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1.
本文利用Coherent GeoLasHD型193 nm ArF准分子激光剥蚀系统和Agilent 7900型四极杆电感耦合等离子体质谱仪, 建立了LA-ICP-MS石榴子石U-Pb定年方法。利用该方法, 对采自冀北地区晚古生代镁铁质-超镁铁质混杂岩体中的异剥钙榴岩和闽西南马坑式铁矿含矿石榴子石矽卡岩这两种岩石中的石榴子石开展U-Pb定年研究。在冀北地区晚古生代镁铁质-超镁铁质混杂岩体中的异剥钙榴岩中, 获得石榴子石下交点年龄为(387.6±5.4) Ma (D496-1, MSWD=1.1, N=30)和(409.3±7.8) Ma (D493-1, MSWD=2.0, N=60), 在马坑铁矿石榴子石矽卡岩中, 获得石榴子石下交点年龄为(128.6±2.1) Ma (ZK7921-b24, MSWD=2.0, N=60)和(128.7±3.2) Ma (ZK7922-b1, 用锆石91500校正, MSWD=1.8, N=42); 在潘田铁矿的石榴子石矽卡岩中, 获得石榴子石的下交点年龄为(128.7±1.7) Ma (PT-b1, MSWD=1.7, N=30)和(132.1±1.3) Ma (PT-b1样品, 用锆石91500校正, MSWD=1.6, N=30)(除了指明使用锆石标样91500校正石榴子石未知样品外, 其他皆用石榴子石标样Willsboro校正石榴子石未知样品的U/Pb分馏)。以上结果与Sm-Nd等时线年龄及前人报道的锆石U-Pb年龄在误差范围内一致。对马坑式铁矿石榴子石矽卡岩U-Pb定年结果表明, 利用石榴子石标样Willsboro和锆石标样91500作为外标样校正同一样品中石榴子石U/Pb同位素分馏, 获得的下交点年龄一致, 206Pb/238U年龄的加权平均值也一致, 说明石榴子石与锆石之间的基体效应较小, 在缺乏石榴子石标样时, 可用锆石标样91500代替。在上述研究基础上分析了石榴子石U-Pb定年方法在矽卡岩型矿床成矿时代研究及异剥钙榴岩年代学研究中的应用潜力, 认为石榴子石U-Pb定年方法在矽卡岩型矿床及异剥钙榴岩年代学研究中具有巨大的应用推广前景, 具有重要的理论指导和实际应用意义。 相似文献
2.
SHRIMP U–Pb dating and laser ablation ICP‐MS trace element analyses of zircon from four eclogite samples from the north‐western Dabie Mountains, central China, provide evidence for two eclogite facies metamorphic events. Three samples from the Huwan shear zone yield indistinguishable late Carboniferous metamorphic ages of 312 ± 5, 307 ± 4 and 311 ± 17 Ma, with a mean age of 309 ± 3 Ma. One sample from the Hong'an Group, 1 km south of the shear zone yields a late Triassic age of 232 ± 10 Ma, similar to the age of ultra‐high pressure (UHP) metamorphism in the east Qinling–Dabie orogenic belt. REE and other trace element compositions of the zircon from two of the Huwan samples indicate metamorphic zircon growth in the presence of garnet but not plagioclase, namely in the eclogite facies, an interpretation supported by the presence of garnet, omphacite and phengite inclusions. Zircon also grew during later retrogression. Zircon cores from the Huwan shear zone have Ordovician to Devonian (440–350 Ma) ages, flat to steep heavy‐REE patterns, negative Eu anomalies, and in some cases plagioclase inclusions, indicative of derivation from North China Block igneous and low pressure metamorphic source rocks. Cores from Hong'an Group zircon are Neoproterozoic (780–610 Ma), consistent with derivation from the South China Block. In the western Dabie Mountains, the first stage of the collision between the North and South China Blocks took place in the Carboniferous along a suture north of the Huwan shear zone. The major Triassic continent–continent collision occurred along a suture at the southern boundary of the shear zone. The first collision produced local eclogite facies metamorphism in the Huwan shear zone. The second produced widespread eclogite facies metamorphism throughout the Dabie Mountains–Sulu terrane and a lower grade overprint in the shear zone. 相似文献
3.
