秦昆结合区两期基性岩墙群^40Ar／^39Ar定年及其构造意义

秦昆(秦岭-昆仑)结合区在不同地质背景基础上于柴达木东缘次级造山带南东段的拉龙洼和满丈岗地区发育了两期基性岩墙群,~(40)Ar/~(39)Ar定年结果分别为393.5Ma和197.5Ma,它们分别代表着东古特提斯洋北支及其北侧秦昆间共和坳拉谷的初始伸展离散和这一构造系统封闭碰撞造山后的伸展垮塌构造岩浆事件。结合区域资料进一步分析表明,晚古生代中晚期东昆南-阿尼玛卿和阿尼玛卿-文县-勉略两支裂谷发育成相互沟通的东古特提斯洋北支时,苦海-赛什塘一支发生“夭折”而形成共和坳拉谷,这一构造系统于三叠纪晚期各区段几乎是同时发生收缩、闭合、碰撞造山,东昆南-阿尼玛卿-文县-勉略有限洋盆的向北俯冲碰撞造山形成统一的秦昆造山带,共和坳拉谷的封闭使西秦岭与东昆仑完成了侧向间的衔接。本文的研究给这一秦昆衔接的动力学演化过程提供了时代依据。     

桐柏山造山带南麓橄长质岩墙（床）群的矿物化学、地球化学特征与成因意义

桐柏山造山带南麓随州枣阳地区的超镁铁镁铁质岩墙(床)群分为性质完全不同的两类:橄长质岩墙(床)群和变质的基性岩墙群。其中橄长质岩墙(床)群未经历过任何变形和变质作用的影响,其侵位时代应不早于区内高压超高压变质作用的时间(220~240Ma),可能属晚造山或后造山岩浆作用的产物。这些橄长质岩墙(床)群中主要造岩矿物为橄榄石、斜长石、斜方辉石和单斜辉石,含少量金云母。其中橄榄石的Fo介于72~78之间,为贵橄榄石;斜长石以倍长石为主,少数为拉长石;单斜辉石主要为普通辉石,少数为透辉石;金云母中含较高的Ti O2(3.36%),属钛金云母变种。橄榄石和普通辉石之间的平衡温度主要介于1120~1165℃之间,普通辉石的结晶温度为971~1079℃。橄长质岩墙(床)群在地球化学上以高Al2O3(17.25%~20.20%)和MgO(9.56%~15.30%)含量、高的MgO/FeO*值(1.26~1.76)以及低Ti O2(0.33%~0.58%)和低碱(Na2O K2O=1.64%~2.23%)为特征,过渡元素的丰度(Cr=70~125μg/g、Ni=251~518μg/g、Co=54~75μg/g)和Mg#值(54~67)均较高,其性质可近似代表上地幔部分熔融所形成的母岩浆。稀土元素含量低(ΣREE=18.66~35.42μg/g),轻、重稀土元素之间的分馏程度较强,(La/Yb)N=4.4~4.7,尤以强的Ba、Sr及Eu正异常(δEu=1.31~1.49)为显著特征,表明其中有较多的富钙斜长石聚集。高场强元素Nb、Ta、U、Th以及Zr和Hf无明显异常,不同于区内早古生代玄武岩所揭示的富集型上地幔的地球化学特征,而与造山带内及前、后陆地区古近纪玄武岩的地球化学特征非常相似,可能表明造山带山根拆沉后,发生过软流圈地幔与大陆岩石圈地幔强烈的相互作用。橄长质岩墙群的出现说明俯冲板块的后缘曾发生过岩石圈范围的强烈拉张作用。     

大火成岩省及其在古大陆重建及裂解研究中的应用

大火成岩省是地质历史上与地球深部过程（如地幔柱）密切相关的极端地质事件,以相对较短时期内形成规模宏大的板内幔源基性岩浆活动为主要特征,对研究全球性大气—海洋环境的巨变与生物灭绝、大规模成矿及超大陆的重建和裂解均有重要意义。从潘吉亚超大陆的裂解过程可以看出,尽管不是所有的大火成岩省都与大陆裂解有关,但几乎每一次重要的裂解事件都伴有大火成岩省的出现,因此大火成岩省对于研究前潘吉亚超大陆,特别是对于缺少古生物等重建标志,古地磁数据偏少的前寒武纪超大陆重建及裂解研究有重要价值。近20多年来随着年代学技术的进步及研究工作的不断深入,大火成岩省在古大陆重建及裂解方面取得了一些重要进展,并发挥了越来越重要的作用。前寒武纪大火成岩省由于经历了多次裂解事件改造及后期抬升剥蚀或覆盖的影响,现今保留的是一些分散在不同大陆上的大火成岩省残片或碎片。用大火成岩省开展古大陆重建及裂解的目标就是通过不同陆块大火成岩省或大规模基性岩浆活动的年代学、岩石学及地球化学对比,结合其他证据,恢复大火成岩省形成时的时空分布特征,据此来确定这些陆块在超大陆中的相对或绝对位置。与其他前寒武纪大陆重建标志相似,大火成岩省也有一些局限性及不确定性,因此应用大火成岩省开展古大陆重建及裂解研究中要与前岩浆期沉积地层中的特殊事件层（如冰碛岩、火山灰、黑色页岩、不整合面、古生物等）对比、岩墙几何学产状、基性岩浆事件序列对比、古地磁、大型构造（如造山带）对比及深部地球物理等多学科方法相结合。     

