Burial Metamorphism of the Ordos Basin in Northern Shaanxi

Burial metamorphism has been found in the Ordos basin of northern Shaanxi. On the basis of a rather intensive study of burial metamorphism of sandstone, it has been shown that the evolution from diagenesis to metamorphism involves four stages: cementation of clay minerals, regrowth of pressolved quartz and feldspar, cementation of carbonates and formation of laumontite. On that basis it has been put forward that the laumontite is formed by burial metamorphism of clay and carbonate minerals. According to the thermodynamic data of minerals, the conditions under which laumontite is formed are T<250℃ and X_(CO_2)<0.17. High-resolution SEM and TEM studies of clay minerals in mudstone show that there occur a mixed layer assemblage of bertherine and illite/chlorite and transformation from bertherine to chlorite. On that basis coupled by the X-ray diffraction analysis the author suggests the following transformation of clay minerals during burial metamorphism: the earliest smectite-kaolinite assemblage changes into the bertherine-illite mixture with increasing depth, then into the illite/chlorite mixed layer assemblage and finally into dispersed individual illite and chlorite. The reaction of the transformation is:smectite+kaolinite+K~+=illite+chlorite+quartz According to the study of the oxygen isotope thermometry of the coexisting illitequartz pair, the temperature of the above transformation is lower than 180℃.     

Burial Metamorphism in the Hamersley Basin, Western Australia

The low-grade metamorphic minerals prehnite, pumpellyite, epidoteand actinolite in rocks of basic and intermediate compositionhave a broad, systematic distribution in the Hamersley Basin.Assemblages of these minerals are wisespread in the FortescueGroup, the lowermost group in the Hamersley Basin. Because ofsunsuitability of rock type no relevant mineral assemblageswere observed in samples from the Hamersley Group. However,metamorphism of this group can be implied from mineral assemblagesin the younger Turee Creek Group, and because the HamersleyGroup conformably overlies the metamorphosed Fortescue Group. Unfolded stratigraphic cross sections show that depth of burialwas the dominant control of increase in metamorphic grade. Fourmetamorphic zones are defined over a relative depth of burialof 9 km. From lowest grade to highest these are: Zone I (ZI)prehnite–pumpellyite zone; ZII, prehnite–pumpellyite–epidotezone; ZIII, prehnite–pumpellyite–epidote–actinolitezone; and ZIV, (prehnite–epidote–actinolite zone.Laumontite, definitive of the zeolite fades is absent but thatpart of the sequence may coincide with rocks of unsuitable composition,or may have been removed by erosion. A large area of prehnite–pumpellyitefades (ZI and ZII) dominates the north side of the basin, whilegreenschist fades (ZIV) dominates the south. Separating thetwo is a curved central strip of pumpellyite-actinolite facies(ZIII). Microprobe data of pumpellyites from the three pumpellyite–bearingzones, ZI, II and III, show two systematic trends: extensivevariation in Al/Fe ratios at any one grade, and a general decreaseof Mg with increasing metamorphism. Consideration of the compositionsof the most abundant pumpellyites in the metabasic rocks showsthat these two trends spread about a more fundamental lineartrend towards AJ-enrichment with increasing metamorphism astotal Fe and Mg decrease. Epidote shows a wide range in Fe content in ZII and ZIII (Ps15to Ps40) crossing the miscibility gap proposed by Raith (1976).In ZIV epidote compositions are more aluminous and restrictedin composition (Ps11 to Ps20). Magnesium has entered the epidotelattice in ZII and ZIII (up to 0–17 ions Mg where £cations = 8) but to only half this in ZIV. Synthesis of the burial model with published experimental workputs constraints on the ancient thermal gradient that existedduring burial metamorphism. For the peak of metamorphic adjustmentfluid pressure appears to have been equal to load pressure.A relatively high gradient of 80 to 100 deg;C/km seems likelyfor the shallow part of the sequence, with a gradient of 40deg;C/km for the deeper part of the sequence, the change beingat about 2–5 km. The prehnite-pumpellyite facies correspondsto a fluid pressure of 0–5 to 1 kilobar and a temperaturerange of about 100 to 300 deg;C. The prehnite-bearing pumpellyite-actinolitefacies is interpreted to have developed at about 1–5 kbover a temperature range of 300 to 360 deg;C. This facies isprobably a low pressure subfacies of the pumpellyite-actinolitefades of Hashimoto (1966).     

