西南三江甘孜-理塘洋晚古生代构造演化：来自理塘蛇绿混杂岩堆晶辉长岩U-Pb年龄的约束

理塘县高城镇西北的擦岗隆洼岩组是一套以玄武岩为基质的蛇绿混杂岩,通过该蛇绿混杂岩中堆晶辉长岩的LA-ICP-MS 锆石U-Pb定年,显示其年龄范围介于349 ~ 274 Ma之间,形成2个年龄组：第一组斜锆石206Pb/238U年龄加权平均值为346±17 Ma,代表堆晶辉长岩早期岩浆活动的时代;第二组斜锆石206Pb/238U年龄加权平均值为286.2±5.1 Ma,代表了堆晶辉长岩晚期岩浆活动的结晶年龄。擦岗隆洼岩组的超基性—基性岩具有N-MORB（正常洋中脊玄武岩）和E-MORB（富集型洋中脊玄武岩）组合特征,指示这些岩石可能形成于洋中脊环境,岩浆物质源自N-MORB所代表的亏损上地幔源区受地幔柱交代混染后而形成的混合源区。研究认为,甘孜−理塘洋最早可能在中泥盆世,受区域地幔柱活动影响开始拉张,导致中咱地块从扬子陆块西缘裂离,并在早石炭世发育形成洋盆。在中晚三叠世甘孜−理塘洋壳开始向西俯冲,并于晚三叠世末闭合,区域进入弧陆碰撞造山阶段。     

Crustal-scale magmatic systems during intracontinental strike-slip tectonics: U, Pb and Hf isotopic constraints from Permian magmatic rocks of the Southern Alps

The Southern Alps host volcano-sedimentary basins that formed during post-Variscan extension and strike-slip in the Early Permian. We present U–Pb ages and initial Hf isotopic compositions of magmatic zircons from silicic tuffs and pyroclastic flows within these basins, from caldera fillings and from shallow intrusions from a 250 km long E–W transect (Bozen–Lugano–Lago Maggiore) and compare these with previously published data. Basin formation and magmatism are closely related to each other and occurred during a short time span between 285 and 275 Ma. The silicic magmatism is coeval with mafic intrusions of the Ivrea-Verbano Zone and within Austroalpine units. We conclude that deep magma generation, hybridisation and upper crustal emplacement occurred contemporaneously along the entire transect of the Southern Alps. The heat advection in the lower crust by injected mantle melts was sufficient to produce crustal partial melts in lower crustal levels. The resulting granitoid melts intruded into the upper crust or rose to the surface forming large caldera complexes. The compilation of Sr and Nd isotopic data of these rocks demonstrates that the mantle mixing endmember in the melts may not be geochemically enriched but has a depleted composition, comparable to the Adriatic subcontinental mantle exhumed to form the Tethyan sea floor during Mesozoic continental breakup and seafloor spreading. Magmatism and clastic sedimentation in the intracontinental basins was interrupted at 275 Ma for some 10–15 million years, forming a Middle Permian unconformity. This unconformity may have originated during large-scale strike-slip tectonics and erosion that was associated with crustal thinning, upwelling and partial melting of mantle, and advection of melts and heat into the crust. The unconformity indeed corresponds in time to the transition from a Pangea-B plate reconstruction for the Early Permian to the Late Permian Pangea-A plate assembly (Muttoni et al. in Earth Planet Sci Lett 215:379–394, 2003). The magmatic activity would therefore indicate the onset of >2,000 km of strike-slip movement along a continental-scale mega-shear, as their model suggests.     

混杂岩基本特征与专题地质填图

混杂岩是造山带内最常见的地质体和最基本的大地构造相单元,是由多种地质作用共同形成的可作为一定比例尺（1：25000或更大）的地质填图单元,并由块体和基质共同构成的内部无序的岩石混杂体。混杂岩的形成和就位构造环境不一致,并非所有混杂岩都具有古板块构造格局指示意义。蛇绿混杂岩和增生杂岩可作为古洋盆和汇聚板块边界的直接判别标志,其结构、组成与时空配置共同记录了古大洋板块地层系统发展及大陆地壳侧向与垂向增生历史。以大洋板块地层、海山理想层序和蛇绿岩"彭罗斯"层序模型为指导,专题地质填图是揭示造山带蛇绿混杂岩和增生杂岩结构、组成及其时空配置关系与形成机制的有效手段。这可为洋-陆转换研究及提高造山带结构深化认知程度提供直接证据,同时指导混杂岩相关矿床勘探与找矿预测。     

中小地震深度分布特征与中长期前兆机理分析

统计了不同震级地震的深度分布，数据显示：震级越低，深度分布的范围越广；随着震级增大，震源深度有集中在地壳中上层的趋势。通过分析中小地震深度分布特征的原因，结合强震发生的条件和特性，对中小地震前兆机理进行了解释。本文的结果和认识对判定中长期强震危险区有一定的应用价值。     

