Rapakivi granite problems: plagioclase mantles and ovoid megacrysts

Mantling of alkali feldspar megacrysts by oligoclase (‘rapakivi texture’) generally can be interpreted as the result of magma mixing, although decompression is a viable interpretation, especially for high-level intrusions. Coexistence of mantled and unmantled crystals can be explained by transfer of mantled crystals (‘antecrysts’) from a mixed (hybrid) rock to a host granitoid devoid of mantled crystals, for example, by disintegration of microgranitoid enclaves. Processes capable of explaining multiple oligoclase shells include repeated increase and release of volatiles, and repeated replenishment by more mafic magma. The shells could be formed by transfer of megacrysts into and out of a magma-mixing zone during flow in dyke-like conduits or in the fronts of mafic flows moving across cumulate layers in plutons. Ovoid megacrysts, which occur especially in Proterozoic rapakivi granitoids, are difficult to interpret but are better explained by growth processes than by magmatic corrosion. The common presence of simple twinning, partial crystal faces, euhedral plagioclase inclusions and granophyre-like intergrowths with quartz favours normal magmatic growth. The common ovoid shapes with local facets could reflect incomplete development of crystal faces, owing to relatively rapid growth. Granophyre-like intergrowths in the ovoids, local granophyre occurring as megacryst rims and in the groundmass, and the common presence of miaroles suggest growth of the ovoids at relatively shallow depth, at conditions of delayed nucleation and consequent undercooling, resulting from accumulation and retention of fluid. Development of the ovoids is independent of plagioclase mantling.     

浙江雁荡山火山-侵入杂岩的岩浆混合作用——暗色包体中长石环带的证据

浙江雁荡山是中国东南部燕山晚期巨型火山-侵入杂岩带的重要组成部分。对其中央侵入相石英正长斑岩的暗色微粒包体中的斑晶和基质斜长石进行了详细的内部结构和成分分析,揭示了斜长石复杂环带的成因和相关的岩浆作用过程。斑晶斜长石由熔蚀的核部和表面干净的幔部组成,边部包裹有钾长石膜。核部斜长石呈浑圆状或港湾状,内部发育筛状结构,An成分显著低于幔部斜长石,代表来自酸性岩浆房中早期结晶的斜长石捕掳晶。同时,幔部斜长石与自形、表面干净的基质斜长石具有类似的An含量,且两者均含有针状磷灰石的包裹体,应结晶自与暗色微粒包体相应的基性岩浆。长石的复杂结构记录了雁荡山火山-侵入杂岩形成过程中的岩浆混合作用和岩浆演化过程。岩浆混合之后的火山喷发活动,造成岩浆房的压力突然减小,温压条件达到钾长石结晶的区域,在石英正长斑岩的斑晶斜长石和暗色包体中的斑晶与基质斜长石外均形成钾长石膜,构成反环斑结构。     

Statistical analysis of granophyric intergrowths in the Main Hill Arkasani Granophyre Pluton, eastern India

The Main Hill Arkasani Granophyre Pluton (MAG), a product of Proterozoic intraplate acid magmatic activity, represents an anatectic melt of the enveloping rocks of dominantly pelitic composition with subordinate trondhjemitic gneiss and basic rocks. Petrography, chemistry, correlation between compositional attributes, areal variation of volume percent granophyric intergrowth, and varimax rotated factor analysis of compositional attributes of these rocks suggest that in the MAG pluton, plagioclase phenocrysts and biotite crystallized first, followed by change of level of emplacement of the magma when the groundmass started crystallizing at a rapid rate. The rapid growth of quartz and alkali feldspar crystallizing from the residual melt gave rise to the ubiquitous granophyric intergrowth in the late stage of crystallization. The alkali-rich residual liquid tended to concentrate toward the margin of the pluton where there is a profusion of granophyric intergrowths.     

