石榴辉石岩的电导率及对岩浆底侵的约束

河北汉诺坝新生代玄武岩携带的石榴辉石岩为岩浆底侵于上地幔顶部40～45 km形成的堆晶岩,是壳-幔过渡带的典型样品。本文使用Solartron 1260阻抗分析仪,测量1.2 GPa和380～900 ℃下石榴辉石岩WD958的电阻率。样品电导率(σ)与温度的关系遵守Arrhenius方程: σ=σ₀exp(-ΔH/kT),其中T是样品的绝对温度,k是Boltzmann常数,指前因子σ₀为97.5 S/m,活化焓ΔH为1.27 eV。使用傅立叶红外光谱仪测定样品中单斜辉石的平均含水量为117×10^-6 H₂O,橄榄石无水,石榴子石的含水量因蚀变无法测定。使用Hashin-Shtrikman平均方法计算样品WD958的电导率,发现样品可看作由含水单斜辉石、无水石榴子石和无水橄榄石组成的高阻集合体,电导率同时受小极化子导电和质子导电的影响。如果原位的石榴辉石岩的矿物含水量保持平衡,其电导率将提高1个数量级,质子导电成为主控导电机制。石榴辉石岩在高温的壳-幔过渡带(～1 000℃)表现为高导层,而在正常地温梯度下,与尖晶石二辉橄榄岩的电导率近似,为高阻层。因此,在岩浆底侵作用下,电性上的壳-幔边界将随着温度和含水量的变化而改变。     

Petrogenesis and metallogenesis of the Yanshanian adakite-like rocks in the Eastern Yangtze Block

Many of the Yanshannian intermediate-acid intrusive rocks related to Cu-Au mineralization in the Eastern Yangtze Block are characterized by high Al2O3, Sr contents, while low in Y, Yb contents, thus with high Sr/Y, and La/Yb ratios, and variational isotope signatures in particular, e.g. εNd(t) = -11.92-1.96, (143Nd/144Nd)i = 0.5120-0.5125, TDM = 0.70-1.71 Ga,(87Sr/86Sr)i = 0.7043-0.7076. The geochemical characteristics of these rocks suggest that: (1) these rocks are geochemically similar to adakite, which might have been stemmed from the partial melting of thickened basaltic lower crust due to basalt underplating; and (2) the high pressure (1.2-4.0 GPa) and high temperature (850-1150℃) surroundings of the lower crust favor both the fluid and adakite-like magma to generation. Not only can the adakite-like magma carry abundant fluid and Cu-Au ore-froming materials, but also can it bring them to the shallow part with ease and contributes to the Cu-Au mineralization.     

花岗岩体的累积生长与高结晶度岩浆的分异

花岗岩成因研究是认识大陆地壳形成和分异的有效方式。野外地质和地球物理观测、岩石学和地质年代学研究以及热演化模拟证明,很多花岗岩体是在数百万年甚至更长的时间跨度内、由多次岩浆累积添加侵位而成的。地壳内可能不存在岩基尺度的大岩浆房,具有流动能力的岩浆体一般规模很小(宽度1000m)。1000m宽的岩浆体冷凝到固相线只需要数千年时间。复式岩体的形成一般要经历三个阶段,即源区岩浆沿岩墙的上升、在脆-韧性地层界面处岩墙转化为岩床以及无数的岩床的垂向堆垛导致侵入体长大。存在于上地壳的岩浆储库,特别是多次先后侵位产生的岩浆体,主体上是晶粥体,其晶体含量高,粘度大,活动性弱,不利于发生对流、分异和混合。当幔源镁铁质岩浆大规模注入到地壳时,使粘稠的晶粥状岩浆受到加热,熔体含量增大,岩浆的粘度降低,引起岩浆体内部的成分分异和不同成分的岩浆之间的混合;当逐渐加厚的熔体层产生了足够大的浮力后,特别是有挥发份加入后,就会快速上升,甚至穿透上部的晶粥体,触发大规模的火山喷发。幔源岩浆的通量越大,地壳岩浆的活动性也越强,大规模的长英质岩浆聚集就可能发生大喷发,形成超级火山。本文提出,只有将侵入岩与火山岩相结合、长英质岩石与镁铁质岩石相结合,重点从侵入体形成的时间长短、岩浆相互作用的规模和频率、岩浆通量的演变、高结晶度的岩浆分异机理、侵入岩与火山岩的关系、地幔热和物质的贡献、挥发份在岩浆分异和火山喷发中的作用等方面入手,开展野外地质、岩石学、地球化学、同位素年代学及岩浆动力学的综合研究,才能深入认识花岗岩的成因机制,深化对大陆地壳形成和演化过程的理解。     

