Niobium-tantalum Fractionation During Slab Subduction:Implications for the Formation of Continental Crust

Compared with the oceanic crust, knowledge about the formation of the continental crust (CC) is relatively poor. Although melting of subducted slabs in the early history of the Earth has been considered as the major way that shaped the chemical characteristics of the CC by most geologists, as the CC shares many characteristics with modern adakites, some geologists argued that Archean TTG was formed in the same way as modern arcs rather than slab melting, whereas others proposed that melting at the bottom of the thickened oceanic crust was more important. Recently,the debate is mainly focused on the unique subchondritic Nb/Ta value of the CC, and particularly, how Nb and Ta fractionated from each other and consequently how, in detail, the CC was built.     

西准噶尔达拉布特地幔橄榄岩弧前成因的矿物地球化学和氧同位素证据

达拉布特蛇绿岩位于中亚造山带西南缘,是古亚洲洋的扩张、俯冲、消减和闭合过程的产物,保留了洋盆形成及构造演化信息。前人对达拉布特蛇绿岩的形成大地构造背景始终未取得统一的认识。为探讨蛇绿岩所代表的构造演化过程,笔者以达拉布特蛇绿岩中的地幔橄榄岩为研究对象,通过详细的矿物地球化学及其氧同位素研究,对达拉布特地幔橄榄岩成因及构造背景提出新的制约。达拉布特方辉橄榄岩中橄榄石Ni/Co值为21~22,Ni/Mn值为3.0~7.8具有部分熔融残余的特征,此外,橄榄石中不相容元素相对于正常地幔橄榄石亏损,表明方辉橄榄岩为部分熔融的残余组分。方辉橄榄岩中尖晶石Cr# 为47~52、TiO2含量0.01%~0.04%,橄榄石Fo为90.34%~90.98%指示方辉橄榄岩经历>20%的部分熔融。方辉橄榄岩中橄榄石δ18 Oolivine值+5.1~+6.2‰、单斜辉石δ18Ocpx值+5.6~+6.9‰,其变化范围较大且整体高于正常地幔中橄榄石和单斜辉石的δ18 O值,矿物间氧同位素分馏系数Δ18OOpx—olivine平均-0.3‰,Δ18OOpx—cpx平均-0.7‰,显著区别于正常地幔中平衡的氧同位素分馏系数,具有明显的交代作用特征。结合方辉橄榄岩橄榄石中亏损的微量元素特征,认为方辉橄榄岩可能受到流体交代作用的影响,俯冲壳源物质脱水形成的高δ18O流体交代地幔橄榄岩导致了矿物与矿物间不平衡的氧同位素特征。通过尖晶石与单斜辉石成分判别,尖晶石与单斜辉石的主量元素具有介于弧前与深海地幔之间的过渡型特征,与俯冲初始阶段形成的地幔橄榄岩相似。综合矿物地球化学与氧同位素特征,笔者认为达拉布特地幔橄榄岩为形成于弧前初始俯冲环境。板块俯冲导致弧前扩张形成新洋壳,上涌的软流圈MORB like熔体与俯冲壳源物质熔融形成熔/流体与地幔橄榄岩相互作用,形成达拉布特地幔橄榄岩。     

滇西保山地块金厂河铁铜铅锌多金属矿床成矿机制——基于流体包裹体和硫、铅同位素证据

滇西金厂河铁铜铅锌多金属矿床位于保山地块北部，是西南"三江"成矿带内已知的大中型矽卡岩型矿床之一，具较高的经济价值及重要科研意义。文章在详细野外地质调查和岩相学观察基础上，对不同成矿阶段、特别是含子矿物的流体包裹体进行了研究，同时结合金厂河矿床S、Pb同位素特征，综合分析金厂河矿床成矿流体和物质的来源与演化，探讨矿床的形成机制。对石英内流体包裹体的研究表明，退化蚀变阶段均一温度为165~335℃，盐度w（NaCl_eq）为1.1%~9.0%；石英硫化物阶段均一温度为151~266℃，WL型包体盐度w（NaCl_eq）为1.9%~18.9%，S型包体盐度w（NaCl_eq）为31.9%~33.5%；碳酸盐阶段均一温度为148~250℃，盐度w（NaCl_eq）为2.6%~9.9%，各阶段均一温度依次降低，盐度先升高，再降低。拉曼光谱分析显示，流体成分为H₂O和极少量N₂，新发现了子矿物斑铜矿（Cu₅FeS₄），流体属H₂O-NaCl体系。S、Pb同位素特征对比研究表明，金厂河矿床各成矿阶段内硫同位素分馏已达平衡状态，矿石硫化物的δ³⁴S值能代表成矿流体的S同位素组成，用来限定其氧化-还原环境，δ³⁴S值为+2.5‰~+11.1‰、均值+5.65‰，硫源为深部幔源岩浆硫和地层硫构成的混合硫；Pb同位素硫化物组成显示，富铀铅、略亏损钍铅，铅源以上地壳铅为主，另含有少量深部壳源乃至幔源岩浆铅，属造山带铅特征，且与S同位素研究成果基本一致。综合研究认为，成矿系统氧化-还原环境的变化及流体沸腾作用的发生是主要的成矿机制。     

