昌乐地区新生代碱性玄武岩中刚玉、尖晶石巨晶岩相学、矿物化学特征

山东昌乐蕴含中国最大的宝石级刚玉—蓝宝石矿床。昌乐刚玉巨晶与玄武岩界限截然,边缘圆化,沿巨晶溶蚀圆化的边缘发育富铝的尖晶石和钛磁铁矿反应环边,显示其为深源捕虏晶的岩相学特征。对刚玉等巨晶及其反应边矿物进行岩相学、电子探针和LA-ICP-MS(Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry)分析能有效揭示其成因及演化历程。昌乐刚玉2组样品(CL-1和CL-2)矿物化学和关键元素特征及Ga/Mg、Cr/Ga、Fe/Ti、Fe/Mg等关键元素比值显示为岩浆型来源,而CL-2组样品明显叠加了后期熔体交代作用的特征。反应边尖晶石具刚玉巨晶向尖晶石巨晶过渡的成分特征,关键元素特征指示两者具强烈亲缘性,尖晶石巨晶是刚玉经历复杂交代作用的产物。昌乐刚玉呈现碱性熔体中结晶的化学成分特征,极有可能源于交代富集的地幔岩低度熔融形成的富铝的碱性熔体。     

阿尔泰山南缘中泥盆世苦橄岩中单斜辉石的成分特征及其地质意义

岛弧苦橄岩非常稀少,但在阿尔泰山南缘却分布有世界上罕见的中泥盆世厚大苦橄岩层。本文对该地区苦橄岩及其共生的玄武岩中单斜辉石的矿物化学成分进行了电子探针分析,结果表明,苦橄岩中的单斜辉石全部为透辉石,玄武岩中的单斜辉石种属有普通辉石和透辉石两类,但玄武岩中的单斜辉石成分呈连续演化关系。根据单斜辉石-熔体平衡原理计算了苦橄岩中单斜辉石斑晶形成的温度和压力,其变化范围主要集中在1350～1387℃和16.6～21.3kPa,相应的深度为55～70km,大致代表了苦橄岩岩浆房的深度。与此不同的是,玄武岩中单斜辉石斑晶结晶的深度较苦橄岩浅,为37～41km,暗示玄武岩浆是在相对低压条件下分离结晶后形成的演化岩浆。基于单斜辉石的成分和演化,确定了该地区火山岩属亚碱性系列,其构造背景为岛弧环境。     

中国东部岩石圈地幔的演化——地幔岩捕虏体微量元素的证据

对中国东部赋存于新生代玄武岩中的地幔岩捕虏体的全岩和单斜辉石等作了主元素和微量元素分析，证实了二辉橄榄岩及其单斜辉石在主元素有连续变化的趋势，反映了具部分熔融后残留相的性质。方辉橄榄岩及其中的单斜辉石的主元素，Ｎｄ／Ｙｂ，Ｔｉ／Ｚｒ和Ｓｒ／Ｚｒ值与二辉橄榄岩的同类矿物是不连续过渡。     

Tracing Lithosphere Evolution through the Analysis of Heterogeneous G9-G10 Garnets in Peridotite Xenoliths, II: REE Chemistry

Following previous publication of major–minor elementdata, this paper presents rare earth element (REE) data forheterogeneous (chemically zoned) garnets belonging to the peridotitesuite of mantle xenoliths from the Jagersfontein kimberlitepipe, South Africa. The rim compositions of the garnets in thehighest temperature–pressure (deepest) deformed peridotitesshow a typical megacryst-like pattern, of very low light REE(LREE) increasing through the middle REE (MREE) to a plateauof heavy REE (HREE) at c. 20 times chondrite; these compositionswould be in equilibrium with small-volume melts of the mid-oceanridge basalt (MORB) source (asthenosphere). With decreasingdepth the garnet rims show increasing LREE and decreasing HREE,eventually resulting in humped relative abundance patterns.A set of compositions is calculated for melts that would bein equilibrium with the garnet rims at different depths. Theseshow decreasing relative abundance of each REE from La to Lu,and the La/Lu ratio of the melts increases with decreasing depthof formation. Modelling of the effects of crystal fractionationshows that this process could largely generate the sequenceof garnet rim and melt compositions found with decreasing depth,including the humped REE patterns in high-level garnets. Consideringthe behaviour of major–minor elements as well as REE,a process of percolative fractional crystallization is advocatedin which megacryst source melts percolate upwards through peridotitesand undergo fractionation in conjunction with exchange withthe peridotite minerals. The initial megacryst melt probablyincludes melt of lithospheric origin as well as melt from theMORB source, and it is suggested that the process of percolativefractional crystallization may form a variety of metasomaticand kimberlitic melts from initial megacryst melts. Repeatedmetasomatism of the lower lithosphere by such differentiatingmelts is suggested by consideration of garnet core compositions.Such metasomatism would progressively convert harzburgites tolherzolites by increasing their CaO content, and this may accountfor the fact that the Cr-rich diamond–garnet harzburgiteparagenesis is commonly preserved only where it has been encapsulatedin diamonds. KEY WORDS: cratonic lithosphere; garnet zoning; mantle xenoliths; megacryst magma; metasomatic melt     

