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1.
Abstract

Variscan convergence produced two-sided (bivergent) crustal-scale thrusting in the Vosges Mountains. In the northern Vosges the central polymetamorphic crystallines were thrust to the NW over Cambrian to Silurian low-grade and very low-grade metamorphic clastics. Synorogenic upper Devonian - lower Carboniferous turbidites and volcanics were folded into NW-vergent structures which display SE-dipping slaty cleavage. The entire sequence shows increasing metamorphism and deformation from NW to SE. Late right-lateral strike-slip faulting along the Lalaye-Lubine fault zone outlasted thrusting. In the southern Vosges a lower Carboniferous turbiditic basin that was fringed on the south by a volcanic arc was tectonically shortened by south-directed tectonic imbrication of slivers of varied rocks including ultramafics, gneissic basement, and synorogenic elastics. The increasing degree of deformation and metamorphism towards the north suggests a thrust contact with the polymetamorphic gneisses of the central Vosges. The final stages of Variscan convergence were accompanied by voluminous granitic plutonism and by faulting along NNE-SSW and E-W-trending strike-slip faults. The tectonic evolution reflects progressive Variscan closure of a previously extended basinal crust in a high-temperature regime.  相似文献   

2.
A petrological and thermobarometric study of the Lago Teleccio hornfelses was undertaken to reconstruct the polymetamorphic evolution and constrain the P–T conditions of Permian contact metamorphism. The Lago Teleccio metasedimentary rocks record a Variscan regional metamorphism characterized by amphibolite facies mineral assemblages including quartz, plagioclase, K‐feldspar (Kfs 1), biotite, garnet (Grt 1) and staurolite; this was followed by a late‐Variscan mylonitization event. Metamorphism of the Variscan metamorphic rocks at the contact with a Permian granitic intrusion produced static recrystallization and/or new growth of quartz, garnet (Grt 2), plagioclase, K‐feldspar (Kfs 2), cordierite, green spinel, biotite and prismatic sillimanite (Contact 1). This thermal event, which occurred at a peak pressure of 0.23–0.35 GPa, temperature of 670–700 °C and aH2O of 0.75–1, was followed either during post‐contact metamorphism cooling or, more likely, during the early‐Alpine metamorphism by the breakdown of cordierite into an anhydrous kyanite + orthopyroxene + quartz assemblage. The poorly developed early‐Alpine eclogite facies metamorphism (Alpine 1) was characterized by relatively anhydrous mineral associations and low strain, which locally produced coronitic and pseudomorphous microstructures in metasedimentary rocks, with scanty formation of jadeite, zoisite and a new high‐pressure garnet (Grt 3). Greenschist facies retrogression (Alpine 2) was characterized by the local development of a chlorite‐ and muscovite‐bearing mineral association, suggestive of aqueous fluid incursion. In the hornfelses, the limited extent of metamorphic overprinting is suggested by the fine grain size of the Alpine mineral associations, which formed at the expense of the Permian contact metamorphic associations, and was favoured by the anhydrous mineralogy of the hornfelses.  相似文献   

3.
In the contact aureole of the Oligocene granodiorite of Cima di Vila, granitic pegmatites of Variscan age were strongly deformed during eo‐Alpine regional metamorphism, with local development of ultramylonites. In the ultramylonite matrix, consisting of quartz, plagioclase, muscovite and biotite, microstructures show grain growth of quartz within quartz ribbons, and development of decussate arrangements of mica. These features indicate that dynamic recrystallization related to mylonite development was followed by extensive static growth during contact metamorphism. K‐feldspar porphyroclasts up to 1.5 cm are mantled by myrmekite that forms a continuous corona with thickness of about 1 mm. In both XZ and YZ sections, myrmekite tubules are undeformed, and symmetrically distributed in the corona, and oligoclase‐andesine hosts have random crystallographic orientation. Myrmekite development has been modelled from the P–T–t evolution of the ultramylonites, assuming that the development of the ultramylonites occurred during eo‐Alpine metamorphism at c. 450 °C, 7.5 kbar, followed by contact metamorphism at c. 530 °C, 2.75 kbar. Phase diagram pseudosections calculated from the measured bulk composition of granitic pegmatite protolith indicate that the equilibrium assemblage changes from Qtz–Phe–Ab ± Zo ± Cpx ± Kfs during the ultramylonite stage to Qtz–Pl(An30–40)–Ms–Kfs–Bt(Ann55) during the contact metamorphic stage. The thermodynamic prediction of increasing plagioclase mode and anorthite content, change of white mica composition and growth of biotite, occurring during the end of the heating path, are in agreement with the observed microstructures and analysed phase compositions of ultramylonites. Along with microstructural evidence, this supports the model that K‐feldspar replacement by myrmekite took place under static conditions, and was coeval with the static growth accompanying contact metamorphism. Myrmekite associated with muscovite can develop under prograde (up‐temperature) conditions in granites involved in polymetamorphism.  相似文献   

