Initial Rocks Research Articles

The implications of fault zone transformation on aseismic creep: Example of the North Anatolian Fault, Turkey

AbstractAseismic creep is observed at surface along several segments of the North Anatolian right‐lateral active fault in Turkey, a major plate boundary between Eurasia and Anatolia. Identifying the mechanisms that control creep and their temporal and spatial change represents a major challenge for predicting the mechanical evolution of active faults, the interplay between creep and earthquakes, and the link between short‐term observations from geodesy and the long‐term fault zone evolution. We combine geological observations, laboratory analyses, and imaging techniques, shedding new light on the mechanism of fault creep along the North Anatolian Fault (NAF) and its time‐dependent change. A clear correlation is shown between shallow creep and near‐surface fault gouge composition: locked segments of the NAF are mostly composed of massive limestones without clay gouges, whereas creeping segments comprise clay gouges that contain low‐friction minerals. Such fault gouges appear to result from a progressive change of initial volcanic host rocks during their deformation. Anastomosing cleavage develops during the first stage of displacement, leading to layering, oblique at first and then subparallel to the fault, which accommodates part of the aseismic creep by pressure solution. Soluble minerals are dissolved, leading to passive concentration of phyllosilicates in the gouges where alteration transformations by fluid flow produce low friction minerals. At the same time damage zones are fractured and fractures are sealed by carbonates. As a result, these mineralogical and structural transformations weaken the gouge and strengthen the damage zone leading to the change from diffuse to localized seismic‐aseismic zones.

Pliocene granodioritic knoll with continental crust affinities discovered in the intra-oceanic Izu–Bonin–Mariana Arc: Syntectonic granitic crust formation during back-arc rifting

A widely held hypothesis is that modern continental crust of an intermediate (i.e. andesitic) bulk composition forms at intra-oceanic arcs through subduction zone magmatism. However, there is a critical paradox in this hypothesis: to date, the dominant granitic rocks discovered in these arcs are tonalite, rocks that are significantly depleted in incompatible (i.e. magma-preferred) elements and do not geochemically and petrographically represent those of the continents. Here we describe the discovery of a submarine knoll, the Daisan–West Sumisu Knoll, situated in the rear-arc region of the intra-oceanic Izu–Bonin–Mariana Arc. Remotely-operated vehicle surveys reveal that this knoll is made up entirely of a 2.6 million year old porphyritic to equigranular granodiorite intrusion with a geochemical signature typical of continental crust. We present a model of granodiorite magma formation that involves partial remelting of enriched mafic rear-arc crust during the initial phase of back-arc rifting, which is supported by the preservation of relic cores inherited from initial rear-arc source rocks within magmatic zircon crystals. The strong extensional tectonic regime at the time of intrusion may have allowed the granodioritic magma to be emplaced at an extremely shallow level, with later erosion of sediment and volcanic covers exposing the internal plutonic body. These findings suggest that rear-arc regions could be the potential sites of continental crust formation in intra-oceanic convergent margins.

The Blagodatnoe gold-sulfide deposit (Yenisei Ridge, Russia): the nature of geophysical anomalies and the succession and causes of formation of petrophysical zoning

