湘赣地区中生代镁铁质岩浆作用与岩石圈伸展

综合分析了华南内部中生代 (178～ 80Ma)镁铁质岩石的年代学和元素同位素地球化学特征。研究表明区内主要发育 4期镁铁质岩浆活动 :2 2 0Ma± ,175Ma± ,12 0～ 15 0Ma ,80～ 90Ma ,其中 2 2 0Ma仅在道县发育辉长岩包体。地球化学上区内 175Ma左右的宁远太阳山和赣中项家碱性玄武岩和 80～ 90Ma的镁铁质岩石主要表现为Hawaii OIB玄武岩的元素同位素地球化学特征 ,而175Ma的其他镁铁质岩石表现为岩石圈地幔属性。 12 0～ 15 0Ma的镁铁质岩石则介于岩石圈地幔端员与软流圈地幔端员之间。时、空上表现为以郴州—临武断裂为界 ,西侧自 170Ma左右的EMI型为主向 80～ 90Ma的OIB型为主迁移演化 ,而东侧则自老而新自EMII型 (175Ma)为主演变为以OIB型 (80～ 90Ma)为主。上述资料暗示区内至少存在 4期强烈的岩石圈减薄作用 ,软流圈物质上涌和岩石圈伸展减薄是华南中生代岩浆作用形成的主要机制。但 2 2 0Ma左右的伸展减薄范围局限 ,而较大规模的岩石圈伸展减薄和软流圈上涌始于 178Ma ,其形成可能与华南印支造山作用的后造山 (或后碰撞 )拉张裂解地球动力学背景有关。同时也暗示郴州—临武断裂可能是界定中生代EMI型扬子岩石圈和EMII型华夏岩石圈地幔的重要边界。     

中国兴蒙—吉黑地区岩石圈结构基本特征

兴蒙—吉黑地区岩石圈由额尔古纳、兴安、松嫩和佳木斯4个古陆块及完达山中生代大陆边缘增生杂岩构成。Nd同位素模式年龄显示,佳木斯陆块时代最老,1500～2200Ma;额尔古纳陆块次之,1000～1600Ma;兴安和松嫩陆块具有相同的Nd模式年龄,500～1200Ma。地球化学示踪分析表明,该区古生代时表层地壳的Nd同位素模式年龄以中元古代为主,而中生代花岗岩的Nd同位素模式年龄主要为新元古代,表明该区深部地壳的年龄较表层地壳的年龄年轻,显示出该区地壳具有下新上老的年龄结构。Os同位素分析同时证明,该区岩石圈地幔也多表现为年轻性质。地震(Vp)速度结构显示,该区岩石圈结构在垂向上具有两个明显的特征:一是与传统意义上的地震岩石圈概念明显不同,该区岩石圈地幔的低速带没有稳定连续的顶界面,低速异常顶界面深浅不一,与高速异常体犬齿交错,某些构造单元之下的低速异常直达Moho,但底界面却十分稳定,深度为230～240km;二是“立交式”速度结构,表现为在地壳范围内,速度等值线总体呈北东向展布;岩石圈地幔的速度等值线呈北北西-近南北向展布;低速异常圈层的速度等值线为近东西向展布。     

Geochemical evolution of magmatism in Archean granite-greenstone terrains

Evolution of Archean magmatism is one of the key problems concerning the early formation stages of the Earth crust and biosphere, because that evolution exactly controlled variable concentrations of chemical elements in the World Ocean, which are important for metabolism. Geochemical evolution of magmatism between 3.5 and 2.7 Ga is considered based on database characterizing volcanic and intrusive rock complexes of granite-greenstone terrains (GGT) studied most comprehensively in the Karelian (2.9–2.7 Ga) and Kaapvaal (3.5–2.9 Ga) cratons and in the Pilbara block (3.5–2.9 Ga). Trends of magmatic geochemical evolution in the mentioned GGTs were similar in general. At the early stage of their development, tholeiitic magmas were considerably enriched in chalcophile and siderophile elements Fe₂O₃, MgO, Cr, Ni, Co, V, Cu, and Zn. At the next stage, calc-alkaline volcanics of greenstone belts and syntectonic TTG granitoids were enriched in lithophile elements Rb, Cs, Ba, Th, U, Pb, Nb, La, Sr, Be and others. Elevated concentrations of both the “crustal” and “mantle-derived” elements represented a distinctive feature of predominantly intrusive rocks of granitoid composition, which were characteristic of the terminal stage of continental crust formation in the GGTs, because older silicic rocks and lithospheric mantle were jointly involved into processes of magma generation. On the other hand, the GGTs different in age reveal specific trends in geochemical evolution of rock associations close in composition and geological position. First, the geochemical cycle of GGT evolution was of a longer duration in the Paleoarchean than in the Meso-and Neoarchean. Second, the Paleoarche an tholeiitic associations had higher concentrations of LREE and HFSE (Zr, Ti, Th, Nb, Ta, Hf) than their Meso-and Neoarchean counterparts. Third, the Y and Yb concentrations in Paleoarchean calc-alkaline rock associations are systematically higher than in Neoarchean rocks of the same type, while their La/Yb ratios are in contrast lower than in the latter. These distinctions are likely caused by evolution of mantle magmatic reservoirs and by changes in formation mechanisms of silicic volcanics and TTG granitoids. The first of these factors was likely responsible for appearance of sanukitoid magmatic rocks in the Late Mesoarchean. Representative database considered in the work includes ca. 500 precision analyses of Archean magmatic rocks.     

