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.     

Strain localization mechanisms in deep-seated layered rocks

More than 160 shear zones of the Norwegian Caledonides and Italian Southern Alps, formed under high-temperature or high-pressure conditions, are analysed with respect to mechanisms of strain localization. In metabasic rocks with a pre-existing (magmatic) layering strain localizes preferentially in mafic layers, although experiments in monomineralic rocks would predict a location in the weaker feldspar-rich layers. In addition, in some of these zones amphibole recrystallizes dynamically, whereas feldspars only show undulatory extinction. These are evidences for strength inversion between feldspars and mafic minerals. This inverse strength behaviour is caused by a reduced grain size of feldspars and sometimes amphiboles in the pre-existing mafic layers, i.e. prior to shearing. The localization of the shear zones is therefore a grain-size-sensitive process (grain-size-dependent softening), nearly independent of the composition of the deformed material. The numerous amphibole–amphibole contacts favour dynamical recrystallization by grain-boundary migration and the rare feldspar–feldspar contacts prevent or suppress the same process in feldspars. Therefore, the feigned inverse strength is mainly caused by the interaction of reduced grain size and grain-boundary effects in the original rock. This behaviour can lead to the preferential localization of shear zones in mafic layers, although they are not favourably oriented with respect to the stress system.     

The Pleasant Bay Layered Gabbro--Diorite, Coastal Maine: Ponding and Crystallization of Basaltic Injections into a Silicic Magma Chamber

The Pleasant Bay layered gabbro–diorite intrusion, locatedon the coast of Maine between Bar Harbor and Machias, is roughlyoval in plan, measuring 12 km by 20 km. Gravity data, contactrelations, and internal layering suggest that it is basinformin structure with a maximum thickness of {small tilde}3 km.Its roof and upper parts have been lost through erosion. Whereit is in contact with underlying granite, the base of the intrusiontypically consists of strongly chilled gabbro with convex-downwardlobate forms, suggesting that the granite was incompletely solidifiedwhen the gabbro was emplaced. Roughly 90% of the exposed rocksare weakly layered gabbro and mafic diorite, both of which varywidely in grain-size and texture. Layers and lenses of medium-grainedleucocratic diorite to granodiorite are widely intercalatedwith the chilled mafic rocks and commonly contain partly digestedmafic inclusions; they also commonly contain zones of pillow-likebodies of gabbro chilled on all margins. The dioritic rocksare consistently topped by gabbroic layers with chilled lobatebases and commonly appear to feed granitic pipes and diapirsinto overlying gabbro. Much of the intrusion can be subdividedinto hundreds of macrorhythmic units (from 1 to 100 m thick)consisting of basally chilled gabbro that grades upward to dioriteor highly evolved leucocratic silicic cumulates. Basaltic dikesare abundant both in the underlying granite and in the layeredgabbro–diorites; they have appropriate compositions tobe feeders for chilled gabbroic layers in the Pleasant Bay intrusion. The layered rocks of the Pleasant Bay intrusion record hundredsof basaltic injections into a chamber with resident silicicmagma. Small injections produced chilled gabbroic layers andpillows within silicic cumulates. Larger infusions of basalticmagma produced temporary compositional stratification and episodesof double-diffusive convection within the chamber. Althoughfractional crystallization produced compositional variationin much of the gabbro, units that grade from chilled gabbroat the base to highly silicic cumulates at the top provide cumulaterecords of magma stratification and hybridization along a double-diffusiveinterface between basaltic and silicic magmas. The intrusionprovides a superb plutonic record of events that have oftenbeen inferred for silicic eruptive centers. Mafic–siliciclayered intrusions comparable with the Pleasant Bay are morewidespread than has generally been appreciated.     

论壳内韧性流层及其构造表现

地球物理探测表明,在上地壳之下有一不均匀分布的壳内低速层,中地壳之中还有一些局部的低速层。现代破坏性地震震源主要集中于10～15km深处,相当于这一低速层之顶部,处于脆韧性过渡带内。从岩石变形的角度看,这个低速层是一个壳内的韧性流变层,以发育近水平的韧性剪切带和褶叠层构造为其特征。它在纵向上和横向上都是不均一的,代表了地壳尺度的韧性剪切带,在地壳构造的演化中起着极重要的作用。     

The hydrothermal system of Central Jebilet (Variscan Belt,Morocco): A genetic association between bimodal plutonism and massive sulphide deposits?