A cold Early Palaeozoic subduction zone in the North Qilian Mountains, NW China: petrological and U-Pb geochronological constraints 总被引：17，自引：1，他引：17
The north Qilian high‐pressure (HP)/low‐temperature (LT) metamorphic belt is composed mainly of blueschists, eclogites and greenschist facies rocks. It formed within an Early Palaeozoic accretionary wedge associated with the subduction of the oceanic crust and is considered to be one of the best preserved HP/LT metamorphic belts in China. Here we report new lawsonite‐bearing eclogites and eclogitic rocks enclosed within epidote blueschists in the North Qilian Mountains. Five samples contain unaltered lawsonite coexisting with omphacite and phengite as inclusions in garnet, indicating eclogite facies garnet growth and lawsonite pseudomorphs were observed in garnet from an additional 11 eclogites and eclogitic rocks. Peak pressure conditions estimated from lawsonite omphacite‐phengite‐garnet assemblages were 2.1–2.4 GPa at temperatures of 420–510 °C, in or near the stability field of lawsonite eclogite, and implying formation under an apparent geothermal gradient of 6–8 °C km?1, consistent with metamorphism in a cold subduction zone. SHRIMP U‐Pb dating of zircon from two lawsonite‐bearing eclogitic metabasites yields ages of 489 ± 7 Ma and 477 ± 16 Ma, respectively. CL images and mineral inclusions in zircon grains indicate that these ages reflect an eclogite facies metamorphism. An age of 502 ± 16 Ma is recorded in igneous cores of zircon grains from one lawsonite pseudomorph‐bearing eclogite, which is in agreement with the formation age of Early Ordovician for some ophiolite sequences in the North Qilian Mountains, and may be associated with a period of oceanic crust formation. The petrological and chronological data demonstrate the existence of a cold Early Palaeozoic subduction zone in the North Qilian Mountains. 相似文献
4.
L. D. Ashwal R. A. Armstrong R. J. Roberts M. D. Schmitz F. Corfu C. J. Hetherington K. Burke M. Gerber 《Contributions to Mineralogy and Petrology》2007,153(4):389-403
Nepheline-bearing gneisses from the 75 km2 Tambani body in the Mozambique Belt of southern Malawi, are miaskitic biotite-nepheline monzodiorites, reflecting an absence
of K-feldspar, alkali amphiboles or pyroxenes, and contain euhedral zircon megacrysts up to 5 cm across. The zircons contain
U = 1–1,860 ppm, Th = 0–2,170 ppm and Y = 400–1,100 ppm, and very low concentrations of all other measured trace elements
except Hf (HfO2 = 0.53–0.92 wt. %). Cathodoluminescence images reveal oscillatory sector growth zoning and no evidence for xenocrystic cores,
indicating that the zircons represent primary magmatic crystallization products that have survived amphibolite grade metamorphism.
U-Pb isotopic analyses (by TIMS) yield an upper intercept age of 730 ± 4 Ma (MSWD = 1.7), which we interpret as the time of
magmatic crystallization of the zircons. This is coincident with 11 SHRIMP spot analyses, which yield a mean age of 729 ±
7 Ma (MSWD = 0.37). Metamorphism, at 522 ± 17 Ma as suggested by monazite, caused partial Pb-loss during local recrystallization
of zircon. Lu-Hf isotopic data for three whole-rock samples of nepheline-bearing gneiss are collinear with those for zircon
megacrysts, and correspond to an age of 584 ± 17 Ma (MSWD = 0.37. We interpret the Lu-Hf array to represent a mixing line
defined by the Hf isotopic signature of primary zircon and that of the rock-forming minerals reset during metamorphic (re-)crystallization;
hence the 584 Ma age is likely geologically meaningless. Given the well-defined association of nepheline syenites (and phonolitic
volcanic equivalents) with continental rifting, we suggest that the Tambani body represents a magmatic product formed at 730 Ma
during the break-up of the Rodinia supercontinent. The 522 Ma age is akin to other Pan-African metamorphic ages that record
collisional suturing events during the final assembly of Gondwana. Zircon-bearing nepheline gneisses thus preserve a record
of intra-continental rifting and of continental collision in southern Malawi. 相似文献
5.
