Fragmentation processes and xenolith transport in a Proterozoic minette dyke,Grenville Province,Québec

 The Mesoproterozoic Rivard minette dyke provides a case example of dyke-parallel fracturing induced by propagation of low-viscosity melts at deep crustal levels. The dyke constitutes a xenolith-choked intrusion breccia and provides samples of an extensive section of the lithosphere underlying the southwestern Grenville Province of Québec at 1.08 Ga. The xenoliths occupy approximately 50% of the volume of the breccia. They are extremely diverse and include pyroxenites and mafic granulites from the upper mantle and lower crust, and Grenvillian gabbroids, gneisses and quartzites. Wall-rock fragments are detached to various extents along dyke-parallel fractures and apophyses. The host lamprophyre magma has a calculated viscosity of ca. 40 Pa·s. Rapid ascent of magma and turbulence are suggested by the presence of large dense fragments and the brittle-to-ductile deformation at the margins of some xenoliths. A minimum ascent velocity of ca. 50 cm·s^–1 is estimated from the settling velocity of a 40 cm long pyroxenite xenolith. The progressive addition of xenoliths increased the effective viscosity of the liquid-solid mixture to 10³ Pa·s during emplacement. This favoured Bingham behaviour and laminar flow of the magma, and xenolith suspension. Field evidence suggests that xenoliths were formed through (1) early fracturing of wall rock by inelastic deformation during dyke propagation, and (2) continuous delamination of wall rock by intrusion of magma along the dyke-parallel fractures. This led to removal of chilled margins, and to fluid infiltration, partial melting and microbrecciation in the wall rock. Pre-existing discontinuities played a minor role in the fracturing process. Xenoliths may thus be abundant in alkaline magmas not only because these magmas ascend rapidly and can transport xenoliths, but possibly also, because their low viscosities promote intense fracturing of wall rock. Received: 10 June 1995 / Accepted: 10 February 1996     

Subsidence breccias in kimberlite pipes—an application of fractal analysis

A particular variety of volcanogenic country rock breccia is described; a contact breccia that has been identified at Venetia, River Ranch and Wimbledon kimberlite pipes. The contact breccia is clast supported with no juvenile kimberlite component, has tightly packed, angular fragments (with occasional rounding of smaller particles), and has a shear-fabric dipping towards the center of each kimberlite pipe or volcanic event. Clasts have preferred orientations parallel to the fabric. Photographs of the breccia in the open pit face and measured data from drill core are analyzed specifically to quantify the clast size distributions and clast shapes by means of fractal analysis. The fractal dimension is one means of characterizing the breccia because the dimension can be specific to a fragmentation mechanism. Clast size distribution fractal dimensions in the coarser particles (greater than circa 3 cm) range from greater than 3 for nonsheared breccia, down to circa 2.3 for the sheared breccia. Breccia characterization based on this fractal analysis suggests that fragmentation occurred initially from confined high-energy explosions, followed by collapse and abrasion by subsequently gravity-induced rockmass subsidence. All studied contact breccias produced a distinctive fractal signature in the finer particles (<3 cm) of circa 1.6 that can be explained by a comminution fragmentation process in that particular particle size range. It is suggested that these subsidence breccias require a substantial volume deficit at depth within the volcanic pipe in order to explain their origin and size. The methodology used in this study could be used to characterize any other volcanic breccia and further model their origins.     

山东栖霞大方山玄武岩中地幔熔体玻璃捕虏体岩相学及岩石化学研究

过去地学界对固体地幔岩捕虏体研究已经相当充分。另外,山东栖霞大方山还存在地幔熔体玻璃捕虏体,其化学成分、矿物成分既不同于固体地幔岩也不同于寄主玄武岩。此玻璃体特别富K,而且早于寄主岩浆,二者间很不平衡。富K玻璃捕虏体往往被玄武岩浆消化殆尽,但在野外仍然可以找到少量残留体供室内研究。此熔体玻璃的存在意味着该区玄武岩浆作用不是一元而是多元岩浆体系,还存在早一期的碱性地幔岩浆。后者消化混染于玄武岩浆之中,会产生富K玄武岩。     

Origin and U–Pb dating of zircon-bearing nepheline syenite xenoliths preserved in basaltic tephra (Massif Central, France)

