Linking deformation, migmatite formation and zircon U–Pb geochronology in polymetamorphic orthogneisses, Sveconorwegian Province, Sweden

Absolute ages of migmatization in the polymetamorphic, parautochthonous basement of the Sveconorwegian Province, Sweden, have been determined using U–Pb ion probe analysis of zircon domains that formed in leucosome of migmatitic orthogneisses. Migmatite zircon was formed by recrystallization whereas dissolution–reprecipitation and neocrystallization were subordinate. The recrystallized migmatite zircon was identified by comparison of zircon in mesosomes and leucosomes. It is backscatter electron‐bright, U‐rich (800–4400 ppm) with low Th/U‐ratios (generally 0.01–0.1), unzoned or ‘oscillatory ghost zoned’, and occurs as up to 100 μm‐thick rims with transitional contacts to cores of protolith zircon. Protolith ages of 1686 ± 12 and 1668 ± 11 Ma were obtained from moderately resorbed, igneous zircon crystals (generally Th/U = 0.5–1.5, U < 300 ppm) in mesosomes; protolith zircon is also present as resorbed cores in the leucosomes. Linkage of folding, synchronous migmatization and formation of recrystallized zircon rims allowed direct dating of south‐vergent folding at 976 ± 7 Ma. At a second locality, similar recrystallized zircon rims in leucosome date pre‐Sveconorwegian migmatization at 1425 ± 7 Ma; an upper age bracket of 1394 ± 12 Ma for two overprinting phases of deformation (upright folding along gently SSW‐plunging axes and stretching in ESE) was set by zircon in a folded metagranitic dyke. Lower age brackets for these events were set at 952 ± 7 and 946 ± 8 Ma by zircon in two crosscutting and undeformed granite–pegmatite dykes. Together with previously published data the present results demonstrate: (i) Tectonometamorphic reworking during the Hallandian orogenesis at 1.44–1.42 Ga, resulting in migmatization and formation of a coarse gneissic layering. (ii) Sveconorwegian continent–continent collision at 0.98–0.96 Ga, involving (a) emplacement of an eclogite unit, (b) regional high‐pressure granulite facies metamorphism, (c) southvergent folding, subhorizontal, east–west stretching and migmatization, all of which caused overprint or transposition of older Mesoproterozoic and Sveconorwegian structures. The Sveconorwegian migmatization and folding took place during or shortly after the emplacement of Sveconorwegian eclogite and is interpreted as a result of north–south shortening, synchronous with east–west extension and unroofing during late stages of the continent–continent collision.     

Discovery of double-peaking potassic volcanic rocks in Langshan Group of the Tanyaokou hydrothermal-sedimentary deposit,Inner Mongolia,and its indicating significance

Since the mid-1980s,Tanyaokou large Zn-Cu-Fe sulfides deposit,located at the southwest end of Langshan-Zhaertaishan-Bayan Obo Mesoproterozoic metallogenic belt in the west section of the northern margin of the North China Platform[1?9](Fig.1),has been confirmed to be submarine volcanic exhalative-sedimentary metamorphosed deposit hosted in the miogeosynclinal mud-carbonaceous formation of the Langshan Group(LG)[1],or submarine volcanic exha-lative-deposition-altered deposit[2]or stratabo…     

Molar‐Tooth Structure from the Mesoproterozoic Wumishan Formation in Lingyuan,Yanshan Region,North China,and Geological Implications

Although its origin has not yet reached a consensus so far, MTS (Molar-Tooth Structure) has been documented for more than 100 years. Current study reports a discovery of MTS from the Mesoproterozoic Wumishan Formation, Lingyuan, Yanshan Region, North China, and the features and geological implications of MTS are further discussed. Here, straitigraphic horizons of MTS’s occurrences show that it was mainly located within the top part of the Wumishan Formation, i.e., limestone unit. Four kinds of morphology of MTS, i.e., fine fusiform, debris, ribbon, ptigmatic and nodular (irregular), were recognized and thought to be highly related to the sedimentary environments and facies. Geochemistry of MTS including oxides, trace elements and C, O and Sr isotopes indicates that the horizons of MTS-bearing is of higher Sr/Ba and Ca/Mg ratios, lower positive δ13C and highly negative δ18O values than the adjacent stratigraphic levels of rare MTS. Lithology, morphology and geochemistry of MTS in the Wumishan Formation suggest that MTS occurs mainly in shallow subtidal near the storm wave base, which is typically characterized by warm temperature, oversaturated calcium carbonate seawater and high organic productivity. Furthermore, occasional enrichment of algae bacteria here is more favorable for the calcification of calcium oozes and catalytic for MTS. C isotope composition of the Wumishan Formation and MTS of this study is well correlated with that of the Mesoproterozoic Belt Supergroup, North America and Riphean, Siberia, suggesting that MTS acts as a sedimentary record responding to global changes and is a perfect indicator in Precambrian stratigraphic correlation worldwide.     

