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1.
对胶东谭格庄地区一地质剖面进行了详细野外观察和锆石SHRIMP年代学研究.剖面主要是由奥长花岗质片麻岩和斜长角闪岩组成,两者互层产出并一起发生褶皱变形,在强变形域两者完全平行化.所以,在强变形地区,不同成分岩性的薄层状互层产出不能作为变质沉积岩的识别标志.奥长花岗质片麻岩(S1105)形成于2496±10Ma,遭受了古元古代早期和晚期变质.两个斜长角闪岩样品(S1238和S0812)都只发育变质锆石,年龄分别为1842±3Ma和1833±13Ma.该剖面岩石记录的两期变质作用在胶东地区具有普遍意义.结合前人研究,作者指出该地区太古宙末存在区域性变质深熔作用,早期地壳受到了强烈的再造;古元古代末的变质作用具有带状分布特点,与造山作用有关.  相似文献   

2.
采用LA-ICP-MS方法,首次对四川西部新元古界苏雄组层型剖面底部豆状熔结凝灰岩进行了锆石U-Th-Pb同位素测定,获得了两组谐和度极高的年龄数据:805.9±4.4 Ma(N=11,MSWD=0.58)和849.4±6.2 Ma(N=6,MSWD=0.34)。其中,805.9±4.4 Ma代表凝灰岩沉积年龄,与前人报道的苏雄组流纹岩SHRIMP锆石U-Pb年龄(803±12 Ma)高度一致,进一步约束了苏雄组底部火山喷发的起始年代。构造岩浆热事件分析表明,805.9±4.4Ma代表了华南扬子古大陆800 Ma左右的火山喷发事件;而849.4±6.2 Ma则与Rodinia超大陆初始裂解(~850Ma)的同裂谷岩浆活动相对应。  相似文献   

3.
The SIMS U-Pb isotopic age of zircons from enderbite gneisses and their metaorthopyroxenite xenoliths in the Pobuzhie granulite complex, Ukrainian Shield (48°13′57.3″ N and 29°59′21.5″ E, WGS84 system), was determined. The chemical compositions of these rocks and composing minerals were studied. Enderbite gneisses contain quartz, antiperthite plagioclase, K-feldspar, clinoenstatite, diopside, amphibole, and a small amount of biotite; accessory minerals are ilmenite and apatite. The age of zircon from enderbite gneiss is estimated at about 3.15 Ga. Metaorthopyroxenites are composed of orthopyroxene, clinopyroxene, phlogopite (up to 6% TiO2), and plagioclase. The age of magmatic zircons from metaorthopyroxenite determined by the upper intercept of the discordia with the concordia is 3485 ± 33 Ma (MSVD = 1.6), and the age of metamorphic zircons is 2742 ± 22 Ma (MSVD = 0.22). Hence, the enderbite gneisses studied pertain to a young group of enderbites in the Pobuzhie granulite complex, while the age of metaorthopyroxenites from xenoliths in these rocks is similar to that of ancient Pobuzhie enderbites and pyroxenites of the Novopavlovsk complex in the Azov Region.  相似文献   

4.
LA-ICP-MS U-Pb zircon dating of the Bozhushan granite in Southeast Yunnan   总被引：1，自引：0，他引：1  
The LA-ICP-MS U-Pb zircon dating of eight typical samples from four units of the Bozhushan granite intrusion in southeastern Yunnan Province, constrains the age of acidic magmatic intrusion in this area. Both the oscillatory zoning and chondrite-normalized REE patterns characterized by LREE-depletion and HREE-enrichment with positive Ce anomaly and negative Eu anomaly indicate the magmatic genesis of these zircons. Eight zircon samples from the Bozhushan granite yielded a mean 206 Pb/ 238 U age of (85.58±1.0) Ma (MSWD=4.1) to (88.10±0.66) Ma (MSWD=1.8). These chronology data suggest an accurate isotopic age for the intrusion of the Bozhushan granite, and are different from the published age data of 48 to 111.5 Ma. The geochronology data of the Gejiu, Dulong and Dachang super-large deposits and related Yanshanian granites indicated that there occurred large-scale granitic magmatism and mineralization events in western Nanling region during the Late Cretaceous.  相似文献   

