共查询到20条相似文献，搜索用时 250 毫秒
1.
新疆西南天山木扎尔特河一带低压泥质麻粒岩岩石学特征、独居石U-Th-Pb定年及其地质意义 总被引：6，自引：5，他引：1
古南天山洋闭合过程中,由于洋壳俯冲产生的岛弧岩浆作用加热大陆地壳,在新疆西南天山木扎尔特一带形成了一套低压高温泥质麻粒岩相变质岩石.本文用Theriak-Domino热力学软件对该套岩石中的堇青石榴夕线石黑云母片麻岩和含夕线石堇青石榴黑云母片麻岩进行了岩石学相平衡计算研究,得到它们峰期变质的温压条件分别是：T=630～674℃,P=5.2～5.5kbar和T=645～684℃,P=5.4～5.7kbar.并采用独居石Th-U-Pb电子探针定年方法,对样品WQ006中的3颗独居石进行了原位年龄测定(38个分析点),得到2组等时线年龄,分别是376±8Ma和280σ8Ma(2σ).结合独居石的岩相学特征,提出了新疆西南天山低压高温麻粒岩相峰期变质作用的时代为280±8Ma,而376±8Ma(2σ)可能为原沉积岩的原岩/成岩年龄.表明西南天山洋壳开始俯冲发生在晚古生代,进一步证明了西南天山造山带俯冲碰撞发生在晚二叠纪之后的观点. 相似文献
2.
浙西南八都杂岩早中生代泥质麻粒岩变质作用及构造意义 总被引：1，自引：0，他引：1
遂昌-大柘泥质麻粒岩出露于华夏地块东北部的浙西南八都杂岩中,该岩石保留了典型的减压反应结构.但其变质演化特点、变质作用时代及构造意义目前尚不明确.通过系统的岩相学、矿物化学和同位素年代学分析,结果表明遂昌-大柘泥质麻粒岩记录了4个阶段的变质矿物组合,其中早期进变质阶段M1的矿物组合为石榴石+黑云母+石英;压力峰期变质阶段M2的矿物组合为石榴石+铝绿泥石+金红石+蓝晶石+刚玉+黑云母+石英±十字石,该矿物组合可能预示着岩石曾经历了超高压变质作用过程;峰期变质阶段M3的矿物组合为石榴石+黑云母+夕线石+石英±钾长石±斜长石±钛铁矿;峰后近等温降压M4-1阶段的矿物组合为石榴石+黑云母+夕线石+堇青石+石英+钛铁矿±尖晶石±斜长石±钾长石;M4-2阶段的矿物组合为石榴石+堇青石+夕线石+斜长石+黑云母+石英±钾长石.相平衡模拟结合传统地质温压计限定其峰期变质阶段的温压条件为T=780~810 ℃、P=8.0~9.2 kbar;峰期后近等温降压的M4-1阶段的温压条件为T=780~860 ℃和P=5.7~6.0 kbar,M4-2阶段的温压条件为T=~700 ℃和P=~4.4 kbar,具有典型的顺时针近等温减压型P-T轨迹特征.LA-ICP-MS U-Pb定年结果表明其麻粒岩相变质作用时代为233.5~238.9 Ma.变质作用历史说明浙西南地体可能卷入了古特提斯洋域内印支-华南-华北板块之间的俯冲-碰撞过程,并经历了早中生代的麻粒岩相变质作用后快速折返至地表. 相似文献
3.
