Exoscopy of Detrital Zircons from Niamey Neoproterozoic Sandstones (Eastern Edge of the West African Craton, Southwestern Niger): Interpretation of Detrital Sediments Provenance

This study focuses on exoscopic analyses of detrital zircon grains of Archean and Paleoproterozoic age, contained in the sediments from the Niamey Neoproterozoic sandstones (Niamey region), with a view to confirming the sediments sources already proposed (Kénéma-Man domain for Archean-age zircons and Baoulé Mossi domain for Paleoproterozoic-age zircons). Exoscopic analysis reveals that Archean zircon grains are more corroded, with rounded to sub-rounded shapes, while Paleoproterozoic zircon grains are less corroded and mostly angular in shape. The strong corrosion of Archean zircon grains, implying long-distance transport, is consistent with the remoteness of the Kenema-Man domain which are the source these sediments. The fact that the Paleoproterozoic zircon grains show little or no wear implies a proximal source of sediments, corresponding to the Baoulé Mossi domain.

Mineral Chemistry,Trace Elements,Isotopic Analysis and Zircon U-Pb Dating in the Hesar Pluton,Northern UDMA,Iran:Implications for Pre-Collisional Magma Mixing

The Hesar pluton in the northern Urumieh-Dokhtar magmatic arc hosts numerous mafic-microgranular enclaves(MMEs).Whole rock geochemistry,mineral chemistry,zircon U-Pb and Sr-Nd isotopes were measured.It is suggested that the rocks are metaluminous(A/CNK=1.32-1.45),subduction-related I-type calc-alkaline gabbro to diorite with similar mineral assemblages and geochemical signatures.The host rocks yielded an U-Pb crystallization age of 37.3±0.4 Ma for gabbro-diorite.MMEs have relatively low SiO_(2) contents(52.9-56.6 wt%)and high Mg^(#)(49.8-58.7),probably reflecting a mantle-derived origin.Chondrite-and mantle-normalized trace element patterns are characterized by LREE and LILE enrichment,HREE and HFSE depletion with slight negative Eu anomalies(Eu/Eu^(*)=0.86-1.03).The host rocks yield(^(87)Sr/^(86)Sr)_(i) ratios of 0.70492-0.70510,positive ε_(Nd)(t)values of+1.55-+2.06 and T_(DM2)of 707-736 Ma,which is consistent with the associated mafic microgranular enclaves((^(87)Sr/^(86)Sr)_(i)=0.705014,ε_(Nd)(t)=+1.75,T_(DM2)=729 Ma).All data suggest magma-mixing for enclave and host rock formation,showing a complete equilibration between mixed-mafic and felsic magmas,followed by rapid diffusion.The T_(DM1)(Nd)and T_(DM2)(Nd)model ages and U-Pb dating indicate that the host pluton was produced by partial melting of the lower continental crust and subsequent mixing with injected lithospheric mantlederived magmas in a pre-collisional setting of Arabian-Eurasian plates.Clinopyroxene composition indicates a crystallization temperature of~1000℃ and a depth of~9 km.

Geochemistry, zircon U–Pb geochronology, and Hf isotopes of S-type granite in the Baoshan block, constraints on the age and evolution of the Proto-Tethys

Geochemistry, zircon U–Pb geochronology, and Hf isotope data for the Early Paleozoic granites in the Baoshan Block reveal the Early Paleozoic tectonic evolution of the Proto-Tethys. The samples are high-K, calcalkaline, strongly peraluminous rocks with A/CNK values of 1.37–1.46, are enriched in SiO2, K2O, and Rb, and are depleted in Nb, P, Ti, Eu, and heavy rare earth elements,which indicates the crystallization fractionation of the granitic magma. Zircon U–Pb dating indicates that they formed in ca. 480 Ma. The Nansa granites have εHf(t) values ranging from-16.04 to 4.36 with corresponding TC DMages of 2.10–0.81 Ga, which suggests the magmas derived from the partial melting of ancient metasedimentary with minor involvement of mantle-derived components. A synthesis of data for the Early Paleozoic igneous rocks in the Baoshan block and adjacent(Tengchong,Qiangtang, Sibumasu, Himalaya, etc.) blocks indicates that these blocks were all aligned along the proto-Tethyan margin of East Gondwana in the Early Paleozoic. The Early Paleozoic S-type granites from Nansa were generated in a high-temperature and low-pressure(HTLP) extensional tectonic setting, which resulted from Andean-type orogeny instead of the final assembly of Gondwana or crustal extension in a non-arc environment. In certain places, an expanding environment may exist in opposition to the tectonic backdrop of the lithosphere’s thickening and shortening, leading the crust to melt and decompress,mantle-derived materials to mix, and a small quantity of peraluminous granite to emerge.

