华南双桥山群和河上镇群凝灰岩中的锆石SHRIMP U-Pb年龄—对江南新元古代造山带演化的制约

通过西澳科庭大学离子探针中心的远程测试,在双桥山群横涌组和安乐林组斑脱岩中获得大量锆石,其SHRIMP U-Pb加权平均年龄为831 Ma±5 Ma(横涌组)、829 Ma±5 Ma（安乐林组）,在河上镇群上墅组中获得加权平均年龄767 Ma±5 Ma。锆石SHRIMP U-Pb年龄表明华南地区广为发育的双桥山群应归入新元古界,该年龄为标定双桥山群在地层柱中的位置提供了准确的年代学依据。     

陆丰南地区断拗转换界面厘定及其地质意义

珠江口盆地经历裂陷阶段向拗陷阶段的转变，同期沉积地层记录了该构造变革和沉积演化过程，为厘定断拗转换界面提供了可能。基于地震反射、沉积旋回特征，识别出陆丰南地区恩平组内部存在全区可追踪对比的构造沉积转换面——T72界面，也是区分上下恩平组界面。对陆丰13洼洼间隆钻井LFA开展上下恩平组碎屑锆石U- Pb 定年、重矿物组分、元素分析，提出恩平组T72界面上下地层物源体系发生转变。结合区域沉积充填特征，表明陆丰南地区T72界面为断- 拗转换界面。具体表现为：下恩平组沉积期湖盆继承文昌期裂陷盆地特征，湖盆水体较深，物源主要为盆地内部的中生代基底凸起，沉积层序明显受盆内低凸起控制，在盆内低凸起四周发育一系列近源的辫状河三角洲，井区主要接受来自惠陆低凸起物源;上恩平组沉积期间，湖盆进入断拗过渡期，湖盆扩大、水体变浅，盆内低凸起四周的断裂活动减弱，沉积层序逐渐向盆内低凸起超覆，盆外太古宙—古生代物源组分供给不断加强，近源的三角洲砂体逐渐向远源的浅水三角洲砂体转化，同时发育宽广的滨浅湖环境，原供给物源的惠陆低凸起为水体淹没，井区接受了北部隆起外的物源。     

张广才岭南部杨木岗组的厘定及物源分析

本文通过岩石组合特征和区域对比，将张广才岭南部西蛤拉河子—大锅盔一带分布的浅变质地层重新厘定为杨木岗组。为了确定杨木岗组的形成时代和沉积物源，进行了碎屑锆石U- Pb年代学和微体古生物地层学研究。锆石大多数呈自形—半自形晶，显示典型振荡岩浆生长环带，暗示其岩浆成因。该地层中测得的两组碎屑锆石U- Pb产生多组谐和年龄，其中PM010- TW样品56个测点最小峰值(谐和)年龄为307 Ma，DB02- TW样品51个测点最小峰值(谐和)年龄为275 Ma；覆盖在杨木岗组之上的中生代二浪河组安山岩的定年结果为181. 1±0. 9 Ma，表明杨木岗组形成于早二叠世晚期。杨木岗组中获取疑源类化石组合出现了新元古代晚期—早寒武世和奥陶纪地层常见分子，结合碎屑锆石年龄结果，反映杨木岗组沉积时周围存在早古生代和中—新元古代地质体。碎屑沉积岩Al2O3/TiO2平均值为24. 44，稀土元素球粒陨石标准化曲线具有轻稀土富集、重稀土稳定和负Eu异常特征，结合碎屑锆石的年龄频数可以看出，确定杨木岗组的沉积物主要来源于沉积盆地周围的晚古生代早期中酸性火成岩，次要物源由沉积盆地周边的早古生代地质体和近地表的中—新元古代地质体提供。佳木斯地块为松嫩- 张广才岭地块上晚古生代地层的形成提供了部分物源，暗示佳木斯地块与松嫩- 张广才岭地块于早二叠世之前已完成拼合。     

