Progressive shortening axis rotation recorded by Variscan synkinematic granites : example of the South Armorican shear zone in the Vendée (France)

The Variscan continental collision is expressed by large shear zones in Western Europe. The synthesis of granite ages, related to different deformation fields in the Vendée area, suggests a geodynamic model for the tectonic evolution of this part of the Variscan belt between 370 Ma – 320 Ma. After the first step of the continental collision, leading to high temperature and anatexis at 375-360 Ma, the south-eastern part of the Armorican massif underwent large dextral shearing along N110-N125 trending shear zones, related to a bulk NNW-SSE shortening direction, up to early Visean time. Large-scale displacements progressively decreased at around 345-340Ma. During middle Visean time, the shortening axis direction rotated towards a NNE-SSW position implying changes in the regional deformation field. The occurrence of N70-N100 sinistral and N110-N130 dextral conjugate shear zones within leucogranites are related to that epoch. Finally, a new dextral shear zone system, trending N130-N150 along the Parthenay shear zone, occurs during late Visean time. This progressive middle Visean change of shortening direction probably corresponds to a major change in the Iberian plate motion and indentation during the Mississipian collision.     

西秦岭宕昌地区晚三叠世酸性火山岩的锆石U-Pb年代学、地球化学及其地质意义

西秦岭造山带在早中生代发育了以印支早期(245~234Ma)和印支晚期(225~205Ma)两期高钾钙碱性花岗岩占绝对优势,并伴有少量火山岩的强烈岩浆活动。本文报道的西秦岭宕昌火山岩总体上是一套以流纹岩为主,并含粗面岩和英安岩的酸性火山岩组合。流纹岩锆石U-Pb测年结果为229Ma,限定其喷发时代为晚三叠世早期。宕昌火山岩属于过铝质的高钾钙碱性-钾玄岩系列,以富集Rb、Ba、Th、U、K并相对亏损Nb、Ta、Ti、P为特征,(⁸⁷Sr/⁸⁶Sr)_i=0.7048~0.7068,ε_Nd(t)=-4.3~-4.0,亏损地幔模式年龄(t_DM)为1.20~1.24Ga。宕昌火山岩起源于下地壳富钾变基性岩的部分熔融,部分岩浆还可能受到了少量地幔或上地壳物质的混染,源区深度约40km。西秦岭地块存在扬子型的中新元古代基底,但不同地段的基底特征存在较大差异。华北板块与扬子板块在西秦岭地区的初始碰撞可能接近229Ma,宕昌火山岩则形成于以整体挤压为主、局部剪切伸展为辅的地壳逐渐加厚的动力学环境。     

江西中部新丰街花岗质岩体LA-ICP-MS锆石U-Pb年代学、元素和Sr-Nd-Hf同位素地球化学特征及岩石成因

对江西中部新丰街花岗质岩体开展了岩相学、LA-ICP-MS锆石U-Pb年代学以及元素和Sr-Nd-Hf同位素地球化学研 究,并探讨了岩石成因及其构造意义。结果表明,新丰街岩体由二云母花岗岩和黑云母花岗组成;两者均形成于晚侏罗世 （~148 Ma）;二云母花岗岩SiO2含量为75.71%~78.36%,为弱过铝质-强过铝质岩石,属高钾钙碱性系列,Mg#变化于0.26~ 0.34,具有较低的Ga/Al比值（绝大部分<2.6×10-4） 和较低的Zr+Nb+Ce+Y含量（<350×10-6）,全岩εNd(t)为-10~-8.2,锆石原 位εHf(t)为-15.7~-9.4;黑云母花岗岩SiO2含量为71.25%~74.41%,主要为准铝质-弱过铝质岩石,也属于高钾钙碱性系列, Mg#变化于0.32~0.37,同样具有较低的Ga/Al比值（绝大部分<2.6×10-4） 和较低的Zr+Nb+Ce+Y含量（<350×10-6）,全岩初始 87Sr/86Sr比值为0.7136~0.7153,εNd(t)为-10.0~-8.9,锆石原位εHf(t)值为-16.5~-10.9。通过综合研究认为二云母花岗岩具有S 型花岗岩特征,是由下地壳中变质泥岩在相对较低温度下发生部分熔融而形成的;黑云母花岗岩具有I型花岗岩特征,是由下 地壳中长英质火成岩在相对较高温度下发生部分熔融而形成的。岩体侵位于由古太平洋板块俯冲引起的陆缘弧构造环境。     

Geological interpretation of gravity profiles through the Karlovy Vary granite massif (Czech Republic)