Central Jebilet (Moroccan Variscan Belt) hosts several Cu and Pb–Zn massive sulphide deposits that are associated with a suite of gabbroic and microgranitic, tholeiitic to alkaline, intrusions emplaced 330 Ma ago. The intrusions and ore bodies form structural lineaments within marine Visean shales that are affected by very low to low-grade post-Visean metamorphism and contemporaneous shortening accompanied by the development of conjugate ductile to brittle shear zones. The ductile shear zones are localised in thermally softened aureoles around magmatic intrusions, while brittle deformation is common far from the intrusions. The intrusions have induced a contact metamorphism that reaches the hornblende hornfels facies, and their emplacement was accompanied by hydrothermal activity that leached base metals from the felsic intrusions.The massive sulphide deposits consist of steeply dipping elongate lenses that are located in shear zones 1–1.5 km away from the intrusions. They are dominated by pyrrhotite (up to 90%), sphalerite, galena, chalcopyrite, pyrite and arsenopyrite forming a mylonitic texture. Their wall rocks are altered to syntectonic mineral assemblages similar to those found in the alteration zones associated with the magmatic intrusions. The massive sulphide deposits located near the felsic intrusions are rich in lead and zinc compared to those located near the mafic intrusions, which are copper deposits. These relationships indicate that the whole Central Jebilet hydrothermal system could be described in terms of a lateral secretion of base metals from source zones (i.e. bimodal intrusions) to discharge zones (i.e. the Jebilet sulphide deposits). The metapelites in the contact metamorphic zone around felsic intrusions contain zincian ilmenite that was probably related to interaction of the host rocks with chlorine-rich fluid carrying zinc and other metals leached from the microgranites.     

Moho topography and lower crustal wide-angle reflectivity around the TESZ in southern Scandinavia and northeastern Europe

The Moho topography is strongly undulating in southern Scandinavia and northeastern Europe. A map of the depth to Moho shows similarities between the areas of the Teisseyre–Tornquist Zone (TTZ) in Poland and the Fennoscandian Border Zone (FBZ), which is partly coinciding with the Sorgenfrei–Tornquist Zone (STZ) in Denmark. The Moho is steeply dipping at these zones from a crustal thickness of approximately 32 km in the young Palaeozoic Platform and basin areas to approximately 45 km in the old Precambrian Platform and Baltic Shield. The Moho reflectivity (P_MP waveform) in the POLONAISE'97 refraction/wide-angle seismic data from Poland and Lithuania is variable, ranging from ‘sharp’ to strongly reverberating signals of up to 2 s duration. There is little or no lower crustal wide-angle reflectivity in the thick Precambrian Platform, whereas lower crustal reflectivity in the thin Palaeozoic Platform is strongly reverberating, suggesting that the reflective lower crust and upper mantle is a young phenomena. From stochastic reflectivity modelling, we conclude that alternating high- and low-velocity layers with average thicknesses of 50–300 m and P-wave velocity variations of ±3–4% of the background velocity can explain the lower crustal reflectivity. Sedimentary layering affects the reflectivity of deeper layers significantly and must be considered in reflectivity studies, although the reverberations from the deeper crust cannot be explained by the sedimentary layering only. The reflective lower crust and upper mantle may correspond to a zone that has been intruded by mafic melts from the mantle during crustal extension and volcanism.     

The Gaojiacun Mafic-Ultramafic Complex （Sichuan, SW China）：Neoproterozoic Magmatic Activity at the Western Margin of the Yangtze Craton

1 Introduction The Gaojiacun intrusive complex is one of the numerous ultramafic-mafic intrusions in Sichuan Province of China. It was mapped during the 1970s and studied mainly by Chinese scientists (e.g. Geological Team 106, 1975; Shen et al., 1986, 1989; CGGJC, 1986; Yang et al., 1993; Li et al., 1995; Shen et al., 2003; Zhu et al., 2004a). Since the year 2000, China has become one of the largest PGE consumers. While the country can produce only less than 1 ton PGE/year, the Chin…     