Microstructures of melt inclusions in anatectic metasedimentary rocks

The occurrence of crystallized and glassy melt inclusions (MI) in high‐grade, partially melted metapelites and metagraywackes has opened up new possibilities to investigate anatectic processes. The present study focuses on three case studies: khondalites from the Kerala Khondalite Belt (India), the Ronda migmatites (Spain), and the Barun Gneiss (Nepal Himalaya). The results of a detailed microstructural investigation are reported, along with some new microchemical data on the bulk composition of MI. These inclusions were trapped within peritectic garnet and ilmenite during crystal growth and are therefore primary inclusions. They are generally isometric and very small in size, mostly ≤15 μm, and only rarely reaching 30 μm; they occur in clusters. In most cases inclusions are crystallized (‘nanogranites’) and contain a granitic phase assemblage with quartz, feldspar and one or two mica depending on the particular case study, commonly with accessory phases (mainly zircon, apatite, rutile). In many cases the polycrystalline aggregates that make up the nanogranites show igneous microstructures, e.g. granophyric intergrowths, micrographic quartz in K‐feldspar and cuneiform rods of quartz in plagioclase. Further evidence for the former presence of melt within the investigated inclusions consists of melt pseudomorphs, similar to those recognized at larger scale in the host migmatites. Moreover, partially crystallized inclusions are locally abundant and together with very small (≤8 μm) glassy inclusions may occur in the same clusters. Both crystallized and partially crystallized inclusions often display a diffuse nanoporosity, which may contain fluids, depending on the case study. After entrapment, inclusions underwent limited microstructural modifications, such as shape maturation, local necking down processes, and decrepitation (mainly in the Barun Gneiss), which did not influence their bulk composition. Re‐homogenized nanogranites and glassy inclusions show a leucogranitic and peraluminous composition, consistent with the results of partial melting experiments on metapelites and metagraywackes. Anatectic MI should therefore be considered as a new and important opportunity to understand the partial melting processes.     

High-pressure minerals in mafic microgranular enclaves: evidences for co-mingling between lamprophyric and syenitic magmas at mantle conditions

Mafic microgranular enclaves, composed of diopside and rare magnesium biotite phenocrysts in a groundmass of diopside, biotite, apatite, Fe-Ti-oxides, and alkali feldspar, are associated with Neoproterozoic Piquiri potassic syenite in southern Brazil. Co-genetic mica and clinopyroxene cumulates present inclusions of pyrope-rich garnet in diopside phenocrysts. Textural evidence, as well as the chemical and mineralogical composition, suggest that enclaves crystallized from a lamprophyric magma and co-mingled with the host syenitic magma. The contrasting temperature between both magmas and the consequent chilling was important for the preservation of some early-crystallized minerals in the mafic magma. Diopside groundmass grains contain micro-inclusions of K-rich augite and phlogopite, and some clinopyroxene phenocrysts and elongate groundmass crystals have potassium-rich cores. The pyrope-rich garnet have high #mg number (67–68), with appreciable amounts of Na₂O and K₂O comparable to pyrope synthesized at 5 GPa. The extremely high K₂O contents of K-rich augite micro-inclusions suggest non-equilibrium with the parental magma, whereas the other K-rich clinopyroxenes are similar to K-clinopyroxenes produced at 5–6 GPa. K-clinopyroxene and garnet in mafic microgranular enclaves suggest that lamprophyric magma started its crystallization at upper mantle conditions, and chilled clinopyroxenes with measurable amounts of K₂O are taken as evidence that co-mingling began still at mantle pressures.     

Sr and Nd isotopic investigations towards the origin of feldspar megacrysts in microgranular enclaves in two I‐type plutons of the Lachlan Fold Belt,southeast Australia