We examined the shape of the Late Variscan Karlovy Vary granite massif located south of the Ohre/Eger graben in Northern Bohemia by reinterpretation of existing gravity data on two perpendicular profiles. The granite body of about 360 km² total outcrop size has the elongation ratio 0.35 with the major axis trending NE-SW. The SW part of the body was crossed in the nineties by the seismic profile 9HR which localized the bottom of granites in a depth of about 10 km. We used this value as a reference datum in our gravity profiles. We positioned one of our profiles along the seismic profile 9HR and the other one perpendicularly, i.e. parallel with the elongation of the outcrop surface. We interpret the shape of the main granite body in the vicinity of Karlovy Vary as a continuous desk whose floor is horizontal (or subhorizontal) and varies along its whole extension about a depth of 10 km. This thickness is approximately identical with that of the Saxothuringian nappes imaged by seismic reflection. The near surface upper contact of the granite body is mildly inclined, and outward dipping. It changes to steep sides or inward inclined contacts in deeper levels. The Lesny-Lysina (Kynžvart) massif is a separate granite body about 324 km thick, not continuously connected with the main Karlovy Vary massif. The gravity curve suggests that granites often enclose in their endocontact large blocks of country metasediments or metabasites the existence of which is partly evidenced by their outcrops outside the line of the profile. The granite body is found density-homogenous. Minor density differences between granite varieties are caused mainly by more intense hydrothermal alterations in younger suite granites. We interpret vertical conduits for the ascent of granitic magmas to be parallel to the Jáchymov-Gera and Ohře (Eger) lineaments or the Mariánské Lázně fault zone as indicated by the elongation of some outcrops. However, they are not clearly imaged from the gravity data. The effect of the depression of the Sokolov basin along the faults parallel with the Ohře (Eger) lineament is shallow and it is not indicated by any change in the floor depth of the granite body. Comparison of the seismicity distribution suggests that the hypocenters occur mostly outside of the granite bodies or near their contact with the country rock.     

Mineralogical feature and geological significance of muscovites from the Longyuanba Indosinian and Yanshannian two-mica granites in the eastern Nanling Range

Emplacement P-T condition estimations using granites are important for understanding metamorphic and erosional processes of orogenic belt.Granites are widespread in South China and a majority of them are peraluminous.Particularly,over 91%of the Indosinian granites exposed in the region are peraluminous in composition.It is extremely hard to determine the pressure of intrusion of these peraluminous granites due to the absence of amphibole,a good mineral barometer commonly identified in metaluminous granites.Muscovite is a common mineral in peraluminous granites,certain kind of it could be used as a mineral barometer to constrain the emplacement pressure of peraluminous granites.In this paper,results of petrographic and geochemical studies of muscovites from the Indosinian and early Yanshanian two-mica granites at the Longyuanba in the eastern Nanling Range are reported.Based on petrographic studies,the primary muscovite can be discriminated from the secondary muscovites.Muscovites from the Indosinian two-mica granites are enriched in Ti,Al,Mg,and Na,and depleted in Fe and Mn.Geochemically,these muscovites were considered as primary,whereas those from the Yanshanian two-mica granites fall into the area of secondary muscovite on discrimination diagrams.Barometer estimations show that pressures calculated for primary muscovites are accurate,but those calculated for secondary muscovites are overestimated.The average pressure of emplacement of the Longyuanba Indosinian two-mica granites is 5.9 kbar,corresponding to~19 km in depth,suggesting that the Indosinian granitic magmas were probably generated by partial melting of a thickened crust root in a compressional tectonic setting.     

小兴安岭东南段重要铅锌多金属、钼矿床的成矿年龄

小兴安岭成矿带东南段是我国东北地区重要的铅锌多金属矿化、钼矿化集中区之一,成矿作用与区内花岗质岩浆侵入作用密切相关,铅锌多金属矿床成因类型为夕卡岩型,钼矿床成因类型为斑岩型.对区内典型矿床进行锆石精确测年研究结果表明,铅锌多金属矿床成矿年龄集中在175.8~209 Ma,鹿鸣钼矿床成矿年龄为176±4 Ma.认为铅锌多金属矿的（主）成矿期为印支晚期-燕山早期,钼矿成矿期为燕山早期.     