大青山-乌拉山变质杂岩带石拐地区富铝片麻岩成因矿物学与变质演化 总被引：11，自引：8，他引：3
大青山-乌拉山变质杂岩中石拐地区富铝片麻岩出露于华北克拉通孔兹岩带中段,包括夕线石榴堇青二长片麻岩、紫苏石榴黑云二长片麻岩和夕线石榴黑云二长片麻岩,与基性麻粒岩彼此呈互层或夹层产出.根据岩相学观察、成因矿物学和变质反应结构的系统研究,结合地质温压计估算以及相平衡模拟的综合分析,揭示石拐地区富铝片麻岩的变质演化可划分为四个变质阶段.其中,早期进变质阶段(M1)矿物组合以石榴石核部及其包裹的细粒矿物黑云母+石英+斜长石±夕线石±钾长石±尖晶石为特征;峰期变质阶段(M2)的稳定的矿物组合为石榴石+基质中粗粒夕线石+黑云母+石英+斜长石+钾长石±磁铁矿±钛铁矿,形成的温压条件为T=840 ～ 860℃,P=10.0～10.5kbar;峰后近等温减压阶段(M3)以石榴石边部发育含堇青石的后成合晶为特征,并发生一系列典型的减压反应:Grt+ Sil+ Qz→Crd、Grt+ Melt→Crd+ Bt+ PI和Grt+ Melt→Crd+ Qz±P1,形成新的矿物组合为石榴石+堇青石+黑云母+斜长石+石英±夕线石±紫苏辉石,相应的温压条件为T=720～ 800℃和P=5.6 ～6.1kbar;晚期角闪岩相降温阶段(M4)的矿物组合是石榴石+石榴边部细粒黑云母+斜长石+石英+磁铁矿±钾长石±钛铁矿,记录的温压条件为T=616 ～661℃和P=3.4 ～5.2kbar.石拐地区富铝片麻岩及相关岩石具有典型的近等温减压的顺时针P-T轨迹,峰后经历了近等温减压和近等压降温的变质演化阶段.上述研究结果表明,石拐地区富铝片麻岩曾卷入到华北克拉通西部的阴山陆块和鄂尔多斯陆块间的俯冲-碰撞造山及随后的快速隆升的演化过程. 相似文献
4.
东南极拉斯曼丘陵出露的中-高级变质岩是普里兹湾基底岩系的主要组成部分,是研究普里兹湾高级区组成与演化的理想对象。本文对拉斯曼丘陵米洛半岛地区的夕线石榴二长片麻岩进行了岩石学、相平衡模拟和地质年代学研究。研究结果表明拉斯曼丘陵夕线石榴二长片麻岩矿物组合为石榴子石+堇青石+斜长石+钛铁矿+夕线石+石英±黑云母±钾长石,经历了格林威尔期(～980Ma)高温变质,其峰期阶段(M1)的矿物组合为石榴子石+斜长石+钾长石+钛铁矿+夕线石+石英,变质温压条件为:～820℃、～9. 6kbar。夕线石榴二长片麻岩的岩浆锆石比变质锆石具有更低的LREE和更高的HREE含量,其原岩的沉积年龄不早于～1073Ma,物源主要为中元古代岩浆岩(S-型花岗岩)。拉斯曼丘陵在格林威尔期经历了陆-陆碰撞,进而快速折返、剥蚀作用的过程。 相似文献
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内蒙古集宁三岔口夕线堇青石榴二长片麻岩变质作用及年代学研究 总被引：7，自引：7，他引：0
三岔口夕线堇青石榴二长片麻岩出露于华北克拉通孔兹岩带东部的集宁变质杂岩中,该岩石保留了典型的减压反应结构。系统的岩相学观察和矿物化学研究结果表明,三岔口地区夕线堇青石榴二长片麻岩保存了两个阶段变质作用的矿物组合及相应的变质反应结构,其中峰期变质阶段M1的矿物组合为石榴石+夕线石+黑云母+石英+斜长石+钾长石+钛铁矿±磁铁矿;峰后减压的M2-1变质阶段矿物组合为石榴石+堇青石+夕线石+石英+斜长石+钾长石+钛铁矿±尖晶石,M2-2阶段的矿物组合为石榴石+堇青石+黑云母+石英+斜长石+钾长石+钛铁矿+磁铁矿±尖晶石。自峰期变质阶段到峰后减压阶段,典型的转变(减压)反应包括:Grt+Sil+Melt→Crd+Bt+Fe-oxide和Grt+Sil→Crd+Spl。相平衡模拟结合传统地质温压计限定峰期变质阶段的温压条件为T=852~862℃,P=9.3~10.2kbar;峰期后近等温减压的M2-1和M2-2阶段的温压条件分别为854~880℃和7.0~7.4kbar,820~848℃和5.3~6.4kbar。三岔口夕线堇青石榴二长片麻岩记录了典型的近等温减压型的顺时针P-T轨迹。阴极发光图像特征显示夕线堇青石榴二长片麻岩存在大量变质锆石,LA-ICP-MS U-Pb定年结果表明,所有变质锆石记录了十分一致的207Pb/206Pb年龄,其变质时代为1912±11Ma。变质作用历史说明内蒙古孔兹岩带东段的集宁地体卷入了华北克拉通西部的阴山陆块和鄂尔多斯陆块之间的俯冲-碰撞,并经历了古元古代(~1912Ma)的麻粒岩相变质作用后快速折返至地表的过程。 相似文献
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山东半岛基性高压麻粒岩的成因矿物学及变质演化 总被引：27，自引：22，他引：5
在山东半岛的平度-莱西-莱阳-烟台一带,广泛出露基性高压麻粒岩,它们呈大小不等的透镜体或不规则脉状体产于TTG片麻岩中,且与超镁铁质岩石密切"伴生"。根据岩相学、矿物相转变、变质反应以及温压条件的系统研究结果,确定山东半岛基性高压麻粒岩经历了四个阶段的变质演化。早期进变质阶段(M1)的标志性矿物组合以石榴石及其内部的细粒单斜辉石+斜长石±石英为特征,记录的温压条件为T=740～770℃、P=0.9～1.0GPa;峰期高压麻粒岩相变质阶段(M2)的矿物组合以基质中石榴石(富Ca核部)+单斜辉石(富Al核部)+斜长石(富Na核部)±石英为特征,记录的温压条件为T=850～880℃、P=1.45～1.65GPa;峰后近等温减压(中压)麻粒岩相退变质阶段(M3),以发生一系列典型减压反应和斜方辉石的大量出现为标志,典型的矿物组合为斜方辉石+单斜辉石+斜长石+磁铁矿±角闪石,记录的温压条件为T=780～830℃、P=0.65～0.85GPa;晚期角闪岩相退变质阶段(M4)以石榴石发生降温反应、形成角闪石+斜长石+磁铁矿±石英等退变矿物组合为特征,记录的温压条件为T=590～650℃、P=0.62～0.82GPa。高压基性麻粒岩的变质演化的P-T轨迹具有顺时针型式,先后经历了近等温减压过程(ITD)和近等压降温过程(IBC)变质演化过程,指示研究区基性高压麻粒岩是古老陆块之间在俯冲-碰撞造山过程中加厚下地壳折返地表的产物。该项成果对于重新认识华北克拉通在早前寒武纪古老陆块的碰撞-拼贴及其演化的动力学过程具有重要科学意义。 相似文献
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冈底斯岩浆弧东段变泥质岩的逆时针P-T-t轨迹与构造机制 总被引：1，自引：1，他引：0
青藏高原南部冈底斯岩浆弧东段出露的中-高级变质岩代表岩浆弧的中-下地壳,是研究岩浆弧深部组成与形成演化的理想载体。本文对冈底斯弧东段布久地区的变泥质岩,即石榴石夕线石云母片岩进行了岩石学、相平衡模拟和地质年代学研究。岩石学和相平衡模拟表明,片岩由黑云母、白云母、石榴石、夕线石、斜长石、石英及少量的钛铁矿、金红石组成,含或不含蓝晶石,经历了早期的低压麻粒岩相变质与部分熔融,变质温压条件为～7kbar和～770℃,中期的高压麻粒岩相变质与深熔作用,变质温压条件为～10. 5kbar和～775℃,以及晚期的角闪岩相退变质与熔体结晶作用,退变质温压条件为～7. 5kbar和～705℃,具有逆时针型的变质作用P-T-t轨迹。本文和现有年代学研究表明,片岩的早期的低压麻粒岩相变质与部分熔融发生在～70Ma,后续高压麻粒岩相变质与深熔发生在50～45Ma,晚期角闪岩相退变质与熔体结晶开始于45Ma,持续到25Ma。大体积幔源岩浆底侵导致的地壳加热与加厚,以及大陆碰撞导致的地壳缩短与加厚是冈底斯弧表壳岩在晚中生代至早新生代被埋藏到下地壳,经历长期持续高温、高压变质与部分熔融的构造机制,而加厚岩石圈地幔在渐新世的拆沉导致了岩浆弧加厚下地壳的折返。 相似文献
