive (mineralogical, geochronological, and geochemical) study of zircons from an eclogitized gabbronorite dike was carried out in order to identify reliable indicators (mineralogical and geochronological) of the genesis of zircons in their various populations and, correspondingly, the age of certain geological events (magmatic crystallization of the gabbroids, their eclogitization, and overprinted retrograde metamorphism). The three populations of zircons separated from two rock samples comprised xenogenic, magmatic (“gabbroic”), and metamorphic zircons, with the latter found exclusively in the sample of retrograded eclogitized gabbroids. The zircons of group I are xenogenic and have a Meso- to Neoarchean age. Mineral inclusions in them (quartz, apatite, biotite, and chlorite) are atypical of gabbroids, and the geochemistry of these zircons is very diverse. The zircons of group II contain mineral inclusions of ortho- and clinopyroxene and are distinguished for their very high U, Th, Pb, and REE concentrations and Th/U ratios. These zircons were formed during the late magmatic crystallization of the gabbroids at temperatures of 1150–1160°C, and their U-Pb age of 2389 ± 25 Ma corresponds to this process. The eclogite mineral assemblages crystallized shortly after the magmatic process, as follows from the fact that the marginal portions of the prismatic zircons contain clinopyroxene inclusions with elevated contents of the jadeite end member. The zircons of group III contain rare amphibole and biotite inclusions and have low Ti, Y, and REE concentrations, low Th/U ratios, high Hf concentrations, contain more HREE than LREE, and have an U-Pb age of 1911 ± 9.5 Ma, which corresponds to the age of the overprinted amphibolite-facies metamorphism.  相似文献   

5.
对大石寨组火山岩的形成时代和形成背景的认识存在较大的分歧。对采自建组剖面底部英安岩样品中的锆石进行阴极发光照相、微区原位LA-ICP-MS微量元素分析和U-Pb测年。锆石U-Pb同位素分析结果显示,锆石发育典型的岩浆震荡环带, 高Th/U(0.51~1.57),轻稀土元素亏损,重稀土元素富集,具有强烈的正Ce异常和强烈的负Eu异常等特征,表明锆石均属于岩浆成因。22个分析点获得的谐和年龄为(314±1)Ma(MSWD=0.88),代表了大石寨组火山岩的形成时代。结合前人的资料,认为大石寨组火山岩为裂陷早期的产物,而晚古生代的沉积岩石组合为在这一基础上发展起来的具有裂陷盆地性质的沉积组合。  相似文献   

6.
文章在对皖南青阳-泾县地区与金-锑多金属矿化有关的中酸性脉岩进行系统调查的基础上,开展了锆石特征及U-Pb定年研究,识别出新生锆石、变质重结晶锆石、捕获锆石和继承锆石共4类锆石,并相应获得一系列年龄值。新生锆石和变质重结晶锆石年龄(125～124 Ma)代表中酸性脉岩形成时代,该年龄值与皖南地区燕山期晚阶段A型花岗岩(130～123 Ma)年龄基本一致,指示二者为同一期岩浆热事件的产物。微量元素特征显示中酸性脉岩源区存在石榴子石残留,而A型花岗岩源区存在斜长石残留,指示源区具双峰式岩浆组合特征,表明该时期皖南地区处于强烈的拉张构造背景下。捕获锆石给出148～142 Ma和136～132 Ma 2组年龄,对应于皖南燕山期早阶段和晚阶段早侵入岩的年龄,暗示脉岩出露区的深部存在隐伏的燕山早期岩浆岩,脉岩岩浆侵位过程中同化混染了早期岩浆岩。继承锆石给出的年龄与扬子板块地壳增生和再造事件中的3期模式增长的年龄相对应,反映与江南过渡带可能有相同的基底物质组成。中酸性脉岩中锆石具有较低的Ce4+/Ce3+比值,表明其岩浆结晶时具有相对低的氧逸度,有利于金、锑的硫化物的形成,进而发育金-锑矿化。区内多金属矿床(点)具有较为明显的分带性,近岩基的中酸性岩株及其接触带发育斑岩-矽卡岩型矿床,成矿元素以钨、钼、铜为主;远离岩基的岩枝或岩脉状中酸性脉岩中及旁侧断裂带内发育脉状矿化,成矿元素以金、银、锑为主,显示出区域成矿作用受断裂构造系统、岩浆活动期次和成矿元素活动性的共同制约。  相似文献   

