甘肃西秦岭两类新生代钾质火山岩:岩石地球化学与成因

甘肃西秦岭地区存在钾霞橄黄长岩和钾质粗面玄武岩(钾玄岩)两类钾质火山岩,出露在甘肃西秦岭礼县、宕昌县等,地理坐标大致相当于104°20′～104°50′E,33°30′～34°10′N。钾霞橄黄长岩是一种不含斜长石,但普遍含有高钛金云母、黄长石、白榴石、霞石的岩石,全岩化学成分具低SiO2和Al2O3,富TiO2、CaO、MgO和高K/Na、高Mg#值的特征;钾质粗面玄武岩(钾玄岩)含有大量斜长石但是缺乏高钛金云母、黄长石、白榴石和霞石,全岩化学中SiO2、Al2O3明显高于前者,而TiO2、CaO、MgO、K/Na和Mg#值要比钾霞橄黄长岩低。钾霞橄黄长岩的全岩K/Ar和金云母单矿物的39Ar/40Ar同位素定年落在7.1～23Ma,而钾玄岩的全岩39Ar/40Ar同位素定年落在9Ma左右,因此它们同为中新世产物。两类钾质火山岩具有相似的富集不相容元素和轻稀土的特征。两类钾质火山岩的初始87Sr/86Sr分别在0.70403～0.70749和0.70412～0.70522;143Nd/144Nd分别在0.51274～0.51294和0.51265～0.51276;εNd分别在1.12～5.95和0.3～2.3。206Pb/204Pb、207Pb/204Pb和208Pb/204Pb分别落到18.3746～18.9986、15.529～15.6693和38.4971～39.4144。在火山岩源区示踪的143Nd/144Nd-87Sr/86Sr,207Pb/204Pb-206Pb/204Pb,208Pb/204Pb-206Pb/204Pb,143Nd/144Nd-206Pb/204Pb,87Sr/86Sr-206Pb/204Pb和Ba/Nb-La/Nb图解中,一致显示两类钾质火山岩具有与OIB相似的地球化学特征,源区可能与地幔柱有关,并具有EM1、DMM和HIMU端员混合特征。结合前人对该区深部地球物理和断裂构造的研究,论证了火山岩的起源与成因,指出作为对印度—欧亚大陆强烈碰撞的吸收与调节,高原下软流圈地幔流沿400km界面向北东方向的侧向流动以及西秦岭周边克拉通块体的阻挡,是形成西秦岭断裂系左行走滑特征和巨大拉分盆地的主要原因,也是导致西秦岭新生代两类钾质火山岩和碳酸岩起源与成因的动力学机制,较好地解释了西秦岭新生代岩浆作用起源深度大,具有地幔柱源的地球化学特征,岩石组合与地球化学有别于高原内部及其周边地区新生代钾质火山岩的原因。     

青藏高原新生代两类超钾质岩石的成因——实验岩石学和地球化学约束

青藏高原分布有羌塘—囊谦—滇西和冈底斯两条新生代钾质-超钾质火山岩带。羌塘—囊谦—滇西超钾质岩浆活动的峰值时间为40～30Ma,主体岩石具有Ⅰ型超钾质岩的高MgO和低CaO、Al2O3含量特征;30～24Ma期间羌塘中、西部出现Ⅲ型钾质-超钾质岩浆活动,主体岩石以贫SiO2、高CaO、Al2O3和低MgO/CaO为特征。冈底斯新生代超钾质火山岩也显示I型超钾质岩的高MgO和低CaO、Al2O3含量特征,其形成时间为25～12Ma。综合超钾质岩石的实验资料,可知区内I型超钾质岩的源区以富硅、富钾流(熔)体交代形成的金云母方辉橄榄岩为主;Ⅲ型钾质-超钾质岩浆源区则以斜辉橄榄岩地幔为主。囊谦—滇西Ⅰ型超钾质岩带空间上严格受红河走滑构造带所控制,40～28Ma出现I型超钾质岩浆活动,16Ma转变为OIB型钾质火山岩。岩浆源区从岩石圈地幔向软流圈演变,暗示大型走滑断裂引起的岩石圈地幔减薄和软流圈上涌是导致交代岩石圈地幔金云母分解熔融产生区内I型超钾质岩浆的主控因素。羌塘中部35～34Ma有软流圈来源为主的钠质碱性玄武岩岩浆的喷发,30～24Ma转变为以岩石圈地幔为主要来源的Ⅲ型钾质-超钾质岩浆活动,岩浆源区从软流圈向岩石圈迁移,指示软流圈上涌伴随的富CO2流(熔)体活动是导致古交代岩石圈地幔升温熔融产生Ⅲ型钾质-超钾质岩浆的主控因素,软流圈上涌可能是俯冲板片断离或岩石圈地幔拆沉作用的结果。     

