Extensional tectonic origin of gneissosity and related structures of the Feiran–Solaf metamorphic belt, Sinai, Egypt

The Feiran–Solaf metamorphic belt consists of low-P high-T amphibolite facies, partly migmatized gneisses, schists, amphibolites and minor calc-silicate rocks of metasedimentary origin. There are also thick concordant synkinematic sheets of diorite, tonalite and granodiorite orthogneiss and foliated granite and pegmatite dykelets. The gneissosity (or schistosity) is referred to as S₁, and is almost everywhere parallel to lithological layering, S₀. This parallelism is not due to transposition. The gneissosity formed during an extensional tectonic event (termed D1), before folding of S₀. S₁ formed by coaxial pure shear flattening strain (Z normal to S₀, i.e. vertical; with X and Y both extensional and lying in S₁). This strain also produced chocolate tablet boudinage of some layers and S₁-concordant sills and veins. S₁ has a strong stretching lineation L₁ with rodding characteristics. Within-plane plastic anisotropy (lower ductility along Y compared to along X) resulted in L₁-parallel extensional ductile shears and melt filled cracks. Continued shortening of these veins, and back-rotation of foliations on the shears produced intrafolial F₁ folds with hinges parallel to the stretching lineation. F₁ fold asymmetry variations do not support previous models involving macroscopic F₁ folds or syn-gneissosity compressional tectonics. The sedimentary protoliths of the Feiran–Solaf gneisses were probably deposited in a pre-800 Ma actively extending intracratonic rift characterizing an early stage of the break-up of Rodinia.     

Surface temperature differences between minerals in crystalline rocks: Implications for granular disaggregation of granites through thermal fatigue

Thermal expansion differences between minerals within rocks under insolation have previously been assumed to drive breakdown by means of granular disaggregation. However, there have been no definitive demonstrations of the efficacy of this weathering mechanism. Different surface temperatures between minerals should magnify thermal expansion differences, and thus subject adjacent minerals to repeated stresses that might cause breakdown through fatigue failure. This work confirms the existence of surface temperature differences between minerals in granitic rocks under simulated short-term temperature fluctuations so as to discriminate their potential for initiating granular disaggregation. The influence of colour, as a surrogate for albedo, and crystal size, as a function of thermal mass are specifically identified because of their ease of quantification. Four rock types with a range of these properties were examined, and subjected to repeated short-term temperature cycles by radiative heating and cooling under laboratory conditions. Results show that while albedo is the main control for overall and individual maximum temperatures, crystal size is the main factor controlling higher temperature differences between minerals. Thus, stones with large differences of mineral sizes can undergo magnified stresses due to thermal expansion differences.     

The Migif–Hafafit gneissic complex of the Egyptian Eastern Desert: fold interference patterns involving multiply deformed sheath folds

The Wadi Hafafit Complex (WHC) is an arcuate belt of orthogneisses, migmatites and other high-grade metamorphic rocks, which marks the boundary between the Central Eastern and the South Eastern Deserts of Egypt. In the WHC, gneissic meta-gabbro outlines macroscopic fold interference patterns characterized by elliptical to irregular culminations cored by gneissic meta-tonalite to meta-trondhjemite. The five main culminations of the WHC have previously been labeled A (most northerly), B, C, D and E (most southerly). A detailed structural investigation of B, C, D and E reveals that these structures are a result of the interference of four macroscopic fold phases, the first three of which may represent a single deformation event. The first folding involved sheath-like fold nappes, which were transported to the N or NW, assisted by translation on gently dipping mylonite zones. The regional gneissosity and mineral extension lineations formed during this folding event. The fold nappes were deformed by mainly open upright small macroscopic and mesocopic folds with approximately NE-trending hinges. As a probable continuation of the latter folding, the sheaths were buckled into large macroscopic folds and monoclines with the same NE-trends. The fourth macroscopic folding resulted from shortening along the NE–SW direction, producing mainly NW–SE-trending upright gently plunging folds. Gravitative uplift is disputed as a component of the deformation history of the WHC. The peculiarities of the fold interference pattern result from the interesting behaviour of sheath folds during their refolding.     