U-Pb zircon geochronology of basaltic pyroclastic rocks from the basement beneath the Xisha Islands in the northwestern South China Sea and its geological significance

As one of the micro-blocks dispersed in the South China Sea(SCS), the basement of the Xisha Islands has rarely been drilled because of the thick overlying Cenozoic sediments, which has led to a confused understanding of the pre-Cenozoic basement of the Xisha Islands. Well CK-1, a kilometer-scale major scientific drill in the Xisha Islands in the northwestern SCS, penetrated thick reefal limestone(0–888.4 m) and the underlying basement rocks(888.4–901.4 m). In this study, we present the zircon U-Pb ages of basement basaltic pyroclastic rocks from Well CK-1 in the Xisha Islands of the northwestern SCS to investigate the basement nature of the Xisha microblock. The basement of Well CK-1 consists of basaltic pyroclastic rocks on the seamount. The zircon grains yielded apparent ages ranging from ca. 2 138.9 Ma to ca. 36 Ma. The old group of zircon grains from Well CK-1 was considered to be inherited zircons. Two Cenozoic zircons gave a weighted mean 206Pb/238U age of(36.3 ± 1.1) Ma,Mean Squared Weighted Deviations(MSWD) = 1.2, which may represent the maximum age of the volcano eruption. The Yanshanian inherited zircons(116.9–105.7 Ma and 146.1–130.2 Ma) from Well CK-1 are consistent with the zircons from Well XK-1, indicating that the basement of Chenhang Island may be similar to that of Well XK-1. We propose that the Xisha micro-block may have developed on a uniform Late Jurassic metamorphic crystalline basement, intruded by Cretaceous granitic magma.

Garnet and Zircon U-Pb Geochronology and Geochemistry Reveal Genesis of the Dafang Au-Pb-Zn-Ag Deposit,Southern Hunan

Garnet is a primary mineral in skarn deposits and plays a significant role in recording copious mineralization and metallogenic information.This study systematically investigates the geochemistry and geochronology of garnet and zircon in the Dafang Au-Pb-Zn-Ag deposit,which represents prominent gold mineralization in southern Hunan,China.Garnet samples with distinct zoning patterns and compositional variations were identified using various analytical techniques,including Backscattered Electron(BSE)imaging,Cathodoluminescence(CL)response,textural characterization,and analysis of rare-earth elements(REE),major contents,and trace element compositions.The garnet was dated U-Pb dating,which yielded a lower intercept age of 161.06±1.93 Ma.This age is older than the underlying granodiorite porphyry,which has a concordia age of 155.13±0.95 Ma determined by zircon U-Pb dating.These results suggest that the gold mineralization may be related to the concealed granite.Two groups of garnet changed from depleted Al garnet to enriched Al garnet,and the rare earth element(REE)patterns of these groups were converted from light REE(LREE)-enriched and heavy REE(HREE)-depleted with positive europium(Eu)anomalies to medium REE(MREE)-enriched from core to rim zoning.The different REE patterns of garnet in various zones may be attributed to changes in the fluid environment and late superposition alteration.The development of distal skarn in the southern Hunan could be a significant indicator for identifying gold mineralization.

