Intrusive Masses Research Articles

Study of Petrogenesis and Tectonomagmatic Environments of Eocene Calc-Alkaline Volcanic Units in South and Southeast of Beroni Village in Southwest Ardestan (Isfahan)

The study area is located in south and southeast of Beroni Village. It contains volcanic rocks including andesitic-basaltic, pyroxene-bearing andesite, andesite, dacite, rhyodacite, rhyolites and Eocene-Oligocene ignimbrites. The volcanic rocks are cut by an intrusive mass with great spreading in the region. According to lithological studies, the calc-alkaline magmas in continental margin arcs are comprised of mantle and fluid crust. The basic elements in the volcanic rocks were studied in terms of petrological indices. According to the results, the metaluminous rocks underwent crustal contamination. Due to chemical reactions between the hydrothermal solution and volcanic host rocks, hydrothermal solutions in volcanic rocks penetrate the surrounding silica rocks and thus some elements such as zinc and barium diffuse in the rocks. In addition, calcium, magnesium and iron have been drawn inwards from the surrounding rocks causing lateral segregation.

Application of U-Th-Pb Phosphate Geochronology to Young Orogenic Gold Deposits: New Age Constraints on the Formation of the Grass Valley Gold District, Sierra Nevada Foothills Province, California

The Grass Valley orogenic gold district in the Sierra Nevada foothills province, central California, the largest historic gold producer of the North American Cordillera, comprises both steeply dipping E-W veins located along lithologic contacts in accreted ca. 300 and 200 Ma oceanic rocks and shallowly dipping N-S veins hosted by the Grass Valley granodiorite; the latter have yielded about 70% of the 13 million ounces of historic lode gold production in the district. The oceanic host rocks were accreted to the western margin of North America between 200 and 170 Ma, metamorphosed to greenschist and amphibolite facies, and uplifted between 175 and 160 Ma. Large-scale magmatism in the Sierra Nevada occurred between 170 to 140 Ma and 120 to 80 Ma, with the Grass Valley granodiorite being emplaced during the older episode of magmatism. Uranium-lead isotope dating of hydrothermal xenotime yielded the first absolute age of 162 ± 5 Ma for the economically more significant N-S veins. The vein-hosted xenotime, as well as associated monazite, are unequivocally of hydrothermal origin as indicated by textural and chemical characteristics, including grain shape, lack of truncated growth banding, lack of an Eu anomaly, and low U and Th concentrations. Furthermore, the crack-seal texture of the veins, with abundant wall-rock slivers, suggests their formation as a result of episodic fluid flow possibly related to reoccurring seismic events, rather than a period of fluid exsolution from an evolving magma. The N-S veins are temporally distinct from a younger 153 to 151 Ma gold event that was previously reported for the E-W veins. Overlapping U-Pb zircon (159.9 ± 2.2 Ma) and 40Ar/39Ar biotite and hornblende (159.7 ± 0.6–161.9 ± 1.4 Ma) ages and geothermobarometric calculations indicate that the Grass Valley granodiorite was emplaced at ca. 160 Ma at elevated temperatures (~800°C) within approximately 3 km of the paleosurface and rapidly cooled to the ambient temperature of the surrounding country rocks (<300°C). The age of the granodiorite is indistinguishable from that of the N-S veins, as recorded by the U-Pb age of xenotime in those veins. Consequently, the N-S veins must have formed between 162 and 157 Ma, the maximum permissive age of magma emplacement and the youngest permissive xenotime U-Pb age, respectively, during an E- to ENE-directed compressional regime. The geochemistry of the Grass Valley granodiorite is consistent with it being the product of arc magmatism. It served as a receptive host for mineralization, but it is has no direct genetic relationship to gold mineralization. Initial uplift of the intrusive mass correlates with the initial voluminous fluid flow event and vein formation at depths of no greater than 3 km. The E-W gold-bearing veins hosted within greenschist-facies country rocks adjacent to the intrusion formed during a second hydrothermal event 5 to 10 million years later than the magmatism and were contemporaneous with a shift to transtensional deformation denoted by sinistral strike-slip faulting.