Crystallization and Layering of the Skaergaard Intrusion

Solidification of large slowly cooled intrusions is a complexprocess entailing progressive changes of rheological propertiesas the crystallizing magma passes through successive stagesbetween a viscous Newtonian fluid and a brittle solid rock.Studies of this transition in the Skaergaard intrusion indicatethat most crystallization took place in an advancing front ofsolidification against the floor, walls, and roof where crystalsnucleated and grew in a static boundary layer, much in the mannerproposed by Jackson in 1961. The non-Newtonian properties ofthe crystallizing magma account for the fact that plagioclase,which was lighter than the liquid, is a major component of rockson the floor, while mafic minerals that were heavier than theliquid accumulated under the roof. Crystals that nucleated andgrew in these zones were trapped by an increasingly rigid zonethat advanced more rapidly than the crystals sank or floated.If any crystals escaped entrapment, they were those of the largestsize and density contrast. The rates of accumulation in different parts of the intrusionwere not governed by rates of gravitational accumulation somuch as by the nature of convection and heat transfer. Cumulatetextures, preferred orientations of crystals, and layering,all of which have been taken as evidence of sedimentation, canbe explained in terms of in situ crystallization. Layering cannothave been caused by density currents sweeping across the floor;it is well developed on the walls and under the roof, lacksthe size and density grading and mineralogical compositionsthat would be expected, and shows no evidence of having beenaffected by obstructions in the paths of the currents. We propose an alternative origin of layering that is based onprocesses governed by the relative rates of chemical and thermaldiffusion during cooling. Intermittent layering resulted fromgravitational stratification of the liquid, and cyclic layeringwas produced by an oscillatory process of nucleation and crystalgrowth. The effects of differentiation during in situ crystallizationare strongly dependent on relative rates of diffusion of individualcomponents, and some of the compositional variations in differentparts of the intrusion can be explained in terms of these differences.     

Textural and chemical evolution of a fractionated granitic system: the Podlesí stock, Czech Republic

The Podlesí granite stock (Czech Republic) is a fractionated, peraluminous, F-, Li- and P-rich, and Sn, W, Nb, Ta-bearing rare-metal granite system. Its magmatic evolution involved processes typical of intrusions related to porphyry type deposits (explosive breccia, comb layers), rare-metal granites (stockscheider), and rare metal pegmatites (extreme F–P–Li enrichment, Nb–Ta–Sn minerals, layering). Geological, textural and mineralogical data suggest that the Podlesí granites evolved from fractionated granitic melt progressively enriched in H₂O, F, P, Li, etc. Quartz, K-feldspar, Fe–Li mica and topaz bear evidence of multistage crystallization that alternated with episodes of resorption. Changes in chemical composition between individual crystal zones and/or populations provide evidence of chemical evolution of the melt. Variations in rock textures mirror changes in the pressure and temperature conditions of crystallization. Equilibrium crystallization was interrupted several times by opening of the system and the consequent adiabatic decrease of pressure and temperature resulted in episodes of nonequilibrium crystallization. The Podlesí granites demonstrate that adiabatic fluctuation of pressure (“swinging eutectic”) and boundary-layer crystallization of undercooled melt can explain magmatic layering and unidirectional solidification textures (USTs) in highly fractionated granites.     

Character and significance of spectacular layering features developed in the thin, alkali-basaltic sills of the Ulvö Gabbro Complex, Sweden

Summary The Ulv? Gabbro Complex consists of alkali-olivine basaltic circular bodies ∼30–80 km in diameter. These intrusions were emplaced at shallow depths (∼3 km) as thin sheets (∼300 m). Among other things, the gabbroic cumulates of the complex display: modal layering, grain-size variations, trough structures, and slump structures. The crystallization sequence is olivine+plagioclase, ulv?spinel, clinopyroxene, and apatite. A nearly continuous exposure across one of these intrusions, the Norra Ulv?n gabbro, is subdivided into: a Lower Zone (LZ), a Rhythmically Layered Zone (RZ) and an Upper Zone (UZ). LZ and RZ were formed at the floor, while UZ grew from the roof downward. Major-element variations in the cores of the cumulus minerals define fractionation trends from the base of the intrusion to the RZ-UZ boundary interpreted as a “sandwich horizon”. Modeling suggests that a significant amount of crystallized interstitial liquid is required to produce the observed stratigraphic relations. Our results suggest that the small size and shallow emplacement depth of the intrusions of the Ulv? Gabbro Complex helped to preserve evidence of primary accumulation processes. However, it is also clear that despite the limited time available postcumulus processes such as diffusional homogenization and compaction of some grains were important. Correspondence: S. ?. Larson, Earth Sciences Centre, Department of Geology, G?teborg University, POB 460, SE 40530 G?teborg, Sweden     

Tectonic setting and petrogenesis of the Çelebi granitoid, (Kırıkkale-Turkey) and comparison with world skarn granitoids

