Pan African layered diorite-tonalite-granodiorite from Sinai massif: an open system fractionation at a subduction zone

This paper reports on the occurrence of layered Pan African dioritetonalite-granodiorite (DTG) rocks. The layering is marked by alternation of melanocratic (M) layers (diorites and tonalites) and leucocratic (L) layers (tonalites and granodiorites). M-samples have cumulus biotite+hornblende+relict pyroxene+plagioclase+K-feldspars+magnetite+apatite, and have transitional calc-alkaline and metaluminous affinities. They were derived from subduction-related magma enriched in Rb, Ba, K and LREE and depleted in Sr and Nb. L-samples have cumulus plagioclase+hornblende. They are enriched in Sr and depleted in Rb, Ba, K, Nb and LREE. They have calc-alkaline and peraluminous affinites.
The formation of the rhythmic layers of DTG composition can be attributed to periodical replenishment of pulses of basic magma into a more evolved acidic magma chamber under open system conditions. Field relations, mineralogy and element concentration among the M- and L-layers indicate that at the subduction zone, the ascending magma was contaminated with lower crustal materials (marginal basin metasediments) which led to LILE-enrichment, Nb-depletion and transition from calc-alkaline to alkaline and from metaluminous to peraluminous affinities as well.     

滦平球状闪长岩岩相学特征和球状构造成因探讨

文章对滦平球状闪长岩作了详细的岩相学研究，并据此提出岩体中球状构造的形成机理。根据球核的不同，可将球分为三种类型，包括以同源岩浆组分为核的球、以异源捕虏体为核的球和没有明显壳层结构的球。球中壳层一方面表现为基本不含角闪石的浅色层和含有一定角闪石的暗色层交替发育的矿物组成韵律，另一方面表现为斑状半自形一它形斜长石梳状层与细粒半自形一自形斜长石层互层的矿物结构韵律。球中斑状斜长石显示清晰的格子双晶，而主岩和基质中的斜长石则为聚片双晶。研究表明，所描述的球状构造是通过岩浆同化捕虏体和岩浆结晶综合作用形成的。球壳的矿物组成韵律和矿物结构韵律受岩浆中水含量变化的控制。     

Small Scale Primary Cumulus Igneous Layering in the Kalka Layered Intrusion, Giles Complex, Central Australia

Small scale primary cumulus layers occur in various parts ofthe Kalka Intrusion in central Australia, and generalized modelshave been developed from them to explain the formation of primarysmall scale layering features in stratiform gabbroic intrusions.Primary small scale layering appears to be produced by repeatedbursts of discontinuous nucleation, followed by differentialgravitational settling of pyroxene or olivine relative to plagioclase.Dependent on the distance of the nucleation zone above the pileof settled cumulus crystals, the layering may be mineral gradedor isomodal as a result of either incomplete or complete separationof cumulus pyroxene or olivine and plagioclase on settling.Reverse or continuous mineral grading may result from settlingoverlap of plagioclase by pyroxene or olivine formed in laternucleation cycles. Conditions of continuous nucleation willgenerally result in massive unlayered sequences, irrespectiveof any differential settling which may occur between phases.     

Metamorphic differentiation at Einasleigh,Northern Queensland

Biotite‐muscovite‐garnet gneisses at Einasleigh contain quartz‐feldspar veins composed of the same minerals as found in the enclosing rock. The vein‐gneiss boundaries are commonly irregular and on a microscopic scale, gradational.

Certain amphibolite layers contain quartz‐feldspar veins composed of the same minerals as found in the amphibolite. Hornblende‐rich extraction zones surround these veins, and material balance calculations show that all or nearly all of the vein‐forming matter was locally derived. Variation in the abundance of hornblende and plagioclase in the amphibolite as a function of distance from a quartz‐feldspar vein can be expressed by error‐function curves, thus suggesting that the mineral‐segregation process was diffusion‐controlled. During the mineral rearrangement, the Na and Ca contents of plagioclase have evidently remained unchanged, but the vein hornblende has become slightly richer in Fe+3, Mg, and Ca, and poorer in Si and Al relative to hornblende in the adjacent amphibolite.

A certain biotite‐plagioclase rock forms layers and boudins in the gneisses and contains pegmatite veins composed of the same minerals as found in the host rock. The plagioclase in these veins is more sodic than that in the host rock while the biotite contains slightly more Ti and Fe+² and less Si and Mg than the biotite of the enclosing rock.