Data on geophysical fields and petrophysical heterogeneity, parageneses and the thermodynamic conditions and age of their formation, and fluid inclusions were used for the genetic reconstruction of petrophysical zoning at the Blagodatnoe gold-sulfide deposit. Petrophysical associations of the preore and ore stages of the deposit formation are clearly reflected in anomalies of the magnetic and natural electric fields and the aureoles of radioactive elements. At the early preore stage (752Ma), reduced solutions with high activity of K, enriched in U, Th, and, probably, Au, were supplied to intensely foliated tectonic zones. Their interaction with initial metasedimentary rocks gave rise to contiguous zones of quartz–muscovite and chlorite metasomatic rocks. Accompanying graphitization led to a high electrochemical activity of the metasomatic rocks, which generated anomalies of up to –300mV in the natural electric field; the most intensely carbonized zones became enriched with U (up to 6.5×10–4%) and, probably, Au. The quartz–muscovite metasomatic rocks accumulated Th and K (up to 29×10–4% and 4%, respectively), whereas the chlorite metasomatic rocks accumulated rock-forming elements (particularly Fe), which led to the compaction of these rocks and the acceleration due to gravity in local positive anomalies. The nonmagnetic character of the fresh pre-ore metasomatic rocks suggests the predominantly pyritic composition of early sulfides. At the ore stage (698Ma), the minerals were deposited from H2O–CO2–As–S solutions at 560 to 315°C. The activity of these solutions caused a redistribution of radioactive elements and a high petrophysical differentiation of the ore-bearing structure. The amplitudes of the anomalies above this structure vary from 500 to 80 nT in the magnetic field and from –130 to +10mV in the natural electric field. It has been found that the hydrothermal fluid hardly affected the polarization properties of graphitized rocks at the maximum temperatures but caused an intense removal of U and the development of magnetic pyrrhotite after pyrite. The temperature decrease in the mineral-forming system was favorable for the formation of siderite. Carbonaceous schists which experienced carbonatization lost their electrochemical activity. The binding of carbon dioxide in the solid phase influenced the migration capability of trace elements and their zonal distribution. With this evolution of the solution, Th accumulated at the lower levels of the mineralized zone, whereas the upper levels of the deposit became enriched with U. Productive gold–arsenopyrite–pyrite–pyrrhotite paragenesis with anomalous magnetic susceptibility evolved at the ore stage. The late galena–sphalerite–chalcopyrite paragenesis (365Ma) was of strictly local occurrence and reduced the magnetic susceptibility of ores.

Onion-like carbon in impact diamonds from the Popigai astrobleme

Phase composition and nanostructure of layered and irregular massive impact diamond grains from the Popigai astrobleme have been investigated by Raman spectroscopy and high-resolution electron microscopy and the conditions of phase transformations are discussed. Several coexisting carbon phases forming tight aggregates have been found, including cubic and hexagonal diamond polymorphs, graphite, amorphous carbon, fullerene-like/onion-like carbon. The latter is described within impact diamonds for the first time. It is proposed that the formation of onion-like carbon, in both layered and massive impact diamond grains, is connected with high-pressure graphite transformation or post-pressure stress cooling which causes the partial back transformation of diamond nanocrystallites to sp2 carbon. However, a possible relict origin of the fullerene-like carbon from the impacted initial sedimentary rocks with fullerenes and fullerene-like substances, like shungite or coal, can not be excluded. The difference between micro- and nanostructures of layered and irregular massive impact grains is presented.

Geochemistry of granitization and magmatic replacement of basic volcanic rocks in the contact aureole of the Yurchik gabbronorite intrusion, Ganal Range, Kamchatka

Geological and geochemical data indicate that the formation of the granulite-like rocks in the contact aureole of the Yurchik gabbronorite intrusion of the Ganal Range, Kamchatka, was caused by the contact metamorphism, metasomatism, and local melting of the initial volcanosedimentary rocks of the Vakhtalka Sequence of the Ganal Group. The temperature in the inner part of the aureole reached 700–800°C and caused the transformation of the basic volcanic rocks of the sequence into two pyroxene-plagioclase, clinopyroxene-amphibole-plagioclase, and amphibole-plagioclase hornfelses, while sedimentary rocks were converted into garnet-biotite ± cordierite hornfelses. The hornfelsed basic volcanic rocks were locally subjected to metasomatic alteration and magmatic replacement with formation of biotite-orthopyroxene-plagioclase metasomatic bodies containing biotite-orthopyroxene-plagioclase ± garnet veinlets and aggregates. During these processes, sedimentary interlayers were converted into garnet enderbites at 700–800°C and 3.2–4.8 kbar. The comparison of the chemical composition of basic volcanic rocks of the Vakhtalka Sequence and their transformation products indicates that the metasomatic alteration and magmatic replacement correspond to siliceous-alkaline metasomatism (granitization) and cause subsequent and uneven influx of SiO2, Al2O3, Na2O, K2O, Rb, Ba, Zr, Nb, and Cl and removal of Fe, Mg, Mn, Ca, Cr, Co, Ti, Y, and S. REE data on basic metavolcanic rocks, hornfelses, and metasomatites suggest that the processes of hornfelsation, metasomatism, and magmatic replacement of the initial volcanic rocks were accompanied by significant increase in LREE and slight decrease in HREE. The Sr and Nd isotope study of the rocks in the aureole showed that the initial basic volcanic rocks of the Vakhtalka Sequence are isotopically close to both mature island arc tholeiites and mid-ocean ridge basalts. The metasomatic alteration and magmatic replacement of volcanic rocks in the aureole lead to the decrease of 143Nd/144Nd and increase of 87Sr/86Sr approximately parallel to mantle array. Pb isotopic ratios in the studied rocks become more radiogenic from initial metavolcanic rocks to metasomatites.

