Trace and rare earth elements as indicators of provenance and depositional environments of Lias cherts in Gumushane,NE Turkey

Trace elements and rare earth elements (REEs) of Lias-aged cherts in the Gumushane area were studied in order to understand their origin and depositional environment. Twenty three chert samples from five stratigraphic sections were analysed by inductively coupled plasma-mass spectrometry, X-ray diffraction, and mineralogical investigation. Lias cherts in the study area are microcrystalline, cryptocrystalline quartz, and megaquartz depending on mineralogical content. Trace elements of the cherts were compared with PAAS, Co, Y, and Th had stronger depletions in the five sections, whereas V, Ni, Zr, Nb, and Hf had smaller depletions. The distribution of Zr, Hf, and Ta yields Zr/Hf, Zr/Ta and Hf/Ta ratios (25/645, 37/665, and 0.18/3, respectively) that differ from those of chondrites and average upper continental crust, suggesting that these elements are likely non-detrital but are sourced from seawater. Th/U ratios range from 0.04 to 0.45 and are lower than those of the upper continental crust (average: 3.9). Lias-aged cherts have low total REE abundances and stronger depletions in five sections of the PAAS and chondrite-normalised plots. The cherts are characterised by a positive Eu anomaly (average: 4.9) and LREE-enrichment (La_N/Yb_N = average: 3.5). In addition, about one-half of the cherts exhibit positive Ce anomaly (range: 0.25–2.58), chondritic Y/Ho values (range: 3.3–60), and low (La/Ce)_N values (average: 1.8). REE and trace element abundance in Lias cherts indicate that these elements were likely derived from hydrothermal solutions, terrigenous sources, and seawater. The REE patterns of the cherts show that they were probably deposited close to a continental margin.     

Provenance and tectonic setting of the Lower Cambrian Niutitang formation shales in the Yangtze platform,South China: Implications for depositional setting of shales

Geochemical compositions of the Lower Cambrian Niutitang Formation shales in the southeastern Yangtze Platform margin were investigated for provenance, tectonic setting, and depositional environment. The shale samples are characterized by higher abundances of large ion lithophile elements (Cs, Ba, and Pb), lower abundances of high field strength elements (Cr, Sc, and Co) and transition elements (Th, Zr, Hf, Nb, and Ta) relative to average shale. North American shale composition (NASC) -normalized rare earth element (REE) patterns are observed, with negative Ce anomalies, negative Eu anomalies, and positive Y anomalies. The chemical index of alteration (CIA) varies from 68.67–74.93. Alkali and alkaline element contents and CIA values suggest that the source rocks have undergone moderate weathering. The index of compositional variability (ICV), Zr/Sc and Th/Sc ratios vary from 0.53 to 1.07, 5.31 to 8.18 and 0.52–1.02, respectively. ICV values and relationships between Zr/Sc and Th/Sc ratios indicate negligible sedimentary recycling. The Al₂O₃/TiO₂ (14–26) and TiO₂/Zr (56–77) ratios imply that the source rocks of the investigated shales had intermediate igneous compositions. However, Cr/V ratios and a La/Th–Hf discrimination diagram suggest that the intermediate compositional signal of the source rocks was derived from a mixture of 75% mafic and 25% felsic igneous rocks rather than intermediate igneous rocks. The major source was the Jiangnan continental island arc with bimodal igneous rocks, lying to the south of the study area, together with a contribution from granites and gneisses uplifted and eroded in the Yangtze Block. Discrimination of tectonic setting using major and trace elements indicates that the source rocks originated in a transitional setting from active continental to passive margin, consistent with the failed intracontinental rift model for the evolution of the South China plate. The Niutitang Formation shales were deposited in a rift basin setting under conditions of anoxic bottom water in a redox-stratified water column, with organic-rich shales prospective for shale-gas production being found in deep-water downslope and basin environments rather than the shallow-water shelf.     

Trace- and rare-earth element geochemistry and Pb–Pb dating of black shales and intercalated Ni–Mo–PGE–Au sulfide ores in Lower Cambrian strata, Yangtze Platform, South China

