冀东太古宙麻粒岩及下地壳分层的讨论

本文提出冀东太古宙麻粒岩可划分为无角闪石或黑云母的辉石亚相和含角闪石或黑云母的角闪石亚相,基于平衡的矿物组合估计的压力和温度,辉石亚相和角闪石亚相分别为800—850℃,9—11kbar,700—750℃,7—9kbar。由麻粒岩亚相推导的冀东下地壳由二层构成,下部层(30—37公里)为辉石亚相,上部层(23—30公里)为角闪石亚相,它与地球物理资料相符合     

山东省麻粒岩与麻粒岩相变质作用——研究现状及对前寒武纪大地构造演化的启示

本文系统收集整理了近年来与山东省境内麻粒岩有关的研究资料,将山东省麻粒岩划分为沂水麻粒岩带、平度—栖霞麻粒岩带、海阳所—威海麻粒岩带三个麻粒岩相带,总结了各带的地质概况、变质温压条件及P—T—t轨迹、变质时代及演化过程,探讨了麻粒岩的成因。认为山东省各麻粒岩带均是多期次麻粒岩相变质作用的产物,其中2.7Ga、2.5Ga两期麻粒岩相变质作用均为中低压条件下的产物,发生时间与区内TTG的形成时间较为一致,P—T—t轨迹为逆时针型,反映当时形成的大地构造背景为地幔底辟引发的拉张环境;而1.9Ga、210Ma两期麻粒岩相变质作用则高压、中低压麻粒岩都有,反映其非均质变质环境的特点,P—T—t轨迹为顺时针型,指示其与俯冲—碰撞造山过程有关。其中1.9Ga期与古元古代胶辽吉活动带发生俯冲—碰撞造山过程有关,而210Ma期则是华北—扬子两大板块三叠纪碰撞产生的榴辉岩的退变质产物。结合省内基础地质研究的最新进展,对山东省前寒武纪大地构造演化作了部分探讨,并建立了年代学格架。认为平度—栖霞麻粒岩带古元古代高压麻粒岩相变质和苏鲁超高压变质带三叠纪的榴辉岩相变质事件可能是胶东地区太古宙绿岩带金元素活化迁移并在中生代成矿的决定性因素。山东省太古宙大地构造演化具有一定的旋回性,每个旋回由绿岩带—基性—超基性侵入体—TTG花岗岩类—热变质事件组成,很好地符合TTG的玄武质岩浆底侵成因模式。自1.9Ga才有高压麻粒岩的形成,暗示了具有一定规模的板块之间的俯冲活动始于1.9Ga之前的古元古代。鲁西陆块与胶辽陆块在古元古代可能并不是两个完全独立的块体,其巨大差异可能是由于两个陆块的大地构造位置差异和后期抬升剥蚀程度不同而引起。     

柳河地区晚太古代高级变质区地质特征及变质作用

笔者在１：５万区调工作中,于吉林省柳河地区首次发现了高级变质区,其变质程度达到麻粒岩相。通过野外地质观察,薄片研究,放射性同位素,ｐ－Ｔ－ｔ轨迹等方法和手段,对该区的变质时代,变质阶段,变质温压及构造环境等方面做了较深入的研究,并综合分析了该区的变质作用。     

The Saxonian Granulite Massif: new aspects from geochronological studies

Geochronological and geochemical analyses were carried out to determine the age of protolith formation and the timing of eclogite and granulite metamorphism of the Saxonian Granulite Massif (SGM). The age of the ultramafic protoliths of the garnet pyroxenites from the rim of the SGM has been determined to be Late Riphean (828 Ma), which together with geochemical and isotope data implies the extraction of the melt from an enriched mantle reservoir with an initial -Nd value of + 3.4. The minimum emplacement age of the mafic to felsic granulites has been dated at 470 Ma. The source material of these granulites reflects an enriched mantle reservoir with an -Nd value of + 2.6 (470 Ma).Based on U/Pb zircon and Sm/Nd analyses (garnet kyanite, apatite, clinopyroxene, whole rock) the granulite metamorphism of the SGM probably occurred at 340 Ma. The time of the eclogite metamorphism for a garnet pyroxenite has been detected at 380 Ma using the Sm/Nd method on minerals (garnet — clinopyroxene—zircon) and whole rock.Geochemical and isotope data clearly indicate a MORB- as well as an IAT-type source for the mafic granulites. During the time of the eclogite and granulite formation the oceanic basin closed in a subduction zone tectonic setting. A terrane or microcontinent model is consistent with these data and further explains the Ordovician, Silurian and Devonian high pressure events detected all over the European Hercynides. In this model, Gondwana derived microcontinents successively collided from the Early Ordovician onwards, causing subduction zone related high pressure metamorphism during closure of oceanic basins.     