扬子克拉通北缘随(州)-枣(阳)地区新元古代变质岩浆岩的地球化学和SHRIMP锆石U-Pb年代学研究

扬子克拉通北缘的随州-枣阳地区是整个秦岭-桐柏山-大别山-苏鲁造山带及其前陆地区受三叠纪(印支期)扬子板块向华北板块深俯冲并发生高压-超高压变质作用影响最小的地区,因而扬子板块北缘的前寒武纪基底在这里得到了较多的保存。它们不仅为研究扬子板块北缘新元古代构造环境和岩石圈性质提供了难得的样品,也为研究造山带内变质杂岩的原岩性质及高压-超高压变质作用过程中元素的地球化学行为提供了参照物。这里出露的前寒武纪基底包括新元古代的变质火山-沉积岩系(随州群)以及大量的超镁铁质-镁铁质岩床群。其中构成随州群的岩性包括变酸性火山岩、变沉积岩及少量的变基性火山岩。超镁铁质-镁铁质岩床群以橄长岩为主,少量辉长苏长岩和辉石岩。本文用SHRIPM锆石U-Pb法测得随州群中变质流纹英安质凝灰岩和变质粗面安山岩及超镁铁质-镁铁质岩床群中橄长岩的侵位年龄分别为763±7Ma、741±7Ma和632±6Ma。随州群火山岩的年龄与桐柏山-大别山-苏鲁造山带内高压-超高压变质杂岩的原岩年龄类似,至于造山带内榴辉岩的原岩年龄是否有类似于橄长岩的侵位年龄(631.5±6.1Ma),尚需进一步研究。地球化学上,随县群的酸性火山岩表现出强烈亏损Sr和高场强元素Nb、Ta,富集Rb、Ba、U、Th、K等强不相容元素和轻稀土元素,轻、重稀土元素之间的分馏程度强((La/Yb)_N=10.44)、弱负Eu异常(δEu=0.75),总体特征类似于大别山-苏鲁造山带内大量出露的黑云斜长片麻岩和裂谷环境下形成的双峰式火山岩的酸性端元。随县群玄武岩的稀土元素之间几乎无分馏((La/Yb)_N=1.78~1.79),高场强元素Nb和Ta相对于La无明显的亏损,总的地球化学特征类似于造山带内大多数的榴辉岩。橄长岩以高Al₂O₃和MgO含量、低TiO₂和碱为特征,轻、重稀土元素之间的分馏程度较强((La/Yb)_N=5.49),尤以强的Ba、Sr及Eu正异常(δEu=1.22)为显著特征,表明其中有较多的富钙斜长石聚集。高场强元素Nb、Ta、U、Th(以及Zr和Hf)无明显的异常,表明其形成过程中未受到大陆地壳的混染,岩浆源区属富集型地幔。     

E) Organisations Nationales

Abstract

Sudden rapid advances or surges of glaciers and sections of smail ice caps are well known. After remaining dormant or in retreat over long periods of time these ice masses suddenly move forward rapidly with speeds about 2 orders of magnitude greater than usual. If such surges were to occur in large sections of the Antarctic ice sheet serious consequences could result. These include a significant rise of sea level, a substantial increase in the high-albedo ice cover around the continent especially in summer, and a cooling of the Antarctic ocean by the additional ice melting.

A numerical model has now been developed which simulates surging in certain glaciers and ice sheets in an apparently realistic manner. This model has been found to give close representations to a number of existing real surging temperate ice masses from small mountain glaciers to large sectors of ice caps. The model reproduces realistically many features of these ice masses such as the period of the surge, the duration, the velocity of advance, the magnitude of the advance, and the changes in ice thickness.

The application of the model to the Antarctic ice sheet is made more difficult by the problem caused by the temperature dependence of the flow properties of ice. This means that for a complete study the interaction with the environment needs to be considered. However, at this stage preliminary calculations indicate a number of features that are relevant to the effect of Antarctic ice surges on the global climate. These include the period between surges, the duration of the surge, the amount of ice advanced and the changes in thickness of the ice sheet.     

