Multiple mantle sources during island arc magmatism: U–Pb and Hf isotopic evidence from the Kohistan arc complex, Pakistan

ABSTRACT This paper presents high-precision U–Pb ages and initial Hf isotopic compositions of zircon from mafic to felsic rocks of the Kohistan Arc Complex, Pakistan. Three magmatic pulses tapping geochemically different reservoirs are distinguished. Partial melting of mantle with MORB-type isotopic characteristics generated 99–92-Ma-old magmas. Plutonism around 85 Ma tapped a more fertile mantle source, most likely consisting of a >600-Ma-old metasomatically enriched mantle, or of mantle contaminated by an old sedimentary component; 82-Ma-old felsic peraluminous dykes have MORB-type isotopic compositions considered to be inherited from remelting earlier magmas in the deep base of the arc. The isotopic results demonstrate several and rather rapid changes in melt source region during arc development. They also show that there was subordinate continental influence and negligible importance of slab components for the Hf budget during the generation of the Kohistan Arc Complex.     

新疆乌伦古富碱侵入岩成因探讨

对沿乌伦古深断裂分布的钠闪石花岗岩、钾长花岗岩、闪长岩、碱性正长岩等富碱侵入体进行了较系统的岩石化学、微量元素、稀土元素地球化学及Ｏ、Ｐｂ、Ｓｒ、Ｎｄ同位素组成研究，确认它们为一类幔源重熔分异型富碱侵入岩类。在分析其物质来源、形成构造环境和成岩条件等特征基础上，用部分熔融、多阶段分离结晶和分离结晶－地壳混染等模型探讨了其成岩机理。     

广西花山花岗岩的岩石学和地球化学特征及成岩物质来源的探讨

本文研究了花山花岗岩的岩石学、地球化学和同位素地质学特征,确定了花山岩体是一个由三期独立的、不同时代、不同成因和不同物质来源的花岗质岩浆岩所组成的复式岩体。第一期印支期牛庙石英二长岩和同安石英二长岩,属以幔源物质为主的壳幔混合来源,由上地幔分异岩浆上侵并同化混染了地壳物质而形成;第二期燕山早期花山主体花岗岩亦属壳幔混合来源,但其壳幔物质比值有所增高。上地幔沿东西向区域性深断裂的上拱和地壳物质(包括沉积组分和火成组分)受热重熔,是该期花岗岩的可能成因;第三期燕山晚期细粒花岗岩小岩体是由以沉积组分为主的地壳物质经部分熔融、重熔或深熔而成。     

The Yermakovsky deposit,Western Transbaikal Region,Russia: Isotopic and geochemical parameters and sources of beryllium-bearing granitoids and other rocks

An isotopic study of igneous and metamorphic rocks has been carried out at the Yermakovsky bertrandite-phenakite-fluorite deposit. It has been established that the model age of the schists pertaining to the Zun-Morino Formation is 1360–1260 Ma. In Nd and Sr isotopic composition, these schists deviate from the isotopic composition of the continental crust and are close in this respect to the enriched mantle reservoir (EM-II). The model age of carbonate rocks of the Zun-Morino Formation is 1330–1020 Ma. The Middle Riphean model age of the Zun-Morino Formation is interpreted as the age of its protolith. According to the Sr and Nd isotopic data, all preore igneous rocks (granitic dikes, gabbroic rocks, and gneissose granite of the Tsagan Complex) were formed with the participation of continental crustal material. Synore basic dikes, alkali leucogranite stock, and syenite intrusion are considered to be mixtures of mantle components (DM+HIMU) and various continental crustal components (Tsagan gneissose granite, crystalline schists, the mean composition of granitoids of the Angara-Vitim batholith as an estimate of average composition of the regional continental crust). Synore igneous rocks are genetically cognate and related to the magmatic activity in the Western Transbaikal Rift Zone presumably formed in the Triassic under effect of a mantle plume.     