7.
董昕  张泽明  王金丽  赵国春  刘峰  王伟  于飞 《岩石学报》2009,25(7):1678-1694
本文对位于青藏高原拉萨地体东南部林芝岩群中的变质岩进行了岩石学和年代学研究。研究表明,林芝岩群由角闪岩相的变质沉积岩和正片麻岩组成。变质沉积岩主要为含石榴石白云斜长角闪片岩、含石榴石云母石英片岩、含石榴石黑云钾长片麻岩、大理岩和石英岩等,代表性矿物组合包括石榴石+斜长石+角闪石+石英+黑云母+白云母,或石榴石+斜长石+钾长石+石英+夕线石+黑云母+白云母。花岗质片麻岩（含二云母片麻岩）的矿物组合是石英+斜长石+钾长石+黑云母+白云母。锆石U-Pb年代学分析表明,变质沉积岩中的碎屑锆石主要为岩浆成因,获得了2708~63Ma的206Pb/238U年龄范围,在~1100Ma和~550Ma出现两个年代峰值。碎屑锆石的变质增生边给出了35Ma的变质年龄。正片麻岩获得了496Ma的锆石结晶年龄和1158Ma的继承年龄。基于上述研究结果、区域对比和相邻变质岩石中获得的多期变质年龄,我们认为林芝岩群的原岩很可能形成在早古生代,其沉积物质主要来源于印度陆块,与特提斯喜马拉雅早古生代的岩石一起同为印度大陆北缘的沉积盖层,在环冈瓦纳大陆周缘造山过程中被寒武纪花岗岩侵入。在新特提斯洋向北的俯冲过程中,林芝岩群经历了晚中生代的安第斯型造山作用,在印度与欧亚大陆的俯冲-碰撞过程中,林芝岩群部分地经历了新生代的变质和岩浆作用再造。本研究证明,林芝岩群并不是传统上认为的拉萨地体的前寒武纪变质基底,其角闪岩相至麻粒岩相变质作用发生在中、新生代。  相似文献   

8.
Ren-Xu Chen  Yong-Fei Zheng  Liewen Xie 《Lithos》2010,114(1-2):132-154
Simultaneous in-situ analyses of trace elements, U–Th–Pb and Lu–Hf isotopes were carried out on distinct domains of zircons in ultrahigh-pressure (UHP) eclogite-facies metamorphic rocks from the main hole of the Chinese Continental Scientific Drilling (CCSD) in the Sulu orogen. For the first time, trace elements are directly linked to Lu–Hf isotopes in metamorphic zircons with reference to their U–Pb dates. This enables methodological integration to distinguish four types of metamorphic zircon: solid-state, replacement and dissolution recrystallizations of protolith zircons, and new growth from the aqueous fluid. Metamorphically grown zircons are characterized by concordant U–Pb ages for the metamorphism, flat HREE patterns typical of the garnet effect, low contents of REE (especially HREE), Y, Nb + Ta and Th + U, high contents of Hf, low (Lu/Gd)N, Lu/Hf and Th/U (< 0.1) ratios, and elevated 176Hf/177Hf ratios relative to solid-state recrystallized zircons. This suggests the effects of both garnet and fluid on the growth of metamorphic zircons. In contrast, metamorphic recrystallization has reset the U–Th–Pb isotope system of protolith zircons to different extents, depending on the extents of fluid action during metamorphism. Solid-state recrystallized zircons exhibit the lowest degrees of resetting and thus almost inherit all geochemical features from the protolith zircons, which are characterized by discordant U–Pb ages close to or below the protolith age, steep MREE–HREE patterns typical of magmatic origin, high contents of trace elements and their ratios, and low 176Hf/177Hf ratios. On the other hand, dissolution recrystallized zircons show the highest degrees of reworking and thus have concordant or nearly concordant U–Pb ages for the metamorphism, steep MREE–HREE patterns, lowered contents of trace elements such as REE, Th, U, Y, Nb, Ta and Ti relative to the protolith zircons, and almost unchanged Hf isotope ratios. Replacement recrystallized zircons display intermediate degrees of reworking and thus have their many features of elements and isotopes in between. While the metamorphic growth in the presence of both garnet and fluid is characterized by both depletion of HREE with flat pattern and the low contents of trace elements, the metamorphic recrystallization in the presence of aqueous fluid is indicated by gradual decreases of MREE to HREE without the flat HREE pattern. Therefore, the simultaneous in-situ analyses of metamorphic zircons have the advantage over single-term analyses in making distinction between the new growth and the different types of recrystallization.  相似文献   