The noble metal distribution in the black shales of the Degdekan gold deposit in Northeast Russia

The mineral and chemical composition of the carbon-bearing rocks of the Late Permian Pionerskaya Formation containing the Degdekan gold deposit has been studied. The bulk contents of Au, Ag, Pt, and Pd in the black shales and their light, sulfide, and electromagnetic fractions were determined by electrothermal atomization. The mineral composition and the phase analysis of the rocks were studied using a scanning electron microscope. Gold is present as fine xenomorphic grains of high fineness with an Fe admixture of up to 4 at %, as well as intergrowths of kustelite and electrum. The Au and Pt contents in the black shales and ores vary in a wide range (g/t): Au 0.01–13.12, Pt 0.001–1.34. The highest Au contents (up to 1748 g/t) were noted in the sulfide fraction. The Pt-bearing phases were not found, whereas a Pt content of about 0.61 wt % was determined using an electron microscope in a carbonaceous matrix. The initial rocks have a steady and low Pt content (less than 0.007 g/t). A stable even Au distribution in the studied rocks was established within 1.14–2.46 g/t. The chemical analysis of the soluble fraction of the carbonaceous matter extracted from the black shales showed the presence of Au 0.375, Ag 3.68, Pt 0.147, and Pd 0.052 g/t. It has been concluded that the carbon-bearing rocks of the Pionerskaya Formation play a resource role in the accumulation of noble metals, whereas economic concentrations of the latters are formed in the course of the superimposed metamorphic-hydrothermal processes.

Sorption treatment of radioactively contaminated natural waters to remove 90Sr and 137Cs using bauxites and products of their processing

Sorption treatment of radioactively contaminated natural waters to remove 90Sr and 137Cs using an alumina production waste, red bauxite slimes, is considered. In radiostrontium sorption, they surpass both the initial bauxite ore (Boksitogorsk deposit) and adjoin rocks (limestone, dolomite, bentonite clay), and owing to porous structure they act as efficient filtering material.

Metasomatic zoning of subcratonic lithosphere in Siberia: physicochemical modeling

Nonisothermal equilibrium physicochemical dynamics has been numerically modeled to estimate the effect of reduced asthenosphere fluids on continental lithosphere profiles beneath the Siberian Platform (SP). When the over-asthenosphere continental mantle is metasomatically changed by reduced magmatic fluids, the following sequence of zones forms: (1) zone where initial rocks are intensively sublimated and depleted by most petrogenic components; the restite in this case becomes carbonated, salinated and graphitized; (2) zone of Si and Fe enrichment and carbon deposition in initial rocks depleted in Na, K, P, Mn; (3) zone of diamond-bearing lherzolites enriched with Na; (4) zone of hydrated rocks enriched with K; (5) zone of hydrated rocks not enriched with petrogenic components. Zone 1 can be responsible for the formation of kimberlite melts, zones 3 and 4 can be substrates of alkaline magma melting, and zone 5 can be the source of mafic tholeiitic magma.

