SHRIMP zircon U–Pb geochronological and whole-rock geochemical evidence for an early Neoproterozoic Sibaoan magmatic arc along the southeastern margin of the Yangtze Block

It has been generally accepted that the South China Block was formed through amalgamation of the Yangtze and Cathaysia Blocks during the Proterozoic Sibaoan orogenesis, but the timing and kinematics of the Sibao orogeny are still not well constrained. We report here SHRIMP U–Pb zircon geochronological and geochemical data for the Taohong and Xiqiu tonalite–granodiorite stocks from northeastern Zhejiang, southeastern margin of the Yangtze Block. Our data demonstrate that these rocks, dated at 913 ± 15 Ma and 905 ± 14 Ma, are typical amphibole-rich calc-alkaline granitoids formed in an active continental margin. Combined with previously reported isotopic dates for the  1.0 Ga ophiolites and  0.97 Ga adakitic rocks from northeastern Jiangxi, the timing of the Sibao orogenesis is thus believed to be between  1.0 and  0.9 Ga in its eastern segment. It is noted that the Sibao orogeny in South China is in general contemporaneous with some other early Neoproterozoic (1.0–0.9 Ga) orogenic belts such as the Eastern Ghats Belt of India and the Rayner Province in East Antarctica, indicating that the assembly of Rodinia was not finally completed until  0.9 Ga.     

新元古代陆缘岩浆弧——塔里木盆地巴楚隆起的基底:来自钻井岩芯的最新证据

塔里木盆地巴楚隆起西缘的同1井(TX1)在寒武系之下钻遇一套火山岩,本文对该套火山岩的岩石类型、形成时代、岩石成因及构造环境进行了详细研究。结果显示,这套火山岩可分为上下两段,分别为杏仁状辉石安山岩和角闪石英安岩,属于钙碱性系列火山岩,形成于大陆边缘弧构造环境。安山岩锆石U-Pb年龄(747±12Ma)表明该岩浆弧发育时代为新元古代。同1井钻揭的火山岩与卡塔克隆起上塔参1井(TC1)钻揭的闪长岩(744.0±9.3Ma~790.0±22.1Ma)和花岗闪长岩(757.4±6.2Ma)成岩时代一致、形成环境相同,表明横亘塔里木盆地中部的古隆起带(巴楚隆起-卡塔克隆起-古城虚隆起)起源于一条新元古代陆缘岩浆弧,塔里木盆地的基底是由新元古代造山作用拼合的基底。以中央隆起带为界,南、北塔里木的基底性质和成盆演化过程可能有所差异。     

东昆仑东段古特提斯洋俯冲作用——乌妥花岗岩体锆石U-Pb年代学和地球化学证据

东昆仑东段发育了一条晚二叠世-中三叠世以花岗质岩石为主的巨型弧岩浆岩。本文对东昆仑东段的中三叠世乌妥花岗岩体开展了岩石学、LA-ICP-MS锆石U-Pb年代学及地球化学研究。结果表明,乌妥花岗岩体主要包括花岗闪长岩、斑状二长花岗岩和正长花岗岩三种岩石类型,对应的锆石U-Pb年龄分别为248Ma、247Ma和245Ma,形成时代为中三叠世。主量元素特征显示其具有高钾钙碱性、准铝质-弱过铝质的I型花岗岩的属性。球粒陨石标准化稀土元素配分图解具轻稀土明显富集、重稀土亏损和Eu弱负异常-强负异常特征。原始地幔标准化微量元素蛛网图显示富集大离子亲石元素(Cs、Rb、Th、Ba、U等)、亏损高场强元素(Nb、Ti、Ta等)、而Zr和Hf无明显异常的特点。乌妥岩体总体具有负的Hf同位素组成特征,花岗闪长岩εHf(t)为-1. 5～+3. 4、斑状二长花岗岩εHf(t)为-4. 1～-0. 6、正长花岗岩εHf(t)为-6. 9～-0. 9。多种微量元素构造环境判别图解表明其形成于类似安第斯型陆缘弧的构造环境。岩石成因研究表明洋壳俯冲带上部壳幔接触过渡部位多次岩浆MASH过程及分离结晶作用是形成乌妥花岗岩体的重要方式。综合前人东昆仑地区有关三叠纪沉积地层及岩浆岩资料,认为布青山-阿尼玛卿古特提斯洋于晚二叠世向北俯冲于东昆仑地块之下,弧岩浆岩记录的布青山古特提斯洋盆向北俯冲过程一直持续到中三叠世晚期。晚三叠世,东昆仑南缘古特提斯洋盆关闭,东昆仑造山带转换为碰撞及后碰撞造山阶段,并形成了区域上具碰撞构造属性的侵入岩。     