Climatic Versus Tectonic Control on Storm Cyclicity in Mesoproterozoic Koldaha Shale, Vindhyan Supergroup, Central India

The 1.2 Ga-old Koldaha shale, central India reveals three orders of depositional cyclicities in its basal storm-dominated shelf succession. Visual appraisal as well as Fourier and MEM analyses concurs in this respect. Only the major storm events at intervals of a few thousands of years have left recognizable imprints. Interbedding of storm sandstones and fairweather shales is apparently climate-controlled. Packaging of about seven such climatic cycles results the second-order cyclicity befitting eccentricity cycles of contemporary scale. Nonetheless, for the erratic storm bed-thickness trends within the cycles some other factor/s might have played a role. The third order cycles are, more dominantly, correlatable with basinal tectonics.     

Sedimentary Features and Their Implications of Microdigital Stromatolites from the Mesoproterozoic Wumishan Formation at the Jixian Section in North China

<正>The Mesoproterozoic Wumishan Formation at the Jixian section in Tianjin is a set of more than 3000-m-thick stromatolitic carbonate succession.In this succession,several lithofacies units,that is,the subtidal stromatolitic biostrome,the thrombolitic bioherm,tidal-flat micritic dolomite and lagoon dolomitic shale,make up many meter-scale cycles of the peritidal carbonate type that have been nominated as the Wumishan cycles.Importantly,many microdigital stromatolites make up the stromatolitic biostrome unit of the Wumishan cycles in the lower part of the Wumishan Formation. These microdigital stromatolites have been grouped as a stromatolitic assemblage by paleontologists, that is,"Pseudogymnosolen mopanyuensis-Scuphus-Yangzhuang columnaris"assemblage.These microdigital stromatolites had also been interpreted as the aragonite(tufa) sea-floor precipitates by sedimentologists,and has further been thought as the special products of the transitional period from the sea-floor aragonite precipitates of the Archean to the clastic and muddy carbonates of the Neoproterozoic.Although there are some restrictions for the stratigraphic meaning of the concept of the stromatolitic assemblage,detailed studies on classification by paleontologists provide an important clue to understand the sedimentological meaning of the microdigital stromatolites.Furthermore,an important and obvious horizon for the end of the microdigital stromatolites was recorded in the Mesoproterozoic Wumishan Formation at the Jixian section,which provides useful information to understand the stromatolite decline occurred at c.1250 Ma and the evolving carbonate world of the Precambrian.     

大兴安岭吉峰科马提岩地质地球化学特征

野外地质调查和室内岩石学研究表明,大兴安岭北段吉峰林场一带变质超基性岩为具有典型鬣刺结构的科马提岩。科马提岩系列由橄榄质科马提岩、玄武质科马提岩及拉斑玄武岩、辉长岩等岩石组成。科马提岩显示了从超镁铁质到镁铁质地球化学趋势,拉斑玄武岩具有从富镁到富铁的趋势,而上覆长英质火山岩则遵循钙-碱趋势。科马提岩稀土配分型式为类似于南非超镁铁质科马提岩的平坦型或轻稀土略富集而重稀土平坦型。科马提岩系列8件样品的Sm-Nd同位素数据构成一条相关性较好的等时线,等时线年龄为1727Ma±74.7Ma,I_Nd=0.510725±0.0000798,ε_Nd(t)=6.94±1.56,表明科马提岩形成于中元古代早期,其源区为亏损的软流圈地幔。这一地壳增生事件可能与松嫩地块从西伯利亚地台南缘裂解有关。      

北京密云元古宙常州沟组之下环斑花岗岩古风化壳岩石的发现及其碎屑锆石年龄

在北京密云地区,最近发现环斑花岗岩(脉)上发育有古风化壳,并被长城系常州沟组砂岩覆盖。风化壳物质的组成主要为来自环斑花岗岩的原地风化残留物和粗碎屑岩,采用SHRIMP和LA-ICP-MS方法,分别获得环斑花岗岩古风化壳碎屑岩的碎屑锆石U-Pb年龄值为(1682±20)Ma、(1708±6)Ma等,与相邻的密云环斑花岗岩年龄相同。这套古风化壳碎屑岩的存在和测年结果显示,长城系常州沟组的底界年龄应小于1682Ma。     