High-pressure metamorphism in the Pohorje Mountains of Slovenia (Austroalpine unit, Eastern Alps) affected N-MORB type metabasic
and metapelitic lithologies. Thermodynamic calculations and equilibrium phase diagrams of kyanite–phengite-bearing eclogites
reveal PT conditions of >2.1 GPa at T<750°C, but within the stability field of quartz. Metapelitic eclogite country rocks contain the assemblage garnet + phengite
+ kyanite + quartz, for which calculated peak pressure conditions are in good agreement with results obtained from eclogite
samples. The eclogites contain a single population of spherical zircon with a low Th/U ratio. Combined constraints on the
age of metamorphism come from U/Pb zircon as well as garnet–whole rock and mineral–mineral Sm-Nd analyses from eclogites.
A coherent cluster of single zircon analyses yields a 206Pb/238U age of 90.7±1.0 Ma that is in good agreement with results from Sm-Nd garnet–whole rock regression of 90.7±3.9 and 90.1±2.0 Ma
(εNd: +8) for two eclogite samples. The agreement between U-Pb and Sm-Nd age data strongly suggests an age of approximately
90 Ma for the pressure peak of the eclogites in the Pohorje Mountains. The presence of garnet, omphacite and quartz inclusions
in unfractured zircon indicates high-pressure rather than ultrahigh pressure conditions. The analysed metapelite sample yields
a Sm-Nd garnet–whole rock scatterchron age of 97±15 Ma. These data probably support a single P-T loop for mafic and pelitic
lithologies of the Pohorje area and a late Cretaceous high-pressure event that affected the entire easternmost Austroalpine
basement including the Koralpe and Saualpe eclogite type locality in the course of the complex collision of the Apulian microplate
and Europe. 相似文献
6.
7.
在大兴安岭中南段巴雅尔吐胡硕地区发现一套碎斑熔岩。根据野外调查,可分为中心相的细微粒斑状石英二长岩(-石英二长斑岩)和边缘相的英安流纹质碎斑熔岩。通过SHRIMP锆石U-Pb测年,获得细微粒斑状石英二长岩的年龄加权平均值为137.4±0.9Ma(MSWD=1.13),英安流纹质碎斑熔岩的年龄加权平均值为135.2±0.8Ma(MSWD=1.17),二者的形成时代均属于早白垩世早期。由元素地球化学分析可知,6件样品均属于过铝质的高钾钙碱性A型花岗岩,且具有完全一致的微量元素蛛网图和稀土元素配分曲线,均具有明显的负Eu异常,均富集大离子亲石元素Rb、Pb和轻稀土元素,亏损高场强元素Nb,Th较富集,Ba、Sr、Eu具有一定亏损,Pb强烈富集,表明岩石来源于地壳部分熔融。在构造环境判别分析的基础上,提出研究区碎斑熔岩属于碰撞后或造山期后的张性构造环境花岗岩(A2型花岗岩),形成于拉张环境,代表了伸展的大地构造背景。结合大兴安岭地区的大地构造演化特征,其形成与蒙古-鄂霍茨克闭合造山后的岩石圈伸展作用有关。 相似文献
8.