拉萨地体北部永珠地区早白垩世岩浆岩地球化学、锆石U-Pb年代学、Hf同位素组成及其地质意义

拉萨地体北部出露大面积早白垩世岩浆岩,对它们的成因和形成机制的研究,有助于揭示拉萨地块白垩纪时期的岩浆作用过程及动力学背景.通过岩石学、地球化学和同位素地质学方法对拉萨地体北带永珠地区早白垩世中-酸性岩浆岩进行了研究.结果显示黑云母二长花岗岩、流纹岩和安山岩的锆石LA-ICP-MS U-Pb年龄分别为118±1.0 Ma、121±0.8 Ma和115±0.8 Ma,代表了其侵入和喷出时代.黑云母二长花岗岩、花岗斑岩和流纹岩为高钾钙碱性过铝质-强过铝质岩浆岩（A/CNK=1.01~1.35）,亏损高场强元素Nb、P、Ti和大离子亲石元素Ba、Sr,富集大离子亲石元素Rb、K和放射性元素U、Th;稀土配分图显示LREE富集,HREE近平坦,Eu明显负异常,为形成于大陆边缘的岛弧岩浆岩特征.黑云母二长花岗岩和流纹岩的锆石Hf初始比值ε_Hf（t）分别为-1.21~3.01和-0.68~5.35,对应的两阶段模式年龄分别为0.99~1.26 Ga和0.84~1.22 Ga,为壳幔混源岩浆.安山岩为高钾钙碱性,亏损Nb、Ta、P、Ti、U和Sr,富集Rb、K和Th,稀土配分图显示LREE富集,HREE近平坦,Eu轻微负异常,为形成于大陆边缘弧的岩浆岩.结合前人研究成果,分析认为永珠地区早白垩世岩浆岩形成于班公湖-怒江特提斯洋壳南向俯冲作用下的大陆边缘弧环境,由俯冲的班公湖-怒江中特提斯洋板片在深部脱水熔融,进而诱发上覆地幔楔部分熔融形成基性岩浆上涌,导致下地壳物质发生部分熔融形成酸性岩浆,它们在上升过程中按不同比例混合,形成中性和酸性岩浆侵入到地下或喷出地表,形成侵入岩和火山岩.      

The Sulu UHP Terrane: A Review of the Petrology and Structural Geology

Petrological and isotopic evidence suggests that the protolith of the Sulu ultrahigh-pressure (UHP) terrane was Precambrian continental crust consisting of granite, granodiorite, gabbro, marble, and basic dikes, with local granulite-facies assemblages. Around 220 Ma this unit of continental rocks was buried to depths up of ～120 km within the mantle. Structures formed during exhumation suggest highly mobile behavior of acidic rocks, even under conditions of very low water activity. Petrological studies show that the Sulu terrane underwent isothermal decompression, which implies relatively rapid exhumation, and suggests that the role of melting during exhumation may have been underestimated. The later stages of exhumation are associated with NW-SE-directed tectonic transport and the formation of at least one major normal detachment.     

Extreme source heterogeneity and complex contamination patterns along the Cameroon Volcanic Line: New geochemical data from the Bamoun plateau

We investigated mafic and felsic volcanic rocks from the Bamoun plateau, a magmatic province located north of Mount Cameroon, in the continental part of the Cameroon Volcanic Line (CVL). Basalts and dacites were probably emplaced more than 40 Ma ago, while basanites represent very young volcanic eruptions. Among the basalts, some of them have suffered crustal contamination during their uprise through the continental crust, and their primary trace element and isotopic compositions have been slightly modified. The formation of the dacites was also accompanied by some crustal contamination. Non-contaminated rocks show that the oldest magmas are transitional basalts formed by relatively high degrees of partial melting of a moderately enriched mantle source, probably containing pyroxenites. Recent basanites were produced by very low partial melting degrees of an enriched mantle source with HIMU composition, but different from the source of the nearby Mount Cameroon lavas. The mantle beneath the CVL is thus very heterogeneous, and the tendency towards more alkaline mafic-ultramafic compositions in the youngest volcanic manifestations along the CVL seems to be a general feature of all CVL.     