Improvement strategy on thermophysical properties of A_(2)B_(2)O_(7)-type rare earth zirconates for thermal barrier coatings applications:A review

The A_(2)B_(2)O_(7)-type rare earth zirconate compounds have been considered as promising candidates for thermal barrier coating(TBC) materials because of their low sintering rate,improved phase stability,and reduced thermal conductivity in contrast with the currently used yttria-partially stabilized zirconia (YSZ) in high operating temperature environments.This review summarizes the recent progress on rare earth zirconates for TBCs that insulate high-temperature gas from hot-section components in gas turbines.Based on the first principles,molecular dynamics,and new data-driven calculation approaches,doping and high-entropy strategies have now been adopted in advanced TBC materials design.In this paper,the solid-state heat transfer mechanism of TBCs is explained from two aspects,including heat conduction over the full operating temperature range and thermal radiation at medium and high temperature.This paper also provides new insights into design considerations of adaptive TBC materials,and the challenges and potential breakthroughs are further highlighted for extreme environmental applications.Strategies for improving thermophysical performance are proposed in two approaches:defect engineering and material compositing.

Effect of Zircon Addition on Properties of Corundum Porous Materials

This paper aims to improve the corrosion resistance of dispersive purging plugs.White fused corundum particles and fine powder,α-Al_(2)O_(3) micropowder,Cr_(2)O_(3) micropowder,Guangxi clay and zircon powder were used as the main raw materials.The mass ratio of white fused corundum particles and fine powder was fixed at 85:15,and 0,1%,2%,or 3%(by mass)of zircon fine powder was added to replace the same amount of white fused corundum fine powder.The corundum porous material was prepared by the particle stacking pore-forming method at 1650℃for 3 h.The effect of the zircon addition on the properties and microstructure of porous materials was investigated.The results show that:after adding zircon,the permeability of the porous material increases,the cold and hot strengths increase obviously,and the expansion rate after firing decreases.When the addition of zircon is 2%,the comprehensive performance of the specimen is optimal with the smallest linear change rate and the highest permeability.

Polymetamorphism of the ultrahigh-temperature granulites in the Rauer Group,East Antarctica:new evidence from zircon SHRIMP U-Pb ages

The Rauer Group is located on the eastern margin of the early Paleozoic Prydz Belt in East Antarctica,and the typical ultrahigh-temperature(UHT,>900℃)granulites outcrop on Mather Peninsula.However,the timing of UHT metamorphism and P–T path of the UHT granulites have long been debated,which is critical to understanding the tectonic nature and evolution history of the Prydz Belt.Thus,both a sapphirine-bearing UHT metapelitic granulite and a garnet-bearing UHT mafic granulite are selected for zircon SHRIMP U-Pb age dating.The results show that metamorphic zircon mantles yield weighted mean^(206)Pb/^(238)U ages of 918±29 Ma and 901±29 Ma for the metapelitic and mafic granulites,respectively,while zircon rims and newly grown zircons yield weighted mean^(206)Pb/^(238)U ages of 523±9 Ma and 532±11 Ma,respectively.These new zircon age data suggest that the UHT granulites may have experienced polymetamorphism,in which pre-peak prograde stage occurred in the early Neoproterozoic Grenvillian orogenesis(1000–900 Ma),whereas the UHT metamorphism occurred in the late Neoproterozoic to early Paleozoic Pan-African orogenesis(580–460 Ma).This implies that P–T path of the UHT granulites should consist of two separate high-grade metamorphic events including the Grenvillian and Pan-African events,which are supposed to be related to assembly of Rodinia and Gondwana supercontinents respectively,and hence the overprinting UHT metamorphic event may actually reflect an important intracontinental reworking.

SHRIMP zircon U-Pb age and O isotopic analysis of the dunite from Kudi ophiolite in the West Kunlun,China

1.Objective The West Kunlun in Xinjiang is located on the northwestern margin of the Qinghai-Tibet Plateau(Fig.1a)and at the junction of the Paleo-Asian tectonic domain and the Tethys tectonic domain.It serves as an important area for the study on the geologic evolution of the Karakorum-West Kunlun due to its special tectonic position.