The Lower Cambrian black shale sequence of the Niutitang Formation in the Yangtze Platform, South China, hosts an extreme metal-enriched sulfide ore bed that shows >10,000 times enrichment in Mo, Ni, Se, Re, Os, As, Hg, and Sb and >1,000 times enrichment in Ag, Au, Pt, and Pd, when compared to average upper continental crust. We report in this paper trace- and rare-earth-element concentrations and Pb–Pb isotope dating for the Ni–Mo–PGE–Au sulfide ores and their host black shales. Both the sulfide ores and their host black shales show similar trace-element distribution patterns with pronounced depletion in Th, Nb, Hf, Zr, and Ti, and extreme enrichment in U, Ni, Mo, and V compared to average upper crust. The high-field-strength elements, such as Zr, Hf, Nb, Ta, Sc, Th, rare-earth elements, Rb, and Ga, show significant inter-element correlations and may have been derived mainly from terrigenous sources. The redox sensitive elements, such as V, Ni, Mo, U, and Mn; base metals, such as Cu, Zn, and Pb; and Sr and Ba may have been derived from mixing of seawater and venting hydrothermal sources. The chondrite-normalized REE patterns, positive Eu and Y anomalies, and high Y/Ho ratios for the Ni–Mo–PGE–Au sulfide ores are also suggestive for their submarine hydrothermal-exhalative origin. A stepwise acid-leaching Pb–Pb isotope analytical technique has been employed for the Niutitang black shales and the Ni–Mo–PGE–Au sulfide ores, and two Pb–Pb isochron ages have been obtained for the black shales (531±24 Ma) and for the Ni–Mo–PGE–Au sulfide ores (521±54 Ma), respectively, which are identical and overlap within uncertainty, and are in good agreement with previously obtained ages for presumed age-equivalent strata.Editorial handling: R. Coveney Jr.     

延边开山屯花岗闪长岩-石英闪长岩体年代学、地球化学特征及对古太平洋板块俯冲作用时限的制约

近年来古太平洋构造域的构造演化备受学者关注。本文报道的延边开山屯地区花岗闪长岩-石英闪长岩体LAICP-MS U-Pb年龄表明其形成时间为早侏罗世早期(198±1Ma),所采样品可根据Zr/Hf值分为高Zr/Hf值组花岗闪长岩和低Zr/Hf值组石英闪长岩。高Zr/Hf值组花岗闪长岩起源深度浅,富集Rb、Th、U、K等大离子亲石元素(LILEs),贫Nb、Ta、Ti等高场强元素(HFSEs),具壳源岩浆的特点。低Zr/Hf值组为壳源岩浆与来自深部的亏损地幔岩浆混合而成,岩石亏损Nb、Ta、Zr、Hf、Ti等高场强元素,具有典型的弧型岩浆地球化学特征。岩体中存在细粒闪长质包体,镜下可见针柱状磷灰石。开山屯岩体属钙碱性系列岩石,结合前人资料,认为其与该地区同时代火成岩组成北-东向分布的早侏罗世活动大陆边缘型火成岩带,而位于该带西侧的小兴安岭-张广才岭地区存在同时代弧后拉张带,两者构成典型的大陆弧与弧后拉张带模型,共同揭示了早侏罗世早期古太平洋板块对东北地区的俯冲作用。     

The Northeast Kingdom batholith,Vermont: magmatic evolution and geochemical constraints on the origin of Acadian granitic rocks

Five Devonian plutons (West Charleston, Echo Pond, Nulhegan, Derby, and Willoughby) that constitute the Northeast Kingdom batholith in Vermont show wide ranges in elemental abundances and ratios consistent with major crustal contributions during their evolution. The batholith consists of metaluminous quartz gabbro, diorite and quartz monzodiorite, peraluminous granodiorite and granite, and strongly peraluminous leucogranite. Contents of major elements vary systematically with increasingSiO<₂ (48 to 77 wt.%). The batholith has calc-alkaline features, for example a Peacock index of 57, and values for K<₂O/Na₂O (<1), K/Rb (60–350), Zr/Hf (30–50), Nb/Ta (2–22), Hf/Ta (up to 10), and Rb/Zr (<2) in the range of plutonic rocks found in continental magmatic ares. Wide diversity and high values of minor- and trace-element ratios, including Th/Ta (0.5–22), Th/Yb (0–27), Ba/La (0–80), etc., are attributed to intracrustal contributions. Chondrite-normalized REE patterns of metaluminous and relatively mafic intrusives have slightly negative slopes (La/Yb_cn<10) and negative Eu anomalies are small orabsent. The metaluminous to peraluminous inter-mediate plutons are relatively enriched in the light REE (La/Yb_cn>40) and have small negative Eu anomalies. The strongly peraluminous Willoughby leucogranite has unique trace-element abundances and ratios relative to the rest of the batholith, including low contents of Hf, Zr, Sr, and Ba, low values of K/Rb (80–164), Th/Ta (<9), Rb/Cs (7–40), K/Cs (0.1–0.5), Ce/Pb (0.5–4), high values of Rb/Sr (1–18) low to moderate REE contents and light-REE enriched patterns (with small negative Eu anomalies). Flat REE patterns (with large negative Eu anomalies) are found in a small, hydrothermally-altered area characterized by high abundances of Sn (up to 26 ppm), Rb (up to 670 ppm), Li (up to 310 ppm), Ta (up to 13.1 ppm), and U (up to 10 ppm). There is no single mixing trend, fractional crystallization assemblage, or assimilationscheme that accounts for all trace elementvariations from quartz gabbro to granite in the Northeast Kingdom batholith. The plutons originated by mixing mantle-derived components and crustal melts generated at different levels in the heterogeneous lithosphere in a continental collisional environment. Hybrid rocks in the batholith evolved by fractional crystallization and assimilation of country rocks (<50% by mass), and some of the leucogranitic rocks were subsequently disturbed by a mild hydrothermal event that resulted in the deposition of small amounts of sulfide minerals.     

