Remagnetization of the Early Paleozoic rocks from southern part of the Huabei basin (II)

The opinions vary on the secondary remagnetization mechanisms of carbonate rocks. The prevalent interpretations of magnetic remanences are either chemical in origin that invoked the large-scale migration of orogenic fluids. or thermoviscous magnetization involved burial diagenetic processes. The paleomagnetic study carried out in northwestern part of Henan Province (north of the Qinling orogenic belt) reveals that 90% of carbonate rocks have suffered from the secondary remagnetization. On the basis of rock magnetic experiments, the assemblage of the magnetic minerals, remanent carriers and its formation process, and the possible mechanisms of remagnetization in these carbonate rocks are discussed.     

Remagnetization in tertiary rocks from the Faeroe Islands

Palaeomagnetic investigations of highly oxidized interbasaltic horizons and associated Lower Tertiary lavas from the Faeroe Islads suggest a long history of magnetization. The remanence directions are composed mostly of two deviating directions of opposite polarities aligned along an axis which palaeomagnetically appears to be younger than that of the original field. This, in addition to the observation that both the baking and baked rocks in most cases show almost identical directional trends upon demagnetization, questions the general validity of the baked contact test. It is suggested that to a major extent post-baking processes, such as low-temperature oxidation and precipitation of ferric oxides, have remagnetized the lavas as well as the tuff layers. Only two of the seven interbasaltic horizons investigated appear to have retained the ‘primary’ magnetization. It is concluded that besides a high bulk oxidation state (induced prior to, or as a result of baking) a “hornfelsic” nature of the baked sediments may be a necessary requirement for establishing primary palaeomagnetic directions in many volcanic regions.     

Early Paleozoic basement diorite of arc-magmatism from Kutch basin,Western India

In this study, we report for the first time an Early Palaeozoic basement diorite from the drilled well Nirona-A in the Banni Half-Graben of the Kutch basin, western India. The ⁴⁰Ar–³⁹Ar dates provided a plateau age of 441.84 ± 2.66 Ma and another pseudo plateau of 441.28 ± 5.82 to 388.08 ± 16.65 Ma for the basement diorite. These ages constrain the basement formation age to the Late Ordovician-Early Silurian period. The obtained basement ages are correlatable with the later...     

Rock magnetic study of the Early Paleozoic limestone from northern part of Henan Province

Rock magnetic study was carried out on the samples of non-remagnetized Lower Paleozoic limestone from Henan Province. These studies suggest that the properties of the natural remanent magnetization are usually dependent on the grain size of magnetic carrier, e.g. the grain size of magnetite mineral (multidomain or single domain). The different contents of the single and multi-domain magnetites could cause the different levels of overlapping of the middle-high and high temperature components. This may be the reason for “easy” or “difficult” separation of magnetic components.     

四川盆地震旦系-下古生界烃源岩热演化模式及主控因素

烃源岩热演化是含油气盆地烃源岩评价的基本内容之一,也是油气动态成藏研究的基础.通过系统分析地层沉积样式,结合盆地热史恢复结果,应用Easy%Ro化学动力学模型,模拟了四川盆地86口代表性钻井和200余口人工井点震旦系-下古生界烃源岩热演化史.结果表明,在盆地不同构造单元,下寒武统和下志留统烃源岩热演化特征存在明显差异,并据此建立了四种热演化模式:①加里东期成熟,早晚二叠世期间快速演化定型,以川南地区下寒武统烃源岩为代表;②加里东期未熟,早晚二叠世期间一次快速演化定型,以川西南下寒武统和川南下志留统烃源岩为代表;③加里东期成熟,晚海西-燕山期再次增熟,以川东、川北地区下寒武统烃源岩为代表;④加里东期未熟,晚海西-燕山期持续增熟,以川中地区下寒武统和川东、川北下志留统烃源岩为代表.通过对比研究沉积速率、热流和增温速率之间的耦合关系,剖析了四川盆地震旦系-下古生界烃源岩热演化的控制因素,即川西南和川南局部地区主要受控于早晚二叠世期间峨眉山地幔柱和玄武岩的异常热效应,而盆地其它地区则受沉积地层埋深增热和盆地热流演化的共同作用,其中沉积地层埋深增热对烃源岩增温效应更加显著.     

