Marble Lenses Research Articles

A preliminary study of geology and skarn of Cemorosewu Area, Bayat, Central Java, Indonesia

We present the first finding of skarn rocks in Cemorosewu area of East Jiwo Hill, Bayat, Central Java, Indonesia. The geological conditions of this area which related to the appearance of the skarn is also reported. The methods used in this research are DEM acquisition using drone to generate basemap for geological mapping and thin section petrographic analyses. The geological map shows that Cemorosewu area consists of metamorphic rocks (mica phyllite, graphite phyllite, with quartzite and marble lenses), sedimentary rocks (carbonate breccia and siltstone), and igneous rock (microdiorite). Based on the field observation and geological map, the regional metamorphic rocks are the oldest units in this area followed by sedimentary rocks and igneous rock which intruded both rocks. Skarn rocks were cropped out as a boulder along the Kluwihan creek with the maximum size of 8 m in length. The skarn consists of garnet, clinopyroxene, zoisite, actinolite, and minor quartz. Metasiltstone and skarn rock are suggested formed by contact metamorphism of microdiorite intrusion. The marble, which lenses within the phyllites, is suggested to be the protolith of the skarn formation in this area.

Metamorphism of sedimentary rocks recognised by 19th century French naturalists: A case study from the Chavanon sequence marbles, Massif Central, France

The Chavanon metamorphic sequence in the Variscan French Massif Central contains marble lenses that have been exploited since at least the 18th century to produce lime. They provided an opportunity to some major pioneers among nineteenth century French geologists to understand how metamorphic rocks are formed. The intimate association between marbles and gneisses led them to propose that the initial pile of rocks was deposited as a single unit under water. They thoroughly described the attitude, folding and mineralogy of the marbles, making their works of great historical value. Originally written in French, their translation into English and the reproduction of original figures from the works they published will be very useful to the international community of geologists interested in the history of their science.

Origin of Au-Rich Carbonate-Hosted Replacement Deposits of the Kassandra Mining District, Northern Greece: Evidence for Late Oligocene, Structurally Controlled, and Zoned Hydrothermal Systems

Abstract The Au-rich polymetallic massive sulfide orebodies of the Kassandra mining district belong to the intrusion-related carbonate-hosted replacement deposit class. Marble lenses contained within the Stratoni fault zone host the Madem Lakkos and Mavres Petres deposits at the eastern end of the fault system, where paragenetically early skarn and massive sulfide are spatially associated with late Oligocene aplitic and porphyritic dikes. Skarn transitions into predominant massive and banded replacement sulfide bodies, which are overprinted by a younger assemblage of boulangerite-bearing, quartz-rich sulfide and late quartz-rhodochrosite vein breccias. The latter style of mineralization is most abundant at the Piavitsa prospect at the western end of the exposed fault system. The sulfide orebodies at the Olympias deposit are hosted by marble in association with the Kassandra fault, where textural and mineralogical similarities to the sulfide bodies within the Stratoni fault zone suggest a genetic relationship. Estimated trapping temperatures and pressures based on fluid inclusion data indicate that carbonate replacement mineralization took place at depths less than about 5.9 km. Carbon and oxygen isotope patterns in carbonate from the Stratoni fault zone support isotopic exchange principally through fluid–wall-rock interaction, whereas decarbonation and fluid-rock exchange reactions were important at the Olympias deposit. Carbonate minerals associated with skarn and replacement sulfide throughout the district have isotopic compositions that are consistent with formation from a hydrothermal fluid of magmatic origin. Lower homogenization temperatures and salinities in the younger quartz-rich sulfide assemblage and quartz-rhodochrosite vein breccias, together with low δ18O values of gangue carbonate, suggest dilution of a primary magmatic fluid with meteoric water late in the evolution of the hydrothermal system in both the Olympias area and the Stratoni fault zone. The replacement sulfide orebodies in the district likely inherited their uniform Pb isotope composition from a late Oligocene igneous source and the isotopically heterogeneous metamorphic basement units. Metal distribution patterns at the scale of the Stratoni fault zone show diminishing Cu concentration with decreasing Pb/Zn and Ag/Au ratios from Madem Lakkos to Mavres Petres and the Piavitsa prospect in the west. The sulfide orebodies at the Olympias deposit exhibit elevated Cu values in the east with increasing Pb/Zn and Ag/Au ratios down-plunge to the south-southwest. Metal concentration and ratios support zoning related to temperature and solubility changes with increasing distance from a probable magmatic source. Structural and igneous relationships, together with fluid inclusion microthermometric and carbon-oxygen isotope data and metal distribution patterns, are supportive of a zoned hydrothermal system that exceeded 12 km along the Stratoni fault zone, sourced by an igneous intrusion to the southeast of the Madem Lakkos deposit. The Olympias replacement sulfide orebodies, associated with the Kassandra fault, resulted from a local hydrothermal system that was likely derived from a concealed igneous intrusion to the east of the deposit.

