【作者】 章军峰；

【导师】 金振民； Green H.W.；

【作者基本信息】 中国地质大学 ， 构造地质学， 2004， 博士

【摘要】 与超高压变质作用密切相关的榴辉岩在俯冲和拆沉过程中占据重要地位，涉及当前地球动力学研究中大陆造山带下地壳拆沉、大洋深俯冲过程中的壳幔拆离作用以及俯冲带中的深源地震成因等一系列地球动力学问题，具有重要的地球动力学意义。然而，国际上目前还没有对榴辉岩在高温高压条件的变形行为进行任何实验研究，这在很大程度上限制了我们对榴辉岩在地幔深部条件下的变形和演化及相关地球动力学问题的认识。本论文主要在榴辉岩高溫高温实验变形领域进行了一些开创性的初步研究工作，包括以下四个方面：超高压榴辉岩流变学实验研究、超高压榴辉岩脱水致裂实验研究、超高压岩石红外光谱（FTIR）研究和超高压榴辉岩电子背散射衍射VEBSD)初步研究。 ●对榴辉岩的流变学实验研究初步确定了干榴辉岩幂律流变学参数为A=10^2.0±0.9，n=3.4±0.5，Q=420±70 kJ／mol and ΔV=19.1±5.5 cm³／mol，榴辉岩的塑性变形主要是通过榴辉岩中弱相—绿辉石和石英来调节的，而石榴石为变形刚性体。榴辉岩与其主要组成矿物岩石及方辉橄榄岩的对比实验研究结果表明：1)绿辉石单矿物岩石和石榴石单矿物岩石强度差异很大，榴辉岩强度是绿辉石单矿物岩石强度的2～3倍，不超过石榴子石岩强度的一半，由于绿辉石流变强度要远低于榴辉岩的流变强度，与湿单斜辉石的流变强度基本相当，说明深俯冲带中应变会主要集中在相对向富绿辉石的区域，榴辉岩中大量低强度的绿辉石的存在使得榴辉岩并不具有前人预见的超高强度；2)逐步增加榴辉岩中石榴石的含量会导致榴辉岩强度稳步升高，作为榴辉岩中的主要应变承载骨架，绿辉石的低强度说明榴辉岩的相对强度也不可能很大，这很好地解释了为什么超高压变质榴辉岩大多经历了很强的变形而不是不变形的高强度刚性体；3)绿辉石的流变强度在同等条件下要比透辉石单晶强度低，但大于多晶硬玉的流变强度，即绿辉石的流变强度介于其两个端元矿物—透辉石和硬玉之间，这进一步肯定了前人关于钠质单斜辉石流变强度低于钙质和钙镁质辉石的观点；4)榴辉岩的强度和方辉橄榄岩的强度基本相当，说明大洋地壳至少在俯冲带的浅部不大可能由于流变强度差异而从底伏的大洋地幔上拆沉出来；5)石榴石相对其他地幔矿物的高流变强度以及其在地幔岩中含量随深度的逐渐增加，说明地幔的流变强度也会随深度的增加而增加，并在上地幔底部的转换带处达到最大，实验研究表明下地幔钙钛矿的流变强度可能较低，因此转换带可能是地幔中的高强度和高粘度层，这为地震层析图象成果展示的深俯冲板舌在地幔转换带附近发生弯曲和偏转提供一个很好的解释。 ●对富结构水的榴辉岩的高温高压脱水实验首次证实了榴辉岩高压结构水脱水反应可以产生类似含水矿物脱水反应的高压致裂作用。当实验变形的温度低于水饱和榴辉岩的熔融温度时，榴辉岩中无流体产生，高压阻止了榴辉岩中脆性破裂现象的发生，榴辉岩表现为韧性变形性质，并且塑性变形强度很大；当变形温度介于“干”和“湿”榴辉岩熔融温度之间，榴辉岩中发生结构水脱水反应和部分熔融，流体和融体的作用导致了榴辉岩中有效压力的下降，在一定差异应力作用下，促进了岩石中微裂隙的打开，大量微裂隙的自我组织，最终导致榴辉岩的高压脆性破裂，并以大量融体充填的Ⅰ型微裂隙和正常的断层碎裂泥为特征，当变形温度高到一定程度时，榴辉岩中大量融体的产生和石榴石的重结晶现象导致榴辉岩流变强度的迅速下降，使得榴辉岩在远未达到脆性破裂强度的低应力条件下即发生塑性流变。富结构水的榴辉岩的塑性流变强度受控于温度和应变速率条件，但其高压脱水脆性破裂强度与溫度和应变速率无关。榴辉岩中结构水的脱水造成了岩石颗粒边界的微量部分熔融，在变形过程中充填到颗粒边界并导致了石榴石中的Ⅰ型微裂隙的形成章军峰:榴辉岩高温高压变形实验研究2003年12月+lJ稽阵宕阴饭裂，只斋妥微重（<1%）的流体就叮能导致岩石的高压脆性破裂。该究成果首次表明，富结构水的榴辉岩的高温高压脱水可以在相当100km深度条件下发生高压致裂，由于榴辉岩是深俯冲洋壳中50到300km深度的主要岩石类型，由此我们认为榴辉岩中结构水脱水致裂机制为解释深源地震成因机制提供了一种全新的思路。由于榴辉岩中名义上无水矿物内结构水的保存并不需要低地温梯度，这对解释板舌深部特别是热板舌中的地震活动具有重要意义。 .我们对天然变形和实验变形榴辉岩矿物中变形晶格优选方位电子背散射衍射 （EBSD）初步研究结果表明:l)无论是天然变形还是实验变形榴辉岩中，石榴石的晶格优选方位都很不明显，湿石榴石晶格优选方位和形状优选方位之间强烈的不匹配现象，说明石榴石中的晶内位错蠕变，即使存在，也没有对石榴石的整体塑性变形起到主导的作用，主导榴辉岩中石榴石塑性拉长的很可能是其他机制，水可以造成榴辉岩中石榴石流变性质的转变，在系统无水变形条件下，石榴石显刚性，但在高温条件下可能通过位错蠕变发生变形，在系统含水条件下，颗粒边界过程将可能主导石榴石的变形，石榴石的流变强度将下降到接近绿辉石的水平;2)绿辉石中存在较强的晶格优选方位，天然变形榴辉岩更多还原