Geoelectromagnetic measurements across the southern Senegal basin (West Africa)

Magnetotelluric and differential geomagnetic sounding surveys, consisting of nine soundings, were performed in 1984 along a 200-km profile across the southern Senegal basin. They were intended to obtain information concerning the resistivity structure of the crust and upper mantle and the distribution of the induced electric currents. Magnetotelluric data indicate that two-dimensional resistivity models are appropriate for the region. The zone above the basement is highly inhomogeneous in geoelectrical structure. Very conductive sediments (0.6-3 ohm m) appear in the Mesozoic-Cenozoic Senegal basin. These sediments lie at depths of up to 4500 m on the west end of the profile. Below this, a modest resistivity material (10–30 ohm m) extends to a maximum depth of about 3000 m. The material at depth on the cast part of the traverse line is thought to be Palaeozoic sediments of the Bove basin. The depth of the magnetotelluric basement lies between about 250 m (in the east) and 4800 m (in the west). The crust is characterised by a drop in electrical resistivity at a depth of 15 km below the east part of the profile. Considering the total section, we observe a general trend towards lower resistivities at depths in excess of 100 km, the transition from 2000 ohm m to about 2 ohm m occurs in the depth range 100 to 175 km. An analysis of the geomagnetic variation field has identified a concentration of telluric current flow beneath the deep basin. It appears that the additional currents flowing in the striking direction of the Senegal basin are largely controlled by sedimentary rocks of high conductivity lying at depths less than 5 km. Model studies show that the local conductivity distribution is able to explain the currents circulating in the thick well-conducting sediments.     

Paleomagnetism of Devonian and Carboniferous sedimentary rocks of Spitsbergen: to the Paleozoic history of the Barents-Kara basin framing

We present the results of studying the paleomagnetic samples of Carboniferous and Devonian rocks collected in 1986–1987 from the Island of Spitsbergen. The paleomagnetic poles determined from these collections are compared to the poles from the coeval rocks of the Russian Platform. On the basis of these results and the available data of the World paleomagnetic database, the paleogeographic layout of Spitsbergen and the Russian Platform in Early Devonian and Early Triassic are reconstructed. The reconstructions demonstrate a common north-northwestern drift of these units from the southern equatorial latitudes in the Early Devonian to the middle northern latitudes with a small (800 km) convergence, compared to this drift, of Spitsbergen with the Russian Platform by the Early Triassic, which is followed by the subsequent retrograde motion.     

New paleomagnetic and magnetic fabric results for Early Cretaceous rocks from the Turpan intramontane basin,east Tianshan,northwest China

~~New paleomagnetic and magnetic fabric results for Early Cretaceous rocks from the Turpan intramontane basin,east Tianshan,northwest China~~     

Origin of volcanic rocks from Stromboli (Italy)

Quaternary volcanic rocks of Stromboli (Italy) can be divided into older calc-alkaline and younger shoshonitic series. The SiO₂ contents of the rocks range from 50% to 61% but the majority of them are basalts. The rocks show systematic variations in chemical composition which correlate with the volcanic stratigraphy, such that, at a given SiO₂ content, K and other incompatible elements such as REE increase with decreasing age. In addition, the La/Yb ratio increases while the K/Rb, K/Ba, Zr/Ce and Zr/Nb ratios decrease towards the top of the volcanic pile. On the other hand, the abundances of transition elements, V, Co, Sc and Zn, like most major elements are broadly similar in comparable rocks of different ages. It is suggested that the parent magmas were derived by partial melting from upper mantle peridotite enriched in incompatible elements by fluids released from the descending oceanic lithosphere. The temporal chemical variations may probably be related to the lengths of time during which fluids were in contact with the upper mantle source.     

Reconstructing the development of a gully in the Upper Kalaus basin,Stavropol region (southern Russia)

The Stavropol region of southern Russia is severely affected by human‐induced gully erosion. A lack of detailed information on the different stages of gully formation resulting from major agricultural expansion c. 100 years ago, is an obstacle for management and containment of these systems. In this study we combine measurements of particle‐bound radionuclides (¹³⁷Cs, ²¹⁰Pb_ex, ²²⁶Ra, ²³²Th and ⁴⁰K) and classical geomorphology to investigate and reconstruct the different phases of development of a gully during the last c. 100 years. We believe the ?rst phase (1) involved an initial incision into the bottom of a small valley (catchment area c. 1 km²) about 100 years ago. A short period of rapid growth was followed by a longer stage of gully stabilization. Subsequent phases were: (2) the period 1954–1960 – re‐incision in the lower gully reach was initiated by a high‐magnitude rainfall event, and a substantial amount of sediment was delivered to the gully fan; (3) c. 1960–1986 – the knickpoint retreated slowly, sediment was redeposited nearby, and the fan surface became stable; (4) 1986–1987 – a dam was built in the gully mouth and breached shortly after construction following 2 days of high rainfall, and substantial sediment accumulated in the gully above the dam and below the spillway channel on the fan surface; (5) 1987–1993 – the knickpoint retreat continued and the lower fan surface was stable until 1993 when the last signi?cant runoff event overlayed it with c. 10 cm of fresh sediment. These detailed reconstructions of gully development stages allow the contribution of high‐magnitude events to gully growth and regional sediment delivery to be assessed. They further guide management actions to prevent such dam failures in the future. Copyright © 2004 John Wiley & Sons, Ltd.     