Many studies have shown systematic correlations between the composition of plutons worldwide and the metal content of associated skarns. This is the first report of similar correlations between the composition of Çelebi granitoid and skarns of the Çelebi district in Central Anatolia, Turkey. The Çelebi district is well known for its polymetallic Fe–W and Cu vein ores. These are hosted by calcic skarn zones. Both exoskarns (pyroxene–garnet) and endoskarns (epidote–pyroxene) occur in the district formed mainly along the granitoid contacts and along the fractures within the marble. Based on mineralogy, petrology and geochemistry, two different igneous rocks were recognized in the Çelebi granitoid, referred to as leucocratic (felsic) and mesocratic (intermediate) Çelebi granitoid. The leucocratic Çelebi occurs as dominant rock type, and is classified as granite. The mesocratic Çelebi is not widespread and is classified as adamellite, tonalite, quartz monzonite and quartz monzodiorite. The mesocratic Çelebi has I-type characteristics, and have subalkaline, calc-alkaline and metaluminous characteristics like most worldwide skarn granitoids.A post-collisional tectonic setting is proposed on the basis of field evidence, the relative timing of intrusions with respect to metamorphic and obducted ophiolitic rocks and trace element geochemistry. The high abundance of La and Ce and the enrichment of V in mafic components suggest that Çelebi granitoids are formed by partial melting of mantle rocks, but have been contaminated by interaction with continental crust involving possible magma mixing processes (i.e. mixing of coexisting felsic and mafic magmas). In the district, the mesocratic type and mafic microgranular enclaves (MME) mainly within leucocratic type represent a mafic underplating magma that was mixed with and/or injected into felsic magma of the leucocratic type.The present study shows that Fe mineralization is associated with mesocratic Çelebi type, whereas W mineralization is associated with leucocratic type. Mesocratic Çelebi granitoid is significantly different from the worldwide average of plutons associated with Fe skarns. In particular, MgO vs. SiO₂, FeOt+CaO+Na₂O/K₂O vs. SiO₂, Fe₂O₃/Fe₂O₃+FeO vs. SiO₂ and V vs. Ni vary from typical values (are lower than values typical for plutons associated with Fe skarns) for plutons associated with Fe skarns. Instead, it resembles the geochemical characteristics of plutons associated with worldwide Cu and possibly Au skarns. This suggests new exploration possibilities for copper and gold in the Çelebi district.     

Petrogenesis of the ~740 Korab Kansi mafic-ultramafic intrusion,South Eastern Desert of Egypt: Evidence of Ti-rich ferropicritic magmatism

The Neoproterozoic Korab Kansi mafic-ultramafic intrusion is one of the largest (100 km²) intrusions in the Southern Eastern Desert of Egypt. The intrusion consists of Fe-Ti-bearing dunite layers, amphibole peridotites, pyroxenites, troctolites, olivine gabbros, gabbronorites, pyroxene gabbros and pyroxene-hornblende gabbros, and also hosts significant Fe-Ti deposits, mainly as titanomagnetite-ilmenite. These lithologies show rhythmic layers and intrusive contacts against the surrounding granites and ophiolitic-island arc assemblages. The wide ranges of olivine forsterite contents (Fo_67.9-85.7), clinopyroxene Mg# (0.57–0.95), amphibole Mg# (0.47–0.88), and plagioclase compositions (An_85.8-40.9) indicate the role of fractional crystallization in the evolution from ultramafic to mafic rock types. Clinopyroxene (Cpx) has high REE contents (2–30 times chondrite) with depleted LREE relative to HREE, like those crystallized from ferropicritic melts generated in an island-arc setting. Melts in equilibrium with Cpx also resemble ferropicrites crystallized from olivine-rich mantle melts. Cpx chemistry and its host rock compositions have affinities to tholeiitic and calc-alkaline magma types. Compositions of mafic-ultramafic rocks are depleted in HFSE (e.g. Nb, Ta, Zr, Th and U) relative to LILE (e.g. Li, Rb, Ba, Pb and Sr) due to the addition of subduction-related hydrous fluids (rich in LILE) to the mantle source, suggesting an island-arc setting. Fine-grained olivine gabbros may represent quenched melts approximating the primary magma compositions because they are typically similar in assemblage and chemistry as well as in whole-rock chemistry to ferropicrites. We suggest that the Korab Kansi intrusion crystallized at temperatures ranging from ~700 to 1100 °C from ferropicritic magma derived from melting of metasomatized mantle at <5 Kbar. These hydrous ferropicritic melts were generated in the deep mantle and evolved by fractional crystallization under high ƒO₂ at relatively shallow depth. Fractionation formed calc-alkaline magmas during the maturation of an island arc system, reflecting the role of subduction-related fluids. The interaction of metasomatized lithosphere with upwelling asthenospheric melts produced the Fe and Ti-rich ferropicritic parental melts that are responsible for precipitating large quantities of Fe-Ti oxide layers in the Korab Kansi mafic-ultramafic intrusion. The other factors controlling these economic Fe-Ti deposits beside parental melts are high oxygen fugacity, water content and increasing degrees of mantle partial melting. The generation of Ti-rich melts and formation of Fe-Ti deposits in few layered intrusions in Egypt possibly reflect the Neoproterozoic mantle heterogeneity in the Nubian Shield. We suggest that Cryogenian-Tonian mafic intrusions in SE Egypt can be subdivided into Alaskan-type intrusions that are enriched in PGEs whereas Korab Kansi-type layered intrusions are enriched in Fe-Ti-V deposits.     