长江中下游成矿带贵池抛刀岭金矿含矿岩体年代学及地球化学研究

贵池抛刀岭金矿位于长江中下游成矿带安庆-贵池矿集区,是该地区首次发现的独立玢岩型金矿床,含矿岩体主要为蚀变矿化的英安玢岩.经过近两年的地质勘查,该矿床已接近大型金矿床规模.通过对抛刀岭金矿含矿岩体锆石LA-ICPMS U-Pb测定,其成岩年龄为146.8±2.4Ma和141.3±1.0Ma(2σ),与长江中下游地区中生代大规模铜金成矿事件时间一致.全岩地球化学研究表明,该岩石为过铝质,富集大离子亲石元素(K、Rb、Pb),亏损高场强元素(Nb、Ta、Ti)和轻稀土富集等地球化学特征.较高的锆石Ce4+/Ce3+和EuN/EuN*显示抛刀岭含矿岩体具有较高的氧逸度特征.构造判别图解显示这套含矿岩石属火山弧花岗岩(VAG),与长江中下游铜金矿床含矿岩体形成于同一构造背景下,即太平洋俯冲作用引起的洋壳和地幔楔熔融形成.     

东北那丹哈达岭中—新生代构造-热演化史:来自(U-Th)/He和裂变径迹热年代学的证据

中国东北的那丹哈达岭地区位于中亚造山带最东部,它的中新生代热演化史是认识陆内造山活动的关键,但该地区相关研究比较薄弱,其中—新生代的热演化史缺乏有效的约束.因此本文应用磷灰石裂变径迹、锆石和磷灰石(U-Th)/He等多种低温热年代学方法,对东北那丹哈达岭地区的侵入岩开展构造热演化历史研究.热年代学数据和热史模拟结果表明,该地区存在早白垩世晚期—晚白垩世(110～80Ma)、古新世—始新世(60～40 Ma)两期快速冷却事件,其冷却速率分别为3.42～4.81℃/Ma和1.43～1.83℃/Ma.结合区域构造和应力分析,我们认为两期冷却事件均受构造活动控制.第一期快速冷却事件是古太平洋板块北西向俯冲引发的构造叠加到鄂霍特莫茨克地块并与东亚大陆边缘碰撞引起;而第二期快速冷却事件是古太平洋俯冲的板片后撤使东亚陆缘处于伸展环境,造成东北大面积的剥露作用引起.这次研究增强了对东亚陆缘中新生代构造-热演化历史的认识,对于理解大陆内部造山带的构造变形过程与机理具有重要意义.     

花东海盆综合地震探测及其重要的构造意义

花东海盆位于欧亚板块与菲律宾海板块交汇处,是现今西太平洋俯冲体系中唯一遗留的早白垩纪洋盆,是研究西太平洋俯冲构造演化与动力学机制的一个关键区域.前期研究发现,关于花东海盆的基础科学问题迫切需要解决:花东海盆的性质、年龄及形成演化历史如何?南北走向的加瓜海脊是花东海盆的东部边界吗?其隆升机制和构造属性如何?花东海盆的西部...     

东至断裂带构造变形特征及其构造演化

东至断裂带是皖西南一条重要的北北东向断裂带。详细的构造解析表明,该断裂带主要经过3期构造变形,分别是发生在晚侏罗世末—早白垩世初的左行平移断层、早白垩世期间的伸展构造和晚白垩世—新生代的右行平移断层。通过断层擦痕矢量反演和断层叠加改造关系分析,认为东至断裂带及其两侧多期构造变形对应的区域应力场分别为近南北向挤压、北北西—南南东向挤压、北西—南东向伸展和近东西向挤压应力场。东至断裂带的形成和演化与郯庐断裂带相似,主要与华南与华北板块俯冲碰撞、伊泽奈崎板块和古太平洋板块向欧亚板块俯冲碰撞与弧后扩张、及印度板块向北碰撞后产生向东的构造挤出等多构造体制共同作用有关。     