胶东金矿区高钾—钾质脉岩地球化学与俯冲—壳幔作用研究

胶东金矿区与金矿成矿伴生的脉岩为一套高钾－钾质脉岩。根据地质、岩相学及与金矿化的时空关系,将主要岩石类型划分为粕斑岩、安山玢岩、英安玢岩类,它们分别形成与金矿化早期、同期和晚期;主要元素成分以富碱高钾、低钛为特征,成分变异具同源岩浆结晶分异演化的一般规律,早期以辉石和橄榄石、中晚期以角闪石和斜长石为主的矿物相分离结晶控制岩浆的演化;岩石明显富Ｂａ、Ｓｒ、Ｒｂ、Ｋ、ＬＲＥＥ等大离子元素、强烈亏损Ｃｒ     

青藏高原拉萨地块西部中新世赛利普超钾质岩石的地球化学与岩石成因

西藏拉萨地块西部赛利普中新世碰撞后超钾质火山岩由中国地质调查局地质填图首次发现,露头呈残丘状集中分布于中新世赛利普盆地.为一套含地幔包体的粗面岩,SiO2含量中等(55.36%～6.70%),高K2O含量(6.70%～7.50%)和K2O/Na2O比值(3.34～4.93).岩石高MgO(6.4%～7.95%)、Cr(174×10-6～421×10-6)、Ni(268×10-6～337×10-6)和Mg#(68～72),岩石为地幔部分熔融的原始岩浆.岩石高度富集大离子亲石元素(LILE)和轻稀土元素(LREE)、具有明显的Nb、Ta、Ti的负异常、富集放射性成因Sr、Pb及Nd同位素(87Sr/86Sr=0.727327～0.727803,206Pb/204Pb=18.705～18.779,207Pb/204Pb=15.731～15.761,208Pb/204Pb=39.775～39.919,143Nd/144Nd=0.511848～0.511861)、较低的εNd值(≈-15)和古老的Nd模式年龄(tDM=2.2～2.4 Ga),这些地球化学特征揭示出赛利普的岩浆源区为富集地幔(EM Ⅱ).将赛利普与拉萨地块西部其他地点和青藏高原北部的北羌塘和西昆仑地区出露的超钾质岩石进行综合对比表明,赛利普超钾质岩石可能为尖晶石相含金云母橄榄岩及少量石榴石相含金云母橄榄岩地幔的低度部分熔融的产物.拉萨地块超钾质岩石的形成可能与印度大陆岩石圈俯冲,或是俯冲的印度大陆地壳前缘撕裂和分段俯冲有关.     

甘肃阳山金矿床花岗岩成因:来自地球化学、Sr-Nd-Pb同位素和年代学的证据

甘肃文县阳山金矿为西秦岭近十年来发现的超大型卡林–类卡林型金矿床,地处扬子板块西北部碧口地体北缘。本文选取阳山矿区与矿体在空间上密切相关的花岗岩样品,进行系统的元素地球化学特征分析,结合前人的年代学和Sr-Nd-Pb同位素数据来探讨其成因。阳山花岗岩SiO_2含量为60.64%~80.77%、Al_2O_3为13.68%~23.71%,K_2O为0.74%~4.32%,A/CNK为1.34~6.02,富集Cs、U、K、Pb等元素,亏损Ba、Nb、La、Ce、Sr、Ti等元素,ΣREE=14.45×10–6~143.14×10–6,平均70.31×10–6,LREE/HREE=1.39~19.52,平均10.12,具有轻稀土富集,重稀土亏损的特征,(La/Yb)N=1.48~37.26,平均17.08,球粒陨石标准化配分模式显示弱负Eu异常(δEu=0.50~1.21,平均0.77),为过铝质钙碱性花岗岩。结合前人对花岗岩、碧口群和泥盆系三河口组的元素地球化学、年代学和Sr-Nd-Pb同位素等测试数据以及区域构造演化历史,认为阳山花岗岩来源于碰撞增厚的地壳物质——碧口群的部分熔融,在上升的过程中与泥盆系发生混染。     