4.
The Schwarzwald is part of the central polymetamorphic crystalline belt of the Variscan Orogen (»Moldanubian Belt«). From north to south it consists of four terranes: the metasedimentary Zone of Baden-Baden, the polymetamorphic Central Schwarzwald Gneiss Complex, the sedimentary — metamorphic Zone of Badenweiler-Lenzkirch, and the Hotzenwald Complex. The largest of these terranes is the Central Schwarzwald Gneiss Complex (CSGC) whose rocks record a history of protracted regional metamorphism and anatectic melt generation. During Variscan convergence between 350 and 325 Ma the CSGC became detached from a high-temperature lower crustal substratum and was emplaced southeastward over Paleozoic clastics, volcanic rocks and crystalline slivers of the Zone of Badenweiler-Lenzkirch and the Hotzenwald Complex. Kinematic indicators suggest that these early convergent movements on retrograde shear zones and the concomitant crustal thickening were superseded by movements on divergent shear zones. The ascent of voluminous granitic plutons from a mid-crustal zone of melt generation into the upper crust was probably triggered by a change in the crustal kinematics from overall convergence to overall divergence at about 325 Ma. In detail this process was probably diachronous. Detachment of upper crust and large scale melt generation in the middle crust of the Schwarzwald was probably facilitated by the tectonic stacking of water-rich pelitic clastics and gneiss slivers, with relatively even proportions of crystalline and pelitic materials.  相似文献   

5.
位于扬子陆块西缘的石棉草科穹状变形变质体,经历了三次变形变质时期：早期为收缩滑脱变形的区域动力变质、中期热隆伸展动热变质和后期岩浆热接触变质。对主期变质进行了划分,利用变质反应、矿物地质温压计及相关的同位素年龄资料,建立了草科穹状变形变质体演化的ＰＴｔＤ轨迹。  相似文献   

6.
Pre-kinematic greenschist facies metamorphism is often observed in granites and basement units of mountain belts, but rarely dated and accounted for in orogenic cycle reconstructions. Studying pre-kinematic alteration is challenging because of its usual obliteration by subsequent syn-kinematic metamorphism often occurring at conditions typical of the brittle–ductile transition. It is, however, to be expected that pre-kinematic alteration has major implications for the rheology of the upper crust. In the 305 Ma-old Variscan basement of the Bielsa massif (located in the Axial Zone of the Pyrenees), successive fluid–rock interaction events are recorded in granites below 350°C. Combined microstructural and petrographic analysis, low-T thermobarometry and in situ U–Th/Pb dating of anatase, titanite and monazite show extensive pre-orogenic (pre-Alpine) and pre-kinematic alteration related to feldspar sericitization and chloritization of biotite and amphibole at temperatures of 270–350°C at 230–300 Ma. This event is followed by a second fluid–rock interaction stage marked by new crystallization of phyllosilicates at 200–280°C and is associated with the formation of mylonitic shear zones and fractures parallel to the shear planes. U–Pb anatase and monazite ages as well as the microtextural relationships of accessory minerals suggest an age for this event at 40–70 Ma, consistent with independent regional geology constraints. The Variscan basement was therefore softened at late to post-Variscan time, at least 150–200 Ma before the main Alpine shortening while Alpine-age compression (c. 35–50 Ma) leads to the formation of a dense net of mylonites. The associated deformation, both distributed at the scale of the Bielsa massif and localized at decametric scale in mylonitic corridors, precedes the strain localization along the major thrusts of the Axial Zone. The Bielsa massif is a good example where inherited, pre-orogenic upper crustal softening controls the deformation patterns in granitic basement units through low-grade metamorphic reactions.  相似文献   