冀北赤城红旗营子群黑云斜长片麻岩的岩石学、地球化学及原岩特征

冀北赤城红旗营子群黑云斜长片麻岩具斑状变晶结构,变基质为中、细粒鳞片粒状变晶结构,片麻状构造,局部保留有变余砂状结构。岩石主要由斜长石(28%~32%)、石英(35%~40%)、黑云母(18%~25%)及少量石榴石(2%~8%)和石墨(1%~4%)等组成,其变质演化可能是一个降温、降压的过程。黑云斜长片麻岩样品具有较宽的常量元素变化范围,稀土元素含量不是很高且变化较大,ΣREE介于49.45×10-6~140.10×10-6之间,具有轻稀土富集、重稀土平坦的球粒陨石标准化稀土配分模式,轻、重稀土元素比值为5.23~9.16,(La/Yb)CN值为5.07~8.70。大多数样品具中等程度的负Eu异常和不太明显的负Ce异常,(Eu/Eu*)CN和(Ce/Ce*)CN值分别为0.63~0.81和0.82~1.01。岩石中Rb、Sr、Cs、Ba、Zr、Hf、Nb、Ta、Th和U等微量元素含量均低于上地壳的平均含量,但Sr/Ba和Th/U值高于上地壳的平均值。红旗营子群黑云斜长片麻岩的地球化学特征表明其原岩应为泥质(或含泥质)砂岩,可能形成于大陆岛弧环境。     

NS935钻孔沉积物不活泼微量元素记录与陆源输入变化探讨

探讨了NS93－5钻孔沉积物的部分高场强元素和大离子亲石元素的含量变化，结果表明，Ti、Rb、Ga、Y、Zr、Nb、Cs、Hf、Ta、Th和稀土元素（REE）相互间含量变化具有很好的相关关系，反映这些元素均基本来自碎屑组分。经Ti和Sc标准化后这些元素含量的变化，以及Zr/Y等特征元素比值和Ge异常指数Ce/Ce、Eu异常指数Eu/Eu的变化，均反映出碎屑输入的通量或者组成的变化，例如火山灰的富集。其中一些变化与气候变化有关，可能反映周边陆壳的风化强度的波动。NS93－5钻孔沉积物的不活泼元素记录，可提供反映输入陆壳物源区的环境演变证据。     

塔里木北部二叠纪长英质火山岩年代学及地球化学特征

大面积分布于塔里木盆地的二叠纪玄武岩构成了面积250000km²的大火成岩省(LIP),长英质火山岩的发现为塔里木二叠纪火山作用的研究打开了新的窗口。本文从塔北地区约5000m深的钻井中收集了4件二叠纪长英质火山岩的样品。通过对其进行锆石U-Pb同位素测试,得出其形成时代为274~282Ma,为塔里木大火成岩省晚期岩浆作用的产物。岩石具有高钾的特征K₂O+Na₂O=7.29%~8.34%,K₂O/Na₂O>1,大部分属于高钾钙碱性系列,且属于过铝质(A/CNK=1.32~1.53)。具有富集LREE和Zr、Hf、Y,亏损Sr、P、Ti、Nb、Ta等特征,微量元素分布曲线形态与地壳相近,具有右倾的稀土元素配分曲线,且显示出一定的负铕异常。通过Sr-Nd-Pb同位素的分析得出其源区有大量地壳物质,这与其具有较高的Th、U含量和与地壳平均值相似的Nb/La、Nb/U、Th/Ta相一致。综合年代学、地球化学特征及构造环境的判断,认为塔北地区二叠纪长英质火山岩形成于地幔柱活动背景下的地壳物质的部分熔融。     

Trace and rare earth element geochemistry of the Upper Triassic mudstones in the southern Ordos Basin,Central China

