Experimental study on the P wave velocity in rocks from lower crust and crust-mantle transitional zone beneath the Hannuoba

Cenozoic basalt-borne mafic granulite-facies plagioclase pyroxenite and eclogite-facies garnet pyroxenite xenoliths from the Hannuoba, as well as nearby Archean terrain granulites, are selected for the experimental study on the P wave velocity at high temperature and high pressure in order to reveal the present-day compositional features for the lower crust and crust-mantle transitional zone. Results show that mafic xenoliths have high Vp (7.0～8.0 km/s), in contrast, the Archean terrain granulites have low Vp (<7.0 km/s). High Vp mafic xenoliths can represent the present-day compositional features for the lower crust and crust-mantle transitional zone beneath the Hannuoba. This provides new evidence for the crust vertical growth and the formation of the crust-mantle transitional zone resulting from the magma underplating. Low Vp Archean granulite still remains the characteristics of the early lower crust.     

Partial melting of the dry mafic continental crust: Implications for petrogenesis of C-type adakites

C-type adakites have been commonly considered as a result of partial melting of the mafic lower continental crust (LCC) at high pressure, as supported by high P-T experiments on hydrous basalts. However, because the mafic eclogitic LCC is generally dry, experiments on water-bearing materials cannot be used to constrain the melting processes of the dry mafic LCC. Due to the lack of systematic melting experimental studies on dry mafic rocks at crustal pressures, MELTs software was applied to simulating melting of the dry mafic LCC at 1–2 GPa. Comparison of model results with experimental data indicates that, when melting degree is greater than 20%, melts from the dry mafic LCC at 1–3 GPa cannot produce the C-type adakitic melt with high SiO₂ content (~70%). Although the limited experimental results about dry mafic rock melting at 1–2 GPa in the literature suggest that low degree melting (<10%) cannot produce silicic melt either, MELTs software simulation shows that, at pressure >1.8 GPa, low-degree melting can produce dacitic melt with high K₂O/Na₂O (~1) if SiO₂ content of the melt is controlled by residual garnet. Furthermore, the simulation also suggests that, if pressure is <1.8 GPa, abundant plagioclase (plg) in the residual phase may decrease SiO₂ content in the melt to below 62%, much lower than that of the C-type adakites observed in eastern China. Given the high P-T conditions required to produce melts with high SiO₂ and extremely low HREE contents, such melts could easily be contaminated by other crustal-derived melts, implying that the C-type adakites from eclogite melting could be less commonly observed in the outcrops than previously believed. Besides the interpretation that garnet fractionates Sr, Y, and REE, high Sr/Y and La/Yb could be also produced by multiple ways such as inheriting the source features and fractional crystallizing clinopyroxene (cpx). Therefore, it may be problematic using high Sr/Y and La/Yb as criteria to identify adakites. Instead, REE patterns with strong depletion of HREE relative to MREE (e.g. high Gd/Yb) could be a better parameter to identify the role of garnet and thus adakites. Finally, geochemical models based on MELTs simulation indicate that Eu anomaly cannot be simply used to constrain the role of plg in magmatism because Eu anomaly in the melt is a function of source characteristics, oxygen fugacity (fO₂) of magmatic systems, and plg/mafic minerals mode ratio.     

堤防截渗墙加固对圩区地下水水质的影响

为了研究圩区堤防截渗墙封闭率对圩区内地下水水质的影响,以江西省赣抚圩区大堤为研究对象,通过野外试验分析,并运用Visual MODFLOW计算软件对大堤在截渗墙不同封闭率条件下地下水水质的变化趋势进行了数值模拟.结果表明:现状封闭率(0.75)不会造成地下水中总氮累积超标,封闭率的合理选择可以缓和堤防加固与圩区水环境之间的矛盾.     

埃提哲尔花岗斑岩及其暗色微粒包体中的斜长石斑晶

位于俄罗斯境内高加索地区的埃提哲尔花岗岩体中广泛分布暗色微粒包体，花岗岩体由黑云母花岗斑岩及暗色微粒包体组成，这两种岩石均含有一种具斑点状结构枋的斜长石斑晶。在这类斜长石玉晶内部可分辨出两个成分变同，这两个界面把斜长石斑晶分割成３部分：具斑点状结构的核部；偏基性且富含细小矿物包裹体的内环带和具正环带结构的酸性外环。     

甘肃白银厂矿田流纹岩的成因及其构造意义

白银厂矿田早-中寒武世流纹岩被认为是大陆裂谷环境或洋内弧岩浆作用的产物。地球化学分析结果显示：流纹岩具有Ti，Nb，Ta明显负异常，Sr和P中等负异常，LREE略富集到富集，相对亏损HFSE，La／Nb值为1～2，La/Th值为1.1～2.5,Th/Ta值高（6～12），部分样品表现为MREE负异常的稀土元素配分特征 ，δu=0.5～1.1,(^87Sr/^86Sr)t=0.7026～0.7048,ENd(t)= 5.60～ 6.14，综合其地球化学特征，推测流纹岩形成应是在大陆地壳基础上，由于与俯冲作用有关的玄武岩浆底侵作用导致镁铁质下地壳发生部分熔融所致。     

丁坝局部冲深计算的理论探讨

多年来,丁坝局部冲深计算大多局限在半经验半理论的回归分析计算方面,由于客观条件的不同,虽计算公式众多,彼此形式及结果也分歧较多。鉴于这种状况,根据流体力学原理、试验现象和数据,从理论分析出发,尝试性地提出了丁坝局部冲深计算理论公式,经与试验结果比较发现：理论公式的结构合理,与试验结果相符,从而为半经验半理论回归公式的进一步研究提供了理论方面的基础。     

陕南镇巴东部地区基性岩墙群的构造意义

应用现代岩石地球化学分析方法,对镇巴东部地区基性岩墙群岩石地球化学特征进行系统分析,分析表明,Ta/Hf值在0.27~0.90之间,Th/Ta值在0.68~1.59之间,这与地幔柱成因的玄武岩系特征(Ta/Hf>0.3,Th/Ta<1.6)相符。根据分析提出早古生代期间扬子板块北缘南秦岭区发生一次重要的地幔热柱岩浆活动,也正是因为这次深部地幔热柱岩浆活动的存在才引发了该区大范围的拉张裂解。此次地幔热柱活动应该是早古生代期间整个南秦岭地区构造演化的主要深部驱动力来源,可能对晚古生代时期勉略洋盆的打开产生着重要的影响,同时对秦岭造山带古生代构造演化过程的研究也具有重要意义。     

上海南汇东滩促淤圈围(四期)工程1#龙口合拢技术的探讨

上海市围涂工程是充分利用长江口岸、长江口三岛滩地、江心滩和杭州湾沿岸滩涂资源，围涂促淤，扩大陆地面积，以解决上海市因经济建设迅猛发展与建设用地紧缺的矛盾。本工程地处长江口，为上海市南汇东潍第四期促淤圈围工程4个围区之一。该围区总面积约5．04km^2，龙口底宽208m，上口宽度240m，一次一个围区面积及龙口宽度之大就全国圈围工程来讲也为数不多。龙口虽是临时工程，但龙口合拢是围堤工程建设的关键进度节点，能否按时合拢决定着圈围工程的成败。本围区于2003年9月开始龙口构筑，2004年3月29日顺利断流，为围涂造地工程施工积累了经验。