Gravity field,surface topography,and volcanic complexes of Kamchatka and its junction with the Aleutian arc

The paper deals with interpretation of global digital maps of gravity anomalies and surface topography for the northwestern Pacific and Kamchatka regions. A transformation procedure is suggested to reveal subtle features of surface topography against high elevation contrasts. Gravity data (free-air and Bouguer anomalies) have important implications for the evolution of the circum-Pacific region and the problems of volcanism and geodynamics in subduction zones. The patterns of gravity anomalies and transformed topography interpreted jointly with onshore and offshore geological data can make a basis for tectonic paleoreconstructions of upper crust and lithospheric mantle structures.     

扬子—华夏构造结合带的演化阶段及对斑岩铜矿形成的控制机制

扬子与华夏古板块均存在太古宙—中元古代古老基底,在构造演化历程中响应全球超大陆聚合-裂解过程,并最终碰撞拼贴形成钦-杭结合带,且经历构造运动之后,其总体演化格架为"两开三合一转折"。结合带内存在元古宙洋壳,以蛇绿岩套与镁铁质-超镁铁质岩石为特征,并以此推断结合带形成于新元古代。钦-杭结合带是斑岩成矿带,斑岩铜矿时空分布具有三段划分特征,且处于170~150 Ma和100 Ma左右两个成矿期。与成矿有关的斑岩主要为钙碱性岩浆系列的中酸性岩,岩石类型主要为花岗闪长斑岩、花岗斑岩和二长花岗斑岩等。钦-杭结合带燕山期的斑岩具岛弧岩浆作用特点,指示了华南地区中生代构造转折事件发生以前的地质演化及太平洋板块俯冲作用,即古俯冲带物质在中生代活化成矿。中侏罗世岩石圈减薄及太平洋板块俯冲的扰动导致上涌软流圈地幔带来大量的热量,这些热量加热上覆元古宙岛弧底部玄武质岩石导致它们发生重熔形成了含矿岩浆,本质上该类矿床带有岛弧俯冲环境的基因。而100 Ma左右斑岩铜矿的形成,则与太平洋俯冲换向所扰动有关。     

西准噶尔马拉苏早泥盆世火山岩LA-ICP-MS 锆石U-Pb 年龄、地球化学特征及其大地构造意义

马拉苏地区早泥盆世地层为一套滨海—浅海相火山—沉积岩系,对其中的火山岩夹层进行锆石U-Pb同位素定年和岩石地球化学研究,定年结果显示有大量的新太古代和中新元古代锆石,表明该区存在古老的大陆地壳物质。火山岩样品SiO2含量为52.38%~69.6%,Na2O含量为2.80%~4.85%,K2O为0.16%~0.96%,TiO2为0.5%~1.96%,Al2O3为14.62%~18.18%,MgO（1.08%~5.75%）变化范围较大,Mg#值在22.92~38之间,具有高钠、低钾的特征,属于钙碱性、低钾拉斑系列。稀土元素总量∑REE=73×10-6~115×10-6,LREE/HREE值为2.66~3.25,具有明显的Eu负异常（0.83~0.92）。玄武安山岩样品相对富集K、Rb、Ba、Sr等大离子亲石元素,亏损Nb、Ta、Zr、Hf等高场强元素,与典型火山弧玄武岩地球化学特征一致。英安斑岩样品也具有富集大离子亲石元素、亏损Nb、Ta、Ti等高场强元素的特征,反映源区可能有较多壳源物质的加入,其稀土元素配分曲线和微量元素蛛网图与玄武安山岩相似,表明其可能为同源岩浆演化的产物。综合研究认为,这套火山岩具有岛弧火山岩特征,形成于板块俯冲拼贴过程中的岛弧环境。     

