China’s crust-and mantle-source pegmatites and their discriminating criteria

According to their material sources, China’s pegmatities can be divided into two broad types: (1) the crust-source pegmatites derived mainly from the upper continental crust, which can be subdivided into metamorphic differentiated, mixed metasomatic and anatectic magmadifferentiated pegmatites, and (2) the mantle-source pegmatites generated from the mantle, including those related to late differentiates and carbonatites. In space, carbonatites are usually accompanied with alkaii syenite. The criteria of discriminating the two-source pegmatites of China are listed as follows:

1. Rock-forming minerals (micas and feldspars);
2. REE distribution patterns;
3. Zr/Hf ratios in zircons; and
4. Species of apatite and REE contents.

     

Stratigraphy,type and formation conditions of the late precambrian banded iron ores in south China

Based on research on the “Xinyu-type” Sinian iron deposits in Jiangxi Province and metamorphosed iron deposits in Jiangkou and Qidong of Hunan, Sanjiang and Yingyangguan of Guangxi, Longchuan of Guangdong and some other areas in Fujian, the authors have come to the following conclusions:

1. The metamorphosed late Precambrian iron ores widespread in south China may be roughly assigned to two ore belts, namely the Yiyang-Xinyu (Jiangxi)-Jiangkou(Hunan)-Sanjiang (Guangxi) ore belt or simply the north ore belt, and the Songzheng(Fujian)-Shicheng (Jiangxi)-Bailing (Longchuan of Guangdong)-Yingyangguan (Guangxi) ore belt or the south ore belt. Tectonically, the former lies along the southern margin of the “Jangnan Old Land”, while the latter along the northwestern border of the “Cathaysian Old Land”.
2. Iron deposits of this type occur exclusively in the same interglacial horizon of the Sinian Glaciation in south China. Above and below the ore bed there lie the glacial till-bearing volcanic-sedimentary layers.
3. Based on sedimentary features, the iron formations can be divided into four types: silica-iron-basalt formation, silica-iron-clastic rock formation, silica-iron-tuff formation and silica-iron-carbonate rock formation, which progressively grade into each other.
4. Iron ores were formed at the late stage of late Proterozoic rifting in neritic environments, with their distribution governed by the rift valleys on the margins of the “Jiangnan Old Land” and “Cathaysian Old Land”. Consequently, intense mafic volcanism as well as weathering and denudation of palaeocontinent during rifting provided material sources for the formation of iron deposits. Meanwhile, warm and humid stationary neritic environment during the south China great glacial period constitutes favorable palaeoclimatologic and palaeogeographic conditions for the deposition of iron ores.
5. The iron formations have undergone regional metamorphism of greenschist-amphibolite facies.

To sum up, the late Precambrian banded iron ores should be of metamorphosed volcano-sedimentary type.     

The origin of basaltic rocks in Niutoushan area— high pressure differentiation

The Niutoushan basaltic cone, consisting of subalkali (quartz-tholeiite and olivine-tholeiite) and alkali basalts, is Late Tertiary in age. Its major characteristics are generalized as follows:

1. Both early subalkali and late alkali bali basalts are formed under the same geological environment.
2. The continuity in chemical composition from subalkali to alkali and the low FeO/MgO in alkali basalts show that they are the products of cognate magmatic differentiation.
3. The change from low REE abundance and weak enrichment of LREE in subalkali to high REE abundance and strong enrichment of LREE in alkali basalts indicates obvious REE enrichment and fractionation during magmatic differentiation. Weak positive Eu anomalies in the REE patterns are indicative of their formation under low oxygen fugacity conditions.
4. According to the calculated values, 70–75% of the primary olivine tholeiitic magma had been separated as subalkaline basaltic magma, the rest residual magma became alkaline basaltic magma. This result is consistent to the field observation that the outcrop area of subalkali basalts is four times as much as that of alkali basalts.
5. The basaltic rocks of Niutoushan show an S-type distribution straddling the thermal barrier on Ol′-Ne′-Qu′ diagram and an evolution tendency for Ne to increase with increasing FeO/MgO. This is in agreement with the melting experimental data on olivine basalts at 10–20 kb.
6. Mantle-derived inclusions (spinel lherzolite) in this area occur in both alkali olivine basalts and olivine tholeiites. The latter is of extremely rare occurrence. The formation temperature and pressure of the inclusions in alkalibasalts and olivine tholeiites have been calculated. The results show that the alkaline basaltic magma was separated from the subalkaline basaltic magma at about 20 kb.

