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1.
对青藏高原西北部班公湖缝合带开展了路线剖面地质调查,在北面日土蛇绿岩带上采得2处镁铁质岩墙(辉长岩和辉绿岩),岩石地球化学分析表明这两处镁铁质岩墙具有平缓的球粒陨石标准化稀土元素分布型式,显示出洋中脊玄武岩特征,说明它们形成于洋中脊环境,其锆石U-PbLAICPMS年龄测定结果分别为(181.9±2.6)Ma(MSWD=11.5)和(184.4±4.4)Ma(MSWD=2.2),表明班公湖中特提斯洋是在早侏罗世晚期打开的。另外,在狮泉河—改则蛇绿岩带上采得橄榄辉绿岩,岩石以K2O(质量分数为1.49%～1.56%)、Al2O3(质量分数为14.76%～15.21%)含量高,Fe2O3/FeO比值高(1.73～2.49),富集大离子不相容元素(Rb、Ba、Th、Sr)和轻稀土元素(LREE)为特点,在基性岩构造环境判别图上显示出洋岛玄武岩的亲合性。该样品给出的(176.2±9.0)Ma(MSWD=3.1)的锆石U-PbLAICPMS年龄说明,班公湖中特提斯洋从早侏罗世晚期打开,经过近10Ma的扩张至中侏罗世早期已变成一个成热的洋盆。  相似文献   

2.
The Naruo porphyry copper-gold deposit(hereinafter referred to as the Naruo deposit) in Tibet is a potentially ultra-large, typical gold-rich porphyry copper deposit, which was recently discovered in the Bangongco-Nujiang metallogenic belt. This study analyzed U-Pb chronology and Hf isotopes of the ore-bearing granodiorite porphyry in the Naruo deposit using the LA-ICPMS dating technique. The results show that the weighted average age is 124.03±0.94Ma(MSWD=1.7, n=20), and 206Pb/238 U isochron age is 126.2±2.7 Ma(MSWD=1.02, n=20), both of which are within the error. The weighted average age represents the crystallization age of the granodiorite porphyry, which indicates that the ore-bearing porphyry in the Naruo deposit area was formed in the Early Cretaceous and further implies that the Neo-tethys Ocean had not been closed before 124 Ma under a typical island-arc subduction environment. The εHf(t) of zircons from the granodiorite porphyry varies from 2.14 to 9.07, with an average of 5.18, and all zircons have εHf(t) values greater than 0; 176Hf/177 Hf ratio is relatively high(0.282725–0.282986). Combined with the zircon age―Hf isotope correlation diagram, the aforementioned data indicate that the source reservoir might be a region that is mixed with depleted mantle and ancient crust, which possibly contains more materials sourced from depleted mantle. Rock-forming ages and ore-forming ages of the Duolong ore concentrate area are 120–124 Ma and 118–119 Ma, respectively, which indicate 124–118 Ma represents the main rockforming and ore-forming stage within the area. The Naruo deposit is located in the north of the Bangongco-Nujiang suture, and it yielded a zircon LA-ICPMS age of 124.03 Ma. This indicates the Bangongco-Nujiang oceanic basin subducted towards the north at about 124 Ma, and the Neo-tethys Ocean had not been closed before the middle Early Cretaceous. It is possible that the crust-mantle mixing formed the series of large and giant porphyry copper-gold deposits in the Bangongco.  相似文献   