Stratabound phyllosilicate zones associated with syntectonic copper orebodies at Mt Isa,Queensland

Three distinctive stratabound phyllosilicate zones are present at the margins of the syntectonic Mt Isa Cu orebodies and their host rock, the ‘silica‐dolomite’, in northwestern Queensland. The zones show close spatial relationships with adjacent stratiform Pb‐Zn ores within the overall host, the dolomitic Urquhart Shale. The Pb‐Zn orebodies may be either sedimentary‐exhalative or, as suggested recently, late diagenetic in origin, whereas the Cu ores were formed during the third regional deformation event. Talc‐stilpnomelane layers within the footwall of the silica‐dolomite, and biotite‐rich layers at the margins and adjacent to the silica‐dolomite are present in sideritic intervals within the dolomitic sequence. These Fe‐carbonate‐rich layers correlate with the Pb‐Zn orebodies, but have a much greater lateral extent. Chloritic layers occur along the hanging wall contacts of several Pb‐Zn orebodies with overlying silica‐dolomite lobes. Microstructural and petrographic studies suggest that the phyllosilicates grew at the silicification stage during early syn‐D3 alteration that also formed the silica‐dolomite and its Cu orebodies. The stratabound distribution of talc, stilpnomelane and biotite is explained by the chemical control of the sideritic beds on silicification. The origin of the Fe‐carbonates is not known and may be either sedimentary or diagenetic. The chlorite zones may have developed because of more extensive fluid‐rock interactions controlled by higher permeabilities along the brittle shale and ductile galena layering within the Pb‐Zn orebodies.     

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.     

层状岩体的成因及成矿作用

层状岩体的典型特征是其具有层状构造和韵律结构。层状镁铁质岩体不仅是研究岩浆分异演化过程和地壳混染作用的最佳天然实验室,其中还广泛赋存重要的稀有金属和贱金属矿床。文中主要总结了不同类型层状岩体形成的构造背景、基本特征、母岩浆特征和韵律层理成因,以及岩浆混合和地壳混染在岩浆演化过程中起到的重要作用。文中还介绍了层状岩体中典型矿床的成矿作用与岩浆演化过程的成因联系。     

Marginal mafic intrusions as indicators of downslope draining of dense residual melts in anorthositic diapirs?

Geochemical and isotopic investigation of three small mafic intrusions (Løyning: 1250 × 150 m, Hogstad: 2000 × 200 m, Koldal: 1250 × 500 m) in the marginal zones of the Egersund-Ogna (Løyning, Koldal) and Åna-Sira massif-type anorthosites (Hogstad) (Rogaland Anorthositic Province, south Norway: 930 Ma) provides new insights into the late evolution of anorthositic diapirs. These layered mafic intrusions are essentially of norite, gabbronorite as well as leuconorite and display conspicuous evidence of subsolidus recrystallization. In Løyning and Hogstad, the modal layering is parallel to the subvertical foliation in the enclosing anorthosite. The northern part of the Koldal intrusion cuts across the foliation of the anorthosite, whereas in its southern part the subvertical layering is parallel to the anorthosite's foliation. The regularity of the layered structures suggests that the layering was initially acquired horizontally and later tilted during the final movements of the diapirs.