Zircon-bearing xenoliths in continental basalts are often interpreted as witnesses of the continental basement uplifted during volcanic eruptions. Nevertheless, their origin is still debated. The Devès basaltic plateau belongs to the alkaline volcanic province of the French Massif Central. In few outcrops, zircon-bearing nepheline syenite xenoliths were preserved. U–Pb dating of the zircon crystals define an age of 956 ± 11 kyr constraining the crystallisation time of the zircons and consequently of the host xenoliths. This age, together with mineral chemistry arguments lead us to conclude that these minerals do not derive from a continental protolith. Rather, they likely result from the crystallisation of a liquid characterised by a nepheline–felspar composition and produced by the differentiation of a basaltic magma or, alternatively, by the low degree partial melting of a metasomatised lithospheric mantle. Such alkaline sialic rock and xenoliths may occur in large volumes at depth and generate the large amounts of zircon megacrysts discovered worldwide in secondary deposits within continental basaltic provinces.     

中国霞石正长岩的地质特征

介绍了中国霞石正长岩的分布，产出的大地构造环境，岩体的岩石组合，岩石的地球化学和造岩矿物组合特征，论述了主要造岩矿物的矿物学特征及产出母岩的关系，以及工业岩石对霞石正长岩的一些指标要求。     

Isotopic geochemical characterization of selected nepheline syenites and phonolites from the Poços de Caldas alkaline complex, Minas Gerais, Brazil

This paper presents and discusses the isotopic data from the hydrothermal studies of the Poços de Caldas Natural Analogue Project. The purpose of these studies was to elucidate the mass transport of relevant elements and isotopes associated with hydrothermal mineralization and alteration at the Osamu Utsumi uranium mine, as applicable to high-temperature radwaste isolation (particularly in the U.S. nuclear waste program). Research efforts were focused on studying the thermal, chemical and hydrologic nature of the palaeohydrothermal regime associated with a breccia pipe at the Osamu Utsumi mine, and related to the geochemical, geochronological and petrological characterization studies of unaltered regional nepheline syenite and phonolite.The regional rocks studies have a vertically elongated δD, δ¹⁸O pattern, which possibly indicates meteoric water/rock interaction. Regression of Rb---Sr whole-rock isotopic data for the regional nepheline syenite and phonolite samples did not produce isochrons. An internal, mineral-separate isochron regression from a nepheline syenite sample, considered representative of unaltered nepheline syenite of the Poços de Caldas plateau, yields an age of 78 Ma, and an initial ratio of approximately 0.7051. The initial ratios of the regional nepheline syenites are possibly indicative of a mantle source for the alkaline magmatism, with some incorporation of old, high Rb/Sr crustal material. The greater-than-mantle values of δ¹⁸O, if not due solely to surficial processes, also appear to require some assimilation of crustal material. Sm---Nd isotopic data for the regional rocks do not define any isochrons, although the nepheline syenite samples conform very well to a calculated reference isochron for 78 Ma and a fixed initial ¹⁴³Nd/¹⁴⁴Nd of 0.512359. The regional phonolite samples lie markedly off this isochron. This is probably due to the phonolite samples having different initial ¹⁴³Nd/¹⁴⁴Nd values. All regional samples lie within the “Mantle Array” trend. Their location within _Nd–_Sr space indicates as asthenospheric Mid Ocean Ridge Basalt (MORB)-type source magma also contaminated by continental igneous and metamorphic rocks (e.g. the Precambrian gneiss surrounding the Poços de Caldas plateau).The rocks studied at the Osamu Utsumi mine from the F4 drillcore have experienced varying degrees of hydrothermal mineralization and metasomatism, and deep weathering. The hydrothermally altered rocks have a quite pronounced δD shift, with only a slight δ¹⁸O shift. The δD-δ¹⁸O trend of the hydrothermally altered F4 samples most likely reflects the variability of temperature, hydrologic flow, mineralogical alteration and, therefore, water/rock interaction and isotopic exchange in the palaeohydrothermal regime.Regression of Rb---Sr whole-rock isotopic data for subsamples from a nepheline syenite xenolith sample yields an age of 76 Ma and an initial ratio of approximately 0.7053. Due to the marked hydrothermal alteration and metasomatism of this sample, the Rb---Sr isotopic system is interpreted as being re-equilibrated and thus the regressed age is the age of the hydrothermal event. Using a versus 1/Sr mixing diagram, distinct trends are seen for hydrothermal alteration, mineralization and weathering. Again, the F4 nepheline syenite samples do not define an Sm---Nd isochron, but conform very well to a calculated model isochron for 78 Ma and an initial ¹⁴³Nd/¹⁴⁴Nd of 0.512365. The Sm---Nd isotopic data also exhibit a possible disturbance by the hydrothermal, metasomatic alteration. A lamproite dyke which crosscuts the hydrothermal alteration in the Osamu Utsumi mine gives an age of 76 Ma, which is essentially the same as the Rb---Sr age of the hydrothermally altered nepheline syenite subsamples.     

