南阿尔金玉苏普阿勒克塔格花岗岩体成因及其对区域早古生代构造演化的启示

【研究目的】 玉苏普阿勒克塔格花岗岩体位于阿尔金造山带南部茫崖蛇绿混杂岩带内,主要由中粗粒似斑状黑云二长花岗岩及中细粒含斑黑云二长花岗岩组成。玉苏普阿勒克塔格岩体的形成时代、成因类型和岩浆物质来源尚不明确,制约了我们进一步认识该岩体形成的地球动力学背景以及南阿尔金造山带早古生代构造演化过程。【研究方法】 因此,本文对玉苏普阿勒克塔格岩体进行了岩石学、锆石U-Pb年代学、全岩地球化学、黑云母矿物化学及Hf同位素组成等方面的研究。【研究结果】 锆石U-Pb年代学研究结果表明该岩体中粗粒似斑状黑云二长花岗岩的锆石U-Pb加权平均年龄为451~447 Ma。结合前期工作获得该岩体中细粒含斑黑云二长花岗岩的年龄（430~423 Ma）,笔者认为玉苏普阿勒克塔格花岗岩体属于早古生代岩浆活动的产物。黑云母矿物化学研究结果表明,玉苏普阿勒克塔格岩体形成于温度较低压力较高环境。根据玉苏普阿勒克塔格岩体两期花岗岩的矿物组成、全岩地球化学特征及形成的物理化学环境,认为该岩体属于I型花岗岩。Hf同位素组成研究结果表明,玉苏普阿勒克塔格岩体两期花岗岩具有相似的Hf同位素组成,暗示它们具有相似的物质来源：源岩以新生地壳的部分熔融为主,在侵位过程中经历了部分古老地壳物质的混染。【结论】 综合玉苏普阿勒克塔格花岗岩体的形成时代、成因类型、物质来源,结合区域构造演化资料,本文认为玉苏普阿勒克塔格岩体形成于与南阿尔金洋北向俯冲有关的活动大陆边缘环境。     

The Formation of the Jiaodong Gold Province

Gold deposits in Jiaodong, termed Jiaodong-type, are tectonically located in the southeastern margin of the North China Craton. Their major features are reviewed in this paper to highlight the differences between Jiaodong deposits and other genetic types of gold deposits. The mineralization was synchronized with asthenosphere upwelling indicated by syn-ore OIB-like mafic dike and large-scale crustal thinning suggested by decrease of Sr/Y from pre-ore to syn-ore granites. Asthenosphere upwelling induced by the roll-back of Paleo-Pacific Plate drove partial melting of lithospheric mantle and devolatilization, which induced the release of the ore-forming fluids. In concomitant with magmatic records, mineralization migrated from the western Jiaobei terrane (133–127 Ma) to the eastern Sulu orogenic belt (114–108 Ma), corresponding to the eastward roll-back of Paleo-Pacific Plate. Gold mineralization in Jiaodong formed in the transitions of ductile to brittle deformation, rapid to slow crustal uplift, and regional compression to extension. In the regional-scale, the gold deposits in the Jiaobei terrane are mostly situated at intersections between NE-trending faults and EW-trending basement faults, and gold orebodies dominantly controlled by the lithologic contacts between Precambrian metamorphic rocks and Mesozoic granites. The mineralization was dominated by the disseminated-veinlet ores related to quartz–sericite alteration in strong cataclasite-breccia zone, with subsidiary thick quartz-sulfide veins developed in secondary fault zones. The ore-forming fluids belong to a H2O–CO2–NaCl±CH4 system and show minor variations in salinity among different types of ore. Structure-fluid feedback involving fluid-rock reaction and hydrofracturing triggered the fluid phase separation and resultant gold deposition. The Jiaodong gold deposits are distinct from orogenic and intrusion-related gold deposits in terms of tectonic setting, origin of ore-forming fluids, and mechanism of gold deposition.     

滇中老街子Pb-Ag多金属矿床的成矿时代及成矿动力学背景探讨：来自硫化物Re-Os同位素证据

滇中老街子Pb-Ag多金属矿床位于红河-哀牢山新生代富碱斑岩带的中段,是与富碱火山-岩浆热液密切相关的典型矿床。首次在该矿床深部发现70m宽的高Cu-Mo异常黄铁矿化蚀变正长斑岩和黄铁矿-辉钼矿矿脉(2073中段3#穿脉),认为该矿床深部存在明显的富碱斑岩-岩浆热液型铜-钼矿化叠加成矿作用。本文选取深部坑道内的辉钼矿-黄铁矿脉、黄铁矿-方铅矿脉和高Cu-Mo异常黄铁矿化蚀变正长斑岩进行辉钼矿-黄铁矿Re-Os同位素成矿年代研究,获得了辉钼矿Re-Os等时线年龄33.71±0.29Ma和加权平均值年龄33.99±0.22Ma,说明滇中老街子斑岩型铜-钼成矿作用时代为渐新世,与Cu-Mo异常黄铁矿化蚀变正长斑岩成岩年龄一致。黄铁矿的初始Os比值(0.3049~1.5549)及其γOs值(140.5~1126)指示老街子Pb-Ag多金属矿床金属成矿物质主要来源于壳幔相互作用而形成的岩浆。老街子Pb-Ag多金属矿床成岩成矿年龄和区域上富碱斑岩-岩浆热液型多金属矿床成岩成矿年龄(30~40Ma)大致相当,表明印度板块与欧亚板块碰撞期后,构造应力场发生转变形成大规模区域左行走滑断裂系统,在稳定块体内部诱发壳幔过渡层发生部分熔融形成富碱岩浆和成矿流体,成矿流体沿着构造薄弱部位形成富碱斑岩-岩浆热液型多金属矿床。     