在岩相学观察和锆石CL图像研究的基础上,利用LA ICP MS原位分析方法,对北秦岭官坡超高压榴辉岩和伴生的石榴石角闪岩(榴闪岩)进行了详细的锆石微区微量元素和U Th Pb同位素分析,在榴辉岩样品中得到变质年龄为(502±11)Ma,原岩结晶年龄>(657±18)Ma;在榴闪岩样品中得到原岩结晶年龄为(791±6)Ma,变质年龄为487~503 Ma,角闪岩相退变质年龄为(366±4)Ma。岩石地球化学研究显示,北秦岭官坡地区的榴闪岩具有低Si(SiO2质量分数为4916%~5078%),高Ti(TiO2质量分数为228%~283%)、富集LREE、LILE和大部分的HFSE元素,不显Nb、Ta负异常的板内玄武岩特征,与北秦岭超高压榴辉岩地球化学特征一致。结合两者的野外产状、岩相学特征、锆石形貌和年代学研究结果,表明本文研究的官坡地区的榴闪岩是超高压榴辉岩在抬升过程中在角闪岩相条件下退变质的产物。综合两者的年代学研究结果,得到北秦岭地区超高压榴辉岩的变质年龄为(502±11)Ma,原岩结晶年龄为(791±6)Ma,角闪岩相退变质年龄为(366±4)Ma。研究得到的(502±11)Ma的榴辉岩相变质年龄与前人得到的该榴辉岩围岩超高压泥质片麻岩的变质年龄(507±38)Ma以及北秦岭松树沟地区的超高压长英质片麻岩的变质年龄485~514 Ma一致,表明它们经历了同期超高压变质作用。而且,榴辉岩(502±11)Ma的变质年龄与其原岩的结晶年龄(791±6)Ma存在近300 Ma的时间间隔,表明原岩具有板内玄武岩性质的北秦岭官坡超高压榴辉岩不可能是秦岭古生代大洋板块深俯冲的产物,而可能是已构造就位的古洋壳或裂谷火山岩在古生代随陆壳一起发生大陆深俯冲作用的产物。 相似文献
9.
《International Geology Review》2012,54(9):1064-1075
ABSTRACTThe Neoproterozoic Kaijianqiao Formation is one of the most important pre-Sturtian rift successions in South China and there has long been a lack of reliable geochronological constraints for its minimum depositional age. In this study, new zircon U-Pb ages of volcaniclastic rocks from the topmost Kaijianqiao Formation are presented. The youngest SHRIMP and LA-ICP-MS zircon 206Pb/238U weighted mean ages of the tuff sample are 715.0 ± 9.8 and 718.8 ± 9.4 Ma, respectively. The youngest LA-ICP-MS zircon 206Pb/238U weighted mean age of the tuffaceous siltstone sample is 720.8 ± 7.4 Ma and represents the maximum depositional age of the topmost Kaijianqiao Formation. The results show that the minimum depositional age of the Kaijianqiao Formation in the western Yangtze Block should be ca. 715 Ma, consistent with other pre-Sturtian rift successions in South China, such as the Banxi Group, Chengjiang, and Liantuo formations. Together with the published zircon U-Pb ages, it is demonstrated that the Sturtian glaciation in South China (Jiangkou glaciation) most likely initiated around 715 Ma. In other Rodinia blocks, like Laurentia and Arabia, the Sturtian glaciation probably started between 712 and 717 Ma, thus our new results further support that the Sturtian glaciation was a rapid and globally synchronous event. Other 206Pb/238U zircon ages display five distinct peaks at ca. 751, 780, 799, 819, and 848 Ma, which corresponded to the tectonic-magmatic events related to the break-up of Rodinia. 相似文献
10.
冀北中低级变质表壳岩的年代学特征及其对华北克拉通构造演化的约束 总被引：1，自引：1，他引：0
华北克拉通在新太古代末期发生克拉通化,形成了现今规模的古陆,大量的太古宙岩石均经历了~2500Ma左右的区域高级变质作用(高角闪岩相-麻粒岩相)。而华北克拉通北部冀北地区出露一套中低级变质(绿片岩相-角闪岩相)的火山-沉积岩系,主要包括胡麻营地区红旗营子表壳岩和大阴山地区单塔子表壳岩中变质程度较低的部分。胡麻营地区红旗营子表壳岩系主要岩石组合为变基性火山岩、绿帘角闪岩、斜长角闪岩、含石榴石斜长角闪岩、角闪斜长片麻岩、黑云斜长片麻岩、黑云角闪斜长片麻岩、黑云二长片麻岩、石英片岩、磁铁石英岩等,SIMS锆石U-Pb定年结果表明斜长角闪岩形成于2486±18Ma(MSWD=1.4),而黑云斜长片麻岩形成于2507±37Ma(MSWD=2.0)。大阴山地区单塔子中低级变质表壳岩系主要由浅变质火山岩、云母石英片岩、斜长角闪岩、磁铁石英岩和大理岩等组成,SHRIMP锆石U-Pb定年结果显示,浅变质火山岩中的变玄武岩形成于2490±19Ma(MSWD=2.0),而变英安岩形成于2502±8Ma(MSWD=0.83)。因此,冀北中低级变质的表壳岩系主要形成于新太古代末期,形成年龄为2507~2486Ma;结合冀东青龙地区新太古代末期(2511~2503Ma)的浅变质火山-沉积岩系(青龙表壳岩),我们认为新太古代末期,中低级变质表壳岩系广泛分布于华北克拉通的核部和边缘地区,此套岩系覆盖在太古宙高级变质杂岩之上,代表华北克拉通化之后的稳定盖层,是克拉通化的主要标志之一。 相似文献
11.