5.
南秦岭迷魂阵岩体LA-ICP-MS锆石U-Pb年代学和Lu-Hf同位素特征   总被引：1，自引：1，他引：0  
位于陕西省的姜家沟-磨沟峡-小岭镇地区的迷魂阵岩体,为南秦岭构造带中一个古老地块内的深成侵入体。该岩体主要由闪长岩、石英闪长岩和花岗闪长岩组成。根据野外地质关系和LA-ICPMS锆石U-Pb定年结果,可将迷魂阵岩体的岩浆作用分为两个阶段:早期岩浆作用阶段主要形成闪长岩,其侵位时代为885±4Ma;晚期岩浆阶段主要形成石英闪长岩-花岗闪长岩,其侵位时代为~737±4Ma。定年锆石原位Lu-Hf同位素分析揭示早期闪长质岩浆主要形成于亏损地幔的部分熔融,晚期石英闪长岩-花岗闪长岩岩浆主要来源于早期闪长质岩浆的结晶分异,并经历了地壳物质的混染或者壳幔岩浆混合作用。  相似文献   

6.
The Erdenetuin-Obo porphyry Cu-Mo deposit was formed at the final stage of development of magmatic activity occasionally manifested in the Late Permian-Early Triassic in the period of at least 40 Ma. Early plutonic (host) and late ore-bearing porphyry intrusive complexes were formed in that period. The plutonic complex is multiphase, while the porphyry complex is polyrhythmical and multiphase within rhythms. The obtained data on the U-Pb isotopic composition (SHRIMP II) of zircons from unaltered rocks of the ore field are discussed: gabbro, diorite, and granodiorite of the plutonic complex and granodiorite-porphyry I and II of the first and second rhythms of the ore-bearing complex, respectively. Zircons of different age levels and genotypes were identified in the course of performed investigations. Gabbro are dominated by postmagmatic (superimposed) zircons with the datings of 239–225 Ma. The age of xenogenic zircon brought out from the basement rocks is estimated at 1146 ± 11 Ma. Zircons occur as magmatic and postmagmatic (superimposed) minerals dated 252–247, 244–233 Ma in diorite and 244–242, 239–224 Ma in granodiorite. The ages of postmagmatic zircons from diorite are partially overlapped by datings of magmatic zircons from granodiorite and granodiorite-porphyry. In the porphyry complex, the datings of magmatic zircons are 240–234 and 222–220 Ma in granodiorite-porphyries I and II, respectively. There are also inherited zircons with datings coinciding with those of magmatic zircons from precursor intrusive rocks. Datings of such zircons are 249–241 and 257–231 Ma for granodiorite-porphyries I and II, respectively. As a whole, zircon datings in all studied igneous rocks forming a virtually uninterrupted range in the period of 257–220 Ma allow us to suggest the relation of the ore magmatic system to the long-living constantly active deep source occasionally delivering melt to the upper levels.  相似文献   

7.
黄陵野马洞基性岩脉中锆石的U-Pb年龄和Hf同位素组成   总被引：1，自引：0，他引：1  
采用激光剥蚀-等离子质谱(LA-ICP-MS)分析技术测定野马洞基性岩脉中锆石的U-Pb年龄和Hf同位素组成,以探讨黄陵地区TTG片麻岩原岩的形成及变质时间、是否存在比崆岭群更古老的地壳等问题。野马洞辉绿岩脉(1850 Ma)侵入TTG片麻岩,并从TTG片麻岩中捕获了大量捕掳晶锆石。捕掳晶锆石岩浆结晶核部的U-Pb年龄分别为2842 Ma、2900 Ma和2949 Ma,指示TTG花岗岩体为复式岩体,其至少经历了2949 Ma、2900 Ma和2842 Ma三期岩浆作用。捕掳晶锆石变质边部的U-Pb年龄为2557 Ma,指示TTG花岗岩体转变为TTG片麻岩,是"水月寺运动"及其构造热事件共同作用的结果,其变形变质的时间为2557～2511 Ma。捕掳晶锆石的εHf(t)为-9.85～0.89、平均值为-4.07,亏损地幔模式年龄TDM为3.6～3.2 Ga,指示黄陵地区存在比崆岭群(3.2 Ga)更古老的陆壳。  相似文献   