Basaltic rocks in Niutoushan were formed through the so-called “high pressure differentiation”, that is, at about 20 kb the crystallization of clinopyroxene and orthpyroxene resulted in the separation of subalkaline basaltic magma from the primary olivine tholeiitic magma, and then the residue gradually became alkaline olivine basaltic magma.     

Rare earth element geochemistry of the Scourian complex N.W. Scotland — Evidence for the granite-granulite link

Medium-to high-pressure granulite facies complexes represent samples of lower crustal material and are, therefore, important in the study of crustal processes. New rare earth element data for the Scourian granulite facies terrain of the Precambrian Lewisian complex of N.W. Scotland indicate that:

1. Overall, the Scourian complex has a light rare earth enriched pattern with a small but distinct positive Eu anomaly;
2. While some rare earth element trends for the complex as a whole (e.g., Σ REE vs SiO₂) are similar to those observed in upper crustal cogenetic sequences, others (e.g., Eu/Eu* vs SiO₂) are reversed;
3. Compared to average upper crust, the Scourian complex is depleted in REE (except Eu) by a factor of 2 to 3.

These new data, along with previously reported major and trace element data, isotopic abundances, and trace element modelling support the hypothesis that the Scourian terrain is the residuum left after genesis and removal of granitic melts.     

Petrogenesis of the Elzevir batholith and related trondhjemitic intrusions in the grenville province of eastern Ontario,Canada

The Elzevir batholith belongs to a suite of trondhjemitic intrusions emplaced at ca. 1,240 Ma in the Grenville Province of eastern Ontario. New major and trace element data, including REE, combined with isotopic and petrographic data indicate that:

1. the batholith has calc-alkalic affinities;
2. the Elzevir parental magma is very similar to that of dacites in the nearby, coeval metavolcanic rocks; the magma formed by partial melting of crustal material at granulite grade;
3. chemical differences between the plutonic and volcanic rocks can be best explained by accumulation of plagioclase in the plutonic environment;
4. fractionation was dominated by plagioclase and quartz, with lesser biotite and epidote, and minor zircon and apatite.

It is suggested that melting of sialic crust took place during the ‘docking’ of a partly-evolved, originally ensimatic arc system against the main cratonic mass to the northwest.     

Kyanite eclogites

Prior experimental work has shown that in the laboratory the mineralogy of eclogites is sensitive to the ratio of CaO ∶ MgO ∶ FeO and that the reaction pyroxene + kyanite?garnet + quartz proceeds to the right at high pressures in rocks rich in magnesium and to the left in rocks rich in calcium and iron. Typical basalts crystallized at high pressure never contain kyanite. The chemistry and mineralogy of a large number of naturally occurring eclogites show they belong to three classes.

1. Kyanite-free magmatic eclogites, rich in magnesium, from:
2. kimberlites
3. dunites and serpentinites.
4. Kyanite-bearing eclogites and grosspydites rich in CaO and low in FeO with intermediate MgO from:
5. kimberlites
6. gneisses.
7. Kyanite-free eclogites of metamorphic origin rich in iron with low magnesium and intermediate amounts of calcium from:
8. glaucophane schists
9. gneisses.

     

Organische Substanzen in präkambrischen Itabiriten und deren Nebengesteinen

Marine sedimentary iron ore samples from various early and middle Precambrian ore provinces of the world (“itabirite, banded ironstones, jaspelites, Quarzbändererze”) have been investigated by means of vaporisation pyrolysis gaschromatography (VPGS) with regard of their contents of organic substances. The main results were:

1. All investigated itabirite samples show, concerning their qualitative hydrocarbon content, a typical distribution in chromatographic diagrams.
2. The amount of pristane and phytane as results of the decomposition of chlorophyll points at a reasonable photosynthetic activity of organisms during the itabirite genesis.

