东喜马拉雅构造结石榴角闪岩变质作用P-T-t轨迹:相平衡模拟与锆石年代学

东喜马拉雅构造结南迦巴瓦杂岩由多种类型的高级变质岩组成,包括片岩、片麻岩、大理岩、石榴角闪岩和基性麻粒岩。石榴角闪岩呈透镜状产出在片麻岩中,可见不连续分布的规模不等的浅色体。石榴角闪岩由石榴石、角闪石、黑云母、斜长石、石英、金红石、钛铁矿和榍石组成,石榴石变斑晶可见由浅色矿物组成的"白眼圈"。岩相学、矿物化学和相平衡模拟表明石榴角闪岩经历了一条顺时针型的P-T演化轨迹,可划分为两个阶段:(Ⅰ)升温升压的进变质阶段,由石榴石和斜长石斑晶记录,峰期矿物组合为石榴石+角闪石+黑云母+斜长石+石英+金红石+钛铁矿。运用石榴石变斑晶边部和斜长石变斑晶边部成分在视剖面图上的投点确定出峰期温压条件为~11.5kbar、790℃,达到了高压麻粒岩相条件,并经历了部分熔融,产生至少9%的熔体;(Ⅱ)降温降压的退变质阶段,由石榴石边部"白眼圈"冠状体记录。运用平均温压法计算冠状体中黑云母+斜长石+角闪石+石榴石组合的形成温压条件为~7kbar、~750℃。该阶段金红石消失,熔体结晶,并与早期矿物发生回反应。在石榴石角闪岩的锆石中获得了从29.2Ma到10.2Ma的连续变质年龄。由于锆石通常在熔体结晶过程中生长,因此确定该组年龄代表石榴石角闪岩退变质年龄。本文和以前的研究结果表明,南迦巴瓦杂岩中的高温和中温麻粒岩相亚单元具有相似的降温降压P-T轨迹,但高温单元具有较高的变质压力条件,表明其俯冲到了更大的深度。     

河南鲁山太华变质杂岩前寒武纪变质作用

太华变质杂岩("太华群")出露于华北克拉通中部造山带南缘,包括自西北向东南依次出露的华山、洛宁、鲁山、舞钢等四个出露区。鲁山地区太华变质杂岩中的斜长角闪岩类,变质演化历史记录得相对最为全面,其中普遍保留了三个阶段的变质矿物组合。第一阶段变质矿物组合(M1)为进变质组合,以石榴子石变斑晶中的包裹体矿物组合(石英+斜长石+角闪石±单斜辉石±钛铁矿)为代表,形成于约710℃/7.8kbar的条件下。第二阶段变质矿物组合(M2)为变质高峰期组合,以石榴子石变斑晶和基质矿物组合(角闪石+斜长石+石英±单斜辉石±钛铁矿)为代表,形成于约730~810℃/8.1~11.7kbar的条件下。第三阶段的矿物组合为退变质组合,以环绕石榴子石变斑晶发育的后成合晶矿物组合(石英+斜长石+角闪石±单斜辉石±钛铁矿)为代表,形成的温度与压力条件约为730~740℃/3.5~5.7kbar。这些斜长角闪岩类记录了顺时针变质作用P-T轨迹,其中包含近等温降压的退变质过程,其中变质峰期变质条件达高角闪岩相到麻粒岩相过渡区。二次离子质谱(SIMS)探针锆石U-Pb定年表明,变质锆石年龄介于1.93~1.87Ga之间。结合其他学者发表的资料,推测华北中部造山带的中段和南段似乎在~1.95Ga即已开始了俯冲-碰撞的造山过程,而且该造山过程几乎延续了150Myr之久。     