闽北地区变质作用基本特征和成矿元素迁移富集规律

闽北地区面型带状分布的巴罗式前进变质带从浅→深→混合岩化带→区域型混合花岗岩呈渐进过渡式的、是和谐的。断裂变质作用缺乏从深→浅的面型带状分布的进变质带,变质级与混合岩化强度带不和谐,呈突变跳跃式。研究表明,成矿元素在区域变质作用过程中难以大规模迁移与富集,混合岩化花岗岩化作用提供了成矿元素迁移富集的物理化学条件;区域型混合花岗岩和边缘型混合花岗岩与岩浆花岗岩（狭义的）在成矿作用方面有明显的差异,表现出别具一格的成矿专属性。     

Zones of Progressive Regional Burial Metamorphism in Part of the Tasman Geosyncline, Eastern Australia

A succession of metamorphic zones can be recognized along theMolong Geanticline, an ancient rise within the Tasman Geosynclineof Eastern Australia. In order of increasing grade, the zonesin the volcanogenic rocks are: (1) an albite-quartz zone, containingneither prehnite, pumpellyite, actinolite, nor biotite; (2)a prehnite zone; (3) a prehnite-pumpellyite zone; (4) an actinolitezone; and (5) a biotite zone. Zeolite associations are not developed.Zones 4 and 5 lie within the greenschist facies. The rocks alongthis geanticline are generally only slightly deformed, lackingcleavage. The grade of this metamorphism within the Molong Geanticlineincreases partly in accordance with increasing stratigraphicdepth and partly increases eastwards towards the adjacent HillEnd Trough. Within the Trough, rocks show metamorphic assemblagesakin to those of Zones 4 and 5 as defined within the MolongGeanticline, but deformation is more extreme and a regionalcleavage is characteristic. In the region about the Hill EndTrough and the adjacent geanticlines there is an apparent variationfrom essentially static burial metamorphism of the geanticlinesthrough transitional types to tectonic dynamothermal metamorphismof the trough. Both metamorphisms are envisaged as having arisenduring the same general period of metamorphism, but deformationof the geanticlines was much less than deformation in the interveningHill End Trough.     

Major Element Chemical Variation in the Eocene Lavas of the Isle of Skye, Scotland

The Lower Eocene lavas of northern Skye are preserved over anarea of approximately 1500 km² in a shallow faulted oval basin.Seventy-four new major element chemical analyses have been madeof specimens showing minimal post-consolidation alteration.These demonstrate that the early volcanics vary from hypersthene-tonepheline-normative basalts; the former containing less TiO₂and P₂O₅ but more K₂O than the latter. The compositions of thesebasalts straddle the low-pressure thermal divide near the criticalplane of silica under-saturation, the normative ‘join’OI–Pl–Cpx; implying that their variation was causedby high-pressure, upper mantle processes. The unusual incompatibleelement pattern of the lavas suggests that a K-rich mantle phase,tentatively taken to be phlogopite, was involved in their genesis.At subvolcanic pressures the spectrum of basaltic magmas wassplit by the join olivine-plagioclase-augite, producing twodivergent trends; from alkali basalts, via relatively Si- andK-poor and Fe- and Ti-rich hawaiites and mugearites to benmoreite,and from hypersthene-normative basalts, via relatively Si- andK-rich and Fe- and Ti-poor intermediates to trachyte. One-atmospheremelting experiments on 21 lavas are used to supplement the chemicalinvestigation of these low-pressure trends. Two flows of aphyric, low-alkali tholeiitic basalt, with compositionsquite distinct from all other Skye lavas, have been discoveredinterleaved among the alkali-rich rocks near the top of thevolcanic pile, at present exposure level. The almost patternlesschemical variation of all the lavas with time suggests thatmagma chambers, as conventionally envisaged, did not exist beneaththe Skye volcanic field, but rather that fissure eruptions werefed from a sponge-like plexus of conduits and small reservoirs,within which magma, affected to varying extents by upper mantleprocesses, remained for varying periods, pursuing low-pressurefractionation trends. The chemistry of the main Skye basaltsand the low-alkali tholeiites, plus available data on otherpost-lava Skye basic igneous rocks, such as the Cuillin LayeredIntrusion, other gabbros in the central complexes, and late-stagealkali dolerite dykes, are all combined in an attempt to ascribethe variation of the basic magmas which approached or reachedthe surface of Skye to the growth, culmination, and waning stagesof a thermal event in the upper mantle beneath that area.     