胶东金矿区高钾-钾质脉岩地球化学与俯冲-壳幔作用研究

胶东金矿区与金矿成矿伴生的脉岩为一套高钾 -钾质脉岩。根据地质、岩相学及与金矿化的时空关系 ,将主要岩石类型划分为煌斑岩、安山玢岩、英安玢岩类 ,它们分别形成与金矿化早期、同期和晚期 ;主要元素成分以富碱高钾、低钛为特征 ,成分变异具同源岩浆结晶分异演化的一般规律 ,早期以辉石和橄榄石、中晚期以角闪石和斜长石为主的矿物相分离结晶控制岩浆的演化 ;岩石明显富 Ba、Sr、Rb、K、L REE等大离子元素、强烈亏损 Cr、Ni及相对亏损 Th、Nb、Ti、Y等高场强元素元素 ,体现初始岩浆起源于富集地幔源区 ;大量的挥发组分及强烈的富集 Ba、Sr及低的 Sr/ Nd比值 ,指示源区为以俯冲陆源沉积为主、有玄武质洋壳参与的脱水、脱气等作用交代早期地幔楔形成的富集地幔源 ,初始岩浆是在中生代印支期形成的富集地幔向燕山期亏损方向演化的特定阶段发生低程度熔融形成 (熔融程度为 6 %～ 8% ) ;就位环境的不同是导致石英脉岩型与蚀变岩型矿区脉岩成岩作用有一定差别的主要原因     

胶西北两类金矿田的高钾-钾质脉岩元素地球化学与成岩作用研究

重点研究了胶西北玲珑、尹格庄两类金矿田的高钾－钾质脉岩,根据地质产状、岩性及与金矿化的时空关系,该脉岩可分为三类：（１）粕斑岩;（２）安山玢岩;（３）英安玢岩类,它们分别形成于金矿化早期、同期、晚期。元素地球化学研究表明：主要氧化物成分变民具岩浆分离结晶的一般规律,早期以辉石为主、晚期以角闪石和斜长石为主的矿物相分离结晶作用控制岩浆成分的演化。岩石富碱、高钾低钛,微量和稀土元素强烈富集大离子元素Ｂ     

青藏高原北部黑石北湖新生代钾质火山岩的成因

昆仑岩带新生代钾质火山活动是青藏高原北缘深部动力学过程的重要记录。有关岩浆源区的性质和岩浆产生的机制一直存在较大的认识分歧。阐述了昆仑岩带黑石北湖钾质火山岩的岩石学和地球化学特征,并与西昆仑康西瓦和泉水沟新生代火山岩进行了成因对比分析。研究表明,昆仑火山岩带玄武质岩石的K2O和Na2O含量变化较大,从钠质过渡到钾质,硅碱图显示玄武质初始岩浆向钾质安粗岩方向演化。黑石北湖火山岩的高场强元素和Sr、Nd同位素组成显示岩浆源区具有EMⅡ的性质。岩石LREE的强烈富集和HREE的强烈分馏指示岩浆来自富集型含石榴子石地幔源区。不相容元素K/Nb、Ba/La等比值具有岛弧火山岩与OIB型玄武岩的过渡特征,指示源区为曾遭受软流圈流（熔）体交代的古俯冲地幔楔。     

青藏高原北部黑石北湖新生代钾质火山岩的成因

昆仑岩带新生代钾质火山活动是青藏高原北缘深部动力学过程的重要记录。有关岩浆源区的性质和岩浆产生的机制一直存在较大的认识分歧。阐述了昆仑岩带黑石北湖钾质火山岩的岩石学和地球化学特征,并与西昆仑康西瓦和泉水沟新生代火山岩进行了成因对比分析。研究表明,昆仑火山岩带玄武质岩石的K2O和Na2O含量变化较大,从钠质过渡到钾质,硅碱图显示玄武质初始岩浆向钾质安粗岩方向演化。黑石北湖火山岩的高场强元素和Sr、Nd同位素组成显示岩浆源区具有EMⅡ的性质。岩石LREE的强烈富集和HREE的强烈分馏指示岩浆来自富集型含石榴子石地幔源区。不相容元素K/Nb、Ba/La等比值具有岛弧火山岩与OIB型玄武岩的过渡特征,指示源区为曾遭受软流圈流（熔）体交代的古俯冲地幔楔。     