Detrital Zircon Records of the Banxi Group in the Western Jiangnan Orogen:Implications for Crustal Evolution of the South China Craton

The Neoproterozoic evolution of the Jiangnan Orogen is important for understanding the tectonic history of South China.As a volcanic-sedimentary sequence developed in the Nanhua rift,the Banxi Group preserves the records of important magmatic and tectonic events linked to the assembly and breakup of the Rodinia supercontinent.In this study,we report the results from whole-rock major-and trace-element concentrations,with zircon LA-(MC)-ICP-MS U-Pb ages,trace elements and Lu-Hf isotopic compositions of sandstones from the Banxi Group.The rocks are characterized by high SiO_(2)(65.88%–82.76%,with an average of 75.50%)contents,moderate(Fe_(2)O_(3)^(T)+MgO)(1.81%–7.78%,mean:3.79%)and TiO_(2)(0.39%–0.54%,mean:0.48%),low K_(2)O/Na_(2)O(0.03–0.40,mean:0.10)ratios and low Al_2O_(3)/SiO_(2)(0.11–0.24,mean:0.15)ratios.The sandstones have highΣREE contents(mean:179.1 ppm),with chondrite-normalized REE patterns similar to the upper crust and PAAS,showing enriched LREE((La/Yb)_N mean:14.85),sub-horizontal HREE curves and mild Eu(Eu/Eu^(*):0.75–0.89,mean:0.81)negative anomalies.Their geochemical characteristics resemble those of passive continental margin sandstones.Most of the zircons are magmatic in origin and yield a U-Pb age distribution with three peaks:a major age peak at 805 Ma and two subordinate age peaks at 1990 Ma and 2470 Ma,implying three major magmatic sources.The Neoproterozoic zircons haveε_(Hf)(t)values ranging from-47.4 to 12.4(mostly-20 to 0),suggesting a mixture of some juvenile arc-derived material and middle Paleoproterozoic heterogeneous crustal sources.The Hf model ages of middle Paleoproterozoic zircons(~1990 Ma)with negativeε_(Hf)(t)values(-12.65 to-6.21,Ave.=-9.8)concentrated around the Meso-Paleoarchean(mean T_(DM)^(C)=3.3–3.1 Ga).For late Neoarchean detrital zircons(~2470 Ma),ε_(Hf)(t)values are divided into two groups,one with negative values(-9.16 to-0.6)with model ages of 3.5–2.9 Ga,the other featuring positive values(1.0 to 3.9)with model ages of 2.9–2.7 Ga,recording a crustal growth event at~2.5 Ga.Neoproterozoic zircons show volcanic arc affinities with partly intraplate magmatic features.We propose that the Banxi Group formed in a rift basin within a passive continental margin setting,which derived detritus from felsic to intermediate rocks from the Yangtze Block and a small amount of arc volcanic rocks.The middle Paleoproterozoic detrital zircon data suggest Columbia-aged basement lies beneath the western Jiangnan orogen.

Report of 2.7 Ga zircon U-Pb age of orthogneiss in the Wenquan metamorphic complex,West Tianshan,China

1.Objective The Wenquan Group is one of the important medium-to low-grade metamorphic units within the Wenquan metamorphic complexes which distributes in Chinese segment of the Tianshan Belt,the southern part of the Central Asian Orogenic Belt.It mainly consists of pre-Neoproterozoic metamorphosed volcanic and sedimentary rocks(Wang B et al.,2014).

Zircon SHRIMP U–Pb geochronology,geochemistry,and geological significance of dacite in the Zhesang gold district,southeast Yunnan Province,China

The Late Paleozoic and Mesozoic tectonic framework of the Nanpanjiang Basin has much been disputed.Herein,the middle-acid volcanic rock,dacite,exposed to the Zhesang gold district,southeast Yunnan Province,has been analyzed.The results show that the dacite belongs to a calc-alkaline series,SiO_(2)contents range from 62.79 to 76.66 wt%.Zircon SHRIMP U–Pb dating of dacite demonstrates that they were formed in the Early Triassic(247.8±1.7 Ma,MSWD=1.2).All samples exhibit enrichment in LILE(e.g.Rb,K,Th,and U),and depletion in HFSE(e.g.Nb,Ta,and Ti),which has the geochemical affinity of I-type granite.La–La/Sm and La–La/Yb discrimination diagrams show that the partial melting,mainly of the mafic lower crust,of rocks,plays a major role in the formation process.The dacite has low initial ^(87)Sr/^(86)Sr ratios(0.706954 to 0.708589)and negative ε_(Nd)(t)values(-11.77 to-10.88).Zircons in dacite have ε_(Hf)(t)values of-16.2 to-8.3,and the two-stage Hf model ages are 1799–2301 Ma,mostly concentrated between 1800 and 1900 Ma,indicating that the magma source area is the reconstructed ancient lower crust mixed with some mantle materials,and crystal fractionation process underwent in the late stage of magma migration.This study reveals that the arc-volcanic rocks of the Early Triassic in the southern margin of the Nanpanjiang Basin were formed by the subduction of the Late Paleozoic ocean basin within the border region between China and Vietnam.