8.
在欧龙布鲁克地块东部地区的正片麻岩中识别出呈透镜状产出的基性麻粒岩,部分已转变为斜长角闪岩。其主要矿物组合为单斜辉石、斜方辉石、斜长石、角闪石等,为典型中低压麻粒岩相组合。锆石SHRIMP U-Pb定年得到基性麻粒岩1928±9Ma的变质年龄,片麻岩围岩得到了1927±20Ma的变质年龄,以及2368±5Ma、2377±7Ma的岩浆结晶年龄。片麻岩锆石Hf同位素数据显示变质锆石及岩浆锆石均具有相似的Hf同位素成分,其二阶段模式年龄为2590~2830Ma,显示其可能源于太古代地壳物质的再造。欧龙布鲁克地块古元古代岩浆及变质演化历史与塔里木克拉通及华北克拉通很高的相似性,预示着在古元古代三者可能具有一定的亲缘性。  相似文献   

9.
报道了鞍山地区东山杂岩带奥长花岗岩和二长花岗岩的锆石SHRIMP U-Pb年龄。中粗粒奥长花岗岩中岩浆锆石的年龄为3329 Ma ± 22 Ma (MSWD=9.6),存在年龄为3687~3784 Ma的残余锆石。细粒奥长花岗岩和二长花岗岩中岩浆锆石的年龄分别为3141 Ma ± 8 Ma (MSWD=1.5)和3142 Ma ± 5 Ma (MSWD=0.35)。研究表明,约~3.3 Ga和3.1 Ga是鞍山地区2个重要的地壳演化阶段。  相似文献   

10.
The Jinchuan ultramafic intrusion in western China hosts the third-largest magmatic Ni–Cu deposit in the world. The crystallization age of the intrusion has long been debated. Here, we present a U–Pb ID-TIMS zircon age of 831.8 ± 0.6 Ma obtained on thermally annealed and chemically etched zircons from a lherzolite sample. The coexisting baddeleyite in the sample is indistinguishable from the age of zircon. Our new results confirm that the emplacement of the Jinchuan ultramafic intrusion was temporally related to the breakup of the Rodinia supercontinent.  相似文献   

11.
抚顺南部早前寒武纪变质杂岩的地质事件序列   总被引：8，自引：7，他引：1  
白翔  刘树文  阎明  张立飞  王伟  郭荣荣  郭博然 《岩石学报》2014,30(10):2905-2924
抚顺南部早前寒武纪变质杂岩是华北克拉通北缘辽北-吉南早前寒武纪变质地块的一个重要组成部分,主要由浑南群石棚子组角闪岩相变质火山岩、火山碎屑岩及相伴生的沉积岩等表壳岩系和侵位于其中的石英闪长质片麻岩、英云闪长质-奥长花岗质-花岗闪长质(TTG)片麻岩和花岗闪长岩-二长花岗岩-钾长花岗岩岩石组合组成。LA-ICP-MS锆石U-Pb同位素分析结果显示,侵位于表壳岩中的石英闪长质片麻岩样品12LN39-3的岩浆结晶年龄为2571±7Ma,指示存在老于该年龄的表壳岩系。英云闪长质片麻岩样品12LN04-1和奥长花岗质片麻岩样品13LB49-3的岩浆结晶年龄分别为2544±4Ma和2550±10Ma,记录了一期重要的英云闪长质-奥长花岗质片麻岩侵位事件。斜长角闪岩(样品12LN25-2)的岩浆结晶的最小年龄为2530±5Ma,指示另一火山喷发阶段。晚期钾长花岗岩样品12LN01-1和奥长花岗质片麻岩样品12LN27-1分别侵位于2522±4Ma和2518±23Ma,说明它们的岩浆作用发生于同一时期。而采自于晚期未变形侵入体的石英闪长岩样品12LN30-2的岩浆结晶年龄为2496±18Ma,与上述表壳岩和深成侵入体的主要变质作用(2510~2470Ma)同期发生。这些年代学结果表明,抚顺南部地区新太古代大规模的铁镁质火山喷发作用在大于2571±7Ma已经发生,紧接着2571±7Ma发生石英闪长质岩浆侵位,在2550±10Ma~2544±4Ma之间发生英云闪长质-奥长花岗质岩浆侵位。接下来铁镁质火山再度喷发(~2530±5Ma),随后为钾长花岗岩和奥长花岗质岩浆的侵位(2522±4Ma~2518±23Ma)。晚期为角闪岩相变质作用时期(2510~2470Ma),伴随一定规模的石英闪长岩侵位。  相似文献   