3.
The Bolong porphyry Cu–Au deposit is a newly discovered deposit in the central Tibetan Plateau, and is ranked as the second largest copper deposit discovered to date in the Bangong‐Nujiang metallogenic belt in China. Three granodiorite porphyry phases occur within the Bolong porphyry Cu–Au deposit. Phyllic alteration is widespread on the surface of the deposit, and potassic alteration occurs at depth, associated with granodiorite porphyries. The copper and gold mineralization is clearly related to the potassic and phyllic alteration. Multiple chronometers were applied to constrain the timing of magmatic–hydrothermal activity at the Bolong deposit. Zircon U–Pb geochronology reveals that the granodiorite porphyry phases were emplaced at ca. 120 Ma. Re–Os data of four molybdenite samples from quartz–molybednite veinlets yielded an isochron age of 119.4 ± 1.3 Ma. The plateau age of hydrothermal K‐feldspar from the potassic alteration zone, analyzed by 40Ar/39Ar dating, is 118.3 ± 0.6 Ma, with a similar reverse isochron age of 118.5 ± 0.7 Ma. Therefore, the magmatic–hydrothermal activity occurred at ca. 120–118 Ma, which is similar in age to the neighboring Duobuza porphyry copper deposit. The period of 120–118 Ma is therefore important for the development of porphyry Cu–Au mineralization in the central Tibetan Plateau, and these porphyry deposits were formed during the final stages of the northward subduction of the Neo‐Tethys Ocean.  相似文献   

4.
Abstract  Abundant small mafic intrusions occur associated with granitoids along the Gangdisê magmatic belt. In addition to many discrete gabbro bodies within the granitoid plutons, a gabbro‐pyroxenite zone occurs along the southern margin of the Gangdisê belt to the north of the Yarlung Zangbo suture. The mafic intrusion zone spatially corresponds to a strong aeromagnetic anomaly, which extends ～1400 km. The mafic intrusions consist of intermittently distributed small bodies and dikes of gabbro and dolerite with accumulates of pyroxenite, olivine pyroxenite, pegmatitic pyroxenite and amphibolite. Much evidence indicates that the Gangdisê gabbro‐pyroxenite assemblage is most likely a result of underplating of mantle‐derived magma. Detailed field investigation and systematic sampling of the mafic rocks was conducted at six locations along the Lhasa‐Xigazê segment of the mafic intrusive zone, and was followed by zircon SHRIMP II U‐Pb dating. In addition to the ages of two samples previously published (47.0±1 Ma and 48.9±1.1 Ma), the isotopic ages of the remaining four gabbro samples are 51.6±1.3 Ma, 52.5±3.0 Ma, 50.2±4.2 Ma and 49.9±1.1 Ma. The range of these ages (47–52.5 Ma) provide geochronologic constraints on the Eocene timing of magma underplating beneath the Gangdisê belt at ca. 50 Ma. This underplating event post‐dated the initiation of the India‐Eurasia continental collision by 15 million years and was contemporaneous with a process of magma mixing. The SHRIMP II U‐Pb isotopic analysis also found several old ages from a few zircon grains, mostly in a range of 479–526 Ma (weighted average age 503±10 Ma), thus yielding information about the pre‐existing lower crust when underplating of mafic magma took place. It is believed that magma underplating was one of the major mechanisms for crustal growth during the Indian‐Eurasia collision, possibly corresponding in time to the formation of the 14–16 km‐thick “crust‐mantle transitional zone” characterized by Vp = 6.85–6.9 km/s.  相似文献   

5.
东昆仑造山带是横亘青海省中部的一条复合造山带,北部祁漫塔格一带有大量NW向延伸的辉长岩、辉绿岩及辉绿玢岩呈岩墙分布.其中,有一期侵入到元古宙—志留纪地质体中的基性岩墙TiO2平均含量为1.63％,为典型的高钛基性岩,岩石地球化学成分与板内玄武岩一致,大离子亲石元素富集,高场强元素较低,微量元素蛛网图呈现出明显的Ba、Ta和Nb“谷”.利用激光等离子体质谱法(LA-ICP-MS),获得辉绿岩中22颗岩浆锆石的206Pb/238U年龄加权平均值为380.3Ma±1.5Ma (MSWD=1.4),精确地表明中晚泥盆世东昆仑—祁漫塔格地区进入陆内伸展阶段,区域构造由志留纪—早泥盆世的碰撞挤压体制转变为后造山伸展环境.  相似文献   