9.
西昆仑上其汗地区火山岩的时代及归属一直存在争议。利用LA-ICP-MS锆石U-Pb定年法对该区火山岩中霏细岩进行了测年。阴极发光图像显示,锆石发育对称的生长震荡环带结构,Th/U大部分大于0.4,显示岩浆锆石的特征。测得火山岩的206Pb/238U加权平均年龄为(335±4.2) Ma,形成时代为石炭纪,可将其划为阿羌岩组。结合区域地质背景资料,该火山岩可能形成于裂谷环境。  相似文献   

10.
The geochemistry of zircons from autochthonous granite gneiss (Lc1) anatectic (Lc3–4) and injection (Lc5) leucosomes has been studied. Neoarchean prismatic zircon grains with cores that reveal oscillatory zoning and are overgrown by a couple of rims have been seen to occur in Lc3–4. The prismatic grains are occasionally modified into isometric grains with block structure by Paleoproterozoic secondary alteration, which is accompanied by the depletion in HREE, Y, Nb, U; enrichment in Ti, Li, LREE; increasing Th: U ratio and Ce anomaly; and decreasing Eu anomaly. The Paleoproterozoic alteration is related to the low-temperature amphibolite-facies metamorphism followed by partial melting. The Neoarchean prismatic zircons were formed under the conditions of high-temperature amphibolite-facies ultrametamorphism at a temperature of ～700°C. Judging by the higher Ce/Ce* ratio, the metamorphic rounded zircons were formed at a higher oxygen fugacity as compared with ultrametamorphic zircons from Lc1 and Lc3–4. Specific variation trends of trace element concentrations in prismatic L1 and L3–4 zircons, occasionally with opposite directions, emphasize their different origin. The former are products of metasomatic granitization completed by selective melting with appearance of dispersed melt drops, while the latter are products of anatexis in the open system and by lit-par-lit migmatization. Prismatic zircons L5 are characterized by rhythmic zoning in the core surrounded by rims. The concordant U-Pb age of rims is 129 Ma; the 206Pb/238U age of cores varies from 2213 to 147 Ma. The appreciable enrichment (by a factor of 2–13) of zircons in all minor elements from the core to the rims is caused by the effect of residual postmagmatic fluid, which not only altered zircons, but also facilitated the recrystallization of granite into a pegmatoid variety.  相似文献   

