Mapping the Dyke Swarms of the Eglab-Yetti Region, Southwestern Algeria

正The Eglab-Yetti region of southwestern Algeria is a main part of the Reguibat shield of the West African craton(WAC)which consists of Archean to Paleoprotzrozoic basement.The region hosts numerous     

Petrogenesis of Early Carboniferous Ultramafic–Mafic Volcanic Rocks in the Southern Changning–Menglian Belt, Southeastern Tibetan Plateau: Implications for the Evolution of the Paleo-Tethyan Ocean

The Changning–Menglian Belt represents the main Paleo-Tethyan Suture in the southeastern Tibetan Plateau, which divides Gondwana- and Eurasia-derived continental fragments from each other. The belt contains ultramafic–mafic volcanic rocks that provide evidence of the tectonic processes which operated during the evolution of the Paleo-Tethyan Ocean. New geochemical data for Early Carboniferous volcanics in the southern Changning–Menglian Belt show that wehrlites have cumulate and poikilitic textures, and contain low-Fo (84.2–87.2) olivine, clinopyroxene with low Mg# values (79.4–85.6), and spinel with high Cr# values (67.8–72.4). Estimated equilibrium temperatures obtained using olivine-spinel Fe-Mg exchange geothermometry range from 978°C to 1373°C (mean = 1205°C; n = 3). These observations combined with a lack of reaction or melt impregnation textures indicate that these units represent magmatic cumulates rather than having formed as a result of mantle-melt reactions. Both wehrlites and basalts in the belt have subparallel rare earth element (REE)-and primitive-mantle-normalized multi-element patterns with slightly positive Nb-Ta anomalies, but negligible Eu and Zr-Hf anomalies. The volcanics have similar Sr-Nd-Pb isotopic compositions with εNd(t) values of 4.2–4.5 (mean = 4.3; n = 3) and 4.0–4.4 (mean 4.2; n = 4), respectively, and also have similar immobile element ratios, such as Nb/La, Nb/U, Th/La, Zr/Nb, Th/Ta, La/Yb, Nb/Th, Nb/Y, and Zr/Y. These characteristics indicate both units have ocean island basalt (OIB)-like geochemical affinities, consistent with the fact that the clinopyroxene in the wehrlites is compositionally similar to OIB-related cumulus clinopyroxene. This suggests that both the wehrlites and basalts were derived from similar parental magmas that underwent generally closed-system magmatic differentiation dominated by fractionation of the olivine and clinopyroxene. This parental magma was likely generated in an oceanic seamount setting from an OIB-type mantle source (i.e., asthenospheric mantle) containing garnet-spinel lherzolite material. Combing this new data with that from oceanic seamount volcano-sedimentary suites derived from previous research enables the identification of a mature late Paleozoic ocean basin between the passive northeastern Gondwanan margin and the northward-migrating microcontinent of Lanping–Simao.     

Genesis of the Cenozoic Sodic Alkaline Basalt in the Xiahe–Tongren Area of the Northeastern Tibetan Plateau and its Continental Dynamic Implications

正Objective The Cenozoic Indo-Asian collision caused significant crustal shortening and plateau uplift in the central Tibet.The extrusion tectonic model has been widely accepted to explain the strike-slip faults around the Tibetan Plateau.Previous studies indicate that the lower crust flow is the     

Metabasic Dyke Swarms in a High-Grade Metamorphic Terrane——A Case Study in the Taipingzhai-Jinchangyu Area,Eastern Hebei Province

Geological and petrological studies indicate that three phases of metabasic dykes are present in theTaipingzhai-Jinchangyu area within the high-grade metamorphic terrane of eastern Hebei. Garnet andhornblende in metabasic dykes of the second and third phases occur separately, forming two mineral aggregateareas gl+cpx+pl±hy and hb+cpx+pl-hy. P_(H_2O) in the rocks appears to be the main factor controlling theformation of the two aggregate areas. Both were formed simultaneously at the same metamorphic temperature. The second-phase basic dykes underwent metamorphism of pyroxene-granulite facies at a temperature ofsome 825C: later the dykes, together with the third-phase basic dykes experienced metamorphism ofamphibole-granulite facies at a temperature of about 750C under pressure of 0.9GPa.     

