Sedimentary records of the late Early Cretaceous compressional setting in the South China block: A case study of the Xingning Basin,Guangdong Province

Recently, some scholars have proposed that the South China Block (SCB) was controlled by a compressive tectonic regime in the middle–late Early Cretaceous, challenging the belief that the SCB was under an extensional setting during the Cretaceous. The Early Cretaceous tectonic setting constraint in the SCB can offer vital insight to clarify the Mesozoic subduction history of the Paleo-Pacific. Therefore, to determine the SCB tectonic regime during the Early Cretaceous, this study investigated sedimentary rocks from the Lower Cretaceous Heshui Formation in the Xingning Basin, a foreland basin located in the southeastern SCB. Provenance analysis was performed using sandstone modal analysis, sandstone geochemical characteristics, and detrital zircon geochronology. Based on the results, we discussed basin sediment sources and the SCB tectonic regime during the Early Cretaceous. The results showed that the maximum Heshui Formation depositional age was 103 Ma ± 1.6 Ma in the Early Cretaceous Albian. Detrital framework modes and geochemical characteristics of sandstone indicated that Heshui Formation's source rocks were granites and sedimentary rocks. The detrital zircon U–Pb ages could be classified into two major and four subordinate age populations. The Wuyi Terrane to the north and southeast coastal regions to the east were the primary potential Heshui Formation source areas. However, the lower and upper sandstones are different in the peak ages, ~437 and ~146 to 104 Ma, respectively, indicating that the major source area shifted from the Wuyi Terrane to the southeastern coastal regions during the late Early Cretaceous. The sandstone modal analysis results indicated that the source area comprised mainly collisional–orogenic material. The SCB was under a compressive tectonic regime during the late Early Cretaceous and this compression action continued until at least 103 Ma ± 1.6 Ma.     

Thrusting of the North Lhasa Block in the Tibetan Plateau

A huge thrust system, the North Lhasa Thrust (NLT), was discovered in the northern Lhasa block of the Tibetan Plateau based on geological mapping of the Damxung region and its vicinity, the Deqen-Lunpola traverse and the Amdo-Bam Co profile. The NLT consists of the Dongqiao-Lunpola thrust (DLT), the west Namco thrust (WNT) and the south Damxung thrust (SDT) and ductile shear zones, ophiolite slices and folds extending in a WNW direction. Major thrust faults of the NLT seem to merge into a single deep-seated detachment of the upper-crust and totally displaced southward as far as 100-120 km. Chronological analyses with 39Ar-40Ar of plagioclase and hornblende, Rb-Sr isochron of minerals and fission-tracks of apatite from mylonite within the WNT yield ages of 174-173 Ma, 109 Ma and 44 Ma, showing 3 periods of thrusting in the north Lhasa block caused by subduction of the Tethys oceanic plate and the India-Eurasia continental collision respectively.     

青藏高原纳木错西缘新元古代中期岩浆事件:对北拉萨地块起源的约束

本文报道了青藏高原北拉萨地块纳木错西缘变质辉长岩和花岗片麻岩的锆石U-Pb定年、岩石地球化学和锆石Hf同位素分析结果。锆石LA-ICP-MS定年结果表明,变质辉长岩和花岗片麻岩的原岩形成时代分别为720±6Ma和732±7Ma,相当于新元古代中期。变质辉长岩为钙碱性系列,具有Nb、Ta和Ti负异常,与岛弧玄武岩类似。变质辉长岩中锆石具有较高的εHf(t)值(+5. 2～+9. 7),应当是源自俯冲环境下相对亏损的地幔楔。花岗片麻岩原岩为I型花岗质岩石,并且具有较为均一的锆石εHf(t)值(-3. 3～+0. 3),可能形成于地壳内古元古代变质火成岩的部分熔融作用。结合区域地质资料,变质辉长岩和花岗片麻岩的原岩应当形成于新元古代中期的洋壳俯冲消减过程。北拉萨地块上的前寒武纪岩浆和变质记录与东非造山带的活动时限较为一致,因而北拉萨地块可能与东非造山带具有亲缘性。     