Changing factors of the coasts of Lake Peipsi, North-Eastern Europe

Lake Peipsi is the fourth largest inland body of water in Europe. Its geological history was controlled by glacial retreat, climate and neotectonic movements. The lake depression has a key position between Russia, Estonia, Latvia and Finland for solving many principal problems related to the deglaciation history and neotectonical movements. The northern part of the meridionally elongated basin is now rising, the central part is stable and the southern part is sinking. These processes caused and are still causing the water to spread from north to south. The resultant flooding of large areas poses many ecological and social problems. Owing to considerable water-level fluctuations, exceeding 3 m, both the surface area and the volume of the lake vary greatly. High-water levels cause remarkable damage on shores and pose a serious threat to the buildings and roads in the immediate vicinity of the lake. Extensive changes on the coast are also due to the action of hummocky lake ice. Lake Peipsi has a variety of shores. Shore types are determined by the coastal topography and lithology of the initial rocks. Shoreline changes and beach erosion have been caused by several geological and hydrometeorological factors (tectonic movements, hardness of rocks, wind regime, wave action, ice-push, water-level fluctuations) and human activity. Warmer winters and concurrent increased cyclonic activity have had noticeable impacts on the lake coasts during last decades.

Enigmatic sedimentary–volcanic successions in the central European Variscides: a Cambrian/Early Ordovician age for the Wojcieszów Limestone (Kaczawa Mountains, SW Poland) indicated by SHRIMP dating of volcanic zircons

AbstractMetamorphosed volcanic and sedimentary successions in the central European Variscides are, in many areas, poorly biostratigraphically constrained, making palaeotectonic interpretations uncertain. In such instances, geochronological data are crucial. Sensitive high resolution ion microprobe (SHRIMP) dating of volcanic zircons from a quartz–white mica schist (interpreted as deformed metavolcaniclastic/epiclastic rock) within the stratigraphically controversial Wojcieszów Limestone of the Kaczawa Mountains (Sudetes, SW Poland), near to the eastern termination of the European Variscides, has yielded an age of 498 ± 5 Ma (2σ error), corresponding to late Cambrian to early Ordovician magmatism in that area and constraining the depositional age of the limestones. The new SHRIMP data are not consistent with the recent revision of the age of the Wojcieszów Limestone based on Foraminifera findings that ascribed them to a Late Ordovician—Silurian or even younger interval. They are though, consistent with sparse macrofossil data and strongly support earlier interpretations of the lower part of the Kaczawa Mountains succession as a Cambrian–Early Ordovician extensional basin‐fill with associated initial rift volcanic rocks, likely emplaced during the breakup of Gondwana. Copyright © 2008 John Wiley & Sons, Ltd.

Petrologic and REE Geochemical Characters of Burnt Rocks

Abstract: The study of burnt rocks is beneficial to the discussion on the tectonic movement, paleoclimate and paleogeography that coal seams are subjected to after they were formed. In order to obtain the basic data on the features of the burnt rocks, a systematic study of petrology and REE geochemistry on burnt rocks in Shenmu, Northern Shaanxi Province has been done, using the methods of SEM, EDS, susceptibility measurements and ICP‐MS. The burnt rocks are divided into two series in the section: the melted rocks and the baked rocks. SEM and EDS analyses reveal that all the minerals show burnt and melted traces, and there are no clay minerals except illite found in the burnt rocks. Susceptibility measurements reveal that the burnt rocks have abnormally high susceptibility values, whereas a geochemical analysis shows that the REE distribution pattern of burnt rocks is similar to that of sedimentary rocks (initial rocks). In the longitudinal section, with increasing degree of burning (from baked rocks to melted rocks), the ΣE gradually decreases, and the total REE of melted rocks is obviously lower than that of baked rocks. Besides, the melted rocks show apparent negative Ce anomalies, while the baked rocks show no anomaly of Ce, and sometimes even show positive anomalies.