The data indicate that significant portions of the vein‐forming matter at Einasleigh were locally derived. The chemistry of some minerals has changed slightly during the segregation process, resulting possibly from different diffusion rates for the different mineral‐forming constituents.     

Modally-Graded Rhythmic Layering in the Skaergaard Intrusion

The origin of igneous layering in the Skaergaard and other intrusionsis the subject of much debate. Currently held theories for itsorigin include gravitational sorting, rhythmic nucleation, andchemical diffusion models. This paper presents data on small-scale,modally-graded, rhythmic layers, one of several distinct typesof layering found in the Skaergaard. These layers occur onlyin the Layered Series of the intrusion, in which they are awidespread, almost ubiquitous feature. They are characterizedby strong variations in modal composition, with magnetite, ilmenite,and olivine concentrated in the lower part of the layers, pyroxenein the middle of the layers, and plagioclase in the upper partof the layers. In addition, the layers are size graded, withthe largest grain sizes of the different minerals generallyoccurring where the mineral is most abundant. The chemical compositionof both plagioclase and Ca-rich pyroxene do not vary in anyregular manner within the layers studied. The forsterite compositionof olivine is consistently higher in the lower part of the layers.This trend, however, correlates very strongly with the modalvariations of olivine and can be explained by minor amountsof subsolidus reequilibration between cumulus and adcumulusolivine, suggesting that the original composition of cumulusolivine within individual layers was constant. These results are consistent with layers formed by some gravitationalsorting process and, in several cases, are at odds with othertheories that have been proposed for the origin of igneous layers.Based on these data and the widespread evidence for currentinteraction (crossbeds, trough-shaped layers, and internallygraded layers), we propose that these layers result from periodicdisruption of a static zone of crystal growth on the floor ofthe magma chamber by convection or density currents. This mechanism,however, is almost certainly not responsible for the formationof all of the types of layering found in the intrusion.     

Metamorphic evolution of oceanic gabbros: recrystallization from subsolidus to hydrothermal conditions in the MARK area (ODP Leg 153)

The 117.38 m of gabbroic core drilled during the Ocean Drilling Program (ODP) Leg 153 at Sites 921 to 924 in the Mid-Atlantic Ridge (MAR) between 23 °N and the Kane Fracture Zone, exhibits a remarkable primary compositional heterogeneity, such as magmatic layering, intrusive contacts and late magmatic veining, which express a succession of magmatic events. Textural indicators suggest that the cooling of the crystal mush occurred in a dynamic environment, with infiltration of progressively evolved liquids. Magmatic features include random shape fabric and magmatic lamination; the subsequent deformational overprint occurred in subsolidus conditions. The ductile deformation, generally concentrated in discrete domains of the gabbro, is associated with continuous re-equilibration of the metamorphic assemblages of (1) olivine + clinopyroxene + orthopyroxene + plagioclase + ilmenite + Ti-magnetite, (2) olivine + clinopyroxene + plagioclase + ilmenite + Ti-magnetite + red hornblende. At lower temperatures brittle deformation prevails and subsequent fractures control the development of metamorphic assemblages: (3) clinopyroxene + plagioclase + red brown hornblende + Ti-magnetite + magnetite (?) + ilmenite, (4) plagioclase + brown hornblende + Ti-magnetite + magnetite + hematite + titanite ± Ti-oxide, (5) plagioclase + green hornblende + magnetite + titanite, (6) plagioclase + actinolite + chlorite + titanite + magnetite, (7) albite + actinolite + chlorite + prehnite ± epidote ± titanite and (8) albite + prehnite + chlorite ± smectite. Assemblages 1 to 8 express increasing water/rock ratios and decreasing degrees of recrystallization.

During the ductile phase, red hornblende is stable and its abundance increases with deformation intensity, possibly as an effect of the introduction of hydrous fluids. During the brittle phase, water diffusion controls the development of the fracture-filling mineral assemblages and re-equilibration of the adjacent rock; temperatures decrease further, as demonstrated by mineral zoning and incompletely re-equilibrated assemblages. The lowest temperatures correspond to the development of hydrothermal assemblages.