12.
This study presents new results on zircons from the enderbite-charnockite rocks of Enderby Land, East Antarctica. U-Pb age of 3981 ± 8 Ma (SIMS SHRIMP II), which was first obtained for a protolith of massive enderbites from Aker Peaks, eastern Napier Mts, suggests that the existence of sialic crust in the study area at 4 Ga. Although there was only one magmatic zircon (of 150 grains analyzed) in the study area known with the oldest age, its significance cannot be overestimated, since it may indirectly evidence the existence of an Early Archean crustal block with a minimum age of 4 Ga, which extends for over 300 km across Enderby Land from its western to eastern part. Based on the U-Pb systematics, REE and trace element distributions in zircons from charnockite and enderbite gneisses, high-aluminous gneisses, and basic granulites, we first revealed that an early high-temperature metamorphic event accompanied by the emplacement of granodiorite intrusions occurred in the vicinity of Aker Peaks at 3620–3630 Ma. Although the 2850–3050 metamorphic overprints are clearly observed in some other areas of Enderby Land and are widely considered to be of critical importance on a regional scale, their metamorphic signatures are apparently absent from the U-Pb systematics of the studied zircon, thus suggesting the presence of similar old zircons in the study area. At the same time, all samples in this study record a 2480–2550 Ma granulite-amphibolite facies overprint represented as new zircon growths or recrystallization of earlier phases.  相似文献   

13.
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   

14.
Zircon formation and modification during magmatic crystallization and high-grade metamorphism are explored using TIMS and LA-ICP-MS U–Pb geochronology, Lu–Hf isotope chemistry, trace element analysis and textural clues on zircons from the Koraput alkaline intrusion, Eastern Ghats Belt (EGB), India. The zircon host-rock is a granulite-facies nepheline syenite gneiss with an exceptionally low Zr concentration, prohibiting early magmatic Zr saturation. With zircon formation occurring at a late stage of advanced magmatic cooling, significant amounts of Zr were incorporated into biotite, nearly the only other Zr-bearing phase in the nepheline syenite gneisses. Investigated zircons experienced a multi-stage history of magmatic and metamorphic zircon growth with repeated solid-state recrystallization and partial dissolution–precipitation. These processes are recorded by complex patterns of internal zircon structures and a wide range of apparently concordant U–Pb ages between 869 ± 7 Ma and 690 ± 1 Ma. The oldest ages are interpreted to represent the timing of the emplacement of the Koraput alkaline complex, which significantly postdates the intrusion ages of most of the alkaline intrusion in the western EGB. However, Hf model ages of TDM = 1.5 to 1.0 Ga suggest an earlier separation of the nepheline syenite magma from its depleted mantle source, overlapping with the widespread Mesoproterozoic, rift-related alkaline magmatism in the EGB. Zircons yielding ages younger than 860 Ma have most probably experienced partial resetting of their U–Pb ages during repeated and variable recrystallization events. Consistent youngest LA-ICP-MS and CA-TIMS U–Pb ages of 700–690 Ma reflect a final pulse of high-grade metamorphism in the Koraput area and underline the recurrence of considerable orogenic activity in the western EGB during the Neoproterozoic. Within the nepheline syenite gneisses this final high-grade metamorphic event caused biotite breakdown, releasing sufficient Zr for local saturation and new subsolidus zircon growth along the biotite grain boundaries.  相似文献   

15.
The Motzfeldt Centre is one of four major alkaline centres belonging to the Igaliko complex, part of the Mid-Proterozoic Gardar province of South Greenland. Motzfeldt comprises syenites and nepheline syenites displaying complex magmatic and subsolidus histories. Ta, Nb and REE-rich zones are associated with a pervasive hydrothermal alteration process in the North and East of the complex, part of a subdivision of the Centre called the Motzfeldt Sø Formation (MSF). The zircons from the MSF show textures that are both magmatic and altered, whereas the textures of pyrochlore have a predominantly subsolidus chemistry. We compare and contrast the radiogenic isotope geochemistry of the zircon and pyrochlore to obtain independent age estimates of the magmatic and hydrothermal episodes. Selected analyses of magmatic zircons are concordant with an age of 1,273 ± 6 Ma. Altered zircons are normally discordant with one intercept at 1,261 ± 28 Ma and the other at 349 ± 49 Ma. This younger age would appear on face value to date the hydrothermal event at Motzfeldt to ~900 Ma after the magmatism. However, the Pb–Pb isotope systematics from altered pyrochlore define an isochron age of 1,267 ± 6 Ma for isotopic closure of the pyrochlore following alteration during the subsolidus. There is no known magmatic event in Southern Greenland at ~350 Ma and discordant points have high common lead. We therefore infer that the younger intercept age is spurious. The overlap between the magmatic U–Pb zircon and the Pb–Pb pyrochlore age suggests that alteration of the Motzfeldt centre occurred shortly after magmatism.  相似文献   