河北宽城地区中元古代高于庄组碳酸盐岩碳氧同位素特征

对河北宽城地区中元古代高于庄组碳酸盐岩的碳氧同位素进行了测定和原始性验证,表明其原始组分保存良好。δ13C、δ18O值分布范围和平均值分别为-5.03‰～0.07‰、-9.92‰～-4.12‰和-0.90‰、-6.58‰,整体上稍低于前人测定的天津蓟县剖面和北京十三陵剖面数据。分析认为:研究区δ13C值主要受有机碳氧化与有机碳的埋藏速率因素影响,有机碳的埋藏速率与蓝绿藻等生物数量关系密切,藻类繁盛的时期一般都具有较高的δ13C值,藻类稀少的时期则具有较低的δ13C值。在浅水潮坪环境中,δ13C值与海平面的变化呈正相关关系;研究区δ18O值则主要受海平面变化影响,与之呈负相关关系;研究区古盐度Z值主要介于120～125之间,相关性分析表明Z值不仅反映氧同位素组成,也反映了碳同位素组成,δ18O和δ13C均与沉积介质的盐度有关,其变化趋势是盐度越大,其δ值越高。     

东阿拉善地块诺尔公群石英岩的碎屑锆石年龄特征——物源区制约及其地质意义

传统上将阿拉善地块东部变质基底"阿拉善岩群"之上的一套以石英岩、浅粒岩、碳酸盐岩及碎屑岩为主的浅变质地层称为诺尔公群,并根据区域地层对比及叠层石化石将其归为"长城纪"。为进一步限定地层时代,对诺尔公群底部3件石英岩进行LA-MC-ICP-MS锆石U-Pb测年,获得的年龄主要集中在2 530~2 500 Ma和1 950~1 850 Ma两个年龄段内,另外还获得少量年龄为~1.69 Ga、~2.0 Ga、~2.15 Ga、~2.35 Ga、~2.7 Ga和~3.4 Ga的锆石。其中,最年轻的碎屑锆石年龄限定了诺尔公群底部石英岩的沉积时代晚于1.69 Ga,结合上覆地层的年龄数据将诺尔公群的沉积时代大致限定为1.69~1.29 Ga,肯定了阿拉善地块存在中元古代地层。石英岩中~2.5 Ga和~1.95 Ga两个显著的碎屑锆石年龄峰符合典型的华北克拉通物源区特征,因此认为,阿拉善地块在太古代-中元古代具有与华北克拉通相似的构造环境,是华北克拉通的一部分。     

天津蓟县中元古界高于庄组中臼齿状构造的层序地层位置及其成因的初步研究

天津蓟县剖面的中元古界高于庄组为一套厚度约为1 600m的碳酸盐岩地层，包括四个段：第一段以潮坪相叠层石白云岩为主；第二段主要为含锰白云岩；第三段发育较多的纹理化石灰岩和泥晶灰岩；第四段则以叠层石岩礁（叠层石生物丘和生物层）的发育为特点。根据岩相到岩相序列可在该套碳酸盐岩地层中识别出L-M型、潮下型、环潮坪型米级旋回层序。根据米级旋回层序的有序垂直叠加形式所反映出的沉积相序列可以把高于庄组划分为13个三级层序（SQ₁至SQ₁₃），并进一步归为4个二级层序。在以灰岩为主的高于庄组第三段中，其中的第三个三级层序（SQ₁₁）中部的灰岩层中发育臼齿状构造。这种臼齿状构造以特别的形态、富含有机质、易硅化等特点可能表明了前寒武纪碳酸盐岩沉积作用的一些基本特征：第一、在浅水环境中发育叠层石而在较深水环境（中缓坡）中发育臼齿状构造，臼齿状构造就象叠层石一样是一种极为特别的与生物沉积作用相关的沉积构造；第二、在发育叠层石的潮坪环境中有利于发生白云石化作用，发育臼齿状构造的地层则以灰岩为主，这从一个侧面反映了前寒武纪白云岩似乎又不是原生白云岩。实际上，这些特征本身即代表了一些前寒武纪沉积学问题，随着研究的深入对这些问题将会得出更加接近自然事实的答案。