Compared with oceanic gabbros from fast-spreading transform environments, high-temperature ductile phases (granulite and amphibolite) are well developed, whereas brittle phases are widespread, as microcracks, prevalent on fracturing associated with discrete veins.     

Development of an augen gneiss terrain

Augen gneisses and related migmatites commonly form extensive areas in deep crystalline terrains and are interlayered with horizons of undoubted metasedimentary rocks. Augen gneiss, migmatite, and hornblende gneiss near McMurdo Sound, Antarctica, is interlayered with calcareous metasedimentary rocks on all scales and passes gradationally into quartzo-feldspathic gneiss. Numerous discontinuous lenses and layers of metasedimentary rocks in augen gneiss have been disrupted by transposition of layering and form tectonic inclusions. Calc-silicate layers have reacted with the surrounding rocks and become partially amphibolitized. Field evidence suggests that augen gneiss formed from quartzo-feldspathic gneiss by local metasomatism that accompanied strong deformation and smoothed out original metasedimentary differences. Locally partial melting took place. The final result is a sequence of augen gneiss, hornblende gneiss, and migmatite with thin amphibolite lenses and layers. The metasedimentary rocks are correlated with lower grade limestones and calcareous arenites including graywackes 125 km to the south. The quartzo-feldspathic gneiss would thus represent highly metamorphosed graywacke and sandstone. Augen gneiss and quartzo-feldspathic gneiss have similar chemical compositions, but augen gneiss seems to be somewhat more mafic. The composition of augen gneiss is more mafic than average continental shield surface rock and less mafic than the average crustal rock. Evidence indicates that augen gneiss and related rocks formed by homogenization of rocks that were originally sedimentary and its composition may reflect the bulk composition of material supplied to the crust earlier in the earth's history.     

Development of igneous layering during growth of pluton: The Tarçouate Laccolith (Morocco)

We discuss the significance of igneous layering with respect to pluton growth processes. The case study is the Tarçouate Laccolith (Morocco), whose core consists of modally layered hornblende granodiorites with high amount of monzodioritic enclaves, contrasting with peripheral, non-layered biotite granodiorites with low amount of enclaves. Rhythmic layering, with modal grading, cross-stratification and trough layering is associated with monzodioritic layers and wraps around mafic enclaves. Its steep dips ≥ 45° result from tilting that occurred above solidus conditions, as indicated by sub-vertical and synmagmatic granite, aplite and monzodiorite dykes cutting across the layering.The systematic association of igneous layering with mafic enclaves in calc-alkaline plutons suggests that layering originates from recurrent injection of mafic magma. Viscosity calculations suggest that the physicochemical properties of magma alone cannot account for the presence of layering in the central hornblende granodiorite and its coeval absence in the peripheral biotite granodiorite of the Tarçouate Laccolith. Intermittent pulses of hot mafic magma into crystallizing granodiorite likely produced thermal perturbations able to trigger local convection, formation of mafic enclaves and development of igneous layering through protracted crystallization.     

Microstructural tectonometamorphic processes and the development of gneissic layering: a mechanism for metamorphic segregation

The Mary granite, in the East Athabasca mylonite triangle, northern Saskatchewan, provides an example and a model for the development of non-migmatitic gneissic texture. Gneissic compositional layering developed through the simultaneous evolution of three microdomains corresponding to original plagioclase, orthopyroxene and matrix in the igneous rocks. Plagioclase phenocrysts were progressively deformed and recrystallized, first into core and mantle structures, and ultimately into plagioclase-rich layers or ribbons. Garnet preferentially developed in the outer portions of recrystallized mantles, and, with further deformation, produced garnet-rich sub-layers within the plagioclase-rich gneissic domains. Orthopyroxene was replaced by clinopyroxene and garnet (and hornblende if sufficient water was present), which were, in turn, drawn into layers with new garnet growth along the boundaries. The igneous matrix evolved through a number of transient fabric stages involving S-C fabrics, S-C-C' fabrics, and ultramylonitic domains. In addition, quartz veins were emplaced and subsequently deformed into quartz-rich gneissic layers. Moderate to highly strained samples display extreme mineralogical (compositional) segregation, yet most domains can be directly related to the original igneous precursors. The Mary granite was emplaced at approximately 900 °C and 1.0 GPa and was metamorphosed at approximately 750 °C and 1.0 GPa. The igneous rocks crystallized in the medium-pressure granulite field (Opx–Pl) but were metamorphosed on cooling into the high-pressure (Grt–Cpx–Pl) granulite field. The compositional segregation resulted from a dynamic, mutually reinforcing interaction between deformation, metamorphic and igneous processes in the deep crust. The production of gneissic texture by processes such as these may be the inevitable result of isobaric cooling of igneous rocks within a tectonically active deep crust.     