胶东谭格庄地区奥长花岗质片麻岩和斜长角闪岩的野外地质和锆石SHRIMP定年 总被引：10，自引：9，他引：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
12.
Hannes K. Brueckner Jane A. Gilotti Allen P. Nutman 《Contributions to Mineralogy and Petrology》1998,130(2):103-120
High-pressure metamorphic assemblages occur in mafic, ultramafic and a few intermediate rocks in a gneiss complex that covers
an area of approximately 400 × 100 km in the North-East Greenland Caledonides. Detailed petrologic and geochronologic studies
were carried out on three samples in order to clarify the P-T-t evolution of this eclogite province. Geothermobarometry yields temperature estimates of 700–800 °C and pressure estimates
of at least 1.5 GPa from an eclogite sensu stricto and as high as 2.35 GPa for a garnet websterite. The eclogite defines a garnet-clinopyroxene-amphibole-whole rock Sm-Nd isochron
age of 405 ± 24 Ma (MSWD 0.9). Isofacial garnet websterites define garnet-clinopyroxene-orthopyroxene-amphibole-whole rock-(biotite)
ages of 439 ± 8 Ma (MSWD =2.1) for a coarse-grained sample and 370 ± 12 Ma (MSWD=0.6) for a finer-grained variety. Overgrowths
on zircons from the fine-grained pyroxenite and the eclogite give a pooled 206Pb/238U SHRIMP age of 377 ± 7 Ma (n=4). Significantly younger Rb-Sr biotite ages of 357 ± 8, 330 ± 6 and 326 ± 6 agree with young Rb-Sr, K-Ar and 40Ar/39Ar mineral ages from the gneiss complex and indicate slow cooling of the eclogitic rocks. High-pressure metamorphism may have
been at least 439 Ma old (Siluro-Ordovician) with cooling through amphibolite-facies conditions in the Devonian and continued
crustal thinning and exhumation well into the Carboniferous. Sm-Nd whole rock model ages indicate the eclogite protoliths
are Early Proterozoic in age, while 207Pb/206Pb SHRIMP ages of 1889 ± 18 and 1981 ± 8 from anhedral zircon cores probably reflect Proterozoic metasomatism. The samples
have negative ɛNd values (−5 to −16) and elevated 87Sr/86Sr ratios (0.708–0.715), consistent with field evidence that the eclogite protoliths were an integral part of the continental
crust long before Caledonian metamorphism. The presence of a large Caledonian eclogite terrane in Greenland requires modification
of current tectonic models that postulate subduction of Baltica beneath Laurentia during the Caledonian orogeny.
Received: 9 October 1996 / Accepted: 7 July 1997 相似文献
13.
阿尔金江尕勒萨依榴辉岩和围岩锆石LA-ICP-MS微区原位定年及其地质意义 总被引：27，自引：0，他引：27
阿尔金江尕勒萨依榴辉岩及其直接围岩——石榴子石黑云母片麻岩锆石的阴极发光图像、微区原位LA-ICP-MS微量元素分析研究表明,榴辉岩锆石内部结构比较均匀,少数颗粒保留斑杂状残核;位于锆石斑杂状残核测点的重稀土相对富集,Th/U比值多大于0.4,为岩浆锆石的特征;位于锆石边部与内部结构均匀颗粒上的测点显示HREE近平坦型或弱亏损型的稀土配分模式,显示了与石榴石平衡共生的变质锆石特征;而石榴子石黑云母片麻岩的锆石具有核-幔-边结构,核部为碎屑锆石,幔部则为与石榴石平衡共生的变质锆石。LA-ICP-MS微区定年获得榴辉岩的变质年龄为(493±4.3)Ma,其原岩形成年龄为(754±9)Ma;石榴子石黑云母片麻岩的变质年龄为(499±27)Ma。榴辉岩的变质年龄滞后于其原岩的形成年龄约250Ma,并且榴辉岩与其直接围岩副片麻岩的变质年龄几乎完全一致,充分表明该超高压榴辉岩的形成是陆壳深俯冲作用的产物。 相似文献
14.