Nature and evolution of the lower crust under central Spain: Inferences from granulite xenoliths (Calatrava Volcanic Field-Spanish central system)

So far, the nature and evolution of the lower crust under central Spain have been constrained mainly on the basis of a heterogeneous suite of granulite xenoliths from the Spanish Central System (SCS). In recent years, ultramafic volcanics from the Calatrava Volcanic Field (CVF) have also provided deep-seated crustal xenoliths which have not been studied in detail. Our data, combining mineral, whole-rock and isotopic geochemistry with U–Pb–Hf isotope ratios in zircons from the CVF and SCS xenoliths, highlight the felsic composition of the lower crust under central Iberia. A number of the Calatrava xenoliths represents Variscan igneous protoliths, which are a minor population in the SCS, and were likely formed by crystallisation of intermediate and felsic melts in the lower crust during the Variscan orogeny (leucodiorite protolith age of 314 ± 3 Ma and leucogranite protolith age of 308 ± 2.5 Ma). U–Pb data of metamorphic zircons show that granulite-facies metamorphism mainly occurred from 299 to 285 Ma in both areas. These ages are slightly younger than those of granitic intrusions that could be genetically related to the granulitic residue, which points to a main role of U–Pb isotope resetting in lower crustal zircons during HT or UHT conditions. The zircon U–Pb–Hf isotopic ratios support the idea that the lower crust in central Iberia consists mainly of Ordovician–Neoproterozoic metaigneous and metasedimentary rocks associated with the Cadomian continental arc of northern Gondwana. These rocks provide evidence of mixing between juvenile magmas and an enriched crustal component, ultimately extracted from an Eburnean crust. Other more evolved components present in detrital zircons are likely related to recycling of Archean crust derived from North Africa cratonic terranes.     

The fate of basaltic enclaves during pyroclastic eruptions: An origin of andesitic ignimbrites

Igneous enclaves, chilled bodies of magma with compositions contrasting with those of their hosts, have long been recognized in felsic plutonic rocks. Similar enclaves occur in felsic pyroclastic rocks despite the apparent difficulty of their survival of the explosive eruption process without fragmentation. The occurrence of andesitic ignimbrites with textural evidence of generation by mechanical mixing of felsic and mafic ash indicates that in some instances basaltic enclaves in felsic magmas that erupted explosively do indeed undergo fragmentation and homogenization with their host. Two exposures of rhyolitic ignimbrite that hosts basaltic enclaves, and of andesitic ignimbrite, in coastal Maine demonstrate the set of conditions necessary for survival of basaltic enclaves during catastrophic explosive eruptions. Relatively lower viscosity of basaltic enclaves compared to the rhyolitic host magma permits vesicle networks to develop as volatiles exsolve from the melt and form bubbles. The vesicle networks provide sufficient permeability for exsolving gases to escape the basaltic magma bodies, hence sparing the basaltic enclaves from fragmentation. If adequate permeability for volatile escape does not develop, the expanding bubbles are trapped within the basaltic enclave and ultimately, with depressurization during rise of the magma to the surface, cause fragmentation of the basaltic magma. In this case, the basaltic ash and the host rhyolitic ash homogenize, producing a hybrid ignimbrite, while the surrounding viscous rhyolitic magma behaves typically, with a small volume of the rhyolitic magma retaining its coherence as pumice bodies while most of the magma fragments shortly after vesiculation to become ash. These observations suggest a distinction between the voluminous andesites associated with subduction zones, for which attainment of intermediate composition occurred as a result of petrologic processes unique to subduction zones, and hybrid andesitic ignimbrites, which are spatially associated with bimodal magmatic systems in a variety of tectonic settings and are the result of mechanical mixing of ash during pyroclastic flow.     

秦岭环斑结构花岗岩中暗色包体的岩浆混合成因及岩石学意义——元素和Nd、Sr同位素地球化学证据

秦岭环斑结构花岗岩中的暗色包体主要为闪长质岩浆包体,SiO2(50％-62％)低,K2O Na2O(7．01％-9．4％) 高,里特曼指数(δ)为5-9,F、过渡性元素和稀土元素富集。包体和寄主岩石的主要氧化物之间具有良好的线性关系、稀土配分曲线和微量元素配分曲线相似,以及活动性组分、高场强元素、轻稀土和同位素特征等显示,寄主岩石和包体之间发生过明显的成分交换,这些成分在二者中大体上趋于平衡。这种特征表明,环斑结构花岗岩岩浆的形成至少与两种岩浆的混合有关。包体的(87Sr／86Sr)i较低(0．70514-0．70624)、εNd(t)值较高(-0．95--3．3)和富过渡性元素的特征揭示,形成包体的原始岩浆为起源于幔源的玄武质岩浆。包体和寄主岩石的关系显示岩浆的混合方式为基性岩浆注入到已经开始结晶的酸性岩浆。这些研究为环斑结构花岗岩是起源于地壳的酸性岩浆和起源于地幔的基性岩浆形成的混合岩浆结晶的产物提供了新证据;同时,也为环斑结构的混合成因研究提供了新思路和途径。     