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新疆西南天山木扎尔特一带低压麻粒岩相变质作用P-T轨迹及其地质意义 总被引：11，自引：5，他引：11
本文首次报道在新疆西南天山木扎尔特一带发现了二辉石麻粒岩和麻粒岩相变质的堇青石榴矽线石片麻岩。二辉石麻粒岩的矿物组合为单斜辉石-斜方辉石-黑云母-角闪石-斜长石-石英。堇青石榴矽线石片麻岩矿物组合为堇青石-矽线石-石榴石-黑云母-斜长石-石英。岩石学和矿物学特征表明它们是典型的低压麻粒岩相变质岩石，其变质作用经历了两期演化：a．峰期麻粒岩相变质，T=681～705℃，P=5．4～5．8kbar；b．峰后角闪岩相退变阶段，T：571～637℃．P=4．7～5．3kbar。其变质作用P-T轨迹具有逆时针近等压降温(IBC)的特点，代表该地区可能为塔里木板块向伊犁．中天山板块俯冲过程中，在陆壳一侧所产生的陆源岩浆弧区域，由于受到下部岩浆热源的影响，在拉伸环境下出现低压麻粒岩相变质。通过分析低压麻粒岩相岩石与其南部高压一超高压变质带的大地构造位置和年代关系，我们认为该地区的低压麻粒岩相变质岩石可能与其南部的西天山高压一超高压变质带组成了双变质带。 相似文献
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东南极梅瑟半岛出露大量的副片麻岩。本文以岩相学和矿物化学为基础,结合相平衡模拟、地质温度计及年代学分析方法对费拉副片麻岩中的含尖晶石榴夕线石片麻岩和梅瑟副片麻岩中的含堇青石榴夕线石片麻岩进行综合研究,结果显示片麻岩经历了超高温(UHT)麻粒岩相变质作用。两个样品峰期前阶段(M1)进变质条件为600～650℃和0.63～0.85GPa;峰期UHT阶段(M2)变质条件为930～1000℃和0.9～1GPa;峰期后变质重结晶阶段(M3)变质条件为610～780℃和0.3～0.39GPa。三个不同阶段的变质条件定义了一致的顺时针的P-T演化轨迹。锆石U-Pb年龄分析结果表明两个片麻岩样品的超高温变质和相关的深熔作用发生在990～920Ma期间的早新元古代格林威尔期,而流体改造与变质重结晶作用发生在～510Ma的早古生代泛非期。结合区域地质特征,超高温变质作用发生的构造背景可能与以放射性元素衰变为热源的大型热碰撞造山带相关。 相似文献
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位于祁连地块北缘的门源-柯柯里杂岩体出露一套早古生代高级变质岩,其中长英质片麻岩的矿物组合以石榴子石+夕线石+黑云母+长石+石英为特征,变基性岩以石榴子石+单斜辉石+角闪石+斜长石+石英为特征,具有典型的高角闪岩相-麻粒岩相组合特征。岩相学观察、相平衡模拟和地质温压计估算结果显示门源地区石榴黑云斜长片麻岩和石榴角闪岩的峰期变质作用的温压条件分别为780℃、9.0kbar和790℃、8.4kbar。相平衡模拟揭示石榴黑云斜长片麻岩经历了近等压降温的逆时针P-T轨迹,可能反映了处于中地壳的岩石经历了与洋壳俯冲相关的弧岩浆岩和弧火山岩所带来大量的热所导致的高温变质及随后的等压冷却过程。锆石U-Pb定年显示门源-柯柯里地区2个高级变质岩的变质年龄和1个闪长岩的弧岩浆作用年龄分别为495±2Ma、493±3Ma和495±3Ma,表明门源-柯柯里一带共同经历了北祁连洋壳俯冲导致的500Ma左右的弧岩浆作用和变质作用。门源-柯柯里高温低压变质带和其北侧百经寺-清水沟一带出露的HP/LT变质岩组成了双变质带,并指示了北祁连洋早古生代时期的向南俯冲。 相似文献
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Reproductive pentoxylalean material from the Albian Triton Point Formation, Fossil Bluff Group of Alexander Island, Antarctica is the youngest record of this group globally. Leaves are referred to Taeniopteris daintreei McCoy, ovulate reproductive organs to Carnoconites cranwelli Harris, and stem material to Pentoxylon sp. Occurring in leaf litter layers on palaeosol surfaces the sedimentology suggests that pentoxylalean plants grew upon areas of the floodplain distal to the river channel, where they covered the forest floor in a similar habit to modern brambles. The forest floor was shared with Equisetales and liverworts, an abundance of ferns [Cladophlebis Brongniart, Gleichenites Goeppert, Phyllopteroides Medwell, Sphenopteris (Brongniart) Sternberg], as well as angiosperms. Bennettitales, Ginkgoites Seward and Nilssonia Brongniart were rare within these communities, as were coniferous trees. 相似文献
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The diagnosis of Estherites corrugatus from the basal part of the Coniacian Second Member of the Nenjiang Formation in Nenjiang County, north-east China is revised following the application of a new preparation technique to some of the carapaces and an examination of specimens under a scanning electron microscope, both of which revealed morphological features on the carapace that had not been recognized previously. Restudy of the type species of the two subgenera Estherites (Euestherites) and Estherites (Parestherites) also revealed details of carapace features not seen hitherto. These indicate that they should be separated from Estherites. As a result, Euestherites is upgraded to genus level and Parestherites is placed in synonymy. The importance of Estherites and Euestherites is considered in the context of Late Cretaceous assemblages of these crustaceans and the three conchostracan provinces (South-West, South-East and North China) that are recognized to have been present in China during the Turonian–Santonian period. 相似文献