Constraints on the depositional age and tectonometamorphic evolution of marbles from the Biharia Nappe System (Apuseni Mountains, Romania)

Abstract Basement rocks from the Biharia Nappe System in the Apuseni Mountains comprise several dolomite and calcite marble sequences or lenses which experienced deformation and metamorphic overprint during the Alpine orogeny. New Sr, O and C-isotope data in combination with considerations from the lithological sequences indicate Middle to Late Triassic deposition of calcite marbles from the Vulturese-Belioara Series (Biharia Nappe s.str.). Ductile deformation and large-scale folding of the siliciclastic and carbonatic lithologies is attributed to NW-directed nappe stacking during late Early Cretaceous times (D2). The studied marble sequences experienced a metamorphic overprint under lower greenschist- facies conditions (316-370 °C based on calcite - dolomite geothermometry) during this tectonic event. Other marble sequences from the Biharia Nappe System (i.e. Vidolm and Baia de Arieș nappes) show similarities in the stratigraphic sequence and their isotope signature, together with a comparable structural position close to nappe contact. However, the dataset is not concise enough to allow for a definitive attribution of a Mesozoic origin to other marble sequences than the Vulturese-Belioara Series.

Carbonatitic and granitic melts produced under conditions of primary immiscibility during anatexis in the lower crust

Carbonatites are peculiar magmatic rocks with mantle-related genesis, commonly interpreted as the products of melting of CO2-bearing peridotites, or resulting from the chemical evolution of mantle-derived magmas, either through extreme differentiation or secondary immiscibility. Here we report the first finding of anatectic carbonatites of crustal origin, preserved as calcite-rich polycrystalline inclusions in garnet from low-to-medium pressure migmatites of the Oberpfalz area, SW Bohemian Massif (Central Europe). These inclusions originally trapped a melt of calciocarbonatitic composition with a characteristic enrichment in Ba, Sr and LREE. This interpretation is supported by the results of a detailed microstructural and microchemical investigation, as well as re-melting experiments using a piston cylinder apparatus. Carbonatitic inclusions coexist in the same cluster with crystallized silicate melt inclusions (nanogranites) and COH fluid inclusions, suggesting conditions of primary immiscibility between two melts and a fluid during anatexis. The production of both carbonatitic and granitic melts during the same anatectic event requires a suitable heterogeneous protolith. This may be represented by a sedimentary sequence containing marble lenses of limited extension, similar to the one still visible in the adjacent central Moldanubian Zone. The presence of CO2-rich fluid inclusions suggests furthermore that high CO2 activity during anatexis may be required to stabilize a carbonate-rich melt in a silica-dominated system. This natural occurrence displays a remarkable similarity with experiments on carbonate–silicate melt immiscibility, where CO2 saturation is a condition commonly imposed.In conclusion, this study shows how the investigation of partial melting through melt inclusion studies may unveil unexpected processes whose evidence, while preserved in stiff minerals such as garnet, is completely obliterated in the rest of the rock due to metamorphic re-equilibration. Our results thus provide invaluable new insights into the processes which shape the geochemical evolution of our planet, such as the redistribution of carbon and strategic metals during orogenesis.