内蒙古赤峰柴胡栏子金矿基性麻粒岩包体特征及其成矿动力学意义

柴胡栏子金矿位于华北板块北缘,属中温热液蚀变岩型金矿。金成矿与矿区北部的早中生代辉石闪长岩体有密切关系。在辉石闪长岩体内发育大量包体,可以分为基性麻粒岩和角闪岩两类包体。包体的地球化学、形成温压条件表明基性岩包体为来源于大陆下地壳的基性麻粒岩包体,来源深度大约相当于下地壳中部-中上部位置,为早中生代时期底侵作用的产物。角闪岩包体来源于下地壳上部-中地壳下部位置,被上升岩浆带至地壳浅部。包体和寄主岩石具有相似的地球化学和氧、铅、锶、钕同位素特征,说明二者具有相同的岩浆来源。基性麻粒岩包体为底侵作用早期形成的堆晶岩受到后续岩浆的烘烤发生麻粒岩化形成。基性麻粒岩和寄主岩石辉石闪长岩与金矿床形成的密切时空关系显示底侵作用对柴胡栏子金矿含矿流体形成、运移和矿质富集有重要控制作用,其中 H2O和CO2等挥发性组分对控制流体形成和演化有至关重要作用。基性麻粒岩包体发育为成柴胡栏子金矿成矿物质来源于深部提供了有力的证据。     

胶辽地块古元古代前造山期深部过程的地质与地球化学制约

通过地质与地球化学资料的综合分析,揭示出胶辽地块古元古代前造山期深部地质过程主要为岩浆底板垫托作用,且具有3个特性,即幕式活动性、样式不对称性及空间上向南的迁移规律。受这一深部过程影响,在壳内引起了2.4Ga的基性岩墙侵位事件,古元古代裂谷带中双峰式火山活动(2.4～1.9Ga),巨量早期二长花岗岩的形成与侵位(主要为2.2～1.9Ga),二长花岗岩的侵位又引发盖层内改造型滑脱变形(2.2～1.9Ga),伴生顺层伟晶岩脉事件及低压高温变质和垂向顺层递增变质带等特殊拉张变质现象。     

Neoproterozoic zircon inheritance in eastern North China craton (China) Mesozoic igneous rocks: derivation from the Yangtze craton and tectonic implications

We propose that inherited Neoproterozoic zircons in Mesozoic igneous rocks from the eastern portion of the North China craton (NCC) were initially derived from the Yangtze/South China block, rather than from the NCC itself. The mechanism that introduced these zircons into the NCC was likely tectonic underplating during Triassic continental subduction/collision of the Yangtze block beneath the NCC. The addition of abundant crustal materials represented by the exotic zircons, probably along the Moho or weak interfaces within the NCC crust, led to the crustal thickening of the NCC. These sialic materials contributed significantly to the Mesozoic igneous rocks, either as source rocks or as contaminants of magmas generated during an extensional environment following crustal thickening. Crustal thickening was spatially linked to lithospheric thinning, with both occurring mainly in the eastern segment of the NCC, suggestive of an intrinsic relationship between thickening and thinning events during Mesozoic evolution of the NCC.     

Study of seismic tomography in Panxi paleorift area of southwestern China--Structural features of crust and mantle and their evolution

Structural features of the typical continental paleorift in Panxiarea are revealed by seismic tomography. (1) In the profile along the minor axis of Panxi paleorift, we found alternating high and low-velocity strips existing at different depths in the crust, presenting itself as a "sandwich" structure. The existence of these high and low-velocity anomaly strips is related to the basal lithology in the rift area. (2) An addition layer with velocity values of 7.1-7.5 km/s and 7.8 km/s exists from the base of lower crust to uppermost mantle and its thickness is about 20 km. Some study results indicate that the addition layer results from the invasion of mantle material. (3) A lens-shaped high-velocity body surrounded by relatively low-velocity material is observed at depths of 110-160 km between Huaping and Huidong in the axis of the paleorift. This is the first time to discover it in the upper mantle of the paleorift. Based on the results of geology, petrology and geochemistry, we infer that the formation of the addition layer and the lens-shaped high-velocity body in the upper mantle are related to the deep geodynamic process of generation, development and termination of the rift. On the one hand, the upwelling of asthenosphere mantle caused partial melting, and then the basaltic magma from the partial melted material further resulted in underplating and formed the crustal addition layer. On the other hand, the high-density content of mineral facies was increased in the residual melted mass of intensely depleted upper mantle, formed by basalt withdrawing. The solid-melt medium in the depleted upper mantle was mainly an accumulation of garnet and peridotite because the heating effect of lithosphere was relatively weakened in the later riftogenesis, so that a lens-shaped high-density and high-velocity zone was produced in the upper mantle. The results indicate that the energy and material exchange between asthenosphere and lithosphere and remarkable underplating would have an important effect on the material state and propagation of seismic wave in the lower crust, crust-mantle interface, asthenosphere and lithosphere. This process possibly is an important mechanism on the growth of continental crust and the evolution of deep mantle.     