江西赣县龙潭下地区物、化探及地质特征与钨多金属矿找矿远景分析

龙潭下地区位于南岭钨锡多金属成矿带东部，寒武纪地层及燕山早期侵入岩体分布广，成矿物质丰富，断裂构造发育，成矿地质条件优越。地表弱磁正异常主要由断裂及沿断裂裂隙充填铁磁质(如含钨石英脉等)和花岗岩等引起，反演推断异常源埋深较大，成矿空间大。化探异常元素组成复杂，各元素异常显示W Sn Mo-Cu Pb Zn-Au Ag Sb高–中、低温热液型组合特征，W、Sn、Mo等主要成矿元素浓集分带明显、富集系数较高，有利于矿床形成。经查证，矿化主要分布在断裂带以及岩体与变质岩内外接触带附近，已知矿点与各元素异常中心高度吻合，找矿潜力。 Longtanxia district is located in the eastern Nanling tungsten tin polymetallic metallogenic belt, the Cambrian strata and the early Yanshan intrusive mass distribution is wide, the ore-forming material is rich, fracture structure, the metallogenic geological condition is superior. The weak magnetic anomaly is mainly composed of fracture surface and the fracture fissure filling ferro-magnetics (such as containing tungsten quartz vein) and granite, etc., the inversion infers that the buried depth of the anomalous source is bigger; the ore-forming space is large. Complex geochem-ical anomaly elements, each element anomalies W Sn Mo, Cu Pb zinc-Au Ag, Sb in the high and low temperature hydrothermal type combination characteristic, W, Sn, Mo etc. of the main ore-forming elements concentration zoning obvious, enrichment coefficient is higher, is advantageous to the ore deposit formation. After verification, the mineralizations are mainly distributed in fault zone and rock and metamorphic rock, near the inner and outer contact zone of known mines with each element anomaly center height, prospecting potential.

Geochemistry of Late Permian picritic porphyries and associated Pingchuan iron ores, Emeishan Large Igneous Province, Southwest China: Constraints on petrogenesis and iron sources

The Late Permian picritic porphyries are spatially and temporally associated with the Pingchuan iron oxide deposit in Emeishan Large Igneous Province (ELIP), southwestern China. The Pingchuan iron deposit carries an estimated ore reserve of 40Mt at ~60wt.% Fe. The iron oxide deposit is associated with an ancient volcanic edifice, which is located at the intersection of the NW- and NE-trending deep faults. The iron ore bodies are intimately associated with the intrusive masses of picritic porphyry, occurring mainly along the contact zone between the picritic porphyries and the Early Permian limestone. The most important ore types are massive and brecciated ores which together make up 90vol.% of the deposit. The massive type ores generally occur as dykes consisting predominantly of magnetite with minor calcite and apatite. The picritic porphyries are characterized by a marked range of SiO2 (37.12–47.39wt.%) and MgO (19.22–29.08wt.%), but show only a minor variation in Na2O+K2O (0.12–1.58wt.%) and TiO2 (0.64–1.44wt.%). The total concentration of rare earth elements (REE) is relatively low (21–83ppm), and shows moderate enrichment in light rare earth element (LREE; [La/Yb]N=3.01–3.63) compared to the HREE. The primitive-mantle-normalized patterns of the rocks are comparable to those of ocean island basalt, possibly indicating a plume source, which is considered to have generated the ELIP. Petrographic observation suggests that the Pingchuan picritic porphyries are virtually primary picritic magma with excess olivine. The primary magma possibly experienced fractional crystallization of Cr-spinel+olivine+clinopyroxene during the magma chamber process, but has not been significantly crustal contaminated. Moreover, the primitive magma was possibly produced by partial melting of the garnet-facies peridotite. In combination with the available information including field observation and geochemical studies, e.g. fluid inclusions and stable isotope data, we infer that the deposit is hydrothermal in origin and the ore-forming fluids are predominantly primary magma-derived fluids which had been released from the picritic porphyry during the emplacement at near the surface and mixed with fluids from country rocks. We envisage that the Pingchuan iron deposit was formed by the shallow emplacement, the aqueous chloride alteration and carbonate solutions and iron precipitation as a continuous process, and thus could be classified as a subvolcanic-hosted iron ore type.

Hornblende gabbros of Wadi Az Zarib area, Central Eastern Desert, Egypt: opaque mineralogy, geochemistry, tectonic setting, and petrogenesis

A small intrusive fresh gabbroic mass intrudes the Neoproterozoic metasediments and Dokhan volcanics of Wadi Az Zarib area, Central Eastern Desert. It is composed of hornblende gabbros and leuco-hornblende gabbros. Their petrography, opaque mineralogy, and geochemistry are addressed to elucidate their tectonic setting and petrogenesis. They represent a subduction-related calc–alkaline magma that evolved in an island arc setting. In terms of maturity, the supposed arc represents an intermediate stage between continental arc and active continental margin. Thermobarometry and physical–chemical data of the parent magma as deduced from compositions of amphiboles, biotite, and plagioclase indicate crystallization temperatures of 931–825 °C at pressures of 6.16–4.01 kbar and H2Omelt of 6.4–5.2 wt%. Data, as presented, argue in favor of fractional crystallization mechanism to be accounted to the present suite to interpret the observed variations. The evolution of the suite from hornblende gabbros to leuco-hornblende gabbros was accompanied by decreasing of MgO, CaO, Cr, and Ni with simultaneous increasing of Al2O3, TiO2, Na2O, K2O, Ba, Rb, Sr, La, and Ce. Residuals calculated during mass balance fractional crystallization modeling suggest that brown and green hornblendes are the main fractionated phases which derived the melt composition towards the leuco-hornblende gabbros.