喜马拉雅造山带核部的变质作用与部分熔融:亚东地区高压泥质麻粒岩的岩石学与年代学研究

喜马拉雅造山带核部的高喜马拉雅结晶岩系是印度大陆深俯冲到欧亚板块之下经历了高压变质作用的产物,记录了喜马拉雅造山带的形成与演化历史。本文对喜马拉雅造山带中段亚东地区高喜马拉雅结晶岩系中的泥质麻粒岩进行了岩石学和锆石U-Pb年代学研究,结果表明泥质麻粒岩经历了复杂的变质演化和部分熔融,可识别出三期变质矿物组合。早期进变质矿物组合为石榴石+斜长石+钾长石+黑云母+白云母+石英,峰期变质矿物组合为石榴石+斜长石+钾长石+黑云母+蓝晶石+石英,晚期退变质矿物组合为石榴石+斜长石+钾长石+夕线石+黑云母+白云母+石英。相平衡模拟表明,该泥质麻粒岩经历了高温、高压的峰期变质条件为800~835℃和12.8~14kbar,在进变质和峰期变质过程中经历了白云母和黑云母脱水熔融,所形成的熔体量至少为5%~8%。麻粒岩的晚期退变质条件为720~740℃和7.6~8.3kbar。这表明泥质麻粒岩经历了一条以高压麻粒岩相峰期变质和降温、降压退变质为特征的顺时针P-T轨迹。锆石U-Pb定年结果表明,麻粒岩相变质和深熔作用发生在28.5~17.0Ma。本研究表明高喜马拉雅结晶岩系的上部构造层位经历了高压麻粒岩相变质作用,而不是以前认为的以高温、低压变质作用为特征,并为喜马拉雅造山带构造演化的研究提供了新的见解。     

拉萨地体东部早侏罗纪变质和深熔作用

位于青藏高原南部的拉萨地体不仅记录了中生代的新特提斯洋俯冲及随后新生代的印度-欧亚板块陆陆碰撞造山作用,而且还记录了晚古生代-早中生代南、北拉萨地体的拼合作用。本文对拉萨地体东部东久地区的片岩和脉体进行了岩石学和锆石U-Pb年代学研究,表明片岩经历了峰期高角闪岩相的变质作用和部分熔融,中压角闪岩相退变质过程以及晚期的降温、降压过程。片岩记录了峰期矿物组合蓝晶石+石榴石+黑云母+斜长石+钛铁矿+石英,退变质矿物组合石榴石+夕线石+堇青石+黑云母+斜长石+钛铁矿+石英,晚期退变质矿物组合堇青石+黑云母+白云母+绿泥石+斜长石+钛铁矿+石英。相平衡模拟研究表明,片岩的峰期变质作用温度、压力条件约为720℃、0.9GPa;退变质条件约为670℃、0.59GPa以及480℃、0.12GPa。全岩地球化学研究表明,含石榴石长英质脉体具有显著的Eu元素正异常(δEu=3.57),为斜长石堆晶的产物。锆石U-Pb年代学表明,片岩和脉体在早侏罗纪的181 Ma和195 Ma发生了变质和部分熔融作用。本文结合已发表研究结果表明,东久地区的高级变质岩可划分出不同的构造岩片,在早侏罗纪先后经历了相似温、压条件的变质作用,为南、北拉萨地体碰撞造山作用的产物。     

辽东半岛北部三家子石榴斜长角闪岩变质演化P-T-t轨迹及其地质意义:来自相平衡模拟与锆石U-Pb定年的约束

石榴斜长角闪岩常蕴含着丰富的变质反应结构,对古元古代造山带构造热演化过程研究具有十分重要的科学意义。胶-辽-吉带辽东半岛北部三家子地区出露的石榴斜长角闪岩,主要以岩脉的形式顺层侵入至南辽河群(里尔峪岩组)含黑云母的斜长片麻岩之中。三家子石榴斜长角闪岩以发育典型的"白眼圈"结构为特征,并保留了三个不同演化阶段矿物组合,峰前矿物组合为分布于石榴子石内部细粒包体矿物,主要包括角闪石+石英+磁铁矿+钛铁矿(M_1);峰期矿物组合为石榴子石+角闪石+单斜辉石+斜长石(An=44~45)+石英+钛铁矿(M_2);峰后退变质阶段矿物组合为角闪石+斜长石(An=64~90)+石英+钛铁矿+磁铁矿+榍石(M_3)。相平衡模拟与矿物对地质温压计算表明,三家子石榴斜长角闪岩峰前变质阶段(M_1)、峰期变质阶段(M_2)与峰后退变质阶段(M_3)稳定的P-T条件分别为:P=0.68~0.72GPa、T=570~600℃,P=0.98~1.01GPa、T=690~710℃,P=0.52~0.61GPa、T=670~700℃。锆石矿物包体扫描电镜分析、锆石CL图像分析与锆石LAICP-MS U-Pb定年的综合研究表明,三家子石榴斜长角闪岩原岩形成时代为2.17~2.06Ga之间;近峰期高压角闪岩相变质的时代为1.96~1.94Ga;峰后近等温减压中-低压角闪岩相退变质时代为1.92~1.83Ga,记录该组年龄的变质锆石微区含有典型的中-低压角闪岩相矿物包体(角闪石+斜长石(An=60~90)+石英+磷灰石)。综合以上分析,本文初步认为三家子石榴斜长角闪岩具有顺时针P-T-t轨迹,其主要特点是从M_1到M_3表现为升温升压的变质过程,并达到变质作用的峰期阶段,峰后开始表现为近等温减压的变质过程,随后可能表现为冷却降温的退化变质过程。因此,它们与胶-辽-吉带古元古代造山作用有关的构造增厚作用有密切关系。     

