Geochemistry of Early–Middle Palaeozoic basalts in the Hodgkinson Province: a key to tectono-magmatic evolution of the Tasman Fold Belt System in northeastern Queensland, Australia

The Palaeozoic Hodgkinson Province in northeastern Queensland, Australia, is host to Late Ordovician to Devonian rock assemblages that contain tholeiitic to calc-alkaline basalts. These basalts occur as massive fault-bounded units interspersed with marine sedimentary rocks and limestones that are metamorphosed to lower greenschist facies in the Ordovician Mulgrave, Silurian Chillagoe and Devonian Hodgkinson formations, respectively. The petrogenetic and Sm–Nd isotope characteristics of these mafic volcanic rocks were investigated to constrain the tectonic setting in which they erupted. Major, trace and rare earth element analyses were carried out on samples from these formations and intrusive dolerites. The mafic rocks can be classified as basalts and basaltic andesites with distinct MORB characteristics. Furthermore, the basalts are characterized by a slight to moderate enrichment in Th and concomitant depletion in Nb, both of which become less pronounced with basalt evolution through time. These features are consistent with decreasing volcanic arc affinity of Silurian and Devonian MORB-type basalts in the Hodgkinson Province. Sm–Nd isotope characteristics of these basalts indicate a change in source region from dominantly sub-continental lithospheric mantle in the Silurian to asthenospheric input in the Devonian. Collectively, the geochemical and isotopic characteristics of the Hodgkinson Province basalts are interpreted to reflect deposition in an evolving back-arc basin setting. The onset of basin extension was initiated in the Silurian. Accelerated basin subsidence occurred throughout the Devonian and was halted by basin inversion in the Late Devonian. Basin evolution was controlled by an eastward stepping subduction zone outboard of the Australian Craton.     

Pb and Nd isotopic composition of the Jigongshan granite: constraints on crustal structure of Tongbaishan in the middle part of the Qinling–Tongbai–Dabie orogenic belt, Central China

The Qinling–Dabie–Sulu belt is the world's largest ultrahigh pressure (UHP) metamorphic belt. The UHP metamorphism is well dated at 220–245 Ma in the Dabie–Sulu belt but at 507 Ma in the Qinling belt. The Tongbaishan is located between the Qinling orogenic belt to the west and the Dabie–Sulu UHP metamorphic belt to the east. It is the key area for studying the tectonic relation between the Qinling and Dabie–Sulu belts and the diachronous UHP metamorphism. The Jigongshan granitic pluton (t=128 Ma) with a total area of 1200 km², composed of monzogranite, was mostly emplaced into the Tongbai complex, an exposed basement in the Tongbaishan. The Jigongshan granites have SiO₂=69.85–72.35%, K₂O/Na₂O=0.87–1.13, A/CNK=0.91–1.03, Rb/Sr=0.14–0.25 and Th/U=3.3–12. Their REE compositions show strongly fractionated patterns with (La/Yb)_N=14–58 and Eu*/Eu=0.79–1.05. The granites are characterized by low radiogenic Pb isotopic composition. The present-day whole-rock Pb isotopic ratios are ²⁰⁶Pb/²⁰⁴Pb=16.707–17.055, ²⁰⁷Pb/²⁰⁴Pb=15.239–15.326 and ²⁰⁸Pb/²⁰⁴Pb=37.587–37.853, which are similar to that of the continental lower crust. Their _Nd(t) values range from −16 to −20, and depleted-mantle Nd model ages (T_DM) from 1.8 to 2.2 Ga. The above evidence indicates that the magma of the Jigongshan granites was derived from the partial melting of the continental crust. The Pb and Nd isotopic compositions of the Jigongshan granites resemble those of the Dabie core complex in the Dabieshan but are distinct from those of the Tongbai complex in the Tongbaishan. Thus, the Dabie core complex would be the magma source of the Jigongshan granites. The result implies that the Dabie core complex is extended to the west and constitutes the unexposed basement underlaying the Tongbai complex in the Tongbaishan.     

