中缅毗邻区金腊Pb-Zn-Ag多金属矿田花岗岩锆石U-Pb定年与地球化学特征

位于中缅毗邻区的金腊铅锌银多金属矿田大地构造上处于保山—掸泰地块东缘,勐统—耿马—西盟元古宙—古生代被动大陆边缘活动带南段。与矿化有关的花岗岩(简称金腊花岗岩)包括老厂似斑状角闪二长花岗岩、勐林山似斑状黑云二长花岗岩和南腊碱长花岗斑岩。文中系统研究了上述岩石的主量元素、稀土元素、微量元素、成矿元素和锆石U-Pb同位素年龄等特征,从构造岩浆演化的角度,探讨上述岩体之间内在联系、成因演化以及与成矿的关系:(1)在金腊花岗岩三种岩石类型中,老厂似斑状角闪二长花岗岩和勐林山似斑状黑云二长花岗岩的锆石同位素U-Pb年龄皆为(45±1)Ma,形成于岩浆结晶分异早期阶段的深成环境,而南腊碱长花岗斑岩的锆石同位素U-Pb年龄为(43.41±0.78)Ma,形成于岩浆结晶分异晚期阶段的浅成环境。(2)主量元素和微量元素(稀土元素和某些微量元素(Zr/Hf、Nb/Ta、Rb/Sr、Rb/Ba、K/Rb、(Rb/Yb)N、Sr*、K*和Zr*)),结合U-Pb同位素定年研究表明,本区花岗岩形成于喜马拉雅同碰撞造山成矿作用末期局部拉张构造环境,并分别代表了构造岩浆演化过程中不同演化阶段岩浆分异结晶的产物。(3)上述三类花岗岩样品皆位于S型花岗岩区,但从老厂似斑状角闪二长花岗岩,勐林山似斑状黑云二长花岗岩,到南腊碱长花岗斑岩,样品分布逐渐远离"I"型花岗岩和"S"型花岗岩的分界线,这表明自老厂似斑状角闪二长花岗岩至勐林山似斑状黑云二长花岗岩,到南腊碱长花岗斑岩幔源组分逐渐减少。(4)相对中国花岗岩,南腊碱长花岗斑岩不仅更富集W、Cu、Bi、Sb、Mo、Sn、Ag、Pb和Au等成矿元素,而且还强烈富集F、B和As等矿化剂元素,因此,碱长花岗斑岩是最有成矿远景的岩体。     

折多山花岗岩时代、成因及其动力学意义

沿鲜水河断裂分布的折多山花岗岩由早期的中细粒花岗闪长岩、二长花岗岩和略晚的主体似斑状二长花岗岩、少量的伟晶岩和细晶岩构成。早期中细粒二长花岗岩的SHRIMP锆石U-Pb同位素定年表明其侵位结晶于18±0．3Ma,使折多山花岗岩岩浆侵位结晶年龄和鲜水河断裂开始活动的时间提前近6Ma。岩石中保存有818±47Ma和156±8Ma的继承锆石．表明存在扬子西缘元古宙和中生代陆壳物质的再循环。在K_2O对SiO_2分类图上,早期的中细粒花岗闪长岩和二长花岗岩落在钙碱性系列区域,似斑状二长花岗岩落在橄榄玄粗岩系列区域,两个系列岩石总体上为铝饱和到过饱和。中细粒花岗闪长岩具有中等的稀土总量(162×10~(-6)～224×10~(-6))、高的(La/Yb)_N(74～118)和明显的正Eu异常,大离子亲石元素富集,Nb、Ta、P和Ti亏损,形成于岛弧钙碱性火山岩低度部分熔融。中细粒二长花岗岩与花岗闪长岩的地球化学特征相似,只是稀土总量和(La/Yb)n较低,形成于杂砂岩部分熔融。主体岩性粗粒似斑状二长花岗岩具有很宽的稀土总量(最高达533×10~(-6)),(La/Yb)_N随着稀土总量增加而增加(最高达523),明显负Eu异常,大粒子亲石元素强烈富集,Nb、Ta、Sr、P和Ti亏损,(~(87)Sr/~(86)Sr)_0=0．7084～0．7133,ε_(Nd)(t)=-5．67～-8．69,形成于元古代上部陆壳物质—砂页岩的较低程度的部分熔融。各类岩石的地球化学特征表明,部分熔融源区的物质组成继承了元古代大陆边缘的某些地球化学特性,经历了高压变质作用,部分熔融残留相主要为石榴石等。部分熔融过程发生于鲜水河断裂早期活动的剪切熔融过程,岩浆作用之前可能发生过强烈挤压而产生了高压变质作用。     