江汉盆地南部热史与下志留统海相烃源岩成熟度演化研究

应用含油气盆地热史模拟系统, 对江汉盆地南部的钻井资料进行了模拟计算, 恢复了研究区的热史和埋藏史. 在此基础上, 正演了下志留统烃源岩成熟度的演化史. 研究结果表明,江汉盆地在印支期(240 Ma)以前处于稳定的低热流(50～55 mW/m2)状态, 印支期后热流开始整体升高. 潜北断裂以北地区的热流在中燕山期(155 Ma)达到峰值(～72 mW/m2), 断裂以南的热流在晚燕山期(40 Ma)达到峰值(～76 mW/m2). 晚喜山期后, 整个研究区的热流快速下降, 盆地开始冷却. 早三叠世末, 下志留统烃源岩在枝江、当阳、沔阳凹陷一带率先进入生油门限, 早侏罗世至早白垩世末烃源岩进入快速增熟期, 成熟度具有北高南低的特征. 晚白垩世末, 烃源岩热演化特征表现为东强西弱. 到了新近纪末, 烃源岩热演化终止. 研究区热史恢复和下志留统烃源岩成熟度演化的研究为合理评估烃源岩生烃量、排烃量和油气资源量提供了科学依据.     

Petrogenetic implications of uranium abundances in volcanic rocks from southern Peru

The content of uranium of andesitic rocks of southern Peru varies according to the distance from the Peru-Chile trench, as has been observed for other trace elements geochemically associated with K. The highest contents found in these rocks may exceed the estimated U content of the upper crust. This enrichment may be related to a variable degree of partial melting and/or crustal contamination of the same source material.     

A geothermal study of the Jharia Gondwana basin (India): Heat flow results from several holes and heat production of basement rocks

Temperatures have been measured in nine boreholes (ranging from 400 to 900 m in depth) in the Jharia Gondwana sedimentary basin of the Indian shield. About two hundred thermal conductivity determinations have been made on core samples from these holes. Temperature profiles, Bullard plots and heat flow profiles of these holes indicate different types of disturbances in the shallow geothermal regime, attributable principally to groundwater movement. Heat flow in the region of the “anticlinal high” is about 0.4 HFU higher than the heat flow in the main synclinal region of the basin. The possible sources for this variation are regional groundwater movement and upwelling of thermal waters through a deep-seated fault/fracture system. The heat flow of 1.9 HFU characterizing the main synclinal region, taken as the regional value for the basin as a whole, has been related to the heat generation of the Precambrian basement rocks. A plot of heat flow vs. heat generation falls in line with three plots for the Precambrian complexes of the Indian shield, indicating the absence of a thermal anomaly due to deeper crustal conditions underneath this basin.     

Geochemistry of basaltic and gabbroic rocks from the West Mariana basin and the Mariana trench

This paper describes the chemistry of 33 basaltic rocks dredged from the West Mariana basin and from the Mariana trench during the R/V “Dmitry Mendeleev” 1976 cruise in the western Pacific.The shipboard investigations were carried out by an international working group of 66 earth scientists under the IGCP Project “Ophiolites” and sponsored by the U.S.S.R. Academy of Sciences, Moscow. The purpose of the expedition was to investigate the structure and composition of the oceanic crust of marginal basins, remnant island arcs and deep-sea trenches. Tholeiitic basalts and gabbros as well as ultramafic rocks in various stages of alteration were dredged from the central part of the West Mariana basin demonstrating the presence of oceanic crust.The Pacific slope of the Mariana trench yielded altered basaltic rocks of tholeiitic and alkalic (? trachybasaltic to shoshonitic) composition. The lower part of the island arc slope contains typical tholeiitic basalts, dolerites and gabbros as well as ultramafics associated with flysch-type sediments. This is strong evidence for the formation of an “ophiolite-schuppenzone”, probably due to subduction of Pacific oceanic crust.Associated with these rocks are amygdaloidal, highly magnesian lavas (similar to boninites), which have not been recognized previously in oceanic ridge basalts.These rocks (together with the dolerites) are interpreted as parts of the Mariana island arc and are thought to be the first stage of island arc development (an immature island arc).     

华南古生代中期地层界面的特征与大地构造意义

华南古生代中期地层界面,指早古生代地层与上覆地层之间的接触界面;这是华南地质发展史上重要的构造转换界面之一,综合研究其分布特征对探讨华南大地构造的演变,特别是广西运动进程的研究具有独特的、基础性的科学意义.文章根据华南14省及1直辖市、115县的126条穿越该地层界面剖面的研究,揭示出其界面上、下地层的时空分布特征,识...     