Discovery of~2.5 Ga zircon U-Pb age from the diabase,Ar Horqin Banner Area,Inner Mongolia:Implications for Neoarchean crystalline basement of Bainaimiao Arc Belt

1.Objective The Bainaimiao Arc Belt(BAB),one of the most important units of the southeastern Central Asian Orogenic Belt(CAOB),locates along the northern margin of the North China Craton(NCC).There is a lack of understanding on the existence,scale,and affinity of the crystalline basement of the BAB.Based on the recent geochronologic studies.

Europium anomalies in detrital zircons reveal the crustal thickness evolution of South China in Early Neoproterozoic

The South China Block(SCB)is formed by the amalgamation of the Yangtze and Cathaysia blocks during the Early Neoproterozoic along the Jiangnan Orogen.However,the precise amalgamation time of these two blocks and the location of the united SCB in the Rodinia supercontinent remain highly debatable.Various tectonic models have been proposed and they may have different implications for the crustal thickness evolution of the central SCB in Early Neoproterozoic.To evaluate these models,this paper uses a recently calibrated Eu/Eu*-inzircon proxy to reconstruct crustal thickness evolution of the central SCB during Early Neoproterozoic.I compiled and screened U–Pb ages and trace elements of 900–700 Ma detrital zircons from the central SCB and then calculated the zircon Eu/Eu*values.The age-binned average zircon Eu/Eu*displays a decreasing trend from 870 to 790 Ma,and thus indicates no significant crustal thickening event occurred during this time interval.This finding seems to be inconsistent with tectonic models that the Yangtze and Cathaysia blocks amalgamated during this time interval.Yet,given that available coupled detrital zircon U–Pb and trace element datasets are very limited,additional studies are warranted to further evaluate this hypothesis.

Preparation of ZrB_(2)-ZrO_(2)-SiC Composite Powder by Carbothermal Reduction from Zircon

Using zircon,boric acid and carbon black as starting materials,ZrB_(2)-ZrO_(2)-SiC composite powder was synthesized by calcining at 1500℃in flowing argon atmosphere.The effects of the soaking time(3,6 and 9 h)and the addition of additive AlF_(3)(0,0.5%,1.0%,1.5%,2.0%and 2.5%,by mass)on the phase composition and the microstructure of the synthesized products were investigated.The results show that:(1)ZrB_(2)-ZrO_(2)-SiC composite powder can be synthesized by carbothermal reduction at 1500℃in flowing argon atmosphere;ZrB_(2) and ZrO_(2) are granular-like,and SiC crystals are fiberous;(2)with the soaking time increasing,the amount of ZrB_(2) increases,the amounts of m-ZrO_(2) and SiC decrease,and the total amount of non-oxides ZrB_(2),SiC and ZrC gradually increases;the optimal soaking time is 3 h;(3)compared with the sample without AlF_(3),the sample with 0.5% AlF_(3) has decreased m-ZrO_(2)amount,noticeably increased ZrB_(2) amount but decreased SiC amount;however,when the addition of AlF_(3) increases from 0.5%to 2.5%,the m-ZrO_(2) amount increases,the ZrB_(2)amount decreases,and the SiC amount changes slightly;the optimum AlF_(3)addition is 0.5%.