6.
The Gangdese magmatic belt formed during Late Triassic to Neogene in the southernmost Lhasa terrane of the Tibetan plateau. It is interpreted as a major component of a continental margin related to the northward subduction of the Neo-Tethys oceanic slab beneath Eurasia and it is the key in understanding the tectonic framework of southern Tibet prior to the India-Eurasia collision. It is widely accepted that northward subduction of the Neo-Tethys oceanic crust formed the Gangdese magmatic belt, but the occurrence of Late Triassic magmatism and the detailed tectonic evolution of southern Tibet are still debated. This work presents new zircon U-Pb-Hf isotope data and whole-rock geochemical compositions of a mylonitic granite pluton in the central Gangdese belt, southern Tibet. Zircon U-Pb dating from two representative samples yields consistent ages of 225.3±1.8 Ma and 229.9±1.5 Ma, respectively, indicating that the granite pluton was formed during the early phase of Late Triassic instead of Early Eocene(47–52 Ma) as previously suggested. Geochemically, the mylonitic granite pluton has a sub-alkaline composition and low-medium K calc-alkaline affinities and it can be defined as an I-type granite with metaluminous features(A/CNK1.1). The analyzed samples are characterized by strong enrichments of LREE and pronounced depletions of Nb, Ta and Ti, suggesting that the granite was generated in an island-arc setting. However, the use of tectonic discrimination diagrams indicates a continental arc setting. Zircon Lu-Hf isotopes indicate that the granite has highly positive εHf(t) values ranging from +13.91 to +15.54(mean value +14.79), reflecting the input of depleted mantle material during its magmatic evolution, consistent with Mg~# numbers. Additionally, the studied samples also reveal relatively young Hf two-stage model ages ranging from 238 Ma to 342 Ma(mean value 292 Ma), suggesting that the pluton was derived from partial melting of juvenile crust. Geochemical discrimination diagrams also suggest that the granite was derived from partial melting of the mafic lower crust. Taking into account both the spatial and temporal distribution of the mylonitic granite, its geochemical fingerprints as well as previous studies, we propose that the northward subduction of the Neo-Tethys oceanic slab beneath the Lhasa terrane had already commenced in Late Triassic(~230 Ma), and that the Late Triassic magmatic events were formed in an active continental margin that subsequently evolved into the numerous subterranes, paleo-island-arcs and multiple collision phases that form the present southern Tibet.  相似文献   

7.
The multi-stage intrusions of intermediate-acid magma occur in the Bangpu mining district, the petrogenic ages of which have been identified. The times and sequences of their emplacement have been collated and stipulated in detail in this paper by using the laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U-Pb dating method. The ages of biotite monzogranite that were formed before mineralization in the southwest of this mining district are 70±1?Ma (mean square of weighted deviates (MSWD) =9.5, n=8) and 60.60±0.31?Ma (MSWD=3.8, n=16), which belong to the late Cretaceous–early Paleocene in age. That means, they are products of an early tectonic-magmatic event of the collision between the Indian and Asian continentals. The ages of ore-bearing monzogranite porphyry and ore-bearing diorite porphyrite are 16.23±0.19?Ma (MSWD=2.0, n=26) and 15.16±0.09?Ma (MSWD=3.9, n=5) separately, which belong to the middle Miocene in age; namely, they are products of the Gangdese post-collision extensional stage when crust-mantle materials melted and mixed as well as magmatic intrusion simultaneously occurred. Some zircons with ages of 203.6±2.2?Ma (MSWD=1.18, n=7) were captured in the ore-bearing diorite porphyrite, which shows that there had been tectono-magmatic events in the late Triassic–early Jurassic. Molybdenum (copper) ore-bodies produced in the monzogranite porphyry and copper (molybdenum) ore-bodies produced in the diorite porphyrite are the main ore types in this ore deposit. The model ages of Re-Os isotopic dating for the 11 molybdenite are 13.97–15.84?Ma, while isochron ages are 14.09±0.49?Ma (MSWD=26). The isochron ages of seven molybdenite from molybdenum (copper) ore with monzogranite porphyry type are 14.11±0.31?Ma (MSWD=5.2). There is great error in the isochron ages of four molybdenite from copper (molybdenum) ore with diorite porphyrite type, and their weighted average model ages of 14.6±1.2?Ma (MSWD=41), which generally represent the mineralization age. The results about the Re-Os isotopic dating of molybdenite in the ore of different types have limited exactly that, the minerlazation age of this ore deposits is about 14.09?Ma, which belongs to the middle Miocene mineralization. The Bangpu deposit has a uniform metallogenic dynamics background with the porphyry type and skarn-type deposits such as Jiama, Qulong and others.  相似文献   