西昆仑新生代钾质火山岩的地球化学及其对源区的约束

对西昆仑大红柳滩、普鲁北和普鲁东新生代火山岩的地球化学研究表明 ,其总体特征相似。但是南带的大红柳滩和北带的普鲁火山岩区 (普鲁北和普鲁东 )存在一定的区别。大红柳滩火山岩的 Ti O2 相对较低 ,但富集碱和 K,同时更加富集轻稀土和大离子亲石元素。在同位素特征上 ,大红柳滩火山岩具有相对低的 Sr同位素比值以及高的εNd值和 2 0 7Pb/2 0 4 Pb、2 0 8Pb/2 0 4 Pb比值。其微量元素和同位素特征指示了火山岩的源区为岩石圈地幔 ,为含有黑云母和角闪石的石榴子石二辉橄榄岩。但在部分熔融前经历了交代富集的过程。原始地幔标准化图解…     

冈底斯基性次火山岩地球化学和Sr-Nd-Pb同位素：碰撞后火山作用亏损地幔源区的约束

青藏高原隆升与伸展构造模式（地幔对流减薄、板片断离和大陆俯冲）认为地幔软流圈上涌为藏南碰撞后火山作用提供了热源,而已有研究成果没有包括碰撞后火山作用软流圈地幔源区的岩石学、地球化学证据,而是集中在富集岩石圈地幔.出露于冈底斯东段达孜-甲马地区的基性次火山岩的^40Ar/^39Ar坪年龄为38.07～42Ma.次火山岩具有高Mg#值和相容微量元素含量（Cr、Ni、V、Sc）,低LILE、和LREE,低LREE/HREE分离.原生样品呈现平坦的稀土配分曲线和LILE亏损的原始地幔标准化曲线,显示出OIB地球化学亲和性.岩石具有低Sr、低Pb和高Nd同位素组成[（^87Sr/^86Sr）i=0.7048～0.7064;（^144Nd/^143Nd） i = 0.5126～0.51286 ;^206Pb/^204Pb = 18.3722～18.5288; ^207Pb/^204Pb = 15.5686～15.6214; ^208 Pb/^204Pb = 38.5203～38.7298],显示出与富集OIB非常相似的Sr-Nd-Pb同位素组成及变化范围.应用我们的分析数据并结合已有研究成果,达孜-甲马基性次火山与adakite、林子宗火山岩、钾质岩-超钾质岩构成了藏南碰撞后火山岩Sr-Nd-Pb同位素组成演化带,基性次火山岩分布于演化带的原生端元,揭示出其岩浆源区与富集岩石圈地幔存在明显的差异.研究结果证明,具有与OIB相似地球化学和Sr-Nd-Pb同位素组成的基性次火山岩的源区是软流圈地幔.由冈底斯带新近纪基性火山作用厘定的软流圈上涌深部过程对洋壳断离模式、基性岩浆底侵作用和西藏高原地壳加厚时限提供了有效的约束.     

Isotopic characteristics of potassic rocks: evidence for the involvement of subducted sediments in magma genesis

The potassic igneous rock suite (with molar K₂O/Na₂O > 1) can be divided into an “orogenic” subgroup that occur in subduction-related tectonic settings and an “anorogenic” sub-group that are confined to stable continental settings. Representatives of both sub-groups possess trace element and isotopic features consistent with the contamination of their magma sources by incompatible element rich and isotopically evolved “metasomatic” components. It is argued here that these metasomatic components are principally derived from subducted lithosphere, including subducted sediments. Most examples of orogenic potassic magmatism (e.g. Italian potassic rocks, Spanish lamproites, Sunda arc leucitites) have trace-element and Sr, Nd and Pb isotopic characteristics consistent with the contamination of their mantle sources by a component derived from marine sediments. Anorogenic sub-group potassic magmas have generally similar incompatible trace element and Sr and Nd isotopic characteristics to those of orogenic potassic magmas, but many examples have unusual Pb isotopic compositions with unradiogenic ²⁰⁶Pb/²⁰⁴Pb. Modern marine sediments characteristically have low U/Pb ratios and the unradiogenic ²⁰⁶Pb/²⁰⁴Pb of anorogenic potassic magmas may have evolved during long-term storage of subducted sediments (or components derived from them) within the subcontinental lithosphere. These unusual Pb isotopic compositions require substantial time periods (> 1 Ga) to have elapsed between the fractionation events lowering the U/Pb ratio (i.e. erosion and sedimentation at the Earth's surface) and subsequent potassic magmatism and it is therefore not surprising that most examples of anorogenic potassic magmatism are not associated with recent subduction processes. Although the eruption of potassic magmas is commonly related to rifting or hotspot activity, these processes do not necessarily play an important role in the genesis of the unusual sources from which potassic magmas are derived.     