从大陆火山岩视角了解深部地幔化学储库的属性

洋岛玄武岩（OIB）在同位素组成上的显著变化表明深部地幔在化学上是高度不均一的,存在EM1、EM2和HIMU等地幔端元。现有OIB的地球化学证据支持EM1、EM2和HIMU源区多存在再循环地壳物质,因此对地幔端元属性的精准约束是探讨壳幔物质循环和地球深部过程的关键。基于近年来来自OIB的Mg、Fe、Zn等金属稳定同位素和橄榄石斑晶元素地球化学方面的观察,本文在重新梳理不同地幔端元在成因上的联系和区别的基础上,把EM1和EM2归类为残余型富集储库（residual-type enriched reservoir）、HIMU归为交代型富集储库（metasomatic-type enriched reservoir）。相对于OIB,大陆幔源火山岩的熔融程度普遍要低得多,更有利于保存富集组分的地球化学信息。另外,洋岛玄武岩主要分布在南半球,大陆火山岩主要分布于北半球,后者可以弥补前者在空间上代表性不足的缺陷。因此,大陆火山岩有潜力成为观察深部地幔储库的新视角。

     

长江中下游—大别造山带中生代火山岩特征及成因

中国中东部长江中下游地区和大别造山带广泛发生了中生代岩浆作用,火山岩均集中在中间阶段,时代分别为135~127 Ma和133~125 Ma。长江中下游地区多个中生代火山盆地发育高钾钙碱性系列双峰式火山岩和中基性橄榄安粗岩系列火山岩。这些岩石富集大离子亲石元素,亏损高场强元素,弱富集Sr-Nd-Hf同位素,具有高放射成因Pb同位素组成,指示地幔源区地壳组分的加入。其中,中基性火山岩起源于富集的岩石圈地幔,受到俯冲大洋板片析出的含水熔体交代,晚阶段的超碱质火山岩起源于类似交代介质交代的软流圈地幔,指示岩浆源区的加深。大别造山带中生代火山岩包括高钾钙碱性系列和超钾质系列,均富集大离子亲石元素,亏损高场强元素,高度富集Sr-Nd-Hf同位素,具有低放射成因Pb同位素组成,与长江中下游地区差异显著。其中,高钾钙碱性系列火山岩起源于交代富集的岩石圈地幔,交代介质为印支期深俯冲的华南陆壳析出熔体,而晚阶段的超钾质火山岩起源更深,是深俯冲的华南陆壳在高压下,多硅白云母分解产生熔体交代的地幔源区。长江中下游地区幔源火山岩记录了俯冲的古太平洋板块的直接物质贡献,而大别造山带地幔源区记录了印支期俯冲陆壳的信息。两个构造单元火山岩早、晚阶段均表现出岩浆源区的加深,长江中下游地区对应了古太平洋板块的低角度俯冲及俯冲板片的回卷(约130 Ma),而大别造山带在古太平洋板块俯冲回卷的动力学机制下,发生造山带的垮塌和岩石圈的拆沉。     

华北陆块南缘汝阳群次火山岩SHRIMP锆石U-Pb年龄及其地质意义

华北陆块南缘汝阳群下部的次火山岩一直被看作是与汝阳群同时代的火山喷发夹层。通过研究测得玄武岩SHRIMP锆石U-Pb年龄为213.5±2.4Ma,并在邻近该次火山岩层上部的紫红色泥岩中发现了褪色的角岩化现象,说明该火山岩是晚三叠世沿汝阳群下部顺层侵入的次火山岩岩床。通过对该次火山岩岩石及其地球化学特征的研究,认为该次火山岩起源于有早期俯冲洋壳或陆壳参与再循环的大陆富集型地幔的部分熔融,它与秦岭造山带几乎同时代形成的超高压榴辉岩、埃达克质岩石、高钾钙碱性花岗岩、环斑花岗岩等共同揭示了秦岭造山带自中三叠世全面碰撞造山之后,由碰撞挤压逐渐转变为伸展拉张的深部动力学过程。     