8.
The Sarek Dyke Swarm (SDS) crops out in the Sarektjåkkå Nappe (SN) of the Seve-Kalak Superterrane in the northern Swedish Caledonides. The SN has two main components: (1) a 4–5 km thick succession of rift-related sedimentary rocks, which is intruded by (2) a suite of tholeiitic dykes (the SDS) constituting 70–80% of the nappe. The nappe was deformed during Caledonian thrusting, but dykes and sedimentary rocks in the interior of the eastern parts of the SN are preserved in a pristine state. The tholeiitic dykes of the SDS commonly occur in sheeted dyke complexes, and up to 11 successive generations can be identified from crosscutting relations. The SN represents the fossil continent–ocean transition between the Baltic craton and the Iapetus Ocean, marking the initiation of seafloor spreading. Bubble-shaped pods and veinlets of diorite are present in the SDS sheeted dyke complexes. The pods are absent in the oldest dykes, but the younger a dyke, the more frequent the pods. The diorite pods are the equivalent of gabbro pegmatites, and both cogenetic and coeval with the dykes. The rapid successive emplacement of tholeiitic magma raised the ambient temperature in the dyke complex, so that crystallization in the youngest dykes mimicked similar processes in gabbro plutons. Six zircon fractions, from the diorite pods including two single grains, were analysed geochronologically by the U–Pb thermal ionization mass spectrometry method. The data yield a linear array of points that are 0.4–0.8% normally discordant, indicating a crystallization age of 608±1 Ma (207Pb/206Pb=607.9±0.7 Ma, MSWD=0.33). This age is inferred to date the onset of seafloor spreading in the Iapetus Ocean along the Baltoscandian margin.  相似文献   

9.
川西金口河地区中元古代峨边群中发育大量基性侵入岩,其具体侵位时间对厘定峨边群的时限、研究扬子西缘前寒武纪构造属性具有重要意义。对峨边群枷担桥组层型剖面中一条辉绿岩脉进行了SHRIMP锆石U-Pb定年。结果表明,该辉绿岩脉的锆石发育清楚且宽缓的内部环带结构,具有较高的Th/U比值,属典型基性岩浆成因的锆石,其206Pb/238U年龄加权平均值为(813.4±8.2)Ma(MSWD=0.56),代表了该辉绿岩脉的侵位时间,也限定了峨边群枷担桥组的上限年龄。综合扬子西缘新元古代中期基性岩浆活动的最新同位素年龄,证实存在2个主要的活动期:830~800Ma和780~745Ma,且830~800Ma期间基性岩浆活动的峰值年龄为(821±2)Ma。这与前人总结的华南和澳大利亚同期基性岩浆活动的年龄分布特征完全一致,暗示川西金口河辉绿岩脉的形成可能与新元古代导致Rodinia超大陆裂解的地幔柱幕式活动有关。  相似文献   