考虑注浆材料流变的隧道施工土层变形计算

在盾构隧道施工过程中背后注浆是必不可少的,注浆材料在逐步卸载的压力作用下由流动状态变化到固体状态,注浆材料的变形特性将与恒载作用有区别。针对南京地铁1号线实际工程盾构隧道施工注浆材料,进行了卸载条件和恒载条件下注浆材料的流变试验,得到了非线性流变的试验曲线和拟合的流变方程;在半无限空间圆孔扩张位移理论解和注浆材料的流变试验结果的基础上,推求得到隧道所在土层的位移解,确定了位移的影响范围,讨论了地表沉降计算结果的合理性,初步建议确定隧道施工应保护区域的确定原则。     

Petrogenesis and tectonic implications of Early Cretaceous andesitic-dacitic rocks,western Qinling(Central China):Geochronological and geochemical constraints

~(40)Ar/~(39)Ar and zircon U-Pb geochronological and whole-rock geochemical analyses for the Laozanggou intermediate-acidic volcanic rocks from the western Qinling orogenic belt,Central China,constrain their petrogenesis and the nature of the Late Mesozoic lithospheric mantle.These volcanic rocks yield hornblende or whole-rock ~(40)Ar/~(39)Ar plateau ages of 128.3-129.7 Ma and zircon U-Pb age of131.3±1.3 Ma.They exhibit Si02 of 56.86-66.86 wt.%,K_2 O of 0.99-2.46 wt.% and MgO of 1.03-4.47 wt.%,with Mg# of 42-56.They are characterized by arc-like geochemical signatures with significant enrichment in LILE and LREE and depletion in HFSE.All the samples have enriched Sr-Nd isotopic compositions with initial ~(87)Sr/~(86)Sr ratios ranging from 0.7112 to 0.7149 and ε_(Nd)(t) values from 10.2 to 6.3.Such geochemical signatures suggest that these volcanic rocks were derived from enriched lithospherederived magma followed by the assimilation and fractional crystallization(AFC)process.The generation of the enriched lithospheric mantle is likely related to the modification of sediment-derived fluid in response to the Triassic subduction/collision event in Qinling orogenic belt.The early Cretaceous detachment of the lithospheric root provides a reasonable mechanism for understanding the petrogenesis of the Laozanggou volcanic sequence in the western Qinling orogenic belt.     

Mechanism of formation of superdeep sedimentary basins: lithospheric stretching or eclogitization?

The superdeep North Caspian, South Caspian, and Barents basins have their sedimentary fill much thicker and the Moho, correspondingly, much deeper than it is required for crustal subsidence by lithospheric stretching. In the absence of large gravity anomalies, this crustal structure indicates the presence under the Moho of a thick layer of eclogite which is denser than mantle peridotite. Crustal subsidence in the basins can be explained by high-grade metamorphism of mafic lower crust. The basins produced by lithospheric stretching normally subside for the first ～100 myr of their history, while at least half of the subsidence in the three basins occurred after that period, which is another evidence against the stretching formation mechanism. According to the seismic reflection profiling data, stretching can be responsible for only a minor part of the subsidence in the Caspian and Barents basins. As for the South Caspian basin, there has been a large recent subsidence event in a setting of compression. Therefore, eclogitization appears to be a realistic mechanism of crustal subsidence in superdeep basins.     