变质作用P—T—t轨迹若干问题评述

着重介绍了目前变质作用P－T─t轨迹研究在变质作用温压确定、变质作用不同阶段时间标定、变质作用动态演化过程的组构标志、变质作用演化热模拟以及变质作用P─T─t轨迹与大地构造环境之间关系等方面所存在的若干问题。     

胶东莱西地区高压基性麻粒岩的变质作用

胶东地区莱西－莱阳－栖霞－带晚太古代花岗片麻碉中,出露相当规模的高压基性麻业岩和超镁铁质岩组合,曾被认为它们是与苏鲁－大别超高压碰撞造山作用有关的一套岩石组合,高在性麻粒岩中可同三期变质矿物组合;早期为Ｇｒｔ核部）＋Ｃｐｘ＋Ｐｌ;中期为Ｇｒｔ（边中）＋Ｃｐｘ＋Ｏｐｘ＋Ｐｌ＋Ａｍｐ＋Ｑ＋Ｍｔ;晚期为Ｃｐｘ＋Ｐｌ＋Ａｍｐ＋Ｑ＋Ｍｔ。早期的石榴石含钱铝榴石和镁铝榴石组分较高,单斜辉石含Ａｌ高。应用矿物温     

Evidence for the Repeated Folding of Isotherms during Regional Metamorphism

Petrologic and structural investigations in a polydeformed terranein southwestern New Hampshire show that more than one stageof noncoaxial folding has significantly affected the metamorphichistory of this region. Detailed examinations of both isograd-isothermpatterns and mineral reaction histories within pelitic rockssuggest that pre-existing, nearly horizontal isotherms werefolded during an early stage of folding about north-south axes.After folding, thermal relaxation resulted in cooling in theanticlinal portions of the early folds and heating in the synclinalregions. This process of isotherm folding and re-equilibrationwas repeated during a later stage of east-west folding, resultingin complex isograd patterns and mineral reaction histories. Detailed mapping of isograd and isotherms in the pelitic rocksof this region shows a complex patten. High grade and high temperatureassemblages occur along early and late synclinal axes, withthe highest grade assemblages occurring at the intersectionof the two synclinal axes. Conversely, the lowest grade assemblagesare found at the intersection of the early and late anticlinalfolds. Four different types of fold intersections result from the twostages of noncoaxial folding. Pelitic rocks at each of thesefour different fold intersections show different and contrastingmineral reaction histories. At the intersections of early andlate synclines the rocks show evidence for continuous heating,while continuous cooling trends are seen at intersections ofearly and late anticlines. Complex reaction histories are observedat the intersections of early synclines and late anticlinesand early anticlines and late synclines, which show heating-coolingand cooling-heating trends, respectively. These results show that over small areas of a metamorphic terranedifferent samples may show widely different P-T paths. Therefore,without careful structural analysis and thorough sampling, tectonicinterpretations based solely on paths determined from a fewwidely scattered samples may lead to erroneous conclusions.     