西准噶尔克拉玛依花岗岩体地球化学特征及其意义

西准噶尔克拉玛依岩体为二长花岗岩和斜长花岗岩等,为海西中期侵入岩体,具有高钠、铝等特点,A/CNK=0.71~0.78,NK/A为0.8~0.89,为偏铝质钙碱性花岗岩.轻稀土元素中等富集,Eu弱亏损,富集大离子亲石元素,Nb、Ta亏损,富Sr,La/Yb值相对低.多种主微量元素和同位素的综合图解分析表明,该岩体为I型花岗岩,与达拉布特断裂西侧同时期的庙尔沟A型花岗岩具有明显的区别,盆地具有较为年轻的地壳基底.克拉玛依岩体为岛弧花岗岩,晚石炭世准噶尔地区仍存在着洋盆体制.     

阿尔泰额尔齐斯带东段酸性岩墙群地球化学特征及其地质意义

野外地质调查发现在阿尔泰南缘额尔齐斯构造带东段、额尔齐斯活动断裂与富蕴-锡泊渡断裂之间发育了一套未变形的酸性岩墙群。岩墙群侵位于海西期片麻岩化花岗岩和上石炭统深变质的额尔齐斯组岩层中,薄片鉴定和岩石地球化学分析确定为具有细晶结构的流纹斑岩。岩石的SiO_2含量为70.9%～75.38%,K_2O Na_2O含量为7.55%～8.99%;大部分样品Na_2O>K_2O,里特曼指数值为1.8～2.5之间,Al_2O_3=12.80%～14.53%,A/CNK=0.9～1.1,NK/A=0.7～0.9,具有准铝质—弱过铝质、低镁、高钾钠、低钙和锶、高(Fe)_(mol)/(Mg)_(mol)特点,具A型花岗岩类特点,属于亚碱性脉岩。岩石轻稀土富集,大离子亲石元素相对富集,具有明显的中等负铕异常,壳源特征明显。锆石U-Pb二次粒子微探针测年显示岩墙群形成于277～286Ma。推断岩墙是阿尔泰海西期造山运动结束后,在拉张构造环境下的地壳局部熔融产物。     

东南极普里兹湾地区花岗岩类的锆石U-Pb年龄、地球化学特征及其构造意义

东南极普里兹湾地区出露大量泛非期花岗质岩类,利用LA-ICP-MS锆石U-Pb原位定年方法测得达尔克花岗岩、蒙罗克尔花岗岩和阿曼达花岗岩的年龄分别为519±2Ma,497±2Ma和498±7Ma。所有普里兹湾地区花岗岩类均具有较高的全碱含量以及K_2O/Na_2O、FeO~T/(FeO~T MgO)和10~4Ga/Al比值,较低的Mg、Ca、Cr和Ni丰度,表现出A型花岗岩的特点,同时还富集大离子亲石元素和稀土元素,不同程度亏损Sr、Nb、Ta、P和Ti,并具有高~(87)Sr/~(86)Sr初始比值和低ε_(Nd)(t)值的同位素特征。研究结果表明,普里兹湾地区的岩浆活动可能与后碰撞的造山作用有关,包括岩石圈减薄、岩浆底侵和地壳伸展作用等。普里兹湾A型后碰撞花岗岩的确定支持普里兹带为碰撞造山带的构造属性。     

晋南中条山古元古代花岗岩的锆石U—Pb年龄

用单颗粒锆石U-Pb稀释法测定了中条山古元古代花岗岩的时代,获得寨子英云闪长质片麻岩、横岭关花岗闪长质片麻岩和烟庄钾长-二长花岗质片麻岩的成岩年龄分别为2321 Ma±2Ma、2256Ma±35Ma和2297Ma±21 Ma.结合区域地质背景和地球化学资料,认为中条山地区可能存在时限为2350～2250Ma的构造-热事件.     