Amphibolitization of metagabbros in the Scottish Highlands

Abstract Compositions of actinolite, hornblende and cummingtonite, together with pyroxene and plagioclase, are studied in basic intrusions in the Dalradian of north-east Scotland, and the Glen Scaddle complex in the West Moine. Amphibolitization is due to influx of water from the country rocks. Pyroxene compositions are found to have adjusted to the regional metamorphic environment. Owing to the difficulty of diffusion of Al and Si, calcic amphiboles are zoned and commonly contain quartz blebs. Discontinuities in zoning give rise to actinolite-hornblende pairs. Compared with north-east Scotland, disequilibrium is less strong in the Glen Scaddle area: in the latter, plagioclase compositions have been greatly changed, Na partition between hornblende and plagioclase is close to equilibrium, the maximum Al content of hornblende is lower and zoning patterns are more consistent. The Fe/Mg ratio in calcic amphiboles varies with Al content, while approaching equilibrium partition with other minerals. Both zoning patterns and Fe/Mg partition with cummingtonite suggest that Fe/Mg of the calcic amphiboles increases more strongly with increasing (Al^vi+Fe³⁺) than can be explained simply by substitution of Al,Fe³⁺ for Mg on M2. Model reactions for amphibole formation are constructed. Cummingtonite formed at lower chemical potential of CaO than actinolite: Ca was exchanged for Mg,Fe between orthopyroxene-derived and clinopyroxene-derived local systems. Both cummingtonite and actinolite were formed because of kinetic constraints, as intermediate reaction products: actinolite-hornblende pairs represent disequilibrium. This work suggests that many occurrences of actinolite with hornblende, where the minerals are zoned, may also be due to diffusion kinetics.     

Middle Miocene thermal metamorphism due to the infiltration of high-temperature fluid in the Sanbagawa metamorphic belt,southwest Japan

 The Middle Miocene Tobe hornfels in the Sanbagawa metamorphic belt, western Shikoku, southwest Japan, is characterized by an abnormally steep metamorphic gradient compared with other hornfelses associated with intrusive bodies. The basic hornfels, originally Sanbagawa greenschist rocks, is divided into the following three metamorphic zones: plagioclase, hornblende, and orthopyroxene. The plagioclase zone is defined by the appearance of calcic plagioclase, the hornblende zone by the assemblage of hornblende+calcic plagioclase+quartz, and the orthopyroxene zone is characterized by the assemblage of orthopyroxene + clinopyroxene + plagioclase + quartz. Calcic amphibole compositions change from actinolite to hornblende as a result of the continuous reactions during prograde metamorphism. Petrographical and thermometric studies indicate a metamorphic temperature range of 300–475°C for the plagioclase zone, 475–680°C for the hornblende zone, and 680–730°C for the orthopyroxene zone. The temperature gradient based on petrological studies is approximately 5°C/m, which is unusually high. Geological and petrological studies demonstrate that the hornfelses were formed by the focusing of high-temperature fluids through zones of relatively high fracture permeability. The steep thermal gradient in the Tobe hornfels body is consistent with a large fluid flux, greater than 8.3 × 10^–7 m³ m^–2S^–1, over the relatively short duration of metamorphism, approximately 100 years. Received: 10 October 1995 / Accepted: 28 May 1996     

Stratigraphy and Petrology of the Mulcahy Lake Layered Gabbro: An Archean Intrusion in the Wabigoon Subprovince, Ontario