The Cimmerian accretionary wedge of Anarak, Central Iran

The occurrence in Iran of several ophiolite belts dating between Late Palaeozoic to Triassic poses several questions on the possible existence of various sutures marking the closure of the Palaeotethys ocean between Eurasia and this Gondwana-derived microplate. In this scenario, the Anarak region in Central Iran still represents a conundrum. Contrasting geochronological, paleontological, paleomagnetic data and reported field evidence suggest different origins for the Anarak Metamorphic Complex (AMC). The AMC is either interpreted, as: (1) relict of an accretionary wedge developed at the Eurasia margin during the Palaeotethys subduction as part of the Cimmerian suture zone of NE Iran, displaced to Central Iran by a large counter-clockwise rotation of the central Iranian blocks; (2) autochthonous unit forming a secondary branch of the main suture zone.Our structural, petrographic and geochemical data indicate that the AMC consists of several metamorphic units also including dismembered “ophiolites” which display different tectono-metamorphic evolutions. Three main ductile deformational events can be distinguished in the AMC. The Morghab and Chah Gorbeh complexes preserve a different M1 metamorphism, characterized by blueschist relics in the S1 foliation of the former unit, and greenschist assemblages in the latter. They share a subsequent similar D2 deformational and M2 metamorphic history, showing a prograde metamorphism with syn- to post-deformation growth of blueschist facies mineral assemblages on pre-existing greenschist facies associations. High pressure, low temperature (HP/LT) metamorphism responsible for the growth of sodic amphibole has been recognized also within marble lenses at the contact between the Chah Gorbeh Complex and serpentinites. Evidence of HP/LT metamorphism also occurs in glaucophane-bearing meta-pillow lavas and serpentinites, which contain antigorite and form most of the “ophiolites” within the AMC. Structural relationships show that the Chah Gorbeh and Morghab units and the “ophiolites” were tectonically coupled within an accretionary wedge before the D2 folding stage. The other units of the AMC lack evidence of HP metamorphism in the area around Anarak, especially the Lakh Marble, a large thrust sheet that occupies the uppermost structural position in the AMC. Available radiometric ages of trondhjemite dikes and stocks that intruded the accretionary wedge, as well as our new data, constrain the subduction event at the end of the Carboniferous, before 290Ma. These data suggest that the AMC is part of an allochthonous crustal fragment belonging to the Variscan belt developed along the southern Eurasian margin before the Cimmerian collision of Iran. Subsequent deformational events that occurred during the Mesozoic and the Cenozoic, up to the Miocene and possibly later, resulted in folding, thrusting and faulting that dismembered the original structure of the wedge accompanying its displacement to the present day position.

Waste disposal on karstic terrain: a case study from the ancient marble quarries in Iznik (Nicaea), Turkey

This study is conducted in Iznik, a city situated in the Marmara region of Turkey. There are number of ancient marble pits in the area, some of which are used as sewage and solid waste dump sites. This study is aimed at investigating the hydrogeological and hydrogeochemical properties of Iznik plain with special emphasis on these waste sites. In terms of geological features, Derekoy metamorphics are basement rocks that consist of graphite schist, muscovite-quartz schist and marble lenses. Since Iznik marble was used as a natural construction material in the area since in the Roman era, 20 ancient marble quarries in different sizes existed in and around the study area. Wastewater from the city of Iznik and surrounding residential areas was disposed in three of these ancient marble quarries. In terms of hydrogeological features, the Iznik marble and alluvium units constitute the main aquifer system in the study area. Because of the highly fractured structure of the Iznik marble, the wastewater in the quarries was able to move rapidly within the rock. Analyses of groundwater samples revealed that the wastewater flowing in the marble unit discharges eventually into the alluvial unit and thereby affecting groundwater quality around the antique quarries. Based on chemical analyses results, the wastewater was Na-Cl water type and contained high concentrations of Na (3260 mg/l), K (903 mg/l) and Cl (10396 mg/l). Samples from wells down gradient of the wastewater source had comparatively higher Na, K and Cl concentrations.

The Neoproterozoic Sergipano orogenic belt, NE Brazil: A complete plate tectonic cycle in western Gondwana

