Final Closure Time of the Paleo‐Asian Ocean: Implication from the Provenance Transformation from the Yangjiagou Formation to Lujiatun Formation in the Jiutai Area,NE China

The Jiutai area is tectonically situated at the eastern segment of the Central Asian Orogenic Belt(CAOB) and is close to the North China Craton(NCC) to the south, serving as an ideal place to investigations of the closure of the PaleoAsian Ocean(PAO). Sandstone samples collected from the Yangjiagou Formation and the Lujiatun Formation in this area have been studied in detail in terms of petrology, geochronology and geochemistry. The maximum depositional time of the Yangjiagou and Lujiatun formations has been constrained to early Middle Triassic(ca. 245 Ma) and middle Late Triassic(ca. 219 Ma), respectively. The Yangjiagou Formation, with a major provenance of dissected island arcs, is dominantly composed of Phanerozoic sediments from Northeastern China(NE China) massifs. The Lujiatun Formation, with major sediments from active continental margins, has a relatively larger proportion of Precambrian sediments, in which the ～1.85 Ga and ～2.5 Ga sediments are typical of the crystalline basements of the NCC and NE China massifs, which were uplifted and eroded during the closure of the PAO. Besides, both formations show the enrichment in LREEs and the depletion in HREEs, the common Eu negative anomalies, and trace element contents similar to that of the upper continental crust. Based on the provenance analysis of these two formations, the final closure time of the PAO in this area is constrained as from the early Middle Triassic(ca. 245 Ma) to the middle Late Triassic(ca. 219 Ma).     

Petrogenesis and PGE Abundances of High‐Cr and High‐Al Chromitites and Peridotites from the Bulqiza Ultramafic Massif,Eastern Mirdita Ophiolite,Albania

The Bulqiza ultmafic massif, which belongs to the eastern Mirdita ophiolite of northern Albania, is world renowned for its high‐Cr chromite resource. The high‐Cr chromitites commonly host in the mantle section, while high‐Al chromitites also present in massive dunite of the mantle‐crust transition zone (MTZ) in this massif. Chromian‐spinel in the MTZ high‐Al chromitites and MTZ dunites have much lower Cr# values [Cr/(Cr+Al)×100] (47.7–55.1 and 46.5–51.7, respectively) than those of chromian‐spinel in the high‐Cr chromitites (78.2–80.4), harzburgites (72.6–77.9) and mantle dunites (79.4–84.3). The high‐Cr chromitites are rich in IPGE relative to PPGE with 0.10–0.45 PPGE/IPGE ratios, whereas the high‐Al chromitites have higher PPGE/IPGE ratios between 1.20 and 7.80. The partial melting degrees of parental magmas for the high‐Cr chromitites are beyond the critical interval (> 25%) and thus prevented sulfide saturation and diluted Pt and Pd in melts, producing high‐Cr chromitites barren of Pt and Pd. However, the degrees for the high‐Al chromitites just enter the critical interval (20–25%) for the effective extraction of PGE from mantle sulfides, which may account for the enrichments of PPGE in high‐Al chromitites. The parental melts of the high‐Cr chromitites have Al₂O₃ and TiO₂ contents of ～10.6–11.4 wt.% and 0.14–0.31 wt.%, whereas the calculated Al₂O₃ and TiO₂ for the high‐Al chromitites are ～14.9–15.9 wt.% and 0.07–0.61 wt.%, respectively. The calculated melts in equilibrium with the high‐Cr chromitites are boninitic‐like, and those with high‐Al chromitites are MORB‐like but with hydrous, oxidized and TiO₂‐poor affinities. To make a compromise between the inconsistence above, we proposed that coexistence of both types of chromitites in the Bulqiza ultramafic massif may reflect that their magma compositions transited from MORB‐like to boninitic‐like in a proto‐forearc setting during subduction initiation. Key words: Chromian‐spinel, Platinum‐group elements, high‐Cr and high‐Al chromitite, Mirdita ophiolite, Albania.     

阿尔金南缘阿克提山岩体锆石U- Pb定年、Hf同位素特征及构造意义

阿克提山花岗岩体位于阿尔金断裂南缘,其形成时代与区域内其他花岗岩体差异较大,该岩体走向明显地受阿尔金断裂带分支断裂控制。锆石LA-ICP-MS U-Pb年代学分析表明,该岩体结晶年龄约为262Ma,其形成可能与南阿尔金地区在华力西期-印支期发生的大规模线性构造运动有关。岩体的岩性主要为石英闪长岩、花岗闪长岩,其暗色矿物以角闪石、黑云母为主,岩石A/CNK值均小于1.1,显示准铝质—弱过铝质特征,属于高钾钙碱性系列;大离子亲石元素Rb、Th、K相对富集,高场强元素Nb、Ta、P、Sr、Ti明显呈负异常;稀土元素配分曲线具有中等负Eu异常,δEu的平均值为0.73,(La/Yb)_N平均值为14.57,说明该花岗岩体岩浆部分熔融程度较高。根据岩石学及地球化学特征可判断该岩体为I型花岗岩。锆石Lu-Hf同位素分析表明,锆石ε_(Hf)(t)值为+1.46～+9.14,均为正值,二阶段模式年龄的峰值平均为943Ma,表明其源岩主要为新元古代新生地壳物质的部分熔融。阿尔金断裂在印支期的走滑运动是从地壳深部的韧性变形开始的,随后在浅地表发生脆性断裂形成大规模走滑断裂带。阿克提山岩体的形成与地壳深部减压熔融有关。     