海洋沉积物的铁和锌同位素测定

介绍海洋沉积物Fe和Zn同位素化学前处理及测定方法,报道南海西部夏季上升流区两个沉积物柱样的Fe和Zn同位素组成。样品采用HF+HNO3+HClO4常压消解,经脱盐后,转化为氯化物形式并经离子交换柱分离纯化后,用多接收器等离子体质谱法测定Fe和Zn同位素比值。该前处理方法可以快捷地实现海洋沉积物的消解、有机质的去除和海盐脱离;结合相关测试流程,可获得较高的δ56Fe(0.10‰,2SD)和δ66Zn分析精度(0.11‰,2SD)。两个沉积物柱样的δ56Fe值(相对于IRMM-014)和δ66Zn值(相对于JMC3-0749C)随深度变化不明显,两柱之间也无明显差异。总体上,南海西部上升流区1～2 ka以来的沉积物δ56Fe值(0.04‰～0.20‰)和δ66Zn值(0.12‰～0.30‰)与已报道的黄土和气溶胶、火成岩以及大部分海洋沉积物接近,明显高于静海相海洋沉积物的δ56Fe值。     

大型花岗岩基形成和演化的深部动力学过程：滴水构造、钾质火山作用与地表地质过程

以美国内华达山脉复合岩基为例,系统评述了与大型花岗岩基的形成、演化相关的深部地球动力学过程及构造地貌学响应。在大陆岛弧环境下,基性岩浆的底侵作用促使下地壳发生角闪岩脱水部分熔融,在岩基根部形成高密度的石榴辉石岩,岩基根部最终发生重力失稳,形成滴水构造;在地貌上反映为滴水构造对应区域的沉降和相应的张性构造,在岩浆作用上则表现为软流圈地幔上涌和残余富集岩石圈地幔的低程度部分熔融,形成钾质火山岩。这种高度动态的深部动力学过程是维持大型花岗岩基地区较高高程或促使这些区域高程骤然增加的重要因素。     

Provenance of the Upper Cretaceous to upper Eocene clastic sediments of the Western Cordillera of Ecuador: Geodynamic implications

The Late Cretaceous–Eocene clastic deposits of the Western Cordillera of Ecuador record significant changes in the source areas, grain size, and location of the depocenters, related to the accretion of oceanic terranes that constitute the present-day Western Cordillera and Coast. Major changes in the source areas occurred in the ?late Maastrichtian and ?late middle Eocene. They are interpreted as corresponding to the accretion of the Guaranda and Macuchi oceanic terranes, respectively. Major increases in the grain sizes occurred in the ?late Maastrichtian, late Paleocene(?), and ?late middle Eocene, and seem to coincide with the accretion of the Guaranda, Piñón, and Macuchi terranes, respectively. The increasing occurrence of plutonic or metamorphic fragments and the westward shift of the depositional areas through the Paleocene–upper Eocene interval indicate an increasing uplift and erosion of the Cordillera Real. Continuous, although jerky, uplift of the latter during the Maastrichtian–Eocene period, supports the idea that the accreted oceanic material contributed to the crustal thickening and relief creation of the Ecuadorian Andes.     