10.
笔者调查发现西藏班公湖-怒江缝合带西段狮泉河-改则-洞错蛇绿岩带北侧和拉果错蛇绿岩带南侧都有岛弧型花岗岩岩基产出。这些岩体岩性上以中粒花岗闪长岩为主,岩石化学上明显富集大离子不相容元素(LILE) Rb、Th、U、K、Pb,亏损高场强元素(HFSE)Nb、Ta、Ti,具有岛弧型岩浆岩的本质特征,指示着班公湖中特提斯洋盆存在双向俯冲。锆石U-Pb LAICPMS定年结果显示岩体在不同构造位置年龄并不一致,位于狮泉河-改则-洞错蛇绿岩带北侧的嘎拉勒和改则北两个岩体锆石206Pb/238U加权平均年龄分别为155.6±1.1Ma(MSWD=1.7)和142.15±0.35Ma(MSWD=2.9),位于拉果错蛇绿岩带南侧的扎布耶北岩体的锆石206Pb/238U加权平均年龄为134.07±0.77Ma(MSWD=1.8),表明班公湖中特提斯洋盆向北俯冲发生在晚侏罗世,而向南的俯冲发生在早白垩世,两者相差约8Ma。岛弧花岗岩浆都是由地幔楔部分熔融而成,岩浆源区经历过来自俯冲板片的沉积物熔体的交代。不同岩体的源区沉积物熔体的交代比例不同:扎布耶北岩体最多,大体在12%~16%之间;嘎拉勒岩体次之,在9%~13%之间;改则北岩体最少,为5%~10%。  相似文献   

11.
蛇绿岩中的辉绿岩岩墙是洋脊扩张的产物．其形成年龄代表了扩张事件的时间,也代表了蛇绿岩的形成时代？对雅鲁藏布江缝合带西段拉昂错蛇绿岩中的辉绿岩岩墙进行锆石SHRIMPU—Pb定年,得出加权平均年龄为120．2Ma±2．3Ma,代表辉绿岩的结晶年龄。结合已有的关于雅鲁藏布江蛇绿岩的形成年龄（西段休古嘎布122．3Ma±2．4Ma,中段大竹卡126．0Ma±1．5Ma、吉定123．0Ma＋_1．8Ma,东段罗布莎162．9Ma±2．8Ma）的报道,表明拉昂错地区特提斯洋海底扩张的时代与体古嘎布地区一致．雅鲁藏布江西段与中段地区洋盆的形成时代一致,但晚于东段的发育时代。这意味着整个东提斯洋盆的发育时代存在东早西晚的特点。  相似文献   

12.
Diabase dyke swarms are widespread in the East Tianshan and Beishan regions. LA-ICP-MS zircon U-Pb ages of these diabase vary from 305 Ma to 278 Ma, showing that these dykes were formed during Late Carboniferous-Early Permian magmatism. All diabase samples are subalkali calc-alkali, characterized by slight LREE and LILEs enrichment, and weak negative Ti, Nb and Ta anomalies. The diabase samples have positive εNd(t) values (>+3), high Sr isotopic compositions (initial 87Sr/86Sr values=0.7030-0.7097), and large variation of Pb isotopic compositions, indicating they were derived from a deplete mantle source. Regional geology and geochemistry evidences indicate that these diabase dyke swarms were generated in a lithosphere extensional setting and had the same magma sources. Initial magmas may be a mixture of depleted asthenosphere mantle and enriched lithospheric mantle during rapid magma ascending.  相似文献   

13.
The Central Asian Orogenic Belt(CAOB) resulted from accretion during the Paleozoic subduction of the PaleoAsian Ocean. The Xilinhot area in Inner Mongolia is located in the northern subduction zone of the central-eastern CAOB and outcropped a large number of late Paleozoic mafic intrusions. The characteristics of magma source and tectonic setting of the mafic intrusions and their response to the closure process of the Paleo-Asian Ocean are still controversial. This study presents LA-ICPMS zircon U-Pb ages and geochemical features of mafic intrusions in the Xilinhot area to constrain the northward subduction of the Paleo-Asian Ocean. The mafic intrusions consist of gabbro, hornblende gabbro, and diabase. Their intrusion times can be divided into three stages of 326–321 Ma, 276 Ma and 254 Ma by zircon U-Pb ages. The first two stages of the 326–276 Ma intrusions mostly originated from subduction-modified continental lithospheric mantle sources that underwent a variable degree partial melting(5–30%), recording the subduction of oceanic crust. The third stage of the 254 Ma mafic rocks also show arc-related features. The primary magma compositions calculated by PRIMELT2 modeling on three samples of ～326 Ma and two samples of ～254 Ma show that these mafic samples are characterized by a variable range in SiO_2(47.51–51.47 wt%), Al_2O_3(11.46–15.55 wt%), ΣFeO(8.27–9.61 wt%), MgO(13.01–15.18 wt%) and CaO(9.13–11.67 wt%), consisting with the features between enriched mantle and lower continental crust. The source mantle melting of mafic intrusions occurred under temperatures of 1302–1351°C and pressures of 0.92–1.30 GPa. The magmatic processes occurred near the crust-mantle boundary at about 33–45 km underground. Combined with previous studies, it is concluded that Carboniferous to early Permian(～326–275 Ma) northward subduction of the Paleo-Asian oceanic crust led to the formation of the mafic magmatism in the Baolidao arc zone. The whole region had entered the collision environment at ～254 Ma, but with subduction-related environments locally. The final collision between the North China craton and the South Mongolian microcontinent may have lasted until ca. 230 Ma.  相似文献   