The Neoproterozoic Sergipano Belt formed by the collision of the Pernambuco-Alagoas Block in the north with the São Francisco Craton in the south, but the timing, duration and mechanics of this amalgamation are poorly understood. The belt is divided into the Canindé, Poço Redondo-Marancó, Macururé, Vaza Barris, and Estância lithostratigraphic domains; the first three are composed of plutonic, volcanic and sedimentary rocks and the last three of sedimentary rocks. Our new field, structural, and geochemical data, and Sm–Nd, Ar–Ar and U–Pb geochronology provide robust constraints for the following evolution. A Mesoproterozoic (∼980–960 Ma) continental arc (Poço Redondo tonalitic gneisses) developed on the margin of the Palaeoproterozoic Pernambuco-Alagoas Block. Extension of this continental block gave rise to (i) A-type crustal granites and associated sedimentary rocks on the stretched, rifted margin of the Poço Redondo-Marancó Domain, (ii) the Canindé rift sequence between the Pernambuco-Alagoas Block and the Poço Redondo/Marancó domain, (iii) a passive margin on the southern boundary of the Pernambuco-Alagoas Block on which sediments were deposited after 900 Ma, (iv) and a second passive margin on the São Francisco Craton. In the Canindé Domain, rifting continued until ca. 640 Ma and led to emplacement of a bimodal association of A-type granite (715 Ma) and continental mafic volcanic rocks, a continental-type layered gabbroic complex (ca. 700 Ma), magma-mingled gabbro/quartz–monzodiorite (688 Ma), and rapakivi granites (684 Ma and 641 Ma). Deformed pillow basalts and interleaved marble lenses are likely ocean floor relicts in the Canindé Domain. Closure of the Canindé oceanic basin began at ca. 630 Ma with the intrusion of arc-type granitic plutons. Convergence of the Pernambuco-Alagoas Block and the São Francisco Craton led to deformation on the passive margins and granite emplacement in the Macururé (628–625 Ma, and 590–570 Ma), Canindé (ca. 621 Ma) and Poço Redondo-Marancó (ca. 625 Ma) domains. A small oceanic basin was most likely subducted beneath the Poço Redondo-Marancó Domain to account for the presence of 602 Ma arc-type volcanic rocks. Shortly after, exhumation of the Pernambuco-Alagoas Block and Canindé, Poço Redondo-Marancó and Macururé domains in the north led to deposition of uppermost clastic sediments in the Estância and Vaza Barris domains in the south, possibly in a foreland basin, and to final thrusting of the continental margin sedimentary rocks onto the São Francisco Craton. Our results indicate that the construction of western Gondwana involved a ca. 300 million years long history of plate breakup and collision.

Formation and evolution processes of the Salanfe W–Au–As-skarns (Aiguilles Rouges Massif, western Swiss Alps)

At Salanfe (Aiguilles Rouges Massif, Swiss Alps) several stratabound W–Au–As-skarn bodies were formed at the contact between pre-Ordovician marble lenses and paragneiss in close proximity to an anatectic leucogranite of Late Palaeozoic age. Anatexis and leucogranite formation occurred at peak metamorphic conditions of P=0.45 GPa and T~650–700 °C. This metamorphic event, dated at 317 Ma in the adjacent Mont Blanc Massif, was related to dextral transpression following continental collision. Structural evidence indicates that the initial stages of skarn formation also occurred under the above P–T conditions. The main stratiform skarn body (Robert) consists of a massive facies close to the marble and of a banded facies on the paragneiss side. The massive facies resulted from the replacement of former dolomite marble, whereas the banded facies resulted mostly from the replacement of thin carbonate layers intercalated in the paragneiss. In the massive facies, the prograde stage skarn is composed by hedenbergite with increasing Fe concentrations through skarn evolutionary stages, whereas in the banded facies it consists of diopside–plagioclase–quartz. The imprint of the retrograde stage is minor and unevenly distributed, with replacement of the pyroxene by magnetite/haematite–calcite–quartz close to the marble and by chlorite–epidote close to the paragneiss. The economic ore minerals are scheelite, arsenopyrite, loellingite and gold and are mainly associated with the prograde skarn stage. Most of the scheelite mineralization occurs during the early prograde stage, probably at temperatures higher than 600 °C, and is associated with pyroxene (hedenbergite 55). Arsenopyrite and loellingite precipitated after scheelite at temperatures between 600 and 350 °C and are associated with hedenbergite>80. Gold occurs both in the final prograde stage and in the retrograde stage. Minor second generation scheelite is associated with epidote and chlorite of the retrograde stage. Lead isotopes indicate that skarn ore minerals were precipitated by a low Th/U fluid like that in equilibrium with the anatectic leucogranite. Oxygen isotopes support the contention that the prograde skarn was formed by a fluid in equilibrium with the leucogranite, mixed with increasing amounts of CO2 approaching the skarn–marble contact. Oxygen and hydrogen isotopes suggest the intervention of a non-magmatic fluid (meteoric, connate?) during the retrograde stage. The Salanfe skarns were formed at deep crustal levels by fluids in equilibrium with an anatectic leucogranite, possibly channelled along permeable paragneiss during regional-scale transpression, and could be the deep expression of a widespread W–Au hydrothermal ore event that occurred along the European Variscides at 310–305 Ma.