New Sr and Nd isotopic data are presented for several large feldspar crystals occurring in microgranular enclaves in the Swifts Creek and Bridle Track plutons, along with analyses of their host enclave groundmass and adjacent granitoid. In the Swifts Creek Pluton several previous studies have concluded that the microgranular enclaves represent admixed, more mafic and more primitive magmas, and new data presented here confirm that. Feldspar megacrysts in the microgranular enclaves have Sr and Nd isotopic signatures that are distinct from the surrounding enclave groundmass and from other enclaves in the pluton and therefore cannot have crystallised in situ. Isotopic compositions of these feldspars are more consistent with their having crystallised in a reservoir similar in composition to the most primitive granitoid analyses. The crystals were then physically transferred from the granitoid magma into the enclave while the latter was still partially liquid, thus invalidating arguments for a porphyroblastic origin. Field, petrographic and geochemical data are consistent with microgranular enclaves in the Bridle Track pluton also originating as admixed, more mafic magmas. However, Sr isotopic compositions of the enclaves are identical, within error, to the host granite and indicate that significant Sr isotopic equilibration has occurred. Nd isotopic compositions of the enclaves extend to slightly higher ¹⁴³Nd/¹⁴⁴Nd_(i) and are consistent with a mingled magma origin followed by major isotopic equilibration. Feldspar phenocrysts in the studied enclave have isotopic compositions indistinguishable from both the enclave groundmass and host granite, preventing an interpretation of their origin using isotopic evidence alone.     

An electron‐optical study of melt‐related microstructures in granulite facies rocks from the Torngat Orogen

Cordierite–quartz and plagioclase–quartz intergrowths in a paragneiss from northern Labrador (the Tasiuyak Gneiss) were studied using SEM, STEM and TEM. The gneiss experienced granulite facies conditions and partial melting during both regional and, subsequently, during contact metamorphism. The microstructures examined all results from the contact metamorphism. Cordierite–quartz intergrowths occur on coarse and fine scales. The former sometimes exist as a ‘geometric’ intergrowth in which the interface between cordierite and quartz appears planar at the resolution of the optical microscope and SEM. The latter exists in several microstructural variants. Plagioclase is present as a minor component of the intergrowth in some examples of both the coarse and fine intergrowth. Grain boundaries in cordierite–quartz intergrowths are occupied by amorphous material or a mixture of amorphous material and chlorite. Cordierite and quartz are terminated by crystal faces in contact with amorphous material. Chlorite is sometimes found on cordierite surfaces and penetrating into cordierite grains along defects. Quartz contains (former) fluid inclusions 10–20 nm in maximum dimension. The presence of planar interfaces between cordierite and the amorphous phase is reminiscent of those between crystals and glass in volcanic rocks, but in the absence of compelling evidence that the amorphous material represents former melt, it is interpreted as a reaction product of cordierite. Plagioclase–quartz intergrowths occur in a number of microstructural variants and are commonly associated with cordierite–quartz intergrowths. The plagioclase–quartz intergrowths display simple, non‐planar interfaces between plagioclase and quartz. Quartz contains (former) fluid inclusions of dimensions similar to those observed in cordierite–quartz intergrowths. The boundary between quartz and enclosing K‐feldspar is cuspate, with quartz cusps penetrating a few tens of nanometres into K‐feldspar, commonly along defects in K‐feldspar and sometimes with very low dihedral angles at their tips. This cuspate microstructure is interpreted as melt pseudomorphs. The plagioclase–quartz intergrowths share some features with myrmekite, but differ in some respects: the composition of the plagioclase (An₃₇Ab₆₂Or₁–An₃₈Ab₆₁Or₁); the association with cordierite–quartz intergrowths; and microstructures that are atypical of myrmekite (e.g. quartz vermicules shared with cordierite–quartz intergrowths). It is inferred that the plagioclase–quartz intergrowths may have formed from, or in the presence of, melt. Inferred melt‐related microstructures preserved on the nanometre scale suggest that melt on grain boundaries was more pervasive than is evident from light optical and SEM observations.     

The melting and crystallisation behaviour of a natural clinopyroxene-ilmenite intergrowth