Oil‐source correlation studies have demonstrated that the crude oils in the Ordos Basin were mainly derived from organic‐rich lacustrine mudstones of the Yanchang Formation. The sedimentology, petrology and organic geochemistry of these mudstones have been studied intensively, but their trace and rare earth element (REE) characteristics have received little attention. In this paper, we present trace and rare earth element data of the Upper Triassic Yanchang Formation mudstones in the southern Ordos Basin to constrain the palaeoenvironment, provenance and depositional setting. Our results show that the REE and trace element concentrations of the Yanchang Formation mudstones are higher than those of the upper continental crust (UCC). The Sr contents and Sr/Ba and Y/Ho ratios of these mudstones indicate the absence of a marine transgression during the sedimentation of the Upper Triassic mudstones. The depositional environment of the Upper Triassic mudstones was slightly oxic as evidenced by the values of Eu/Eu*, Ce/Ce*, Ce_anom, δU, U/Th, V/Cr and Ni/Co. The UCC‐normalized distribution pattern of REEs, spider diagrams, the ratios of related elements, the bivariate diagrams of Th/Sc–Zr/Sc and La/Th–Hf and the ternary plots of La–Th–Sc and Th–Sc–Zr/10 signify that the provenances of the Chang9–7 mudstones were mainly derived from a continental island arc, whereas the provenances of the Chang6–3 mudstones were mainly derived from a mixture of continental island arc and active continental margin, and the latter contain less recycled materials. Combined with the previous studies of detrital zircon dating and petrography of the Yanchang Formation sandstones in the southern Ordos basin, we propose that the Qinling orogenic belt served as one of the primary source regions occurring between the Chang7 and Chang6 periods, corresponding to the initial uplift of the west Qinling Mountains due to the collision between the Yangtze and North China blocks. Copyright © 2014 John Wiley & Sons, Ltd.     

The Earth’s tungsten budget during mantle melting and crust formation

During silicate melting on Earth, W is one of the most incompatible trace elements, similar to Th, Ba or U. As W is also moderately siderophile during metal segregation, ratios of W and the lithophile Th and U in silicate rocks have therefore been used to constrain the W abundance of the Earth’s mantle and the Hf-W age of core formation. This study presents high-precision W concentration data obtained by isotope dilution for samples covering important silicate reservoirs on Earth. The data reveal significant fractionations of W from other highly incompatible lithophile elements such as Th, U, and Ta. Many arc lavas exhibit a selective enrichment of W relative to Th, U, and Nb-Ta, reflecting W enrichment in the sub-arc mantle via fluid-like components derived from subducting plates. In contrast, during enrichment by melt-like subduction components, W is generally slightly depleted relative to Th and U, but is still enriched relative to Ta. Hence, all arc rocks and the continental crust exhibit uniformly low Ta/W (ca. 1), whereas W/Th and W/U may show opposite fractionation trends, depending on the role of fluid- and melt-like subduction components. Further high-precision W data for OIBs and MORBs reveal a systematic depletion of W in both rock types relative to other HFSE, resulting in high Ta/W that are complementary to the low Ta/W observed in arc rocks and the continental crust. Similar to previous interpretations based on Nb/U and Ce/Pb systematics, our Ta/W data confirm a depletion of the depleted upper mantle (DM) in fluid mobile elements relative to the primitive mantle (PRIMA). The abundance of W in the depleted upper mantle relative to other immobile and highly incompatible elements such as Nb and Ta is therefore not representative of the bulk silicate Earth. Based on mass balance calculations using Ta-W systematics in the major silicate reservoirs, the W abundance of the Earth’s primitive mantle can be constrained to 12 ppb, resulting in revised ratios of W-U and W-Th of 0.53 and 0.14, respectively. The newly constrained Hf-W ratio of the silicate Earth is 25.8, significantly higher than previously estimated (18.7) and overlaps within error the Hf-W ratio proposed for the Moon (ca. 24.9). The ¹⁸²Hf-¹⁸²W model age for the formation of the Earth’s core that is inferred from the ¹⁸²W abundance and the Hf/W of the silicate Earth is therefore younger than previously calculated, by up to 5 Myrs after solar system formation depending on the accretion models used. The similar Hf/W ratios and ¹⁸²W compositions of the Earth and the silicate Moon suggest a strong link between the Moon forming giant impact and final metal-silicate equilibration on the Earth.     