Stenian A-type granitoids in the Namaqua-Natal Belt,southern Africa,Maud Belt,Antarctica and Nampula Terrane,Mozambique:Rodinia and Gondwana amalgamation implications

We carried out SHRIMP zircon U-Pb dating on A-type granitic intrusions from the Namaqua-Natal Province,South Africa,Sverdrupfjella,western Dronning Maud Land,Antarctica and the Nampula Province of northern Mozambique.Zircon grains in these granitic rocks are typically elongated and oscillatory zoned,suggesting magmatic origins.Zircons from the granitoid intrusions analyzed in this study suggest~1025-1100 Ma ages,which confirm widespread Mesoproterozoic A-type granitic magmatism in the Namaqua-Natal(South Africa),Maud(Antarctica) and Mozambique metamorphic terrains.No older inherited(e.g.,~2500 Ma Achean basement or~1200 Ma island are magmatism in northern Natal)zircon grains were seen.Four plutons from the Natal Belt(Mvoti Pluton,Glendale Pluton,Kwalembe Pluton,Ntimbankulu Pluton) display 1050-1040 Ma ages,whereas the Nthlimbitwa Pluton in northern Natal indicates older 1090-1080 Ma ages.A sample from Sverdrupfjella,Antarctica has~1091 Ma old zircons along with~530 Ma metamorphic rims.Similarly,four samples analysed from the Nampula Province of Mozambique suggest crystallization ages of~1060-1090 Ma but also show significant discordance with two samples showing younger~550 Ma overgrowths.None of the Natal samples show any younger overgrowths.A single sample from southwestern Namaqualand yielded an age of~1033 Ma.Currently available chronological data suggest magmatism took place in the Namaqua-Natal-MaudMozambique(NNMM) belt between~1025 Ma and~1100 Ma with two broad phases between~1060-1020 Ma and 1100-1070 Ma respectively,with peaks at between~1030-1040 Ma and~1070-1090 Ma.The age data from the granitic intrusions from Namaqualand.combined with those from Natal,Antarctica and Mozambique suggest a crude spatial-age relationship with the older1070 Ma ages being largely restricted close to the eastern and western margins of the Kalahari Craton in northern Natal,Mozambique.Namaqualand and WDML Antarctica whereas the younger 1060 Ma ages dominate in southern Natal and western Namaqualand and are largely restricted to the southern and possibly the western margins of the Kalahari Craton.The older ages of magmatism partially overlap with or are marginally younger than the intracratonic Mkondo Large lgneous Provinee intruded into or extruded onto the Kalahari Craton,suggesting a tectonic relationship with the Maud Belt.Similar ages from granitic augen gneisses in Sri Lanka suggest a continuous belt stretching from Namaqualand to Sri Lanka in a reconstituted Gondwana,formed during the terminal stages of amalgamation of Rodinia and predating the East African Orogen.This contiguity contributes to defining the extent of Rodinia-age crustal blocks,subsequently fragmented by the dispersal of Rodinia and Gondwana.     

海西至印支期郯庐断裂带的性质--据中国东部石炭纪至三叠纪的岩相古地理特征分析

文中通过对晚石炭世至早三叠世华南和华北地块古地理特征以及地层学证据的分析,认为中国东部的郯庐断裂带自海西期以来经历了两个主要发展阶段：第一阶段是广义的郯庐断裂带发展阶段,在海西期它是扬子地块北东缘呈宽缓弧形展布的边缘裂陷槽(或盆地)的边界;在印支期由于扬子地块与华北地块的碰撞,成为两地块的对接边界,具有逆冲推覆的性质,属广义的特提斯构造域。第二发展阶段从燕山期以来,发展成为一条平移断裂带,属于狭义的环太平洋构造域的平移系统。自晚石炭世至早三叠世的中国南方及华北东南部的岩相古地理资料显示了扬子地块与华北地块的对接始于晚二叠世早期,地块的抬升自南向北、自南东向北西方向呈迁移趋势;印支期的郯庐断裂带是一条北东、北北东展布的缓‘S’形的地块拼贴边界,在现今的郯庐断裂带上表现为残留的由北北西向南南东的斜向逆冲推覆的性质,表现为大别苏鲁造山带的中上部构造层的变形,即张八岭构造带及前陆褶皱冲断带的变形;燕山期以来则为众所周知的狭义的郯庐断裂带即郯庐平移断裂系统的一部分。     

秦岭造山带是印支碰撞造山带吗?