7.
该文以老君山锡多金属成矿区为研究对象,根据与成矿相关的地质作用的组合特征（加里东期火山喷流沉积成矿作
用、印支期区域变质作用和燕山期花岗岩叠加改造成矿作用）,厘定了三大成矿系列。通过不同成矿系列矿床的稀土元素地
球化学对比研究,发现各成矿系列矿床的稀土元素配分模式具有明显区别如下：Ⅰ. 加里东期海底火山喷流沉积-印支期变
质成矿系列,分布于老君山花岗岩体的北东部外围,其稀土元素配分模式与海相火山喷流沉积岩(岛弧拉斑玄武岩)类似;
Ⅱ. 加里东期海底火山喷流沉积-印支期变质-燕山期花岗岩热液叠加改造成矿系列,分布于老君山花岗岩接触带附近,具
海底火山喷流沉积岩和花岗岩并存的稀土元素配分模式;Ⅲ. 燕山期花岗岩热液成矿系列,位于老君山花岗岩体及接触带,
其稀土元素配分模式与地壳重熔型花岗岩一致。  相似文献   

8.
Analysis of the conodont colour alteration index and the Kübler index of illite allowed us the characterization of four types of very low‐ or low‐grade metamorphism in the Cantabrian Zone (CZ) and determination of their regional and temporal distribution. These types are: (1) an orogenic Variscan metamorphism present only in restricted areas of the western and north‐western parts of the CZ where epizonal conditions are reached; (2) a burial metamorphism that appears in the basal part of some nappes, where anchizonal conditions are sometimes achieved; the thermal peak preceded emplacement of the nappes; (3) a late‐Variscan metamorphism in the southern and south‐eastern parts of the CZ; a cleavage, cutting most of the Variscan folds, is associated with this metamorphism, which has been related to an extensional episode; (4) a contact metamorphism and hydrothermal activity associated with minor intrusive bodies. The extension continued after the Variscan deformation giving rise to hydrothermal activity during Permian times.  相似文献   

9.
Hot collisional orogens are characterized by abundant syn-kinematic granitic magmatism that profoundly affects their tectono-thermal evolutions. Voluminous granitic magmas, emplaced between 360 and 270 Ma, played a visibly important role in the evolution of the Variscan Orogen. In the Limousin region (western Massif Central, France), syntectonic granite plutons are spatially associated with major strike–slip shear zones that merge to the northwest with the South Armorican Shear Zone. This region allowed us to assess the role of magmatism in a hot transpressional orogen. Microstructural data and U/Pb zircon and monazite ages from a mylonitic leucogranite indicate synkinematic emplacement in a dextral transpressional shear zone at 313 ± 4 Ma. Leucogranites are coeval with cordierite-bearing migmatitic gneisses and vertical lenses of leucosome in strike–slip shear zones. We interpret U/Pb monazite ages of 315 ± 4 Ma for the gneisses and 316 ± 2 Ma for the leucosomes as the minimum age of high-grade metamorphism and migmatization respectively. These data suggest a spatial and temporal relationship between transpression, crustal melting, rapid exhumation and magma ascent, and cooling of high-grade metamorphic rocks.Some granites emplaced in the strike–slip shear zone are bounded at their roof by low dip normal faults that strike N–S, perpendicular to the E–W trend of the belt. The abundant crustal magmatism provided a low-viscosity zone that enhanced Variscan orogenic collapse during continued transpression, inducing the development of normal faults in the transpression zone and thrust faults at the front of the collapsed orogen.  相似文献   

