Emplacement of the Ardara pluton (Ireland): new constraints from magnetic fabrics, rock fabrics and age dating

The Ardara pluton as part of the Donegal batholith was intruded into Neoproterozoic metasediments and metadolerites at mid-crustal levels. The emplacement mechanism of the Ardara granite is very controversial, and mechanisms ranging from diapirism, ballooning and stoping followed by nested diapirism have been proposed. Magnetic fabrics, rock fabrics and K/Ar dating of micas are used here to constrain the emplacement history. The compositional zoning of the Ardara pluton is clearly reflected in the different bulk magnetic susceptibilities between the outer quartz monzodiorite and the central granodiorite, whereas the intervening tonalite is of intermediate nature. The magnetic carriers are characterized by the anisotropy of the magnetic susceptibility (AMS), thermomagnetic measurements and through high field analyses (HFA). The separation of the ferrimagnetic and paramagnetic contributions revealed that biotite and magnetite control the AMS in the quartz monzodiorite. Both minerals are oriented in such a way that their summed contribution is constructive and originates from the shape fabric of magnetite and the texture of biotite. Biotite is responsible mainly for the AMS in the tonalite and granodiorite. The magnetic foliation can be directly related to the macroscopic foliation and also to the D4 structures in the country rocks. The foliation is consistent with the geometry of the roughly circular shape and has a mostly steep to vertical dip. Towards the central granodiorite the magnetic foliation dies out, although plagioclase texture measurements indicate a weak magmatic shape fabric. With the exception of the tail, the K_max axes (magnetic lineation) vary from steeply to gently plunging. The so-called lineation factor is approximately 1.01 and therefore points to a less significant axial symmetry. These observations coincide with strain estimates on mafic enclaves that show a very consistent pattern of K ∼0 flattening strain. Texture analyses of biotite and quartz additionally support the observations made by the strain analyses and the magnetic fabric data. Microstructural investigations give evidence that the fabrics are associated with the emplacement over a range of temperatures from truly magmatic to high-temperature solid-state conditions. The age of the intrusion is still under discussion, but a new cooling age was determined by K/Ar dating of biotite at 403.7±8 Ma corresponding to a temperature range between 450 and 300°C. For a mylonite along the southern contact between the Ardara pluton and the country rock a K/Ar muscovite age of 378.8±7 Ma indicates a minimum age for the shear zone when the Ardara pluton must have already been cooled down below 350±50°C. Received: 28 January 1999 / Accepted: 28 December 1999     

大别山双河片麻状花岗岩岩体石英组构特征及应变分析

对大别山双河片麻状花岗岩岩体中的石英C轴组构、黑云母(001)解理极点和石英颗粒三维应变分别进行系统测量。结果显示,双河岩体在侵位和后期演化过程中以挤压变形为特点,岩体经过NW-SE向和NE-SW向两次挤压,NW-SE向挤压强度较大,奠定了岩体基本的构造形态。结合区域应力场特征分析得出,双河岩体在早三叠世至早侏罗世应力方位为NW-SE向挤压,中侏罗世及后期为NE-SW向挤压,应力场发生顺时针变化。      

西天山喀拉达拉基性岩体及其构造背景

在中国西天山伊什基里克山一带,发育有一系列橄榄辉长岩、橄长岩、辉长岩、辉绿岩床和岩墙群,同位素年龄为２７７～２９７Ｍａ。其中哈拉达拉层状橄榄辉绿长岩－辉绿辉长岩体（又称大桥岩体）规模最大,约２５．５ｋｍ＾２。根据近年来的实地研究,认为该岩体形成于伊犁盆地石炭一二叠纪裂谷带发育的构造背景之下,既有大量碱性玄武岩和火山碎屑岩,又有一定数量的基性一超基性杂岩和层状辉长侵入体,经历了稳定－亚稳定－稳定的演     

下扬子地区姚村A型花岗岩年代学、地球化学特征及岩石成因

姚村岩体位于下扬子江南造山带东北端,主要由中心相的中粗粒正长花岗岩和边缘相的细粒似斑状正长花岗岩组成.本文对该岩体进行了详细的锆石U-Pb年代学、主量元素、微量元素以及Nd-Hf同位素研究.LA-ICP-MS锆石U-Pb定年表明姚村岩体的形成年龄为(127.6±1.4)Ma,为燕山晚期岩浆活动的产物.岩石地球化学研究表...     

