秦岭群裂谷岩浆建造地球化学型式及其成因探讨

秦岭群形成于早元古宙华北南缘裂谷化环境中。其下部有一套岩浆岩建造,包括基性和酸性的火山岩或侵入岩。地球化学资料表明,基性岩具有OIB型和CNB型两种地球化学型式,酸性岩具有壳幔混合型地球化学型式。这些变化起因于不同的源区和岩石成因过程,但本质上取决于岩石圈拉张程度和地幔流能量。     

边缘海盆地的形成机制及其对中国东南地质研究的启示

虽然有关边缘海的成因解释有“捕获机制”、弧后扩张(被动、主动)机制、陆内应力传播机制等,但迄今尚未有一种机制能统一解释所有边缘海的成因。边缘
海的成因不仅是经典的沟-弧-盆二维剖面问题,而应是一个包括平面图上大陆板块的变形在内的三维问题。边缘海的研究能给我国东部、特别是东南地区地质研究提供有益的启示,它包括:慎重鉴别古岛弧、对变质带、古洋壳及洋盆规模;充分注意小块体之间的碰撞,古转换断层在本区晚中生代岩浆活动、大陆增生过程中起重要作用;相关盆地内的沉积物记录着丰富的大陆碰撞、造山作用的信息。     

大陆构造样式及大陆动力学模式初探

根据大陆构造的最新研究成果,总结了大陆构造的基本型式,提出了大陆五层两域的构造模型,并从物质流变和构造变形的有机联系出发,探讨了大陆动力学的层流隆陷机制。     

A new metallogenic model of the Panzhihua giant V–Ti–iron oxide deposit (Emeishan Large Igneous Province) based on high-Mg olivine-bearing wehrlite and new field evidence

The Panzhihua layered intrusion hosts a giant V–Ti–iron oxide deposit with ore reserves estimated at 1333 Mt. Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) U–Pb zircon dating of comagmatic anorthosite yields a crystallization age of 259.77 ± 0.79 million years, coeval with the Emeishan flood basalts. Recently, we identified a small wehrlite dike in microgabbroic rocks and marbles. The wehrlite consists of high-Mg olivine phenocrysts with up to 90.44 wt.% Fo. Incompatible element-normalized patterns between bulk wehrlite and clinopyroxenes in gabbro suggest that they are cogenetic. The Panzhihua parental magma is estimated to have been picritic (～10 wt.% FeO and ～16 wt.% MgO), produced by partial fusion of garnet peridotite. Much of the melting occurred in garnet-facies mantle at an initial melting temperature of about 1530°C and pressure of ～3.4 GPa, suggesting involvement of a mantle plume. The degree of partial melting was rather modest and could have been generated by plume–lithosphere interaction or ascending plume-derived melting contaminated by lithospheric mantle. Field relationships show sharp contacts between the massive ores and gabbro, between wehrlite and fine-grained gabbro, and between disseminated ores and gabbro. Considering the entire intrusion, which is locally cut by dikes or veins of anorthosite, together with the occurrence of a breccia made up of gabbro clasts cemented by disseminated ores, we suggest that different types of magmas were generated by liquid differentiation in a deeper-level chamber. This differentiation could have resulted from double-diffusive convection cells, with melt later intruding into a higher-level chamber, rather than by crystal settling or in situ growth on the floor of the intrusion. However, rhythmic layering produced by in situ crystallization only occurs in the middle of the Panzhihua intrusion and was caused by periodic fluctuation in water pressure.     

塔里木二叠纪基性-中性-酸性岩浆岩的年代学及其地质意义

塔里木西部柯坪地区出露的早二叠纪玄武岩与盆地内其他地区广泛分布的玄武岩共同构成一个面积约0.25Mkm²的溢流玄武岩省。巴楚地区出露的早二叠纪辉绿岩、正长岩、石英正长岩与石英正长斑岩等多种岩浆岩构成了连续的基性-中性-酸性岩浆序列。野外露头观察表明,巴楚麻扎尔塔格地区的基性-中性-酸性岩浆岩为近同时侵位。柯坪塔格开派兹雷克组顶部层位玄武岩的锆石SHRIMP U-Pb年龄为279.0±4.5Ma, 该年龄值可限定该区二叠纪溢流玄武岩喷发的最晚时限。巴楚麻扎尔塔格石英正长斑岩脉的锆石SHRIMP U-Pb年龄为273.0±3.7Ma, 与前人测定的巴楚辉绿岩脉和正长岩的锆石年龄在误差范围内一致,表明巴楚地区的基性-中性-酸性岩浆岩为近同时侵位,时间在275Ma左右。综合已发表的塔里木二叠纪岩浆岩高精度年代数据,本文提出：整个塔里木大火成岩省岩浆作用的持续时间约为20Myrs; 291~287Ma喷发的巨量溢流玄武岩可能构成塔里木大火成岩省的主体,是地幔柱头部熔融的直接产物; 283~272Ma侵位的巴楚、一间房和塔北地区的其它类型岩浆岩体积较小,为塔里木大火成岩省晚期岩浆作用的产物。     

