西藏冈底斯带门巴地区印支期花岗岩地球化学特征及其构造意义

门巴地区晚三叠世花岗岩主要出露在西藏冈底斯构造带的弧背断隆上,主要岩石类型为黑云角闪花岗闪长岩和黑云二长花岗岩。岩石为钙碱性,具有富SiO2、K2O的特点。K2O/Na2O平均为1.13,相对富钾。Al2O3变化于13.27%～15.53%之间,A/CNK平均为1.0,为准铝质岩石。花岗岩体稀土元素总量∑REE变化于99.5×10-6～294.72×10-6之间,轻稀土元素富集,重稀土元素亏损,具弱至中等的负Eu异常。微量元素以富K、Rb、Ba、Th等大离子亲石元素和亏损Nb、Ta、Y、Yb等高场强元素为特征。岩石学和岩石地球化学研究表明,该时代的花岗岩具I型花岗岩的特点,形成于岛弧环境,是新特提斯洋早期俯冲作用的产物。同时它也暗示着冈底斯岩浆弧带在晚三叠世就已成雏形。     

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。     

西藏冈底斯东部门巴地区晚白垩世埃达克质岩的岩石成因

本文首次报道了西藏冈底斯东部门巴地区晚白垩世花岗闪长岩的岩石学、锆石U-Pb定年和Hf同位素、全岩元素与Sr-Nd同位素地球化学数据。门巴花岗闪长岩锆石U-Pb定年结果为88.3Ma。花岗闪长岩的SiO2含量为66.4%～68.5%,K2O含量为3.1%～4.1%,A/CNK为0.90～0.99,属于高钾钙碱性准铝质岩石。岩石强烈富集轻稀土元素,(La/Yb)N为25.3～46.6,具弱的负Eu异常(δEu=0.83～0.91),高Sr(725×10-6～766×10-6)、高Sr/Y比值(Sr/Y=52.2～65.2),具有埃达克岩的特征,本文称之为门巴埃达克质岩石。门巴埃达克质岩石εNd(t)值为-4.1～-4.3,(87Sr/86Sr)t值为0.70671～0.70672。εHf(t)范围为-3.4～-1.2,Hf同位素地壳模式年龄为tCDM=1.2～1.4Ga。结合门巴埃达克质岩的主、微量元素、同位素以及含有闪长质包体等特征,判断门巴埃达克质岩石主要来源于古老下地壳物质的部分熔融,可能是来源于下地壳物质的熔体与幔源岩浆(如底侵的玄武岩浆)发生混合作用的结果。通过与在南冈底斯发现的埃达克岩的成因及地球动力学背景的对比,本文认为新特提斯洋洋脊俯冲作用不仅为镁铁质下地壳的熔融提供了能量,而且在岩浆形成演化过程中也起到了重要作用。     

Slumping in the Marra Mamba Supersequence Package in the southern Hamersley Province,Western Australia

Detailed mapping of the Hardey Syncline region at the Marra Mamba Supersequence Package closure, in the southwestern part of the Hamersley Province, has indicated that a hiatus in parts of the Jeerinah Formation was probably caused by submarine slumping during sedimentation. A similar hiatus at the base of the Jeerinah Formation farther west, noted on air photographs, is attributed to the same cause. Removal by slumping, and redeposition westwards, could account for olistostromes which were previously recorded in the Jeerinah Formation at Mt Edith and Mt de Courcey on the southern rim of the Wyloo Dome.     

西藏门巴地区地质旅游资源和开发建议

西藏门巴地区地质旅游资源十分丰富,其地理位置、交通条件、自然和社会环境都十分有利于旅游业的发展。地质旅游资源包括沉积与构造、地貌景观、自然变动遗迹和泉四大亚类,分布在6条旅游路线上。这些地质旅游资源大部分是首次发现的,需进一步开发、评价和保护。     

Marra Mamba Iron Formation stratigraphy in the eastern Chichester Range,Western Australia

The Marra Mamba Iron Formation is the basal member of the Hamersley Group in Western Australia and is host to major iron‐ore deposits in its upper Mt Newman Member. Previous studies have suggested that the Marra Mamba Iron Formation in the eastern Chichester Range was deposited in an isolated basin behind a barrier reef and was greatly reduced in thickness. New drilling covering a large part of the Chichester Range shows that only the lower part of the Nammuldi Member, the lowest unit of the Marra Mamba Iron Formation, remains over most of the range. However, 15 m of the Mt Newman Member, the complete MacLeod Member and the upper part of the Nammuldi Member were drilled in small north‐south synclines preserved near Mulga Downs homestead. This drilling shows that the Marra Mamba Iron Formation is similar in all respects to other occurrences in the Hamersley Province. There is no evidence of facies change or reduced thickness and therefore no evidence for a northern limit to the deposition of the Hamersley Group. Apart from the small synclines near Mulga Downs homestead, the Fortescue River alluvial deposits abutting the Chichester Range are underlain by the Roy Hill Shale Member of the Jeerinah Formation.