The Mulcahy Lake gabbro is an Archean layered intrusion of tholeiiticbulk composition located in the Wabigoon subprovince. The intrusionis 6 km thick at the thickest and is exposed over an area of63 km². It intrudes basaltic to siliceous volcanics of the CrowLake-Savant Lake greenstone belt and is intruded by the Atikwabatholith. Zircon U-Pb data indicate crystallization at 27322+1·0/–0·9m·y. Principal phases are plagioclase, orthopyroxene, augite (andpigeonite in iron-rich rocks), olivine, hornblende and magnetite.Olivine is confined to several horizons. Apatite and then zirconare prominent accessory phases at advanced stages of fractionation.Plagioclase, pyroxenes and olivine are cumulate phases. Hornblendeis invariably an intercumulus phase. Magnetite is ubiquitousthroughout the intrusion, generally as a cumulate phase, andforms centimeter thick layers in fractionated rocks. Fractionationfollowed a tholeiitic trend with iron enrichment in the liquid. The intrusion is divided into lower, mixed, middle, upper andmarginal zones. The lower and middle zones are 2·0 and2·5 km thick respectively. The upper zone is approximately1 km thick, and the marginal zone is measured in hundreds ofmeters. A 200 m thick mixed zone is interposed between the lowerand middle zones. The base of the lower zone consists of ultramaficunits containing olivine of Fo₈₂. The top of the zone has olivineof Fo₂₈. Fractionation of the lower zone, from the floor up,was interrupted by the introduction of pristine liquid whichmixed with more dense and cooler residual liquid in the chamberto form the mixed zone. Further introduction of several minorpulses of liquid constructed the lower part of the middle zone.The upper part of the middle zone was constructed from a majorpulse of liquid plus several minor pulses each of which is representedby reversals in cryptic layering. The upper zone consists ofultramafic to iron-rich gabbro cumulates formed by cooling throughthe roof plus horizons formed by influx of pristine liquid.Marginal zone rocks represent cooling through the walls of theintrusion. Rhythmic layering is well developed in lower and middle zonecumulates. Petrofabric data show that orthopyroxene has a lineationin the plane of layering and parallel to structures suggestiveof flow. Plagioclase laths also have a preferred orientationin many cumulates and in unlayered gabbros as well. Flow, possiblylaminar, of liquid-crystal material is suggested and may belinked to the ultimate development of layering. Pressure during the course of crystallization probably was greaterthan 2 and less than 5 kb. Temperatures estimated from pyroxenesvaried from approximately 1200 to 1000 °C.f_o², is not wellconstrained but was sufficient to allow the formation of thinlocal magnetite cumulates late in the crystallization. The primarymelt was hydrous as indicated by the presence of hornblende.It is very unlikely that the melt was saturated with water duringcrystallization of the cumulate phases.     

角闪斜长片麻岩流变力学特性研究

为了解小湾水电站枢纽区角闪斜长片麻岩的流变力学特性,采用岩石全自动三轴流变伺服仪对角闪斜长片麻岩进行三轴流变力学试验。试验结果表明,强度较高的角闪斜长片麻岩会发生流变现象,尤其在高应力水平条件下,其流变特性明显;外荷载超过岩样的长期强度时,随着时间的逐渐增加试样变形经历典型流变3个阶段,并最终加速流变破裂。依据岩样稳态流变阶段的流变速率与应力水平的关系,提出了一种确定长期强度的方法。在Burgers流变模型中加入统计损伤,并用Mohr-Coulomb准则确定的岩石长期强度作为开始进入非线性加速流变阶段的阀值,得到相应的非线性损伤流变模型。利用岩石全程三轴流变试验结果,采用优化后的算法,对非线性损伤流变模型相应的参数进行了辨识,结果表明,建立的岩石统计损伤非线性流变变模型与流变试验结果吻合较为理想,可以较为准确地反映角闪斜长片麻岩的流变力学特征     

Diffusion in coronas around clinopyroxene: modelling with local equilibrium and steady state,and a non-steady-state modification to account for zoned actinolite-hornblende