广州小北路黑人聚居区社会空间分析

1978 年改革开放带来中国城市的快速转型。近年来, 针对经济转型背景下中国大城市 的社会空间研究很多, 但对在全球化影响下出现的跨国移民族裔聚居区的研究尚不多见。对 广州黑人聚居区展开研究, 以越秀区小北路为例, 探讨其社会空间特征与演进机制。研究采 用微观视角, 针对典型案例进行问卷调查和半结构式访谈。研究表明, 广州黑人聚居区与西 方同类族裔社会区存在诸多差异。小北路黑人聚居区因全球化下新的“自下而上”的跨国经 济联系而生, 因广州城市的商贸文化、宗教历史、贸易网络和地理气候条件而兴, 其跨国移 民多为来自西非地区的族裔散居者/ 漂泊者(Diaspora), 其人口构成异质多元, 且流动性强。 小北路的社会空间机制既包含被动隔离也包含主动聚居, 其经济形态已经开始向“族裔聚居 区经济”(ethnic enclave economy) 发展。     

Genesis and Mixing/Mingling of Mafic and Felsic Magmas of Back-Arc Granite: Miocene Tsushima Pluton, Southwest Japan

The Middle Miocene Tsushima granite pluton is composed of leucocratic granites, gray granites and numerous mafic microgranular enclaves (MME). The granites have a metaluminous to slightly peraluminous composition and belong to the calc‐alkaline series, as do many other coeval granites of southwestern Japan, all of which formed in relation to the opening of the Sea of Japan. The Tsushima granites are unique in that they occur in the back‐arc area of the innermost Inner Zone of Southwest Japan, contain numerous miarolitic cavities, and show shallow crystallization (2–6 km deep), based on hornblende geobarometry. The leucocratic granite has higher initial ⁸⁷Sr/⁸⁶Sr ratios (0.7065–0.7085) and lower ε_Nd(t) (?7.70 to ?4.35) than the MME of basaltic–dacitic composition (0.7044–0.7061 and ?0.53 to ?5.24), whereas most gray granites have intermediate chemical and Sr–Nd isotopic compositions (0.7061–0.7072 and ?3.75 to ?6.17). Field, petrological, and geochemical data demonstrate that the Tsushima granites formed by the mingling and mixing of mafic and felsic magmas. The Sr–Nd–Pb isotope data strongly suggest that the mafic magma was derived from two mantle components with depleted mantle material and enriched mantle I (EMI) compositions, whereas the felsic magma formed by mixing of upper mantle magma of EMI composition with metabasic rocks in the overlying lower crust. Element data points deviating from the simple mixing line of the two magmas may indicate fractional crystallization of the felsic magma or chemical modification by hydrothermal fluid. The miarolitic cavities and enrichment of alkali elements in the MME suggest rapid cooling of the mingled magma accompanied by elemental transport by hydrothermal fluid. The inferred genesis of this magma–fluid system is as follows: (i) the mafic and felsic magmas were generated in the mantle and lower crust, respectively, by a large heat supply and pressure decrease under back‐arc conditions induced by mantle upwelling and crustal thinning; (ii) they mingled and crystallized rapidly at shallow depths in the upper crust without interaction during the ascent of the magmas from the middle to the upper crust, which (iii) led to fluid generation in the shallow crust. The upper mantle in southwest Japan thus has an EMI‐like composition, which plays an important role in the genesis of igneous rocks there.     

滇西马厂箐岩体及其中深源包体地球化学特征

云南马厂箐岩体是滇西喜马拉雅期富碱侵入岩带的重要组成部分,具有高钾富碱的特征,属于高钾钙碱性或钾玄岩岩石系列。该岩体中首次发现镁铁质深源暗色包体,寄主岩及包体的对比研究为揭示富碱岩浆的性质、起源及成岩成矿的地球动力学机制提供了可靠的地质依据。包体大部分呈浑圆状或次圆状,微细粒结构,包体中含针状磷灰石,部分包体中含有寄主岩石中的钾长石和石英斑晶。包体及寄主岩的主量和微量元素协变图呈不同程度的直线变异关系,而且稀土、微量元素曲线形态相近,显示了包体和寄主岩在地球化学特征上既有相似性,又具有明显非同源性的特点。以上特征表明,马厂箐岩体中的暗色包体是壳幔岩浆混合的产物,幔源岩浆的混合注入是这套岩浆成矿的关键因素。