The anhydrous solidus of a natural clinopyroxene-ilmenite intergrowth from the kimberlite pipe at Monastery was found to be 1300° C at 20 kb increasing to 1470 ° C at 47 kb. The slope of the solidus is 6 ° C/kb, with a constant melting interval of approximately 140 ° C. Clinopyroxene was the liquidus phase in all but one case, and the ilmenite-out curve coincided with the beginning of melting. Controlled cooling experiments resulted in liquidus clinopyroxene crystals surrounded by intergrowths of clinopyroxene and ilmenite. The experimental and natural intergrowths are texturally, crystallographically and chemically similar. These results support the view that the natural clinopyroxene-ilmenite intergrowths found in kimberlites are a product of eutectic crystallisation in the upper mantle. Compositional similarities between the liquidus clinopyroxene and discrete clinopyroxene nodules from kimberlite suggest that the latter are possibly related to the clinopyroxene-ilmenite intergrowths by fractional crystallisation. Differences between the temperatures of equilibration obtained for clinopyroxenes from the natural intergrowths, and the beginning of melting on the clinopyroxene-ilmenite join, may be the result of hydrous melting in the former case, or as a consequence of the natural intergrowths existing as mantle veins or pegmatites at the ambient temperature prior to their incorporation in the kimberlite.     

Experimental studies At 5–12 GPa of the Ondermatjie hypabyssal kimberlite

Liquidus and sub-liquidus phase relationships are reported for melts formed from an aphanitic kimberlite composition crystallized at 5–12 GPa and 900–1400 °C. The liquidus phase over the pressure range investigated is forsteritic olivine. This is followed with decreasing temperature by olivine plus garnet as the initial sub-liquidus solid phase assemblage. Supra-solidus assemblages consist of olivine+garnet+clinopyroxene+Mg-ilmenite+liquid at 5–7 GPa or olivine+garnet+clinopyroxene+hematite–ilmenite solid solutions (+/−perovskite)+liquid at 8–12 GPa. Phlogopite forms as a near-solidus phase only at 900 °C and 6 GPa. Orthopyroxene does not form at any temperature and pressure. All garnets formed at 6–7 GPa are Ti-rich almandine–grossular–pyrope solid solutions and not Cr-pyrope, whereas garnets formed above 8 GPa are Ti- and Fe³⁺-rich and have no natural counterparts. Quenched liquids are represented by magnesite at 10–12 GPa and Mg–Ca-carbonates at lower pressures. In addition to forming discrete crystals, Mg-ilmenite and hematite–ilmenite solid solutions occur as lamellar intergrowths that are identical in texture to naturally occurring intergrowths. Mg-ilmenite compositions at 6–7 GPa are similar to those of the natural occurrences, whereas clinopyroxenes are richer in Ca. The effects of graphite versus platinum capsules on the oxygen fugacity of the experimental charges and the composition of the olivine, clinopyroxene, Fe–Ti-oxides and garnets formed are described. These experimental data are interpreted to indicate that kimberlite magmas are unlikely to be formed by very small degrees of partial melting of a simple homogeneous carbonated garnet lherzolite mantle. It is proposed that kimberlite magmas form by extensive partial melting of metasomatized mantle, i.e. mineralogically complex carbonate-bearing veins in a lherzolitic/harzburgitic substrate, and that lamellar ilmenite–clinopyroxene intergrowths represent the products of non-equilibrium growth in kimberlite magma.     