Lithospheric structure and crust–mantle decoupling in the southeast edge of the Tibetan Plateau

Convergence between the Indian plate and the Eurasian plate has resulted in the uplift of the Tibetan Plateau, and understanding the associated dynamical processes requires investigation of the structures of the crust and the lithosphere of the Tibetan Plateau. Yunnan is located in the southwest edge of the plateau and adjacent to Myanmar to the west. Previous observations have confirmed that there is a sharp transition in mantle anisotropy in this area, as well as clockwise rotations of the surface velocity, surface strain, and fault orientation. We use S receiver functions from 54 permanent broad-band stations to investigate the structures of the crust and the lithosphere beneath Yunnan. The depth of the Moho is found to range from 36 to 40 km beneath southern Yunnan and from 55 to 60 km beneath northwestern Yunnan, with a dramatic variation across latitude 25–26°N. The depth of the lithosphere–asthenosphere boundary (LAB) ranges from 180 km to less than 70 km, also varying abruptly across latitude 25–26°N, which is consistent with the sudden change of the fast S-wave direction (from NW–SE to E–W across 26–28°N). In the north of the transition belt, the lithosphere is driven by asthenospheric flow from Tibet, and the crust and the upper mantle are mechanically coupled and moving southward. Because the northeastward movement of the crust in the Burma micro-plate is absorbed by the right-lateral Sagaing Fault, the crust in Yunnan keeps the original southward movement. However, in the south of the transition belt, the northeastward mantle flow from Myanmar and the southward mantle flow from Tibet interact and evolve into an eastward flow (by momentum conservation) as shown by the structure of the LAB. This resulting mantle flow has a direction different from that of the crustal movement. It is concluded that the Sagaing Fault causes the west boundary condition of the crust to be different from that of the lithospheric mantle, thus leading to crust–mantle decoupling in Yunnan.     

Aeolian desertification and its causes in the Zoige Plateau of China’s Qinghai–Tibetan Plateau

Studies and efforts to control aeolian desertification in China have focused on the arid and semiarid lands in the north. However, the aeolian desertification that is occurring on the high-altitude Qinghai–Tibetan Plateau, which has a cold and humid climate, has received attention only in recent years. In this paper, we report the results of monitoring this aeolian desertification between 1975 and 2005 and of our analysis of its causes on the Zoige Plateau, which is located in the northeastern part of China’s Qinghai–Tibetan Plateau. Aeolian desertified lands expanded at a compound annual rate of 4.07% between 1975 and 2005. They expanded most rapidly between 1975 and 1990, at an annual rate of 7.73%. Factors responsible for this expansion include increasing temperature, decreasing precipitation, over-grazing, drainage of water systems, and land reclamation for agriculture. Increasing temperature, over-grazing, and the drainage of water systems were the key factors. The climatic variations between 1975 and 2005 were not sufficient by themselves to lead to aeolian desertification. Human disturbances such as over-grazing and drainage of water systems must thus have been primarily responsible for the observed changes, and human behavior must be adjusted to control the expansion of aeolian desertification and rehabilitate the desertified lands.     

Genesis of Dolomite from Ma55–Ma510 Sub-members of the Ordovician Majiagou Formation in the Jingxi Area in the Ordos Basin

We clarified three stages of dolomitization and secondary changes by studying the petrology and geochemistry characteristics of dolomite from the Ma55–Ma510 sub-members of the Ordovician Majiagou Formation in the Jingxi area in the Ordos Basin: (1) Syngenetic microbial dolomitization is characterized by formation of dolomite with a mainly micrite structure and horse tooth-shape dolomite cements. (2) Seepage reflux dolomitization during the penecontemporaneous period superposed adjustment functions such as recrystallization and stabilization in the middle-deep burial stage, forming dolomites mainly consisting of micro crystal and powder crystal structure. (3) Powder dolomite, fine dolomite, and medium-coarse crystalline dolomite formed in pores and fractures in the middle-deep burial stage. The secondary concussive transgression-regression under a regressive background is an important condition for the occurrence of many stages of dolomitization in the study area. The basin was an occlusive epicontinental sea environment in the Ma5 member of the Ordovician Majiagou Formation sedimentary period. In the sediments, sulfate content was high, which is conducive to the preservation of microbial activity and microbial dolomitization. Micritic dolomite formed by microbial dolomitization provides good migration pathways for seepage reflux dolomitization. Affected by evaporation seawater with increased Mg/Ca ratio, seepage reflux dolomitization was widely developed and formed large-scale dolomite, and underwater uplifts and slopes are favorable areas for dolomite. In the middle-deep burial stage, dolomitizing fluid in the stratum recrystallized or stabilized the previous dolomite and formed a small amount of euhedral dolomite in the pores and fractures.     