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In this study we report similar biotic response patterns in planktic foraminiferal assemblages, whether in association with volcanism, impacts or climate change at the end of the Cretaceous and early Tertiary. During and after each type of catastrophe two groups dominate high stress assemblages: (1) the small Guembelitria species, which are interpreted as having thrived in eutrophic surface waters where other species rarely survived; and (2) the low oxygen tolerant small Heterohelix species, which thrived at times of an expanding oxygen minimum zone associated with high nutrients and a stratified water column. The ecosystem collapse appears to be primarily the result of high macro- and micronutrient influx (from impacts, volcanism and erosion) leading to eutrophication and phytoplankton blooms (i.e., primary producers) that result in toxic conditions for foraminifera. Once nutrients decrease due to consumption by phytoplankton, the first opportunistic foraminifera, the Guembelitria, appear and graze on phytoplankton, rapidly reproduce (heterochronic acceleration) and increase populations exponentially. With nutrient depletion Guembelitria populations rapidly decrease leading to ecologic niches for other generalists and ecosystem recovery. Small low O2 tolerant heterohelicid populations mark this second stage, followed by small trochospiral and planispiral species. With further environmental recovery, increasing competition, niche development, and restoration of a well-stratified watermass, oligotrophic conditions are restored opening habitats for large, highly specialized species and a return to normal diverse assemblages. Such highly stressed ecological successions are observed in association with mantle plume volcanism in the Indian Ocean, Andean volcanism in Argentina and shallow inland seas in Egypt and Madagascar during the late Maastrichtian, the K-T impact, volcanism during the early Danian, and intense upwelling and climate extremes. We present a simple model to explain the ecological succession and recovery phases that follow major biotic perturbations. 相似文献
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A review of the tetrapod (amphibian and amniote) record across the Permo-Triassic boundary (PTB) indicates a global evolutionary turnover of tetrapods close to the PTB. There is also a within-Guadalupian tetrapod extinction here called the dinocephalian extinction event, probably of global extent. The dinocephalian extinction event is a late Wordian or early Capitanian extinction based on biostratigraphic data and magnetostratigraphy (the extinction precedes the Illawara reversal), so it is not synchronous with the end-Guadalupian marine extinction. The Russian PTB section documents two tetrapod extinction events, one just before the dinocephalian extinction event and the other at the base of the Lystrosaurus