华北陆块南缘汝阳群次火山岩SHRIMP锆石U-Pb年龄及其地质意义

华北陆块南缘汝阳群下部的次火山岩一直被看作是与汝阳群同时代的火山喷发夹层。通过研究测得玄武岩SHRIMP锆石U-Pb年龄为213.5±2.4Ma,并在邻近该次火山岩层上部的紫红色泥岩中发现了褪色的角岩化现象,说明该火山岩是晚三叠世沿汝阳群下部顺层侵入的次火山岩岩床。通过对该次火山岩岩石及其地球化学特征的研究,认为该次火山岩起源于有早期俯冲洋壳或陆壳参与再循环的大陆富集型地幔的部分熔融,它与秦岭造山带几乎同时代形成的超高压榴辉岩、埃达克质岩石、高钾钙碱性花岗岩、环斑花岗岩等共同揭示了秦岭造山带自中三叠世全面碰撞造山之后,由碰撞挤压逐渐转变为伸展拉张的深部动力学过程。     

Elemental and Nd-Sr isotopic geochemistry of granitoids from the West Junggar foldbelt (NW China), with implications for Phanerozoic continental growth

We report elemental and Nd-Sr isotopic data for two coeval postcollisional magmatic suites (∼300 Ma), the Miaogou and Karamay suites, from West Junggar foldbelt (NW China), aiming to determine their source regions and implications for continental growth. The Miaogou rocks, monzonitic to granitic in composition, show low Mg# (<0.30) and depletion of HFSE, and are highly depleted in isotopic compositions, with I_Sr ranging from 0.7035 to 0.7045, ε_Nd(300 Ma) from + 8.4 to + 6.6 and young Nd model ages (0.37-0.70 Ga) that coincide with the ages of the ophiolites in the area. These features suggest that the Miaogou rocks were derived essentially by partial melting of a juvenile basaltic lower crust formed in the early to middle Palaeozoic. The Karamay suite contains a variety of rock types from gabbroic diorite to granite. These rocks show enrichment of LREE and LILE and depletion of HFSE, and have chemical composition (e.g., Mg# = 0.63-0.41) rather different from the Miaogou suite, though isotopically they are indistinguishable. This suggests that the parental magma of the Karamay suite originated from melting of a young lithospheric mantle that had previously been metasomatized during the Palaeozoic subduction. The West Junggar magmatic rocks represent production of juvenile continental crust in the Phanerozoic, triggered by upwelling of asthenophere in an extensional regime. The basement beneath the area is dominated by “trapped” Palaeozoic arc series and oceanic crust.     

Systematic variations in the composition of volcanic rocks in tectono-magmatic seamount chaines in the Brazil Basin

Data on the composition of rocks in linear tectono-magmatic rises in the Brazil Basin indicate that volcanic rocks in the Vitoria—Trindade seamount chain were derived from a mantle reservoir unevenly enriched in phosphorus under the effect of melts close to subalkaline picrobasalt. These melts contained much of the EM I mantle component because the plume material was contaminated with continental lithospheric component. A long-lived isotopic homogeneity of the source is typical of the whole structure, including the Trindade and Martin Vaz Islands and the Abrolhos Plateau. The analogous isotopic ratios of rocks at the Fernando de Noronha Islands are reportedly explained by a similar mechanism of melt derivation and the similar evolution of the mantle plume material, which was originally situated beneath the South American continent. Compared to the melts of volcanic rocks of all other seamounts discussed herein, the parental melts of volcanics at the Victoria—Trindade Seamounts were derived at lower degrees of melting of enriched source material at a greater depth. The overwhelming majority of volcanic rocks at the northern chain of the Bahia Seamounts were produced by melts generated with the involvement of material of the HIMU type. At the same time, one of our rock samples was derived from a source of composition close to DM with a certain admixture of enriched material like EM I. The mantle source of rocks of the Pernambuco Seamounts consisted of a mixture of DM and HIMU material with a certain admixture of EM I (or, perhaps, EM II). The 10°–11° S Seamounts were formed near the MAR axial zone at the decompressional melting of chemically homogeneous mantle source that consisted of DM material with an admixture of EM I (or, perhaps, EM II) component.     

