浙江中部芙蓉山花岗斑岩及包体岩石地球化学研究

花岗岩的源区、温压条件及与其他岩石的共生组合的研究可以限定其形成构造背景,了解其形成的深部动力学过程。本文对浙江中部中生代芙蓉山花岗斑岩及其暗色包体开展了全岩主微量元素、锆石U-Pb年代学和Hf同位素、Ti温度计和全岩Sr-Nd同位素研究,探讨芙蓉山花岗斑岩的成因类型、源区特征及其与镁铁质包体之间的关系,并进一步限定其构造背景。锆石U-Pb定年结果显示芙蓉山花岗斑岩加权平均年龄为133.6±1.6 Ma (MSWD = 2.1,2σ),镁铁质包体加权平均年龄为130.3±4.2 Ma (MSWD = 5.8,2σ),其成岩年龄基本一致,形成于早白垩世。全岩地球化学表明芙蓉山花岗斑岩属于亚碱性系列,同时具有低的Ga/Al(<2.6),高的Na2O含量(2.75%-4.5%,平均3.78%),较高的全碱含量(Na2O + K2O = 7.93%-8.75%),以及较低的锆石Ti饱和温度特征（631-690℃）,这些特征显示芙蓉山花岗斑岩为I型花岗岩,而非A型花岗岩。芙蓉山花岗斑岩中锆石Hf同位素有比较大的范围[εHf(t)=-10.9~-1.1],全岩[n(87Sr)/ n(86Sr)]i = 0.7062-0.7078,相对富集的εNd(t)=-5.6 ~ -4.7值;而包体表现为均一的[n(87Sr)/ n(86Sr)]i=0.7071-0.7079和弱富集的εNd(t)=-3.8 ~-2.8特征。野外及岩相学和元素地球化学特征显示典型的岩浆混合特征。镁铁质包体源区可能来自于俯冲交代地幔,芙蓉山花岗斑岩则形成于古老富钾地壳熔体和交代地幔熔体混合后的结晶分异。混合模型计算表明混合比例为：~80%的地幔端元和~ 20%地壳端元。浙江中部芙蓉山富钾I型花岗斑岩与镁铁质包体共生可能指示其形成于古太平洋板块俯冲后撤初始弱伸展拉张的构造背景。     

新疆东天山黄山东岩体岩石地球化学特征与岩石成因

黄山东岩体位于东天山北部的土墩-黄山-图拉尔根镁铁-超镁铁质岩带中段,受康古尔塔格-黄山深大断裂控制,是由辉长闪长岩、角闪辉长岩、橄榄辉长岩、辉长苏长岩及二辉橄榄岩组成的复式岩体。主量元素化学组成部分属拉斑玄武岩系列,也有部分属钙碱性系列。大部分岩石相对富集LREE、适度亏损高场强元素(Nb、Ta、Ti)。元素地球化学和Nd、Sr、Pb同位素体系证明,岩浆遭受了5%～8%的中-上地壳物质的同化混染作用。岩浆源区由被消减板片交代的地幔楔物质和软流圈地幔物质组成。岩石圈根部拆沉和软流圈地幔上涌过程中生成了黄山东岩体。富铁硅酸盐的分离结晶作用对硫化物富集并分凝起到了关键性作用,成矿过程应该是岩浆自身演化过程的有机组成部分。     

云南牟定二台坡岩体中钪的发现及其意义

二台坡岩体赋存低品位小型铁矿床,然其伴生稀贵金属元素钪已达特大型矿床工业指标。钪主要赋存于磁铁辉石岩相及含杆辉岩相中,载体矿物是钛磁铁矿、钛普通辉石及钛铁矿。钪是岩浆早期结晶分异、熔离的产物。元谋牟定地区是找矿远景区。     

The magmatic plumbing system beneath El Hierro (Canary Islands): constraints from phenocrysts and naturally quenched basaltic glasses in submarine rocks