Major and trace element variation in ilmenite in the Skaergaard Intrusion: petrologic implications

Ilmenite separates from the floor (LS), roof (UBS), and wall (MBS) sequences of the Skaergaard Intrusion were analyzed for major and trace elements using DCP-AES and ICP-MS techniques. In all three sequences, FeO progressively increases, and MgO and Al₂O₃ progressively decrease with differentiation. Although trace element abundances are, in general, higher in UBS ilmenite than in MBS and LS ilmenite, all three sequences have similar trends for trace element abundance vs. crystallization. Ba, Cs, Rb, Sr, Th, U, Y, and the REEs are excluded elements in ilmenite, and remained at low abundances during differentiation. Cr, Ni, Sc, and V are included elements in ilmenite and other mafic phases, and decreased during differentiation. V contents in ilmenite, however, do not decrease significantly until the upper part of the middle zone, suggesting that magnetite did not begin to affect the magma differentiation trend until much later than when it first appears in the intrusion. Hf, Nb, Ta, and Zr, which are strongly excluded elements in silicates, are included elements in ilmenite. The element ratios Zr/Hf, Y/Ho, Nb/Ta, and U/Th are relatively constant in Skaergaard ilmenite from different parts of the intrusion, suggesting that fluid transport did not significantly effect these elements during differentiation or post-solidification cooling. Calculated partition coefficients for ilmenite in the Skaergaard Intrusion are similar to those reported from previous studies of lunar and terrestrial basalts and kimberlites, and for most elements are significantly lower than those reported for ilmenite in rhyolitic magma. Similar D_i's for Zr, Hf, Nb, and Ta suggest that ilmenite crystallization did not significantly affect Zr/Nb or Hf/Ta in the Skaergaard magma, but the ratios of Zr, Hf, Nb, or Ta to other high field strength elements, such as Th, U, Y, or the REEs, may have been altered by ilmenite fractionation.     

吉南天桥地区基性岩墙与其围岩TTG片麻岩的成因及构造意义

吉南地区太古宙基底中发育大量早前寒武纪基性岩墙群,是陆壳伸展的直接证据。对白山市东部天桥太古宙基底出露区内基性岩墙及其围岩进行了锆石U-Pb定年和地球化学分析,以确定该期伸展事件的形成机制及地质意义。天桥地区基性岩墙岩性为斜长角闪岩,侵位于TTG片麻岩中。英云闪长质片麻岩(TN1)中锆石具核-边结构,岩浆核的LA-ICP-MS测年结果为2500±6Ma,指示其形成于新太古代末期。天桥岩墙(TN3)中的锆石内部结构与TN1相同,酸性岩浆核的SHRIMP测年结果为2490±17Ma,与TN1在误差范围内一致,表明这些锆石不是基性岩墙原生锆石,而是岩墙侵位过程中在围岩中捕获的锆石,但根据岩墙仅侵位在太古宙基底中且变质程度高于周围古元古界老岭群,将其侵位年龄大致限制在新太古代末期-古元古代早期。地球化学特征显示,基性岩墙具有低SiO_2、Na_2O、K_2O含量,高CaO、MgO含量,A/CNK=0.56～0.59,属于准铝质的拉斑玄武岩系列岩石,∑REE低、配分曲线平坦,富集LILE(Rb、Ba和K),亏损HFSE(Th、U、Nb和Ta),具有与原始地幔相同的Nb/Ta、Zr/Hf比值及接近地壳的Nb/U、Ta/U比值,指示其岩浆可能来源于地幔且在上升过程中受到地壳混染,形成于板内伸展环境。TTG片麻岩具有中等的SiO_2和MgO含量,高Al_2O_3和Na_2O含量以及低CaO含量,A/CNK=1.00～1.14,属弱过铝质的钙碱性系列岩石,∑REE低、具有右倾的REE配分曲线,轻稀土富集、重稀土亏损,富集LILE(Rb、Ba、K和Sr),强烈亏损HFSE(U、Nb、Ta、Sm和Ti),其岩浆可能来源于变质玄武质岩石和极少量沉积岩的部分熔融,结合邻区TTG的研究成果,认为其形成于与俯冲相关的活动大陆边缘环境。前人研究表明,新太古代晚期板块构造体制可能已经启动,结合我们以往研究,认为新太古代晚期华北克拉通东北部可能发生了弧陆碰撞造山运动,天桥岩墙的侵位标志着新太古代末期至古元古代早期之间华北克拉通东北部进入造山后伸展环境,可能是对新太古代造山运动结束的响应。     