白云岩埋藏成岩作用过程中的金属元素与同位素再分配规律与受控因素

前人在白云岩岩石地球化学研究过程中注意到,在开放体系下埋藏成岩作用过程中白云石金属元素含量与同位素组成将发生再分配,然而到目前为止还没有系统的理论来解释这一现象。根据化学平衡原理,提出离子半径理论控制白云岩在埋藏过程中的金属元素与同位素组成再分配规律,基本概念为大半径离子含量减少,小半径离子含量升高。根据这一理论,结合Ca、Mg、Fe、Mn、Sr等+2价离子的离子半径数值得出推论,在埋藏压实成岩作用过程中白云石将向着有序度升高的方向转化,白云石中的Fe、Mn含量升高,Sr含量降低,同位素比值87Sr/86Sr升高。该结论与前人资料有着高度的一致性。     

Petrogenesis of Late Archaean Flood-Type Basic Lavas from the Klipriviersberg Group, Ventersdorp Supergroup, South Africa

The Klipriviersberg Group is a small continental flood-typetholeiitic suite forming the basal unit of the Ventersdorp Supergroup,an undeformed late-Archaean supracrustal sequence covering 200000km² in the SW part of the Kaapvaal Craton. From the base up,the Klipriviersberg Group consists of the Westonaria, Alberton,Orkney, Jeannette, Loraine, and Edenville formations, with amaximum combined thickness of 1–8 km. Samples were obtainedfrom several borehole cores in the Klerksdorp goldfield closeto the type area of the Klipriviersberg Group. This sample suiteincludes a detailed collection from a 1–36-km core intersection.These samples reveal that the Westonaria Formation is absentfrom the succession and that the Jeannette Formation is representedby a thin sequence of breccias. The remaining units are builtof lavas ranging from siliceous picrite basalts to tholeiiticandesites in composition. All lavas exhibit the effects of greenschistfacies metamorphism but igneous textures are well preservedand reveal that most lavas are aphyric to sparsely porphyritic.A well-developed geochemical stratigraphy exists in the lavasequence and is characterized by an upward trend from evolvedto primitive lavas. The Alberton (mg-number = 0.53–0.43;Zr= 108–137 ppm) and Orkney (mg-number = 0.51–O.42;Zr = 90–110ppm) lavas have a narrow compositional rangeand are more differentiated than the compositionally more variableLoraine/Edenville lavas (mg-number = 0.75 0.47; Zr = 34–97ppm). Unusual inter-element correlations in the Alberton andOrkney lavas suggest that magma mixing processes were importantin their petrogenesis. Compositional variation in the Loraine/Edenvillelavas is consistent with simple fractional crystallization dominatedby orthopyroxene. Collectively, all these lavas are light rareearth element (LREE)-enriched and have negative Nb-Ta, P, andTi anomalies on mid-ocean ridge basalt (MORB)- or primitivemantle-normalized abundance diagrams. In this they resemblemafic lavas from modern subduction-zone environments and manyArchaean and early Proterozoic siliceous high-magnesium basalts.The Klipriviersberg lavas also have initial Nd-isotopic compositionsthat cluster about the chondritic uniform reservoir (CHUR) valueregardless of degree of differentiation. It is unlikely thatcrustal contamination has influenced the composition of theKlipriviersberg lavas to any significant extent, and their geochemicalfeatures have probably been inherited from lithospheric mantlesources.     

Foidite and Meimechite Lavas of Polar Siberia (Some Questions of Petrogenesis)

Doklady Earth Sciences - For the Permian–Triassic foidite and meimechite lavas of Polar Siberia, both the whole-rock petrochemistry and geochemistry and that of melt inclusions in olivine...     