16.
本文报道了黑龙江省张广才岭"新元古代"一面坡群唐家屯组流纹岩和长石石英砂岩以及杨木岗组粉砂质泥岩中锆石的U-Pb定年结果,结合上覆早侏罗世太安屯组流纹岩的定年结果,讨论了一面坡群的形成时代及其构造属性。采自该群绝大多数锆石均呈自形-半自形晶,显示出典型岩浆生长环带或条痕状吸收,暗示其岩浆成因。定年结果表明,采自一面坡群唐家屯组底部流纹岩的形成时代为295±2Ma,而上部长石石英砂岩产生了234Ma、260Ma、273Ma、291Ma、309Ma和327Ma多组谐和年龄;杨木岗组粉砂质泥岩产生了226Ma、267Ma、362Ma、411Ma、485Ma和783Ma多组谐和年龄;覆盖在杨木岗组之上的中生代太安屯组流纹岩的定年结果为189±1Ma。从上述定年结果可以看出,一面坡群唐家屯组流纹岩形成于早二叠世,而其上部的长石石英砂岩与杨木岗组粉砂质泥岩的形成时代类似(226~234Ma)——即其形成于晚三叠世-早侏罗世之间(226~189Ma),而非先前所确定的新元古代。基于杨木岗组中碎屑锆石的年龄频数,杨木岗组的沉积物源主要来自于周边晚古生代地质体以及次要的早中生代、早古生代和新元古代地质体。此外,早二叠世流纹岩与同时代玄武质岩石共同构成了双峰式火山岩组合,可能暗示一种伸展环境。  相似文献   

17.
北大别片麻岩中锆石U—Pb年龄离子探针初步测定   总被引：24，自引：5，他引：19  
陈道公 Delou.  E 《岩石学报》2000,16(2):199-202
对北大别鹿吐石和道士冲二个片麻岩中锆石进行了离子探针Ｕ－Ｐｂ年龄测定。二个样品１３个颗粒的２０个有效测定点的年龄表明,鹿吐石片麻岩主要分布在２３３￣３９４Ｍ道士冲片麻岩在３０７￣７１８Ｍａ之间。道十冲片麻岩年龄的一致性程度要高于鹿叶石,两个样品均无低于２２０Ｍａ的年龄信息。由于存在年龄的不一致性,这些年龄可能代表了由古老岩浆锆石和三叠纪的超高压变质锆石或者早白垩纪岩浆锆石所代表的混合、增生锆石。Ｈ  相似文献   

18.
The Sakharjok Y-Zr deposit in Kola Peninsula is related to the fissure alkaline intrusion of the same name. The intrusion ∼7 km in extent and 4–5 km2 in area of its exposed part is composed of Neoarchean (2.68–2.61 Ma) alkali and nepheline syenites, which cut through the Archean alkali granite and gneissic granodiorite. Mineralization is localized in the nepheline syenite body as linear zones 200–1350 m in extent and 3–30 m in thickness, which strike conformably to primary magmatic banding and trachytoid texture of nepheline syenite. The ore is similar to the host rocks in petrography and chemistry and only differs from them in enrichment in zircon, britholite-(Y), and pyrochlore. Judging from geochemical attributes (high HSFE and some incompatible element contents (1000–5000 ppm Zr, 200–600 ppm Nb, 100–500 ppm Y, 0.1–0.3 wt % REE, 400–900 ppm Rb), REE pattern, Th/U, Y/Nb, and Yb/Ta ratios), nepheline syenite was derived from an enriched mantle source similar to that of contemporary OIB and was formed as an evolved product of long-term fractional crystallization of primary alkali basaltic melt. The ore concentrations are caused by unique composition of nepheline syenite magma (high Zr, Y, REE, Nb contents), which underwent subsequent intrachamber fractionation. Mineralogical features of zircon-the main ore mineral—demonstrate its long multistage crystallization. The inner zones of prismatic crystals with high ZrO2/HfO2 ratio (90, on average) grew during early magmatic stage at a temperature of 900–850°C. The inner zones of dipyramidal crystals with average ZrO2/HfO2 = 63 formed during late magmatic stage at a temperature of ∼500°C. The zircon pertaining to the postmagmatic hydrothermal stage is distinguished by the lowest ZrO2/HfO2 ratio (29, on average), porous fabric, abundant inclusions, and crystallization temperature below 500°C. The progressive decrease in ZrO2/HfO2 ratio was caused by evolution of melt and postmagmatic solution. The metamorphic zircon rims relics of earlier crystals and occurs as individual rhythmically zoned grains with an averaged ZrO2/HfO2 ratio (45, on average) similar to that of the bulk ore composition. The metamorphic zircon is depleted in uranium in comparison with magmatic zircon, owing to selective removal of U by aqueous metamorphic solutions. Zircon from the Sakharjok deposit is characterized by low concentrations of detrimental impurities, in particular, contains only 10–90 ppm U and 10–80 ppm Th, and thus can be used in various fields of application.  相似文献   