14.
西藏夏瓦地区位于特提斯喜马拉雅构造带中东部,广泛发育大量近东西向的基性脉岩,这些基性脉岩对于研究该区域地质构造演化具有重要的意义。本文对夏瓦地区基性脉岩开展了系统的岩石学、年代学、地球化学等研究。夏瓦基性脉岩的岩石类型以辉绿岩和辉长玢岩为主。锆石U-Pb年代学揭示基性脉岩结晶年龄为146～145 Ma。夏瓦基性脉岩具有低SiO2(47.20%～50.54%)和高Mg~#值(39.78～53.79)特征,富集Ti、Fe、P元素,属碱性系列,富集轻稀土元素(LREE),高场强元素(Nb、Ta、Zr、Hf、Th)相对富集,显示出似OIB的地球化学特征,指示夏瓦基性脉岩来源于软流圈地幔的部分熔融,形成于大陆边缘裂谷背景下。结合区域大火成岩省基性岩的发育,认为夏瓦地区基性脉岩是大陆边缘裂谷背景下Kerguelen地幔柱作用的产物。夏瓦基性脉岩的结晶年龄虽然早于Kerguelen地幔柱活动的峰期(132 Ma),但可能属于地幔柱峰期之前的小规模岩浆活动。  相似文献   

15.
班- 怒带东段丁青蛇绿岩中镁铁质岩石年代学及构造背景   总被引：1，自引：0，他引：1  
丁青蛇绿岩位于班公湖-怒江缝合带东段,分为东、西两个蛇绿岩体,丁青西蛇绿岩体缺乏基性岩年代学研究。对丁青西地质填图显示,蛇绿岩主要由方辉橄榄岩、纯橄榄岩、辉绿岩、玄武岩及辉长岩组成。其中玄武岩、辉长岩及辉绿岩出露在宗白区域,玄武岩和辉绿岩与下侏罗统沉积岩呈构造接触,辉长岩呈岩脉侵入到下侏罗统沉积岩中。岩石地球化学研究表明,玄武岩和辉长岩同属于碱性基性岩石,其中玄武岩具有典型洋岛玄武岩的稀土和微量元素特征,可能形成于与地幔柱有关的洋岛环境。在玄武质凝灰岩中挑选出的锆石测年,获得U-Pb年龄为198.7±3.8Ma,属早侏罗世。辉长岩的稀土和微量元素含量低于典型洋岛玄武岩,但其REE和微量元素具有OIB的特征,与典型OIB相比,辉长岩的HREE发生了一定程度富集。辉长岩锆石的U-Pb年龄为164.3±2.6Ma,认为辉长岩在形成过程中受到了软流圈地幔和岩石圈下部LVZ中富集熔体的共同作用,其形成于大陆边缘裂谷环境;辉绿岩成分属于拉斑系列岩石,其REE和微量元素曲线显示辉绿岩同时具有N-MORB和E-MORB的特征。辉绿岩锆石U-Pb年龄为114.2±1.3Ma,其形成晚于玄武岩。结合区域地质,认为辉绿岩形成于受地幔柱影响的弧后扩张脊环境。本研究提供了丁青西蛇绿岩新的年代学和岩石学证据,为探讨丁青蛇绿岩的形成和演化历史提供了新的证据。  相似文献   