南羌塘增生过程的中-晚三叠世岩浆记录:藏北玛依岗日-角木日地区基性岩墙

古特提斯洋向北俯冲形成增生杂岩,它们向南增生构成了南羌塘增生地体,且增生期内发育的岩浆岩可以用来研究南羌塘的增生过程。在南羌塘增生杂岩带内,由北向南,发育有玛依岗日(MG)辉长岩墙、角木日(JM)辉长辉绿岩墙。锆石U-Pb测年结果显示,两者分别形成于237. 1±2. 3Ma和230. 7±1. 8Ma,为南羌塘增生期内岩浆岩。它们的岩石地球化学特征均介于OIB与E-MORB之间,富集Ti、Nb,以及LREE和LILE。Nd-Pb同位素结果显示两者均起源于富集地幔源区。Sm/YbLa/Yb图解显示,两者均起源于尖晶石-石榴石二辉橄榄岩源区,相对于JM辉长辉绿岩MG辉长岩部分熔融程度较高。两者Mg#和Ni、Cr特征表明,MG辉长岩分异程度高于JM辉长辉绿岩。Th/Nb-La/Nb图解显示,相对于JM辉长辉绿岩MG辉长岩经历了一定程度的地壳混染。综合研究认为,MG辉长岩和JM辉长辉绿岩为古特提斯洋壳俯冲过程中软流圈上涌的两次岩浆活动的产物,并受俯冲洋壳和地幔楔影响。MG辉长岩和JM辉长辉绿岩记录了南羌塘的增生过程,它们为相关研究提供了岩浆岩证据。     

Palaeozoic suturing of eastern and western Tasmania in the west Tamar region: Implications for the tectonic evolution of southeast Australia

A new tectonic model for Tasmania incorporates subduction at the boundary between eastern and western Tasmania. This model integrates thin‐ and thick‐skinned tectonics, providing a mechanism for emplacement of allochthonous elements on to both eastern and western Tasmania as well as rapid burial, metamorphism and exhumation of high‐pressure metamorphic rocks. The west Tamar region in northern Tasmania lies at the boundary between eastern and western Tasmania. Here, rocks in the Port Sorell Formation were metamorphosed at high pressures (700–1400 MPa) and temperatures (400–500°C), indicating subduction to depths of up to 30 km. The eastern boundary of the Port Sorell Formation with mafic‐ultramafic rocks of the Andersons Creek Ultramafic Complex is hidden beneath allochthonous ?Mesoproterozoic turbidites of the Badger Head Group. At depth, this boundary coincides with the inferred boundary between eastern and western Tasmania, imaged in seismic data as a series of east‐dipping reflections. The Andersons Creek Ultramafic Complex was previously thought of as allochthonous, based mainly on associations with other mafic‐ultramafic complexes in western Tasmania. However, the base of the Andersons Creek Ultramafic Complex is not exposed and, given its position east of the boundary with western Tasmania, it is equally likely that it represents the exposed western edge of autochthonous eastern Tasmanian basement. A thin sliver of faulted and metamorphosed rock, including amphibolites, partially separates the Badger Head Group from the Andersons Creek Ultramafic Complex. Mafic rocks in this package match geochemically mafic rocks in the Port Sorell Formation. This match is consistent with two structural events in the Badger Head Group showing tectonic transport of the group from the west during Cambrian Delamerian orogenesis. Rather than being subducted, emplacement of the Badger Head Group onto the Andersons Creek Ultramafic Complex indicates accretion of the Badger Head Group onto eastern Tasmania. Subsequent folding and thrusting in the west Tamar region also accompanied Devonian Tabberabberan orogenesis. Reversal from northeast to southwest tectonic vergence saw imbricate thrusting of Proterozoic and Palaeozoic strata, possibly coinciding with reactivation of the suture separating eastern and western Tasmania.     

北山大地构造深部成因与浅部成矿特征遥感综合分析

大地构造受地球深部动力活动控制这个观点早已为人们所接受.本文通过对地处新疆、甘肃、内蒙三省(区)交汇部位的北山及附近地区大地构造的综合分析,认为北山地区在构造上形成于塔里木、中朝与蒙古三大板块构造汇聚挤压环境,但对其影响最为主要的是塔里木板块的向北移动造成的北山地区的古洋闭合.同时,进一步的深部构造运动使塔里木板块向北俯冲北山造山带,造成北山造山带的断裂构造具有多向北倾的特点.这种区域性的由南向北挤压俯冲构造对于浅部成矿具有重要影响.已知的许多矿床均分布于规模较大的中酸性岩基北侧,并多与北倾逆冲断裂带及小岩株(群)关系密切,由于与断裂构造具有多向北倾,使得与成矿作用有关的岩基总体上也具有向北倾的特征,并导致后来的成矿岩浆能够侵入就位在岩基北缘,这为人们在类似地区寻找与岩浆活动有关的矿床提供了一种重要找矿方法和线索.