LA-ICP-MS zircon U-Pb dating from granitoids in southern basement of Songliao basin:Constraints on ages of the basin basement

Seven LA-ICP-MS zircon U-Pb datings from granitoids in the southern basement of the Songliao basin were done in order to constrain the ages of the basin basement. The cathodoluminescence (CL) images of the zircons from seven granitoids indicate that they are euhedral-subhedral ones with striped ab-sorption and obvious oscillatory zoning rims. The dating results show that a weighted mean 206Pb/238U age is 236±3 Ma for quartz diorite (sample No.T6-1) located in the western slope of the basin,that weighted mean 206Pb/238U ages are 319±1 Ma (2126 m) and 361±2 Ma (1994 m) for diorite (sample No.YC1-1) and granite (sample No.YC1-2) located in northern part of southeastern uplift of the basin,respectively,and that weighted mean 206Pb/238U ages are 161±5 Ma,165±2 Ma,165±1 Ma and 161±4 Ma for samples Q2-1,SN121,SN122,and SN72 granitoids located in southern part of southeastern uplift of the basin,respectively. The statistical results of ages suggest that the middle Jurassic granitoids con-stitute the main part of basement granitoids,and that the Hercynian and Indo-Sino magmatisms also occur in the basin basement. It is implied that the Songliao basin should be a rift one formed in the intracontinent or active continental margin settings in the late Mesozoic after the Middle Jurassic orogeny took place.     

Matrix hydrodynamic properties of carbonate rocks from the Betic Cordillera (Spain)

Laboratory tests performed on 181 rock samples from boreholes drilled in different areas of the Betic Cordillera allow us to calculate their hydraulic conductivity and open porosity values. Higher values are generally associated with Miocene calcareous sandstones, although hydraulic conductivity reaches its highest values in some isolated limestone and dolostone samples. Lower values were found in marly limestones and marbles. Specific yield ranged from 0 to 0·0798, with a mean value of 0·00579. A total of 79 samples did not release water during the specific yield test, while another 11 samples released water for more than 30 min. Such wide ranges of variation show the great diversity of behaviour that the matrix of the carbonate rocks can have, referring to water storage and transfer, and its influence on pollutants spread, for example. A weak relation between interconnected porosity and hydraulic conductivity was found. The relation between interconnected porosity and specific yield is slightly stronger, except in the case of the dolomites, where a high correlation was found. No dependence on depth was found for hydraulic conductivity and interconnected porosity. Copyright © 2004 John Wiley & Sons, Ltd.     

Volatiles in submarine volcanic rocks from the spreading axis of the East Scotia Sea back-arc basin

The volatile content of glassy pillow rims from East Scotia Sea back-arc basin (BAB) lavas are unlike those of mid-ocean ridge (MOR) pillow-rim glasses, although non-volatile compositions of the two rock groups overlap. The East Scotia Sea samples have three to ten times greater water contents and nearly twice the average CO₂ and Cl contents of MOR samples; F contents are similar. S contents are only one-third those from MOR samples. H₂O and CO₂ contents of glassy pillow rims from Mariana island arc andesites are similar to those in the BAB lavas studied. Nevertheless, volatiles in the East Scotia Sea BAB magmas are probably not directly derived from the subducted slab, because there is no seismic evidence that the slab extends within 200 km of the spreading axis of the East Scotia Sea. Available data do not preclude the possibility that the magmas were contaminated by seawater prior to eruption or that the mantle under the East Scotia Sea spreading center is volatile-rich. The volatiles may have been added to the mantle during an earlier period of subduction, perhaps during the initial formation of the East Scotia Sea basin.     

SHRIMP zircon U-Pb age and significance of Early Paleozoic volcanic rocks in East Kunlun orogenic belt, Qinghai Province, China

A great deal of practical data in recent years have proved that the East Kunlun orogenic belt and even the China central orogenic belt are complex orogenic belts that underwent polycycle orogenic evolvement[1―7]. Each orogenic cycle has left a compositional print, the multi-period ophiolites[4―6] and various types of tec-tono-magmatic production in the same orogenic belt. There is a suite of shallow metamorphic volcanic rocks in the Nuomuhong area in the east part of the East Kunlun orogen…     

Geochemistry of calc-alkaline volcanic rocks from southeastern Iran (kouh-e-shahsavaran)

The Upper Tertiary to Quaternary volcanic complex of Kouh-e-Shahsavaran in southeastern Iran is composed of calc-alkaline rocks of island are type (high-alumina basalts, basic andesites, andesites and dacites) even though it was emplaced on the continental basement. The volcanic rocks of the complex are genetically related and were probably derived by low-pressure fractional crystallization of high-Al basalts. The anomalously high content of Sr in some rocks probably reflects an accumulation of plagioclase. The trace element data are consistent with the origin of the parental magma by partial melting of an “enriched” upper mantle peridotite.