自生伊利石40Ar/39Ar法定年技术及气藏成藏期的确定

为了确定天然气藏的成藏期,进行了自生伊利石的^40Ar／^39Ar法同位素定年实验．实验重点解决了自生伊利石的^40Ar／^39Ar法定年的几个技术难题：第一,粘土矿物的提纯,避免伊利石以外的含K矿物混入;第二,自生伊利石与碎屑伊利石的分离;第三,克服核反冲造成的Ar原子丢失．利用冷冻一加热循环碎样技术获得高纯度的粘土矿物;通过阶段加热得到的年龄谱可以区分自生伊利石与碎屑伊利石;利用“显微包裹”技术有效克服了核照射反冲问题．对鄂尔多斯盆地北部苏里格气田研究发现。二叠系储层中的伊利石有2种年龄图谱：一种只有自生伊利石的坪年龄;另一种图谱既有自生伊利石的坪年龄,也有碎屑伊利石的年龄,形成二阶式的图谱．通过自生伊利石的形成时间推断,天然气的最早充注时间晚于169～189Ma．实验的结果表明,冷冻加热循环碎样技术可以有效地避免伊利石以外的含K矿物混入．是获得高纯度粘土矿物的关键技术;自生伊利石^40Ar／^39Ar法定年技术可以用于确定天然气藏的成藏期.     

吕梁—晋北地区晚前寒武纪镁铁质岩墙群侵位方式的磁组构证据

华北克拉通区内的吕梁-晋北地区广泛发育晚前寒武纪镁铁质岩墙群。该地区近EW向和NW-NNW向岩墙的磁组构测量结果表明,岩墙群磁各向异性与岩墙侵位的关系密切,磁化率的长轴能指示其岩浆侵位的流向。该区的岩墙群的岩浆源位于东侧的燕辽拗拉槽处,岩浆沿岩墙走向以一定仰角由ESE(或E)朝NW-NNW(或W)向流动,具板内裂谷模式。     

辽宁瓦房店金刚石矿田金伯利岩侵位机制分析

辽宁省瓦房店金刚石矿田位于华北陆块辽东新元古代-古生代坳陷带。区内各时代地层均有出露,其中新元古界出露面积最大。区内断裂构造发育,较大的有北北东向的金州断裂,已发现的金伯利岩体基本上分布在该断裂以西。矿田内金刚石矿均为金伯利岩型,已发现100多个金伯利岩体,划分成4条矿带,已提交4个大型原生金刚石矿床和3个近源小型金刚石砂矿床,资源量占全国的一半以上,是我国重要的金刚石矿集区,其中50号金伯利岩管因其出产的金刚石质量优越而在宝石界享有盛誉。但本区的金伯利岩绝大部分是20世纪70年代、80年代发现的,为了更好地开展金刚石勘查工作,对本区金伯利岩的成矿条件和控矿因素进行了研究,金伯利岩体的平面分布位置表明,瓦房店地区的金伯利岩体成群、成带分布,既有岩管也有岩脉,以岩脉为主,岩管约占20%左右,岩体大小不等、形态各异,钻孔控制的岩管、岩脉大多具有向下延伸突然中断的特征,钻孔中见到的金伯利岩显示,很多金伯利岩管底界平直或具有多个水平标高上出现平移错动的现象,典型岩管、岩脉与等轴或近等轴状构造盆地的关系密切。通过对区内金伯利体岩空间分布特征、岩体形态特征进行分析,并探讨了本区金伯利岩的侵位过程和...     

海沟岩体的成岩时代及成因探讨

本文在研究海沟岩体地质、岩石化学、地球化学特征的基础上,对海沟岩体的成岩时代和成因进行了探讨。同位素结果表明,海沟岩体的就位年龄为167Ma-185.6Ma,属燕山早期第一阶段岩浆活动的产物;岩体稳定同位素特征分析、侵位特征以及稀土元素特征的研究,确定海沟岩体为幔、壳混源型。     

Chandrayaan-1 observation of distant secondary craters of Copernicus exhibiting central mound morphology: Evidence for low velocity clustered impacts on the Moon

Analysis of the Chandrayaan-1 Terrain Mapping Camera image of a 20 km×27 km area in the Mare Imbrium region revealed a cluster of thousands of fresh and buried impact craters in the size range of 20-1300 m. A majority of the large fresh craters with diameter ranging from 160 to 1270 m exhibit near-circular mounds (30-335 m diameter and 10-40 m height) in the crater floor, and their size depends on the host crater size. The origin of this cluster of secondary craters may be traced to Copernicus crater, based on global lunar image and the analysis of Chandrayaan-1 Hyper Spectral Imager data. Our findings provide further evidence for secondary crater formation by low-velocity impact of a cloud of clustered fragments. The presence of central mounds can also distinguish the secondary craters from the primary craters and refine the chronology of lunar surface based on counting of small craters.     