11.
广西东南部钦防海槽地区晚古生代硅质岩十分发育。对硅质岩主量元素、微量元素及稀土元素的分析结果表明:该区晚古生代硅质岩含有较高的SiO2,硅化程度较高;除上泥盆统弗拉斯阶及下石炭统硅质岩具有较低的Al/(Al+Fe+Mn)值(0.14～0.24;0.07～0.81)及较高的U/Th值(0.34～5.09;0.16～10.1)外,其他层位硅质岩具有较高的Al/(Al+Fe+Mn)值(0.4～0.95)及较低的U/Th值(0.1～2.1);上泥盆统硅质岩稀土元素比值Ce/Ce*、LaN/YbN及LaN/CeN值分别为0.78～1.08、0.4～1.52和0.88～1.35,而下石炭统硅质岩具有较低的Ce/Ce*值(0.55～0.91)、LaN/YbN值(0.12～1.8)及较高的 LaN/CeN值(1.12～1.79),中下二叠统硅质岩Ce/Ce*、LaN/YbN及LaN/CeN值分别为1.01～1.62、0.72～2.71和0.62～1.9。钦防海槽晚古生代硅质岩地球化学特征表明:晚泥盆世钦防海槽发生扩张,热液活动比较强烈,处于大陆边缘海环境;早石炭世钦防海槽再次发生扩张,热液活动强烈,硅质岩形成于远离陆源的深海海盆环境;早中二叠世,钦防海槽逐渐收缩,海水变浅。钦防海槽晚古生代并没有出现真正的洋壳,可能属于晚古生代古特提斯分支洋盆的一个坳拉槽盆地。  相似文献   

12.
Simultaneously in-situ analyses of U–Pb isotopes and trace elements were carried out for zircons, in combination with the in-situ analyses of trace elements in coexisting minerals, from low-T/UHP metagranite in the Dabie orogen. The results provide geochemical evidence for the existence of supercritical fluid during continental subduction-zone metamorphism. The zircons are categorized into three types based on their patterns of REE distribution. Type I zircons show increasing enrichment from La to Lu, with prominent positive Ce anomalies and negative Eu anomalies, which are typical of magmatic zircon. Some of them display regular or blurred oscillatory-zoned texture and apparent 206Pb/238U ages of 341 to 780 Ma, suggesting metamorphic modification by solid-state recrystallization with no significant involvement of metamorphic fluid. Type II zircons share similar Th, U and HFSE contents and REE patterns to Type I zircons. However, they exhibit blurred oscillatory-zoned texture or are unzoned, have apparent 206Pb/238U ages of 348 to 709 Ma, and are LREE-enriched relative to Type I zircons. This suggests that they underwent metamorphic reworking by replacement recrystallization in the presence of metamorphic fluid. The LREE enrichment is due to the presence of microscale LREE-bearing mineral inclusions (such as apatite, monazite or epidote) in the zircons. Type III zircons, representing the majority of the present analyses, are characterized by spongy texture and consistent enrichment of LREE, HREE, Th, U and HFSE relative to Type I zircons. They yield nearly concordant U–Pb ages close to the discordia lower-intercept. The consistent enrichment of trace elements relative to the magmatic zircon indicates involvement of a special UHP metamorphic fluid that has a strong capacity to extract significant amounts of LREE, HREE, Th, U and HFSE from such accessory minerals as allanite, garnet, rutile and zircon. Because these minerals are stable in the field of hydrous melt in granite–water systems, they are not able to be decomposed during the exhumation of deeply subducted continental crust. Thus, a supercritical fluid is suggested to transport the LREE, HREE, Th, U and HFSE in the accessory minerals to recrystallized zircons. The mechanism of dissolution recrystallization is responsible for the spongy texture and the very high concentration of trace elements in this type of metamorphic zircons. Therefore, the action of supercritical fluid is evident under the low-T/UHP metamorphic conditions.  相似文献   

13.
蒙古戈壁阿尔泰省巴音陶勒盖地区地处南戈壁-阿尔泰构造带南缘,区内构造活动活跃,中酸性侵入岩比较发育。采用LA-ICP-MS锆石U-Pb测年技术,对区内二长花岗岩中的锆石进行同位素年龄研究。阴极发光图像显示,锆石颗粒多为自形-半自形,且有明显的初始岩浆振荡环带,少见蚀变微区,结合较高的Th/U值(0.65~1.79),断定其为典型的岩浆成因锆石;30颗锆石的~(206)Pb/~(238)U年龄加权平均值为300.2±0.9Ma,显示二长花岗岩成岩时代为晚石炭世,代表了华力西中期的一次构造-岩浆事件,为约束南戈壁-阿尔泰构造带晚石炭世深成岩浆活动时限提供了新证据。  相似文献   