10.
Late Variscan granites intruded Brioverian (Upper Proterozoic) and Lower Paleozoic pelitic sequences to the north of the South Armorican shear zone. In the vicinity of the granites, Brioverian garnet micaschists contain pre/syn-S2 assemblages with garnet + staurolite and post-S2 assemblages with staurolite ± andalusite. Andalusite appeared pre/syn- and post-S2 in garnet-free micaschists. The garnets in the Brioverian micaschists are zoned with increasing Mg and decreasing Mn and Ca from core to inner rim. Only poor garnet zonations occur in Paleozoic hornfelses of enclaves in the Rostrenen granite. The results of a microstructurally controlled application of garnet–biotite geothermometers and garnet–plagioclase geobarometers are similar to P–T trends obtained by the Gibbs method of garnet zonation modelling in the system NCFMnMASH. The P–T paths of a pre/syn-S2 regional metamorphism are clockwise between 500–550°C/8 kbar and 700°C/5 kbar, followed by cooling decompression. They contrast with isobaric contact metamorphism between 500 and 700°C at 2.5–3 kbar in Paleozoic hornfelses. This points to a two-stage Variscan metamorphism with a pre-granitic pressure-dominated event in the Brioverian micaschists, followed by Late Variscan contact metamorphism, and suggests the existence of a pre-granitic tectonic boundary between the micaschists and overlying low-grade sequences.  相似文献   

11.
The structure and tectonics of the Aga Zone are considered. It is shown that this zone is a system of tectonic nappes thrust over the Argun microcontinent. The zone is composed of two rock complexes related to the Variscan and Kimmerian structural stages. The Variscan stage (Silurian(?)-Early Carboniferous) comprises structural elements that correspond to the continental slope; the oceanic basin proper; the active continental margin, including an accretionary wedge; and an island arc and backarc basin. The Devonian age of the ophiolites of the Shilka Belt is specified. The formation of this set of tectonic units is related to the Middle Paleozoic pulse of the opening of the Mongolia-Okhotsk paleobasin. The Kimmerian stage (Middle Carboniferous-Early Jurassic) is characterized by a different style of structural evolution. A system of separate troughs filled with flyschoid sequences was formed on the Variscan basement. The unstable setting related to shortening and closure of the paleobasin brought about the spatial migration of sedimentation zones and the development of intraformational breaks in sedimentation, as well as unconformities. This stage was completed in the Lias by the general uplift of the territory and the formation of Jurassic and Cretaceous mollase along its periphery. The Aga allochthonous mass was ultimately formed in the Middle Jurassic. This event is recorded in emplacement of Middle-Late Jurassic granitic plutons that blocked the nappes. The granitic-metamorphic layer was formed in the Paleozoic and Early Mesozoic at the margin of the Aga Zone upon its conjugation with the adjacent continental masses; this layer is related to crustal anatexis. The bulk of the granitic rocks of the Aga Zone were generated in the Middle and Late Jurassic due to the collision of the North Asian continent with the Argun microcontinent.  相似文献   

12.
河南省洛宁县三官庙金矿床构造控矿特征   总被引：1，自引：0，他引：1  
陈全树 《西北地质》2011,44(2):110-116
三官庙金矿位于太华群与熊耳群接触带附近的似斑状花岗岩体南侧,矿床明显受断裂构造控制。矿区内拆离断裂带和层间滑脱断裂蚀变带发育,含金蚀变破碎带中的构造岩石为含矿主岩。矿体形态以较稳定的似层状为主,金矿物为银金矿等;金的载体矿物为黄铁矿、石英和方铅矿。围岩蚀变为硅化、黄铁矿化、方铅矿化、绿泥石化和绢云母化等,矿化类型属蚀变岩型金矿。在多期次强烈构造-岩浆活动、区域变质和混合岩化作用过程中,Au元素得以沿层间滑脱断裂蚀变带富集成矿。  相似文献   