Swarm Theory, Hydraulic Fracturing, and the Emergence of Natural Spacing within Dyke Swarms

正Like all swarming behavior in Nature,the selforganization expressed by the emergence of a natural spacing among a swarm’s members should be considered its most fundamental characteristic,and grasping the     

Distribution,stock, and influencing factors of soil organic carbon in an alpine meadow in the hinterland of the Qinghai–Tibetan Plateau

Understanding the spatial distribution, stocks, and influencing factors of soil organic carbon (SOC) is important for understanding the current situation of SOC in alpine meadow ecosystems on the Qinghai–Tibetan Plateau (QTP). We sampled 23 soil profiles to a depth of 50 cm in a 33.5 hm\(^{2}\) plot in a typical meadow on the central QTP. The distribution, stock and influencing factors of SOC was then analyzed. The mean density of soil carbon content (SOCD) was 2.28 kg m\(^{-2}\) with a range of 5.99 kg  m\(^{-2}\). SOCD in the 0–10 cm layer was 3.94 kg m\(^{-2}\) and decreased quadratically with depth. The total stock of SOC to a depth of 50 cm was ca. 2950 t, the 0–10 and 0–30 cm layers accounting for 38 and 80%, respectively. SOCD varied moderately spatially and was distributed more homogeneously in the 0–10 and 40–50 cm layers but was more variable in the middle three layers. SOCD was significantly correlated positively with soil-water content, total porosity, and silt content and negatively with soil pH, bulk density, stone content and sand content. This study provides an important contribution to understanding the role of alpine meadows in the global carbon cycle. It also provides field data for model simulation and the management of alpine meadow ecosystems.     

Geological Characteristics and Genesis of the Jiamoshan MVT Pb–Zn Deposit in the Sanjiang belt,Tibetan Plateau

The carbonate-hosted Pb–Zn deposits in the Sanjiang metallogenic belt on the Tibetan Plateau are typical of MVT Pb–Zn deposits that form in thrust-fold belts. The Jiamoshan Pb–Zn deposit is located in the Changdu area in the middle part of the Sanjiang belt, and it represents a new style of MVT deposit that was controlled by karst structures in a thrust–fold system. Such a karst-controlled MVT Pb–Zn deposit in thrust settings has not previously been described in detail, and we therefore mapped t...     

The 2405 Ma and 2310 Ma Mafic Dyke Swarms in the Karelian Craton: Age,Chemical and Sr-Nd Isotope Composition,and Tectonic Setting

正Until recently the time period 2400-2200 Ma in the Erath history was characterized by low amount of age data for igneous events,and was even considered as‘magmatic age gap’(Condie et al,2009;Eriksson and Condie,2014).     

Mapping the Dyke Swarms of the Eastern Desert,Egypt

正The Eastern Desert of Egypt hosts numerous undeformed to slightly deformed mafic dyke swarms which have previously been poorly characterized.Systematic use of full resolution Google Earth?images yields an initial     

藏北羌塘地区地层新资料

新的牙形石化石资料证实在羌塘北部地区有泥盆纪查桑组地层，它是该盆地保存的最古老海相沉积地层，羌塘地区中西部广大地区上侏罗统索瓦组顶部产有丰富的以Virgatosphinctes和Aulacosphinctes两属为主的菊石化石，可初步建立5个菊石组合，其中Berriasella和Blanfordiceras菊石的出现使最高海相层位上延至提塘阶顶部或贝利阿斯阶，而圆笠虫(Orbitolina)、似异卷虫（Heterohelix）出现可能反映有海相白垩纪地层的存在，在晚三叠世肖茶卡组中发现有Epigondolella牙形石动物群，这是我国晚三叠世最高位牙形石带又一产地，首次在双湖比隆组油页岩剖面顶部发现产Harploceras sp.菊石化石的层位，时代属早上托阿尔斯（Toarcian）。解决了这一特殊沉积地层单位长期争论的时代归属问题，并指出它与早侏罗世海侵高峰期全球缺氧事件有关。     

藏南基性岩墙群的地球化学特征

藏南江孜—哲古一带广泛分布有北西向和近东西向产出的基性—中基性岩墙群,它们是新特提斯洋晚期发生大规模扩张的产物。本文对这些基性岩脉进行了较为系统的元素地球化学和Pb-Sr-Nd同位素研究。元素地球化学研究表明,该岩体略为富集LREE,Eu的亏损不明显,表明岩浆结晶分异作用较弱;LILE的富集和HFSE的亏损均不明显,所有样品均以相对富集Ta、Ce和亏损La、Y和Yb为特点。铅同位素206Pb/204Pb比值和207Pb/204Pb比值变化范围较小,分别为18.330～18.717和15.504～15.674;而208Pb/204Pb值和μ值相对变化较大,分别为37.664～39.156和9.296～9.931;初始87Sr/86Sr值变化范围较大,为0.7044～0.7105;岩石的εNd(t)值变化范围为-4.49～ 6.77,绝大多数为正值。微量元素地球化学和Nd-Sr-Pb同位素研究结果表明,藏南地区基性岩墙群可能是由来自亏损地幔源的岩浆与来自富集地幔源(EM)的岩浆混合的产物,其快速侵位造成岩浆分异作用较差,而岩浆的不均匀混合导致LILE富集与HFSE亏损均不明显,并产生较大的初始87Sr/86Sr比...     