STRATIGRAPHY OF THE GOIÁS AND FAINA GREENSTONE BELTS, CENTRAL BRAZIL: A NEW PROPOSAL

The southcm part of the Archaean terranes of the State of Goias, Central Brazil, contains a NW-SE greenstone belt synclinorium, 150 km long and averaging 7 km wide, which was previously interpreted as one single belt. New lithological and structural data show that the belt consists of two allochthonous segments, the Goias and Faina Greenstone Belts, with contrasting stratigraphic sequences juxtaposed by a NE-trending dextral fault. The contact bctween the graniloid complexes and the supracrustal sequences is either tectonic or intrusive, indicating that the complexes are not basement of the Greenstone Belts. The Goias and Faina Greenstone Belts have common lower sections (Serra de Santa Rita Group), composed, from base to top, of metakomatiites (Manoel Leocadio Formation) and metabasalts (Digo-Digo Formation). In the Goias Belt, the Digo-Digo Formation is subdivided into a Lower and an UpperMember, of metabasalts and felsic metapyroclastics respectively, the latter being absent in the Faina Belt. In both belts, metabasalts are succeeded by thick metasedimentary sequences. The Fazenda Paraiso Group, subdivided into the lower Fazenda Limeira and the upper Fazenda Cruzeiro Formations, represents the metasedimentary record of the Goias Greenstone Belt. The Fazenda Limeira Formation has a Lower Memberof carbonaceous schist and an Upper Member of metachert, banded iron formation, calc-schist and marble. The Fazenda Cruzeiro Formation consists of siliciclastic metarhythmites. The Furna Rica Group represents the metasedimentary record of the Faina Greenstone Belt, and is subdivided, from base to top, into the Fazenda Tanque, Serra de Sao Jose, and Corrego do Tatu Formations. The Fazenda Tanque Formation rests unconformable on metabasalts of the Serra de Santa Rita Group, and contains a Lower Member of orthoquartzites with lenses of metacpnglomerate, an Intermediate Member of metapelites, and an Upper Member of carbonaceous schists and iron formation. The Fazenda Tanque Formation contains a Lower Me mberof orthoquartzites with minor marble lenses, and an Upper Member of metapelites. The Corrego do Tatu Formation comprises a Lower Member of marbles and an Upper Member of banded iron formations. The sedimentary contrast between the Greenstone Belts is interpreted as due to distinct paleogeographic settings and deppsitional environments. In the Goias Belt, sedimentation took place in a deep, yet progressively shallower, marine environment, whilst in the Faina Belt the sedimentation took place in a shelf environment with two transgressive and progressively deeper marine cycles. Geochemical data of clastic metasedimentary units show that the Sm/Nd model age (TDM) decreases from 3.1 to 2.8 Ga across the sedimentary record, interpreted as the age range of the source-area, and that the clastic load may be explained by a major contribution from mafic/ultramafic rocks, with increasing felsic contribution towards the top of the sequences. The core of the Goias Greenstone Belt synclinorium also contains a narrow zone of metaconglomerates and orthoquartzites (Serra do Cantagalo Sequence) of Sm/Nd model age of 2.3 Ga. This sequence is overturned and overlies the greenstone belt via a thrust fault. Both the Archaean and Protcrozoic rocks delineate the synclinorium of the Goias Belt, indicating that the structure is not of Archaean, but, at most, of Palaeoproterozoic age.

The mineralogy, petrology, metamorphic PTDt trajectory and exhumation mechanism of blueschists, south Tianshan, northwestern China