13.
德新银铅锌矿床位于拉萨地块中部隆格尔-工布江达断隆带中段,是冈底斯银铅锌多金属成矿带上典型的热液型矿床之一。区内的铅锌矿化主要赋存于围岩裂隙和花岗斑岩中。利用LA-ICP-MS锆石U-Pb测年方法对区内含矿花岗斑岩进行年龄测定,获得~(206)Pb/~(238)U年龄加权平均值为57.7±0.5Ma(MSWD=1.6,n=15),代表花岗斑岩及与之相关的铅锌矿化形成年龄。综合区域地质背景,认为德新矿区与花岗斑岩有关的铅锌矿化形成于印度-亚洲大陆主碰撞时期,与纳如松多隐爆角砾岩型铅锌矿属同一岩浆成矿系统,冈底斯中西段存在(近)主碰撞阶段成矿事件。  相似文献   

14.
曾祥平 《云南地质》2003,22(3):260-266
多年对滇东地质勘查和矿业开发研究中发现，区内各金矿床(体)的赋存部位，多沿被动大陆边缘华力西期不整合面分布。由于区内华力西期不整合面明显受到边缘NS两侧两条深大断裂控制，多期NE～SW向左行剪切挤压作用，使不整合面上下地块以其为差异变形轴，旋转产生多期构造剥离或错移，在不整合面上下层位中形成较发育的构造剥离空间、张裂带或板劈理带。这些略呈线型分布的构造挤压带，在地史发展过程中，卷入多期构造变形后演变成局限断裂陷槽或槽沟环境，沉积了一套有别于其它区域的岩性组合。边缘深大断裂多期次海底基性火山喷发或喷流作用。除提供了幔源、壳源含金物质和含矿热液的活化与迁移的通道外，再次卷入构造变形的这些局限断裂陷槽或槽沟又成为区域金成矿物质聚集的有利场所，含矿热液选择沉积其内或堆积其中的细碎屑岩、粘土质岩、含碳质泥质岩类、不纯碳酸盐岩进行交代与沉淀，不受层控的影响，形成了滇东沿华力西期不整合面或接触带金成矿带。  相似文献   

15.
四川丹巴穹状变质地体   总被引：1，自引：0，他引：1  
四川西部丹巴地区最为引人注目的地质构造是穹状变质地体的发育。华北、扬子和羌塘三个板块之间的南北向和东西向双向收缩,引起区内发育了大小不等的十几个穹状变质地体,自北而南有马奈、春牛场、丹巴、公差、格宗等变质穹隆。多数穹隆的核部出露的是前寒武纪的片麻岩和混合岩,例如春牛场侵入体。其中的片麻岩原岩、黑云母和角闪石质片麻岩均属本区最老的岩石。它不整合于志留纪地层之下,年代学研究证明其年代属新元古代(大约865~785Ma)。混合岩从形态上说,多为条带状,偶见角砾状的角闪石质混合岩。但是也有不少穹隆,核部是花岗岩类。岩石化学研究证明,它们大多属于S型花岗岩,仅个别为I型。穹状变质地体的外围变质带可分三类:(1)巴罗带型区域递增变质带,有的显示变质带的倒转;(2)巴肯型变质带;(3)低级区域变质带,多数是中压绿片岩相。巴罗带变质的泥质岩,多数变晶矿物如黑云母、十字石、石榴石均具早期低级变质矿物的定向包裹物,显示明显叠加变质的信息。变质泥质岩的∑REE=(195~274)×10-6,(La/Yb)n=0·811~1·917。稀土配分曲线和微量元素蛛网图具Nb、P、Ti负异常,显示大陆地壳的特征,是陆缘碎屑物质区域变质产物。巴肯带出露于丹巴以北,主要变质泥质岩是夕线石片麻岩类,常见铁铝榴石而少见堇青石,说明原岩富铁贫镁,局部出现锌铁尖晶石。由北侧的巴肯带到巴罗型变质的公差穹隆到南部的格宗穹隆变质带是从高温到低温连续变化的。因之,我们倾向于认为松潘—甘孜造山带的东南缘是一个规模较大的、呈NE向分布的低—中压区域变质带,总体是一条热轴,垂直走向,向东南温度逐步降低。据前人同位素年龄资料:M1巴罗型区域变质发生于约210~205Ma,马奈花岗岩U-Pb锆石年龄为(197±6)Ma。M2巴肯型变质与岩体侵入有关,年龄约为164Ma。M3喜马拉雅期重结晶的黑云母年龄约为30Ma。总之,丹巴变质穹隆的形成是青藏高原东北部地质构造演化中重要的一幕,其主要活动期起于印支晚期最后结束于喜马拉雅期的隆升和挤出。依据低压高温变质带的空间分布,推测本区印支末期存在一NE向的热轴,同时也是S型花岗岩体的出露区。至于木里一带穹隆与丹巴穹隆在变质程度上的差异,应当归因于印支晚期的陆壳增厚过程中,北倾南倒逆冲剪切造成区域热流的不均一性。也说明了青藏高原东北缘在喜山期隆升之前具有复杂的构造变质历史。  相似文献   