It appears likely that organic life has influenced and promoted the genesis of the iron ore deposits of the itabirite type.     

Charriage crustal,anatexie et decrochements ductiles dans les Maures orientales (Var,France) au cours de l'orogenese varisque

The kinematics of the deformational events recorded in the catazonal gneisses within the eastern part of the Maures massif (Variscan basement of Provence, southeastern France) has been established. These events can be correlated with both the metamorphic and the magmatic evolution, and the orogenic history of the eastern Maures then consists of the following stages:

1. -HP-metamorphism preserved only in relict eclogites and quartzites rich in calc-silicates,
2. -Catazonal metamorphism involving anatectic melting, broadly contemporaneous with large-scale horizontal transport towards the NNE. During this event, the lithologic units were disrupted and intensively mylonitized.
3. -Intrusion of a first generation of anatectic granitoids.
4. -A second tectonic event under epi-to mesozonal metamorphism conditions is responsible for the northward displacement of the eastern Maures relative to the western part along a sinistral strike-slip fault 4 km wide (Ramatuelle — Plan de la Tour fault) in which the early anatectic granitoids have been mylonitized. Outside the fault zone, this event is marked by upright to W-vergent open folds trending N-S, i.e. parallel to the transport direction.
5. -A moderate cataclastic reactivation of the Ramatuelle — Plan de la Tour fault with a dextral sense of shear, locally accompanied in the eastern part of the area by minor lowtemperature thrusting towards the south.
6. -Intrusion of a second generation of anatectic granites about 320 Ma ago.

The metamorphic, magmatic and tectonic evolution of the eastern Maures suggests a continuous orogenic history in an area of rapid crustal thickening by large-scale thrusting within the continental crust. This evolution may be related to the development of continental subduction during the continent/continent collision responsible for the Variscan orogeny in southern Europe.     

Geochemistry and origin of granitic rocks,Scourian Complex,NW Scotland

Concordant granite sheets from the granulite facies Scourian Complex, N.W. Scotland exhibit the following features:

1. a common planar fabric with their host pyroxene granulites;
2. the presence of an exsolved ternary feldspar phase;
3. a low-pressure, water-saturated minimum composition;
4. K/Rb ratios (450–1,350) distinctly higher than most upper crustal granites but similar to the surrounding granulites;
5. low absolute concentrations of the rare earth elements (REEs), light REE enrichment, and large positive Eu anomalies.

It is proposed that the granite sheets have originated by anatexis of gneisses undergoing granulite facies metamorphism — gneisses that were already essentially dry and depleted in incompatible elements. Their unusual trace element chemistry may be explained by either disequilibrium melting and/or sub-solidus reequilibration of the granite sheets with the surrounding gneisses. Isotopic and trace element data suggest that cross-cutting, potash-rich pegmatites represent reworking of the granite sheets during a later amphibolitization.     

Die Ultrabasitmassive im Präkambrium des Staates Goiäs/Brasilien

The Brazilian state of Goiás and parts of Mato Grosso represent the centre of the Brazilian shield, between the Guaporé craton in the west and the São Francisco craton in the east. In contrast to opinions before, early Precambrian gneisses and migmatites form only small parts of the shield complex whereas middle and late Precambrian sequences dominate. From tectonic position, an eugeosynclinal central zone can be distinguished accompanied by younger miogeosynclinal series to the east and west. The ultrabasic massifs of Goiás can be differentiated into following types:

1. alpinotype serpentinised dunites and peridotites,
2. gabbro-pyroxenite-peridotite associations,
3. pyroxenite-metagabbro associations,
4. dunite-pyroxenite-alkalisyenite associations,
5. lamprophyre dykes.

Nos. 1, 2 and 3 are of Precambrian age whereas 4 and 5 are surely of post-Devonian, probably of Cretaceous age and can be attributed to the phase of Paraná volcanism. The most economic types at the moment (Nos. 1 and 2) yield Ni, Cu and Co in lateritic enrichment, Cr, Pt and related elements as primary constituents. Latest, the types 4 and 5 cause special interest. From their association with carbonatites they are related to the Araxá complex of Minas Gerais and anticipate mineralizations of Nb, Ta, Ti and P.     

