韧性剪切带的变形变质与同构造熔融作用——以中祁连地块宝库河韧性走滑剪切带为例

宝库河韧性剪切带是发育在中祁连地块北缘上的一条向北陡倾,走向近东西,宽约6 km的右行平移型韧性剪切带.剪切带内岩石原岩为泥质岩、基性岩和花岗岩,变质程度达角闪岩相,变形变质温度在685～763±46℃之间,压力在0.62～0.83±0.13 GPa范围内.其内长英质条带非常发育,规模变化较大,分布局部相对集中且受剪切带控制,走向与剪切带一致,平行于叶理,孤立无根,并在后期递进变形过程中发生不同程度的糜棱岩化、布丁化和褶皱,主要成分为长石和石英,明显不同于韧性剪切前或后侵入的花岗岩脉或岩体.长英质条带特征、REE配分模式及剪切带内岩石的变形变质温度说明剪切带内发育的长英质条带与基体是同源的,是在剪切应变过程中剪切热使围岩内部分物质发生动态熔融形成的,是同构造熔融作用的产物.     

Charnockitic magmatism in southern India

Large charnockite massifs cover a substantial portion of the southern Indian granulite terrain. The older (late Archaean to early Proterozoic) charnockites occur in the northern part and the younger (late Proterozoic) charnockites occur in the southern part of this high-grade terrain. Among these, the older Biligirirangan hill, Shevroy hill and Nilgiri hill massifs are intermediate charnockites, with Pallavaram massif consisting dominantly of felsic charnockites. The charnockite massifs from northern Kerala and Cardamom hill show spatial association of intermediate and felsic charnockites, with the youngest Nagercoil massif consisting of felsic charnockites. Their igneous parentage is evident from a combination of features including field relations, mineralogy, petrography, thermobarometry, as well as distinct chemical features. The southern Indian charnockite massifs show similarity with high-Ba-Sr granitoids, with the tonalitic intermediate charnockites showing similarity with high-Ba-Sr granitoids with low K₂O/Na₂O ratios, and the felsic charnockites showing similarity with high-Ba-Sr granitoids with high K₂O/Na₂O ratios. A two-stage model is suggested for the formation of these charnockites. During the first stage there was a period of basalt underplating, with the ponding of alkaline mafic magmas. Partial melting of this mafic lower crust formed the charnockitic magmas. Here emplacement of basalt with low water content would lead to dehydration melting of the lower crust forming intermediate charnockites. Conversely, emplacement of hydrous basalt would result in melting at higher {ie565-01} favoring production of more siliceous felsic charnockites. This model is correlated with two crustal thickening phases in southern India, one related to the accretion of the older crustal blocks on to the Archaean craton to the north and the other probably related to the collision between crustal fragments of East and West Gondwana in a supercontinent framework.     

Possible spatial variability in the Selwyn Block of Central Victoria: evidence from Late Devonian felsic igneous rocks

The Neoproterozoic to Cambrian Selwyn Block in Central Victoria forms the mainly unexposed basement to the Paleozoic metasediments, granitic rocks and felsic volcanic complexes of the Melbourne Zone of the Lachlan Orogen. The Late Devonian felsic rocks are largely products of partial melting of the Selwyn Block, and their chemistry implies that their sources were most probably arc-related andesite, dacite, volcaniclastic greywackes and some pelites. When plotted against the median longitudes of the plutons and volcanic complexes, the average values for ⁸⁷Sr/⁸⁶Sr_t and ?Nd_t (at 370 Ma) reveal broad trends interpreted to reflect possible compositional and/or age structure in the Selwyn Block. Assuming that the trends are real, from W to E, I-type sources are progressively less crustally evolved, probably younging eastward. The S-type sources show no trend in ?Nd_t, suggesting that there was efficient sediment mixing. The ⁸⁷Sr/⁸⁶Sr_t values, however, become more evolved eastward (opposite in sense to the apparent variation in the I-type sources). This is interpreted as the original Selwyn Block sediments having been more pelitic eastward, perhaps suggesting a deepening of the basin in this direction, as well as structurally upward in the succession. The opposite senses of variation highlights the spatial separation of the S- and I-type sources and suggest that the granitic magmas here are unlikely to represent any sort of mixing continuum.     