Stream Sediments Geochemical Exploration in the Northwestern Part of Wadi Allaqi Area, South Eastern Desert, Egypt

Geochemical stream sediments survey was conducted in the northwestern part of Wadi Allaqi area, Eastern Desert, Egypt. The area comprises Precambrian metasediments, intermediate metavolcanics, gabbro, and serpentinites, with intrusive masses of granites and quartz-porphyry and invaded by several quartz veins. The −1.0-mm size fraction is analyzed for As, Cu, S, Mo, Pb, Zn, Co, Ni, Rb, Ba, Sr, Nb, V, U, Th, Cr, Zr, La, Ce, Nd, and Y. The geochemical survey is supported by heavy minerals study in the −0.125 + 0.0625-mm fraction. The geochemical data were statistically investigated using Q-mode cluster and R-mode factor analyses as well as the enrichment factor. Factors 1 (Zr, Nb, Nd, La, and Y), 2 (V, Sr, and Zn), and 4 (Ba and Rb) are mainly controlled by the lithological characters of the rocks hosting Au-sulfide mineralizations and their accompanied hydrothermal alteration zones. In the mineralization Factor 3 (Cu, S, As, Ce, and Mo), arsenic, Cu, S, and Mo are direct indicators, while Ce is indirect one for the Au-sulfide mineralizations. The Cu–S–As–Mo association with Pb and Zn anomalies in the stream sediments draining the quartz-porphyry point to its porphyry copper mineralization. Cobalt and Ni (Factor 5) are pathfinders for the Fe- and Cu-sulfides, whereas Zn and Pb of Factor 8 are additional pathfinders for the Au-sulfide mineralizations. The southern stream sediments having high U/Th ratios with U–Mo association and draining granites traversed by pegmatites, as well as the stream sediments draining Um Garayat area and the quartz-porphyry stock with high abundance of monazite, zircon, epidote, sphene, and ilmenite, could signify sources of U and Th (Factor 7). Two watershed areas have distinct enrichment factors for arsenic suggesting unexplored extensions of Au-sulfide mineralization linked to the Allaqi shear-zone. The enrichment of the mineralization Factor 3 in the drainage system is mainly controlled by the prevailed mechanical dispersion for the hosting heavy minerals in such arid region with minor role of hydromorphic dispersion. The chemistry and mineralogy of the stream sediments are evidently allied to the drained bedrocks and their hosted mineralizations that signify a promising area for detailed exploration.

Three-dimensional modeling of borehole data cored from engineering rock mass

Vast data from the drilling and geophysical prospecting are reliable original information to describe the space state of engineering rock mass, and one of the main difficulties in three-dimensional (3D) modeling of engineering rock mass is the processing of the primary data. From the view-point of 3D modeling, the engineering rock masses are classified as four basic types according to their geometric characteristics of geologic structure: (1) continuum rock mass; (2) discontinuous rock mass; (3) overturned fold rock mass and (4) intrusive rock mass. Because drilling data are very important to describe the characters of multi-scale of the spatial data for rock mass, the rule of how to process drilling data is developed to help appropriately display them in the viewpoint of 3D space. According to the characteristics of rock mass layers, the processing method of drilling data for 3D modeling of engineering rock masses, along with the layer thicknesses, is also proposed, including the evaluation rules and the extensive direction for original borehole data. By this method, the typical 3D data modeled is completed and the model form of the engineering rock mass is developed. By this example, it is finally verified that the method presented is successful and feasible to process 3D engineering rock mass.