The least differentiated compositions of plagioclase and orthopyroxene in the three intrusions (An₅₉–En₆₈ in Løyning, An₄₉–En₆₄ in Hogstad and An₄₄–En₆₁ in Koldal) and the REE contents in apatite (Hogstad) indicate that their parent magmas were progressively more differentiated in the sequence Løyning–Hogstad–Koldal. Isotopic data (Løyning: ⁸⁷Sr/⁸⁶Sr: 0.70376–0.70457, ε_Ndt: + 6.8 to + 2.7; Hogstad: ⁸⁷Sr/⁸⁶Sr: 0.70537–0.70588, ε_Ndt: + 2.1 to − 0.5; Koldal: ⁸⁷Sr/⁸⁶Sr: 0.70659–0.70911, ε_Ndt: + 3.5 to − 1.6) also indicate that in this sequence, parent magmas were characterized by a progressively more enriched Sr and Nd isotopic signature. In Løyning, the parent magma was slightly more magnesian and anorthitic than a primitive jotunite; in Hogstad, it is a primitive jotunite; and, in Koldal, an evolved jotunite. Given that plagioclase and orthopyroxene of the three intrusions display more differentiated compositions than the orthopyroxene and plagioclase megacryts of the enclosing anorthosites, it is suggested that the parent magmas of the small intrusions are residual melts after anorthosite formation which were entrained in the anorthositic diapir during its rise from lower crustal chambers.

Calculated densities of primitive jotunites (2.73–2.74 at FMQ, 0.15% H₂O, 200 ppm CO₂, 435 ppm F, 1150 °C, 3 kb) and evolved jotunites (2.75–2.76 at FMQ, 0.30% H₂O, 400 ppm CO₂, 870 ppm F, 1135 °C, 3 kb) demonstrate that they are much denser than the plagioclase of the surrounding anorthositic crystal mush (2.61–2.65). Efficient migration and draining of dense residual melts through the anorthositic crystal mush could have taken place along sloping floors (zones of lesser permeability in the mush), which occur along the margins of the rising anorthositic diapirs. This process takes into account the restricted occurrence of the mafic intrusions in the margins of the massif anorthosites. In a later stage, when the anorthosite was nearly consolidated, the residual melts were more differentiated (evolved jotunites) and could have been extracted into extensional fractures in the cooling and contracting anorthositic body in a similar way as aplitic dikes are emplaced in granitic plutons. As in the Rogaland Anorthositic Province, these dikes are much more abundant than the small mafic intrusions, collection and transport along dikes was probably more efficient than draining through the crystal mush.     

HT/LP metamorphic zoning in the eastern Voronezh Crystalline Massif: Age and parameters of metamorphism and its geodynamic environment

The Vorontsovskii terrane of the Eastern Sarmatian orogen underwent HT/LP metamorphism at temperatures of 430–750°C and pressures of 3–5 kbar. The TIMS monazite age of this metamorphism is 2067 ± 9 Ma and corresponds to the most probable age range (2050–2080 Ma) when large volumes of mafic and granitoid intrusions were emplaced. The time spans of the magmatic activity and metamorphic event are closely similar, which suggests that the melts could have served as sources of metamorphic heat. However, geological data on the relations between the metamorphic zones and magmatic bodies (the largest of the mafic, diorite, and granitoid intrusions are hosted in zones of low-temperature metamorphism) and the occurrence of relict metamorphic mineral assemblages and crystallization foliation in metapelite xenoliths in these intrusions suggest that the intrusions were emplaced after the metamorphism. The most probable reason for the HT/LP metamorphism was an increase in the heat flux in the course of viscous deformations and folding in the warm lithosphere of the young Paleoproterozoic Vorontsovskii terrane during collision processes.     

津巴布韦克拉通构造变形特征分析

根据津巴布韦克拉通北部地区的构造变质特征及矿物测年数据,表明太古界地壳基底形成和演变分为两个阶段.早期约在26.7亿年,Shamva-Bindura绿岩带内发育平行层理的剪切带,在长英质片麻岩核组成的大型推覆体之间形成洋壳和火山岛弧物质的叠瓦状堆积,结果是炽热长英质和铁镁质岩层的堆积使得地壳的演变暂时达到均衡状态,岩层堆积厚度达35 km.后期大约在26.0~26.2亿年炽热岩层的冷却收缩导致应变产生,沿垂直方向形成走向滑动断层带,为达到地热均衡,大面积的岩层发生熔蚀,并伴有花岗岩体底辟作用.岩层熔蚀和底辟作用产生二次临时变质作用和多种应变形态,也最终导致克拉通冷凝固化.     

良山丹霞地貌风景区的地质简况及其旅游开发价值

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