Petrology and petrogenesis of carbonatitic rocks in syenites from central Anatolia,Turkey

The late Cretaceous A-type Karaçay?r pluton in Central Anatolia, Turkey, intrudes and entrains xenoliths of Palaeozoic limestone. Carbonatitic magmatic rocks within the syenite have been previously interpreted (Schuiling in Nature, 192:1280, 1961) to result from metasomatic alteration and syntectic melting of marble. Carbonatites and associated calcite-syenites exhibit mineralogical characteristics (Ab-rich plagioclase, Ba-rich K-feldspar, low-Mg# biotite) that are petrogenetically more evolved than the host syenitic suite. Geochemically, carbonate-rich magmatic rocks are greatly enriched in Sr, Ba, Th, and REE and have higher LREE/HREE ratios than either syenites or marbles. In terms of O-C-Sr-Pb isotope ratios, the carbonatite/calcite-syenite suite form a consistent and geochemically coherent group that is distinct from the marble country rock and xenolith population, but similar to some of the syenitic, and particularly the nepheline syenite components of the Karaçay?r pluton. Other silicate magmatic rocks are geochemically, isotopically, and geochronologically different, suggesting the pluton is composite. Overall, the mineralogical and geochemical characteristics of the carbonatites are incompatible with binary mixing of syenite and marble but are consistent with derivation of carbonatite from petrogenetically evolved foid syenite. Carbonate–silicate rock types have modal variations compatible with an origin by fractional crystallisation, rather than by liquid immiscibility.     

Petrogenesis of carbonate-bearing nepheline syenites and carbonatites from Southern Victoria Land, Antarctica: origin of carbon and the effects of calcite-graphite equilibrium

Chemically-evolved carbonate-bearing nepheline syenites are intruded into basement metasediments of the Koettlitz Group on Dismal and Radian Ridges in the Pipecleaner Glacier region of Southern Victoria Land, Antarctica. Whole rock XRF data from the Dismal Nepheline Syenite defines a broad trend which is consistent with the removal of a cumulate fraction of approximate composition 70% hedenbergite, 15% nepheline, 10% titanite and 5% apatite. Stable isotope, major and trace element and mineralogical characteristics of the syenites are very similar to those of cross-cutting calcite-rich dykes indicating derivation from closely-related source magmas. The general association of carbonatites and nepheline syenites with extensional environments, suggests that the Dismal and Radian Ridge nepheline syenites and carbonatite dykes indicate a period of early Paleozoic (531 Ma) rifting or intrusion into localised tensional structures in an overall compressional regime. Assimilation of marble by the syenite magmas is evidenced by abundant rafts and xenoliths within the Dismal Nepheline Syenite, however, carbon and oxygen isotopic ratios from syenite and carbonatite calcites are distinctly lighter than values from the marble country rock indicating a magmatic source. Graphite and calcite commonly occur as aggregates in the Dismal Nepheline Syenite suggesting equilibrium between these two carbon-bearing phases. Isotopic fractionation between calcite and graphite, via the equilibrium: C + O₂ = CO₂ has resulted in enrichment of the ¹³C isotope in calcites from the Dismal Nepheline Syenite.     

Structure of the Ethiopian lithosphere: Xenolith evidence in the Main Ethiopian Rift

The lithospheric and sublithospheric processes associated with the transition from continental to oceanic magmatism during continental rifting are poorly understood, but may be investigated in the central Main Ethiopian Rift (MER) using Quaternary xenolith-bearing basalts. Explosive eruptions in the Debre Zeyit (Bishoftu) and Butajira regions, offset 20 km to the west of the contemporaneous main rift axis, host Al-augite, norite and lherzolite xenoliths, xenocrysts and megacrysts. Al-augite xenoliths and megacrysts derived from pressures up to 10 kb are the dominant inclusion in these recent basalts, which were generated as small degree partial melts of fertile peridotite between 15 and 25 kb. Neither the xenoliths nor the host basalts exhibit signs of carbonatitic or hydrous (amphibole + phlogopite) metasomatism, suggesting that infiltration of silicate melts resulting in pervasive Al-augite dyking/veining dominates the regional lithospheric mantle. Recent geophysical evidence has indicated that such veining/dyking is pervasive and segmented, supporting the connection of these Al-augite dykes/veins to the formation of a proto ridge axis. Al-augite xenoliths and megacrysts have been reported in other continental rift settings, suggesting that silicate melt metasomatism resulting in Al-augite dykes/veins is a fundamental processes attendant to continental rift development.     