Early Paleoproterozoic magmatism in the Quanji Massif, northeastern margin of the Qinghai-Tibet Plateau and its tectonic significance: LA-ICPMS U-Pb zircon geochronology and geochemistry

The Quanji Massif is located on the north side of the Qaidam Block and is interpreted as an ancient cratonic remnant that was detached from the Tarim Craton. There are regionally exposed granitic gneisses in the basement of the Quanji Massif whose protoliths were granitic intrusive rocks. Previous studies obtained intrusion ages for some of these granitic gneiss protoliths. The intrusion ages span a wide range from ~ 2.2 Ga to ~ 2.47 Ga. This study has determined the U-Pb zircon age of four granitic gneiss samples from the eastern, central and western parts of the Quanji Massif. CL images and trace elements show that the zircons from these four granitic gneisses have typical magmatic origins, and experienced different degrees of Pb loss due to strong metamorphism and deformation. LA-ICPMS zircon dating yields an upper intercept age of 2381 ± 41 (2σ) Ma from monzo-granitic gneiss in the Hudesheng area and 2392 ± 25 (2σ) Ma from granodioritic gneiss in the Mohe area, eastern Quanji Massif, and 2367 ± 12 (2σ) Ma from monzo-granitic gneiss in the Delingha area, central Quanji Massif, and 2372 ± 22 (2σ) Ma from monzo-granitic gneiss in the Quanjishan area, western Quanji Massif. These results reveal that the intrusive age of the protoliths of the widespread granitic gneisses in the Quanji Massif basement was restricted between 2.37 and 2.39 Ga, indicating regional granitic magmatism in the early Paleoproterozoic, perhaps related to the fragmentation stage of the Kenorland supercontinent. Geochemical results from the granodioritic gneiss from the Mohe area indicate that the protolith of this gneiss is characterized by adakitic rocks derived from partial melting of garnet-amphibolite beneath a thickened lower crust in a rifting regime after continent-continent collision and crustal thickening, genetically similar to the TTG gneisses in the North China Craton. This suggests that the Quanji Massif had a tectonic history similar to the Archean Central Orogenic Belt of North China Craton during the early Paleoproterozoic. We tentatively suggest that the Quanji Massif and the parental Tarim Craton and the North China Craton experienced rifting in the early Paleoproterozoic, after amalgamation at the end of the Archean. The Tarim Craton and North China Craton might have had close interaction from the late Neoarchean to the early Paleoproterozoic.     

塔里木盆地盆内震旦系特征

结合前人对盆地周边露头研究成果,根据钻井和地震资料给出了盆地上、下震旦统的沉积体系分布。以阿满地区相对隆起为界,震旦系厚度总体上为东西厚、中间薄,沉积中心分布在盆地的东、西两端,并以东北角的库鲁克塔格山前地带沉积最厚,下震旦统达3400m,上震旦统也达2000m以上。早震旦世塔里木盆地主要发育了塔东和塔西两个沉积沉降凹陷,构成了塔东海洋冰川—浅海沉积体系区、塔西滨浅海沉积体系区、阿满海岸沉积体系区以及塔西南浅海—次深海沉积体系区,塔东和塔西两个沉积区以阿满海岸沉积区为界近乎对称分布。晚震旦世是在早震旦世沉积填平补齐的基础上的继承性沉积。     

鄂西渝东地区石柱古隆起构造沉积演化

石柱古隆起区的构造演化可划分为加里东、海西—早印支、晚印支—早燕山、晚燕山—早喜马拉雅、晚喜马拉雅等五个阶段。海西期—早印支期、晚印支期—早燕山期是石柱古隆起形成并继承性发展的重要时期 ,发育了石炭系黄龙组、二叠系、三叠系、侏罗系等地层序列 ,厚度大于 5 - 0 0 0 -m ,形成了多套生、储、盖成藏组合。三叠纪末的挤压 (晚印支运动 )对石柱古隆起的叠加导致了古圈闭的最终形成 ,圈闭高度达 10 0 -m左右 ,隆起幅度达 4 0 0～ 5 0 0 -m左右。此时 ,下志留统、下二叠统烃源岩埋深均大于 2 - 6 0 0 -m ,已进入生气、生油高峰阶段。三叠纪及侏罗纪石柱古隆起是油气运移的有利指向区及聚集区 ,侏罗纪末—白垩纪 (燕山晚期 )则是石柱古隆起圈闭的油气重新运移聚集的重要时期。石柱古隆起及周缘地区成藏条件优越 ,区内现今的圈闭群可认为是天然气勘探的有利靶区。     

Spatial variations in chemical compositions along Langtang–Narayani river system in central Nepal