Ascent-driven crystallisation of dacite magmas at Mount St Helens, 1980–1986

We introduce a novel scheme that enables natural silicic glasses to be projected into the synthetic system Qz-Ab-Or-H₂O in order to relate variations in volcanic glass chemistry to changing pressure (P) and temperature (T) conditions in the sub-volcanic magma system. By this means an important distinction can be made between ascent-driven and cooling-driven crystallisation under water-saturated or undersaturated conditions. In samples containing feldspar and a silica phase (quartz or tridymite), quantitative P-T estimates of the conditions of last equilibrium between crystals and melt can be made. Formation of highly silicic melts (i.e. >77 wt% SiO₂) is a simple consequence of the contraction of the silica phase volume with decreasing pressure, such that high silica glasses can only form by crystallisation at low pressure. Resorption of quartz crystals appears to be a further diagnostic feature of decompression crystallisation. Groundmass and inclusion glasses in dacites from the 1980-1986 eruption of Mount St Helens volcano (WA) span a wide range in SiO₂ (68-80 wt%, anhydrous). The compositions of the least evolved (SiO₂-poor) inclusions in amphibole phenocrysts record entrapment of silicic liquids with Е.4 wt% water, corresponding to a water saturation pressure of ~200 MPa at 900 °C. The compositions of more evolved (higher SiO₂) plagioclase-hosted inclusions and groundmass glasses are consistent with extensive ascent-driven fractional crystallisation of plagioclase, oxide and orthopyroxene phenocrysts and microlites to low pressures. During this polybaric crystallisation, plagioclase phenocrysts trapped melts with a wide range of dissolved water contents (3.5-5.7 wt%). Magmas erupted during the Plinian phase of the 18 May 1980 eruption were derived from a large reservoir at depths of ̈́ km. Subsequent magmas ascended to varying depths within the sub-volcanic system prior to extraction. From glass chemistry and groundmass texture two arrest levels have been identified, at depths of 0.5-1 and 2-4 km. A single dome sample from February 1983 contains groundmass plagioclase, tridymite and quartz, testifying to temperatures of at least 885 °C at 11 MPa. These shallow storage conditions are comparable to those in the cryptodome formed during spring 1980. The corresponding thermal gradient, А.2 °C MPa^-1, is consistent with near-adiabatic magma ascent from ~8 km. We argue that the crystallisation history of Mount St Helens dacite magma was largely a consequence of decompression crystallisation of hot magma beyond the point of water saturation. This challenges the conventional view that phenocryst crystallisation occurred by cooling in a large magma chamber prior to the 1980-1986 eruption. Because the crystallisation process is both polybaric and fractional, it cannot be simulated directly using isobaric equilibrium crystallisation experiments. However, calculation of the phase proportions in water-saturated 910ᆣ °C experiments by Rutherford et al. (1985) over the pressure range 220-125 MPa reproduces the crystallisation sequence and phenocryst modes of Mount St Helens dacites from 18 May 1980. By allowing for the effects of fractional versus equilibrium crystallisation, entrained residual source material, and small temperature differences between nature and experiment, phase compositions can also be matched to the natural samples. We conclude that decompression of water-saturated magma may be the dominant driving force for crystallisation at many other silicic volcanic centres.     

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.     

山东金伯利岩中榄橄石的研究

本文百次研究了山东金伯利岩中橄榄石的产状、含量、大小、世代、形态、颜色、环带、矿物包体、折光率、2V、化学成分、端员组分特征及红外光谱和穆斯堡尔谱特征,并分析研究了橄榄石的成因。指出了无色—浅绿色的、含MgO、Cr2O3、NiO高的橄榄石是找金刚石矿的指示性矿物。     

Crystallization and resorption in plutonic plagioclase: Implications on the evolution of granodiorite magma (Gęsiniec granodiorite, Strzelin Crystalline Massif, SW Poland)

Granodiorite from the Gęsiniec Intrusion, Strzelin Crystalline Massif, SW Poland contains complexly zoned plagioclases. Five chemically and structurally distinct zones can be correlated among crystals: ‘cores’ (25–35% An), inner mantles (40–45% An), outer mantles (40–25% An), resorption zones (35–50% An) and rims (35–30% An). Good structural and chemical (major and trace elements) correlation of zones between crystals indicates that zonation was produced by changes in conditions of crystallization on a magma chamber scale. Plagioclase, being the liquidus phase, records a time span from the beginning of crystallization to emplacement and rapid cooling of granodiorite as thin dykes.

Crystallization began with the formation of inner mantles. The paucity and different sizes of inner mantles suggests slow crystallization in high temperature magma. Normally zoned inner mantles were formed under increasing undercooling. Compositional trends in mantles suggest closed system crystallization.

The major resorption zones were caused by injection of less evolved magma as indicated by the strontium increase in plagioclase. The injection triggered a rapid rise of magma and plagioclase crystals facilitating mixing but also inducing fast, kinetically controlled growth of complex multiple, oscillatory zonation within resorption zones. The ascent of magma caused decompression melting of plagioclase and produced melt inclusions within inner mantles—the ‘cores’. The decompression range is estimated at a minimum of 2 kbar. Emplacement of granodiorite as thin dykes allow rapid cooling and preservation of magmatic zonation in plagioclases. Melt inclusions crystallized completely during post-magmatic cooling.