招平断裂带中段深部结构构造——来自地震剖面的证据

玲珑花岗岩岩基东侧的招平断裂带是胶西北矿集区的主要控矿构造之一。布设于招平断裂带中段大尹格庄矿区的地震剖面,揭示了招平断裂带中段深部的结构构造特征,地震剖面显示,玲珑岩基内部有多个类似"拱弧构造"的岩浆侵入形成的弧形界面,由此推测存在2~3期的岩浆活动。剖面显示了3期构造组合样式,第1期为以招平断裂带和宋家河断裂为组合倾向SE的铲式断裂,底部似汇入统一的构造滑脱带;第2期为倾向NW的错断了招平断裂带的反倾伸展断裂;第3期为NE向延伸以花状构造为特征的走滑断裂。前两期反映构造发育处于伸展环境,第3期显示为左行走滑。招平断裂带深部延伸存在分支复合现象,形成由2个主裂面夹持的透镜状岩片。地震反射剖面数据的获得对深刻理解该区域中生代岩浆构造演化过程和深部找矿具有重要的借鉴意义。     

西准噶尔南部庙尔沟岩体晚石炭世花岗闪长斑岩岩石成因及其动力学背景

位于西准噶尔南部的庙尔沟岩体主体由碱长花岗岩和少量紫苏花岗岩组成.本文在前人工作基础上,以岩体东南边缘新发现的花岗闪长斑岩为研究对象,开展岩石学、年代学和Hf同位素以及全岩地球化学研究,确定花岗闪长斑岩形成时代、揭示岩石成因类型及源区属性、探讨其与碱长花岗岩和紫苏花岗岩岩浆演化成因联系及其形成的深部动力学过程.锆石U-...     

Mesoproterozoic Continental Arc Type Granite in the Central Tianshan Mountains: Zircon SHRIMP U-Pb Dating and Geochemical Analyses

The Central Tianshan belt in northwestern China is a small Precambrian block located in the southern part of the Central Asia Orogenic Belt （CAOB）, which is considered as ＂the most voluminous block of young continental crust in the world＂ that comprises numerous small continental blocks separated by Paleozoic magmatic arcs. The Precambrian basement of the central Tianshan Mountains is composed of volcanic rocks and associated volcano-sedimentary rocks that were intruded by granitic plutons. Geochemical analyses demonstrate that the granitic plutons and volcanic rocks were generated in the Andean-type active continental arc environment like today＇s Chile, and the zircon U-Pb SHRIMP dating indicates that they were developed at about 956 Ma, possibly corresponding to the subduction of the inferred Mozambique Ocean under the Baltic-African super-continent.     

香草坪花岗岩体年代学和地球化学特征

香草坪岩体主要岩性为中粗粒斑状黑云母花岗岩,SHRIMP锆石U-Pb法年龄为211±2Ma,属于印支期岩浆作用产物。岩石地球化学特征研究表明,其ACNK,微量元素模式图曲线形态总体向右倾,以及轻稀土富集、Eu明显亏损的稀土配分模式曲线特征,与华南S型花岗岩的地球化学特征相似。该岩体高(87Sr/86Sr)i和低εNd(t)的特点,表明其可能是前寒武系中等成熟度的基底岩石经部分熔融形成的。该区基底岩石较高的铀含量,可为香草坪岩体提供充足的铀元素,形成铀元素的初始富集。     

Porphyroblast rotation during crenulation cleavage development: an example from the aureole of the Mooselookmeguntic pluton, Maine, USA

In the low‐pressure, high‐temperature metamorphic rocks of western Maine, USA, staurolite porphyroblasts grew at c. 400 Ma, very late during the regional orogenesis. These porphyroblasts, which preserve straight inclusion trails with small thin‐section‐scale variation in pitch, were subsequently involved in the strain and metamorphic aureole of the c. 370 Ma Mooselookmeguntic pluton. The aureole shows a progressive fabric intensity gradient from effectively zero emplacement‐related deformation at the outer edge of the aureole ～2900 m (map distance) from the pluton margin to the development of a pervasive emplacement‐related foliation adjacent to the pluton. The development of this pervasive foliation spanned all stages of crenulation cleavage development, which are preserved at different distances from the pluton. The spread of inclusion‐trail pitches in the staurolite porphyroblasts, as measured in two‐dimensional (2‐D) thin sections, increases nonlinearly from ～16° to 75° with increasing strain in the aureole. These data provide clear evidence for rotation of the staurolite porphyroblasts relative to one another and to the developing crenulation cleavage. The data spread is qualitatively modelled for both pure and simple shear, and both solutions match the data reasonably well. The spread of inclusion‐trail orientations (40–75°) in the moderately to highly strained rocks is similar to the spread reported in several previous studies. We consider it likely that the sample‐scale spread in these previous studies is also the result of porphyroblast rotation relative to one another. However, the average inclusion‐trail orientation for a single sample may, in at least some instances, reflect the original orientation of the overgrown foliation.     

湘南中山地区中棚岩体地球化学特征与铀成矿

通过对中棚岩体地球化学特征的研究,探讨其与铀矿化的关系。中棚岩体属于过铝质花岗岩,高钾钙碱性岩石系列。分析认为,岩体具有S型花岗岩特征,为陆壳物质部分熔融的产物,原岩可能为寒武系浅变质砂岩。构造环境判别认为,岩体属于后碰撞花岗岩范畴,在岩石圈伸展作用构造环境下由寒武系部分熔融所形成。岩体的原岩铀含量较高,为富铀的中棚岩体形成奠定了基础,也为后期成矿提供了充足的铀源。