Two porphyry Cu-Mo prospects in northern Sonora, Mexico (Fortuna del Cobre and Los Humos) located within the southwestern North American porphyry province have been dated in order to constrain the timing of crystallization and mineralization of these ore deposits. In Fortuna del Cobre, the pre-mineralization granodiorite porphyry yielded an U-Pb zircon age of 76.5?±?2.3 Ma, whereas two samples from the ore-bearing quartz feldespathic porphyry were dated at 74.6?±?1.3 and 75.0?±?1.4 Ma. Four molybdenite samples from Los Humos porphyry Cu prospect yielded a weighted average Re-Os age of 73.5?±?0.2 Ma, whereas two samples from the ore-bearing quartz monzonite porphyry gave U-Pb zircon ages of 74.4?±?1.1 and 74.5?±?1.3 Ma, showing a Late Cretaceous age for the emplacement of this ore deposit. The results indicate that Laramide porphyry Cu mineralization of Late Cretaceous age is not restricted to northern Arizona as previously thought and provide evidence for the definition of NS trending metallogenic belts that are parallel to the paleo-trench. Porphyry copper mineralization follows the inland migration trend of the magmatic arc as a result of the Farallon slab flattening during the Laramide orogeny. 相似文献
15.
柴北缘HP-UHP变质带多期构造热事件的再厘定:鱼卡地区高压变泥质岩锆石和独居石U-Pb定年 总被引：3，自引：3，他引：0
柴北缘高压-超高压变质带西段鱼卡地区变泥质岩中夹有榴辉岩透镜体,已有的研究显示变泥质岩的变质程度也达到了榴辉岩相,并与榴辉岩一起经历了高压-超高压变质作用,是柴北缘曾经历早古生代大陆深俯冲作用的直接证据,也是研究柴北缘大陆深俯冲过程重要的岩石"探针"。本文选择柴北缘西段鱼卡超高压变质单元中的3件蓝晶石榴白云母石英片岩HP变泥质岩样品分别进行了SHRIMP、LA-ICP-MS锆石和原位独居石U-Pb定年。样品Q06-1-2的锆石给出了920±18Ma(MSWD=1.3)的加权平均年龄,其CL图像特征和极低的Th/U比显示其为变质年龄,代表了与罗迪尼亚超大陆碰撞拼合相关的变质事件。样品A03-11-2.2的锆石给出了450±7Ma(MSWD=0.2)的年龄,认为其代表变泥质岩的榴辉岩相变质年龄。样品A03-14-11的薄片原位独居石定年给出了439±8Ma(MSWD=0.072)的加权平均年龄,结合岩相学观察,认为其可能为榴辉岩相峰期之后的早期退变质年龄。这些资料显示柴北缘鱼卡地区早古生代大陆深俯冲的时限为440~450Ma。结合已有研究资料,鱼卡高压变泥质岩记录了新元古代早期和早古生代两期变质事件,进一步证明了柴北缘地区经历了格林威尔期和早古生代两期造山事件 相似文献
16.