Ni–Cu–(PGE) magmatic sulfide deposits in the Yangliuping area,Permian Emeishan igneous province,SW China

The Ni–Cu–PGE sulfide deposits in the Yangliuping area, SW China, are hosted in mafic–ultramafic sills. The four mineralized sills are located in the Yangliuping tectonic dome and intrude Devonian carbonaceous marble, graphitic schist. The sills are 200–300 m thick and 1,000–2,000 m in strike length and now consist chiefly of serpentinite, talc schist, tremolite schist, and meta-gabbro. Disseminated Ni–Cu sulfide mineralisation occurs in the serpentinite in the lower parts of the sills. Massive sulfide mineralisation is located in the base of the sills and in the footwall along fractures beneath the mineralized serpentinite. Although the sulfide ores have been modified by hydrothermal activity, there are relict cumulate textures in the disseminated sulfides indicating a magmatic origin for the ores. The Yangliuping Intrusions and the Dashibao Formation have similar primitive-mantle normalized trace element and platinum group element (PGE) patterns, indicating that they are derived from a common parental magma type. The positive correlation between Cu concentrations and Cu/Zr ratios of the Dashibao Formation basalts indicates that the chalcophile elements were removed before eruption. We propose that fractional crystallization of the Yangliuping magma accompanied by the introduction of S and CO₂ from the wall rocks caused the magma to become S-saturated leading to the segregation of magmatic sulfides that became enriched in Ni–Cu–(PGE). The sills acted as conduits for the overlying Dashibao Formation basalts with the sulfide liquid, along with early crystallizing olivine and pyroxene, segregating from the magma as it passed through the conduits prior to eruption.Editorial handling: H.E. Frimmel     

Another look at anisotropy in geostatistics

A thorough geostatistical data analysis includes a careful study of how the data's second-order variation, as characterized by the semivariogram, depends on the relative orientation of data locations. If the semivariogram depends on only the (Euclidean) distance between locations, then the semivariogram is isotropic; otherwise, it is anisotropic. In this article, I take another look at the modeling of anisotropy in geostatistics. A new, more specific classification of types of anisotropy is proposed. More importantly, some heretofore inadequately understood implications of the dependence of various semivariogram attributes on direction are discussed, and the wisdom of some current practices for modeling the direction-dependence of these attributes is questioned.     

The Graveyard Point Intrusion: an Example of Extreme Differentiation of Snake River Plain Basalt in a Shallow Crustal Pluton

The Graveyard Point intrusion is the only known example of awell-exposed differentiated mafic pluton associated with thelate Miocene–Pleistocene magmatism of the western SnakeRiver Plain (SRP). It is exposed in a 6 km by 4 km area adjacentto the Oregon–Idaho border, and exposures range in thicknessfrom 20 to 160 m. The thicker parts of the intrusion are stronglydifferentiated and contain a 25–60 m thick section ofwell-laminated cumulus-textured gabbros that grade upward intopegmatoidal ferrogabbro. Evolved liquids formed sheets of Fe-richsiliceous granophyre. At least two injections of magma are indicatedby abrupt discontinuities in the rock and mineral compositions,and by the lack of mass balance between the bulk intrusion andits chilled borders. The laminated gabbros are interpreted tohave formed from a tongue of augite and plagioclase crystalsthat were carried in with the second pulse of magma. Followingthe final emplacement of the intrusion, in situ differentiationproceeded through a two-stage process: the ferrogabbros areexplained as interstitial liquids forced out of the crystalmush by compaction, and the siliceous granophyres are interpretedto be residual liquids that migrated out of the partly crystallizedferrogabbros in response to the exsolution of volatiles. Becausethe geochemical trend inferred for the mafic to intermediatecomposition liquids in the Graveyard Point intrusion is similarto the trend for many western Snake River Plain lavas, the plutonmay be a good model for shallow sub-volcanic magma chamberselsewhere in the SRP. However, some western SRP lavas containanomalously high concentrations of P₂O₅ , which are best explainedby mixing within the active crustal mush column or with partialmelts of previously formed differentiated mafic intrusions. KEY WORDS: Snake River Plain; mafic intrusions; tholeiitic; sill; granophyre     

Quaternary Intrusions from the Zhongjiannan Basin, South China Sea: Their Relationship with the Hainan Mantle Plume and Influence on Hydrocarbon Reservoirs

The transportation of magma in sedimentary basins often occurs through extensive dyke-sill networks. The role of sills on the plumbing system in rifted margins and the impact of sills on hydrocarbon reservoirs of prospective sedimentary basins has long been an area of great industrial interest and scientific debate. Based on 2D seismic reflection, we present data on how the sills emplaced to form a magmatic plumbing system of the volcanic system for the Zhongjiannan Basin(ZJNB). The results show...