Application of Sr isotopes to geochemical mapping and provenance analysis: The case of Aichi Prefecture, central Japan

Geochemical maps expressing areal distributions of chemical elements in the earth’s land surface have been published in several countries in relation to various global environment issues. The authors have applied a radiogenic isotope ratio, 87Sr/ 86Sr, to geochemical mapping in order to understand the geological origin, transportation and dispersion system of chemical elements in the earth’s land surface. The Sr isotope ratio is a useful tracer for distinguishing the geological origin of surficial deposits, especially in areas where surface exposure of bedrocks is low, because it is not significantly altered by the processes of weathering and transportation. Most bedrocks in the Japanese islands are covered by plants, soils and urban areas. In this study, 142 of 1219 stream sediments (<180 μm) collected from the northeastern part of Aichi Prefecture, in the central part of Japan (75 km × 30 km), were analyzed. Their Sr isotope ratios range from 0.7086 to 0.7315 with an average of 0.7129, except for one sample. This average is higher than the mean of the upper crust of the Japan Arc (the Japanese Island Crustal Composite, JICC), 0.7077. This difference can be attributed to the below-average presence of young volcanic rocks, generally having lower 87Sr/ 86Sr values, and the above-average presence of granitic rocks, in the study area compared with the surface exposure of the Japan Arc. The first factor controlling the distribution of Sr isotope ratios is the bedrock distributed around the sampling points. Regional variation in the 87Sr/ 86Sr value shows that it is higher in the western and southeastern parts, where sedimentary rocks and metamorphic rocks are distributed, and that it is lower mainly in the central part, where granitic rocks are distributed. The 87Rb/ 86Sr– 87Sr/ 86Sr plot for stream sediments more clearly reveals the differences and similarities of bedrocks. In some locations, the distribution of Sr isotope ratios does not correspond to that of bedrocks on the geological map. One reason is the existence of unmapped bedrock, for example, small intrusive masses of granite. The other is fluvial transportation and dispersion. The distribution of the isotope ratios suggests that some stream sediments include surficial deposits from a few km upstream. Application of the Sr isotope ratio to geochemical mapping is useful for revealing both the distribution of unexposed bedrocks and the transportation of surficial deposits. Information on unexposed bedrocks will be expected to contribute to the improvement of geological mapping.

Magma ponding in the Karakoram shear zone, Ladakh, NW India

Granitic melt migration and pluton emplacement are commonly closely associated with transcurrent shear zones. The processes that link granites to shear zones are not yet fully understood. The dextraltranspressive Karakoram shear zone in Ladakh, NW India, exposes anatectic rocks where synkinematic melt migration and ponding at kilometer scale were controlled by competency contrasts. Metasedimentary rocks and a dominantly granodioritic calcalkaline intrusion underwent fl uid-present partial melting at upper-amphibolite facies to produce leucogranite sheets and irregular intrusive masses dated at 21–14 Ma. Leucogranitic magmas ponded in the lowpressure strain shadow of the competent grano dioritic calc-alkaline pluton, giving rise to (a) migmatitic rocks that are pervaded by irregular leucogranite intrusions at a scale of meters or tens of meters, and (b) the growth of the Tangtse pluton, a kilometer-scale sheeted complex. Thus, magmas accumulated during shearing and anatexis in a low-pressure strain shadow within the Karakoram shear zone. This magma provided a readily available magma source that could have been tapped to feed larger plutons at shallower levels by modifi cations in the pressure distribution accompanying changes in shear zone geometry and kinematics. We conclude that shear zones tapping anatectic regions act as magma pumps, creating and destroying magma traps at depth as they evolve, and leading to incremental magma addition to upper-crustal plutons.

Evidence for slab melt/mantle reaction: petrogenesis of Early Cretaceous and Eocene high-Mg andesites from the Kitakami Mountains, Japan