西藏蒙亚啊铅锌矿床矽卡岩型矿体白云母Ar-Ar年代学研究及其地球动力学意义

西藏蒙亚啊矿床是冈底斯北缘Pb-Zn-Ag-Cu-Fe-Mo多金属成矿带内发育的Pb-Zn矿床的典型代表.对该矿床形成时代的精确厘定,是正确理解矿床成因机制,准确评价成矿潜力的关键。目前蒙亚啊矿床成矿时代的研究主要针对矿区内的花岗斑岩岩体或矿石矿物铅同位素展开,并存在较大的争议。本文基于成矿作用类型的差异,将蒙亚啊矿床的矿体进一步划分为矽卡岩型矿体和斑岩型矿体,而最具经济价值的硫化物成矿作用为矽卡岩型矿化。在此基础之上,通过云母Ar-Ar测年对蒙亚啊矽卡岩型矿体开展了定年工作。蒙亚啊矽卡岩型矿体中与闪锌矿共生的白云母的Ar-Ar同位素测年显示,其Ar-Ar坪年龄为54.60±0.40 Ma,正等时线年龄为54.45±0.67 Ma。根据白云母与矿石矿物的共生关系可知,该年龄应代表了蒙亚啊矿区矽卡岩型矿体的成矿时代。综合前人的研究结果,笔者认为:蒙亚啊Pb-Zn矿床存在两期成矿事件,但主成矿期形成于印度-欧亚陆-陆碰撞的主碰撞阶段,而新特提斯洋壳板片的回卷可能是矿床形成的深部动力学机制。     

Fault zone characteristics,fracture systems and permeability implications of Middle Triassic Muschelkalk in Southwest Germany

Fault zone structure and lithology affect permeability of Triassic Muschelkalk limestone-marl-alternations in Southwest Germany, a region characterized by a complex tectonic history. Field studies of eight fault zones provide insights into fracture system parameters (orientation, density, aperture, connectivity, vertical extension) within fault zone units (fault core, damage zone). Results show decreasing fracture lengths with distances to the fault cores in well-developed damage zones. Fracture connectivity at fracture tips is enhanced in proximity to the slip surfaces, particularly caused by shorter fractures. Different mechanical properties of limestone and marl layers obviously affect fracture propagation and thus fracture system connectivity and permeability. Fracture apertures are largest parallel and subparallel to fault zones and prominent regional structures (e.g., Upper Rhine Graben) leading to enhanced fracture-induced permeabilities. Mineralized fractures and mineralizations in fault cores indicate past fluid flow. Permeability is increased by the development of hydraulically active pathways across several beds (non-stratabound fractures) to a higher degree than by the formation of fractures interconnected at fracture tips. We conclude that there is an increase of interconnected fractures and fracture densities in proximity to the fault cores. This is particularly clear in more homogenous rocks. The results help to better understand permeability in Muschelkalk rocks.     

胶东中生代动力学演化及主要金属矿床成矿系列

胶东处于古特提斯和太平洋两大成矿域的结合部位,经历了强烈的中生代构造岩浆活动和重要金属矿床成矿作用。本文通过对中生代各期岩浆活动背景、典型矿床成因类型的分析判别,结合区域地质构造特征,总结提出胶东中生代成岩成矿动力学演化主要受扬子板块、华北板块碰撞造山作用和古太平洋板块俯冲作用影响,并可划分为晚三叠世陆陆碰撞造山期,中侏罗世被动陆缘向活动陆缘转换、地壳增生期,早白垩世早期地壳增生向垮塌转换期,早白垩世中期岩石圈大规模拆沉、壳幔强烈作用期,早白垩世晚期陆缘弧俯冲作用期,和晚白垩世早期弧后岩石圈强烈伸展期六个演化阶段,分别对应着区域~205Ma、160~155Ma、135~125Ma、125~115Ma、115~100Ma、100~90Ma等六期主要金及多金属成矿作用;形成的贵金属及有色金属矿床具有较为明显的时空分布规律,大致包括6个矿床成矿系列,分为9个亚系列和16个矿床式。自西向东,可划分出莱州西部、招远-平度、栖霞-蓬莱-福山、胶莱盆地东北缘、牟平-乳山、文登-威海、荣成等7个贵金属、有色金属成矿区(带)。区内造山型金矿仍是找矿的重点,斑岩型有色金属矿床可望取得突破,中低温热液脉型多金属矿床应受到充分重视。     