16.
中天山卡瓦布拉克地区侵入岩类广泛发育,并多处被中-基性岩脉穿插。LA-ICP-MS锆石U-Pb测年获得闪长岩和辉长岩脉年龄分别为296.1±2.5 Ma(MSWD=1.4),299.5±2 Ma(MSWD=1.07),均侵位于早二叠世。岩石学和岩石地球化学分析结果表明该区中-基性岩脉具有富集轻稀土元素(LREE)和大离子亲石元素(Rb,U,Th)、亏损高场强元素(Nb,Ta,Ti)的地球化学特征,SREE为69.19~234.62,LREE/HREE=3.97~11.18。闪长岩脉锆石(176Hf/177Hf)值为0.280 618~0.283 329,对应的εHf(t)值为-14.57^+25.06。研究数据表明,该区中-基性岩脉可能起源于亏损岩石圈地幔,原始岩浆在侵位过程中发生了分离结晶作用,同时还受到了地壳混染作用的影响。基于卡瓦布拉克地区中-基性岩脉的野外地质特征和地球化学分析结果,并结合区域地质资料,认为它们形成于南天山洋闭合之后的后碰撞伸展构造背景。  相似文献   

17.
班公湖?怒江结合带西段班公湖地区蛇绿岩组合较为完整,由超镁铁质岩、镁铁质堆晶岩、辉绿岩脉、铁镁质熔岩、放射虫硅质岩等组成。这套蛇绿岩所代表的洋盆打开时间一直争论不休。该蛇绿岩中MOR型辉长岩的LA-ICP-MS锆石U-Pb定年显示,18个测点的~(206)Pb/~(238)U加权平均年龄为231.5±2.6 Ma,代表了辉长岩的结晶年龄。地球化学分析表明辉长岩SiO_2含量在46.12%~47.85%之间,TiO_2(1.52%~1.71%),具低的K_2O(0.19%~0.25%)、Al_2O_3(12.97%~13.51%)和Fe_2O_3/FeO比值(0.19~0.24);微量元素蛛网图与洋中脊玄武岩类似,Zr、Nb、Ta、Hf丰度与N-MORB相当或略高,相对富集Ba、Pb、Sr等大离子亲石元素,无Ta-Nb负异常,在基性岩构造环境判别图上显示出洋脊玄武岩的亲合性;REE配分曲线具有平缓型特点,介于E-MORB与N-MORB之间;各种地球化学特征与洋脊玄武岩类似,表明该辉长岩是在班公湖洋盆扩张阶段形成的。结合前人研究成果,本文认为班公湖中特提斯洋盆初始打开时间至少为中三叠世。  相似文献   

18.
The Mianlue Belt is located in the central China and regarded as a tectonic mélange belt, representative of suture zone formed in the early Mesozoic between North and South China Blocks. However, its forming age has been controversial for a long time. This study provides the reliable LA‐ICPMS U‐Pb zircon dating and in‐situ Hf isotopes from the meta‐volcanics, gabbro and plagiogranite in this belt. Of which, two meta‐andesites yielded the ages of 934 Ma and 933 Ma, two meta‐dacites give the ages of 999 Ma and 873 Ma, and another plagiogranite define an age of 923 Ma, respectively, evidently older than those previously thoughted. Their Hf isotopes are quite coincident. The andesites have the εHf(t)= ‐0.03 ～ +11.76 with the TDMC of 1032‐1629 Ma, εHf(t) = +9.81～ +16.06 and the TDMC = 867‐1185 Ma for dacite and εHf(t) = +8.65 ～ +16.66 and the TDMC = 805～1123 Ma for plagiogranite, suggesting an important crustal growth in the early Neoproterozoic. In addition, the gabbro yielded younger ages of 224‐213 Ma and variable ages from 1225 Ma to 280 Ma in some inherited zircons, consistent with early Mesozoic mafic dykes in South Qinling belt. Their TDMC range from 1.2 to 2.7 Ga, indicating early‐middle Proterozoic crustal residence involved minor Neo‐Archean crustal materials. All new results lead us to propose that most of components in the Mianlue Belt were generated in the early Neoproterozoic and related to the subduction process in the northern margin of the Yangtze block, representative of an important juvenile crust growth in South Qinling during the Neoproterozoic.  相似文献   