国内外一些学者认为秦岭是一个印支碰撞造山带.但迄今为止,秦岭尚未发现三叠纪或古生代延续到三叠纪的洋盆存在的任何痕迹.秦岭泥盆系-三叠系为滨、浅海相沉积,没有远洋沉积,更没有镁铁质和超镁铁质岩石及与之密切相关的放射虫硅质岩组成的蛇绿岩套.泥盆系与下伏地质体之间有一个清楚的区域性角度不整合.商丹断裂并不是印支期,而是加里东期的板块缝合带;其两侧,中朝板块南缘和扬子板块北缘均有十分清楚的加里东造山作用的记录.沉积于扬子板块北缘的中上泥盆统刘岭群的放射性铅同位素组成与北秦岭相近,碎屑锆石年龄谱系亦证明其物质主要来自中朝板块南缘的北秦岭造山带.所谓勉略印支缝合带中的勉略和三里岗蛇绿混杂岩中的镁铁质岩,同位素测年均为元古代之产物,后者又被南华系-震旦系沉积覆盖.所谓勉略缝合带,实为一区域性大断裂带.早古生代,其北侧属扬子板块北部被动边缘;南侧为扬子板块核心部分的扬子准地台(小克拉通).所以,秦岭的印支造山作用,并不是洋盆消失后的陆陆碰撞造山作用,而是海盆消失后的中朝与扬子2个小陆块间逆冲-叠覆造山作用.作为秦岭东延的大别山超高压变质带被认为是秦岭印支碰撞造山的重要证据之一,但大别山超高压变质岩是在造山作用过程中动态超高压条件下形成的,仅用简单的静岩压力来计算其形成深度,显然是不符合实际情况的.野外地质观察、构造地质学、变质岩石学、同位素地质学、地球化学、地球物理学以及物理实验等方面的实际资料和研究结果均说明超高压变质作用并不是在上地幔而是在地壳内进行的.南秦岭-大别山的地壳构造层次,上地壳自上而下依次为:未变质的沉积岩层、绿帘-蓝片岩层、高压变质岩层、超高压变质岩层;下地壳为未卷入超高压变质作用的麻粒岩相-高角闪岩相变质杂岩.含柯石英的超高压单位只是位于上地壳下部的厚约10～12km的席状构造岩片.初步认为上地壳这一从低压到高压再到超高压的构造系统,是印支造山期间,南秦岭-大别山的上地壳以下地壳顶部为主剪切滑动面,多层次剪切作用造成的.上地壳下部的超高压变质岩,则可能是强烈剪切引起的频繁地震的震源区瞬时超高压作用的结果.     

贺兰山古元古代花岗岩年代学及地球化学特征:对华北克拉通西部孔兹岩带形成和演化的制约

华北克拉通西部贺兰山地区基底岩石以古元古代孔兹岩系和S型花岗岩为主,该区的S型花岗岩是探索华北克拉通西部陆块孔兹岩带地质演化的关键研究对象之一。本文主要以贺兰山北部的片麻状含石榴黑云母花岗岩和似斑状石榴黑云母花岗岩为研究对象,通过详细的岩石学、锆石U-Pb年代学、Hf同位素以及全岩地球化学等方法来讨论其岩石成因和地质意义。结果表明,片麻状含石榴黑云母花岗岩和似斑状石榴黑云母花岗岩侵位年龄分别为～2. 0Ga和～1. 97Ga,属于古元古代时期不同阶段岩浆作用的产物。片麻状含石榴黑云母花岗岩具有较低SiO_2(60. 88%～65. 42%),高Al_2O_3(16. 84%～19. 35%)、Fe_2O_3~T(5. 94%～6. 14%)、K_2O (3. 25%～5. 10%)、A/CNK (1. 75～2. 34)和Mg#(47～51),低Na_2O (1. 14%～2. 45%),以及较高的稀土元素含量(∑REE=262×10-6～290×10-6)和明显的Eu负异常(δEu=0. 30～0. 55)。似斑状石榴黑云母花岗岩具有高SiO_2(71. 93%～74. 98%)、K_2O (4. 58%～6. 98%)、Al_2O_3(13. 25%～14. 79%)和Na_2O (3. 13%～3. 94%),低Fe2OT3(0. 69%～1. 39%),A/CNK值为1. 08～1. 14,且具有相对较低的Mg#值(17～43)和稀土元素总量(∑REE=35. 3×10-6～127×10-6),Eu负异常和正异常(δEu=0. 07～3. 58),二者均属于过铝质S型花岗岩。贺兰山两期花岗岩明显富集大离子亲石元素(如Rb、U等),亏损高场强元素(如Nb、Ta等)。早期片麻状含榴黑云母花岗岩和晚期似斑状石榴黑云母花岗岩的εHf(t)值分别为-1. 1～+2. 6和-2. 1～+3. 8,对应的两阶段Hf同位素模式年龄(tDM2)分别为2. 45～2. 68Ga和2. 36～2. 73Ga,远大于其锆石的结晶年龄。此外,贺兰山花岗岩具有相对高的Al_2O_3/TiO_2和低的CaO/Na_2O、Rb/Sr、Rb/Ba比值,表明其原岩可能主要以变沉积岩为主。上述结果表明贺兰山地区古元古代S型花岗岩的岩浆起源于古老地壳变沉积岩物质,可能有少量新生地壳物质的加入。综合区域地质资料,本文研究认为贺兰山地区在古元古代晚期为活动大陆边缘的构造环境,片麻状含石榴黑云母花岗岩和似斑状石榴黑云母花岗岩分别形成于阴山地块和鄂尔多斯地块碰撞前和同碰撞的构造背景,表明这两个微陆块的碰撞时间可能要稍早于1. 95 Ga。