The zonation styles of plutonic plagioclase differ markedly from volcanic ones suggesting different magma evolution. Zones in plutonic plagioclase are well correlated indicating crystallization in quiescent magma where crystals accumulation and compositional magma stratification may occur. Crystals probably did not travel between different regimes. Resorption occurred but as single albeit complex episodes. Good correlation of zones in plutonic plagioclases allows a distinction between the main processes controlling zonation and superimposed kinetic effects.     

Assimilation and fractionation in adjacent parts of the same magme chamber: Vandfaldsdalen macrodike,East Greenland

The Vandfaldsdalen macrodike, which lies in the Skaergaard region of East Greenland, is a remarkably zoned fossil magma chamber, with a granophyric cap overlying cumulate gabboros. The intrusion is distinctly bimodal, with a large compositional discontinuity at the contact between the gabbro and granophyre. Although the exposed part of the macrodike is in contact with Tertiary basalts and sediments, the granophyre originated by assimilation of xenoliths derived from the underlying Archean basement. Sr and Nd isotopic ratios throughout the cumulate sequence are remarkably similar, indicating insignificant contamination of the gabbro by the granophyre. Modelling of the compositional effects of cooling and crystallization indicate that the cumulate pile resulted from fractional crystallization, with the complicating effects of trapped liquid and post-cumulus fractionation. The uppermost rocks in the mafic part, of the chamber (SiO₂=62%; FeO^*=12.4%) resulted from about 85% fractional crystallization. A transgressive sill of strongly fractionated magma (SiO₂=67%; FeO^*=8.8%) formed from extracted intercumulus liquid that was the result of 90% fractional crystallization of the original magma. Mass-balance indicates that typical granophyre is made up of about 75% dissolved xenoliths, by weight, and 25% mantle-derived basaltic magma. The magmas were not measurably affected by material exchange across the interface between the gabbro and granophyre. This magma chamber evolved by both assimilation and fractional crystallization, but the residual liquids formed by fractional crystallization were unaffected by assimilation. Heat exchange between were unaffected by assimilation. Heat exchange between the two parts of the chamber was obviously important, but there was insignificant material exchange. The inability of fractional crystallization and assimilation to affect the same liquid is related to the dynamic behavior of this particular magma chamber, particularly the buoyancy of granophyre relative to evolving tholeiitic magma.     

Feldspar geothermometry of the Hell Canyon Pluton,Boulder Batholith,Montana

The bulk compositions of the groundmass alkali feldspar from the Hell Canyon Pluton is 0.146mole% albite. The composition of the outermost zone of the oscillatory zoned plagioclase is 0.686 mole% albite, whereas the most calcic cores have a composition of 0.43 mole% albite. The structural state of the alkali feldspar is near orthoclase. Both composition of coexisting feldspars and structural state of the alkali feldspar are nearly constant throughout the pluton.Exsolved albite in the alkali feldspar have a composition of 0.965 mole% albite and the orthoclase host has a composition of 0.032 mole%. Singe crystal X-ray studies indicate that the albite intergrowths are coherent with the host.Equilibrium temperatures derived from the coexisting feldspar average 554 ° C; about 150 ° C, too low for the minimum solidus temperatures for reasonable emplacement pressures (2 kb). If this minimum solidus temperature is assumed, then the alkali feldspar has lost about 0.15 mole% albite. This loss was most likely caused by hydrothermal solutions associated with the crystallizing magma and equilibrated at about 550 ° C. However, based on the coherent albite intergrowths and the orthoclase structure state it can be inferred that the system was relatively free of volatiles below 500 ° C. Final equilibirium between orthoclase host and albite intergrowths occurred at about 311 ° C.     

