The bimodal rift-related Juscelândia volcanosedimentary sequence in central Brazil: Mesoproterozoic extension and Neoproterozoic metamorphism

The Barro Alto Complex and Juscelândia volcanosedimentary sequence are exposed in the central part of the Neoproterozoic Brasília belt of central Brazil. The former is a large (approximately 150 km long), boomerang-shaped, mafic-ultramafic, layered complex formed by two different intrusions metamorphosed under granulite facies. These rocks are tectonically overlain by rocks of the Juscelândia volcanosedimentary sequence, represented mainly by biotite-gneiss and amphibolite, or amphibolite facies metamorphic equivalents of rhyolite and basalt, respectively. New SIMS U–Pb zircon data and Sm–Nd isochron data presented herein help clarify the igneous and metamorphic evolution of the Juscelândia volcanosedimentary sequence, as well as its relationship with the Barro Alto Complex. Zircon grains from two biotite gneisses were analyzed by SIMS (SHRIMP) and indicate Mesoproterozoic dates, approximately 1.28 Ga, interpreted as the time of bimodal volcanism in a tectonic setting transitional between a continental rift and an ocean basin. Metamorphism is constrained by Sm–Nd garnet-whole-rock isochrons for garnet amphibolite and pelitic schists of the Juscelândia sequence, as well as for clinopyroxene-garnet amphibolite and garnet granulite of the Barro Alto Complex, which give ages between 0.74 and 0.76 Ga, in agreement with SIMS dates for metamorphic zircon rims. These new data are significant, because they establish that a single metamorphic event affected both the Barro Alto Complex and the Juscelândia sequence. Based on these new data, we present a modified tectonic model for the Brasília belt.     

Cretaceous isochron ages from K–Ar and Ar / Ar dating of eclogitic rocks in the Tso Morari Complex, western Himalaya, India

The Tso Morari Complex, which is thought to be originally the margin of the Indian continent, is composed of pelitic gneisses and schists including mafic rock lenses (eclogites and basic schists). Eclogites studied here have the mineral assemblage Grt + Omp + Ca-Amp + Zo + Phn + Pg + Qtz + Rt. They also have coesite pseudomorph in garnet and quartz rods in omphacite, suggesting a record of ultrahigh-pressure metamorphism. They occur only in the cores of meter-scale mafic rock lenses intercalated with the pelitic schists. Small mafic lenses and the rim parts of large lenses have been strongly deformed to form the foliation parallel to that of the pelitic schists and show the mineral assemblages of upper greenschist to amphibolite facies metamorphism. The garnet–omphacite thermometry and the univariant reaction relations for jadeite formation give 13–21 kbar at 600 °C and 16–18 kbar at 750 °C for the eclogite formation using the jadeite content of clinopyroxene (X_Jd = 0.48).

Phengites in pelitic schists show variable Si / Al and Na / K ratios among grains as well as within single grains, and give K–Ar ages of 50–87 Ma. The pelitic schist with paragonite and phengite yielded K–Ar ages of 83.5 Ma (K = 4.9 wt.%) for paragonite–phengite mixture and 85.3 Ma (K = 7.8 wt.%) for phengite and an isochron age of 91 ± 13 Ma from the two dataset. The eclogite gives a plateau age of 132 Ma in Ar/Ar step-heating analyses using single phengite grain and an inverse isochron age of 130 ± 39 Ma with an initial ⁴⁰Ar / ³⁶Ar ratio of 434 ± 90 in Ar/Ar spot analyses of phengites and paragonites. The Cretaceous isochron ages are interpreted to represent the timing of early stage of exhumation of the eclogitic rocks assuming revised high closure temperature (500 °C) for phengite K–Ar system. The phengites in pelitic schists have experienced retrograde reaction which modified their chemistry during intense deformation associated with the exhumation of these rocks with the release of significant radiogenic ⁴⁰Ar from the crystals. The argon release took place in the schists that experienced the retrogression to upper greenschist facies metamorphisms from the eclogite facies conditions.     

Precambrian fluvial deposits: Enigmatic palaeohydrological data from the c. 2–1.9 Ga Waterberg Group, South Africa

Precambrian fluvial systems, lacking the influence of rooted vegetation, probably were characterised by flashy surface runoff, low bank stability, broad channels with abundant bedload, and faster rates of channel migration; consequently, a braided fluvial style is generally accepted. Pre-vegetational braided river systems, active under highly variable palaeoclimatic conditions, may have been more widespread than are modern, ephemeral dry-land braided systems. Aeolian deflation of fine fluvial detritus does not appear to have been prevalent. With the onset of large cratons by the Neoarchaean–Palaeoproterozoic, very large, perennial braided river systems became typical. The c. 2.06–1.88 Ga Waterberg Group, preserved within a Main and a smaller Middelburg basin on the Kaapvaal craton, was deposited largely by alluvial/braided-fluvial and subordinate palaeo-desert environments, within fault-bounded, possibly pull-apart type depositories.