16.
川西丹巴穹状变质体的P－T－t－d演化   总被引：1，自引：0，他引：1  
在扬子地台西缘松潘－甘孜复合造山带东部的丹巴地区，发育一系列穹状变质体。据近年的研究，这些变质体至少经历了四期变形及相伴随的三期变质作用。利用变质反应、角闪石和二云母的矿物化学，矿物地质温压计及相关的同位素年龄资料，建立了丹巴变质体演化的Ｐ－Ｔ－ｔ－ｄ轨迹。此轨迹呈顺时针形式，反映了变质分带属于叠加复合型，黑云母带－蓝晶石带属于中压型。矽线石带则属低压高温型，即变质作用曾经历了增温增压，增温降压和恒压降温三个演化阶段，表明变质体具陆－陆碰撞的地壳加厚和随后花岗岩浆活化上侵的回返上隆的演化特点，这和区域大地构造背景相吻合。  相似文献   

17.
In an attempt to better constrain the timing of Variscan HP-HT metamorphism in the SE Bohemian Massif we have dated zoned zircons from a garnet-kyanite granulite of granitic composition from the Dunkelsteiner Wald Massif, Lower Austria, by means of sensitive high-resolution ion microprobe (SHRIMP) technique. In order to combine isotopic information with crystal growth textures, CL and BSE images were systematically taken from the dated zircons. A characteristic threefold concentric zoning was found in many zircons. This involves pre-Variscan protolithic cores followed by two distinct metamorphic/anatectic overgrowth shells of Variscan age. The inner overgrowth shell is characterized by a weak CL but bright BSE signal, and yields high contents of uranium (0.1 to 0.2 wt.%). A pooled U-Pb Concordia age for this zone is 342.0?±?3.0?Ma (n?=?11, MSWD?=?0.12). The second, outer, overgrowth shell is always bright in the CL image, dark in the BSE image, and has generally low uranium contents (mostly <500?ppm). A pooled U-Pb Concordia age for this zone is 337.1?±?2.7?Ma (n?=?11, MSWD?=?0.22). These results imply that the Variscan HT crystallisation history of the Moldanubian granulites took place over a period of a few million years and was not an extremely rapid subduction-exhumation process. SHRIMP measurements in the protolithic cores yield a cluster of (sub)concordant ages between ??390 and 460?Ma and a few outliers at higher ages mostly represented by cores in cores. Core domains, which are large, homogeneous and with undisturbed igneous oscillatory zoning, yielded preferentially ages between 430 and 460?Ma. We therefore consider that granitic protolith formation took place at that time. The still older inner cores are interpreted as inherited into the granitic melt.  相似文献   