浙江江山上墅组玄武粗安岩中单斜辉石环带特征及指示意义

岩浆化学混合作用长期通过岩石地球化学进行识别,一直缺乏直接的矿物学证据。针对这一问题,我们对浙江江山碗瑶乡上墅组内薄层状玄武粗安岩夹层中的单斜辉石矿物环带开展了系统的矿物学研究工作,探讨其岩石成因、构造背景和岩浆动力学过程,为江山地区新元古代岩浆构造演化过程提供新的证据。研究表明,单斜辉石以斑晶和基质两种形式产出,斑晶发育核-幔-边结构,其中幔部和边部包含有较多的钛铁矿、单斜辉石和斜长石等矿物包裹体,表明经历了复杂的岩浆活动过程。电子探针分析表明,基质和斑晶辉石都属于普通辉石,斑晶辉石从核部、幔部到边部成分呈现规律性变化,CaO和MgO含量增加、Al₂O₃和Na₂O、K₂O含量逐渐降低,但是基质辉石具有相对较低的Ti、Al和Na含量,且与斑晶边缘成分类似。单斜辉石原位微量元素组成表明,斑晶单斜辉石从核部到边部具有相似的特征,呈现Th、La、Nd和Ta等元素相对富集,Nb、K、Ti和Pb等元素相对亏损的特点。温压计算结果显示斑晶和基质的形成温度较低(1040.3～1122.3℃),斑晶核部结晶深度较深...     

北京西山中生代火山岩中单斜辉石矿物化学及成因意义

本文对北京西山中生你火山岩中单斜辉石的矿物化学特点进行了研究,确定了矿物种属,计算了晶胞体积、密度和平均折射率。对单斜辉石－熔体平衡进行了判别,计算了平衡温度、压力和氧逸度,进而估算了岩浆房的深度。依据单斜辉石的成分和演化,结合岩石化学和地球化学特点,对火山岩的系列以及岩浆形成机制进行了讨论,探讨了岩浆活动的构造背景。     

福建省新生代玄武岩中辉石巨晶的矿物学研究

本文对我国福建省三条断裂带上产出的辉石巨晶进行了成分、结构、差热及穆斯堡尔谱分析,提出辉石巨晶有黑色和绿色两种类型,每种辉石巨晶均由两个相组成,两个相的存在决定了辉石巨晶的许多特殊性质。黑色辉石巨晶与玄武岩有明显的成因联系,而绿色辉石巨晶则没有。     

Exsolution Lamellae in Olivine Grains of Dunite Units from Different Types of Mafic‐Ultramafic Complexes

Exsolution microstructures in olivine grains from dunite units in a few selected tectonic environments are reported here. They include lamellae of clinopyroxene and clinopyroxene-magnetite intergrowth in the Gaositai and Yellow Hill Alaskan-type complexes, clinopyroxene-magnetite intergrowth in the K?z?lda? ophiolite, and chromite lamellae in the Hongshishan mafic-ultramafic intrusive complex. These lamellae commonly occur as needle-or rod-like features and are oriented in olivine grains. The host olivine grains have Fo contents of 92.5–92.6 in the Gaositai complex, 86.5–90.1 in the Yellow Hill complex, 93.2–93.4 in the K?z?lda? ophiolite and 86.9–88.3 in the Hongshishan complex. Clinopyroxene in the rod-like intergrowth exsolved in olivine grains in the Gaositai and Yellow Hill is diopside with similar major element compositions of Ca O(23.6–24.3 wt%), SiO_2(52.2–54.0 wt%), Al_2O_3(0.67–2.15 wt%), Cr_2O_3(0.10–0.42 wt%) and Na_2O(0.14–0.26 wt%). It falls into the compositional field of hydrothermal clinopyroxene and its origin is thus probably related to reaction between dunite and fluids. The enrichment of the fluids in Ca~(2+), Fe~(3+), Cr~(3+) and Na+, resulted in elevated concentrations of these cations in olivine solid solutions via the reaction. With decreasing temperature, the olivine solid solutions altered to an intergrowth of magnetite and clinopyroxene. The Fe~(3+) and Cr~(3+) preferentially partitioned into magnetite, while Ca~(2+) and Na+ entered clinopyroxene. Since the studied Alaskan-type complexes and ophiolite formed in a subduction environment, the fluids were probably released from the subducted slab. In contrast, the exsolved chromite in olivine grains from the Hongshishan complex that formed in post-orogenic extension setting can be related to olivine equilibrated with Cr-bearing liquid. Similarly, these lamellae have all been observed in serpentine surrounding olivine grains, indicating genetic relations with serpentinization.     