郯庐断裂带南段构造演化与邻区成矿作用--从年代学资料得到的启示

郯庐断裂带为我国东部北北东—北东向岩石圈深大断裂带。在其南段的邻区,分布有胶东金矿区、鲁西铁金矿区以及长江中下游的铁铜（金）成矿带和苏北油气田等矿产。从已获得大量测年资料分析,可将构造—岩浆—成矿事件分为3个阶段： 1）早期（800～550 Ma）：自元古宙泛古陆裂解和随之而来的加里东期古地震和局部海西期喷流作用,证实地壳运动早已拉开了它的“构造活动序幕”,此阶段继承和构建了华北克拉通岩石圈先存的构造薄弱带,从成矿作用而论,此阶段可称为“成矿预备场地”;  2）主期（180～110 Ma）：在三叠纪期间,扬子板块和华北板块陆—陆碰撞和中生代西太平洋板块向欧亚板块俯冲等,造就了现今郯庐断裂与分支断裂的构造格架。其中主峰形成时间为140～120 Ma±,在这个阶段广泛有钙碱性系列、富钾高锶钙碱系列、碱性系列岩浆和后期成矿热液,沿着有利构造空间剧烈活动和成矿就位。本区壳幔混熔型和壳源同熔型花岗岩,是地壳减薄和下地壳拆沉、底侵的产物,而不是单一的构造运动的结果。此阶段可称为断裂构造—岩浆活动—成矿作用的鼎盛期;  3）晚期（60 Ma～现代）：沿东西向与北北东向（或北东向）断裂交汇处,存在橄榄玄武岩喷侵,为石油和天然气的形成提供了热源和物源,此期可称其为构造—岩浆活动的衰变期。     

Seismic evidence for thick and underplated late Archaean crust of eastern Dharwar craton

The deep crustal structure of eastern Dharwar craton has been investigated through τ-p extremal inversion of P-wave travel times from a network of seismographs recording quarry blasts. Travel times have been observed in the distance range 30–250 km in a laterally homogeneous lithospheric segment Main features of the inferred velocity-depth relationship include: (a) 29 km thick combined upper and middle crust velocity varying from 6 km/s to 7 km/s, with no observable velocity discontinuity in this depth range; (b) a lower crust (∼ 29–41 km) with velocity increasing from 7.0 to 7.3 km/s; (c) an average upper mantle velocity of 8.1 km/s; and (d) presence of a 12 km thick high velocity crustal layer (7.4 – 7.8 km/s) in the depth range 41–53 km, with a distinct velocity gradient marking a velocity increase of 0.4 km/s. The anomalous 53 km thick crust is viewed as a consequence of magmatic underplating at the base of the crust in the process of cratonization of the eastern Dharwar craton during late Archaean. The underplated material reflects here with the velocity of 7–3 to 7–8 km/s below the depth of 40 km. Our proposition of magmatic underplating is also supported by the presence of large scale I-granitoid, a product of partial melting of the upper mantle material.     

华北晚中生代和新生代玄武岩中单斜辉石巨晶来源及其对寄主岩岩浆过程的制约:以莒南和鹤壁为例

本文对华北克拉通晚中生代和新生代碱性玄武质岩石中的单斜辉石巨晶进行了主、微量元素和Sr-Nd同位素的综合研究,发现晚中生代和新生代单斜辉石巨晶存在明显的主、微量元素和同位素组成上的差异.新生代单斜辉石巨晶有Al-普通辉石和次透辉石两类;而中生代单斜辉石巨晶只有Al-普通辉石.新生代单斜辉石SiO2含量高、REE配分型式为上凸型、LILE和放射性元素含量高,并具有比寄主碱性玄武岩更亏损的Sr和Nd同位素组成;而中生代单斜辉石SiO2含量低、REE配分型式为LREE富集型、LILE和部分HFSE以及放射性元素含量低,并具有比寄主碱性玄武岩稍富集的Sr和Nd同位素组成;巨晶的结构、矿物成分和地球化学特征,以及Mg-Fe在熔体与单斜辉石间的分配状况皆说明,新生代碱性玄武岩中单斜辉石巨晶是碱性玄武岩浆在高压下结晶的,因此二者是同源的;而中生代单斜辉石巨晶是被寄主岩浆偶然捕获的捕虏晶,是不同源的.华北新生代单斜辉石巨晶存在于碱性玄武岩和拉斑玄武岩中,它们具有比寄主碱性玄武岩更亏损的Sr和Nd同位素组成,说明即使是碱性玄武岩也不能完全代表软流圈来源的原始岩浆,其在上升过程中或多或少存在同位素组成富集的物质的混入.同时,拉斑玄武岩不是碱性玄武质岩浆直接结晶分异的产物,亦不是完全由部分熔融程度的不同造成的.拉斑玄武岩中存在岩石圈地幔物质的贡献或是岩浆房内碱性玄武质岩浆受地壳混染作用的结果.