古岛弧地体的俯冲是南羌塘增生杂岩形成的重要机制:来自日湾茶卡洋岛的证据

洋内岛弧及微陆块的俯冲增生是形成增生杂岩的重要机制。本文通过对南羌塘地区日湾茶卡组进行野外实测地质剖面,开展沉积特征、古生物化石、碎屑组分模式、碎屑锆石测年等研究,发现:(1)日湾茶卡为近源沉积,最年轻碎屑锆石年龄峰值为325～375 Ma,但在龙木错-双湖古特提斯大洋周边陆块均未发现源区,其真正物源应为其下伏的望果山组火山岩;(2)日湾茶卡组内珊瑚化石丰度虽然高,但分异度非常低,其沉积位置应是一个相对突出的孤立位置。根据日湾茶卡组下伏望果山组火山岩所具有的洋内岛弧地球化学特征,并与同期SSZ型蛇绿岩组成的类似洋内俯冲的大地构造体系对比,本文认为日湾茶卡组与其下伏的望果山组火山岩共同组成了泥盆纪—石炭纪由洋内俯冲形成的古岛弧地体。根据碎屑锆石分布型式的相似性,本文进一步认为猫儿山地区部分南羌塘增生杂岩的源岩为日湾茶卡组。因此,日湾茶卡洋岛应曾经历过俯冲增生作用:浅部发生前端"刮削作用"形成冈玛错地区有变形但无变质的日湾茶卡组及望果山组,俯冲到深部的日湾茶卡组则发生高压变质作用并在后期折返至增生杂岩的浅部层次。因此,本文认为在南羌塘增生杂岩的形成过程中,日湾茶卡古岛弧地体的俯冲与增生也起到了重要的作用。     

Adakite-like granitoids of Songkultau: A relic of juvenile Cambrian arc in Kyrgyz Tien Shan

The early Paleozoic Terskey Suture zone,located in the southern part of the Northern Tien Shan domain in Kyrgyzstan,comprises tectonic slivers of dismembered ophiolites and associated primitive volcanics and deepmarine sediments.In the Lake Songkul area,early-middle Cambrian pillow basalts are crosscut by the Songkultau intrusion of coarse-grained gneissose quartz diorites and tonalites with geochemical characteristics typical for high-SiO2 adakites(SiO2>56 wt.%,Al2O3>15 wt.%,Na2 O>3.5 wt.%and high Sr/Y and La/Yb ratios).The Songkultau granitoids have positive initialεNd(+3.8 to+6.4)andεHf(+12.3 to+13.5)values indicating derivation from sources with MORB-like isotopic signature.Volcanic formations,surrounding the Songkultau intrusion,have geochemical affinities varying from ocean floor to island arc series.This rock assemblage is interpreted as a relic of an early-middle Cambrian primitive arc where the adakite-like granitoids were derived from partial melting of young and hot subducted oceanic crust.An age of 505 Ma,obtained for the Songkultau intrusion,shows that hot subduction under the Northern Tien Shan continued until middle Cambrian.The primitive arc complexes were obducted onto the Northern Tien Shan domain,where the Andean type continental magmatic arc developed in Cambrian and Ordovician.Formation of the Andean type arc was accompanied by uplift,erosion and deposition of coarse clastic sediments.A depositional age of ca.470 Ma,obtained for the gravellites in the Lake Songkul area,is in agreement with the timing of deposition for lower Ordovician conglomerates elsewhere in the Northern Tien Shan,and corresponds to the main phase of the Andean type magmatism.The Songkultau adakites in association with surrounding ocean floor and island arc formations constitute a relic of a primitive Cambrian arc and represent a juvenile domain of substantial size identified so far within the predominantly crustal-derived terranes of Tien Shan.On a regional scale this primitive arc can be compared with juvenile Cambrian arcs of Kazakhstan,Gorny Altai and Mongolia.     