Orogenic evolution of the Hellenides: new aspects

This progress report is based on investigations of the tectonometamorphic development of crystalline complexes and is restricted to a few key problems of the Hellenides:

1. ‘Hinterland’. Rhodopia is strongly affected by Alpidic metamorphism, granitoid intrusions, orogenic deformation and intracrustal delamination. Therefore, close relations between the Balkanides and Hellenides have to be considered.
2. External zones. The Phyllite-Quartzite Series probably originated in a Late Palaeozoic rift within Apulia. In Middle Triassic times rifting stopped and the area became the basement on an extended carbonate platform (Late Triassic-Liassic). From the Dogger to Palaeocene, parts of that platform subsided, forming the Ionian pelagic basin. The Eocene orogenesis within the central Hellenides then caused an inversion of the buried Phyllite-Quartzite rift zone, whereas from the Late Oligocene onwards the previous rift zone underwent continental (A-) subduction. Finally, uplift of the Phyllite-Quartzite Series and nappe emplacement started in Miocene times.
3. Late orogenic intracrustal shearing. Structural analyses of crystalline complexes of Attica have shown that neotectonic extension and the large vertical displacement of the Aegean region were caused by low angle faulting and large-scale shearing within the deeper crust, probably along former overthrust planes.

These results reveal the mechanisms of intracratonic tectonics, remobilization of continental crust and intracrustal detachment throughout the evolution of the Hellenides.     

Laihunite—A new iron silicate mineral

Laihuite reported in the present paper is a new iron silicate mineral found in China with the following characteristics:

1. This mineral occurs in a metamorphic iron deposit, associated with fayalite, hypersthene, quartz, magnetitc, etc.
2. The mineral is opaque, black in colour, thickly tabular in shape with luster metallic to sub-metallic, two perfect cleavages and specific gravity of 3.92.
3. Its main chemical components are Fe and Si with Fe³⁺>Fe²⁺. The analysis gave the formula of Fe Fe _1.00 ³⁺ ·Fe _0.58 ²⁺ ·Mg _0.03 ²⁺ ·Si_0.96O₄.
4. Its DTA curve shows an exothermic peak at 713°C.
5. The mineral has its own infrared spectrum distinctive from that of other minerals.
6. This mineral is of orthorhombic system; space group:C _2h ^/5 ?P2₁/c; unit cell:α=5.813ű0.005,b=4.812ű0.005,c=10.211ű0.005,β=90.87°.
7. The Mössbauer spectrum of this mineral is given, too.

     

Geochemistry of uranium in sediments of the Bohai Gulf,China

The following conclusions can be drawn from the work reported in this paper:

1. Sixteen samples were determined for uranium by spectrophotometric method. The uranium content in the sea floor sediments of the Bohai Gulf ranges from 1.6 to 6.3 ppm, with an average of 4.3 ppm.
2. Statistical data show close relationship between U concentration and grain size. Relatively larger amount of uranium was found accumulated in mud than in sand. The bulk of uranium is assumed to be derived from terrestrial detrital minerals.
3. A positive correlation between U and Fe is recognized. Similar relation also can be seen between U and Al. The plot of U concentration vs. Fe is linear, and can be expressed by the linear regression equation:Y=?0.37+1.35X. The plot of U against Al gives an equation ofY=?2.48+1.01X.
4. The average U/C_org. ratio for these sediments is 7×10^?4, and the average ratios of U/P, U/Mn, and U/CaCO₃ are 100×10^?4, 50×10^?4 and 2×10^?4, respectively.
5. Compared with the abundances of other shelf sediments, the average concentration of U in the area under consideration is close to that of sediments on the selves of Japan and the Gulf of Mexico, and the Black Sea. Uranium concentration in the Bohai Gulf sediments is comparable to that of the continental crust, but differs from that of deep-sea clay.