Palaeoproterozoic granitic magmatism in the northern segment of the Jiao-Liao-Ji Belt: implications for orogenesis along the Eastern Block of the North China Craton

As the northern segment of the Jiao-Liao-Ji Belt (JLJB), the Palaeoproterozoic Liaoji Belt is a key region for deciphering the formation and evolution of the North China Craton (NCC). In this study, we present the geochronology, geochemical, and isotopic studies on the monzogranitic gneiss, which is one of the major lithotectonic elements of the Liaoji Belt. LA-ICP-MS zircon U–Pb dating reveals that the studied monzogranitic gneisses were formed in the period of 2213–2178 Ma. They are in tectonic contact with the Palaeoproterozoic volcano-sedimentary rocks in the field. The monzogranitic gneisses belong to the high-K calc-alkaline series, and are metaluminous to peraluminous. They have 10,000 Ga/Al ratios of 2.63–3.14 with an average of 2.90, and are thus classified as aluminous A-type granites. Their ε_Nd(t) values vary from ?3.4 to +2.5, indicating heterogeneous source region. The monzogranitic gneisses are characterized by enrichment in LREE and LILE (e.g. Rb, Ba, Th, and K) and depletion in HREE and HFSE (such as Nb, Ta, and Ti), and are typical to magmatism in active continental margins formed in a subduction-related tectonic setting. Taking into account their A-type affinity and regional geological data, we suggest that the monzogranitic gneisses were most probably generated in a local extensional back-arc environment during subduction.     

吉林南部太古宙英云闪长质片麻岩类的特征及成因

吉林南部太古宙英云闪长质片麻岩类主要由石英闪长质、英云闪长质、花岗闪长质和奥长花岗质片麻岩组成。地质学、岩相学、地球化学研究表明，它们是同源岩浆通过结晶分异作用形成的，最主要的分离矿物相是角闪石和斜长石。岩浆来源于下地壳角闪质岩石的部分熔融作用。     

Long-term erosion on granitic roadcuts based on exposed tree roots

Exposed roots were used to estimate soil and bedrock erosion on the cut slopes of a 45-year-old road constructed in granitic soils of the Idaho Batholith. The original roadcut surface was defined by projecting a straight line from the toe of the cut past the end of the exposed root to the intersection of a straight line projected along the surface of the hillslope. A cross-sectioning technique was then used to determine erosion to the present roadcut surface. A total of 41 exposed root sites were used to estimate erosion on a 1350 m-long section of road. Average erosion was 1·0 and 1·1 cm/year for soil and bedrock respectively. Buttressing by tree roots caused lower erosion rates for soil as compared to bedrock. Both soil and bedrock erosion rates showed statistically significant correlations with the gradients of the original cut slope. The bedrock erosion data provide a reasonable estimate of the disintegration rate of exposed granitic bedrock exhibiting the weathering and fracturing properties common to this area. The road is located in a study watershed where long-term sediment yield data are available. Sediment data from adjacent study watersheds with no roads were compared to sediment data from the roaded watershed to estimate the long-term increase in sediment yield caused by the road. The increase amounts to about 2·4 m³/year. This figure, compared to the average annual on-site road erosion, provides an erosion to sediment delivery ratio of less than 10 per cent. Based on study results, road construction and maintenance practices are suggested for helping reduce roadcut erosion.     

Two-dimensional geostatistical modeling and prediction of the fracture system in the Albala Granitic Pluton, SW Iberian Massif, Spain

We use a fracture index distribution method of geostatistical modelling and prediction to characterize quantitatively the fracture system in two-dimensions (2D) in the Mina Ratones area, located in the Albalá Granitic Pluton (SW Iberian Massif). The fracture index (FI) is a quantitative estimate of the fracture density in discrete domains. To validate the results of geostatistical modeling a detailed structural map of the area was also made on a scale of 1:1000. The resulting grids, expressed as pixel-maps, describe the continuous value of the FI in 2D for the whole Mina Ratones area. Based on the modelled distribution of the FI and their correlation with mapped faults, we distinguish two structural domains in the studied area: elongated bands of fracture zones with high FI values and romboidal blocks located between them with low FI values. The separation between both domains is gradual. Though a threshold value of the FI that separate both structural domains is not clearly defined, the fracture zones generally present FI>1 and the individualized blocks FI<0.50. As a consequence, the obtained grid of the FI permits the quantitative structural classification of the granitic massif in 2D and understanding fault zone architecture in the Mina Ratones area.     