大兴安岭南段朝克乌拉地区晚侏罗—早白垩世辉长岩年代学和地球化学研究

基于内蒙古朝克乌拉地区开展的1∶5万区域地质调查工作,发现一期以辉长岩为主的基性岩浆活动。研究表明：朝克乌拉地区辉长岩锆石U- Pb年龄为142.6±0.5 Ma、145.1±2.1 Ma,显示岩石为晚侏罗—早白垩世产物。辉长岩为拉斑系列岩石,具有低硅、低钛、贫碱且高钠低钾特点,分异作用极弱,Eu异常不明显,微量元素Ba、Ta、Sr、Hf富集, Rb、Th、Nb、P亏损。辉长岩岩石地球化学显示其岩浆源区为亏损尖晶石二辉橄榄岩地幔发生30%以上的部分熔融形成,存在流体影响的地球化学特征。岩浆在演化过程中存在一定程度的分离结晶作用,地壳混染作用极其微弱。通过与贺根山蛇绿岩配套岩系的类比,结合研究区古生代至中生代构造背景和贺根山缝合带壳幔电性结构全面分析,推断晚侏罗—早白垩世辉长岩具有的岛弧印记特征为贺根山洋发育时期古岛弧环境的反映,辉长岩只是伸展背景下幔源岩浆沿深大断裂上涌作用的产物。     

冈底斯岩浆弧东段沉积岩的晚白垩世变质作用及其构造意义

位于青藏高原南部的冈底斯岩浆弧形成于中生代新特提斯洋俯冲过程中,并在印度与欧亚大陆碰撞过程中叠加了新生代岩浆作用和变质作用。冈底斯岩浆弧东段出露的中、新生代变质岩是研究其深部组成与形成演化的理想窗口。本文对冈底斯东段米林田兴村地区的变沉积岩,即石榴夕线黑云片岩、含石榴钙硅酸盐岩、黑云斜长片麻岩和大理岩进行了岩石学和锆石U-Pb年代学研究。研究结果表明,石榴夕线黑云片岩由夕线石、黑云母、石榴石、斜长石、钾长石、石英和钛铁矿组成,经历了中压麻粒岩相变质作用,变质条件为810~820℃和6.4~7.8kbar。锆石年代学研究表明,石榴夕线黑云片岩、含石榴钙硅酸盐岩和大理岩经历了87~83Ma的变质作用。本文和现有研究表明,冈底斯弧东段林芝和米林地区的高压麻粒岩相变质岩分布区代表该岩浆弧的下地壳,而包括本文研究点在内的中压麻粒岩相到角闪岩相变质岩分布区为其中地壳组成部分。本文认为晚白垩世大体积幔源岩浆的注入和随后的新特提斯大洋岩石圈平俯冲,导致了冈底斯岩浆弧地壳的生长、加厚和底冲,使上地壳的沉积岩和岩浆岩运移到中-下地壳,并经历了高温变质与部分熔融,形成了分布在上地壳的花岗岩。这表明岩浆弧的新生地壳在晚白垩世新特提斯洋俯冲过程中发生了再造。以长英质岩石为主的表壳岩进入深地壳很可能是岩浆弧中-下地壳由基性转变成中性成分的重要方式。