Retrograde coronas of Caledonian age, between clinopyroxene and plagioclase in the Jotun Nappe Complex, Norway, illustrate the effects of diffusion kinetics on mineral distributions among layers and on the compositions of hornblende-actinolite. One corona type comprises a symplectite of epidote + quartz adjacent to plagioclase, and a less well-organized intergrowth of amphibole + quartz replacing clinopyroxene. The observed mineral proportions imply an open-system reaction, but the similarity of Al/Si ratios in reactant plagioclase and product symplectite indicates approximate conservation of Al₂O₃ and SiO₂. The largest inferred open-system flux is a loss of CaO, mostly derived from consumption of clinopyroxene. The approximate layer structure, Pl|Ep + Qtz|Hbl + Qtz|Act±Hbl + Qtz|Cpx, is modelled using the theory of steady-state diffusion-controlled growth with local equilibrium. To obtain a solution, it is necessary to use a reactant plagioclase composition which takes into account aluminous (epidote) inclusions. The results indicate that, in terms of Onsager diffusion coefficients L _ii , Ca is more mobile than AL (L _CaCa/L _AlAl3.) (where means greater than or approximately equal to). This behaviour of Ca is comparable with that of Mg in previously studied coronas around olivine. Si is non-diffusing in the present modelling, because of silica saturation. Oxidation of some Fe²⁺ to Fe³⁺ occurs within the corona. Mg diffuses towards its source (clinopyroxene) to maintain local equilibrium. Other coronas consist of two layers, hornblende adjacent to plagioclase and zoned amphibole + quartz adjacent to clinopyroxene. In the zoned layer, actinolitic hornblende forms relict patches, separated from quartz blebs by more aluminous hornblende. A preliminary steady-state, local-equilibrium model of grain-boundary diffusion explains the formation of low-Al and high-Al layers as due to Al immobility. Zoning and replacement are qualitatively explained in terms of evolution of actinolite to more stable aluminous compositions. This is modelled by a non-steady-state modification of the theory, retaining local equilibrium in grain boundaries while relatively steep zoning profiles develop in grain interiors through slow intracrystalline diffusion. Replacement of actinolite by hornblende does not require a change in P–T conditions if actinolite is a kinetically determined, non-equilibrium product. The common preservation of a sharp contact between hornblende and actionolite layers may be explained by ineffectiveness of intracrystalline diffusion: according to the theory, given sufficient grain-boundary Al flux, a metastable actinolite + quartz layer in contact with hornblende may be diffusionally stable and may continue to grow in a steady state.     

塔里木板块东北部坡一镁铁质-超镁铁质层状侵入体岩石成因

坡一侵入体位于塔里木板块东北部坡北岩体内,是该岩体第三阶段岩浆活动形成的十几个小侵入体中的一个,锆石U-Pb年龄为278±2Ma。该侵入体属于以超镁铁质岩石为主的层状岩系,堆晶结构与韵律性堆晶层理非常发育。岩浆分异充分,形成了从纯橄岩到石英闪长岩的多种岩石类型。在超镁铁质岩石中,所有的橄榄石和大部分斜方辉石是堆晶相,少量斜方辉石是填隙相,大部分单斜辉石、褐色普通角闪石和黑云母是填隙相。在镁铁质岩石中,橄榄石和斜方辉石全部是堆晶相,单斜辉石与斜长石既可以是堆晶相,也可以是填隙相;褐色普通角闪石、黑云母和石英均为填隙相。超镁铁质岩石属拉斑玄武岩系列,镁铁质岩石属钙碱性系列。侵入体中大量存在的捕掳体、微量元素地球化学、Nd-Sr同位素组成的EMⅡ型演化趋势,充分证明了同化混染作用伴随岩浆演化过程而逐渐增强,并不断促进了岩浆的分异,而且导致了岩石化学系列的转化。PGE和亲硫元素地球化学以及硫同位素组成证明,硫主要来自于岩浆,硫化物形成于岩浆阶段,岩浆未经历过早期硫化物熔离作用,硫化物熔离起始于橄榄岩相结晶的晚期阶段,并伴随着此后的岩浆演化过程而继续熔离。硫化物熔离是岩浆自身演化和同化混染共同作用的结果。橄榄石Fo分子含量和全岩FeO含量显示,原生岩浆是苦橄质岩浆;源区物质应该是石榴石辉石岩;岩浆生成于地幔柱轴部。在塔里木板块东北部还存在分别来自于软流圈和亏损型大陆岩石圈地幔的二叠纪岩浆岩,它们都应该是塔里木大火成岩省的组成部分。     