Cambrian sedimentary dykes in the Proterozoic basement of the Västervik area (southeast Sweden): episodic formation inferred from macro- and microfabrics

Fabrics of Cambrian sedimentary dykes formed in Proterozoic granites of the Västervik area (Southeast Sweden) evidence repeated opening/filling and mineralisation/cementation events under varying conditions. Diagnostic features include (1) wall-parallel boundaries between epiclastic fillings and (2) early formed dyke sediments that appear as lithoclasts in subsequently formed sedimentary fillings. The psammitic components mostly consist of well-rounded quartz grains related to a coastal environment and fragments from the granitic host rock. Platy calcitic fragments embedded in the epiclastic matrix originally formed as microveins within already-lithified dyke sediments and the adjacent host rock. Convex downward-pointing, internal sagging structures, together with the preferred orientation of compositional boundaries and long axes of grains/rock fragments parallel to the dyke walls, are interpreted as the result of suction-controlled flow of unconsolidated fillings during episodes of downward dyke growth. Pressure solution of quartz grains are evidence of extensional phases with dyke propagation that were interrupted by phases of horizontal compression normal to the dyke walls. The N–S and NE–SW striking sedimentary dykes formed by opening of a pre-existing joint set during NW–SE oriented rifting during the Cambrian.     

Isotopic Geochronology of the Xianghongdian Alkaline Complex, Northern Margin of the Dabie Mountains, China

To the north of the Qinling-Dabie orogen there exists a Mesozoic (K1) granite-syenite belt called the Beihuaiyang granite-syenite belt, in which the Xianghongdian alkaline complex outcrops in the Devonian Foziling schist and also intruded into the slightly earlier volcanic rocks. This alkaline complex is composed of syenite, alkaline syenite and nepheline syenite, but the nepheline syenite occurs inside the alkaline syenite in the pod-like, irregular, lenticular or layered shape. Researches on the petrogenesis of the alkaline complex and the relationship between the alkaline syenite and the nepheline syenite must rely on precise isotopic ages. Previous researches have reported iso-topic dating results merely by the 40Ar/39Ar method on hornblende and biotite separated from the alkaline syenite and nepheline syenite. In view of this, the authors have made fairly detailed isotopic geochronological studies of the alkaline syenite and nepheline syenite by using Rb-Sr isochron for the whole rock and minerals,     

IMPACT BRECCIAS AND IMPACTITES OF THE KARA AND UST'-KARA ASTROBLEMES

Seventeen upper-mantle ultramafic xenoliths from the Lower Quaternary Tal Khodr Imtan cinder cone in southern Syria have revealed a dominant protogranular texture of nine spinel lherzolites, two spinel harzburgites, four spinel dunites, one spinel olivine websterite, and one spinel clinopyroxenite. The lherzolites, harzburgites, and dunites contain Cr-diopside and brown-red picotite, with a basanitic host rock; the websterite and clinopyroxenite contain Ti-Al-augite and Cr-hercynite. A lherzolite to dunite depletion trend is shown in the abundance of intermediate- and lightrare-earth elements (IREE and LREE) and from analytical data of dunitic olivine, with Ca, Al, Fe, Cr, and Si being the most depleted elements. The depletion probably resulted from successive partial melting. The scoriaceous basanite shows enrichments in REE and trace elements from a plume; the basanitic coating (around ultramafic xenoliths) increases in Mg/Mg+Fe⁺² and concentrations of Al₂O₃, TiO₂, and Na₂O by contamination from peridotitic olivine, and also from eclogite-gabbro and nephelinite near the bottom of the rifted crust.