     

Altération supergène des protores à grenats manganésifères dans quelques gisements d'Afrique de l'Ouest

The wet tropical weathering of Precambrian spessartite quartzites leads to the formation of secondary oxidized ores. These ores result from different successions of superimposed elementary processes, among them:

garnet hydrolysis: Mn, Ca, and Fe are leached, and only an amorphous, silicoaluminous phase remains.

lithiophorite neoformation: Added manganese combines with alumina of the amorphous, Si-Al products.

leaching: Voids appear in the central part of the weathered garnets. They may widen, leaving only the quartz matrix with polyhedric voids.

epigenic replacement: The quartz is replaced by cryptomelane, while the garnets may remain unweathered.

The various ore types are characterized by the sequence of transformations and by the volumes that each transformation affects. These transformations are old because they are observed not only in situ ores but also in removed detrital ores. In the same way the major part of the accumulated stocks of oxidized ores must be old.     

REE Geochemistry and Mineralization Characteristics of the Zudong and Guanxi Granites,Jiangxi Province

Abstract The Zudong and Guanxi granites are original rocks of the ion adsorption-type HREE and LREE deposits in weathering crust of granites. The ∑REE ¹ ∑REE=REE+Y.
value and LREE ² LREE=∑(La-Eu) and HREE=∑(Gd-Lu)+Y.
/ HREE ratio of the Zudong granite are 264 ppm and 0.81-0.24 respectively, and the average Y/∑REE ratio is 35.8-54.5%. This is mainly due to magmatic crystallization and evolution and deuteric metasomatism (albitization, muscovitization and fluorite-doveritization). These alterations resulted in endogenic mineralizations of yttrium-group REE fluorine carbonates, silicates and arsenates. The Guanxi granite is characterized by LREE enrichment (the average LREE/HREE ratio is 2.43).     

La Formation glaciaire de la Mambéré (République Centrafricaine): Reconstitution paléogéographique et implications à l'échelle du Paléozoique africain

The Mambéré Formation constitutes a horizontal unit located in the western and southwestern part of the Central African Republic. Stratigraphical and sedimentological study provides strong argument to attribute these deposits a glacial origin. A palaeogeographical reconstruction has been outlined in order to precise the age of the formation. Two main categories of glacial deposits have been recognized:

glaciogenic deposits made of basal tills (with facetted pebbles) and flow tills (with flattened blunt pebbles);

reworked glacial deposits formed of sandstones and conglomeratic sandstones, in continuous beds, lenses or isolated blocks, together with siltstones and bedded sandstones.

The southerly provenance of the detrital material is demonstrated by quartz grain surface analysis and heavy-mineral study. This material results essentially from the dismantling of the Precambrian Schistoquartzitic Complex and secondarily from the Granitogneissic Complex. According to the palaeomagnetic polar paths and the migration of the glacial centers over the African continent during the Palaeozoic, the Mambéré Formation may be attributed a Lower Silurian age by reference to similar formations observed in Cameroon or a Lower Carboniferous age by comparison with the glacial formations reported from Niger and Egypt.     

U-Pb zircon and monazite dating of a mafic-ultramafic complex and its country rocks

U-Pb data on zircons from the largest mafic-ultramafic body (6×2 km) of the French Central Massif (Sauviat-sur-Vige) yield the following age results: Primary magmatic crystallization of the gabbroic and peridotitic protoliths took place in the Cambro-Ordovician (496±25/17 m.y.). Variable transformation under eclogite facies conditions was Hercynian (320±29/36 m.y.). The same age pattern, derived by U-Pb monazite analyses, was found also for the immediate country rocks, i.e. kyanite bearing, coarse-grained metagranites occurring to the W and N of the Sauviat massif. Due to the fact that there is no regional Hercynian high-grade metamorphism in this part of the French Central Massif (e.g. Duthou 1977; Bernard-Griffiths 1975), both mafic-ultramafic complex as well as immediate felsic country rocks must have been emplaced tectonically into pre-Hercynian (Acadian±Caledonian) crustal rocks. The cause for such a Hercynian tectonism is thought to be due to continent-continent collision of the Spanish with the Armorican plate. Preliminary U-Pb zircon results on one eclogite sample taken about 50 km S of the Sauviat complex indicate also an Early Palaeozoic age for the magmatic protolith and a Hercynian transformation into eclogite, combined with and/or followed by tectonic emplacement. However, opposite to the continental Sauviat massif, we are probably dealing here with oceanic material, possibly deposited in an Early Palaeozoic marginal sea basin. Thus, subduction and tectonic emplacement of oceanic crust into continental crust terminated in this area also in the Hercynian. Concerning U-Pb systematics of zircon and monazite the following conclusions can be drawn:

1. U-Pb systems of primary magmatic zircons of mafic and ultramafic rocks are only weakly disturbed during static eclogite facies metamorphism (T>820° C; P> 15 kbar);
2. New- and overgrowth of zircon during eclogite facies metamorphism seems to be the major cause for the degree of discordance;
3. Amphibolitization of metagabbros and eclogites had no effect on the degree of discordance of zircon;
4. U-Pb systems of monazites remained undisturbed during intense weathering of the mother rock.

     

The Araguainha astrobleme/Central Brazil

The following facts have supported the origin of the Araguainha circular structure in Central Brazil by a meteoritic impact:

1. the almost circular contour
2. the impact-morphologic sequence including a central uplift, ring walls and a basin rim of escarpments
3. outcrops of suevites and mixed breccias
4. the evidence of shock metamorphism
5. the presence of shatter cones, and
6. negative anomalies of the total intensity of the magnetic field at the center of the ring structure.

     

Les granites du Massif Central Français: étude comparée des leucogranites et granodiorites

Chemical and mineralogical compositions of granitic rocks of the French Massif Central enable us to classify them into two distinct groups: (i) leucogranites, (ii) granodiorites or quartz-monzonites, separated by a natural gap. The differences between these two groups are not only chemical and mineralogical, but are also reflected by:

conditions of crystallization as deduced from compositions and from thermal metamorphism of xenoliths.

nature of inclusions and, especially, the occurrence of basic fine-grained igneous xenoliths restricted to the granodiorite — quartz-monzonite group

relations between intrusive and autochtonous migmatitic masses;

levels of formation and of intrusion; the granodioritic and quartz-monzonitic magmas have originated at deeper horizons of the earth's crust than the leucogranitic one, but have risen higher.

In our opinion these differences are linked, and the specific characteristics of the granodiorites and quartz-monzonites are related to the presence of basic igneous inclusions. These inclusions are relics of deep basic intrusions in the earth's crust, which have overheated the surrounding acid rocks beyond the minimum temperature of melting. Thus granodioritic or quartz-monzonitic melts were produced and were able to rise to superficial levels. These inclusions are missing in leucogranites. The compositions of the latter and their poor mobility suggest a wet anatectic origin in minimal PT conditions.     

Untersuchung von suspendiertem Material in den Hydrothermallaugen des Atlantis-II-Tiefs

Suspended matter was separated from the hydrothermal brines of the Atlantis-II Deep in the Red Sea. Contents of iron, manganese, copper, and zink collected on membrane filters were measured by X-ray-flourescence, and the main results were:

1. Metal concentrations in suspended form were lower by 10^?1 to 10^?5 compared to the dissolved concentrations of the brines.
2. Suspended copper and zink were enriched most pronounced in the deeper brine zone — markedly so in the SW-basin, values there ranged between 10 and 30μg/l, one Zn-value was as high as 60μ/l, the other basins contained mostly less than 1 to 10μg/l.
3. Iron, copper, and zink in the deeper brine zone and in few cases also in the 50°-brine were suspended in the form of sulfide compounds. This conclusion is based only on the slightly purple-, green-, and blue-colored gray hues of the material on the filters and its rapid oxidation upon contact with the air.
4. In the transition zone of the 50°/60°-brine iron hydroxides were highly concentrated with values ranging up to 1000μg Fe/l.
5. Suspended manganese is found only within the transition zone of brine to the normal sea water, where up to 200μg Mn/l in form of darkbrown manganese hydroxides were found.
6. Concurrently with the increased thermal activity since 1966 more strongly reducing conditions within the brines seemed apparent which were caused by discharge of higher amounts of Sulfides into the basins. Increased precipitation of heavy metal sulfides was found to be most pronounced in the SW-basin.