鲁西前寒武纪基性岩墙群

鲁西地区发育大规模的中元古代基性岩墙群,侵位于早前寒武纪结晶基底内,未变形未变质,以北北西向和近南北向为主,是鲁西前寒武纪最引人注目的一种伸展构造标志。鲁西基性岩墙群属于板内大陆裂谷拉斑玄武岩系列,与板内大陆裂谷活动密切相关,代表一种非造山岩浆活动。该岩墙群富集大离子亲石元素(L IL E) ,略亏损高场强元素(HFSE) ,比较富集Cr、Ni,明显亏损Th。稀土元素配分模式为轻稀土略富集的右倾曲线。鲁西基性岩墙群的微量元素和稀土元素特征反映源区为略富集的地幔。从时空分布特征、构造特征、岩石化学相关性、岩浆源区和古构造应力场分析,鲁西基性岩墙群与燕辽—中条拗拉槽系有一定的成生联系。     

青藏高原东北缘循化-化隆地区新生代沉积古地理演化

通过详细分析和对比循化-化隆盆地及邻区新生代沉积序列,将研究区新生代沉积古地理演化划分为3个阶段:(1)渐新世,西秦岭逆冲带、拉脊山逆冲带抬升,循化-化隆盆地压陷下沉,接受以砾岩夹砂岩为主的洪冲积扇和山区河流相沉积,称他拉组。他拉组古流向主要为北东向及北北西向,与盆地南缘的西秦岭北缘隆起带基本垂直或平行,西北边缘古流向为南东东向。(2)中新世—上新世早期,经过渐新世洪冲积填平,很快进入河流三角洲充填阶段,并在中新世早期进入湖泊环境接受沉积。中新世是循化-化隆盆地、临夏盆地、定西-兰州盆地湖盆面积扩展最大期,循化-化隆盆地往东与临夏盆地连通,总体上构成向东张开的大型压陷盆地。这一时期,盆地多以湖泊和湖相三角洲、冲洪积平原相沉积为主。盆地在循化、尖扎、化隆等地发育大面积咸水湖相沉积。盆地四周发育冲洪积扇和冲洪积平原相沉积,在同仁往北、民和往南发育河流相、三角洲相沉积。(3)上新世3.6 Ma左右,为临夏组之上的积石组砾岩发育期。由于周缘山系强烈逆冲与盆地整体隆升,导致积石组与下伏临夏组何王家段之间发育沉积间断与不整面,区域上同仁盆地、甘加盆地、桑科盆地和定西-兰州盆地全部结束沉积。3.6 Ma以后盆地内分布的上新世积石组巨砾岩是该时期周缘山系进一步隆升的沉积响应。循化-化隆盆地与临夏盆地被积石山隆起带分割,盆地主要物源为拉脊山隆起带和西秦岭隆起带。盆地在循化县尕楞乡宗务一带发育的积石组冲洪积扇砾岩堆积最厚。     

晚新生代以来青藏高原中部环境演化过程的地球化学证据

青藏高原中部发育一系列独立的内流断陷湖盆，其中位于南亚季风区边缘的错鄂湖盆发育了巨厚的湖泊沉积，是研究高原内部地质环境变迁与构造-气候旋回的理想场所。本文对青藏高原中部错鄂孔197m沉积岩心Sr、Rb、Zr含量及其比值以及沉积物有机质δ^13C、TOC含量进行了分析，同时结合岩性、粒度、粘土矿物组成，初步建立了青藏高原中部2．8Ma来岩石化学风化的相对强度和古环境演化过程。研究表明，2．8Ma来青藏高原中部经历了三次剧烈的环境演化过程：在约197～170m(2．8～2．5Ma)之间，低Sr含量，高Rb／Sr和Zr／Rb值记录了较弱的化学风化过程，δ^13C、TOC含量反映气候前期暖干，后期偏凉湿，揭示冷湿气候的云杉林突然增加，沉积岩性明显变粗，反映了一次强烈的构造隆升运动；在约170～38．5m(约2．5～0．8Ma)之间，高Sr含量，低Rb／Sr和Zr／Rb值记录流域经历较强的化学风化过程，δ^13C、TOC含量反映高原中部总体处于温湿或凉湿的环境下，沉积岩性较细，高原总体处于相对剥蚀夷平阶段；在38．5～0m(约0．8～0Ma)段，Sr含量处于相对低值，Rb／Sr值相对较高，化学风化相对较弱，δ^13C、TOC含量表明高原中部处于较寒冷的环境之中，沉积岩性的再次明显变粗表明高原中部又经历了一次强烈的构造运动，并使高原中部整体隆升到4000m以上，奠定了现代高原面的基本格局。