Differences in the REE and trace-element concentrations among the peridotite xenoliths, the basanite host rock, and websterite indicate at least three different depths for their parent sources. The ultramafic inclusions in the basanitic host rock, as well as xenoliths in a carbonatite dike, suggest a deeper source for the carbonatite magma. At least part of the enrichment of the plume probably was accomplished by the subducted Tethys oceanic crust, suboceanic litho-sphere, and eclogite-gabbro. The thick plateau basalt in southern Syria indicates heavy and deep fracturing, and the extrusions of successive magmas from the upper mantle created a stretching and thinning in the continental crust. The proximity of this plateau basalt area to the Dead Sea-Jordan River Valley Rift, together with the source of the ultramafic xenoliths, points to a possible close relationship between the Red Sea Rift and the fracturing (offshoot rifting) in southern Syria.     

滇西六合正长斑岩和花岗岩包体中锆石U-Pb年代和微量元素地球化学研究

六合正长斑岩是滇西新生代富碱斑岩的典型代表,以发育各类深浅不一的岩石包体而闻名,其中包括了同期的花岗岩包体;主岩和包体所蕴含的深部地质过程对于富碱岩浆的形成和演化具有重要约束作用。本文以六合正长斑岩和其中花岗岩包体的锆石微量元素地球化学为研究重点,结合主岩主量元素、锆石U-Pb年龄、Ti温度、锆石氧逸度的对比研究,分析和探讨富碱岩浆的形成和演化过程。研究表明:锆石U-Pb年龄所限定的六合富碱岩浆活动时限为(37.89±0.96)~(35.87±0.58) Ma;而花岗岩包体代表的同期富硅酸性岩浆活动时限为(38.21±0.44)~(36.21±0.36) Ma;两者几乎同步活动。锆石微量元素成因判别图显示主岩锆石主要位于钾镁煌斑岩范围,暗示富碱岩浆源区具地幔属性;花岗岩包体锆石大多位于花岗岩范围,两者在花岗岩范围有部分重叠,证实了主岩锆石和花岗岩包体锆石结晶时存在共同的生长环境。Ce^(Ⅳ)/Ce^(Ⅲ)比值估算和Ti温度计算显示,在36.42~36.41 Ma范围内富碱岩浆氧逸度和锆石结晶温度发生突变,指示在该时限内富碱岩浆及所裹挟的花岗质岩浆发生了混合作用;氧逸度的振荡降低反映了后期幔源岩浆的补充。综合研究认为,六合地区来自富集地幔的喜山期富碱岩浆底侵熔化上覆深部地壳长英质岩石形成花岗质岩浆,随即以富碱岩浆为主与少量花岗质岩浆不均一混合并同步结晶成岩形成含有花岗岩包体的正长斑岩。     

Reactions between mantle xenoliths and host magma beneath La Palma (Canary Islands): constraints on magma ascent rates and crustal reservoirs

Spinel-bearing peridotitic mantle xenoliths from the 1949 eruption on La Palma were modified mineralogically and chemically during prolonged reaction with their host magma. The magmatism that brought the peridotites to the surface caused two distinct generations of xenolith fractures: (1) Old fractures are characterized by crystalline selvages with cumulus textures towards the host magma, or by polymineralic veins. They are accompanied by 0.9–2 mm wide diffusion zones where peridotite olivine became less forsteritic through diffusive exchange with the host magma. Old fractures represent most of each xenolith's surface. (2) Young fractures show no selvages and only narrow diffusion zones of <0.02 mm width. Calculations based on a model of Fe-Mg interdiffusion give an age of 6 to 83 years and <4 days for old and young fractures, respectively. A combination of these data with fluid inclusion barometry indicates that selvages and veins formed during xenolith transport rather than representing wall-rock reactions or mantle metasomatism. The results provide ample evidence for prolonged storage of the xenoliths in the crust, constraining a multi-stage magma ascent: Years to decades prior to eruption, ascending magma ruptured peridotitic wall-rock possibly through hydraulic fracturing and stoping around magma reservoirs. Magma batches transported the peridotite xenoliths to the crust at ascent rates exceeding 0.2 ms⁻¹. The xenoliths and their host magma stagnated during at least 6 years in possibly sill-like reservoirs at 7–11 km depth. The xenoliths became deposited and subsequently embedded in a mush of settled phenocrysts, while selvages and veins crystallized until the eruption commenced. At the end of the eruption, the xenoliths were finally transported to the surface within hours to days. Decompression during the rapid ascent induced internal stresses and caused renewed fragmentation of the xenoliths, producing the young fractures. Received: 25 August 1997 / Accepted: 25 November 1997     