Geochemistry of the Eocene Felsic Porphyric Rocks and High-Mg Potassic Rocks along JARSZ: Implication for the Tectonic Evolution in Eastern Tibet

<正>Eocene felsic porphyric rocks and the high-Mg potassic volcanic rocks(HMPR) occur along the Jinshajiang-Ailao Shan-Red River shear zone(JARSZ) in eastern Tibet.Compared with the HMPR,which are generally believed to be sourced from an enriched mantle,the felsic porphyric rocks show similar K_2O contents,enrichment in LREE and LILE,particularly radiogenic isotope(e.g.Sr and Nd) features much similar to the former,implying generation of the felsic porphyric rocks most likely related to the HMPR,although they both have clearly different major and trace element compositions. The close relationship in spatial-temporal distribution and similar Sr-Nd characteristics between the felsic porphyric rocks and HMPR in eastern Tibet indicate that both of them were possibly formed by a similar tectonic process(event).Combining the basic dikes in southern and eastern Tibet,we suggest that the break-off of north-dipping Neo-Tethyan slab in southern Tibet during 50-40 Ma,triggered formation of high-Mg potassic magma.This led to developing felsic porphyric magma production by partial melting of underplating HMPR in the lower crust,or fractionation crystallization of the high-Mg potassic magmas.The break-off of slab in the Eocene may also have contributed to the abundant ore-forming material related to earlier subduction events,resulting in formation of the porphyric deposits along JARSZ in eastern Tibet.     

西藏尼玛盆地油页岩的发现及其地质意义

<正>青藏高原在长期复杂的演化过程中发育了一系列时代与类型不同的含油气盆地,但由于古生代以来冈瓦纳大陆裂解和特提斯洋的开启-闭合-造山作用的影响,特别是新生代以来印度-欧亚板块强烈碰撞造山和高原隆升作用的影响,含油气盆地受到了不同程度的改造,加之恶劣的自然环境,青藏高原成为中国目前油气地质研究和勘探程度最低的地区。     

青岛青山湾-垭口变沉积岩岩石地球化学特征及其物质来源

青岛青山湾—垭口出露一套浅变质的海相陆缘碎屑沉积岩,上覆岩浆锆石的LA-ICP-MS U-Pb年龄为(118.9±3.3)Ma,成熟度较低,与中新生代造山带砂岩类似。稀土元素球粒陨石化标准配分曲线均为轻稀土明显富集的右倾型,表明,研究区碎屑岩具有相同的物源区和构造背景,与活动陆缘的稀土元素配分曲线非常接近。Ce具弱的负异常,指示碎屑岩为大陆边缘海相成因,(La/Ce)N(PAAS)表明陆缘碎屑物质对该区碎屑岩有明显的贡献。主微量元素构造环境判别图解表明,研究区碎屑岩具有活动陆缘和大陆岛弧的特征,经历了相对简单的沉积演化过程;结合稀土元素配分曲线的特征,该区碎屑岩更可能是活动大陆边缘沉积形成。主微量元素分析揭示研究区碎屑岩物源富长英质岩石,镁铁质岩石作为物源也参与到成岩过程中。     

估算钙碱性长英质侵入岩结晶压力的地质压力计评述

本文对四种常见的矿物压力计（角闪石全铝压力计、黑云母压力计、榍石压力计和“石英”压力计）的适用性进行了经验性检验。基于国内外典型钙碱性长英质岩体的矿物化学和全岩化学成分数据,采用不同压力计估算岩体侵位压力,并与角闪石全铝压力计结果对比。结果表明：角闪石六次配位Al压力计与广泛使用的角闪石全铝压力计的估算值存在系统相关性;榍石压力计计算的压力与角闪石全铝压力计的估计结果相关性较为显著;基于Qtz—Ab—Or简并花岗岩体系的“石英”压力计可以用于估计准铝质—弱过铝质钙碱性（狭义）花岗岩的结晶压力。但是,经验性的黑云母压力计和基于实验数据的黑云母压力压计均不能很好地复现角闪石全铝压力计的结果。黑云母压力计不适用于估算钙碱性侵入岩的结晶压力。     

The Central Bundelkhand Archaean greenstone complex,Bundelkhand craton,central India: geology,composition, and geochronology of supracrustal rocks