Revisiting Early–Middle Jurassic igneous activity in the Nanling Mountains,South China: Geochemistry and implications for regional geodynamics

Early–Middle Jurassic igneous rocks (190–170 Ma) are distributed in an E–W-trending band within the Nanling Tectonic Belt, and have a wide range of compositions but are only present in limited volumes. This scenario contrasts with the uniform but voluminous Middle–Late Jurassic igneous rocks (165–150 Ma) in this area. The Early–Middle Jurassic rocks include oceanic-island basalt (OIB)-type alkali basalts, tholeiitic basalts and gabbros, bimodal volcanic rocks, syenites, A-type granites, and high-K calc–alkaline granodiorites. Geochemical and isotopic data indicate that alkaline and tholeiitic basalts and syenites were derived from melting of the asthenospheric mantle, with asthenosphere-derived magmas mixing with variable amounts of magmas derived from melting of metasomatized lithospheric mantle. In comparison, A-type granites in the study area were probably generated by shallow dehydration-related melting of hornblende-bearing continental crustal rocks that were heated by contemporaneous intrusion of mantle-derived basaltic magmas, and high-K calc-alkaline granodiorites resulted from the interaction between melts from upwelling asthenospheric mantle and the lower crust. The Early–Middle Jurassic magmatic event is spatially variable in terms of lithology, geochemistry, and isotopic systematics. This indicates that the deep mantle sources of the magmas that formed these igneous rocks were significantly heterogeneous, and magmatism had a gradual decrease in the involvement of the asthenospheric mantle from west to east. These variations in composition and sourcing of magmas, in addition to the spatial distribution and the thermal structure of the crust–mantle boundary during this magmatic event, indicates that these igneous rocks formed during a period of rifting after the Indosinian Orogeny rather than during subduction of the paleo-Pacific oceanic crust.     

Geochemistry and geochronology of mafic rocks from Bamenda Mountains (Cameroon): Source composition and crustal contamination along the Cameroon Volcanic Line

Mafic rocks from the Bamenda volcanic province along the Cameroon Volcanic Line have been dated from 17 to 0 Ma. Associated with some trachytes and rhyolites, this volcanism covers a period of more than 25 Ma. The studied rocks are basalts to mugearites. Most of them have been contaminated by continental crust during their transit to the surface. The oldest rocks are the most contaminated. One group of samples shows high Eu, Sr and Ba contents. This characteristic is not due to crustal contamination process, but has a mantle source origin. We argue that these characteristics have been acquired by mixing of melts formed by partial melting of mantle pyroxenites with melts formed in mantle peridotites. Such pyroxenites have been observed as mantle xenoliths in the Adamaoua province, and their chemical and isotopic compositions are consistent with such a model.     

四川木落寨稀土矿床与成矿有关的花岗岩问题及其意义

通过对四川木落寨稀土矿床及与成矿有关的花岗岩的岩石特征、岩石化学以及稀土元素地球化学研究表明，与成矿有关的岩石是英碱正长岩，而不是前人认为的黑云母花岗岩。岩石Pb-Sr-Nd同位素研究说明矿区英碱正长岩及矿石具深源、甚至幔源特征，不同于通常具壳源特征的黑云母花岗岩。木落寨矿床与邻近的冕宁牦牛坪大型稀土矿床的成矿岩石相同，都为英碱正长岩，它们的矿床特征也相似。木落寨英碱正长岩的厘定对研究区域岩浆活动、寻找同类型的稀土金属矿产有重要意义。     