Geochemistry of magmatic rocks and time constraints on deformational phases and shear zone slip in the Méiganga area,central Cameroon

The Méiganga area is situated in the Adamawa–Yadé domain (AYD) of the Pan-African fold belt in Cameroon. The AYD is characterized by abundant plutonic rocks that intruded Palaeoproterozoic gneisses. It is cut by the transcurrent Central Cameroonian Shear Zone (CCSZ). The studied area is made up of metadiorite (MD), pyroxene-bearing granite (PGr) and biotite-muscovite granite (BMGr), hosted in a metasedimentary sequence of amphibole-biotite gneisses. The granitoids are metaluminous to slightly peraluminous, and mainly of I-type (ASI ≤ 1.1), representing a high-K calc-alkaline to shoshonitic suite. They were derived from crustal protoliths (BMGr), from rocks of mantle origin (MD, PGr), and/or from the remelting of metabasalt or metatonalite (MD, PGr). Four successive deformational phases (D1, D2, D3, and D4) are present in the Méiganga area. The S1 foliation is formed by tectonic transposition of the S0 primitive surface represented by contacts between the gneiss and intercalated amphibolites. The synmigmatitic D2 deformational phase is characterized by S2 banded schistosity, S2/C2 sinistral shear planes, and F2 folds with axes parallel to a L2 stretching lineation. Imprints of the D2 and subsequent deformational phases are similar in the metadiorite and host rocks, implying syn-D2 emplacement and crystallization of the metadiorite; therefore 614–619 Ma ²⁰⁷Pb/²⁰⁶Pb zircon evaporation ages obtained for this rock type date the syn-D2 magmagenesis. Similarly, the D3 phase of the PGr is 601 ± 1 Ma, dated by the ²⁰⁷Pb/²⁰⁶Pb evaporation method. D4 is a late-stage brittle deformational phase. Sinistral movement of the CCSZ is associated with D2, whereas its latest activity, characterized by dextral slip, cannot be older than emplacement of the 558 ± 2 Ma BMGr (²⁰⁷Pb/²⁰⁶Pb zircon evaporation age).     

Physical volcanology and geochemistry of Palaeoarchaean komatiite lava flows from the western Dharwar craton,southern India: implications for Archaean mantle evolution and crustal growth

ABSTRACT

Palaeoarchaean (3.38–3.35 Ga) komatiites from the Jayachamaraja Pura (J.C. Pura) and Banasandra greenstone belts of the western Dharwar craton, southern India were erupted as submarine lava flows. These high-temperature (1450–1550°C), low-viscosity lavas produced thick, massive, polygonal jointed sheet flows with sporadic flow top breccias. Thick olivine cumulate zones within differentiated komatiites suggest channel/conduit facies. Compound, undifferentiated flow fields developed marginal-lobate thin flows with several spinifex-textured lobes. Individual lobes experienced two distinct vesiculation episodes and grew by inflation. Occasionally komatiite flows form pillows and quench fragmented hyaloclastites. J.C. Pura komatiite lavas represent massive coherent facies with minor channel facies, whilst the Bansandra komatiites correspond to compound flow fields interspersed with pillow facies. The komatiites are metamorphosed to greenschist facies and consist of serpentine-talc ± carbonate, actinolite–tremolite with remnants of primary olivine, chromite, and pyroxene. The majority of the studied samples are komatiites (22.46–42.41 wt.% MgO) whilst a few are komatiitic basalts (12.94–16.18 wt.% MgO) extending into basaltic (7.71 – 10.80 wt.% MgO) composition. The studied komatiites are Al-depleted Barberton type whilst komatiite basalts belong to the Al-undepleted Munro type. Trace element data suggest variable fractionation of garnet, olivine, pyroxene, and chromite. Incompatible element ratios (Nb/Th, Nb/U, Zr/Y Nb/Y) show that the komatiites were derived from heterogeneous sources ranging from depleted to primitive mantle. CaO/Al₂O₃ and (Gd/Yb)_N ratios show that the Al-depleted komatiite magmas were generated at great depth (350–400 km) by 40–50% partial melting of deep mantle with or without garnet (majorite?) in residue whilst komatiite basalts and basalts were generated at shallow depth in an ascending plume. The widespread Palaeoarchaean deep depleted mantle-derived komatiite volcanism and sub-contemporaneous TTG accretion implies a major earlier episode of mantle differentiation and crustal growth during ca. 3.6–3.8 Ga.     