19.
U-Pb age and isotope-geochemical features were determined for zircon from kyanite gneisses and amphibolites of the Chupa Sequence of the Belomorian mobile belt (BMB) of the Fennoscandian shield. The cores of the zircon from the gneisses marks the Neoarchean events within 2700–2800 Ma known in the BMB, while those of the amphibolites correspond to the age of magmatic crystallization (2775 ± 12 Ma). The inner rims of zircon from the amphibolites and gneisses likely record two different Neoarchean metamorphic events (2650 ± 8 and 2599 ± 10 Ma, respectively). The outer rims record Paleoproterozoic metamorphism with an age of 1890 Ma, which formed the modern appearance and mineral assemblages of the rock association. The value of δ18O in the zircon from the gneiss is 8.6‰ in cores, slightly decreases to 8.0‰ in inner rims, and sharply decreases to 3.9‰ in outer rims. The value of δ18O in the zircon from the amphibolite is around 6.2‰ in cores, increases up to 8.6 in inner rims, and decreases to 5.2‰ in outer rims. A significant decrease of δ18O is likely related to the anomalous composition of Svecofennian metamorphic fluid restricted to local shear zones. The geochemical features of the zircons in combination with their morphology and anatomy make it possible to distinguish zircon generations of different age and change in metamorphic environments.  相似文献   

20.
Several types of growth morphologies and alteration mechanisms of zircon crystals in the high-grade metamorphic Ivrea Zone (IZ) are distinguished and attributed to magmatic, metamorphic and fluid-related events. Anatexis of pelitic metasediments in the IZ produced prograde zircon overgrowths on detrital cores in the restites and new crystallization of magmatic zircons in the associated leucosomes. The primary morphology and Th-U chemistry of the zircon overgrowth in the restites show a systematic variation apparently corresponding to the metamorphic grade: prismatic (prism-blocked) low-Th/U types in the upper amphibolite facies, stubby (fir-tree zoned) medium-Th/U types in the transitional facies and isometric (roundly zoned) high-Th/U types in the granulite facies. The primary crystallization ages of prograde zircons in the restites and magmatic zircons in the leucosomes cannot be resolved from each other, indicating that anatexis in large parts of the IZ was a single and short lived event at 299 ± 5 Ma (95% c. l.). Identical U/Pb ages of magmatic zircons from a metagabbro (293 ± 6 Ma) and a metaperidotite (300 ± 6 Ma) from the Mafic Formation confirm the genetic context of magmatic underplating and granulite facies anatexis in the IZ. The U-Pb age of 299 ± 5 Ma from prograde zircon overgrowths in the metasediments also shows that high-grade metamorphic (anatectic) conditions in the IZ did not start earlier than 20 Ma after the Variscan amphibolite facies metamorphism in the adjacent Strona–Ceneri Zone (SCZ). This makes it clear that the SCZ cannot represent the middle to upper crustal continuation of the IZ. Most parts of zircon crystals that have grown during the granulite facies metamorphism became affected by alteration and Pb-loss. Two types of alteration and Pb-loss mechanisms can be distinguished by cathodoluminescence imaging: zoning-controlled alteration (ZCA) and surface-controlled alteration (SCA). The ZCA is attributed to thermal and/or decompression pulses during extensional unroofing in the Permian, at or earlier than 249 ± 7 Ma. The SCA is attributed to the ingression of fluids at 210 ± 12 Ma, related to hydrothermal activity during the breakup of the Pangaea supercontinent in the Upper Triassic/Lower Jurassic. Received: 7 July 1998 / Accepted: 4 November 1998  相似文献   

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