东南极拉斯曼丘陵泥质麻粒岩变质作用演化

普里兹湾拉斯曼丘陵代表了东南极一条重要的早古生代的～530Ma泛非期(Pan-African)高级构造活动带。然而,该区早期的晚元古代的～1000Ma格林维尔期(Grenvellian)高级变质作用的演化历史至今仍有争论。该区呈透镜状产出的泥质麻粒岩峰期矿物组合(M1)为石榴石+堇青石+斜方辉石+钾长石+石英,峰期石榴石变斑晶发育堇青石或堇青石+斜方辉石反应边(M2)。利用Thermocalc程序在KFMASH模式体系对该泥质麻粒岩进行的定量模拟表明,其峰期矿物组合是由反应石榴石+黑云母+石英=堇青石+斜方辉石+钾长石+熔体形成的。利用Themocalc平均P-T计算方法获得峰期M1变质P-T条件为～0.9GPa和～900℃,而叠加的M2组合反映了一个减压冷却的过程,其变质P-T条件为～0.7GPa和800～850℃。结合已有的年代学数据,认为该区泥质麻粒岩的峰期M1矿物组合反映晚元古代(～1000Ma)格林维尔期挤压D1构造事件,而叠加的M2矿物组合与M3蠕虫状结构则形成于早古生代泛非期(～530Ma)D2～D3高级扭压剪切构造期间。该扭压事件导致了面状高低应变带的发育以及进步花岗岩和伟晶岩的侵入。     

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

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M_4-1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;M_4-2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M_4-1阶段的温压条件为T=780~860 ℃和P=5.7~6.0 kbar,M_4-2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.      

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

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.     

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

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。     

阿尔泰造山带晚古生代高温变质作用与塔里木地幔柱活动的成因联系:来自泥质和镁铁质麻粒岩的证据

阿尔泰造山带是中亚造山带(CAOB)的重要组成部分,在其南缘出露有高温泥质和镁铁质麻粒岩,确定其P-T轨迹对于理解阿尔泰造山带南缘在晚古生代的构造演化历史具有重要意义。通过对该区乌恰沟泥质和镁铁质麻粒岩样品精细的岩相学观察,显示其变质矿物组合分别为石榴石+堇青石+黑云母+斜长石+石英+磁铁矿+钛铁矿±斜方辉石±尖晶石±钾长石,斜方辉石+单斜辉石+角闪石+黑云母+斜长石+钾长石+石英+钛铁矿+磁铁矿。使用传统温压计和平均温压计算方法对其变质峰期及峰期后的P-T条件进行了计算,并利用Thermocalc软件对其中两个泥质麻粒岩进行了P-T视剖面图模拟,确定了其峰期变质条件为770~865℃和3.0~5.1kbar,并得到了两条峰期后近等压冷却(IBC)的P-T演化轨迹。这样近等压冷却的P-T轨迹表明高温变质作用可能发生于一个总体伸展的大地构造背景之下。已有年代学数据支持该区高温变质事件发生于二叠纪(270~280Ma),与塔里木地幔柱活动的时间(~275Ma)高度一致。这说明阿尔泰二叠纪高温变质事件可能与塔里木地幔柱有着密切的成因联系。因此,我们认为二叠纪地幔柱活动引起的幔源岩浆底侵和加热可能提供了阿尔泰南缘高温-超高温变质作用所需的热源。     

Palaeoecology and taxonomy of Pentoxylales from the Albian of Antarctica

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.     

Elastic properties and stability of coexisting 3T and 2M 1 phengite polytypes

The elastic properties of coexisting natural 3T and 2M ₁ phengite samples (Cima Pal, Sesia Zone; Val Savenca; Western Alps, Italy) with similar chemical compositions have been studied by room temperature–high pressure powder diffraction, using synchrotron radiation on the ID9A beam-line at ESRF (Grenoble, France). The P–V curves have been modelled by the Birch–Murnaghan model; a third-order expansion fitted to the experimental data yields for 3T and 2M ₁ K ₀=60.4(±0.7) GPa, K′=5.79(±0.11) at V ₀=703.8851 ų, and K ₀=57.3(±1.0) GPa, K′=6.97(±0.24) at V ₀=938.8815 ų, respectively. The relative stability of 3T vs. 2M ₁ has been explored as a function of pressure and temperature in terms of configuration and deformation contributions to the Gibbs energy, using the elastic properties determined here and other thermodynamic parameters from earlier investigations. The results presented agree with the hypothesis of stability of the 3T polytype in the high pressure regime.     

