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141.
太古宙约占地球已有演化历史的三分之一强,这一时期涉及到大陆地壳起源、陆壳的巨量生长和稳定以及板块构造作用的启动、建立等诸多最根本的全球性重大地质事件。太古宙岩石在华北克拉通南部的涑水、登封、太华、霍邱和五河等杂岩中广泛出露,这为解析上述重大科学问题提供难得的素材。近十年来,在华北克拉通南部古生代-中生代火山岩或早前寒武纪变沉积岩中陆续发现冥古宙-古太古代的捕获/碎屑锆石,暗示南部地块依然尚存地球形成最初期的陆壳物质。根据华北克拉通南部太古宙岩石年龄统计结果显示有2850~2700Ma和2580~2480Ma两个突出年龄区间,对应的峰值年龄分别为~2.76Ga和~2.52Ga。其中~2.76Ga的岩石主要出露于南部的鲁山、霍邱、五河和中条山地区。此外,在华北克拉通诸多地区,诸如怀安、阜平、五台、中条等地区的花岗质片麻岩和变质沉积岩中也均发现年龄为~2.76Ga的碎屑锆石或者继承锆石,暗示华北克拉通2.85~2.70Ga岩石的分布似乎比现今出露范围更为广泛。与整个华北克拉通类似,2.58~2.48Ga岩石亦在克拉通南部广泛分布,尤其是嵩箕地区的登封杂岩几乎全部是由新太古代晚期的岩石组成。~2.52Ga是华北克拉通南部,乃至整个克拉通太古宙地壳演化最突出、最重要的岩浆-构造事件,明显有别于全球其它诸多典型克拉通。已有的同位素资料研究表明华北克拉通南部,乃至整个克拉通在太古宙经历了两期明显的地壳生长事件:一期发生在2.85~2.70Ga左右,以形成于此时期的涑水杂岩中花岗质岩石和鲁山太华片麻岩系中深成侵入岩和斜长角闪岩为代表;另一期发生在2.58~2.48Ga,以登封杂岩、涑水杂岩以及小秦岭地区太华杂岩中~2.52Ga各类花岗质岩石和变基性岩为代表。华北克拉通正是经过这两期陆壳巨量生长事件之后完成初始的克拉通化。我们在登封杂岩中识别出形成于俯冲汇聚环境的TTG质片麻岩、类似于赞岐岩的变闪长岩和具有N-MORB地球化学特征的变基性火山岩,提出其构成"新太古代构造混杂岩",标志着新太古代末期具有现代体制的板块构造在华北克拉通南部已经开始启动。最近,在登封杂岩中识别出的新太古代双变质带也支持上述观点。 相似文献
142.
板片俯冲过程将地表碳带入地球内部,火山作用将深部碳输送至地球外部圈层,两者构成了地球深部碳循环的主要方式,进而影响了地史时期的气候变化。我国东北新生代火山活动被认为是太平洋板片深俯冲作用的产物,板片俯冲导致岩浆源区强烈的碳酸盐组分交代作用,进而使东北亚上地幔成为一个新生代时期的巨型深部碳库,它的活动和释放将会对全球的气候与环境变化造成重要影响。然而,有关该深部碳库向当今大气圈输送CO_2气体的规模及其演化过程尚不清楚,从而影响了进一步定量评估该碳库在全球变化研究中的地位和作用。针对上述科学问题,本文对我国东北长白山、五大连池和阿尔山火山释放CO_2气体的规模与成因进行了研究。结果表明,东北新生代火山区的土壤CO_2释放通量介于9.6~41.2g·m~(-2)·d~(-1)之间,每年向当今大气圈释放CO_2气体约为2.1Mt(其中,长白山火山区为0.94Mt,五大连池火山区为1.2Mt)。气体地球化学研究证实,长白山与五大连池火山气体均起源于太平洋板片深俯冲环境;但是,与长白山相比,五大连池火山气体具有较高比例壳源组分贡献。阿尔山火山气体的成分与长白山和五大连池火山区存在着明显的差异,它们以N_2为主( 95%),并且其δ~(15)N_(N_2)值高于空气值(1.3‰~1.9‰),~3He/~4He比值较低(0.14~0.18R_A),δ~(13)C_(CO_2)较轻(-13.7‰~-6.2‰),表明壳源富氮有机沉积物的贡献占比较大的比例。上述特征进一步表明,阿尔山火山气体在上升经过地下水时可能滞留了较长时间,混染了大量的陆壳组分,其源区并未受到太平洋板片俯冲物质的显著影响。 相似文献
143.
144.