Quantification of radiation damage in natural and synthetic zircon by Raman spectroscopy:application to low-temperature thermochronology

Due to its ubiquitous occurrence in igneous,metamorphic,and sedimentary rocks and its wide application in geochronology and geochemistry,zircon has become the most widely used accessory mineral in the geological community.Nevertheless,the decay of U and Th causes radiation damage to the zircon structure,resulting in various degrees of metamictization,which can affect the accuracy of U–Pb dates and Hf and O isotope results.If the degree of zircon radiation damage can be quantified,the influence on geochemical analyses can be evaluated,and the results can be corrected more precisely.In this paper,synthetic and natural zircon crystals with different crystallization ages were selected for Raman spectroscopy analysis,cathodoluminescence imaging,and determination of the U and Th concentrations.The results show that Raman FWHM(full width at half bandmaximum)and Raman shift correlate with alpha dose(Da)ofzirconsfollowingtheseequations,FWHM=44.36(±2.32)×[1-exp(-2.74×Da)]-+1.7(±0.19),Raman Shift=-6.53×Da+1007.69.Analysis of synthetic zircon crystals shows that doped REEs(rare earth elements and P)can also lead to an increase in the FWHM.However,this effect can be ignored for natural zircon samples with REE contents at a normal level of hundreds to a few thousand ppm.The FWHM and Raman shift can be used as proxies to measure the degree of zircon radiation damage.Using the updated equations to calculate the latest age when zircon began to accumulate radiation damage,a more accurate and more meaningful“radiation damage age”can be obtained.

Zircon U-Pb geochronology,geochemistry and tectonic implication of volcanic rocks from Manketouebo Formation in Keyihe area of northern Great Xing'an Range

Zircon U-Pb isotope dating and whole-rock geochemical analyses were undertaken for the rhyolite,rhyolitic lithic crystal tuff and dacitic tuff from the Manketouebo Formation in the Keyihe area,in order to constrain their genesis and tectonic significance.Zircon LA-ICP-MS U-Pb data indicate that the rhyolite and rhyolitic lithic crystal tuff were formed during 137±5 Ma and 143±1 Ma,respectively.These volcanic rocks have high SiO2(70.03%–76.46%)and K2O+Na2O(8.10%–9.52%)contents,but low CaO(0.03%–0.95%)and MgO(0.07%–0.67%)contents,which belong to the peraluminous and high-K calc-alkaline rocks.They are enriched in light rare earth elements(REEs),and exhibit fractionation of light over heavy REEs,withδEu values of 0.37–0.83.The volcanic rocks are enriched in LILEs(e.g.,Rb,U and K)and depleted in HFSEs(e.g.,Nb,Ti,P and Ta).The chemical composition suggests that these volcanic rocks formed by partial melting of crust material.Combined with previous regional research results,the authors consider that the volcanic rocks of the Manketouebo Formation in the Keyihe area were formed under an extensional environment related to the closure of the Mongolia–Okhotsk Ocean.

东昆仑大格勒农场南辉长岩锆石U-Pb年代学、地球化学特征及地质意义

东昆仑志留纪岩浆活动强烈,记录了东昆仑造山带原特提斯洋演化晚期大洋俯冲阶段与碰撞阶段转换地质过程。本次研究对东昆仑大格勒农场南志留纪基性侵入岩开展了详细的岩相学、岩石地球化学、锆石U-Pb年代学和Hf同位素研究,以确定其形成时代、岩石成因和构造环境。研究结果表明大格勒农场南辉长岩的结晶年龄为430.2±2.4 Ma,形成时代为中志留世。辉长岩SiO_(2)(51.40%~53.66%)含量较低,MgO(18.56%~22.16%)和FeO^(T)(7.54%~10.64%)含量高,Mg^(#)为79~82。微量元素富集Rb、Th、Ba、Cs等大离子亲石元素(LILE),亏损Nb、Ta、Ti等高场强元素(HFSE),具有高Ni(276~973μg/g)、Cr(2152~3574μg/g)和Sc(35.5~53.1μg/g)含量,低Nb/Ta值(3.23~8.68)和εHf(t)值(-7.3~-3.6)。大格勒农场南辉长岩的地球化学组成具有岛弧基性岩特征,其成因可能是流体和俯冲沉积物加入的富尖晶石橄榄岩部分熔融的结果。综合区域资料表明,大格勒农场南辉长岩形成于东昆仑原特提斯洋俯冲晚期阶段,进一步限定原特提斯洋晚期俯冲与碰撞转换时限为中志留世,随后转入碰撞造山阶段。