内蒙古阿鲁科尔沁旗林西组泥岩地球化学特征及构造意义

阿鲁科尔沁旗陶海营子剖面以灰、灰黑、黄绿色的砂板岩组合为特征,是林西组典型剖面之一,含双壳、叶肢介、植物和孢粉化石,结合碎屑锆石年龄,时代为晚二叠世晚期。本次所研究的林西组样品以暗色泥岩为主,主量元素平均含量SiO_2=62.90%,Al_2O_3=16.85%,MgO=1.78%,CaO=1.01%,Na_2O=1.64%,K_2O=4.0%,K_2O/Na_2O为1.10～5.66;A1_2O_3/(CaO+Na_2O)比值为3.57～12.69;稀土元素ΣREE介于139.85×10~(-6)～231.65×10~(-6)之间,平均值为190.74×10~(-6),δEu值介于0.49～0.87之间,平均值为0.61,表现为铕的负异常,Ce介于0.91×10~(-6)～0.96×10~(-6)之间,为负异常,LREE富集,HREE亏损,微量元素以亏损Nb、Ta、Sr,富集Rb、Ba、La、Ce、Pb、Nd、Sm为特征。以上分析表明林西组形成于靠近大陆岛弧的活动大陆边缘构造背景。依据碎屑锆石年龄谱值信息,林西组沉积物源具多样性和复杂性,除东北各地块,同时存在华北板块和西伯利亚板块的物源信息,说明林西组沉积时期华北板块与西伯利亚板块可能已经开始俯冲过程。     

蒸发岩中硫同位素的地球化学特征及其沉积学意义——以思茅盆地MZK- 3井为例

思茅盆地目前是中国唯一的含古代固体钾盐矿床的沉积盆地,其钾盐形成时代、物源特征、海侵方向等多年来一直存在争议。本文依据海相硬石膏的形成条件、存在形式、同位素分馏机理,重点分析了盆地内MZK-3井蒸发岩硫同位素的地球化学特征。结果表明:①岩盐中的硬石膏在蒸发盆地析岩盐阶段即可形成,单独成层的硬石膏是由原始沉积的石膏经历了沉积埋藏升温进而脱水而成;②岩盐中硬石膏的硫同位素值具有"双峰"特征,分别为14‰～16‰和8‰～10‰或6‰～8‰,这体现了硫酸盐的双重来源——原始海水中的硫酸盐和陆源淡水输入的硫酸盐或火山活动提供的硫源;③硬石膏层的硫同位素在区域上具有对比性,结合~(87)Sr/~(86)Sr值的特征,认为其代表了海相的沉积环境;④硬石膏层的硫同位素值平面上由南向北降低,可能反映了在此方向陆源淡水或火山活动对蒸发岩盆地的影响逐渐增强,进而说明这可能也是海侵的方向。可见对硬石膏硫同位素的研究,不仅在沉积学上能揭示物源、沉积环境、海侵方向等信息,更能对研究区钾盐矿床勘查提供有益的参考。     

基于集对分析法的喀斯特地区水资源安全动态变化及原因分析——以贵州省为例

根据中国西南喀斯特地区特殊的水文水资源特点,从水资源赋存、供水状况、用水状况、水资源利用效率、水资源管理等五个方面构建了水资源安全评价指标体系,选择SPA（集对分析法）对贵州省2001-2015年水资源安全状况进行了动态评价。结果表明:贵州省2001-2007年水资源处于临界安全状态,2008-2015年水资源整体处于安全和较安全等级（除2009干旱年份外）。从影响水资源安全的5大要素类的变化看,水资源赋存受自然环境因素影响显著,H2值较不稳定;供水状况因受水利工程建设的影响,2001-2015年H2值由0.53上升到0.90;用水状况因生产用水和生态环境用水的提高,H3值由2001年的0.87下降为2015年0.20;水资源利用效率因万元GDP用水量以及农田亩均灌溉用水量的显著下降,H2值由2001年的0.26增加到2015年的0.88,向安全和较安全等级转变明显;水资源管理受生活污水处理率、森林覆盖率显著提高的影响,H2值由2001年的0.09增加到2015年的0.58,由原先的极不安全等级提升到临界安全等级,但在某些方面,如万元GDP污染物减排率等仍有较大的提升空间;供水状况、水资源利用效率、水资源管理这几个方面均表明人类行为对水资源安全影响显著。      

西藏班戈县保吉乡嘎热扎地中——晚三叠世珊瑚化石的发现及意义

在西藏班戈县保吉乡嘎热扎地发现了一套碎屑岩和灰岩互层的地层,产珊瑚和苔藓虫化石。珊瑚化石共计7属3种及4未定种,分别为Procyclolites zizaensis Deng&Zhang,Chondrocoenia?sp.,Parastraeomorpha?sp.,Pamiroseris?sp.,Astraeomorpha multisepta Melnikova,Volzeia sublaevis(Laube&Volz),Radiophyllia?sp.,时代为中-晚三叠世。该套地层的发现表明,措勤盆地存在中-晚三叠世地层,为该地区的古地理研究提供了地层古生物学证据。     

基于L1范数的波阻抗反演

提出了一种新的波阻抗反演方法,首先根据地下反射系数序列是一个稀疏脉冲序列这一假设,将地震波阻抗反演中的目标函数表示为L1范数约束下的L1范数优化问题,并将L1范数用一个平滑函数来近似,从而使得目标函数函数可微。然后将L1范数约束下的基追踪问题转化为无约束最优化问题,并利用基于导数的局部优化方法求解无约束优化问题,求得反射系数,进而计算得到波阻抗。模型和实际资料计算结果均表明该方法可行。