Deformation history and timing of granite emplacement in the Butchers Hill — Helenvale region,northern Hodgkinson Province,Queensland

Granite plutons of the Whypalla Supersuite in the Butchers Hill — Helenvale region of north Queensland were intruded into the upper crust of the Hodgkinson Formation during contractional deformation associated with the Permian‐Triassic Hunter‐Bowen Orogeny. A four‐stage structural history has been resolved for the area, with fabric overprinting relationships, porphyroblast‐matrix microstructural geometries and isotopic ages being consistent with granite emplacement during D₄ shortening at ca 274 Ma. Microstructural relationships suggest the possibility of a minor syn‐D₃ phase of granite emplacement. The deformation‐emplacement history of the Butchers Hill — Helenvale area is consistent with that recognised regionally for the Hodgkinson Province, indicating province‐wide synchronous syntectonic granite intrusion during a major phase of contractional deformation. Intense syn‐emplacement deformation partitioning was ongoing in the country rocks during progressive D₄ and was associated with upward translation of country rock from the microscale to the macroscale along D₄ cleavages and shears. Kinematic indicators show that this progressive uplift, at the scale of the area examined, was east‐side‐up.     

Highly retentive core domains in K-feldspar and their implications for 40Ar/39Ar thermochronology illustrated by determining the cooling curve for the Capoas Granite,Palawan, The Philippines

K-feldspar from the late Miocene Capoas Granite on Palawan in The Philippines appears to contain highly retentive diffusion domains that are closed to argon diffusion at near-solidus temperatures during cooling of this ～7 km-diameter pluton. This is an important result, for K-feldspar is commonly considered not retentive in terms of its ability to retain argon. Closure temperatures for argon diffusion in K-feldspars are routinely claimed to be in the range ～150–400°C but the release of ³⁹Ar from irradiated K-feldspar during furnace step-heating experiments in vacuo yields Arrhenius data that imply the existence of highly retentive core domains, with inferred closure temperatures that can exceed ～500–700°C. These high closure temperatures from the Capoas Granite K-feldspar are consistent with the coincidence of ⁴⁰Ar/³⁹Ar ages with U–Pb zircon ages at ca 13.5 ± 0.2 Ma. The cooling rate then accelerated, but the rate of change had considerably slowed by ca 12 Ma. Low-temperature (U–Th)/He thermochronology shows that the cooling rate once again accelerated at ca 11 Ma, perhaps owing to renewed tectonic activity.     

The upper Hawasina nappes in the central Oman Mountains: stratigraphy,palinspastics and sequence of nappe emplacement

Abstract

In the Oman mountains, a succession of sedimentary decollement nappes, the Hawasina nappes, is sandwiched between the Samail ophiolite nappe and its underlying melange and the “autochthonous” sequences of the Arabian platform. The sediments of the Hawasina nappes document the Mesozoic evolution of the northeastern Arabian continental margin and the adjacent Tethys Ocean. In earlier paleogeographic reconstructions, based on simple telescoping of the tectonic units, the upper Hawasina nappes represent the distal part and the lower nappes the proximal part of the margin. New stratigraphic data suggest a revision of the paleogeography and a more complex model for nappe emplacement in the central Oman mountains. The lower Hawasina nappes with their Jurassic and Cretaceous base of slope and basin sediments (Hamrat Duru, Wahrah) form the original cover of part of the upper Hawasina nappes. In the latter (Al Ayn, Haliw), Triassic pelagic sediments, locally overlain by massive sandstone successions are preserved. Complete Mesozoic sequences with pelagic Cenomanian sediments as youngest dated elements are found in the highest Hawasina units (Al Aridh and Oman Exotics). The stratigraphic data indicate polyphase thrusting in the central Oman mountains. Downward propagation of thrusting in front of the Samail is responsible for cutting the original stratigraphie sequence into a number of thrust-sheets, involving successively older and more external formations. This kind of thrust propagation eventually leads to the observed superposition of originally lower stratigraphie units onto their original cover. Regional deformation of the nappe contacts in post-nappe culminations (J. Akhdar, Saih Hatat) is related to ramp-flat-systems in the Arabian foreland.