东昆仑祁漫塔格阿确墩地区侵入岩U-Pb年代学、地球化学及其地质意义

祁漫塔格地区岩浆岩的成岩时代和形成环境的确定能对东昆仑造山带加里东期构造演化时限加以约束.对祁漫塔格西北部阿确墩地区石英闪长岩和二长花岗岩进行了年代学和岩石地球化学研究,结果显示,石英闪长岩属准铝质-弱过铝质钙碱性系列岩石;轻重稀土分馏明显,具中等-轻微铕负异常（δEu=0.79~0.90）;相对富集Rb、K、Hf、Zr、Tb、Nd等元素,不同程度地亏损Ba、P、Ti、Nd、Ta、Y;具有I型花岗岩类特征.二长花岗岩属弱过铝质钙碱性系列岩石;轻重稀土分异程度极大,具明显铕负异常（δEu=0.42~0.45）;富集大离子亲石元素（如Rb、K、La、Ce、Nd、Tb等）,亏损高场强元素（P、Ti、Nd、Ta）和Ba、Sr、U等元素;为高分异I型花岗岩.Nd/Th、Nb/Ta、Mg#值等指标显示石英闪长岩为壳源特征且受到幔源岩浆的影响,推测是幔源岩浆底侵地壳物质发生部分熔融形成的;二长花岗岩则是壳源的,可能与幔源岩浆底侵诱发的上地壳物质部分熔融有关,且经历了强烈的结晶分离作用.石英闪长岩和二长花岗岩的LA-ICP-MS锆石U-Pb年龄分别为448.8±3.9 Ma和405.2±3.6 Ma,代表其形成时代.石英闪长岩总体显示出与俯冲消减作用有关的岛弧岩浆岩地球化学特征;二长花岗岩在构造环境图解中显示为碰撞背景,但微量元素与同碰撞花岗岩典型特征不符,综合分析认为形成于后碰撞构造背景下.结合区域构造演化,推测东昆仑祁漫塔格地区在晚奥陶世持续处于俯冲消减环境中,早泥盆世之前进入后碰撞造山阶段.      

Geodynamic significance of the Raspas Metamorphic Complex (SW Ecuador): geochemical and isotopic constraints

The Raspas Metamorphic Complex of southwestern Ecuador is regarded as the southernmost remnant of oceanic and continental terranes accreted in the latest Jurassic–Early Cretaceous. It consists of variably metamorphosed rock types. (1) Mafic and ultramafic rocks metamorphosed under high-pressure (HP) conditions (eclogite facies) show oceanic plateau affinities with flat REE chondrite-normalized patterns, Nd_{150 Ma} ranging from +4.6 to 9.8 and initial Pb isotopic ratios intermediate between MORB and OIB. (2) Sedimentary rocks metamorphosed under eclogitic conditions exhibit LREE enriched patterns, strong negative Eu anomalies, Rb, Nb, U, Th, Pb enrichments, low Nd_{150 Ma} values (from −6.4 to −9.5), and high initial ⁸⁷Sr/⁸⁶Sr and ^206,207,208Pb/²⁰⁴Pb isotopic ratios suggesting they were originally sediments derived from the erosion of an old continental crust. (3) Epidote-bearing amphibolites show N-MORB affinities with LREE depleted patterns, LILE, Zr, Hf and Th depletion, high Nd_{150 Ma} (>+10) and low initial Pb isotopic ratios.The present-day well defined internal structure of the Raspas Metamorphic Complex seems to be inconsistent with the formerly proposed interpretation of a “tectonic mélange”. The association of oceanic plateau rocks and continent-derived sediments both metamorphosed in HP conditions suggests that the thin edge of the oceanic plateau first entered the subduction zone and dragged sediments downward of the accretionary wedge along the Wadatti–Benioff zone. Subsequently, when its thickest part arrived into the subduction zone, the oceanic plateau jammed the subduction processes, due to its high buoyancy.In Ecuador and Colombia, the latest Jurassic–Early Cretaceous suture involves HP oceanic plateau rocks and N-MORB rocks metamorphosed under lower grades, suggesting a composite or polyphase nature for the latest Jurassic–Early Cretaceous accretionary event.     

Characteristics of Rare Earth and Trace Element Patterns in Bedded Cherts from the Bottom of the Lower Cambrian in the Northern Tarim Basin, Northwest China and Their Genetic Significance