Analysis of 3.3 Ga tonalite–trondhjemite–granodiorite (TTG) series granitoids and greenstone belt assemblages from the Bundelkhand craton in central India reveal that it is a typical Archaean craton. At least two greenstone complexes can be recognized in the Bundelkhand craton, namely the (i) Central Bundelkhand (Babina, Mauranipur belts) and (ii) Southern Bundelkhand (Girar, Madaura belts). The Central Bundelkhand greenstone complex contains three tectonostratigraphic assemblages: (1) metamorphosed basic or metabasic, high-Mg rocks; (2) banded iron formations (BIFs); and (3) felsic volcanics. The first two assemblages are regarded as representing an earlier sequence, which is in tectonic contact with the felsic volcanics. However, the contact between the BIFs and mafic volcanics is also evidently tectonic. Metabasic high-Mg rocks are represented by amphibolites and tremolite-actinolite schists in the Babina greenstone belt and are comparable in composition to tholeiitic basalts-basaltic andesites and komatiites. They are very similar to the metabasic high-Mg rocks of the Mauranipur greenstone belt. Felsic volcanics occur as fine-grained schists with phenocrysts of quartz, albite, and microcline. Felsic volcanics are classified as calc-alkaline dacites, less commonly rhyolites. The chondrite-normalized rare earth element distribution pattern is poorly fractionated (La_N/Lu_N = 11–16) with a small negative Eu anomaly (Eu/Eu* = 0.68–0.85), being characteristic of volcanics formed in a subduction setting. On Rb – Y + Nb, Nb – Y, Rb – Ta + Yb and Ta – Yb discrimination diagrams, the compositions of the volcanics are also consistent with those of felsic rocks formed in subduction settings. SHRIMP-dating of zircon from the felsic volcanics of the Babina belt of the Central Bundelkhand greenstone complex, performed for the first time, has shown that they were erupted in Neoarchaean time (2542 ± 17 Ma). The early sequence of the Babina belt is correlatable with the rocks of the Mauranipur belt, whose age is tentatively estimated as Mesoarchaean. The Central Bundelkhand greenstone complex consists of two (Meso- and Neoarchaean) sequences, which were formed in subduction settings.     

赣中变质岩带的Sm—Nd,Rb—Sr同位素年代研究

赣中变质岩带主要由斜长(云母)变粒岩、十字石榴云母片岩、云母石英片岩夹斜长角闪岩组成。斜长角闪岩Sm-Nd全岩等时线年龄为1113±19Ma,相当于该变质岩带的原岩形成年龄。ε_(Nd)(t)值为2.4±0.1,说明岩浆起源于亏损程度较低的地幔源区:726.6±1.1Ma、403.1±6.4Ma的Rb-Sr等时线年龄表明赣中地区在新元古代末期、加里东期经历了一次强烈的构造变质热作用,其地质、地球化学特征及时代可与浙闽地区陈蔡群、建瓯群进行对比。因此赣中变质岩带并非长期公认的华南加里东褶皱带,应是华夏地块的一部分,这一事实对华南大地构造单元的划分及构造演化具有重要意义。     

长英质岩石的流变学特征及其影响因素

在总结岩石变形机制与岩石流变学实验进展的基础上,讨论了岩石流变学数据的重复性。虽然高温高压流变学实验积累了大量的数据,但中、上地壳长英质岩石和早期获得的石英集合体的流变实验数据重复性比较差,而近年来发表的石英、长石的流变学实验数据重复性相对较好。虽然利用经验理论模型,根据端元组分可以拟合两相矿物集合体的流变律,但并不能满足定量确定复杂组分和特殊流变性的长英质岩石流变参数的需要。因此,利用长英质岩石流变参数估计大陆地壳流变强度剖面时,即使在相同地温和应变速率条件下,给出的流变曲线、脆-塑性转化带深度也有一定差别,还需要通过大量实验给出更精细的长英质岩石流变学实验数据。根据近年来流变学实验研究的新进展,讨论了在实验室条件下影响长英质岩石流变的各种因素,重点分析了流体、岩石成分、样品粒度和组构对流变的影响。微量结构水对岩石流变有显著的弱化作用,而熔体对流变的影响与熔体含量和分布相关,只有熔体呈薄膜状湿润颗粒边界时,熔体的弱化作用才显著。成分对岩石流变的影响不仅体现在样品的应力指数等流变参数的变化方面,还体现在样品从半脆性变形向塑性变形的转化温度方面。粒度主要影响岩石的变形机制,其中,细粒样品在扩散蠕变域具有应力与粒度线性负相关特性,是理想的应力计,可以用来定量确定韧性剪切带的流变强度;而在位错蠕变域,应力与粒度没有依存关系,这为将实验室条件得出的流变数据外推估计地壳流变提供了重要依据。组构和各向异性是地壳中岩石存在的普遍现象,但关于层状组构对多相矿物组成的岩石流变影响的研究非常少,需要通过新的实验来深入研究。