宇宙成因核素测年方法及其在地球科学中的应用

宇宙成因核素地表暴露测年方法,是近年来迅速发展起来的一种新的同位素地质年代学方法.宇宙成因核素主要是由来源于银河系的宇宙射线与暴露于地表的物质作用形成的,作用机制主要包括裂变、中子捕获和介子反应.产生宇宙成因核素宇宙射线粒子主要是次生快中子、热中子和负慢介子,由于这些宇宙射线粒子在空间分布上的不同,地球上不同纬度、海拔高度和深度处的宇宙成因核素生成速率也表现出较大的差异.地表物质中宇宙成因核素浓度除了受到核素生成速率和地表物质的暴露时间制约外,还与地表侵蚀速率密切相关,此外,地磁场强度、遮蔽、化学风化及样品的几何位置等也会对核素浓度产生一定影响,在求算样品的地表暴露年代时,应对这些因素进行相应的校正.宇宙成因核素地表暴露测年技术的理论和方法日臻完善,目前它已被广泛到第四纪冰川、撞击坑、火山地貌、断层面等地学问题中来.     

西域砾岩宇宙成因核素26Al/10Be埋藏定年探索

西域砾岩是分布于青藏高原北缘天山、昆仑山一带的一套晚新生代砾石堆积地层,获知其可靠的年代信息对于理解高原隆升及地貌演化过程尤为关键.宇宙成因核素26Al/10Be埋藏测年是近年来发展起来的同位素测年新技术.本文对帕米尔高原西昆仑山山前的两个西域砾岩剖面采集的砾石样品进行了26Al/10Be埋藏测年分析.简单快速埋藏模式计算获得亚马勒剖面西域砾岩下部混合砾石样品的埋藏年龄＞3.5Ma,对该剖面西域砾岩上部5个样品加权平均得到的年龄为2.11+0.60/-0.46Ma;另一处奥依塔克西域砾岩生长地层底部砾石样品的埋藏年龄在0.46 ～ 7.12Ma.研究表明,研究区的西域砾岩堆积具有穿时性.本文是26Al/10Be埋藏测年方法应用于西域砾岩年代学研究的一次有益探索和尝试,并对在西域砾岩26Al/10Be埋藏测年中存在的问题进行了探讨.     

Ages of Detrital Zircon from Siliciclastic Successions South of the São Francisco Craton, Brazil: Implications for the Evolution of Proterozoic Basins

In this work we report ²⁰⁷Pb/²⁰⁶Pb LA-ICPMS ages of 152 detrital zircons from lower greenschist facies quartzites from Proterozoic basin successions of the southern border of the São Francisco Craton, southern Minas Gerais State, Brazil. These are the intracratonic São João del Rei basin, the intraplate continental margin Andrelândia basin, and the Serra do Ouro Grosso sequence, developed on a crystalline basement older than 1.8 Ga, and deformed and metamorphosed during the Brasiliano Orogeny, ca. 0.59–0.50 Ga. The data constrain both the ages of the sources and the interval of sedimentation. The detrital zircons of the Serra do Ouro Grosso sequence were derived predominantly from the erosion of a Neoarchean crust, 2.5–2.8 Ga old, with only one grain showing a Paleoproterozoic age (2, 245±83 Ma) older than the Transamazonian event. Zircons extracted from a shelf quartzite of the lowermost sequence of the São João del Rei basin indicate derivation from the 1.8–2.2 Ga Transamazonian crust, with subordinate contribution from the 2.5–2.9 Ga Archean crust. The 1, 809±41 Ma age is interpreted as the maximum limit for sedimentation in this basin. The results confirm the regional correlation with the Espinhaço Rift successions. The zircons extracted from an autochthonous quartzite of the Andrelândia sequence yielded ages in the 1.0–2.2 Ga range, with a modal class at 1.2–1.3 Ga. Only two of the forty analyzed zircons yield Archean ages. The youngest zircon yields 1, 086±85 Ma. The zircons from the allochthonous quartzite yield ages between 1.0–2.7 Ga, with a modal class at 2.1–2.2 Ga. Only five of 45 analyzed grains yield Archean ages. The youngest zircon has an age of 1, 047±77 Ma. The results indicate that the detrital sediments deposited during the second marine flooding event of the Andrelândia sedimentation were mainly derived from the erosion of Mesoproterozoic and Paleoproterozic rocks. The 1, 047±77 Ma age is interpreted as the maximum depositional age for the described association.     