assemblage. However, generic diversity across the latter extinction remains essentially the same despite a total evolutionary turnover of tetrapod genera. The Chinese and South African sections document the stratigraphic overlap of Dicynodon and Lystrosaurus. In the Karoo basin, the lowest occurrence of Lystrosaurus is in a stratigraphic interval of reversed magnetic polarity, which indicates it predates the marine-defined PTB, so, as previously suggested by some workers, the lowest occurrence of Lystrosaurus cannot be used to identify the PTB in nonmarine strata. Correlation of the marine PTB section at Meishan, southern China, to the Karoo basin based primarily on magnetostratigraphy indicates that the main marine extinction preceded the PTB tetrapod extinction event. The ecological severity of the PTB tetrapod extinction event has generally been overstated, and the major change in tetrapod assemblages that took place across the PTB was the prolonged and complex “replacement” of therapsids by archosaurs that began before the end of the Permian and was not complete until well into the Triassic. The tetrapod extinctions are not synchronous with the major marine extinctions at the end of the Guadalupian and just before the end of the Permian, so the idea of catastrophic causes of synchronous PTB extinctions on land and sea should be reconsidered. 相似文献
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A well preserved assemblage of compressed, straight, circular to sinuously coiled megascopic and helical carbonaceous fossils and other varied megascopic morphoforms are known from the Early Mesoproterozoic Rohtas Formation, Semri Group within Vindhyan Supergroup exposed in Katni district of central India. These megascopic remains are preserved as impressions, compressions, partially mineralized remains, and/or epi-relief. Some of the forms are typical filamentous empty sheaths and others are trichomes, with cell like entities under various stages of degradation. This study, based on fresh collections and also of the topotype material of the helically coiled megascopic fossils, straight forms and related fossilized remains occurring as epi-relief from Katni indicate that the two morphotaxa are distinct entities and possibly appear to be prokaryotes. Grypania spiralis and Katnia singhii are most likely of cyanobacterial origin. Spirally coiled and circular fossils, with epi-relief, and which probably represents a tissue grade organism, are considered as Spiroichnus beerii Mathur, 1983. Linear sheet-like carbonaceous solitary form has been placed in the morphotaxon Proterotainia and described as P. katniensis n. sp. Certain rare circular, carbonaceous forms are considered as Chuaria sp. A few circular disc-like forms found in the assemblage are treated as dubiofossils. 相似文献