Trace-element study and age dating of zircon from chromitites of the Bushveld Complex (South Africa)
Marina Yudovskaya Judith Kinnaird Anthony J. Naldrett Nickolay Rodionov Anton Antonov Sergey Simakin Dmitry Kuzmin 《Mineralogy and Petrology》2013,107(6):915-942
The layered Bushveld Complex hosts a number of chromitite layers, which were found to contain significant amounts of zircon grains compared with adjacent silicate rocks. Cathodoluminescent-dark, partially metamict cores and transparent rims of composite zircon grains were analyzed for trace elements with SIMS and LA-ICPMS techniques. The cores are enriched in REE, Y, Th and U and are characterized by distinctly flatter REE patterns in contrast to those of the rims and transparent homogenous crystals. Zircon from the different stratigraphic units has specific Th/U ratios, the highest of which (1.5–4) occurs in a Merensky Reef zircon core. The Ti content of Bushveld zircon ranges from 12 to 52 ppm correlating to a crystallization temperature range of 760–930 °C. The geochemical characteristics of the first zircon generation are consistent with its high-temperature crystallization as the first major U, Th and REE acceptor from a highly-evolved residue of the high-Mg basalt magma, whereas the rims and coreless crystals have crystallized from percolating intercumulus liquid of new influx of the same magma. U-Pb SHRIMP dating of zircon cores and rims does not reveal a distinguishable difference between their ages indicating the absence of inherited zircon. Concordia ages of 2,051?±?9 Ma (2σ, MSWD?=?0.1) and 2,056?±?5 Ma (2σ, MSWD?=?0.05) for zircons from the Merensky Reef and the Upper Platreef located equally near the top of the Critical Zone are in agreement with published ages for the Merensky Reef. Zircon from the deeper-seated Lower Group, Middle Group and Lower Platreef chromitites yields younger concordia ages that may reflect prolonged late-stage volatile activity. 相似文献
17.
《International Geology Review》2012,54(2):171-195
abstractAn integrated study including petrography, mineral chemistry, metamorphic P–T path modelling, and zircon U–Pb dating was conducted on a granitic gneiss and enclosed eclogite from South Dulan, North Qaidam UHP (ultrahigh-pressure) belt. The result shows that the granitic gneiss underwent a clockwise P–T path with a peak-P stage at 655–745°C, 30–34 kbar, and a subsequent peak-T stage at 815–870°C, 14–18 kbar, which is similar to the P–T estimates reported for coesite-bearing continental-type eclogites in this region. The enclosed eclogite resembles an olivine–pyroxene-rich cumulate in Qaidam block. It has a similar prograde P–T path with the country gneiss and experienced a peak-P stage of 682–748°C at 27–34 kbar. Zircon U–Pb dating yields an eclogite-facies metamorphic age of 447 ± 2 Ma for the granitic gneiss and 445 ± 6 Ma for the enclosed eclogite. These ages agree with metamorphic ages obtained from paragneisses (427–439 Ma), coesite-bearing continental-type eclogites (430–451 Ma), and UHPM (ultrahigh-pressure metamorphic) oceanic crust–mantle sequence (440–445 Ma) from South Dulan, as well as UHP eclogites, garnet peridotite, and gneisses from other units (460–420 Ma) within this belt reported by others. Similar metamorphic ages as well as P–T evolution documented in gneisses and intercalated eclogites imply that both rocks experienced a coeval UHP event. Summarizing all the published geochronology data, we argue that the North Qaidam UHP belt was mainly formed by continental deep subduction at ~460 to ~420 Ma. The UHPM oceanic crust-mantle sequence in South Dulan may represent oceanic lithosphere in the transition zone between oceanic and continental crust, which was dragged upward by the exhumed continental rocks after break-off of the dense oceanic crust. 相似文献
18.
The Seve Nappe Complex of the Scandinavian Caledonides is thought to be derived from the distal passive margin of Baltica which collided with Laurentia in the Scandian Phase of the Caledonian Orogeny at 430–400 Ma. Parts of the Seve Nappe Complex were affected by pre-Scandian high- and ultrahigh-pressure metamorphism, in a tectonic framework that is still unclear, partly due to uncertainties about the exact timing. Previous age determinations yielded between ~ 505 and ~ 446 Ma, with a general trend of older ages in the North (Norrbotten) than in the South (Jämtland). New age determinations were performed on eclogite and garnet–phengite gneiss at Tjeliken in northern Jämtland. Thermodynamic modelling yielded peak metamorphic conditions of 25–27 kbar/680–760 °C for the garnet–phengite gneiss, similar to published peak metamorphic conditions of the eclogite (25–26 kbar/650–700 °C). Metamorphic rims of zircons from the garnet–phengite gneiss were dated using secondary ion mass spectrometry and yielded a concordia age of 458.9 ± 2.5 Ma. Lu–Hf garnet-whole rock dating yielded 458 ± 1.0 Ma for the eclogite. Garnet in the eclogite shows prograde major-element zoning and concentration of Lu in the cores, indicating that this age is related to garnet growth during pressure increase, i.e. subduction. The identical ages from both rock types, coinciding with published Sm–Nd ages from the eclogite, confirm subduction of the Seve Nappe Complex in Northern Jämtland during the Middle Ordovician in a fast subduction–exhumation cycle. 相似文献
19.