Characterising deep vadose zone water movement and solute transport under typical irrigated cropland in the North China Plain 下载免费PDF全文
Understanding the dynamics and mechanisms of soil water movement and solute transport is essential for accurately estimating recharge rates and evaluating the impacts of agricultural activities on groundwater resources. In a thick vadose zone (0–15 m) under irrigated cropland in the piedmont region of the North China Plain, soil water content, matric potential, and solute concentrations were measured. Based on these data, the dynamics of soil water and solutes were analysed to investigate the mechanisms of soil water and solute transport. The study showed that the 0–15‐m vadose zone can be divided into three layers: an infiltration and evaporation layer (0–2 m), an unsteady infiltration layer (2–6 m), and a quasi‐steady infiltration layer (6–15 m). The chloride, nitrate, and sulphate concentrations all showed greater variations in the upper soil layer (0–1 m) compared to values in the deep vadose zone (below 2 m). The average concentrations of these three anions in the deep vadose zone varied insignificantly with depth and approached values of 125, 242, and 116 mg/L. The accumulated chloride, sulphate, and nitrate were 2,179 ± 113, 1,760 ± 383, and 4,074 ± 421 kg/ha, respectively. The soil water potential and solute concentrations indicated that uniform flow and preferential flow both occurred in the deep vadose zone, and uniform flow was the dominant mechanism of soil water movement in this study. The piston‐like flow velocity of solute transport was 1.14 m per year, and the average value of calculated leached nitrate nitrogen was 107 kg/ha?year below the root zone. The results can be used to better understand recharge processes and improve groundwater resources management. 相似文献
145.
In September 1979, rain-induced complex landslides occurred in two separate localities along the slopes of the Akovolwo Mountains near Jato-Aka in the Kwande Local Government area of Benue State, Nigeria.The two movements developed on bedrock slopes covered by a veneer of colluvium and (or) residuum no more than 1.5 to 2m deep. Each started as a slump and graduated into a debris flow. The slump, extending downhill a short distance away from the head scarp, had been reconstituted into a debris flow as the initial movement (sliding movement) of the soil mass caused remoulding of the saturated moving mass into viscous debris-laden mud. The resulting mass moved partly along a pre-existing mountain stream channel and partly along a fresh channel it cleared in the savanna forest. At the break of slope, the mass of boulders was dumped and a trail of smaller fragments littered the channels towards the Katsina-Ala River. 相似文献
146.
147.
W. Franke 《International Journal of Earth Sciences》1993,82(3):505-515
The brachyanticlinal structure of the Saxonian Granulite Complex was formed, or at least overprinted, by large-scale crustal extension, which is documented in an extreme reduction of the metamorphic profile and a pressure-temperature path with nearly isothermal decompression. the granulite is situated within a south-west-northeast trending belt characterized by basaltic volcanism and an intrabasinal, sediment-producing rise, both active in Frasnian times. These features are taken to reflect at least part of the extension which has exhumed the high grade rocks; however, Cambro-Ordovician rifting possibly initiated the process. The Frasnian palaeogeography and geophysical data suggest that the axis of the granulite dome extends at least 150 km towards the south-west. The overall setting strongly resembles the metamorphic core complexes of the western USA, but the exhumation of rocks preserving evidence of metamorphic pressures of 10–12 kbar requires much higher extensional strain, possibly due to the longer period of stretching. Early Carboniferous compression and minor post-collisional extension (Late Carboniferous to Permian) have not significantly modified the earlier geometry. U-Pb and Sm-Nd data ranging between 380 and 340 Ma are interpreted to record retrogression during uplift rather than peak metamorphism. 相似文献
148.
149.
滇中郝家河砂岩铜矿的地球化学及其成因意义 总被引:1,自引:0,他引:1
本文论术字滇中生代地层与矿区及春外围马头山组中铜的时空分布与迁移富集规律,揭示了矿床的沉积-后期叠加改造成因。成矿作用经历了同生沉积形成矿源层,后期叠加改造形成矿体的过程。后期改造中铜元素发生了大量迁移,矿质主要来自矿体之下伏紫色矿源层。 相似文献
150.
Kalyani Chatterjea 《地球表面变化过程与地形》1994,19(7):585-607
The nature and rates of fluvial and slope processes change over time and space as urbanized areas replace forested land in Singapore. Storm-based and time-based data, from undisturbed rainforests, heavily disturbed construction sites, urban grass-covered slopes and an experimental plot, are collected to observe the impact of rainwater on the soil moisture conditions, surface microtopography, runoff generation, sediment movement, and ground lowering in the three different categories of land use. The undisturbed forested environment is characterized by high throughfall (58% of total rainfall) and frequent negative soil moisture suctions. The slow and unconcentrated overland flow during heavy storms is restricted by the forest floor microtopography. No rills develop. Ground lowering is recorded as 3·2–3·4 mm a?1. But sediment movement is episodic and suspended sediment concentrations in overland flow are 172–222 mg l?1. During urban construction, gully development is rapid on the bare slopes, runoff generation, voluminous, and sediment-laden discharges (5200–75498 mg l?1) lead to sediment plumes at channel mouths. Ground lowering rates are measured at 132·4 mm a?1. Once grass-covered, runoff carries less suspended sediment (800 mg l?1) and ground lowering rates are reduced, but depend on the condition of the cover, ranging from 0·2 to 8·2 mm a?1. As urban development continues, environments are altered both in time as well as spatially. 相似文献