豫西宜阳地区TTG质片麻岩锆石U-Pb定年和Hf同位素地质学

本文报道对华北地块南缘、豫西宜阳穆册TTG质片麻岩的地球化学、锆石U-Pb定年及Hf同位素研究。宜阳穆册TTG质片麻岩SiO_2含量为59～73%,Al_2O_3>15%、CaO、MgO、Al_2O_3、TiO_2、Na_2O/K_2O与SiO_2的含量呈负相关,表明在岩浆演化中有镁铁矿物和斜长石的分离,岩浆成分随着分异而富钾。P_2O_5与SiO_2含量呈正相关,反映出磷灰石的分离。轻重稀土分异明显((La/Yb)_N=9～23);重稀土相对于轻稀土轻度分异;无明显的Eu异常(Eu*=0.95～1.07)。岩石富Rb、Sr、Ba、K等大离子亲石元素,亏损Nb、Ta等高场强元素。低Y(13.25～25.27μg/g)和Yb(1.27～1.98μg/g);高Sr(507.14～606.85μg/g)及Sr/Y(34.15～39.97),上述地球化学特征一致表明,研究区片麻岩为一套高铝的TTG质岩石。这套片麻岩可能是在角闪岩相条件下部分熔融的产物。利用LA-ICP-MS锆石U-Pb法测得TTG质片麻岩的年龄为2.3Ga。片麻岩中的锆石大部分具负ε_(Hf)(t)值(最高达-4.95),揭示其源区可能主要为古老陆壳物质;部分锆石的ε_(Hf)(t)为正值,说明在其形成过程中有一定比例的亏损地幔物质的加入。宜阳TTG质片麻岩的Hf同位素数据表明,其两阶段模式年龄主要2.57～3.01Ga之间,平均值为2.82Ga,所以说穆册灰色片麻的源区可能存在年龄为2.8Ga左右(或更老)的地壳物质,说明代表该区基底的太华杂岩形成时代可能为2.8Ga(或更老)。虽然华北地块陆壳最主要的增生时代被认为发生在2500Ma,但宜阳TTG质片麻岩的研究结果和其他研究者的研究成果都说明华北地块大陆地壳在古元古代早期仍然存在相当规模的陆壳增生。     

大别山超高压榴辉岩和花岗片麻岩中锆石Lu-Hf同位素研究

对大别山超高压榴辉岩和花岗片麻岩进行了锆石Lu-Hf同位素分析,结果为原岩来源提供了制约,表明扬子陆块在Rodinia超大陆裂解时的裂谷岩浆活动中发生了显著的陆壳生长。对这些锆石的不同区域进行的U-Pb和Lu-Hf同位素分析和比较表明,不同成因的锆石在~(206)Pb/~(238)U年龄、初始Hf同位素组成、Th/U及Lu/Hf比值等方面具有明显的差异。与年龄较老的岩浆核部和幔部相比,年轻的变质增生边具有低的Th/U和Lu/Hf比值但高的ε_(Hf)(t)值。不同成因锆石的Th/U和Lu/Hf比值存在着正相关性,表明变质作用对锆石的U-Th-Pb和Lu-Hf同位素体系有着相似的影响。高级变质作用有时能够引起岩浆锆石增生边~(176)Hf/~(177)Hf比值的显著升高,导致变质新生颗粒或增生边类似于新生地壳的高ε_(Hf)(t)值假象。对榴辉岩和片麻岩锆石核部的分析发现,镁铁质和长英质原岩在大约750Ma左右形成一个双峰式火山岩套,另外包含少量的年龄约为2.15Ga的陆壳。初始Hf同位素组成可分成两组:第一组具有正的ε_(Hf)(t)值,为5.9±0.9～12.9±0.7;第二组ε_(Hf)(t)值在零左右,为-4.3±0.5-2.3±0.3。正的ε_(Hf)(t)值与较年轻的模式年龄相对应,负的ε_(Hf)(t)值与古元古代模式年龄相对应。前者表明,在扬子陆块北缘裂谷岩浆作用将亏损地幔物质加入到大陆地壳中,同时在新元古代中期的裂谷构造带中存在同时期的壳-幔相互作用。因此,在扬子陆块北缘新元古代中期裂谷岩浆活动中,既有新生地壳生长和即时再造,也有古老地壳再造。     

多覆盖地区花岗岩接触带及其控矿特征的遥感研究

本文主要介绍应用TM图像对西秦岭地区的中川岩体进行研究的成果,即研究在多植被、多黄土覆盖的复杂条件下,如何依据接触带蚀变岩的遥感图像像元亮度值及实验室测得的光谱曲线特征,区分不同的岩体接触带类型,并探讨它们对金矿的控制作用。