新疆西准噶尔夏尔莆岩体岩浆混合的岩相学证据

夏尔莆岩体由寄主岩石、微粒镁铁质包体和中基性岩墙群组成,具丰富、典型的岩浆混合岩相学特征.野外露头,寄主岩石中暗色矿物分布不均并发育暗色矿物集合体、微小的镁铁质包体和不均匀混合条带;包体具有明显的塑性变形,与寄主岩石或界线截然或渐变过渡,常发育反向脉和寄主岩石中的长石巨晶(捕虏晶);中基性岩墙群与微粒镁铁质包体紧密共生并延伸方向基本一致,发育寄主岩石中的长石捕虏晶,被寄主岩的反向脉横切.在镜下,包体与寄主岩混合带中均发育斜长石异常环带和多种不平衡矿物共生现象,包体中发育针状磷灰石.这些特征表明镁铁质包体和中基性岩墙群来源于与寄主岩石同一岩浆事件的基性岩浆,并与其发生了强烈的岩浆混合作用.岩相学特征为夏尔莆岩体岩浆混合成因提供了重要佐证.     

Trace element modelling of the origin and evolution of the Oyaca‐Boyalik volcanic rocks (NE of Haymana,Ankara, Turkey)

Lower Miocene Boyalik volcanic rocks, situated approximately 80 km south of Ankara, exhibit both alkaline and calc‐alkaline characteristics. Alkaline products are trachybasaltic and trachyandesitic, whereas calc‐alkaline products are dacitic. The phenocrysts in the dacites consist primarily of plagioclase and hornblende, with lesser amounts of biotite. The groundmass contains plagioclase and quartz microcrysts. Trachyandesites are mainly composed of plagioclase and biotite phenocrysts with a groundmass of alkali feldspar microlites and minor clinopyroxene microcrysts. Trachybasalts are mainly composed of olivine and plagioclase phenocrysts, with minor clinopyroxene phenocrysts associated with alkali feldspar, plagioclase and clinopyroxene microlites and microcrysts in the groundmass. Oxides are common accessory phases in all products. Boyalik volcanic rocks have essentially homogeneous incompatible trace element patterns with variable Nb and Th anomalies, enrichment in Rb, Ba, K, La, Ce and Nd, and positive Sr anomalies. Some trace element ratios (e.g. Ba/Ta, Ba/Nb, Th/U and Ce/Pb) are variable among the series. For instance, dacites and trachyandesites have higher Ba/Ta (724–2509), Ba/Nb (45–173) and Th/U (3.5–8.7) and lower Ce/Pb (7.1–3.9) values than the trachybasalts. Trace element data indicate that the series are chemically distinct but probably were derived from a common lithospheric mantle source via variable degrees of partial melting. The magmas then underwent a process of evolution involving assimilation and fractional crystallization (AFC) during ascent to the surface. Although trachyandesites and dacites were generated from a lithospheric mantle source via ～1% and ～1.5% to ～5% degrees of partial melting, respectively, trachybasalts were derived from the same source via higher degrees of partial melting (～20%) with neglegible crustal contamination. Boyalik volcanism is linked to an intracontinental transpressional setting. However, the overall geochemical features are consistent with derivation from a mantle source that records earlier Eocene subduction between the Sakarya continental fragment and the K?r?ehir block during time.     

Mineralogy of the upper mantle: A review of the minerals in mantle xenoliths from kimberlite