峨眉山大火成岩省白马层状侵入体的成因

峨眉山大火成岩省与二叠纪晚期的峨眉山地幔柱作用有关。白马层状侵入体是峨眉山大火成岩省赋存超大型Fe-Ti-V氧化物矿床的镁铁-超镁铁质侵入体之一。白马侵入体橄榄辉长岩和橄长岩全岩Mg O与Cr、Ni的相关性表明白马母岩浆经历了较高程度的分离结晶作用。原始地幔标准化微量元素图解和球粒陨石标准化稀土元素图解总体具有显著的Sr、Eu和Ti正异常,Zr-Hf负异常;而Nb、Ta既有正异常,也有负异常,这些特征与岩石中磁铁矿、单斜辉石和斜长石等矿物的堆晶作用有关。(87Sr/86Sr)i=0.704232～0.704855,平均值为0.704706;εNd(t)=1.40～3.94,平均值为2.41。Sr-Nd同位素组成落于峨眉山苦橄岩和高钛玄武岩的范围内,混合模拟计算表明白马母岩浆经历了10%～30%硅质大理岩围岩的混染。因此,白马Fe-Ti-V氧化物矿床的形成受母岩浆的组成、分离结晶作用及大理岩围岩的混染等多种因素共同制约。     

粤西黄岭岩组片麻状花岗质岩石的剪切深熔机理

粤西黄岭岩组片麻状花岗质岩石属于云开地块的基底岩系(2600Ma～1700Ma)。自下而上发育3种均造样式：下部透入性剪切变形带；中部平直均匀变形带；上部为揉皱带。应变分析表明，近水平分层剪切作用以及剪切应变带的向上迁移形成了上述不同构造样式，化学分析显示不同构造样式的花岗质岩石具有陆壳改造S型花岗岩特点。它们可能是形成于浅海一半深海的碎屑原岩在原地或准原地剪切深熔的结果。剪切深熔深度约13km～15km，峰期变形在2000Ma～1700Ma。     

A correlation between the Large Igneous Provinces and mass extinctions: constraint on the end-Guadalupian mass extinction and the Emeishan LIP in South China,eastern Tethys

ABSTRACT

Well-documented outcrops around the Emeishan Large Igneous Province (ELIP) in South China, eastern Tethys, encompassing the end-Guadalupian mass extinction have been investigated. Correlatable sections recording the event exhibit very similar lithological characters, positive-then-negative C isotope excursions and massive biotic demise. Detailed analyses of the fossil record and carbon isotopic variations were carried out on the Guadalupian–Wuchiapingian Boundary sections over the inner, middle, outer zones of the ELIP and its margin. Due to a pronounced decrease in marine habitat area and the environmental and ecological change over this part of the Tethys, the biota crisis records show the loss of numerous tropical invertebrate taxa, and exhibit fewer genera and smaller testing sizes and low productivity. The biota crisis was a sustainable and gradual reduction in diversity over the Capitanian. The associated carbon isotopic data reveal unusually high δ¹³C(carb) values before the late Capitanian, representing higher primary productivity (or buried rate) and more ¹³C-enriched CO₂ released by hydrothermal carbonate breakdown from the upper crust into the sediments at that time. Subsequently, an accelerated negative excursion across the boundary and the gradual excursion with low carbon isotope amplitude favours an increased influx of light ¹²C sourced by the volcanism around the eastern Tethys. The very similar time–space relation between the biota crisis and the Emeishan volcanism confirms that volcanic eruptions may have triggered the biota crisis event in South China. Intensive volcanism could result in detrimental environmental and ecological stresses, habitat loss, organic material splitting, or the emission of light carbon and thermal fluid (or aerosol), implying that the losses of the shallow-marine invertebrates either occurred geologically instantaneously or in a series of closely spaced crises coinciding with the initial phase of ELIP formation. These findings in South China may reveal the causal relation between mass extinctions and LIPs in a global context.