周向应力在地壳运动中的作用

地壳运动的驱动力一直存在争议。目前虽然提出了很多假说,但这些假说所描述的驱动力数量级均较小,不足以推动地壳运动;另外,大量实际地应力测量表明,水平主压应力在三个地应力分量中最大,被看作地壳“异常”压力,其机理也没有统一的认识。因此,有必要弄清楚地壳运动的动力来源是什么及为什么会出现这种水平应力占主导的现象。受背斜构造或石拱桥的侧向支撑的启发,通过地球模型受力分析得出,地壳作为球壳在自重下会相互挤压,在圆周方向产生很强的周向应力。周向应力大于重力,且由重力派生,和实测的地应力特征是一致的。推测该应力在20 km深处约为900 MPa,足以驱动板块运动(>500 MPa)。因软流圈是可流动的,其上面的岩石圈只要存在薄弱带,该应力就会释放,板块之间从而产生相对运动。整个洋壳和拱桥类似,在该力的作用下,会在俯冲带处下插至陆壳深部,俯冲带就是岩石圈的薄弱区,它因此会承担部分甚至全部洋壳的重量。最后提出,没有独立于重力的、可独立起作用的构造力或碰撞力,周向应力是地壳运动的唯一具有足够数量级的驱动力。     

Depth and region dependence of b-value for micro-aftershocks of the May 12th, 2008 Wenchuan earthquake and its tectonic implications

Micro-aftershocks with magnitude range of 1.5?4 around the Wenchuan earthquake epicenter, the southern part of the Longmenshan fault zone, exhibit good frequency-magnitude linear relationships, thus enabling b-value analysis. The average b-value for micro-aftershocks of M1.5?4 from July to December of 2008 in our local study region is about 0.88, similar to the b-value for all aftershocks of M3.0?5.5 from May, 2008 to May, 2009 along the whole Longmenshan fault zone. The similarity between the local and regional b-values possibly indicates that the southern part of the Longmenshan fault zone has similar seismogenic environment to the whole Longmenshan fault zone. Alternatively, it may also imply that b-values derived from all events without consideration of structural variation can not discriminate local-scale tectonic information. The present study shows that the b-value for the Wenchuan earthquake micro-aftershocks varies with different regions. The b-value in southwest of the Yingxiu town is higher than that in the northeast of the Yingxiu town. The high b-value in the southwest part where the Wenchuan earthquake main shock hypocenter located indicates that the current stress around the hypocenter region is much lower than its surrounding area. The b-values are also dependent on depth. At shallow depths of < 5 km, the b-values are very small (~0.4), possibly being related to strong wave attenuation or strong heterogeneity in shallow layers with high content of porosity and fractures. At depths of ~5?11 km, where most aftershocks concentrated, the b-values become as high as ~0.9?1.0. At the depth below ~11 km, the b-values decrease with the depth increasing, being consistent with increasing tectonic homogeneity and increasing stress with depth.

     

Geodynamic framework of large volcanic fields highlighted by SRTM DEMs: Method evaluation and perspectives exampled on three areas from the Cameroon Volcanic Line

This study is a part of a wider investigation to evaluate how much can be learnt by using low-cost methods such as systematic moderate-resolution remote sensing to map and quantify geological structures at the regional scale on very large volcanic provinces only partly studied in the field. Volcanic-centre and cinder-cone distribution, faults and structural lineaments are mapped combining Shuttle Radar Topography Mission (SRTM), Digital Elevation Models (DEMs) and Landsat satellite images. As an example of the method, we present the interpretation of structural data and morphological features of three contrasted areas from the Cameroon Volcanic Line (Tombel graben, Upper Benue valley, and Ngaoundéré area) for which local field studies are available for comparison. At a local scale, this remote-sensing method of mapping displays good to excellent correlations with previously published data and, by itself, it allows one to constrain the structural setting of each area. Numerical treatment of vent and cinder-cone localisation can be related to tension fractures (T direction), whereas numerical treatment of the lineaments constrains the associated fault system to a single transtensional (strike-slip + extension) Riedel type fracture network. The first results on the Cameroon Volcanic Line are promising and could be used at a larger scale on numerous volcanic provinces for which field data are not yet available.     

内蒙古呼伦贝尔盟博克图南部变质堆晶角闪辉长岩的U-Pb锆石年龄及地质意义

博克图南部的变质堆晶角闪辉长岩锆石U-Pb年龄测定结果为2096±36Ma,表明本区的变质堆晶角闪辉长岩形成于古元古代.结合岩石化学及岩石地球化学特征研究表明,岩石可能形成于俯冲环境,是造山作用过程中弧岩浆活动的早期产物,标志着区内一个新的构造岩浆旋回的开始,不能作为古元古代末期大陆裂解的产物.