Eocene mafic volcanism in northern Anatolia: its causes and mantle sources in the absence of active subduction

Eocene mafic volcanic rocks occurring in an E–W-trending, curvilinear belt along and north of the Izmir–Ankara–Erzincan suture zone (IAESZ) in northern Anatolia, Turkey, represent a discrete episode of magmatism following a series of early Cenozoic collisions between Eurasia and the Gondwana-derived microcontinents. Based on our new geochronological, geochemical, and isotope data from the Kartepe volcanic units in northwest Anatolia and the extant data in the literature, we evaluate the petrogenetic evolution, mantle melt sources, and possible causes of this Eocene volcanism. The Kartepe volcanic rocks and spatially associated dikes range from basalt and basaltic andesite to trachybasalt and basaltic trachyandesite in composition, and display calc-alkaline and transitional calc-alkaline to tholeiitic geochemical affinities. They are slightly to moderately enriched in large ion lithophile (LILE) and light rare earth elements (LREE) with respect to high-field strength elements (HFSE) and show negative Nb, Ta, and Ti anomalies reminiscent of subduction-influenced magmatic rocks. The analysed rocks have ⁸⁷Sr/⁸⁶Sr_(i) values between 0.70570 and 0.70399, positive ?_Nd values between 2.7 and 6.6, and Pb isotope ratios of ²⁰⁶Pb/²⁰⁴Pb_(i) = 18.6–18.7, ²⁰⁷Pb/²⁰⁴Pb_(i) = 15.6–15.7, and ²⁰⁸Pb/²⁰⁴Pb_(i) = 38.7–39.1. The ⁴⁰Ar/³⁹Ar cooling ages of 52.7 ± 0.5 and 41.7 ± 0.3 Ma obtained from basaltic andesite and basalt samples indicate middle to late Eocene timing of this volcanic episode in northwest Anatolia. Calculated two-stage Nd depleted mantle model (T_DM) ages of the Eocene mafic lavas range from 0.6 to 0.3 Ga, falling between the T_DM ages of the K-enriched subcontinental lithospheric mantle of the Sakarya Continent (1.0–0.9 Ga) to the north, and the young depleted mantle beneath central Western Anatolia (0.4–0.25 Ga) to the south. These geochemical and isotopic features collectively point to the interaction of melts derived from a sublithospheric, MORB-like mantle and a subduction-metasomatized, subcontinental lithospheric mantle during the evolution of the Eocene mafic volcanism. We infer triggering of partial melting by asthenospheric upwelling beneath the suture zone in the absence of active subduction in the Northern Neotethys. The geochemical signature of the volcanic rocks changed from subduction- and collision-related to intra-plate affinities through time, indicating an increased asthenospheric melt input in the later stages of Eocene volcanism, accompanied by extensional deformation and rifting.     

阿尔泰喀纳斯早泥盆纪钾质富镁英安岩:俯冲沉积物熔体交代

阿尔泰造山带喀纳斯地区出露一套钾质富镁英安岩,锆石U-Pb定年显示其形成于早泥盆世(411±4 Ma)。该套英安岩富SiO_2(64.71～67.77 wt%),MgO(3.28～3.53 wt%)和K_2O(2.12～2.75 wt%),贫Na_2O(1.70～1.85wt%),K_2O/Na_2O比值介于(1.20～1.53),具较高的Al_2O_3(14.12～15.33 wt%)和较低的CaO(1.12～1.28 wt%),ASI指数1.8,显示过铝质钙碱性岩浆系列特征,明显不同于初始的高镁安山质岩浆。样品具有中等的Cr(134×10~(-6)～153×10~(-6))、Co(17.47×10~(-6)～18.65×10~(-6))、Ni(71.64×10~(-6)～80.76×10~(-6))含量,富集大离子亲石元素Rb、K和高场强元素U、Th、Pb及LREE,发育Sr、Ba及TNT(Ta、Nb、Ti)负异常,显示俯冲带岩浆岩地球化学特征。其高的Mg~#(51.71～52.47)和低的ε_(Nd)(t)值(-4.6～-4.2)说明该套火山岩可能来自遭受强烈交代作用的地幔楔部分熔融。样品具有较高的Ba/La、Pb/La比值和较低的Th/Yb、Nb/Y比值,显示地幔交代作用以俯冲沉积物熔体的交代作用为主。以上地球化学特征说明,喀纳斯钾质富镁英安岩可能主要是受俯冲沉积物熔体交代的上覆地幔橄榄岩部分熔融的产物,阿尔泰泥盆纪处于俯冲作用相关的活动大陆边缘构造环境。此外,该套火山岩的形成时代制约了其下覆的哈巴河群沉积上限,显示哈巴河群沉积于早泥盆世之前。     