Metamorphism of the Witwatersrand gold fields: conditions during peak metamorphism

Abstract Pelitic assemblages from all major Witwatersrand gold fields record metamorphic conditions of the greenschist facies, with minimal regional grade changes over at least 200 km strike length. Diagnostic metamorphic assemblages are less common in the volumetrically dominant quartzites, the actively-exploited auriferous conglomerates and some of the regionally persistent metapelitic horizons. Bulk rock composition has been a major control on assemblage development.
Key metapelitic assemblages include pyrophyllite, chloritoid, chlorite and muscovite in each gold field, with less common metamorphic biotite. Accessory minerals are pyrite, tourmaline, rutile and zircon. The abundance of chloritoid and pyrophyllite in thin shaly units, together with their minor, but widespread, distribution in quartzites and conglomerates, indicate that metamorphic temperatures reached 350°C ± 50°C in all the gold fields. Pressures are less-well constrained, 1–2 kbar being inferred. Outside the gold fields, higher grades are indicated by andalusite and kyanite near granitoid domes and later intrusions.
The temperatures during peak metamorphism and the abundance of pyrite provide ideal conditions to (re)mobilize gold and may explain its secondary textural features.     

乌力吉木仁河水文要素时间尺度变异分析

由于气候变化和人类活动的影响,水文要素的分布形式和参数可能在统计意义上产生了变异。为了揭示水文要素在时间尺度上的变异关系,将乌力吉木仁河梅林庙测站1956~2000年的逐月径流资料,按照不同的时间尺度划分为季度、汛/非汛期和年径流序列,利用水文变异诊断系统分别对其进行变异诊断,并查阅相关文献得出剧烈的人类活动是导致变异的主要原因。通过归纳不同时间尺度的变异诊断结果得出,对于按照季度尺度划分的径流序列,不同月份径流量占年径流量的比例是影响季度变异的主要因素,在变异程度方面,第一季度的变异程度最大,而第三季度的变异程度则最小;对于汛/非汛期尺度划分的径流序列,其变异则受所包含月份的综合影响,且月径流量比例系数越大,其影响程度也越大;对于年径流序列,其变异则主要受到第三季度或汛期径流的影响。     

中国蓝闪片岩相的变质作用

本文论述了中国蓝闪片岩的分布、变质条件及其构造位置。中国的蓝闪片岩从中晚元古代开始,各变质期均有出现。根据矿物组合,可分为两类:第一类蓝闪片岩常含有高压矿物,如硬柱石、硬玉和文石以及蓝闪石、绿纤石、黑硬绿泥石、多硅白云母、红帘石等,属高压亚绿片岩相,称蓝闪—硬柱石片岩相,形成温度约250—350℃,压力大于500—800MPa,甚至可达1200MPa。此类蓝闪—硬柱石片岩相多代表海洋板块古消减带。第二类蓝闪片岩的常见矿物为蓝闪石、青铝闪石或镁钠闪石、黑硬绿泥石、红帘石和绿片岩相中的绿帘石、阳起石、绿泥石、白云母、有时还有黑云母、铁铝榴石和钠质辉石。形成温度约350—450℃,压力500—800MPa。此类蓝闪绿片岩相虽处于活动带,但与板块构造没有直接关系。我国西藏南部和内蒙温都尔庙属第一类,但大部分蓝闪片岩带属第二类。     

Metamorphism during Ophiolite Emplacement the Petrology of the St. Anthony Complex

The St. Anthony Complex consists of the White Hills Peridotite(the ultramafic remnant of an ophiolite), and an underlyingmetamorphic aureole which grades from metagabbro and hornblendegranulite through amphibolite, epidote amphibolite, and greenschistinto undeformed volcanic rocks at the base of the sequence.The petrology and mineralogy of the complex show that the basalperidotite mylonites recrystallized at 900–950 °C,the coronitic metagabbros at 850–900 °C, two-pyroxeneamphibolites at 860 °C, marbles at 680 °C, epidote amphibolitesat 550–650 °C, and greenschists at 350–550 °C.The metamorphic pressures range from 7–10 kb at the peridotitecontact to 3–5 kb in the greenschists and amphibolites,and less than 2 kb in the volcanics. The geology suggests thatthe metamorphic rocks originally formed from a strip of turbidites,alkali basalts, tholeiites, and gabbros adjacent to a continentalmargin. This sequence was subjected to dynamothermal metamorphismresulting from a combination of conduction of heat from theoverlying ultramafic rocks and shear heating. The rocks weregradually accreted onto the base of the overriding ophiolitesheet, forming the composite metamorphic sequence now observed.     