Palaeohydrological data obtained from earlier work in the Middelburg basin (Wilgerivier Formation) are compared to such data derived from the correlated Blouberg Formation, situated along the NE margin of the Main basin. Within the preserved Blouberg depository, palaeohydrological parameters estimated from clast size and cross-bed set thickness data, exhibit rational changes in their values, either in a down-palaeocurrent direction, or from inferred basin margin to palaeo-basin centre. In both the Wilgerivier and Blouberg Formations, calculated palaeoslope values (derived from two separate formulae) plot within the gap separating typical alluvial fan gradients from those which characterise rivers (cf. [Blair, T.C., McPherson, J.G., 1994. Alluvial fans and their natural distinction from rivers based on morphology, hydraulic processes, sedimentary processes, and facies assemblages. J. Sediment. Res. A64, 450–489.]). Although it may be argued that such data support possibly unique fluvial styles within the Precambrian, perhaps related to a combination of major global-scale tectono-thermal and atmospheric–palaeoclimatic events, a simpler explanation of these apparently enigmatic palaeoslope values may be pertinent. Of the two possible palaeohydrological formulae for calculating palaeoslope, one provides results close to typical fluvial gradients; the other formula relies on preserved channel-width data. We suggest that the latter will not be reliable due to problematic preservation of original channel-widths within an active braided fluvial system. We thus find no unequivocal support for a unique fluvial style for the Precambrian, beyond that generally accepted for that period and discussed briefly in the first paragraph.     

Shape and distribution analysis of Merensky Reef potholing, Northam Platinum Mine, western Bushveld Complex: implications for pothole formation and growth

Syn-magmatic removal of the cumulate pile during the formation of the Bushveld Complex resulted in “potholes”. Erosion progressed downward in the cumulate pile, resulting in a series of steep, transgressive contacts between locally conformable potholed reefs in the regional pothole sub-facies of the Swartklip Facies in the western limb of the Bushveld Complex. The deepest of these potholes, “third-order” or “FWP2” potholing, occurs where the base of the Merensky Cyclic Unit transgresses the Upper Pseudo-Reef Chromitite marker horizon. The base of a FWP2 pothole on Northam Platinum Mine consists of an unconformable stringer Merensky Chromitite overlain by a medium-grained, poikilitic orthopyroxenite and underlain by either a pegmatitic harzburgite or the medium-grained Lower Pseudo-Reef Anorthosite. Detailed shape and distribution analysis of FWP2 potholes reveals underlying patterns in their shape and distribution which, in turn, suggest a structural control. The ratio between pothole short vs long axes is 0.624 (N=1,385), although the ratio increases from 0.48 to 0.61 in the long axis range 10 to 60 m, then decreases from 0.61 to 0.57 from 61 to 100 m, increasing again from 0.57 to 0.61 from 101 to 400 m, suggesting that there is not a simple relationship between pothole shape and size. Shape (circularity, eccentricity, and dendricity) analysis of a subset of 638 potholes indicates that potholes with long axes <100 m have an elliptical, average normalized shape, elongate on a 120–150° orientation. Potholes with long axis lengths >100 m have an average normalized shape that is bilobate and elongate on a 120° orientation. The average aspect ratio (short axis length divided by long axis length) of potholes is highest for potholes with long axis lengths >100 m and lowest for potholes with long axis lengths between 35 and 60 m. The most common long axis orientation for potholes with long axis lengths <100 m is 150° but 120° for long axis lengths >100 m. Fractal analysis indicates that the distribution of pothole centers is controlled neither by a single nor several interacting fractal dimensions. Autocorrelation (Fry) analysis of the distribution of pothole centers shows recurring pothole distribution trends at 038, 070, and 110° for potholes over the full range of long axis lengths, while the trends of 008 and 152° occur in potholes with long axes lengths between 60 and 100 m. Chi-squared (X ²) analysis of the locations of pothole centers suggests that the distribution of small potholes is highly non-uniform but becomes exponentially more uniform with increasing pothole size. The model which best fits the observed shape and distribution analysis is a combination of protracted independent growth and “nearest neighbor” merging along specific orientations. For instance, the clustered distribution of original pothole centers resulted in merged potholes with long axes lengths of up to 60 m, exhibiting short vs long axes ratios of 0.61, preferred orientations of 150°, and alignment along 010 and 150° trends. Further independent growth allowed for merging of similar-sized (and smaller) neighboring potholes, generating potholes with long axes of up to 100 m in length, a preferred long axis orientation of 150°, and alignment along 010, 040, 075, and 150°. Subsequent preferential merging occurred along a 120° trend, thereby preserving a bilobate form. This implies that while pothole initiation and enlargement may be driven by a “top-down” (i.e., possibly thermomechanical) process, an underlying linear or structural catalyst/control is revealed in changes in pothole shape during enlargement and, furthermore, in the preferred trends along which potholes merged over a considerable period, possibly concomitant with adjustment of major structures in the footwall to the Bushveld Complex and pulses into the magma chamber.     