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The b-value of the Gutenberg–Richter’s frequency–magnitude relation and the p-value of the modified Omori law, which describes the decay rate of aftershock activity, were investigated for more than 500 aftershocks in the Aksehir-Afyon graben (AAG) following the 15 December 2000 Sultandagi–Aksehir and the 3 February 2002 Çay–Eber and Çobanlar earthquakes. We used the Kandilli Observatory’s catalog, which contains records of aftershocks with magnitudes ≥2.5. For the Çobanlar earthquake, the estimated b-values for three aftershock sequences are in the range 0.34 ≤ b ≤ 2.85, with the exception of the one that occurred during the first hour (4.77), while the obtained p-values are in the range 0.44 ≤ p ≤ 1.77. The aftershocks of the Sultandagi earthquake have a high p-value, indicating fast decay of the aftershock activity. A regular increase of b can be observed, with b < 1.0 after 0.208 days for the Çay–Eber earthquake. A systematic and similar increase and decrease pattern exists for the b- and p-values of the Çobanlar earthquakes during the first 5 days. 相似文献
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Early Maastrichtian larger foraminifera of the Qahlah Formation, United Arab Emirates and Sultanate of Oman border region 总被引：2，自引：0，他引：2
The Early Maastrichtian (Late Cretaceous) Qahlah Formation is the oldest stratigraphical unit lying upon the eroded top of the obducted Semail Ophiolite, of Santonian–Campanian age, in the Northern Oman Mountains. It crops out as a series of low hills on the western flank of the mountains, which are located along the boundary between the United Arab Emirates and the Sultanate of Oman. This paper presents the results of an investigation into the bio- and lithofacies of the Qahlah Formation in the Jabal Huwayyah section (at the boundary between the Al Ain area of the United Arab Emirates and the Buraimi area of Oman). The data collected were used to determine the age and depositional environment of the rock unit studied. In addition, the section is lithostratigraphically correlated with other sections in the study area. Microfossils from Jabal Huwayyah are represented mainly by the larger benthic foraminifera Loftusia elongata, L. morgani, Pseudorbitolina marthae, Orbitoides media, Omphalocyclus macroporus and Lepidorbitoides minor. These foraminifera are reported for the first time from this section. The diversity and abundance of fauna, particularly within the calcareous siltstone bed near the top of the formation (unique to the Huwayyah area), reflect a shallow marine depositional environment. 相似文献
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The diagnoses of the conchostracan genera Cratostracus and Porostracus are revised following a re-examination of their type species under a scanning electron microscope. In Cratostracus each growth line has a serrated lower margin. Radial lirae branch into smaller ridges that merge with each other to give the upper part of each growth band in the ventral region of the carapace a slightly undulating surface. Cross-bars are present between these lirae. In Porostracus the radial lirae on the growth bands near the umbo are long and relatively widely spaced. There is a fine reticulum between them in the umbonal part of the carapace and cross-bars on the ventral surface. The ornament of Porostracus indicates that it can be assigned to the family Halysestheriidae rather than to the Afrograptidae, in which it has been placed previously. Morphological analysis of most of the species of the two genera and closely related Orthestheria (Migransia) under the scanning electron microscope is required before their stratigraphic potential can be fully realized. 相似文献