Lower Cretaceous and Eocene igneous rocks in the Kitakami Mountains, northeast Japan, attract special interest because of the occurrence of adakitic rocks. In addition, high-Mg andesites (HMAs) were discovered from the Lower Cretaceous dike rocks and from the Eocene Jodogahama rhyolitic rocks in the Kitakami Mountains. Lower Cretaceous HMAs occur as several small dikes (less than 5 m thick, rarely up to 70 m) from Ubaishi, Gongen, Shizugawa, Numazu, and Oshika, southern Kitakami Mountains, intrude slightly older than the Lower Cretaceous plutonic rocks. Among these HMAs, the HMA dike from Gongen, characteristically contains ultramafic inclusions. Eocene HMA, a small intrusive mass (observed maximum diameter is 300 m) consisting of aphyric andesite and olivine andesite, occurs only in Matsuhashi, Iwaizumi, northern Kitakami Mountains. The Eocene HMA is associated with hornblende rhyolite belonging to the Eocene Jodogahama rhyolitic rocks. The Lower Cretaceous and Eocene HMAs show similar petrochemical characteristics to those of Cenozoic adakite (high LREE/HREE ratios and Sr contents, low Y and HREE contents) except higher Cr, Ni, and Mg contents. In addition, the Eocene HMAs and the ultramafic inclusions in the Lower Cretaceous HMAs show less radiogenic Nd–Sr isotopic characteristics than those from the adakitic granites in Kitakami, and similar to those of the Cenozoic adakite. On the other hand, the Miocene Setouchi HMAs in southwestern Japan are characterized by lower Sr/Y ratios, less fractionated REE patterns, weak negative Eu anomalies, and more radiogenic Nd–Sr isotopic characteristics than those of Cenozoic adakite. The ultramafic inclusions from Gongen have clinopyroxene characterized by high Sr concentrations and high LREE/HREE ratios. These clinopyroxenes are considered to be crystallized during interaction of slab derived adakitic melt with overlying mantle peridotite. The olivine phenocrysts of the Eocene HMA show extremely high NiO contents (maximum 0.58 wt.%), which may be an evidence for slab melt/mantle reaction. Concludingly, the petrochemical features of HMA magmas in Kitakami can be explained by reaction of slab derived adakitic melt with overlying mantle peridotite to equilibrate with mantle olivine. The difference between the chemical compositions of the Kitakami and Setouchi HMAs can be explained by the difference in compositions of initial slab melt resulted from the difference in depth of slab melting.

Fusion of surface relief data with high spectral and spatial resolution satellite remote sensor data for deciphering geological information in a mature topographic terrain

For deciphering a greater amount of geological information from satellite remote sensor data, high spectral resolution ‘multi-spectral optical’ remote sensor data and high spatial resolution ‘panchromatic optical’ or ‘radar microwave’ remote sensor data are conventionally merged by data fusion. In a mature topographic terrain, topographic expression, i.e. variation in surface relief over the area, plays an important role in reflecting lithological variation and exposed/subsurface structural features. If the topographic information from such a terrain is merged with high spectral resolution multi-spectral and higher spatial resolution panchromatic satellite remote sensor data by an appropriate data fusion method, the visual interpretability of geological information in the final image product will be significantly improved. This method may also be useful in identifying linear features that are reflected through subtle variation in topographic elevation, but suppressed in the satellite data due to parallelism of the sun azimuth with the trends of these features or due to high sun elevations. In the present work, surface relief data derived from the existing Survey of India topographic base map have been merged with high spectral resolution Landsat-5 thematic mapper (TM) data and higher spatial resolution IRS-1C panchromatic data of Holenarsipur Supracrustal Belt, Karnataka, India, using a data fusion method. The method employs mainly the Intensity-Hue-Saturation (IHS) technique incorporating the concept of the High-Pass-Filter (HPF) technique of data fusion. In this technique, although the spatial details of the original panchromatic data are not being exploited to their fullest potential, the geological information reflected through subtle topographic expression are highlighted well, for example, boundaries of resistant rock formations, regional fold closures and folded layers in resistant rock formations, intrusive granitic masses and mafic-ultramafic intrusive bodies, and regional trends of the planar structural fabrics (regional foliation, fault and joint planes).

Zeolites in Burial Diagenesis and Low-grade Metamorphic rocks

Many zeolite assemblages originated during burial and subsequent low-grade metamorphism of rocks, particularly those of volcaniclastic origin. Eskola (1939) first suggested this type of zeolitization and applied the term “zeolite facies” to assemblages formed under such low-temperature and low-pressure conditions. Although several definitions of the “zeolite facies” have been proposed over the years (e.g. Fyfe et al. 1958), the phrase as used here includes both low-grade metamorphic assemblages and zeolite assemblages formed successively during burial. The regional occurrence of zeolites in sedimentary rocks was first recognized by Coombs (1954), who made detailed petrographic studies of Triassic tuffaceous greywackes and tuffs of andesitic to rhyolitic in composition in the Taringatura area of New Zealand. Coombs recognized a zonal arrangement of heulandite and analcime near the top of the sequence and heulandite and laumontite near the bottom. He also recognized a similar progressive zeolitization with depth in clastic sediments in other orogenic belts. The zeolite assemblages described by Coombs represent a rather advanced stage of alteration, as opposed to an earlier stage of zeolitization that was subsequently described in Neogene silicic volcaniclastic rocks from Japan. In the latter rocks, zones (from top to bottom) of fresh glass, clinoptilolite + mordenite, analcime + heulandite, laumontite, and albite were recognized (Utada 1965, 1970). Progressive zeolitization has also been reported as a result of the thermal effects of intrusive masses (see Seki et al. 1969) and around volcanic calderas (Utada and Ito 1989). Recently, Boles (1991) described the low-temperature formation of laumontite and stilbite in fractures and faults related to post-Jurassic folding and uplift of the Southland Syncline, New Zealand. The physico-chemical conditions of zeolite crystallization in burial diagenesis and low-grade metamorphic environments can vary widely. Both borehole data (see Iijima 1995) and experimental results (Liou 1970; 1971a,b,c …