Nutrient Release from Weathering of Purplish Rocks in the Sichuan Basin, China

Purplish soils having high fertility with mineral nutrients inherited from the parent rock are widely distributed in the hills along the Yangtze River, especially in the Sichuan Basin. Pot and field weathering experiments were conducted to mimic rock weathering and nutrient release processes in order to better understand soil fertility and nutrient compensation. Three types of purplish rock formations formed in the Jurassic period, Shaximiao (J 2s), Suining (J 3s), and Penglaizhen (J 3p), as well as one type formed in the Cretaceous period, the Chengqiangyan group (K 1c), were used in this study. Results showed that the soil formation rate was in the range from 11.2 to 19.6 mm every year, and rock weathering was in the order of J 3s > J 3p > J 2s > K 1c. Because more rock surface was exposed to sunlight and rainfall in field conditions, pot weathering was slower than field weathering. Nutrient release rates increased with rock weathering and was in the order similar to that of rock weathering: J 3p > J 3s > J 2s > K 1c. Potassium release was the most important in all rocks; after 2 years of weathering, 19.4% to 46.9% of K was released from the initial parent rocks, which suggested that K release from weathering could meet most of the crop K requirement in purplish soils. Thus, rapid release of nutrients from weathering of purplish rocks was key to nutrient replenishment and fertility of purplish soils.

Prolonged KREEP magmatism on the Moon indicated by the youngest dated lunar igneous rock

Primordial solidification of the Moon (or its uppermost layer) resulted in the formation of a variety of rock types that subsequently melted and mixed to produce the compositional diversity observed in the lunar sample suite. The initial rocks to crystallize from this Moon-wide molten layer (the magma ocean) contained olivine and pyroxene and were compositionally less evolved than the plagioclase-rich rocks that followed. The last stage of crystallization, representing the last few per cent of the magma ocean, produced materials that are strongly enriched in incompatible elements including potassium (K), the rare earth elements (REE) and phosphorus (P)--termed KREEP. The decay of radioactive elements in KREEP, such as uranium and thorium, is generally thought to provide the thermal energy necessary for more recent lunar magmatism. The ages of KREEP-rich samples are, however, confined to the earliest periods of lunar magmatism between 3.8 and 4.6 billion years (Gyr) ago, providing no physical evidence that KREEP is directly involved in more recent lunar magmatism. But here we present evidence that KREEP magmatism extended for an additional 1 Gyr, based on analyses of the youngest dated lunar sample.

Fluorine mineralisation from burning coal spoil-heaps in the Russian Urals

Anhydrous iron, aluminum and fluorine-rich paralavas were found in the burned spoil-heaps of the Chelyabinsk coal basin, Russia. The rocks contain tridymite, anorthite, ferroan fluorine-bearing cordierite, fluorine-bearing mullite, periclase, fluorapatite, micas of the F-biotite–F-phlogopite series, fluortopaz, fluorite, and sellaite. The fluorine-rich minerals formed as a result of local thermal reactions of sedimentary carbonates and silicates with gaseous fluorine. During coal combustion fluorine concentrates in the annealed ankeritic marls where the increase of F is hundreds of times over its concentration in the initial sedimentary rocks. The formation of MgF2 and CaF2 promotes local melting at relatively low temperatures (T < 1000 °C) with the residuum consisting of two immiscible liquids. One crystallises as the fluorides, the other as fluorine-substituted analogues of the hydrosilicates, which under the extremely dry conditions, produce minerals containing extremely high F-contents.

Timing of late Cenozoic tidewater glaciation in the far North Pacific

The onset of late Cenozoic glacial events in the far North Pacific Ocean is recorded by ice-rafted debris in the Yakataga Formation in the Gulf of Alaska. The dating of these events is controversial. Ages based on molluscs suggest that initial late Cenozoic tidewater glaciation occurred in the early middle Miocene (15-16 Ma). Previous work on planktic foraminifera indicates that this event is no older than late Miocene, probably latest Miocene (5-6 Ma). Resolution of this problem is important because the Yakataga Formation is the thickest, most complete, and best exposed repository of late Cenozoic glaciomarine rocks in the northern hemisphere. Investigation of this record has broader implications for global temperature gradients, paleoceanographic development of the far North Pacific, and northern hemisphere glacial history. New planktic foraminiferal data from onshore and offshore Yakataga Formation sections in the Gulf of Alaska are compared to a regional depositional and chronostratigraphic framework in order to test whether the Yakataga record is anomalous or consistent with the paleoclimatic record of the North Pacific. This comparison reconfirms that initial glaciomarine rocks of the Yakataga Formation are no older than late Miocene. New paleomagnetic data suggest that the base of the Yakataga Formation is within the lower Gilbert polarity chron, consistent with the planktic foraminiferal dating and K/Ar dates on glauconite. Correlation of the Yakataga record to key offshore sections in the far North Pacific (Deep Sea Drilling Project [DSDP] Sites 178, 183, 192) provides additional insights into the paleoclimatic significance of these rocks. Uplift of the Alaska coastal ranges, a necessary prelude to alpine glaciation, occurred during late Miocene time. Diatom biofacies (DSDP Sites 183, 192) indicate warm middle Miocene conditions, climatic deterioration during the late middle and early late Miocene time, and initial tidewater glaciation between 5.0 and 6.7 Ma. A relatively warm mid-Pliocene interval succeeded this initial glaciation, which was, in turn, followed by hemisphere-scale glaciation beginning at ∼2.5 Ma. This second phase of Yakataga glaciation may have begun as early as 3.0-3.5 Ma. Glaciomarine rocks in the basal Yakataga Formation mark an important climatic event in the North Pacific Ocean. They are latest Miocene in age, consistent with regional climatic information, and not a middle Miocene climatic anomaly.