Mesozoic lithospheric mantle of the northeastern Siberian craton (evidence from inclusions in kimberlite)

Several thousand clinopyroxene, garnet, and phlogopite inclusions of mantle rocks from Jurassic and Triassic kimberlites in the northeastern Siberian craton have been analyzed and compared with their counterparts from Paleozoic kimberlites, including those rich in diamond. The new and published mineral chemistry data make a basis for an updated classification of kimberlite-hosted clinopyroxenes according to peridotitic and mafic (eclogite and pyroxenite) parageneses. The obtained results place constraints on the stability field of high-Na lherzolitic clinopyroxenes, which affect the coexisting garnet and decrease its Ca contents. As follows from analyses of the mantle minerals from Mesozoic kimberlites, the cratonic lithosphere contained more pyroxenite and eclogite in the Mesozoic than in the Paleozoic. It virtually lacked ultradepleted harzburgite-dunite lithologies and contained scarce eclogitic diamonds. On the other hand, both inclusions in diamond and individual eclogitic minerals from Mesozoic kimberlites differ from eclogitic inclusions in diamond from Triassic sediments in the northeastern Siberian craton. Xenocrystic phlogopites from the D’yanga pipe have ⁴⁰Ar/³⁹Ar ages of 384.6, 432.4, and 563.4 Ma, which record several stages of metasomatic impact on the lithosphere. These phlogopites are younger than most of Paleozoic phlogopites from the central part of the craton (Udachnaya kimberlite). Therefore, hydrous mantle metasomatism acted much later on the craton periphery than in the center. Monomineral clinopyroxene thermobarometry shows that Jurassic kimberlites from the northeastern craton part trapped lithospheric material from different maximum depths (170 km in the D’yanga pipe and mostly < 130 km in other pipes). The inferred thermal thickness of cratonic lithosphere decreased progressively from ~ 260 km in the Devonian-Carboniferous to ~ 225 km in the Triassic and to ~ 200 km in the Jurassic, while the heat flux (Hasterok-Chapman model) was 34.9, 36.7, and 39.0 mW/m², respectively. Dissimilar PT patterns of samples from closely spaced coeval kimberlites suggest different emplacement scenarios, which influenced both the PT variations across the lithosphere and the diamond potential of kimberlites.     

Oceanic plateau accretion inferred from Late Paleozoic greenstones in the Jurassic Tamba accretionary complex, southwest Japan

Abstract The Jurassic Tamba accretionary complex is divided into two tectono‐stratigraphic suites (Type I and II nappe groups), which are further divided into six complexes (nappes) each of which is characterized by a rock sequence of Late Paleozoic greenstone/limestone, Permian to Jurassic chert and Jurassic terrigenous clastic rocks. The mode of occurrence of the greenstone is divided into two types. The major basal type occurs as a large coherent slab associated with Permian chert and limestone, constituting the basal part of each complex, and the minor mixed type occurs as fragmented allochthonous greenstone blocks and lenses mixed with chert, limestone and sandstone in the Jurassic mudstone matrix. Most of the basal greenstones have uniform geochemical characteristics, which indicate enriched‐mid‐oceanic ridge basalt (MORB) affinity. Their geochemical compositions are akin to the reported Permo‐Carboniferous and Triassic oceanic plateau basalts. Mixed greenstones are divided into two petrochemical types: (i) tholeiitic basalt with normal‐MORB affinity, which is predominant in the uppermost complex of the Type II suite (upper nappe group); and (ii) tholeiitic and alkalic basalts of oceanic island or seamount origin, which are common in all complexes of the Tamba Belt. Geochemical characteristics of the greenstones thus vary in accordance with their occurrences and the structural units to which they belong. This relationship reflects the difference in topographic relief and crustal thickness of the accreted oceanic edifices – the remnants of thick oceanic plateau crust tended to accrete to the continental margin as a large basal greenstone body, whereas thin normal oceanic crust with small seamounts or oceanic islands accreted as mixed greenstones because of their mechanical weakness. The Type II suite (upper nappe group) contains the basal and mixed greenstones, whereas the Type I suite (lower nappe group) includes only mixed greenstones. This distinction may reflect the temporal change of subducting edifices from a thick oceanic plateau to a thin normal oceanic crust, and suggests that the accretion of a large oceanic plateau may be responsible for building accretionary complexes with thick basal greenstones slabs.