U-Pb zircon ages from a craton-margin archaean orogenic belt in northern Zimbabwe

In northern Zimbabwe, the Archaean Pfunzi Orogen comprises an east-west-trending belt of migmatitic gneisses separating the Zimbabwe Craton from the Pan-African Zambezi Belt farther north. Previously available Rb-Sr dates for the Pfunzi Orogenic Belt have been interpreted to record formation of granulites at ca 3.0 Ga and subsequent amphibolite-facies metamorphism at ca 2.6 Ga. Here the first conventional IDTIMS U-Pb zircon dates for the Pfunzi Belt are reported, from orthogneisses in the eastern part of the belt in northeastern Zimbabwe. Two intrusive masses of granitic and tonalitic gneiss, together with a granitic leucogneiss inferred to have formed by leucosome segregation during migmatisation, yield crystallisation ages of ca 2.62 Ga. These results are interpreted to date a major tectonothermal event along the northern margin of the Zimbabwe Craton, involving regional amphibolite-facies metamorphism and migmatisation, as well as emplacement of compositionally diverse granitoid plutons and retrogression of older granulites. This event is roughly coeval with orogenesis in the Limpopo Belt along the southern craton margin and with emplacement of the craton-wide ca 2.6 Ga Chilimanzi Granite Suite.

Geology and geochemistry of reworking gold deposits in intrusive rocks of China—II. Gold deposits and their genesis

Gold deposits in intrusive masses include the veinlet dissemination, quartz vein and veinlet dissemination + vein types. They are distributed in fracture zones along the endocontact zone of a batholith or in the centre and edge of a stock. The metallogenic epochs are Yenshanian, Hercynian, Archean, Proterozoic and Himalayan. The gold deposits are characterized by a big difference in time span between gold mineralization and the formation of host masses. Ore-forming materials were derived from the masses and auriferous strata and ore-forming fluids came from meteoric and formation waters. When circulating water was heated by ascending heat flow, gold would be extracted, concentrated and transported from auriferous rocks and then precipitated in the masses during the late tectonic movement. Finally gold deposits were formed in the intrusive masses.

Results of New Field and Geochemical Studies of the Volcanic and Intrusive Rocks of the Maymecha-Kotuy Area, Siberian Flood-Basalt Province, Russia