甘孜- 理塘洋可能存在北向俯冲：来自松潘- 甘孜道孚- 炉霍岩浆岩的证据

松潘- 甘孜造山带巨厚的三叠系复理石沉积盖层给探讨其基底性质、俯冲- 碰撞过程和深部岩浆作用增添了难度，使得带内广泛出露的花岗质岩体和少量镁铁质岩体成为解开松潘- 甘孜造山带构造演化谜团的重要研究对象。锆石U- Pb定年结果表明道孚花岗闪长岩形成于223. 5~217. 4 Ma，炉霍二长岩结晶年龄为219. 4 Ma，辉长岩为218. 9 Ma，均属于晚三叠世岩浆活动的产物。化学成分研究结果表明，花岗质岩石都表现出I型花岗岩特征，其中炉霍二长岩具有较高的Ba、Sr含量，相对较高且均一的εHf(t)值（-3. 69~-1. 65），表明其可能来源于富集玄武质新生下地壳的熔融。道孚花岗闪长岩具有分散的εHf(t)值（-13. 51~0. 41），野外和地球化学特征指示其形成于幔源熔体与古老壳源熔体的混合。辉长岩在微量元素蜘蛛图上具有类似的模式，不同程度富集Ba、Sr、Pb、Th和U元素，出现Nb、Ta、Ti元素亏损，具有岛弧玄武岩的特征，来源于流/熔体交代的地幔楔部分熔融。综合区域已有资料，我们对甘孜- 理塘洋的演化历史提出新的认识，认为甘孜- 理塘洋不仅仅存在向南俯冲的可能性，同时也具有北向俯冲的历史，晚三叠世道孚- 炉霍岩浆岩的形成受控于甘孜- 理塘洋向北边松潘- 甘孜地块俯冲的背景之下。     

The timing of ultrahigh-temperature metamorphism in Southern India: U–Th–Pb electron microprobe ages from zircon and monazite in sapphirine-bearing granulites

We report here U–Pb electron microprobe ages from zircon and monazite associated with corundum- and sapphirine-bearing granulite facies rocks of Lachmanapatti, Sengal, Sakkarakkottai and Mettanganam in the Palghat–Cauvery shear zone system and Ganguvarpatti in the northern Madurai Block of southern India. Mineral assemblages and petrologic characteristics of granulite facies assemblages in all these localities indicate extreme crustal metamorphism under ultrahigh-temperature (UHT) conditions. Zircon cores from Lachmanapatti range from 3200 to 2300 Ma with a peak at 2420 Ma, while those from Mettanganam show 2300 Ma peak. Younger zircons with peak ages of 2100 and 830 Ma are displayed by the UHT granulites of Sengal and Ganguvarpatti, although detrital grains with 2000 Ma ages are also present. The Late Archaean-aged cores are mantled by variable rims of Palaeo- to Mesoproterozoic ages in most cases. Zircon cores from Ganguvarpatti range from 2279 to 749 Ma and are interpreted to reflect multiple age sources. The oldest cores are surrounded by Palaeoproterozoic and Mesoproterozoic rims, and finally mantled by Neoproterozoic overgrowths. In contrast, monazites from these localities define peak ages of between 550 and 520 Ma, with an exception of a peak at 590 Ma for the Lachmanapatti rocks. The outermost rims of monazite grains show spot ages in the range of 510–450 Ma.While the zircon populations in these rocks suggest multiple sources of Archaean and Palaeoproterozoic age, the monazite data are interpreted to date the timing of ultrahigh-temperature metamorphism in southern India as latest Neoproterozoic to Cambrian in both the Palghat–Cauvery shear zone system and the northern Madurai Block. The data illustrate the extent of Neoproterozoic/Cambrian metamorphism as India joined the Gondwana amalgam at the dawn of the Cambrian.     

The bimodal rift-related Juscelândia volcanosedimentary sequence in central Brazil: Mesoproterozoic extension and Neoproterozoic metamorphism

The Barro Alto Complex and Juscelândia volcanosedimentary sequence are exposed in the central part of the Neoproterozoic Brasília belt of central Brazil. The former is a large (approximately 150 km long), boomerang-shaped, mafic-ultramafic, layered complex formed by two different intrusions metamorphosed under granulite facies. These rocks are tectonically overlain by rocks of the Juscelândia volcanosedimentary sequence, represented mainly by biotite-gneiss and amphibolite, or amphibolite facies metamorphic equivalents of rhyolite and basalt, respectively. New SIMS U–Pb zircon data and Sm–Nd isochron data presented herein help clarify the igneous and metamorphic evolution of the Juscelândia volcanosedimentary sequence, as well as its relationship with the Barro Alto Complex. Zircon grains from two biotite gneisses were analyzed by SIMS (SHRIMP) and indicate Mesoproterozoic dates, approximately 1.28 Ga, interpreted as the time of bimodal volcanism in a tectonic setting transitional between a continental rift and an ocean basin. Metamorphism is constrained by Sm–Nd garnet-whole-rock isochrons for garnet amphibolite and pelitic schists of the Juscelândia sequence, as well as for clinopyroxene-garnet amphibolite and garnet granulite of the Barro Alto Complex, which give ages between 0.74 and 0.76 Ga, in agreement with SIMS dates for metamorphic zircon rims. These new data are significant, because they establish that a single metamorphic event affected both the Barro Alto Complex and the Juscelândia sequence. Based on these new data, we present a modified tectonic model for the Brasília belt.     