辽宁南部万福变质核杂岩的发现及其区域构造意义

根据宏观与微观构造测量,结合白垩纪沉积盆地组成与结构、区域岩浆活动性及其测年资料等的综合分析,揭示出在辽宁南部辽南变质核杂岩东侧存在另一个变质核杂岩构造,即万福变质核杂岩。该核杂岩具有典型的三层结构:拆离断层带由不同层次构造岩构成,上盘为元古宇岩石,下盘是太古宇岩石和就位于其中的同构造花岗质侵入体。万福变质核杂岩形成于早白垩世,与辽南变质核杂岩构成一个变质核杂岩对,两者在很多方面具有相似性。该变质核杂岩对的厘定可能为阐明华北晚中生代岩石圈的力学和流变学属性以及岩石圈减薄过程提供了依据。     

粤北石人嶂钨矿区角闪辉长岩地球化学、年代学特征及其地质意义

石人嶂钨矿区发育的角闪辉长岩与钨矿密切共生,研究该矿区对认识粤北地区区域地质演化、深部岩浆信息以及区域基性岩与成钨作用关系研究意义重大,但到目前为止,尚未开展相关系统的研究。文章报道了角闪辉长岩的锆石和磷灰石U-Pb年代学、地球化学资料。磷灰石U-Pb年龄指示角闪辉长岩的结晶年龄为（171±14） Ma,形成于燕山早期晚阶段古太平洋板块俯冲背景下的岩石圈伸展的构造环境,其中,锆石为捕获锆石,具有3组年龄,分别记录了区域古元古代陆核增生,新元古代古华南板块的聚合和裂解,早中生代华南板块与华北板块,印支地块聚合的后造山伸展等多起构造事件。角闪辉长岩具有低Si、K、Mn,高Ca、Fe、Mg、Na、Ti的特征,富集Rb、Ta、Zr、Ti等,亏损Ba、Nb、Sr、Hf、Be、Cs、U、Th等元素,表明其源区具有E-MORB型富集地幔的属性。角闪辉长岩的形成主要受控于软流圈地幔的部分熔融。同时,由于角闪辉长岩的高氧逸度的特征及其较石人嶂钨矿成矿更早,角闪辉长岩的存在可能主要是为后期的钨矿化的叠加提供部分Ca、Fe等元素,并为富矿流体的运移提供通道。     

北部湾涠洲岛橄榄岩中熔体包裹体——大陆岩石圈地幔交代作用的新证据

根据北部湾涠洲岛晚更新世火山岩中尖晶石二辉橄榄岩的矿物化学和形成条件推测,它们是来自地下约40km-50km的大陆岩石圈地幔样品。涠洲岛地幔橄榄岩中同时出现高硅（64％～68％）和低硅（49％～57％）两种熔体成分,它们普遍富碱、Al2O3和H2O、CO2等挥发组分（2％-5％）。研究认为,两种熔体存在不同的成因机制。高硅熔体可能与软流圈上升的玄武质岩浆和斜方辉石反应或下地壳物质循环进入地幔后的部分熔融有关。低硅熔体直接提供了富硅熔体与橄榄石发生交代作用的证据。这可以用富SiO2熔体＋Ol→贫SiO2熔体＋Opx反应解释,其结果是消耗橄榄石生成斜方辉石。涠洲岛橄榄岩中两种熔体的发现,进一步提供了发生在大陆岩石圈地幔流（熔）体与橄榄岩交代作用的普遍性、复杂性,以及地幔熔体多样性的新证据。     

Early Cretaceous rift-related volcanism in the Songliao Basin,NE China – A geochemical study

Following the amalgamation of the Siberian and North China Cratons, NE China, as part of the Central Asian Orogenic Belt (CAOB), underwent Late Mesozoic lithospheric extension that was associated with volcanic activity. The Songliao Basin is the most important rift structure formed during these processes and contains voluminous volcanic rocks interlayered with sedimentary infill. Mafic-to-intermediate lavas are associated with felsic ones. This study focusses on the geochemical compositions of the less-widespread Early Cretaceous mafic-to-intermediate lavas in the Songliao Basin and compares them with the more abundant felsic rocks. Two mafic-to-intermediate magma series, one with alkaline and the other with sub-alkaline affinity, were identified. High MgO and Cr contents, low Th/Nb and La/Nb ratios, and variable but depleted Nd isotope compositions indicate that both magma suites were most likely formed by the melting of enriched upper mantle sources. Sub-alkaline mafic-to-intermediate rocks and I-type rhyolites define a co-genetic magma series. This rock suite was produced by the melting of subduction-modified lithospheric mantle and subsequent magma evolution as well as crustal melting during lithospheric extension. Alkaline mafic-to-intermediate rocks and A-type rhyolites form another co-genetic magma suite that was produced under within-plate conditions from an OIB-type mantle source, supposed to be the heterogeneous shallow asthenosphere and/or the lower lithosphere. Decompression partial melting of this mantle source requires a relatively thin lithosphere. The development of alkaline mafic rocks and A-type rhyolites as typical bimodal volcanic assemblage reflects that lithospheric thinning below the Songliao Basin reached its maximum, whereas basin rifting terminated afterwards.