The importance of ultramafic and eclogitic xenoliths in kimberlite as representing the rocks and minerals of the upper mantle has been widely perceived during the last decade. Studies of the petrology and mineral chemistry of these mantle fragments as well as of inclusions in diamond, have led to significant progress in our understanding of the mineralogy and chemistry of the upper mantle. For example, it is now known that textural differences in the ultramafic xenoliths (lherzolite, harzburgite, pyroxenite and websterite) are partially reflected in chemical differences. Thus xenoliths that display a ‘fluidal’ texture, indicative of intense deformation are less depleted in Ca, Al, Na, Fe and Ti than those xenoliths in which granular textures are predominant. It is believed this relative depletion may indicate the sheared (fluidal texture) xenoliths are representative of primary, undifferentiated mantle. This material on partial melting would produce ‘basaltic-type’ material, and leave a residuum whose chemistry and mineralogy is reflected by the granular xenoliths.Also present in kimberlite are large single phase xenoliths that may be either one single crystal (xenocryst, megacryst) or an aggregate of several crystals of the same mineral (discrete xenolith, or discrete nodule). These large single phase samples consist of similar minerals to those occurring in the ultramafic xenoliths but chemically they are distinct in being generally more Fe-rich. The relation between these xenocrysts to their counterparts in the ultramafic xenoliths is unknown. Also unknown, at the present time, is the exact relation between diamond and kimberlite. Evidence obtained from study of the mineral inclusions in diamond suggests that diamond forms in at least two chemically distinct environments in the mantle; one eclogitic, the other, ultramafic. Interestingly, this suggestion is true for diamonds from worldwide localities.The mineral-chemical results of studies on xenoliths and inclusions in diamond have been convincingly interpreted in the light of experimental studies. It is now possible based on several different geothermometers and barometers to determine relatively reasonable physical conditions for the final genesis of many of these mantle rocks. For the most part the final equilibration temperatures range between 1000 and 1400°C and pressure in the region 100–200 km. These conditions are consistent with an upper mantle origin. Future studies will undoubtedly attempt to more concisely, and accurately, define these conditions, as well as understand better the chemical and spatial relationship of the rock-types in the mantle.     

Mafic xenoliths from Egyptian Tertiary basalts and their petrogenetic implications

Mineralogical data, coupled with whole-rock major and trace element data of mafic xenoliths from two occurrences of the Egyptian Tertiary basalts, namely Abu Zaabal (AZ) near Cairo and Gabal Mandisha (GM) in the Bahariya Oases, are presented for the first time. Chemically, AZ basalts are sodic transitional, while those of GM are alkaline. In spite of the different petrographic and geochemical features of the host rocks, mafic xenoliths from the two occurrences are broadly similar and composed essentially of clinopyroxene, plagioclase, alkali feldspar, and Fe–Ti oxides. The analytical results of host rocks, xenoliths and their minerals suggest that the xenoliths are cognate to their host magmas rather than basement material. The mafic xenoliths are olivine-free and contain alkali feldspar contrary to the phenocryst assemblage of the host rocks, confirming that they are not cumulates from the host magma. The geochemical and mineralogical characteristics show that the precursor magmas of these xenoliths are more fractionated and possibly contaminated compared to those of the host rocks. Estimated crystallization conditions are  1–3 kbar for xenoliths from both areas, and temperature of  950–1100 °C vs. 920–1050 °C for AZ and GM, respectively. These cognate xenoliths probably crystallized from early-formed, highly-fractionated anhydrous magma batches solidified in shallow crustal levels, possibly underwent some AFC during their ascent, and later ripped-up during fresh magma pulses. The xenoliths, although rare, provide an evidence for the importance of crystal fractionation at early evolution of the Egyptian Tertiary basalts.     

Diffusion-controlled biotite breakdown reaction textures at the solid/liquid transition in the continental crust

Two-phase quartz intergrowths with garnet, cordierite and tourmaline occur commonly in prograde high-temperature migmatites, granulites, as well as in the last crystallization stages of biotite granites. Structural, microtextural and mineralogical data show that they result from the breakdown of biotite in the presence of a melt phase associated with incongruent dissolution of feldspars into the melt and silica release (giving quartz in silica saturated rocks). Biotite breakdown and growth of Al-rich ferromagnesian minerals, occurring at the solid–liquid transition in the crust (early melting or final crystallization), is kinetically controlled by Fe and Mg mass transport, the network-forming cations Si and Al being locally compensated for by feldspar dissolution/crystallization. This process leads to significant changes with respect to equilibrium dehydration-melting reactions wherein quartz is a reactant and K-feldspar a reaction product. Therefore, quartz inclusions commonly occurring in garnets from granulite-facies metapelites and metagraywackes are not simply grains passively included during garnet growth. They may also correspond to newly crystallized phases. Resorption of feldspar may lead to more alkaline melt and to crystalline residue richer in Al than expected under equilibrium conditions. Hence, excess alumina in granulite-facies rocks is not necessarily related to initial alumina-rich whole-rock compositions (as currently considered), but may be due, at least partly, to kinetics of melting.