变质锆石成因的岩相学研究——高级变质岩U-Pb年龄解释的基本依据

岩相学在研究高级变质岩 (含深熔片麻岩 )的锆石中起着重要的作用 ,其内容主要包括锆石的形貌和表面特征 ,锆石在岩石中的赋存状态 ,与主晶的关系和内部结构等方面。基于大别山地区的一些榴辉岩相岩石、麻粒岩相岩石和片麻状花岗质岩石中锆石的研究 ,阐述了继承锆石、变质锆石、深熔锆石、变质增生、深熔增生和后期流体改造的基本岩相学特征。结合大别山地区的 4个实例 ,说明了高级变质岩中锆石的岩相学研究方法和步骤 ,并初步提出了高级变质岩锆石U -Pb年龄解释的岩相学标志。     

鄂东矿集区宝团山金铜矿床地质特征、赋矿岩体锆石U-Pb年龄和Hf同位素组成及其地质意义

宝团山矿床位于鄂州市沙窝乡宝团村,是鄂东矿集区北缘鄂城岩体内部新发现的金铜矿床。矿体主要赋存于石英二长岩中,受断裂构造控制。对典型矿石样品开展了矿物学特征的研究,同时对与成矿关系密切的石英二长岩进行了锆石U-Pb和Hf同位素分析。结果显示,矿石中金以赋存于黄铜矿中的可见金和黄铁矿中的不可见金为主,原生含铜矿物主要为黄铜矿。石英二长岩LA-ICP-MS锆石U-Pb年龄为136±1Ma,ε_(Hf)(t)值介于-21.9~-7.2之间,暗示石英二长岩岩浆源区为壳幔物质混合的产物。通过对比研究认为,宝团山矿床可能代表了鄂东矿集区内存在一期受构造控制的金铜矿化事件。该矿床位于程潮矽卡岩型铁矿的外围,与长江中下游成矿带已报道的铁矿床外围存在金铜矿床/矿化现象一致。因此在区域金矿勘查中,建议注意寻找岩体中受断裂控制的热液型金矿床,同时铁矿床及其外围金矿床之间的联系值得深入思考。     

延边小西南岔大型富金铜矿床矿区内暗色脉岩的锆石年代学及其地质意义

本文对小西南岔富金铜矿床矿区内与成矿空间密切的成矿前辉绿辉长岩、成矿期闪长玢岩岩脉进行了单颗粒锆石SHRIMP、LAICPMS UPb年龄的测定。实验数据表明：辉绿辉长岩的16个锆石或单点有387.0±11.8Ma、292.0~251.1Ma (加权平均值270±14Ma, n=10)、129.8±2.6 Ma、107.0～95.4Ma（加权平均年龄为103±13Ma,n=3）和46.8±2.6Ma五组谐和年龄; 闪长玢岩的11个锆石、12个单点仅获一组谐和年龄,变化在108～98Ma之间（加权平均值102.1±2.2Ma）。结合CL图像特征,初步厘定成矿前辉绿辉长岩形成于早二叠世晚期（270±14Ma,n=10;273±14Ma,n=6）,岩浆上侵过程中捕获了少量的泥盆世碎屑锆石,在成岩后的早白垩世、新生代始新世中期曾被热液改造;而闪长玢岩则形成于早白垩世晚期。考虑到伴生闪长玢岩的年龄与辉绿辉长岩的107.0～95.4Ma相匹配,且辉绿辉长岩的热液锆石年龄在103~95.4Ma之间,进而限定该富金铜矿床的成矿热事件应发生在108～98Ma,成矿作用发生在晚中生代地壳强烈伸展、减薄作用末期,经晚白垩世—古近世的后期抬升被剥露于现今地表,其成矿滞后于中国东北部晚中生代岩浆热液金矿床大规模成矿峰期（120Ma）约10Ma。     

东秦岭钼矿Re-Os同位素年龄及其成矿动力学背景

东秦岭钼矿带位于华北克拉通南缘 ,集中分布于陕西省的金堆城地区、河南省栾川县南泥湖 -三道庄 -上房沟、嵩县雷门沟地区。钼矿床类型主要为斑岩型、斑岩 -矽卡岩型 ,少量热液碳酸盐岩脉型。采用电感耦合等离子体质谱仪法对南泥湖钼矿田、雷门沟钼矿床等 4个矿床 8件辉钼矿进行了 Re-Os同位素年龄测定 ,获得南泥湖矿床的辉钼矿 Re- Os模式年龄为 14 1.8± 2 .1Ma;三道庄矿床的辉钼矿 Re- Os模式年龄为 14 4 .5± 2 .2～ 14 5 .0± 2 .2 Ma,平均为 14 5 .0± 2 .2 Ma;上房沟矿床的辉钼矿 Re- Os模式年龄为 14 3.8± 2 .1～ 14 5 .8± 2 .1Ma,平均为 14 4 .8± 2 .1Ma;6件样品的等时线年龄为 14 1.5±7.8Ma(2σ) ;雷门沟钼矿床的 2件辉钼矿样品的 Re- Os模式年龄为 131.6± 2 .0～ 133.1± 1.9Ma,平均为 132 .4± 2 .0 Ma。结合前人资料 ,认为东秦岭钼矿的形成时代局限于 2 2 1.5± 0 .3～ 132 .4± 2 .0 Ma之间 ,主要出现在 2 2 1.5± 0 .3Ma左右和 14 4 .8± 2 .1～ 132 .4± 2 .0 Ma时限之间 ,其对应的地球动力学背景分别为华北克拉通与扬子克拉通的碰撞造山后陆内造山和伸展过程、中国东部构造体制大转换时期     