Major and Trace Element Variations in the Tertiary Lavas of Eastern Iceland and their Significance with respect to the Iceland Geochemical Anomaly

Major and trace element analyses of 130 lavas, including rareearth elements (REE) for 90 of them, are used to investigategeochemical variation in this lava series. The lavas are dividedinto six compositional groups, ranging from high magnesia basaltto rhyolite, on the basis of Fe/Mg ratio and the phenocrystphase assemblage. The high magnesia basalts range from tholeiiticto transitional and show a correlation between silica saturationand light REE enrichment. This chemical variation cannot beexplained in terms of open or closed system crystal fractionationinvolving the observed phenocryst phases, but is probably theresult of a two (or more) stage mantle process. When the observedvertical chemical zonation and subsequent erosion of the lavapile are taken into account, it is apparent that a range ofbasaltic compositions has been available for eruption throughoutthe history of the lava pile. The evolved (low Mg) lavas belong to a spectrum of closed systemcrystal fractionation trends derived from each of the high magnesiabasalts. However, silicic and intermediate lavas are all derivedfrom the more light REE enriched basalts. The crystal fractionationmodel is quantified using a least squares mixing computer programfor the major elements and the Rayleigh equation for the REE.The importance of minor phases (i.e. iron-titanium oxides andapatite) in controlling both major and trace element behaviourin the residual liquid during crystal fractionation is established.The large calculated bulk solid/liquid distribution coefficientsfor most of the analysed trace elements, at certain compositionalintervals in the series, suggest that few, if any, of the elementscan be considered as totally incompatible. In detail the chemicalvariation in the silicic and intermediate lavas cannot be producedby the partial melting of a basaltic source. Hybrid intermediatelavas can be distinguished chemically from differentiates becausethey lie on linear trends between basalt and rhyolite for mostelements. Intermediate differentiates are only formed in a narrowcrystallization interval which, together with their need tobe superheated in order to reach the surface, probably explainsthe paucity of these compositions in the lava pile.     

青藏高原重大冻土工程的基础研究

青藏高原是我国乃至世界高海拔多年冻土区的典型代表。伴随着青藏铁路的建成通车,西藏自治区迎来了新一轮经济发展,迫切需要新建高速公路、输变电线路、输油气管道工程等。这些拟建工程与已建的青藏公路、青藏铁路、格拉输油管道、兰西拉光缆等工程均聚集于宽度不足10km范围内的青藏工程走廊。在这狭长的冻土工程走廊内,已修建或拟建的各种冻土构筑物相互影响,多因素耦合叠加,加速区域内的冻土退化,而冻土融化必将影响到工程的稳定性和生态环境退化。再加上全球气候变化的影响,其变化程度更加剧烈。面对国家需求,国家重点基础研究发展项目"青藏高原重大冻土工程的基础研究"于2012年4月正式启动。该项目旨在揭示气候变化与人类工程活动加剧背景下冻土变化及灾害时空演化规律,建立冻土工程稳定性和服役性能评价体系,提出冻土工程灾害防治理论与控制对策,为冻土构筑物群灾害应急预案和重大冻土工程建设提供科学决策依据。     

四川省主要构造单元及其特征

四川省大地构造具有明显的两分性,东部扬子准地台,具有典型的基底与盖层二元结构.基底具双层模式,下层为结晶基底,上层为褶皱基底.盖层包括南华系-中三叠统,以碳酸盐岩为主,富含扬子古生物群.西部为松潘-甘孜地槽褶皱系,包括巴颜喀拉冒地槽和沙鲁里山优地槽.