同源岩浆不同期次之间混合产生的暗色包体——以北拉萨地块中部晚白垩世桑心日岩体为例

本文在研究西藏北拉萨块体中段桑心日岩体中的暗色包体时发现了一种具有特殊岩石成因的暗色包体。暗色包体呈椭球状,在暗色包体和寄主岩的接触面上通常形成一个明显的可能由风化作用造成的间隙面。暗色包体为二长玢岩-花岗闪长玢岩,寄主岩为花岗岩,暗色包体明显较寄主岩更基性,更富Na_2O、CaO、MgO和Fe_2O_3~T。暗色包体和寄主岩具有明显不同的稀土元素特征,暗色包体的稀土元素含量变化较大,最基性样品具有最高的稀土含量,随着基性程度的降低稀土元素含量明显下降。随着岩浆的进一步演化,岩浆向花岗岩方向演化,稀土含量又逐步升高。寄主岩和暗色包体具有基本一致的微量元素组成,具有典型的弧岩浆岩的特征,富集Rb、Cs、K等大离子亲石元素和Th、U,亏损Nb、Ta、Ti等高场强元素。此外,暗色包体和寄主岩具有明显的Ba、Sr的负异常。暗色包体成岩年龄为75. 6±1. 2Ma,寄主岩的成岩年龄为71. 8±0. 6Ma,暗色包体成岩年龄较寄主岩早约4Myr。两者具有一致的锆石原位Lu-Hf同位素特征。综合以上岩相学、年代学、元素地球化学和同位素地球化学证据,我们认为桑心日暗色包体和寄主岩来源于同源母岩浆,初始岩浆在母岩浆房中经历了不同程度的含钾角闪石结晶分离作用,并沿早期较弱的构造裂隙侵入到地壳的某一层位,随着构造活动进一步加剧,经过进一步分异母岩浆大规模上侵,并将早先侵位处于半塑性状态的暗色包体侵吞、裹挟至近地表。桑心日暗色包体最可能的成因模式可以解释为同源岩浆不同期次间的物理混合。     

拉萨地块措勤-隆格尔地区铁矿床成岩时代、岩石成因及构造环境指示

为深入了解中北部拉萨地块构造背景,利用LA-ICP-MS技术对洛布勒铁矿床成矿花岗闪长岩锆石进行了U-Th-Pb同位素测定,分析了隆格尔、洛布勒铁矿床侵入岩岩石地球化学和Sr-Nd-Pb同位素组成.获得洛布勒花岗闪长岩锆石U-Pb年龄为111.3±1.6 Ma(MSWD=0.61,n=9).隆格尔和洛布勒铁矿床侵入岩高硅(66.63%~69.02%和64.33%~64.82%)、富碱(全碱为5.91%~6.40%和5.81%~6.05%)、低A/CNK(0.91~0.97和0.94~0.95)、SiO2与P2O5含量负相关;稀土元素总量较低(∑REE为123.11×10^-6~148.83×10^-6和96.17×10^-6~101.92×10^-6),球粒陨石标准化配分模式图右倾,弱Eu负异常(0.70~0.82和0.79~0.81),富集大离子亲石元素Rb、Th、U、K、Pb等,亏损Ba和高场强元素Nb、Ta、Sr、Ti等.隆格尔花岗岩全岩和斜长石(206Pb/204Pb)t为18.474和18.626,(207Pb/204Pb)t为15.657和15.722,(208Pb/204Pb)t为38.592和39.145,(87Sr/86Sr)i为0.704 757 6和0.707 047 3,(143Nd/144Nd)i为0.512 281和0.512 339,εNd(t)为-4.13和-2.99,tDM2为1.15 Ga和1.24 Ga;洛布勒花岗闪长岩(206Pb/204Pb)t比值为18.281,(207Pb/204Pb)t比值为15.616,(208Pb/204Pb)t比值为38.369,(87Sr/86Sr)i为0.706 551 4;(143Nd/144Nd)i为0.512 309,εNd(t)为-3.62,tDM2为1.20 Ga.结果表明,措勤-隆格尔铁矿床成矿侵入岩为中钾-高钾钙碱性岩I型花岗岩,为早白垩世晚期岛弧岩浆活动产物,岩浆源于地壳物质部分熔融,岩浆演化过程经历了壳幔岩浆混合和围岩混染.结合前人研究成果,通过对比白垩纪中北部拉萨地块和南部羌塘地块成矿事件的差异,提出中北部拉萨地块113±3 Ma岩浆活动和Fe(-Cu)成矿事件与向南俯冲的班公湖-怒江洋壳发生断离有关.     

涪陵页岩气田江东区块优快钻井技术研究与应用

涪陵页岩气田江东区块地质构造复杂、地层倾角大,存在机械钻速低、定向钻井托压严重、水平井轨迹控制困难等问题,给该地区优快钻井作业带来了挑战。本文通过总结现场施工作业及技术攻关实践,分析总结了江东区块复杂地质条件下相适应的钻井新工艺、新工具,形成了一套适用于涪陵页岩气田江东区块的优快钻井技术。江东区块钻井周期从开发前期的235 d缩短至9033 d,机械钻速、储层钻遇率不断提高的同时,定向钻井托压、钻头提前失效等工况逐渐减少。通过4年发展,涪陵页岩气田江东区块优快钻井集成技术逐渐成熟,基本满足了深层页岩气的开发需要。     