The Maymecha-Kotuy area, comprising ∼70,000 km2 of the northern part of the Siberian flood-basalt province, is of unusual interest because it appears to be the only such province in the world where high-Ti, alkaline-ultramafic rocks with associated carbonatites predominate over basaltic extrusive and intrusive rocks. New field and geochemical studies of the igneous rocks of this area were initiated with the goals of (1) correlating them with the magmatic formations of the well-studied Noril'sk area and (2) reconstructing the entire magmatic evolution of Siberian flood-basalt volcanism. This report presents the first complete stratigraphic section for the volcanic sequence of the Maymecha River Basin (the most extensive in the Maymecha-Kotuy area), based on flow-by-flow mapping and sample collection. The geochemical and lithologic characteristics of the volcanic and intrusive rocks are thoroughly documented and show an unusually broad range in composition–e.g., SiO2 and MgO contents range from 40 to 70 and from <1 to 38 wt%, respectively. New geochemical data, considered together with earlier paleomagnetic data, indicate that as much as 3000 m of the Maymecha-Kotuy sequence consists of lavas younger than those preserved near Noril'sk. Thus, the combined thickness of the lavas and tuffs that constitute the Siberian flood-basalt province is estimated as ∼6500 m. As at Noril'sk, the volcanic sequence lies with unconformity on sedimentary rocks of the Tungusskaya Series. At Maymecha, the sequence begins with a major, basaltic-tuff unit and an overlying suite of low-Ti, tholeiitic basalts, but the magmatism then became more diverse, with several alternations of low-Ti and high-Ti lavas, and evolved to eruption of high-Ti trachybasalts, melanephelinites, trachyandesites, trachytes, trachydacites, and trachyrhyodacites. The uppermost ∼1400 m of the sequence consists of high-Ti, limburgitic and picritic lavas, as well as Mg-rich (23 to 38 wt%), high-Ti, meymechitic lavas, apparently unique in the world. Graphic evidence of the volcanic nature of the meymechites is presented here for the first time. The numerous dikes and several sills of the Maymecha River Basin are subdivided into seven types, which can be geochemically related to the volcanic sequence and in some cases can be reliably correlated with magmatic rocks of the Noril'sk area. Preliminary indications are that the complex body commonly referred to as the Guli intrusion is, in fact, an intrusive-volcanic complex occupying ∼2000 km2, in which a large, laccolithic intrusive mass of dunitic to peridotitic rocks was breached by a central volcanic edifice from which flowed ankaratrite and picritic ankaratrite lavas. Cutting the ankaratrites and picritic ankaratrites of the complex, but intimately related to it, is a variety of intrusive, alkaline rock types and two carbonatite bodies that may be related to a central feeder zone. We recognize four magma types as products of Siberian flood-basalt volcanism and refer to them herein as low-Ti-I, low-Ti-2, moderate-Ti, and high-Ti. In contrast to the Noril'sk area, where high-Ti magmas constitute <1 vol% of the igneous rocks, their proportion is ∼50 vol% in the Maymecha River Basin. These high-Ti rocks can be subdivided into trachybasaltic, melanephelinitic, and meymechitic rock series; clearly related to these series, judging from trace-element characteristics, are associated trachyandesitic, trachytic, and felsic lavas. In general, the trachyandesitic, trachytic, and felsic lavas appear to be related to the trachybasaltic series through fractionation, but some of the felsic tuffs display characteristics that suggest their relation to the melanephelinitie series. Previous investigators have concluded that meymechitic magma formed by low degrees of mantle-peridotite melting (<7%) at pressures corresponding to depths of 250 to 300 km. The distinctions between the trachybasaltic, melanephelinitic, and meymechitic rock series must relate to different sources and conditions of melting. It is remarkable that no geochemical signature of crustal contamination can be recognized for any of the high-Ti rocks, including the felsic lavas and tuffs. We conclude that the alternation of low-Ti and high-Ti magma types observed in the lower half of the volcanic sequence in the Maymecha River Basin resulted from repetitive expulsion from discrete mantle sources through independent plumbing systems.

A preliminary study on the tectonics and evolution of the Fujian Coast Changle—Nanao metamorphism zone

The Fujian coast Changle—Nanao metamorphism zone rocks are composed of gneiss, schist and hornblendite which had gone through metamorphism of amphilbolite facies, and followed the large-scale intrusive mass of gneissic granite. The zone was originally composed of Early Palaeozoic continental margin and island arc volcanic and sedimentary rocks. In 180-150 Ma, the zone underwent ductile shear deformation and amphibolite facies metamorphism; and in 130-80 Ma, rose rapidly and collided with the Southeastern China continental margin volcanic rocks along the NE trending ductile shear belt. The above geologic setting laid the present Fujian coast tectonic foundation whose formation and evolution are known to be interrelated with the subduction and collision of the Taiwan. Central Range.

Large-scale rhyolite peperites (Jurassic, southern Chile)

Development of a Jurassic volcano-tectonic rift basin in the southern Andes created a setting in which thick, rhyolitic volcaniclastic sequences accumulated in submarine environments and were penetrated by hypabyssal intrusions during or shortly after deposition. In the Ultima Esperanza District of southern Chile, extensive masses of peperite were produced when rhyolite magma underwent quenching, disruption, and commingling with wet, unconsolidated sediments during intrusion at shallow levels beneath the sea floor. The peperite forms discordant intrusive masses with volumes of up to several cubic kilometers, in which large, widely spaced, coherent rhyolite feeder pods are surrounded by, and grade into closely packed and dispersed peperite. Closely packed peperite consists of tightly fitting clasts separated by sediment-filled fractures. In dispersed peperite, the sediment forms a matrix surrounding large masses of fractured rhyolite and smaller more widely separated rhyolite clasts; evidence of in situ quench fragmentation is well preserved on both outcrop and thin-section scales. Thin sections show that clast margins and, in some cases, entire small clasts underwent cooling-contraction granulation, releasing shards of quenched rhyolite and fragments of phenocrysts into the adjacent sediment. Interaction between magma and wet sediment was non-explosive and involved fluidization of the host sediments, creating space for the intruding magma and causing pervasive injection of highly mobile sediment along thermal contraction cracks in quench-fragmented rhyolite. The ability of the magma to undergo complex intermixing with large volumes of sediment, with widespread preservation of in situ fragmentation textures, is interpreted to reflect a relatively low magma viscosity, presumably caused by retention of volatiles in the magma at the ambient pressures involved. Beds of redeposited peperite within the rift-basin fill indicate that some of the intrusive peperite masses reached the sea floor, undergoing slumping and mass flow. The peperites were thus an important local source of coarse-grained debris during the evolution of the basin.