花岗岩研究的若干新进展与主要科学问题

花岗岩研究进入到新的时期,需要寻找新的研究思路和研究方法。本文总结了近十年来国际上有关花岗岩前沿研究的新进展,归纳总结了七个方面的主要研究内容,包括:1)地球早期花岗岩与大陆演化;2)源区的不均一性与不平衡熔融;3)岩浆成分变化的同位素示踪;4)壳-幔岩浆混合与花岗岩的形成;5)地壳热带与中酸性岩的形成;6)花岗岩岩体的生长和结晶时间;7)金属稳定同位素在花岗岩研究中的应用。最后在此基础上对花岗岩的研究趋势,建议利用新视角、新方法等着重开展花岗岩源区和岩浆深部过程的精细研究,并揭示花岗岩与早期地壳形成之间的联系。     

Petrogenesis of the Xihuashan Granite in Southern Jiangxi Province,South China: Constraints from Zircon U-Pb Geochronology,Geochemistry and Nd Isotopes

Mesozoic granitic intrusions are widely distributed in the Nanling region,South China.Yanshanian granites are closely connected with the formation of tungsten deposits.The Xihuashan granite is a typica...     

Geochemistry and Petrogenesis of Granitoids at Sharang Eocene Porphyry Mo Deposit in the Main‐Stage of India‐Asia Continental Collision,Northern Gangdese,Tibet

The Sharang porphyry Mo deposit is the first discovered Mo porphyry‐type deposit in the Gangdese Metallogenic Belt. The orebody is hosted by the Eocene multi‐stage composite intrusive complex which is emplaced in the Upper Permian Mengla Formation and cut by the Miocene dykes. Granite porphyry is recognized as the ore‐bearing porphyry in the complex, which consists of quartz diorite, quartz monzonite, granite, prophyritic granite and post‐mineral lamprophyre. Granodiorite porphyry and dacite porphyry intrude the granite porphyry. Geochemical data indicate that Sharang complex has a High‐K calc‐alkalinc to shoshonitic, metaluminous to slightly peraluminous composition. The Sharang complex rocks are enriched in large ion lithophile elements, depleted in high‐field strength elements, Nb, Sr, P and Ti. REE patterns show slight enrichments in light REE relative to heavy REE and weak negative Eu anomalies. All rocks in this complex have a wide range of initial ⁸⁷Sr/⁸⁶Sr ratios (0.705605～0.712496) and lower ε_Nd(t) values (?0.61～?7.80). The geochemical data suggest highly oxidized‐evolved magma and old continental materials may have been the magma source for the Sharang intrusive complex that host porphyry Mo mineralization. Eocene pre‐ore and ore‐forming rocks at Sharang may have formed by partial melting of mantle wedge and by mixing with old continental crust at the lower crust level. In contrast the post‐ore rocks may have formed by partial melting of enriched lithospheric mantle.     

地壳深俯冲与富钾火山岩成因

富钾火山岩是一类兼具壳幔双重地球化学特征的特殊岩石组合 ,它们不可能由亏损或原始地幔所派生 ,成岩过程中必须有地壳物质的参与 ,将地壳物质引入富钾火山岩成岩过程的主要动力机制即是深俯冲作用。洋壳和陆壳均可以通过俯冲进入地幔 ,俯冲地壳物质析出流体对地幔岩石的交代作用是导致富钾火山岩具特殊地球化学特征的主要原因。根据对大别—苏鲁造山带南北两侧晚中生代富钾火山岩的实例研究 ,表明该区火山岩的形成均受到了俯冲洋壳析出流体的交代作用 ,但造山带北侧富钾火山岩的形成还叠加了俯冲的扬子陆壳析出流体的交代作用 ,是多次富集事件综合作用的结果。文中还对富钾火山岩成因研究中值得进一步深入探索的问题进行了讨论。