Geochemistry, mineralogy, and radioactivity of Upper Cretaceous Abu Khruq Ring Complex, South Eastern Desert, Egypt

The upper Cretaceous Abu Khruq ring complex (ARC) is located in the South Eastern Desert between latitudes 24°00′10′′ and 24°03′15′′ N, and longitudes 33°54′50′′ and 33°58′ E and has a roughly circular shape with a diameter of 7 km. ARC is built up by major extrusion of alkaline volcanic rocks comprising mainly rhyolite porphyry and alkaline trachyte rocks at the center of the ring complex followed by successive intrusions of alkaline gabbro and syenitic rocks comprising quartz syenite (oversaturated), syenite (saturated), and nepheline syenite (undersaturated). Petrographical and geochemical studies were carried out for the rocks of the forming ARC. For mineralogical and radioactive investigations, samples were collected from the most promising locations representing the hematitized nepheline syenite, nepheline syenite pegmatites, and quartz syenite. The most important minerals comprise: phosphuranylite, zircon, monazite, xenotime, plumbopyrochlore, pyrite, huttonite, apatite, REE mineral, rutile, and atacamite. The hematitized nepheline syenite is the most U- and Th-rich rocks, where eU content in this rock ranges from 375 to 788 ppm with an average 502 ppm and the average eTh content is 2,345 ppm ranging from 1,918 to 3,067 ppm. The pegmatite syenite and quartz syenite contain relatively low concentrations of U and Th, where the average eU content are 11 and 16 ppm and average eTh contents are 27 and 327 ppm, respectively.     

Metamorphism of Proterozoic agpaitic nepheline syenite gneiss from North Singhbhum Mobile Belt, eastern India

Sushina nepheline syenite gneisses of Early Proterozoic North Singhbhum Mobile Belt (NSMB), eastern India suffered regional metamorphism under greenschist-amphibolite transitional facies condition. The Agpaitic Sushina nepheline syenite gneisses consist of albite, K-feldspar, nepheline (close to Morozewicz-Buerger composition), aegirine, biotite, epidote, piemontite, sodalite, cancrinite, natrolite and local alkali amphibole. Accessory phases include zircon, hematite, magnetite, rare pyrochlore and occasional eudialyte and manganoan calcic zirconosilicates. Mineral chemistry of albite, K-feldspar, nepheline, aegirine, alkali amphibole, natrolite and zirconium silicate minerals are described. The detailed textural features together with chemical data of some minerals indicate metamorphic overprint of these rocks. A new reaction is given for the genesis of metamorphic epidote. Metamorphic piemontite suggests greenschist facies metamorphism under high fO₂ (Hematite-Magnetite buffer). Up to 15.34 mol% of jadeite component in aegirine suggests that the metamorphic grade of the nepheline syenite gneiss reached at least to greenschist-amphibolite transitional facies or higher. Nepheline geothermometry suggests temperature of metamorphism <500 °C, which is consistent with greenschist facies metamorphism of surrounding chlorite-biotite-garnet phyllite country rock.     

Emplacement kinematics of nepheline syenites from the Terrane Boundary Shear Zone of the Eastern Ghats Mobile Belt,west of Khariar,NW Orissa: Evidence from meso- and microstructures

Nepheline syenite plutons emplaced within the Terrane Boundary Shear Zone of the Eastern Ghats Mobile Belt west of Khariar in northwestern Orissa are marked by a well-developed magmatic fabric including magmatic foliation, mineral lineations, folds and S-C fabrics. The minerals in the plutons, namely microcline, orthoclase, albite, nepheline, hornblende, biotite and aegirine show, by and large, well-developed crystal faces and lack undulose extinction and dynamic recrystallization, suggesting a magmatic origin. The magmatic fabric of the plutons is concordant with a solid-state strain fabric of the surrounding mylonites that developed due to noncoaxial strain along the Terrane Boundary Shear Zone during thrusting of the Eastern Ghats Mobile Belt over the Bastar Craton. However, a small fraction of the minerals, more commonly from the periphery of the plutons, is overprinted by a solid state strain fabric similar to that of the host rock. This fabric is manifested by discrete shear fractures, along which the feldspars are deformed into ribbons, have undergone dynamic recrystallization and show undulose extinction and myrmekitic growth. The shear fractures and the magmatic foliations are mutually parallel to the C-fabric of the host mylonites. Coexistence of concordant solid state strain fabric and magmatic fabric has been interpreted as a transitional feature from magmatic state to subsolidus deformation of the plutons, while the nepheline syenite magma was solidifying from a crystal-melt mush state under a noncoaxial strain. This suggests the emplacement of the plutons synkinematic to thrusting along the Terrane Boundary Shear Zone. The isotopic data by earlier workers suggest emplacement of nepheline syenite at 1500 +3/−4Ma, lending support for thrusting of the mobile belt over the craton around that time.     