Low-pressure deformation of a small high-level pluton: The syn-tectonic Davys Creek Granite from the southeastern Lachlan Fold Belt, N.S.W.

The Davys Creek Granite (DCG) is a small dyke-assembled granophyre intrusion within greenschist-facies metasediments of the eastern Lachlan Fold Belt, Australia. The granite is a high-level, late syn-tectonic intrusion. Strain in the granite is generally weak but strongly heterogeneous from the microscopic to regional scales. The dominant structure is a crude spaced crack-like cleavage (S) produced by brittle-ductile transitional behaviour involving simultaneous plastic deformation of quartz and groundmass intergrowths and subcritical intergranular crack growth. Strain heterogeneity in the DCG is related to 1. (1) timing of intrusion with respect to waning tectonic forces, 2. (2) strain softness variations due to primary grain-size variations, 3. (3) variable access of aqueous fluids. These factors are typical of epizonal conditions of low confining pressure, abundance of aqueous fluids and rapid and variable cooling rates for small intrusive masses. Subparallel high-temperature solid-state plastic and magmatic foliations were not identified. The weakly-deformed bulbous DCG is contrasted with the nearby, probably older mainly mylonitic gneissic, elongate Wyangala and Wologorong batholiths. Mylonites are confined to initially finest-grained microgranites in the DCG. Deformation style differences relate to lower strain and lower confining pressure associated with DCG deformation. The dyke-like intrusion style of the DCG and above plutons is similar though the DCG is not apparently related to regional-scale ductile shear zones. The absence of complex deformation in the DCG aureole may reflect the lack of a related ductile shear zone.

Structure of remanent magnetization and magnetic mineralogy of the Cheviot lavas (lower Devonian), northeast England

Demagnetization data and magnetomineralogical results from the Lower Devonian Cheviot lavas have revealed a complex remanence structure associated with extensive low-temperature alteration of the primary magnetic oxides. The original cooling magnetization has apparently been replaced by chemical remanences acquired in one or more dual polarity fields. Superposition of normal and reverse field components appears to have created fairly stable, but what may be palaeomagnetically irrelevant, directions which, if not recognized or experimentally resolved, may easily lead to incorrect conclusions. Evaluation of the total data base suggests that the characteristic palaeomagnetic axis is sub-horizontal with an approximately N–S declination. The age range of this magnetization may vary from Lower Devonian to Lower Carboniferous. The new experimental results throw into question some previous palaeomagnetic conclusions regarding the British Lower Old Red Sandstone lavas. Further critical study of the fossil magnetization of Lower Devonian rocks from Britain should be on intrusive masses, as such rocks are likely to have escaped at least the extensive surface oxidation, and associated remagnetization complexities, that have apparently developed in the extrusive suites.

Tertiary deep crustal ultrametamorphism in the Hidaka metamorphic belt, northern Japan

The Main Zone of the Hidaka metamorphic belt is an imbricate stack of crustal material derived from an island arc in which a sequence of units with increasing metamorphic grade from low to high structural levels is exposed. The basal part of the metamorphic sequence underwent granulite facies metamorphism with peak P–T conditions of 7kbar, 870°C. In this zone pelitic granulite includes leucosomes which consist mainly of orthopyroxene‐plagioclase‐quartz.To test whether the leucosome was derived by partial melting of the surrounding pelite, melting experiments of the pelitic granulite were carried out for water‐saturated and dry systems at 7 kbar and 850°C. The chemical composition of the leucosome produced during these runs shows a peraluminous S‐type tonalitic affinity and is located very close to the tie‐line between the average melts produced in water‐saturated systems and the average composition of the residual orthopyroxene + plagioclase. This therefore suggests that the lecosome in pelitic granulite was formed by incipient anatexis at close to the highest P–T condition of the Main Zone.The age of the crustal anatexis is determined by the Rb‐Sr whole rock isochron method for garnet‐cordierite‐biotite gneiss (host rock), garnet‐orthopyroxene‐cordierite gneiss (restite) and S‐type tonalite (melt). This gives an age of 56.0 Ma with an initial 87Sr/86Sr ratio of 0.705711. The S‐type tonalite magmas that form large intrusive masses in the Main Zone were probably generated by crustal anatexis in deeper parts of the crust at the same time (late Palaeocene).