A thermobarometric and petrologic study of basanites erupted from young volcanic cones along the submarine portions of the three El Hierro rift zones (NE-Rift, NW-Rift and S-Ridge) has been performed to reconstruct magma plumbing and storage beneath the island. Mineral-melt thermobarometry applied to naturally quenched glass and clinopyroxene rims yields pressures ranging from 350 to 1070 MPa with about 80% of the calculated pressures being in the range of 600–800 MPa. This corresponds to a depth range of 19–26 km, implying that the main level of final crystal fractionation is within the uppermost mantle. No systematic dependence between sample locality and fractionation pressures could be observed. Olivine and clinopyroxene crystals in the rocks are complexly zoned and have, on an inter-sample as well as on an intra-sample scale, highly variable core and rim compositions. This can best be explained by mixing of multiply saturated (olivine, magnetite, clinopyroxene, ilmenite), moderately evolved magmas with more mafic magmas being either only saturated with olivine + spinel or with olivine + spinel + clinopyroxene. The inter-sample differences indicate derivation from small, isolated magma chambers which have undergone distinct fractionation and mixing histories. This is in contrast to oceanic intraplate volcanoes situated on plumes with high melt supply rates, e.g. Kilauea Volcano (Hawaii), where magma is mainly transported through a central conduit system and stored in a shallow magma chamber prior to injection into the rift zones. The plumbing system beneath El Hierro rather resembles the magma storage systems beneath, e.g. Madeira or La Palma, indicating that small, intermittent magma chambers might be a common feature of oceanic islands fed by plumes with relatively low fluxes, which results in only limited and periodic magma supply.     

Mineralogy, whole-rock and Sr-Nd isotope geochemistry of mafic microgranular enclaves in Cretaceous Dagbasi granitoids, Eastern Pontides, NE Turkey: Evidence of magma mixing, mingling and chemical equilibration

Rocks of the Late Cretaceous Dagbasi Pluton (88-83 Ma), located in the eastern Pontides, include mafic microgranular enclaves (MMEs) ranging from a few centimetres to metres in size, and from ellipsoidal to ovoid in shape. The MMEs are composed of gabbroic diorite, diorite and tonalite, whereas the felsic host rocks comprise mainly tonalite, granodiorite and monzogranite based on both mineralogical and chemical compositions. MMEs are characterized by a fine-grained, equigranular and hypidiomorphic texture. The common texture of felsic host rocks is equigranular and also reveals some special types of microscopic textures, e.g., oscillatory-zoned plagioclase, poikilitic K-feldspar, small lath-shaped plagioclase in large plagioclase, blade-shaped biotite, acicular apatite, spike zones in plagioclase and spongy-cellular plagioclase textures and rounded plagioclase megacrysts in MMEs. Compositions of plagioclases (An₃₃-An₆₀), hornblendes (Mg#=0.77-1.0) and biotites (Mg#=0.61-0.63) of MMEs are slightly distinct or similar to those of host rocks (An_12-57; hbl Mg#=0.63-1.0; Bi Mg#=0.50-0.69), which suggest partial to complete equilibration during mafic-felsic magma interactions.The felsic host rocks have SiO₂ between 60 and 76 wt% and display low to slightly medium-K tholeiitic to calc-alkaline and peraluminous to slightly metaluminous characteristics. Chondrite-normalized rare-earth element (REE) patterns are fractionated (La_cn/Lu_cn=1.5-7.3) with pronounced negative Eu anomalies (Eu/Eu*=0.46-1.1). Initial ε_Nd(i) values vary between −3.1 and 1.6, initial ⁸⁷Sr/⁸⁶Sr values between 0.7056 and 0.7067.Compared with the host rocks, the MMEs are characterized by relatively high Mg-number of 22-52, low contents of SiO₂ (53-63 wt%), low ASI (0.7-1.1) and low to medium-K tholeiitic to calc-alkaline, metaluminous to peraluminous composition. Chondrite-normalized REE patterns are relatively flat [(La/Yb)_cn=1.4-3.9; (Tb/Yb)_cn=0.9-1.5] and show small negative Eu anomalies (Eu/Eu*=0.63-1.01). Isotope signatures of these rocks (⁸⁷Sr/⁸⁶Sr_(i)=0.7054-0.7055; ε_Nd(i)=-1.0 to 1.9) are largely similar to the host rocks. Gabbroic diorite enclaves have relatively low contents of SiO₂, ASI; high Mg#, CaO, Al₂O₃, TiO₂, P₂O₅, Sr and Nb concentrations compared to dioritic and tonalitic enclaves.The geochemical and isotopic similarities between the MMEs and their host rocks indicate that the enclaves are of mixed origin and are most probably formed by the interaction between the lower crust- and mantle-derived magmas. All the geochemical data suggest that a basic magma derived from an enriched subcontinental lithospheric mantle, interacted with a crustal melt that originated from dehydration melting of the mafic lower crust at deep crustal levels. The existence of compositional and textural disequilibrium and the nature of chemical and isotopic variation in these rock types indicate that magma mixing/mingling between an evolved mafic and a granitic magma was involved in their genesis. Microgranular enclaves are thus interpreted to be globules of a more mafic magma probably from an enriched lithospheric mantle source. Al-in-amphibole estimates the pluton emplacement at ca. 0.3-3.8 kbar, and therefore, magma mixing and mingling must have occurred at 3.8 kbar or below this level.     