西藏拉萨地块盐湖石英闪长岩成因：锆石SHRIMP U- Pb年代学、地球化学及Sr- Nd- Pb- Hf同位素的制约

西藏阿里盐湖地区发育不同类型的火成岩,通常被认为形成于班公湖-怒江特提斯洋南向俯冲板片发生断离的构造背景。本文报道了盐湖石英闪长岩的锆石SHRIMP U-Pb年龄,全岩地球化学以及Sr-Nd-Pb-Hf同位素数据。盐湖石英闪长岩的锆石~(206)Pb/~(238)U加权平均年龄为118.5±1.9Ma,属早白垩世晚期。石英闪长岩样品主量元素表现为富钠的钙碱性岩石系列,A/CNK值介于0.84～0.89之间,属准铝质。微量元素富集Rb、K、Th、U等大离子亲石元素(LILE)和轻稀土元素(LREE),强烈亏损Nb、Ta、Zr、Hf、P、Ti等高场强元素(HFSE)和重稀土元素(HREE),无明显的Eu异常(δEu=0.81～0.98),无白云母和碱性暗色矿物,属准铝质未分异的I型花岗岩。盐湖石英闪长岩的(~(87)Sr/~(86)Sr)_i值为0.7045～0.7048,ε_(Nd)(t)值介于+0.5～+2.1之间,ε_(Hf)(t)值介于+7.9～+13之间,Hf同位素模式年龄t_(DM2)变化于283～674Ma之间。基于同位素以及岩石地球化学数据,表明盐湖石英闪长岩很可能是具亏损地幔印记的新生下地壳在角闪岩相发生部分熔融作用形成的。结合本文以及区域上的研究资料,盐湖石英闪长岩的形成时代和岩石地球化学特征与盐湖复式岩体中的花岗岩存在较大差异,具较低的锆石饱和温度(596～614℃),表明该岩体的形成尚未受到幔源物质上涌带来的热作用影响,很可能形成于板片断离前,班公湖-怒江特提斯洋持续南向俯冲的板片发生折返,俯冲角度变陡的深俯冲背景。     

扬子地块北缘新元古代望江山层状岩体矿物成分和铂族元素特征:对岩浆演化过程和构造环境的制约

望江山层状岩体位于扬子地块北缘新元古代汉南杂岩带中,岩体从底部到顶部由超镁铁质岩过渡为中性岩:底部主要由辉石岩和橄长岩组成;中部为辉长苏长岩和辉长岩;上部为辉长岩和闪长岩。研究以中部岩相带橄榄辉长苏长岩、辉长苏长岩和辉长岩为对象,通过主要矿物的主微量元素和全岩主微量元素的分析,查明望江山岩体来源于尖晶石二辉橄榄岩组成的大陆下岩石圈地幔,并且地幔源区受到了来自俯冲板片流体的交代,岩体中部带的母岩浆为拉斑玄武质岩浆。钛铁矿—磁铁矿矿物对成分计算表明,母岩浆在形成时具有较高氧逸度。通过单斜辉石压力计得到岩体的侵位深度约为12.9~18 km。对岩体母岩浆橄榄石分离结晶过程的模拟计算表明,中部带橄榄石为母岩浆经过~28%分离结晶的产物。此外,铂族元素（PGE）组成暗示岩体并未经历过大规模的硫化物熔离,可能与缺乏地壳物质混染有关。岩体中单斜辉石与岛弧环境堆晶岩中单斜辉石成分相似,不同于裂谷环境中堆晶单斜辉石的成分;同时,全岩Th/Yb和Nb/Yb比值也与岛弧玄武岩比值相似,因此矿物和全岩成分均说明望江山层状岩体应形成于岛弧环境。研究认为扬子北缘在新元古代长期的俯冲过程中,大洋板片断离导致软流圈上涌,提供热源使交代大陆下岩石圈地幔部分熔融形成具有岛弧特征的镁铁质岩浆,在局部伸展环境中上升侵位形成汉南杂岩带中镁铁—超镁铁质层状岩体。      

扬子西缘小相岭地区苏雄组古火山机构的发现及意义

通过详细的野外地质调查,发现扬子西缘小相岭地区苏雄组火山岩由巨厚的酸性-中酸性熔岩、火山碎屑岩和少量基性熔岩组成,在小相岭山脊两侧首次厘定4处古火山机构。其中,阳糯雪山古火山机构最壮观,是由火山颈相、溢流相、爆发相和火山沉积相构成的完整古火山机构。通过系统的地质剖面研究,发现该古火山机构存在2次喷发亚旋回和15个火山韵律,2次喷发亚旋回之间出现了1次明显的火山间歇期。小相岭地区古火山机构的发现,进一步厘定了苏雄组火山岩的喷发方式和构建特征,对进一步研究苏雄组酸性火山碎屑岩的起源及新元古代陆内裂谷作用具有重要意义。     

Stress Field Estimations for Colima Volcano, Mexico, Based on Seismic Data

 For first time, during 1991, seismic activity was recorded during an eruption at Colima volcano. We analyze these data to obtain a stress pattern using a composite focal mechanism technique. From the analysis of regional seismicity, the Tamazula Fault and the Armeria River appear as active features and the dip of the slab east of the Jalisco Block is approximately 12°. Southwest of Colima volcano a vertical alignment of seismic events was observed. We estimate five different composite focal mechanism solutions from our data set, which indicate a change of the stress field at the volcano after the 1991 eruption. These solutions suggest that the stress field in the volcanic edifice was controlled by stresses related to the emplacement of magma superimposed on the regional stress field. No evidence of active local faults in the volcanic edifice was found. We propose a model for the eruptive process that involves tilting of the volcanic edifice. Received: 15 October 1995 / Accepted: 26 October 1998