14.
Several lines of isotopic evidence - the most direct of which is from Hadean Jack Hills zircons - suggest a very early history of crust formation on Earth that began by about 4.5 Ga. To constrain both the fate of the reservoir for this crust and the nature of crustal evolution in the sediment source region of the Jack Hills, Western Australia, during the early Archean, we report here initial 176Hf/177Hf ratios and δ18O systematics for <4 Ga Jack Hills zircons. In contrast to the significant number of Hadean zircons which contain highly unradiogenic 176Hf/177Hf requiring a near-zero Lu/Hf reservoir to have separated from the Earth’s mantle by 4.5 Ga, Jack Hills zircons younger than ca. 3.6 Ga are more radiogenic than -13ε (CHUR) at 3.4 Ga in contrast to projected values at 3.4 Ga of -20ε for the unradiogenic Hadean reservoir indicating that some later juvenile addition to the crust is required to explain the more radiogenic younger zircons. The shift in the Lu-Hf systematics together with a narrow range of mostly mantle-like δ18O values among the <3.6 Ga zircons (in contrast to the spread towards sedimentary δ18O among Hadean samples) suggests a period of transition between 3.6 and 4 Ga in which the magmatic setting of zircon formation changed and the highly unradiogenic low Lu/Hf Hadean crust ceased to be available for intracrustal reworking. Constraining the nature of this transition provides important insights into the processes of crustal reworking and recycling of the Earth’s Hadean crust as well as early Archean crustal evolution.  相似文献   

15.
Zircon is a key mineral in geochemical and geochronological studies in a range of geological settings as it is mechanically and chemically robust. However, distortion of its crystal lattice can facilitate enhanced diffusion of key elements such as U and Pb. Electron backscatter diffraction (EBSD) analysis of ninety-nine zircons from the Lewisian Gneiss Complex (LGC) of northwest Scotland has revealed five zircons with lattice distortion. The distortion can take the form of gradual bending of the lattice or division of the crystal into subgrains. Zircon lattices are distorted because of either post-crystallisation plastic distortion or growth defects. Three of the five distorted zircons, along with many of the undistorted zircons in the population, were analysed by ion microprobe to measure U and Pb isotopes, Ti and REEs. Comparison of Th/U ratio, 207Pb/206Pb age, REE profile and Ti concentration between zircons with and without lattice distortion suggests that the distortion is variably affecting the concentration of these trace elements and isotopes within single crystals, within samples and between localities. REE patterns vary heterogeneously, sometimes relatively depleted in heavy REEs or lacking a Eu anomaly. Ti-in-zircon thermometry records temperatures that were either low (~700 °C) or high (>900 °C) relative to undistorted zircons. One distorted zircon records apparent 207Pb/206Pb isotopic ages (?3.0 to +0.3 % discordance) in the range of ~2,420–2,450 Ma but this does not correlate with any previously dated tectonothermal event in the LGC. Two other distorted zircons give discordant ages of 2,331 ± 22 and 2,266 ± 40 Ma, defining a discordia lower intercept within error of a late amphibolite-facies tectonothermal event. This illustrates that Pb may be mobilised in distorted zircons at lower metamorphic grade than in undistorted zircons. These differences in trace element abundances and isotope systematics in distorted zircons relative to undistorted zircons are generally interpreted to have been facilitated by subgrain walls. Trace elements and isotopes would have moved from undistorted lattice into these subgrain walls as their chemical potential is modified due to the presence of the dislocations which make up the subgrain wall. Subgrain walls provided pathways for chemical exchange between crystal and surroundings. Only five per cent of zircons in this population have lattice distortion suggesting it will not have a major impact on zircon geochronology studies, particularly as three of the five distorted zircons are from strongly deformed rocks not normally sampled in such studies. However, this does suggest there may be a case for EBSD analysis of zircons prior to geochemical analysis when zircons from highly deformed rocks are to be investigated.  相似文献   