东昆仑二叠系格曲组火山岩年代学、地球化学特征及其构造意义

东昆仑古特提斯造山带经历了从洋壳俯冲到陆-陆碰撞及后碰撞伸展的造山过程,但对古特提斯洋闭合时间一直存在争议,争论的焦点集中在晚二叠世格曲组沉积盆地原型及中上二叠统之间不整合关系代表的构造事件。本文对东昆仑红石山地区格曲组火山岩开展了锆石U-Pb年代学与岩石地球化学研究,结果表明,岩石富集Rb、Th、Ba等大离子亲石元素(LILE)而亏损Nb、Ta、Ti、P等高场强元素(HFSE),具有大陆岩浆弧的地球化学特征。火山岩具有高Ba/Th比值(53.6)、Th/Ce比值(0.23)、Nb/Ta比值(18.35)及低Dy/Yb比值(1.42),表明晚二叠世火山岩由枕状玄武岩和洋底沉积物等洋壳物质俯冲至60 km左右深度时熔融所形成。锆石U-Pb定年表明,格曲组下部碎屑岩段流纹质凝灰岩和英安质沉凝灰岩206Pb/238U加权平均年龄分别为(257.5±2.5)Ma和(256.2±4.8)Ma,上部灰岩段粗安质晶屑凝灰岩206Pb/238U加权平均年龄为(251.8±2.3) Ma。结合区域上已报道的格曲组及花岗岩研究成果,认为古特提斯洋在晚二叠世开始向北俯冲消减,沉积于弧前盆地的格曲组为古特提斯洋向北初始俯冲的沉积响应,而格曲组与下伏地层之间的角度不整合关系记录了古特提斯洋晚二叠世开始向北俯冲这一构造事件。

柴达木盆地东北部拉合根地区中生代火山岩的发现及其地质意义:来自地球化学与锆石年代学证据

柴达木盆地北缘东部地区哇洪山—温泉断裂与宗务隆山南缘断裂之间地带出露系列火山岩,由于其中一部分缺乏精确的地质年代学制约,导致其地层归属难以确定,因此该地区大地构造单元划分尚存争议。本文基于详细的野外调查,在柴达木盆地北缘东部拉合根地区新识别出一套中生代火山岩,岩石类型为流纹岩、中酸性角砾熔岩及英安岩。对代表性流纹岩样品进行LA-ICP-MS锆石U-Pb定年,获得^(206)Pb/^(238)U加权平均年龄为(248.3±1.2)Ma(MSWD=0.30,n=20),表明岩石形成于早三叠世。全岩元素组成分析结果显示,该火山岩为准铝质高钾钙碱性岩石,轻重稀土元素分馏明显,轻稀土元素富集、重稀土元素亏损,δEu平均值为0.63,显示为弱负异常;微量元素呈现Rb、Ba等大离子亲石元素和Th、U、Hf和Zr等高场强元素相对富集,Sr强烈亏损,Ta、Nb相对亏损的特征,具有典型岛弧火山岩地球化学特征。锆石εHf(t)值分布范围为-19.62~-3.07,对应的t_(DM2)为2421~1639 Ma,表明岩石来自古元古代地壳物质的部分熔融。综合岩石年代学和地球化学特征,认为该火山岩为早三叠世阿尼玛卿洋向北俯冲于东昆仑地块,导致地壳基底(如达肯大坂岩群)发生深熔作用的产物。通过区域对比,提出该火山岩与鄂拉山岩浆弧洪水川组火山岩的地球化学性质较为相似,因此将其厘定为下三叠统洪水川组火山岩。该火山岩的发现指示拉合根地区大地构造单元属于东昆仑造山带鄂拉山岩浆弧。研究成果为柴达木盆地北缘东部地区大地构造划分提供了新的数据资料支撑。