19.
In this study (U‐Th)/He dating of the Penglai zircons, which occur as abundant megacrysts in Neogene alkaline basalts in northern Hainan Province, south‐eastern China, was undertaken. A weighted mean age of 4.06 ± 0.35 Ma (2s) with a mean square weighted deviation (MSWD) of 1.79 was obtained from eighteen fragments of four zircon megacrysts using single‐crystal laser fusion He determinations and the U‐Th isotope dilution (ID) method. The (U‐Th)/He ages are consistent, homogeneous and systematically slightly younger than the preferred 206Pb/238U age of 4.4 ± 0.1 Ma (95% confidence interval) determined by ID‐TIMS and subsequently published U‐Pb results. The U‐Pb isotopic system in zircon has a high closure temperature of ~ 900 °C, and the preferred U‐Pb age may record both the time since eruption and the zircon residence time in the magma chamber. In contrast, the closure temperature of the zircon (U‐Th)/He system is ~ 190 °C and the zircon megacrysts were brought quickly to the surface by the host basaltic magma. Thus, the (U‐Th)/He age represents the timing of the eruption. Based on the unlimited quantity, large grain size, mostly weak broad zoning, rapid cooling and homogenous (U‐Th)/He ages, we consider the Penglai zircons suitable for use as a reference material in (U‐Th)/He isotope geochronology.  相似文献   

20.
Although the U–Pb zircon chronometer has been widely used for dating metamorphism in moderate‐ to high‐grade rocks, it is generally difficult to link the U–Pb age of zircon to specific metamorphic reactions. In this study, the initial Hf isotopic composition of secondary zircon is compared with the evolution of Hf isotopic composition of the bulk sample, back‐projected from the measured value through time. This approach may enhance the interpretation of radiometric ages performed on metamorphic mineral assemblages. Here, U–Pb, Sm–Nd and Lu–Hf geochronology and thermobarometry have been integrated and applied to two metamorphosed diabase dykes in the Sveconorwegian orogen, SW Sweden. The dykes are located ～5 km east of the NNE‐trending Göta Älv deformation zone in the Idefjorden terrane, and trend parallel to this zone. The Lunden dyke is recrystallized into a coronitic, granulite facies assemblage. U–Pb isotopic analyses of baddeleyite in this dyke indicate an emplacement age of c. 1300 Ma. Thermobarometric techniques applied to garnet and omphacitic clinopyroxene coronas indicate high‐pressure metamorphism at ～15 kbar and ～740 °C. The growth of polycrystalline zircon at the expense of baddeleyite occurred at 1046 ± 6 Ma. The identical Hf isotopic composition of polycrystalline zircon and baddeleyite shows that the baddeleyite‐to‐zircon transition took place before Hf equilibration among the other metamorphic minerals and, hence the c. 1046 Ma age of polycrystalline zircon sets an upper age limit of metamorphism of this sample. The Haregården dyke is recrystallized into a granoblastic transitional upper amphibolite to granulite facies assemblage. The estimated P–T conditions are ～10 kbar and ～700 °C. Analyses of small (～30 μm), clear and round zircon in this sample yield a Concordia U–Pb age of 1026 ± 4 Ma, which is indistinguishable from the Lu‐Hf and Sm‐Nd mineral isochron ages of 1027 ± 9 and 1022 ± 34 Ma, respectively. This type of secondary zircon plots at the lower end of the Lu‐Hf isochron and indicates simultaneous growth with garnet at c. 1026 Ma, a time when Hf isotopic equilibrium among minerals must have been reached.  相似文献   

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