【Abstract】 Eclogite serves as a reliable marker of high-pressure environments in collision zone terranes. The eclogitization of subducted crust and its reincorporation into the mantle are a critical part of the cycle of mantle convection. The transformation of basalt/granulite/gabbro to eclogite may contribute to the subduction and delamination of continental crust at depths, may produce a strong layer that separates from the rest of the lithosphere, and has been suggested to affect the distribution of intermediate-focus earthquakes. However, largely for technical reasons, there has been no experimental investigation on the deformation of eclogite at high temperature and high pressure. As a consequence, the deformation behavior of eclogite and its role in the deeper levels of subduction zones and mantle remain poorly understood. To address this important problem, we have performed preliminary studies on eclogite under high temperature and high pressure, including an experimental investigation on the rheology of UHP eclogite, an experimental investigation on the hydroxyl dehydration embrittlement of UHP eclogite, a FTIR study on the hydroxyl in UHP eclogite and associated rocks and a preliminary EBSD study on the deformation microstructure of UHP eclogite.We have conducted experiments at strain-rates of 10-4 -10-5/s, pressures of 2.5-3.5 GPa, and temperatures of 1300-1700K to determine the power flow law parameters for deformation of eclogite: A = 102.0 0.9, n = 3.4±0.5, Q = 420±70 kJ/mol and V = 19.1 ±5.5 cm3/mol. Our observations suggest that the strength contrast among garnet, omphacite and quartz is garnet > omphacite > quartz. The strain of eclogite is mostly accommodated by the deformation of weak quartz and omphacite, while garnet serves as a rigid body. We also conducted preliminary experiments on the strengths of the two minerals that constitute eclogite. These preliminary data show that (i) The two principal minerals of eclogite have greatly different strengths; the creep strength of eclogite appears to be comparable with that of harzburgite, the rock type located immediately below the oceanic crust, within experimental error, but 2-3 times that of omphacitite and no more than half that of garnetite; (ii) Progressive increase of garnet results in a smooth increase in strength, closely consistent with theoretical expectations; (iii) Eclogite has strength comparable to harzburgite; this equality is achieved because the great strength of polycrystalline silicate garnet is compensated by the weakness of omphacite. The latter is surprising from the experimental side, given previous data on diopside and enstatite showing strength much greater than olivine, but is consistent with predictions made from micro structures of naturally deformed eclogites. These results explain why eclogites from subduction-zone terranes exhibit evidence of extensive flow rather thanremaining undeformed eternally, (iv) These data further suggest that, during subduction, delamination of the oceanic crust from the underlying mantle due to contrasting rheologies is unlikely, except perhaps at depths of the mantle transition zone, (v) The extraordinary strength of polycrystalline silicate garnet supports previous suggestions that the lower portion of the mantle transition zone (-500-700 km) may be a layer of enhanced viscosity for it has the highest garnet contents in the upper mantle. We have performed preliminary deformation experiments at 3 GPa on a reconstituted natural eclogite that contains a significant hydroxyl concentration in both pyroxene and garnet. These preliminary data revealed faulting phenomena under stress within hydroxyl-enriched eclogite at temperatures between the H2O-saturated and dry solidi. The flow behavior of these eclogites is controlled by temperature and strain rate but their failure strength is not temperature or strain rate sensitive. Microstructural observations suggested exsolution of dissolved H2O to grain boundaries and within abundant Mode I microcracks followed by very small amounts of melting as ultra-thin films更多还原