Fluid control of charnockitization

The reactions Bt + Qtz = Opx + Kfs + H 2O, Bt 1 + Qtz + (K 2O) = Or + Bt 2 +( n H 2O), Mg-Ts(Opx) + Otz + (K 2O) = Or + En, Pl 1 + (K 2O) = Pl 2 + Or + (Na 2O) and Prg + En + Qtz + (K 2O) = Or + Ed recorded in unique rock textures and mineral compositions from gneissic complexes of some granulite facies terrains have been studied. It is shown that chemical potentials of the perfectly mobile CO 2, H 2O and K 2O in a metamorphic fluid govern the charnockitization process along the retrograde P- T paths typical of granulite facies rocks. The sensors indicating the contribution of thermodynamic parameters to the process are discussed. In many complexes charnockitization took place at a a fl H 2O similar to or even higher than that for initial gneisses. The origin of arrested charnockites in the Precambrian gneissic complexes is considered to be CO 2/1bH 2O fluid/gneiss interaction, which causes migration of the majority of the basic components (MgO, FeO, CaO, TiO 2 etc.) from initial metamorphic rocks toward a transition zone; the scale of the migration depends strongly on the intensity of the interaction. The migration of the elements from a given rock volume propagates the shift of its bulk composition toward the charnockite (Kfs + Qtz + Fs eutectic and subsequent melting. This model may explain the existence of zoned metabasite-enderbite-charnockite complexes of Precambrian granulite facies terrains. The arrested charnockites are a small-scale model for the formation of these complexes.

Ultrametamorphism within the amphibolite-granulite transition zone, upper Aldan River, Siberia

The sequence of metamorphic events and granite formation in the Nimnyr block of the Aldan shield is described. Metamorphic assemblages, polymigmatites and granite varieties developed in the upper Aldan River are the results of two tectonothermal events corresponding to the second, so called “Aldan”, cycle (∼3.3-3.1 Ga) and the fourth, or “Ungra”, cycle (2 Ga) occurring during the five-cycle history of the Aldan shield. For the second cycle a prograde sequence from the amphibolite facies through a transition zone to the biotite-garnet-cordierite-orthoclase subfacies and then the garnet-cordierite-hypersthene-orthoclase subfacies of the granulite facies is established. Temperatures and pressures are in the range of 680–850°C and 5–7 kbar (biotite-garnet-cordierite geothermobarometer data). Metamorphism of the Ungra cycle is also inhomogeneous, and similar to that of the second cycle. Enderbite- and tonalite-gneisses, charnockite- and granite-gneisses ( γ 1), stromatic and stockwork migmatites with leucosomes γ 2 and γ 3, respectively, anatectic and removed charnockites and granites were formed during the Aldan cycle. Migmatites with leucosomes γ 4 and removed charnockites ( γ 5) are related to the Ungra cycle. The mineral assemblages of granitoids and migmatite leucosomes vary systematically with the metamorphic assemblages. Comparison of the simultaneous leucosomes of the different metamorphic zones suggests a decrease in Si and increase in Ti, Ca, Mg, Fe from amphibolite to granulite facies. The biotite compositions and lattice ordering of K-feldspar also change regularly. Three stages are recognized in the sequence of granite formation related to the second cycle: (1) replacement of initial rocks by granitoids, (2) leucosome formation in stromatic and stockwork migmatites, and (3) mobilization of granite materials. These stages are also recognized in the fourth cycle. We consider the predominant processes of granite formation: metasomatism, anatexis together with replacement, and rheomorphism. Densities of CO 2-inclusions suggest that a decompressional process forms all the more or less displaced granites, whereas conditions generating granite- and charnockite-gneisses were closer to those of metamorphism. During the formation of the metamorphic zone, pressure decreased by at least 3 kbar., corresponding to an isothermal uplift up to 10–11 km.