SHIFTS IN THE WHOLESALE TRADE STATUS OF U.S. METROPOLITAN AREAS*

Considerable instability exists among large U.S. metropolitan areas in their status as wholesale trade centers. Differences in wholesale trade growth rates, changes in national shares, and shifts in rank position reflect not only population growth patterns, but also structural changes in the national economy. In the latter instance, the wholesale product mix of SMSAs is an important factor in explaining their differential fortunes within the wholesaling hierarchy.     

Magma-induced strain localization in centrifuge models of transfer zones

Scaled centrifuge experiments have been used to investigate the dynamic relations between deformation and magma distribution in rift-related transfer zones. The physical models were built using suitable analogue materials, such as sand to represent the brittle upper crust, various kinds of silicone mixtures to simulate the lower crust and upper mantle and glycerol to reproduce magma. Models simulated the development of transfer zones across pre-existing glycerol reservoirs placed at the base of the analogue continental crust. In plan view, different geometries, dimensions and positions of subcrustal reservoirs were reproduced in three different sets of experiments; to compare results, models were also performed without magma-simulating glycerol.Set 1 experiments, incorporating a narrow rectangular glycerol reservoir, show that the low-viscosity material is able to localise deformation into the overlying crust, giving rise to discrete transfer zones. This concentrated surface deformation corresponds at depth to major magma accumulation. Set 2 experiments, with an initial wide squared glycerol reservoir, show instead that deformation is distributed across the whole model surface, corresponding at depth to relatively minor magma accumulation. Set 3 experiments explored various positions of a small squared reservoir that invariably localised faulting in the overlying analogue brittle crust at the onset of model deformation.The overall model behaviour suggests that magma distribution at depth can effectively control the strain distribution in the overlying crust and the deformative pattern of transfer zones. Strain distribution, in turn, may control magma emplacement as localized deformation would favour major accumulation of magma at transfer zones. Coupled to a strong thermal weakening of the country rocks, this process may ultimately lead to a positive feedback interaction between magma and deformation.     

汉诺坝榴辉岩相石榴辉石岩——岩浆底侵作用新证据

汉诺坝榴辉岩相石榴辉石岩捕虏体的产状、层状堆晶构造和粒状变晶结构，以及具有幔源成因的矿物学、REE地球化学特征和温压条件表明，其深部过程是由幔源岩浆底侵于上地幔顶部，形成镁铁质堆晶岩，再经榴辉岩相变质而成，它与下地壳底部的麻粒岩相镁铁质堆晶岩，提供了岩浆底侵作用和壳－幔过渡带存在的新证据。     

Crystal size distributions and other quantitative textural measurements in lavas and tuff from Egmont volcano (Mt. Taranaki), New Zealand

 The size, shape and orientation of plagioclase crystals have been quantified in a tuff and series of andesite lavas from the active Egmont volcano (Mt. Taranaki), New Zealand. Linear crystal size distributions (CSDs) show that if the magma had several components, then only one provided the crystals. The slope of the CSD indicates that the earliest lavas measured had a residence time of ∼50 years in the magma chamber for a growth rate of 10^–11 cm/s. Subsequent lavas had slightly longer residence times (50–75 years), but the following series returned to 50-year residence times. The youngest magmas, from both Egmont summit and the parasitic Fantham's Peak, have the shortest residence times of ∼30 years. Variations in residence time may reflect changes in the magma chamber shape or depth, or the temperature of the surrounding rocks. Crystal shapes and zonation suggest that crystallization occurred in a bottle-shape magma chamber, and not in a narrow conduit. If future eruptions use the same magma chamber as the most recent eruptions, then a delay of approximately 30 years can be expected between refilling and eruption. Received: 25 October 1995 / Accepted: 19 April 1996     

全球主要地震区中源和深源大地震的18．6年轮回

分析全球中源和深源大地震与月球交点运动的关系，确定了１５个地区的中源和深源大地震有明显的１８．６ａ地震轮回。在这１５个地区中，除兴都库什地区外，其余地区的边界与俯冲带的分段性有密切关系。这一发现进一步证实了岩浆潮致上涌可调制地震活动的岩浆潮假设，并表明岩浆潮致上涌的影响范围在深部和浅部明显不同，中源和深源地震的发生很可能和岩浆上涌有关。