Survival of eclogite xenolith in a Cretaceous granite intruding the Central Dabieshan migmatite gneiss dome (Eastern China) and its tectonic implications

Investigation of an eclogite xenolith, discovered in a Cretaceous granite from the Central Domain of the Dabieshan massif in eastern China, yields new petrological insights into the high to ultrahigh-pressure metamorphism, experienced by the Qinling-Dabie orogen. Prior to inclusion as a xenolith in the granite during the Early Cretaceous, this eclogite xenolith had recorded a complex metamorphic evolution that complies with subduction and exhumation processes experienced by the continental crust of the South China Block. Well-preserved mineral parageneses substantiate the prograde and retrograde stages revealed by inclusions in porphyroblastic garnet and zoned minerals such as garnet, omphacite and amphibole in the matrix. The relatively low P/T re-equilibration during a late metamorphic stage was textually inferred by the presence of aluminous and calcic-subcalcic amphiboles such as katophorite, edenite, taramite and pargasite as main matrix phases. According to our U/Pb, Rb/Sr and new ⁴⁰Ar/³⁹Ar geochronological results, namely109 ± 1 and 112 ± 2 Ma plateau ages for muscovite and amphiboles, respectively, two successive but distinct cooling stages account for the thermal history of the granite–migmatite gneiss dome that forms the Central Dabieshan Domain. We argue that prior to the Cretaceous doming, the Central Dabieshan Domain experienced a tectono-metamorphic evolution similar to that observed in the high-pressure to ultra high-pressure units developed in the Southern Dabieshan Domain and Hong’an massif.     

湖南泥盆纪露头层序地层研究

湖南及相邻地区泥盆纪地层发育，其地层划分对比清楚，沉积类型丰富，地层裸露良好，是进行露头层序地层研究的理想场所。本文运用层序地层学的理论和方法，在详细的沉积学研究和精细的地层划分对比基础上，对湖南泥盆纪进行了精细的露头层序地层研究，详细阐述了层序建立和各种界面的露头标志，探讨了湖南泥盆纪的层序划分、海平面变化及全球对比，从而解释泥盆里各沉积层序的形成控制因素。     

Zircon Crystal Morphology, Trace Element Signatures and Hf Isotope Composition as a Tool for Petrogenetic Modelling: Examples From Eastern Australian Granitoids

In situ laser ablation inductively coupled plasma mass spectrometryanalysis of trace elements, U–Pb ages and Hf isotopiccompositions of magmatic zircon from I- and S-type granitoidsfrom the Lachlan Fold Belt (Berridale adamellite and Kosciuskotonalite) and New England Fold Belt (Dundee rhyodacite ignimbrite),Eastern Australia, is combined with detailed studies of crystalmorphology to model petrogenetic processes. The presented examplesdemonstrate that changes in zircon morphology, within singlegrains and between populations, generally correlate with changesin trace element and Hf-isotope signatures, reflecting the mixingof magmas and changes in the composition of the magma throughmingling processes and progressive crystallization. The zircondata show that the I-type Kosciusko tonalite was derived froma single source of crustal origin, whereas the S-type Berridaleadamellite had two distinct sources including a significantI-type magma contribution. Complex morphology and Hf isotopevariations in zircon grains indicate a moderate contributionfrom a crustal component in the genesis of the I-type Dundeerhyodacite. The integration of data on morphology, trace elementsand Hf isotope variations in zircon populations provides a toolfor the detailed analysis of the evolution of individual igneousrocks; it offers new insights into the contributions of differentsource rocks and the importance of magma mixing in granite petrogenesis.Such information is rarely obtainable from the analysis of bulkrocks. KEY WORDS: granite source origins; zircon Hf isotopes; zircon petrogenesis; zircon morphology; zircon U–Pb ages