The sedimentary sequence of the Lower Cambrian is a key interval to reveal the early evolution history of the Earth and there occur widespread cherts worldwide. These cherts possibly carry important information to decipher the breakup process of the Rodinia Continent. Black rock series at the bottom of the Lower Cambrian in the Northern Tarim Basin, China, is composed of black shales interbedded with thin-bedded cherts. Ten chert samples were systematically collected from two outcrops at Xiaoerbulak and Sogatbulak, 8.8 and 7.5 m thick respectively. The cherts were crushed, and were analyzed for trace element and rare earth concentrations. Trace elements such as V, Cu, Zn, U, Pb, Ba, Cd, Ag, Mo,As and Sb are highly enriched, and others such as Rb, Zr, Cs, Hf, Ta, W, T1, Bi and Th are highly depleted in the cherts.These trace element patterns suggest that the cherts may be of deep crustal origin. The low ratios of Th/U and Rb/Sr further suggest that the cherts are of earth interior sources or received hydrothermal input during their deposition. Chondritenormalized Eu/Eu* value markedly decreases upward in the section from 5.54 at the lowermost to 0.73 at the top, and NASC-normalized Eu/Eu* value decreases from 8.05 to 1.03. The relatively high Eu/Eu* ratio for the cherts from the northern Tarim Basin is most likely due to a hydrothermal input (e.g., Eu/Eu*-10). The systematic decrease of Eu/Eu* ratio from the bottom to the top of the section reflects that the hydrothermal input is the largest in the lowermost portion of the section and gradually decreases upward. The chondrite-normalized Ce/Ce* ratio ranges from 0.42 to 0.83, with an average of 0.60. North American Shale Composite (NASC)-normalized Ce/Ce* ratio ranges from 0.42 to 0.79, with an average of 0.57. Negative Ce anomalies are distinct. EREEs in the cherts generally increase from 10.50 ppm at the bottom to 35.97 ppm at the top of the sampled section. NASC-normalized (La/Lu)N ratio decreases from 2.72 at the bottom to 0.67 at the top. NASC-normalized (La/Ce)N ratio increases from 1.36 at the bottom to 3.13 at the top. These REE patterns are very similar to those for the cherts deposited in the pelagic ocean-basin floor in the Franciscan Complex exposed at Marin Headlands, California (F-MH chert) (Murray et al., 1991). These geochemical signatures are inconsistent with our previous sedimentological data, which suggests a continental shelf setting. Based on multiple lines of evidence including high TOC content in the concomitant black shales, phosphorite at the bottom of black rock series, regional rise of sea level,and beginning of the southern Tianshan Ocean geotectonic cycle, the authors infer that the hydrothermal fluid was carried to the continental shelf by upwelling from a divergent pelagic ocean floor setting.     

豫西伊川地区中元古界兵马沟组泥质岩地球化学特征及其环境与物源示踪

华北克拉通南缘古-中元古代构造-沉积演化的俯冲说、裂解说之争由来已久,兵马沟组作为熊耳群形成后的第一套碎 屑沉积岩层,对解释上述过程有重要意义。该文通过对豫西伊川地区的兵马沟组泥质岩微量及稀土元素的地球化学特征分析, 探讨了其沉积环境、物源,进而揭示了其发育的大地构造背景。结果表明：（1）兵马沟组泥质岩稀土元素含量高,球粒陨石标准 化后轻重稀土分异明显,δEu负异常,δCe异常不明显,富集Rb、Th、La、Ce、Nd、Zr、Hf 等,较上地壳平均值富集Sc、V、Cr、Co、Ni、 Rb;（2）Ceanom指数、V/V + Ni反映了泥质岩沉积时的还原环境,Sr/Ba、B/Ga表明其为由陆相至海相的过渡型沉积;（3）Cr/Zr、Th/U 等元素比值关系反应其物源无深部物质加入,La/Yb-Ce、Co/Th-La/Sc图解表明物源为中酸性火山岩及少量稳定陆壳物质; （4）Th-Sc-Zr/10、Th-Co-Zr/10图解表明其物源主要来自大陆岛弧。综合上述分析结果,结合区域地质信息,可以得出中元古界 兵马沟组形成于大陆岛弧弧后盆地,熊耳群分布区为中元古代俯冲成因的大陆岛弧区。     

西天山伊宁县北琼阿希河谷火山岩地球化学特征及构造背景探讨

对西天山伊宁县北琼阿希河谷出露的火山岩进行了地球化学特征研究,结果表明,该地区火山岩属于钙碱性火山岩系列,LREE强烈富集[(La/Yb)N介于2.76~7.03之间],具有弱的Eu负异常,样品的强不相容元素(Cs、Rb、Ba、Th、U)LILE均明显富集,总体在原始地幔的10倍以上,显示了岛弧火山岩的Nb-Ta负异常特征。但是,其(Th/Nb)N、(La/Nb)N的比值远远大于1,Ba/Nb、La/Nb的变化范围较大,Zr/Nb、Zr/Hf接近N-MORB的比值,Ta/Nb却接近上地壳的比值,暗示该地区的火山岩岩浆在上升过程中受到一定程度的地壳混染,所有样品均显示为中钾-高钾系列,这种钾质类型的优势地位也反映了陆壳基质在岩浆活动中占有重要的地位。样品的Zr均大于300、Zr/Y均大于4,显示了板内玄武岩的成分特征,在Zr-Zr/Y判别图解中,大多数样品投到板内玄武岩的构造环境区域内。结合区域上的地质特征,认为该地区在泥盆世一直处于抬升阶段,在石炭纪时碰撞结束并进入碰撞后伸展阶段,局部地区产生一定的裂谷化特征。琼阿希河谷的火山岩的Nb-Ta负异常特征应为地壳混染引起,火山岩的形成环境为碰撞后伸展的构造环境。     