Detrital zircon reference for the North China block

U–Pb analyses of 250 single detrital zircons from Upper Proterozoic to Ordovician strata collected from the Zhuozi Shan in north-central China provide a detrital zircon reference for the North China block, a major crustal entity in the Asian tectonic collage. The results, which range in age from 1.72 to 2.97 Ga, shed new light on the age of the crystalline basement in North China, much of which is covered by younger sedimentary units. In addition, this detrital zircon reference can be used to help determine the provenance of clastic sedimentary units and for assessing validity of paleogeographic and regional tectonic models that include the complex history of Asian continental amalgamation, terrane accretion, and subsequent translation that is ongoing today.     

Reference Materials in Geoanalytical Research -Review for 2004 and 2005

This review gives an overview of the use and development of reference materials of geochemical and environmental interest in the literature of the years 2004 and 2005. In these years the performance of existing methods has been improved and new geochemical applications using new techniques have been developed. Accordingly, there was an increasing need for new reference materials, especially for in situ microanalysis and for precise stable isotope measurements. In addition, there was a notable trend for further characterisation of existing reference materials, mainly for the platinum-group elements. This review focuses on five topics: reference materials for platinum-group elements, reference glasses for in situ microanalysis, zircon reference materials, isotopic reference materials, and the development and certification of reference materials.     

2004–2005 Analytical Developments in Secondary Ion Mass Spectrometry

This review of the literature from 2004 and 2005 concerning secondary ion mass spectrometry (SIMS) highlights the contribution the technique has made in the fields of petrology, geochronology, cosmochemistry and material sciences. In petrology, much research was devoted to the measurement of stable isotopes and trace elements by developments in multicollection acquisition, with emphasis on low atomic mass number elements. Elements studied in particular were S (in sulfides), O (in garnets), C (in sedimentary organic matter), Cl (in glasses) and Si. Novel applications of SIMS to geochronology have included the measurement of young zircon grains by the U-Pb and U-Th decay methods. An increasing number of studies have combined U-Pb geochronology with the measurement of trace elements or stable isotopes in zircon.     

Early Cretaceous U–Pb zircon ages for the Copiapó plutonic complex and implications for the IOCG mineralization at Candelaria, Atacama Region, Chile

Four of the major plutons in the vicinity of the Candelaria mine (470 Mt at 0.95% Cu, 0.22 g/t Au, 3.1 g/t Ag) and a dike–sill system exposed in the Candelaria open pit have been dated with the U–Pb zircon method. The new geochronological data indicate that dacite magmatism around 123 Ma preceded the crystallization of hornblende diorite (Khd) at 118 ± 1 Ma, quartz–monzonite porphyry (Kqm) at 116.3 ± 0.4 Ma, monzodiorite (Kmd) at 115.5 ± 0.4 Ma, and tonalite (Kt) at 110.7 ± 0.4 Ma. The new ages of the plutons are consistent with field relationships regarding the relative timing of emplacement. Plutonism temporally overlaps with the iron oxide Cu–Au mineralization (Re–Os molybdenite ages at ∼115 Ma) and silicate alteration (ages mainly from 114 to 116 and 110 to 112 Ma) in the Candelaria–Punta del Cobre district. The dated dacite porphyry and hornblende diorite intrusions preceded the ore formation. A genetic link of the metallic mineralization with the quartz–monzonite porphyry and/or the monzodiorite is likely. Both of these metaluminous, shoshonitic (high-K) intrusions could have provided energy and contributed fluids, metals, and sulfur to the hydrothermal system that caused the iron oxide Cu–Au mineralization. The age of the tonalite at 110.7 Ma falls in the same range as the late alteration at 110 to 112 Ma. Tonalite emplacement may have sustained existing or driven newly developed hydrothermal cells that caused this late alteration or modified ⁴⁰Ar/³⁹Ar and K/Ar systematic in some areas.