Occurrence and significance of pyrochlorins in a lake sediment

The occurrence of pyrochlorophyll a and pyrophaeophytins a and b in the bottom sediments of a small eutrophic lake (Priest Pot, Cumbria, U.K.) is reported on the basis of HPLC coinjection with standards. Assignment of pyrophaeophytins a and b was confirmed by FAB mass spectrometric and nuclear magnetic resonance studies of the isolated components and comparison with standards. The co-occurrence of pyrochlorins with their non-pyro counterparts, even in a surface sediment, shows that decarbomethoxylation can occur at the very earliest stages of the Treibs diagenetic pathway linking the chlorophylls and sedimentary porphyrins.     

Scaling variables and interpretation of eigenvalues in principal component analysis of geologic data

The dominant feature distinguishing one method of principal components analysis from another is the manner in which the original data are transformed prior to the other computations. The only other distinguishing feature of any importance is whether the eigenvectors of the inner product-moment of the transformed data matrix are taken directly as the Q-mode scores or scaled by the square roots of their associated eigenvalues and called the R-mode loadings. If the eigenvectors are extracted from the product-moment correlation matrix, the variables, in effect, were transformed by column standardization (zero means and unit variances), and the sum of the p-largest eigenvalues divided by the sum of all the eigenvalues indicates the degree to which a model containing pcomponents will account for the total variance in the original data. However, if the data were transformed in any manner other than column standardization, the eigenvalues cannot be used in this manner, but can only be used to determine the degree to which the model will account for the transformed data. Regardless of the type of principal components analysis that is performed—even whether it is Ror Q-mode—the goodness-of-fit of the model to the original data is given better by the eigenvalues of the correlation matrix than by those of the matrix that was actually factored.     

Early Maastrichtian larger foraminifera of the Qahlah Formation, United Arab Emirates and Sultanate of Oman border region

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.     

西藏同卡地区高压麻粒岩的发现、PT轨迹及其大地构造意义

在班公湖-怒江缝合带的同卡地区发现了高压麻粒岩,它经历了至少两期变质作用,即高压麻粒岩相和退变质角闪岩相变质作用。峰期高压麻粒岩相矿物组合为:石榴石-单斜辉石-斜长石,退变质角闪岩相矿物组合:角闪石-斜长石。地质温压计的计算结果表明高压麻粒岩相的温压条件为600～750℃,11.0～13.0kb;退变质角闪岩相的温压条件为500～600℃,6.0kb左右,高压麻粒岩具有顺时针退变质的P-T轨迹特点。     

Spatial and seasonal variation of salt ions under the influence of halophytes,in a coastal flat in eastern China

The high salinity of coastal saline field is a key factor limiting the reclamation. Halophytes have been utilized in the reclamation of saline land. The study area is in Yancheng, China. An analysis of the concentrations of Rb, Cs, Sr, and Ba, the ratio of Rb/Cs, and Sr/Ba in soils in autumn shows that the soil of this study area has great homogeneity. Artemisia halodendron, Gossypium hirsutum, and Sesbania cannabina were selected as the reclamation plants in the present study. In order to know the spatial-temporal distribution of soil salinity, the influence of plant-specific vegetation, and the difference of desalination among these halophytes in coastal flat, the authors analyze the soil-layers and seasonal variation in salt ions. Sodium chloride was accumulated in 0–5 cm topsoil with no vegetation during the winter and spring. The effect of desalinization of halophytes is significant. Of the three plant species, Sesbania cannabina has the greatest desalinization. The difference of ions composition of soils covered with various plant species is significant. It can be concluded that halophytes have better amelioration of coastal soil salinity. Special attention should be paid to the selection of plant species and measures to plant and cultivate crops in the reclamation of saline land.     

Biotic effects of environmental catastrophes at the end of the Cretaceous and early Tertiary: Guembelitria and Heterohelix blooms

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.     

Breakage assessment through cluster analysis of joint set orientations of exposed benches of opencast mines

Summary This paper substantiates, through three different case studies, that the breakage assessment of opencast bench blasting can be made scientifically through the analysis of joint set orientations of different exposed benches in association with the corresponding face direction. Joint orientations, dip and strike of bedding planes have a significant influence on rock breakage. They can have decelerating effects on blasting performance, which may be rectified by geological mapping, and subsequently modifying the blast design parameters.