The origin of brown hornblende in the Artfjället gabbro and dolerites

Brown hornblende occurs in minor amounts in the Artfjället gabbro and dolerites, except in quartz-dolerites where a pale green hornblende occurs. In the gabbro, brown hornblende is mostly Ti-bearing pargasite or kaersutite. It occurs along veins of orthopyroxene, as rims around and blebs in pyroxenes, with orthopyroxene in coronas between olivine and plagioclase and in coronas between ilmenite and plagioclase. In the olivine-dolerites and orthopyroxene-dolerites brown hornblende is ferroan titanian pargasite or ferroan kaersutite. The pale green hornblende in the quartz-dolerites is a magnesio-hornblende. The hornblendes in the dolerites are interstitial or granular, in some dolerites occurring as coarse oikocrysts. It is proposed that under certain conditions the Ti content of hornblende can be used as a thermometer, derived from experimental data of Helz (1973). Microstructures, compositions and formation temperatures (< 1,040° C) show that the brown hornblende in the gabbro is not magmatic, but of subsolidus origin. Probably it formed as a result of the introduction of water into the gabbro during a deformation event that occurred early in the cooling history of the gabbro. Least-squares modelling of hornblende formation indicates that all magmatic minerals must have participated in the reaction and that the reaction probably was not isochemical. Microstructures, compositions and formation temperatures (1,030-965° C) of brown hornblende in the dolerites are consistent with late-stage crystallization from the magma. For the pale green hornblende in the quartz-dolerites a magmatic origin is likely, but cannot be proven.     

新疆西南天山小哈拉军山辉长岩体岩石学及地球化学研究

出露于西南天山特克斯县东南的小哈拉军山辉长岩体主要由角闪辉长岩和含磁铁矿辉长岩组成。角闪辉长岩主要由单斜辉石、角闪石和斜长石组成;含磁铁矿辉长岩主要由斜长石和磁铁矿组成。这两类岩石都含少量方解石。这些方解石均呈独立矿物出现,且与长石、单斜辉石或黑云母平衡共生。这些方解石是原生碳酸盐矿物（不是后期热液蚀变的产物）。两类岩石属于钙碱性系列,富集轻稀土元素,亏损重稀土元素,轻重稀土之间的分异比较明显,但不显示Eu异常。辉长岩相对富集大离子亲石元素而亏损高场强元素。岩石学和地球化学研究表明,小哈拉军山辉长岩是碳酸盐化地幔极低程度部分熔融的产物,斜长石堆晶过程对岩浆演化起重要作用。     

云南金平马鞍山钛磁铁矿

钛磁铁矿产于角闪辉石岩、斜长角闪岩、角闪岩等变基性-超基性杂岩体中,是在深部构造活动控制和后期变质作用下形成的晚期结晶分异型矿床。     

Mineral growth in melt conduits as a mechanism for igneous layering in shallow arc plutons: mineral chemistry of Fisher Lake orbicules and comb layers (Sierra Nevada,USA)

Different processes have been proposed to explain the variety of igneous layering in plutonic rocks. To constrain the mechanisms of emplacement and crystallization of ascending magma batches in shallow plutons, we have studied comb layers and orbicules from the Fisher Lake Pluton, Northern Sierra Nevada. Through a detailed study of the mineralogy and bulk chemistry of 70 individual layers, we show that comb layers and orbicule rims show no evidence of forming through a self-organizing, oscillatory crystallization process, but represent crystallization fronts resulting from in situ crystallization and extraction of evolved melt fractions during decompression-driven crystallization, forming a plagioclase-dominated cres-cumulate at the mm- to m-scale. We propose that the crystal content of the melt and the dynamics of the magmatic system control the mechanisms responsible for vertical igneous layering in shallow reservoirs. As comb layers crystallize on wall rocks, the higher thermal gradients will increase the diversity of comb layering, expressed by inefficient melt extraction, thereby forming amphibole comb layers and trapped apatite + quartz saturated evolved melt fractions. High-An plagioclase (An₉₀–An_97.5) is a widespread phase in Fisher lake comb layers and orbicule rims. We show that a combination of cooling rate, latent heat of crystallization and pressure variations may account for high-An plagioclase in shallow melt extraction zones.     

弓长岭鞍山群太古代基底岩石特征

弓长岭鞍山群呈北西至南东方向分布，位于中朝陆块东端，该群各类型岩石构成华北陆块古老的变质结晶基底。文中详细地记述了岩浆岩及混合岩、硅质板岩和长英质变粒岩、交超基性岩、交基性火山岩、变泥质岩、铁矿床、变辉长辉绿岩岩墙群的岩石特征和产出特征及成岩时代。本区为低压区域浅变质类型，演化与成岩时代约在太古代的中、晚期(3400～2500Ma)。