拉萨南冈底斯花岗岩地球化学特征及构造背景

正拉萨地体记录了多期次碰撞造山和特提斯洋演化的关键信息(许志琴等,2006)。拉萨地体南缘发育大量晚白垩系—古近系岩浆活动且规模较大,冈底斯南部酸性火山岩为典型代表,为印度大陆—亚洲大陆碰撞演化及新特提斯洋向北俯冲消减提供证据。早期学者认为古新世花岗岩类由于新特提斯洋板块俯冲所造成。之后部分研究者通过系统研究林子宗火山岩认为印度—亚洲板块碰撞促     

Major and trace element, and Sr-Nd isotope constraints on the origin of Paleogene volcanism in South China prior to the South China Sea opening

Paleogene volcanic rocks crop out in three sedimentary basins, namely, Sanshui, Heyuan and Lienping, in the attenuated continental margin of south China. Lavas from the Sanshui basin which erupted during 64-43 Ma are bimodal, consisting of intraplate tholeiitic basalt and trachyte/rhyolite associations. Similar to Cretaceous A-type granites from the nearby region, the felsic member shows peralkaline nature [Na₂O + K₂O ≈ 10–12%; (Na + K)/Al≈ 0.98−1.08], general enrichment in the incompatible trace elements and significant depletion in Ba, Sr, Eu, P and Ti. Although both types of the Sanshui lavas have rather uniform Nd isotope compositions [_Nd(T) ≈ +6 to +4]that are comparable to Late Cenozoic basalts around the South China Sea, the felsic rocks possess apparently higher initial Sr isotope ratios (I_Sr up to 0.713) and form a horizontal array to the right in the Nd vs. Sr isotope plot. Closed system differentiation of mantle-derived magmas in a ‘double diffusive’ magma chamber is considered for the bimodal volcanism, in which the trachytes and rhyolites represent A-type melts after extensive crystal fractionation in the upper portion of the chamber. Such A-type melts were later contaminated by small amounts (1–3%) of upper crustal materials during ascent. On the other hand, composition of lavas in the other two basins varies from tholeiitic basalt to andesite. Their Sr and Nd isotope ratios [I_Sr ≈ 0.705 to 0.711; _Nd(T) ≈ +1 to − 5] and generally correlative Nb-Ta depletions suggest a distinct magma chamber process involving fractional crystallization concomitant with assimilation of the country rock. We conclude that these Paleogene volcanic activities resulted from the lithospheric extension in south China that migrated southwards and eventually led to opening of the South China Sea during 30-16 Ma.     

Geochemical and isotopic studies of aeolian sediments in China

The Sr and Nd isotopic, rare earth element (REE) and major element compositions, together with mineral and grain‐size proportions, are reported for aeolian loess deposits and desert sands from several Chinese localities. The study was carried out in order to examine regional variations in the isotopic and geochemical features of these aeolian sediments, and to constrain the provenance of Chinese loess. Samples include loesses from the Tarim and Junggar basins and desert sands from the Taklimakan desert in north‐west China, loess from the Ordos area and desert sands from the Tengger and Mu‐us deserts in north‐central China, as well as loess and desert sands from the Naiman area, north‐east China. REE distributions show minimal variation among the Chinese loess deposits, whereas those for the desert sands show regional variations. New isotopic data document a latitudinal variation in Sr and Nd isotopic features for the loesses and desert sands. The Naiman and Junggar loesses have distinctly lower ⁸⁷Sr/⁸⁶Sr ratios and higher ε_Nd(0) values than the loesses from the Tarim Basin, the Ordos area and the Loess Plateau. Among the desert sands, the Naiman samples have higher ε_Nd(0) values than the Taklimakan, Tengger and Mu‐us samples. Isotopic data suggest that loesses of the Loess Plateau were supplied from the Tarim Basin loesses and Taklimakan Desert sand, and that the Naiman loesses were supplied from the Junggar Basin loesses. The latitudinal variation in the loesses and desert sands may be partly explained by isotopic variations reported previously for moraines from the Tianshan and west Kunlun Mountains, which are possible sources for the loesses and desert sands. These inferences on the provenance of the loesses and desert sands are consistent with the dust transport pattern over East Asia.