蚌埠隆起区古元古代钾长花岗岩的成因: 岩石地球化学、锆石U-Pb年代学与Hf同位素的制约

对蚌埠隆起区庄子里和磨盘山钾长花岗岩进行了系统的年代学和地球化学以及锆石Hf同位素的研究, 以便对其岩石成因进行约束.研究结果表明, 庄子里和磨盘山钾长花岗岩中锆石发育震荡生长环带, 且具有较高的Th/U比值(0.13~1.47), 反映了岩浆成因特征.对庄子里和磨盘山钾长花岗岩中岩浆锆石进行的LA-ICP-MSU-Pb定年结果(上交点年龄) 分别为2104±20Ma和2196±190Ma, 这表明蚌埠隆起区钾长花岗岩的形成时代为古元古代.钾长花岗岩的SiO₂和K₂O含量分别介于69.65%~77.95%和4.98%~5.17%之间; 该类岩石富集轻稀土元素和Zr、Hf、Rb、Th、U等元素, 明显亏损Ba、Sr、Eu、P和Ti等元素; 它们的ε_Nd(t) 值变化于-3.4~+3.2之间, Nd的模式年龄变化于2.31~2.79Ga之间; 钾长花岗岩中锆石的ε_Hf(t) 值和Hf同位素两阶段模式年龄分别介于-5.1~+7.8和2.26~2.83Ga之间.上述特征表明, 蚌埠隆起区钾长花岗岩的原始岩浆起源于有少量古老地壳物质涉入的新生下地壳的部分熔融.庄子里和磨盘山钾长花岗岩为A型花岗岩, 形成于伸展的构造背景.      

小兴安岭二龙山林场堆晶辉长岩地球化学特征及构造环境

小兴安岭北部二龙山林场辉长岩的主量、微量和稀土元素的测试分析结果显示: 二龙山辉长岩为钙碱性系列、偏铝质岩石; δEu 正异常,Sr 元素含量富集明显,具有堆晶辉长岩特征; Rb /Sr、Nb / Ta、LILE/HFSE、Th /Ta、Nb /U 和Nb /La 比值特征均显示,辉长岩岩浆来自受到俯冲流体交代的地幔源区。Nb /Zr、Th /Nb、Th /Ta 和Ta /Hf 比值特征及对Th /Hf --Ta /Hf 构造环境判别图解投点表明,二龙山辉长岩形成于陆内拉张环境。     

北秦岭新元古代前属于扬子板块的地球化学证据

秦岭是扬子板块和华北陆块间的复合造山带,其中存在两个古缝合带,北面的新元古代到早古生代商－丹缝合带介于北秦岭和南秦岭之间,南面的晚古生代勉－略缝合带形成于南秦岭与扬子陆块北缘之间,属于确知的扬子陆块内部打开形成的类型。北秦岭在地壳增生历史,元古宙上地慢性质,以及元古宙以来地幔相对富Nb、Ta、Cu,高Rb／Sr、Ba／Sr、Ba／La、Th／La、Nb／Ta比值和相对贫Fe、Mo方面均与南秦岭和扬子陆块北段一致。而与华北板块南段明显不同。变玄武岩类的Pb同位素三维空间拓扑图和铀εNd（t）－206Pb／204Pb图显示,区域一级地球化学界面位于北秦岭与华北陆块的分界处,二级界面才是商丹－缝合带。构造侵位于该带中的松树沟蛇绿岩<1000 Ma士)中已发现并存着N—MORB和E—MORB型变拉斑玄武岩。表明它们的岩浆分别来自亏损地幔和深部地幔热柱源区。松树沟蛇绿岩与勉,略蛇绿岩中的N—MORB型岩石具有与此相似的同位素组成和特征元素对比值,暗示松树沟蛇绿岩所代表的古洋壳也应属于扬子板块内部型。加之,新元古代之前秦岭只存在裂谷系而无板块结合带,故可确定北秦岭原来应属于扬子板块的组成部分。对秦岭群和宽坪群变拉斑玄武岩及松树沟E—MORB和N—MIORB进行地球化学对比的结果,既能够支持北秦岭是在扬子板块的洋壳洋岛基础上发展形成的微陆块的推断,又可解释北秦岭幔源岩石具有特高Th／La、Yb／Hf、Sc／Th比值和壳幔更富于放射成因钳的原因。此外。对本研究结果在秦岭造山带发展动力学方面的意义也进行了初步讨论。