Surface water samples were collected from Langtang Lirung glacier outlet point to the Narayani river system in central Nepal in order to investigate the role of elevation in the variation of chemistry along the drainage networks. The chemistry of Langtang–Narayani river system was dominated by sulfide oxidation coupled with carbonate dissolution and weathering of silicate minerals. Calcium and magnesium concentrations were relatively higher than other cations and the sum of both species strongly correlated with alkalinity, supporting the dissolution of carbonate and dolomite as the dominant source for these ions. Aluminosilicate minerals primarily as albite and anorthite appeared as dominant silicate minerals within the drainage basin. Bisiallitization was the dominant type of weathering within the entire drainage system. Hydrogen ion concentration was lower in the low elevation sites than in high elevation sites reflecting the more consumption of carbon dioxide in the low elevation sites due to enhanced chemical weathering rates. Furthermore, major solutes like sum of base cations, silicon as well as alkalinity increased in concentration in the lower elevation sites. All regulating factors appeared to be directly related to elevation and hence elevation appeared to be the prime factor for the variation in chemical species along the Langtang–Narayani river system. Toshiyuki Masuzawa: deceased.     

川西喜马拉雅期碰撞造山带岩浆碳酸岩的地幔源区特征——Pb-Sr-Nd同位素证据

对形成于大陆碰撞带内的川西碳酸岩进行了详细的Pb-Sr-Nd同位素分析.结果表明,川西碳酸岩具有非常负的εNd值（-3.2～-18.7）和高的（^87Sr/^86Sr）i值（0.706020～0.707923）,以及较宽的^207Pb/^204Pb值（15.362～15.679）和^208Pb/^204Pb值（38.083～39.202）的特征,明显不同于世界上由非造山作用形成的碳酸岩.它们的Sr-Nd、Pb-Pb、Sr-Pb和Nd-Pb同位素特征表明大多数碳酸岩来源于EM Ⅰ与EMⅡ之间的一种混合地幔,与元古宙不同质量比的深海/陆源沉积物和下覆的似MORB由俯冲作用引起的洋壳的再循环有关,而少数碳酸岩则受到地壳物质混染的结果.此外,通过动力学背景分析可以得出,富集地幔EM Ⅰ与EMⅡ之间的熔融产生了川西碳酸岩以及同时代的富钾岩浆岩,这种熔融可能是由于印度大陆板片与扬子大陆板片俯冲引起的新生代软流圈物质的上涌产生的,并发生于青藏高原东缘始新世-渐新世分界线的从转换压扭变形向转换张扭变形的转变过渡的构造背景下.     

渤张断裂带的构造机制及其地质意义

渤张断裂带形成于中生代,为剪张性断裂带。中新生代以来渤张断裂带一直活动,并控制中新生代沉积和火山活动。中生代至老第三纪早期(42Ma 之前)渤张断裂带为右旋走滑的剪张性断裂带,而老第三纪中晚期以来(42Ma 以来)渤张断裂带转为左旋走滑的剪张性断裂带,并控制了渤海盆地的油气分布和现代的地震活动。近期,沿渤张断裂带地震活动频繁,说明渤张断裂带仍在活动。     

平湖油气田油气成藏新模式

通过对区域构造演化、沉积背景、盖层特性以及油气生成和运聚条件等的分析,结合前人研究成果,重新建立了平湖油气田的成藏模式.平湖油气田含上部渐新统花港组和下部始新统平湖组两套储层,呈上油下气的油气分布特征.古新世-始新世的断块阶段在区域上初步形成了大断裂控制下的断背料构造,在平湖组形成圈闭.中新世中期(平湖组沉积后期)发生...     

地应力钻孔施工关键技术探讨

汶川地震后,地应力研究受到重视,应力孔施工也日渐增多。本文依据多个项目实践经验,总结了地应力钻孔施工中的一些主要技术问题,介绍了钻孔结构设计按照终孔直径选择、监测仪器安装以及地层条件来综合考虑的方法。特别是从保证技术套管的使用和起拔安全,保障仪器安装精度的扩孔技术,冲洗液的选择与控制,应力测量与应力监测仪器安装的技术保障措施等应力孔施工关键技术方法方面探讨和总结了该类型钻孔施工技术特点和技术经验,系统总结了孔内应力测量和监测仪器安装技术方法,对同类项目的开展具有一定的借鉴和现实指导意义。