冈底斯带门巴花岗岩同位素测年及其对新特提斯洋俯冲时代的约束

本文应用单颗粒锆石SHRIMP U—Pb法和角闪石^40Ar／^39Ar法，对出露在冈底斯构造带上的门巴花岗岩体进行了同位素测年，时代分别为207Ma和215Ma，表明它们形成于晚印支期。地质与地球化学研究显示该时期形成的岩体主要为黑云角闪花岗闪长岩和黑云母二长花岗岩，它们形成于岛弧环境，与其南侧墨竹工卡一带出露的晚三叠世钙碱性火山岩（叶巴组）共同构成与板块俯冲有关的特征性岩石组合，揭示新特提斯洋的俯冲时代应早于晚三叠世。     

Phase Equilibria of the Lyngdal Granodiorite (Norway): Implications for the Origin of Metaluminous Ferroan Granitoids

The Proterozoic (950 Ma) Lyngdal granodiorite of southern Norwaybelongs to a series of hornblende–biotite metaluminousferroan granitoids (HBG suite) coeval with the post-collisionalRogaland Anorthosite–Mangerite–Charnockite (AMC)suite. This granitoid massif shares many geochemical characteristicswith rapakivi granitoids, yet granodiorites dominate over granites.To constrain both crystallization (P, T, fO₂, H₂O in melt) andmagma generation conditions, we performed crystallization experimentson two samples of the Lyngdal granodiorite (with 60 and 65 wt% SiO₂) at 4–2 kbar, mainly at fO₂ of NNO (nickel–nickeloxide) to NNO + 1, and under fluid-saturated conditions withvarious H₂O–CO₂ ratios for each temperature. Comparisonbetween experimental phase equilibria and the mineral assemblagein the Lyngdal granodiorite indicates that it crystallized between4 and 2 kbar, from a magma with 5–6 wt % H₂O at an fO₂of NNO to NNO + 1. These oxidized and wet conditions sharplycontrast with the dry and reduced conditions inferred for thepetrogenesis of the AMC suite and many other rapakivi granitesworldwide. The high liquidus temperature and H₂O content ofthe Lyngdal granodiorite imply that it is not a primary magmaproduced by the partial melting of the crust but is derivedby the fractionation of a mafic magma. Lyngdal-type magmas appearto have volcanic equivalents in the geological record. In particular,our results show that oxidized high-silica rhyolites, such asthe Bishop Tuff, could be derived via fractionation of oxidizedintermediate magmas and do not necessarily represent primarycrustal melts. This study underlines the great variability ofcrystallization conditions (from anhydrous to hydrous and reducedto oxidized) and petrogenetic processes among the metaluminousferroan magmas of intermediate compositions (granodiorites,quartz mangerites, quartz latites), suggesting that there isnot a single model to explain these rocks. KEY WORDS: ferroan granitoids; crystallization conditions; experiments; Norway; Sveconorwegian; Bishop Tuff     

Early Paleozoic gabbro-granitoid associations in eastern segment of the Mongolian-Okhotsk foldbelt (Amur River basin): Age and tectonic position

Magmatic rocks of the Pikan and Un’ya massifs situated in eastern segment of the Mongolian-Okhotsk foldbelt are studied using isotopic-geochronological (U-Pb zircon dating) and geochemical methods. Two rock complexes different in age are recognized in the Pikan massif: the high-Al gabbro-tonalite association of the Middle Ordovician (468 ± Ma) and granodiorite-granite association of the Late Silurian-Early Devonian (415 ± 7 Ma). The Late Ordovician age (454 ± 5 Ma) is established for leucocratic granites of the Un’ya massif. As is suggested, the Pikan and Un’ya massifs are “allogenic blocks” detached from continental framework of the Mongolian-Okhotsk foldbelt and tectonically emplaced into the foldbelt structure at the last stage of its development.