PRELIMINARY STUDY ON THE OPHIOLITIC MELANGE IN THE YALU TSANGPU GRAND CANYON AREA

PRELIMINARY STUDY ON THE OPHIOLITIC MELANGE IN THE YALU TSANGPU GRAND CANYON AREA     

桂东鹰扬关构造混杂岩物质组成及变形特征

鹰扬关构造混杂岩出露于湘-粤-桂三省(区)交界处,构造上位于扬子陆块与华夏地块结合带西南段,在形成过程中保存了丰富的物质记录和构造演化信息,是研究扬子陆块与华夏地块拼合造山过程及相互关系的重要对象。对鹰扬关构造混杂岩物质组成及变形特征的详细研究表明,其由基质与岩块组成,基质为变质含凝灰质细碎屑岩,岩块为变质火山(碎屑)岩、微晶石英岩与大理岩等;根据野外变形特征及各期变形的叠加改造关系,于鹰扬关构造混杂岩中识别出5期构造变形,并建立了5期构造的相对变形序列。S₁、S₂代表晋宁期变形作用(D₁～D₂); S₃及NNE向断层左行走滑运动指示加里东期NW-NNW向挤压作用(D₃);印支期及早燕山期W-NWW向挤压作用(D₄)对前期构造进行了改造, NNE向断层发生右行走滑运动;晚燕山期的伸展作用主要表现为正断层活动(D₅)。变形序列的建立为区域构造过程的探讨和对比提供了新的基础资料。     

共轭伸展褶劈理夹角的定量解析

９０ 年代初以来,中国地质学者先后在浙、皖、赣、湘的“板溪群”及震旦系中发现了古生代的生物化石。因此,再次将“板溪群”是否是前震旦纪的一个变质地层单位、江南古陆是否存在以及华南大地构造格局是否需要重新厘定等问题提了出来。１９９３ 年笔者于赣东北蛇绿混杂岩带内的“双桥山群”岩块中,首次发现了含晚古生代的放射虫硅质岩。１９９６ 年又在该混杂岩带多处的“双桥山群”岩块和“下震旦统志棠组”中找到了含晚古生代的放射虫硅质岩,还首次在“登山群拔竹坑组”中发现了早古生代的几丁虫化石。经数年野外多次观察和对室内化石鉴定、岩石地球化学测试结果以及岩石学特征的综合分析表明,所发现的含晚古生代放射虫硅质岩及其岩石组合是一套洋盆深水相沉积;登山群拔竹坑组,是早古生代的一套浅水陆棚 深水陆棚 半深海相沉积。结合同行学者在邻区“板溪群”研究方面取得的新成果,笔者认为：“板溪群”不是前震旦纪的一个变质地层单位,而是一个复杂的构造岩带,江南古陆是不存在的,华南从早古生代到中生代早期还经历了一次洋盆演化过程,现在的构造格局是印支运动奠定的     

广西十万大山前陆冲断推覆构造

通过十万大山盆地内地震剖面资料和ＴＭ遥感图象的地质构造解译,结合重力资料和野外地质观察及构造分析,阐述了十万大山前陆冲断推覆构造的发育特征和前陆盆地的构造演化。前陆冲断推覆构造由３个不同的构造变形带组成：卷入海西和印支期花岗岩体的逆冲断裂带、充填中生代陆相沉积并发生构造滑脱的前陆盆地和对应于华南准地台的前陆腹地。冲断推覆构造的形成和演化是与中、晚古生代钦州海槽晚二叠世的褶皱回返和中生代相继的构造复活密切联系的,它经历了３期主要构造应力作用事件：晚二叠世海西运动晚幕为冲断推覆构造的雏形期,晚三叠世印支运动晚幕的近ＳＮ向挤压是陆相前陆盆地的发育期;早白垩世末期燕山运动主幕ＮＷ—ＳＥ向挤压是现今十万大山前陆冲断推覆构造的成型期。     

鄂北大洪山地区“花山群”的解体

岩性—构造填图结果表明,大洪山区的原“花山群”实际上包含３套不同时代、不同环境的物质建造,应予以解体。其中襄广断裂带内分布的以基性火山岩为主的一套岩系是构造侵位的蛇绿混杂岩,称之为花山蛇绿混杂岩;大洪山以西的耿集—板桥火山—沉积岩系,是南秦岭随县群的构造岩块;剔除这两者之后的碎屑沉积岩系定为真正意义的花山群,沉积时代为晚元古代。     

Geochemical and isotopic evidence for the age,orogenic setting and petrogenesis of the Nychum Volcanic association,North Queensland

Rb‐Sr and K‐Ar age determinations agree with palaeobotanical evidence in assigning an age of 270 Ma (Lower Permian) to the Nychum Volcanics, North Queensland. Geological and geochemical evidence (moderate ‐REE, La/Yb, Th, Zr/Y, Hf/Yb, Sc/Ni) indicate that the volcanics were erupted on a thin, active continental margin. Five magma groups are present: high‐alumina basalts, andesite‐dacites, acid rocks, tholeiitic andesites, and pitchstone with a high La/Yb ratio. The high‐alumina basalts and calc‐alkaline andesites have a parent‐daughter (source rock‐partial melt) relationship. The acid rocks share high HREE, Zr, Hf, Zn, Sc, Fe/Mg and low Al with, and may be fractionated from, the tholeiitic subduction‐zone andesites. Low‐pressure fractional crystallisation is evident in all five magma groups.