云南个旧似长石正长岩类及其岩石成因

云南个旧碱性岩体主要的岩石类型有碱性正长岩和似长石正长岩,其中,似长石正长岩中出现大量似长石矿物霞石、方钠石和碱性暗色矿物。本文根据矿物成分及特征,将这些似长石正长岩进一步划分为黑榴霞石方钠正长岩、霞石方钠正长岩、霞石正长岩及方钠霞石正长岩4类。岩石地球化学结果表明,4类岩石的地球化学行为整体表现出过碱质岩的特征,K2O+Na2O含量很高,为钾玄岩系列,同时表现出钾质的特点。分异指数高,呈现高度分异演化特点。稀土元素变化大,轻重稀土元素分异明显,富集轻稀土元素,亏损重稀土元素。微量元素富集大离子亲石元素Th、U及Zr、Hf等高场强元素,亏损Ba、Sr大离子亲石元素,而P和高场强元素Ta、Ti亏损,同时Cr、Co、Ni含量非常低,具有中等的负Eu异常和微弱的负Ce异常。研究表明,似长石正长岩在岩浆演化过程中表现出明显的分离结晶作用特征,且岩浆起源温度较高,约为835℃,起源较深。个旧似长石正长岩为A型岩套A1亚型,结合构造判别图解,认为其可能来自角闪石或者金云母相矿物存在的富集地幔,形成于燕山晚期伸展的构造背景,岩浆在较高温度下高度结晶分异,并在侵位过程中伴随陆壳成分的混染。     

冀北印支期碱性岩浆活动及其地球动力学意义

冀北印支期碱性岩浆活动及其地球动力学意义张招崇（中国地质科学院地质研究所，北京１０００３７）王永强（中国地质科学院西安地质矿产研究所，西安７１００５４）关键词碱性岩浆印支期冀北地球动力学燕山地区是否存在印支期的构造、岩浆活动一直存在着争论。过去多数人...     

Fluid–rock interaction in autoliths of agpaitic nepheline syenites in the Ilímaussaq intrusion, South Greenland

Within the 1.16 Ga old Ilímaussaq intrusion, up to 700 m large autoliths occur in one stratigraphic unit of the layered floor series of agpaitic nepheline syenites (kakortokites). These autoliths consist of two different rock types: augite syenite and naujaite (agpaitic nepheline syenite). All three rock types show a number of alteration features related to the entrapment of the autoliths in the kakortokite magma caused by the interaction with a fluid phase.

In the kakortokites, the oxidation of primary arfvedsonite to aegirine and fluorite is restricted to the close proximity to the autoliths. Close to the surrounding kakortokite, the primary mafic phases of the augite syenites (augite, fayalite, Fe–Ti oxides) are completely replaced by arfvedsonite, aenigmatite, biotite, aegirine and fluorite. The decomposition of primary hastingsite to spectacular aegirine–augite–nepheline–aenigmatite symplectites can be observed up to several meters inside the autoliths. Additionally, fluorite formed at grain boundaries of primary nepheline. In the naujaite autoliths, primary arfvedsonite is replaced by aegirine–biotite intergrowths and abundant aenigmatite is occasionally replaced by Ti-rich aegirine and Fe–Ti oxides.

The mineral reactions in the autoliths are used to decipher details of the late to post-magmatic processes in a peralkaline syenitic intrusion. Mineral equilibria record an evolution governed by falling temperature (620 to ca. 500 °C) and increasing relative oxygen fugacity from FMQ + 1 to above FMQ + 4. Quantification of the observed mineral reactions reveals the infiltration of the autoliths with an oxidizing fluid phase rich in Na and F and minor addition of K. Volatiles (H and F) and in some cases also Fe, Ti and Ca (± Mg) released from primary autolith phases were mainly just relocated within the autoliths.