J. X. ZHANG C. G. MATTINSON S. Y. YU J. P. LI F. C. MENG 《Journal of Metamorphic Geology》2010,28(9):955-978
Coesite‐bearing eclogites from >100 km2 in the southern Dulan area, North Qaidam Mountains (NQM) of western China, contain zircon that records protolith crystallization and ultra high pressure (UHP) metamorphism. Sensitive High‐Resolution Ion Microprobe (Mass Spectrometer) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry U–Pb analyses from cathodoluminescence (CL)‐dark zircon cores in a coesite‐bearing eclogite yield an upper intercept age of 838 ± 50 Ma, and oscillatory zoned cores in a kyanite‐bearing eclogite gave a weighted mean 206Pb/238U age of 832 ± 20 Ma. These zircon cores yield steep heavy rare earth element (HREE) slopes and negative Eu anomalies that suggest a magmatic origin. Thus, c. 835 Ma is interpreted as the eclogite protolith age. Unzoned CL‐grey or ‐bright zircon and zircon rims from four samples yield weighted mean ages of 430 ± 4, 438 ± 2, 446 ± 10 and 446 ± 3 Ma, flat HREE patterns without Eu anomalies, and contain inclusions of garnet, omphacite, rutile, phengite and rare coesite. These ages are interpreted to record 16 ± 5 Myr of UHP metamorphism. These new UHP ages overlap the age range of both eclogite and paragneiss from the northern Dulan area, suggesting that all UHP rock types in the Dulan area belong to the same tectonic unit. Our results are consistent with slow continental subduction, but do not match oceanic subduction and diapiric exhumation UHP model predictions. These new data suggest that, similar to eclogites in other HP/UHP units of the NQM and South Altyn Tagh, protoliths of the eclogites in the Dulan area formed in a continental setting during the Neoproterozoic, and then subducted to mantle depth together with continental materials during the Early Palaeozoic. 相似文献
20.
内蒙古长山壕金矿区花岗岩同位素年代学研究及地质意义 总被引：3，自引：1，他引：2
长山壕金矿床是近年来在内蒙古中西段找到的一特大型金矿床。尽管金矿化在中元古界白云鄂博群变质沉积岩内呈层状、似层状和透镜状产出,但是与各类花岗岩类侵入岩具有密切空间分布关系。本次研究采用LA-ICP-MS方法分别对有关花岗岩类侵入岩进行了系统年代学研究,花岗斑岩和二长花岗斑岩样品的锆石U-Pb年龄值分别为290.9±2.8Ma (MSWD=1.4)和287.5±1.9Ma (MSWD=2.4);2件黑云母花岗岩样品的年龄值分别为267.9±1.2Ma (MSWD=0.95)和274.0±2.3Ma (MSWD=1.4)。根据上述同位素年龄数据,同时结合金矿区野外地质调查和岩(矿)相学证据,可以认为,长山壕金矿区花岗斑岩和二长花岗斑岩及相关金矿床的形成时间为早二叠世早期,并且遭受到早二叠世晚期-中二叠世早期构造-岩浆活动的叠加改造,矿区切穿含金矿脉黑云母花岗岩体的存在就是很好的例证。强烈的中酸性岩浆作用为金矿床的形成提供了动力、热力和物质来源,初步研究结果表明,长山壕金矿床是海西期构造-岩浆作用及相关流体活动的产物,属于与侵入岩有关的中温热液脉型金矿床。 相似文献