Initiation of the neomagmatism in East Anatolia

In Eastern Anatolia, the neotectonic episode commenced with the complete elemination of the neo-Tethyan ocean floor as a result of collision between Arabia and Eurasia during Early Miocene time. After the collision, continuing convergence initiated a new tectonic regime and coeval magmatic activity. The post-collisional convergence caused crustal shortening and thickening as evidenced by regionwide development of E-W trending folds and thrusts, and conjugate strike-slip faults. The related volcanic activity has progressed in three cycles. In the initial cycle weakly-alkaline volcanic rocks were produced that are known as the Solhan volcanics. They outcrop at the western end of an E-W striking neotectonic depression called the Muş basin, which constitutes the western prolongation of the Lake Van basin. The basin is a thrust-bounded depression that developed out of an original syncline by the disruption of its limbs. The lavas of the first cycle reached the surface along N-S aligned extensional structures. In the second volcanic cycle a widespread K-type Ca volcanism occurred through crustal contribution during the Late Miocene-early Pliocene time when the continuing north-south convergence caused substantial shortening and thickening of the continental crust. The last volcanic cycle produced a thick volcanic cover consisting of rocks of alkaline affinity during the Pleistocene and the Quaternary when the thickened crust began to accommodate the ongoing north-south shortening by east-west extension after the formation of the East and North Anatolian transform faults.

Petrology of the anorthositization zones in the Precambrian of the Aldan-Stanovoy Shield

Here we report results on the anorthositization zones occurring in Precambrian formations in the Aldan-Stanovoy Shield; by anorthositization we mean a set of processes in which the initial rocks are altered in such a way that the composition and structure approach those of anorthosites. Barth (1956) introduced the term anorthositization when he pointed out that metasomatic anorthositization may play the decisive part in the production of anorthosites. Michot (1955) showed that metasomatic anorthositization had occurred in the Precambrian basites of Norway. Subsequently, anorthositization zones were identified and examined by Hiitanen in 1963 in the Precambrian basement of the Cordillera, by Zhdanov in 1968 in the granulite series in the Kola Peninsula, and by Bogdanova in the basement of the Russian sheet. Anorthositization zones within the Aldan-Stanovoy Shield were first identified and described by Moskhin (1962), who later examined them in more detail in collaboration with Dagelayskaya (Moshkin and Dagelayskaya, 1964; Moshkin et a l . , 1967; Dagelayskaya, 1975). Bazhenova (1967) also examined these zones west of the region.

METASOMATIC ZONALITY AND GENESIS OF SAPPHIRINE-BEARING ROCKS IN THE BUG REGION

Precambrian sapphirine-bearing rocks were discovered in the core of a borehole on the River Bug in the zone of the metasomatic changes of the pyroxene body. Their origin is a particular case of regional granitization and takes place in connection with the formation of charnockite. The area of modified pyroxenite, 1.7m thick in the core, has a symmetrical zonal structure and is subdivided into six subzones (in the direction to the center). I. Pyroxenite, serpentinized, carbonatized pyroxenite (initial rocks); II. Diopside-phlogopite-anorthite; III. Corundum-anorthite phlogopite; IV. Sapphirine -corundum- microcline -biotite; V. Corundum-sillimanite-sapphirine-microcline; VI. Sillimanite-sapphirine-corundum-prismatine-microcline. The minerals are not unstable together, some are replaced by others with a general tendency to forming a microcline rock. --Author's English Summary.