18.
The Paleozoic massif of Tichka in the southern part of the Western High Atlas of Morocco constitutes a structural transition between the Meseta and the Anti-Atlas domains. It was affected by a complex network of fractures noticeable at different scales. Using Landsat ETM+ imagery permits detecting the main fracture directions. Various techniques of lineament’s extraction were applied, including the colored compositions, spectral band ratios, and directional filters applied to the principal component analysis. Lineament’s extraction is based on visual interpretation and completed by field observations. The resulted map allows recognizing at least four trending fracture system, with average N-S, NE-SW, E-W, and NW-SE orientations. The surrounding rocks of the granitic massif show a high fracture density. Tectonic indicators show that this massif is initially affected by NW-SE Variscan tectonic extension, followed by NW-SE Variscan compression. This regime is being maintained until the late Variscan period corresponding to the relaxation of the NW-SE major Variscan stress. A clockwise rotation of the latter stress, which became N-S to NNE-SSW, related to the late Variscan deformation, is responsible for reworking preexisting faults.  相似文献   

19.
Several small outcrops along the western Rhinegraben escarpment expose rocks which represent the western prolongation of the so-called Mid-German Crystalline Rise. This basement ridge separates the Rhenohercynian and Saxothuringian zones of the Variscan belt of Europe and thus marks the boundary between the external and the internal zones. The variable rock association includes an orthogneissamphibolite complex, weakly deformed low grade sediments (?Devonian and Visean), and a number of different syn- to post-orogenic granodioritic to granitic intrusives, all crosscut by Late Lower Carboniferous undeformed lamprophyric dikes and unconformable overlain by Permian sediments and volcanics. Largely isothermal decompression during coaxial fabric evolution in the orthogneiss complex marks an early stage of deformation possibly due to crustal attenuation. Peak metamorphism (amphibolite/greenschist facies) in the other sequences with only minor orogenic shortening is succeeded by retrogressive strike-slip deformation associated to peak intrusive activity. The encountered typically low-P high-T metamorphism, the predominant strike-slip type kinematic pattern, and the preservation of parts of the Devono-Carboniferous sedimentary cover of the Rise preclude major crustal thickening and subsequent exhumation. An exception is the probably thrust-bounded juxtaposition of the Albersweiler orthogneisses and Burrweiler schists which is supported by their respective PT-paths. The orogenic imprint in the sedimentary cover of the crystalline rise appears to be thermal rather than strain-induced, suggesting a dominant role of the abundant pre- to late-orogenic intrusives. The essential aspects of this sequence of related structural and thermal events as well as the rock type association suggest a largely submarine incipient magmatic arc type of orogenic environment for this part of the Variscan belt. Its evolution probably started during the Upper Devonian on a disintegrating continental platform and proceeded through the Lower Carboniferous continental collision with the Rhenohercynian zone entailing a concomittant switch in deformation mode of the upper plate.  相似文献   

20.
The Teplá Crystalline unit (TCU), western Bohemian Massif, proves highly suitable for studying the effects of differential metamorphic reworking on the U–Th–Pb systematics in monazite, as the overprint of Variscan regional metamorphism onto high-grade Cadomian paragneisses intensifies progressively towards the northwest. Although variably hampered by scarcity, small size, and low uranium contents of monazite, isotope dilution–thermal ionisation mass spectrometry of monazite from paragneisses from the garnet, staurolite, and kyanite zones of the TCU gives a narrow 206Pb/238U age range from 387 to 382 Ma for Variscan peak metamorphism. These data are supported by 382–373 Ma monazite ages derived from electron microprobe analyses. Inheritance of older components in grains from the central TCU imply major “resetting” of pre-Variscan monazite around 380 Ma, possibly due to widespread garnet growth during Variscan metamorphism, which led to the consumption of pre-Variscan high-Y monazite and subsequent growth of new low-Y monazite. Concordant 498–494 Ma monazite ages in a migmatitic paragneiss close to the adjacent Mariánské Lázně Complex (MLC) grew in response to metagabbro emplacement in the MLC from 503 to 496 Ma and not during either Cadomian or Variscan regional metamorphism. Backscatter imaging and electron microprobe analyses reveal that discordant monazite of the migmatite comprises a mix of various age domains that range from ca. 540 to 380 Ma. Combined evidence presented here suggests that instead of Pb loss by volume diffusion, the apparent resetting of the U–Th–Pb systematics in monazite rather involves new crystal growth or regrowth by recrystallisation and dissolution/reprecipitation.  相似文献   

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