16.
对大石寨组火山岩的形成时代和形成背景的认识存在较大的分歧。对采自建组剖面底部英安岩样品中的锆石进行阴极发光照相、微区原位LA—ICP—MS微量元素分析和U-Pb测年。锆石U—Pb同位素分析结果显示,锆石发育典型的岩浆震荡环带,高Th／U（0．51～1．57）,轻稀土元素亏损,重稀土元素富集,具有强烈的正Ce异常和强烈的负Eu异常等特征,表明锆石均属于岩浆成因。22个分析点获得的谐和年龄为（314±1）Ma（MSWD=0．88）,代表了大石寨组火山岩的形成时代。结合前人的资料,认为大石寨组火山岩为裂陷早期的产物,而晚古生代的沉积岩石组合为在这一基础上发展起来的具有裂陷盆地性质的沉积组合。  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.
For the first time in Russia, a Hadean zircon grain with an age of 3.94 Ga (ID-TIMS) has been discovered in high-aluminous garnet granulites of the Aldan Shield among the U–Pb zircons with an age from 1.92 Ga. In this connection, the problems of its parental source, the petrogenesis of granulites that captured this zircon, and the mechanism of occurrence of these deep rocks in the upper horizons of the crust have been solved. The comparison of the geochemistry of garnet granulites and the middle crust has shown that the granulites are enriched in the entire range of rare-earth elements (except for the Eu minimum), as well as in Al2O3, U, and Th and are depleted in the most mobile elements (Na, Ca, Sr). In the upper part of the allitic weathering zone of the middle crust, which formed under conditions of arid climate, this zircon grain was originated from the weathered granites from the middle crust. In the latter case, they were empleced discretely in the upper granite–gneiss crust under high pressure conditions (the rutile age is 1.83–1.82 Ga). The zircon with an age of 3.94 Ga is comparable to the Hadean zircons from orthogneisses of the Acasta region (Canadian Shield, 4.03–3.94 Ga).  相似文献   

19.
Undeformed felsic to mafic igneous rocks, dated by U–Pb zircon geochronology between 311 and 255 Ma, intrude different units of the Oaxacan and Acatlán metamorphic complexes in southwestern Mexico. Rare earth element concentrations on zircons from most of these magmatic rocks have a typical igneous character, with fractionated heavy rare earths and negative Eu anomalies. Only inherited Precambrian zircons are depleted in heavy rare earth elements, which suggest contemporaneous crystallization in equilibrium with metamorphic garnet during granulite facies metamorphism. Hf isotopic signatures are, however, different among these magmatic units. For example, zircons from two of these magmatic units (Cuanana pluton and Honduras batholith) have positive εHf values (+3.8–+8.5) and depleted mantle model ages (using a mean crustal value of 176Lu/177Hf = 0.015) (T DMC) ranging between 756 and 1,057 Ma, whereas zircons from the rest of the magmatic units (Etla granite, Zaniza batholith, Carbonera stock and Sosola rhyolite) have negative εHf values (?1 to ?14) and model ages between 1,330 and 2,160 Ma. This suggests either recycling of different crustal sources or, more likely, different extents of crustal contamination of arc-related mafic magmas in which the Oaxacan Complex acted as the main contaminant. These plutons thus represent the magmatic expression of the initial stages of eastward subduction of the Pacific plate beneath the western margin of Gondwana, and confirm the existence of a Late Carboniferous–Permian magmatic arc that extended from southern North America to Central America.  相似文献   

20.
大型硅质火山作用(喷发体积约102～104 km3)的岩浆系统是地壳尺度的,经历了复杂的起源、运移、存储、补给和喷发等过程.揭示岩浆从起源到喷发过程中的结晶分异、堆晶、晶体-熔体分离、地壳混染、岩浆补给、晶粥活化等岩浆作用的细节是认识硅质火山岩浆系统演化的关键.锆石中Th、U、Ti、Hf和REE等微量元素的含量和系统变...  相似文献   

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