地震属性分析中水平切片的应用

利用地震数据体中含有的丰富地质信息,可以有效地进行储层预测。层拉平水平切片地层学解释方法是先将地震剖面目的层段做层拉平处理,然后对切片顺地层层面追踪,勾绘出各种地层现象,并以地层学的观点加以解释。该方法在吉林探区扶新南部地区的应用中,对于识别河道的展布趋势取得了较好的效果,而且发现了多期叠置的扇型沉积体。另外,还可指导反演数据体的解释,做到对岩性体从定性到定量的描述。     

藏北若拉岗日岩群变质火山岩的基本特征及构造属性

藏北的若拉岗日岩群，因处于拉竹龙-金沙江缝合带南缘，其成生背景及构造属性历来倍受关注。白头山、岗盖日、狮头山等地的若拉岗日岩群主要为一套变质中基性火山岩系，不同地段夹杂大理岩、变质砂板岩、硅质岩、超基性岩等小岩片或块体，分别具有洋岛型、洋脊型及岛弧型火山岩的基本特征，并构成白头山西以岛弧火山岩为主的弧火山岩片，岗盖日以洋脊型变玄武岩为主夹杂硅质岩、超基性岩小岩片而共同组成的蛇绿混杂岩片及白头山、狮头山以洋岛型火山岩为主夹杂碳酸盐岩块体的大洋海山岩片，从而指出该区若拉岗日岩群为一套不同构造属性的火山岩与大理岩、硅质岩、超基性岩等相伴的构造混杂岩，其中既有某大洋的产物，又有某洋岛及陆缘岛弧的产物。     

冲积平原区土壤元素组成特征及其示踪作用

应用土壤元素组成特征研究基础地质问题是覆盖区多目标地球化学调查工作后项重要任务。通过对珠江三角洲，江汉平原，成都盆地3个冲积平原区土壤元素组成特征的研究，探讨了其对土壤成壤母质特性的示踪作用，为今后开展类似的研究工作提供了一种思路。     

Geochronological constraints on the evolution of the Embu Complex, São Paulo, Brazil

Petrologic and geochronological work was carried out on a roadside outcrop of amphibolite facies orthogneisses near São Lourenço da Serra, about 50 km southwest of São Paulo City. These orthogneisses belong to the Embu Complex, within the Neoproterozoic Brasiliano Orogenic Cycle mobile belts of SE Brazil. The outcrop consists of predominantly foliated biotite tonalites and granodiorites, which were cut by granitic veins and pegmatites prior to final deformation. SHRIMP U/Pb measurements on zircons from one granodioritic–tonalitic gneiss indicate magmatic crystallization of the protolith at 811±13 Ma (MSWD=1.0). Zircons with dates of ca. 2000 and ca. 1000 Ma in this rock are interpreted as inherited from older crust. One zircon analyzed from the gneiss and three zircons from a discordant pegmatitic vein indicate an event at 650–700 Ma, perhaps related to the intrusion of the pegmatites. A regression of Rb–Sr whole rock data for four biotite gneisses yielded an imperfect isochron, giving an apparent age of 821±68 Ma and an elevated initial ⁸⁷Sr/⁸⁶Sr ratio of 0.719±0.005. The elevated initial ⁸⁷Sr/⁸⁶Sr ratio and the inherited zircons indicate involvement of older crust in the genesis of the gneisses. Rb–Sr feldspar and whole rock pairs yield ca. 560 Ma tielines, giving the time of final cooling below 300–350 °C, and the cessation of medium-grade metamorphism and ductile deformation. These results document a series of tectono-thermal events spanning 250 million years during the Brasiliano Orogenic Cycle. They relate to ca. 800 Ma magmatic arc activity and later allochthonous terrane assembly during closure of the Adamastor Ocean, resulting in the accretion of Western Gondwana.     

圆弧状沉积盆地与软土单覆盖层出平面地表运动对比

采用解析方法，通过圆弧状沉积盆地与同样参数的软土均匀单覆盖层的对比，研究沉积盆地对平面SH波二维散射效应和其出平面地表运动。数值对比结果表明：（1）波垂直入射时盆地中央地表位移幅值反应，其随频率变化形式与软土单覆盖层的地表运动有较强的对应性，且此时单覆盖层地表位移幅值为其下降；（2）盆地的二维散射效应特点显著波的汇聚作用使盆地中的地表运动显著放大，总体上较软土单覆盖层的地表位移幅值要大，其频域内的最大反应则明显大于单覆盖层情况，但最大反应在盆地表面出现的位置依赖于入射波角度、盆地深宽比和盆地的软硬程度；（3）沉积盆地中地表位移运动有强烈扭转效应，且随盆地变软和入射波频率增高而增强。