The south Tianshan blueschist belt extends for about 200 km along the south Tianshan suture from Changawuzhi to the upper Kekesu river. The high-pressure—low-temperature (high- P/low- T) metamorphic rocks contain sodic amphibole, garnet, phengite, albite, quartz, epidote, chlorite and stilpnomelane. These blueschists occur as small blocks, lenses, laminae and bedded slabs or sheets in greenschist facies country rocks. A few dismembered ophiolitic nappe slices and marble lenses are interlayered in the high- P/ T metamorphic belt. Metabasite blueschists are mainly E-type MORB, less commonly N-type. The protoliths of the meta-sedimentary rocks are graywackes, which contain volcaniclastic material of E-type MORBs, and probably some continental basement materials, derived from the Tarim plate. These protoliths may represent subduction complexes or meta-melanges, which formed within an accretionary wedge on the southern side of the Yili-Central Tianshan plate (YCTP) at the end of the early Paleozoic. The mineral assemblages include sodic amphibole (mainly crossite) + garnet + phengite + epidote + chlorite + albite, estimated to have crystallized at 450°C and 9–10 kbar. These conditions correspond to the epidote-blueschist facies and are the peak metamorphic conditions. The prograde metamorphism is represented by lawsonite-blueschist facies and the retrograde is greenschist facies. The metamorphic PTDt trajectory of the blueschists runs clockwise and reveals that the metamorphism revolved from laumontite facies (?), lawsonite-blueschist facies (?), epidote-blueschist facies (408 Ma?) to greenschist facies (345 Ma), during the creation and exhumation of the blueschists in the south Tianshan suture. We have recognized six stages of deformation in the blueschists. The earlier three stages were concomitant with metamorphism, the later three postdated the metamorphism. On the basis of the deformation characteristics of the blueschists and coherent rocks, the regional geology and the metamorphic PTDt path we discuss the exhumation mechanism of the blueschists. The post-collisional ductile thrust within the suture probably exhumed the blueschists and coherent rocks, allowing them to escape retrograde overprinting in the greenschist facies. The influence of erosion on the exhumation of blueschists could not be considered directly and quantitatively because of the absence of Devonian strata and the few exposures of Early Carboniferous clastic rocks, all without detritus of blueschists.

Stratigraphy of the greenstone belts, Crixás region, Goiás, central Brazil

Archean rocks in the Crixás region, central Brazil, comprise polydeformed low-grade metamorphic greenstone belt volcano-sedimentary sequences of 2.8 Ga arranged in three subparallel belts separated by domal granitoid gneiss blocks. From west to east the supracrustal belts are named Crixás, Guarinos and Pilar de Goiás. A revised stratigraphy is proposed for each belt. The Crixás Group is composed of: (a) the Córrego Alagadinho Formation, consisting of an approximately 500 meter thick sequence of komatiites interlayered with banded iron formations; (b) the Rio Vermelho Formation (350 m), locally pillowed metabasalts with intercalations of oxide and/or silicate facies banded iron formations; and (c) the Riberrão das Antas Formation (700 m), with a lower sub-unit of carbonaceous phyllite-manganiferous metachert assemblage, a middle carbonaceous phyllite-marble assemblage, and an upper metapelite sub-unit. The Guarinos Group is subdivided into: (a) the Serra do Cotovelo Formation (500 m), made up of komatiites with intercalations of carbonaceous phyllite; (b) the Serra Azul Formation (300 m), consisting of metabasalts with intercalations of carbonaceous phyllite; (c) the São Patricinho Formation (300 m), metagraywacke interlayered with chlorite phyllites and locally banded iron formations and orthoquartzite; (d) the Aimbé Formation (50 m), a stratigraphic marker made up to diverse banded iron formations; and (e) the Cabaçal Formation (400 m), consisting of carbonaceous phyllite. The Pilar de Goiás Group contains four sub-units: (a) the Córrego Fundo Formation (900 m), komatiites interlayered with thin banded iron formations; (b) the Cedrolina Formation (500 m), consisting of metabasalts with thin iron and/or manganese formation layers; (c) the Boqueirão Formation (200 m), made up of 50 meters of lower metachert grading up into calc-silicate rocks with dolomitic marble lenses close to the top; and (d) the Serra do Moinho Formation (400 m), composed of phyllite with variable proportions of carbonaceous matter. Each of the greenstone belts has a gross stratigraphic framework made up of a lower volcanic and an upper sedimentary sequence. Major stratigraphic differences occur among the upper sedimentary sections of the three belts; these are interpreted as resulting from different paleographic settings. Vertical facies change from deep to shallow water facies in each belt. Lateral facies change from an outer shelf euxinic environment in the Crixás greenstone belt, eastward into slope graywackes in the Guarinos greenstone belt, and deep water cherts and calc-silicate rocks in the Pilar de Goiás greenstone belt. A carbonaceous sequence on top of both the Guarinos and Pilar de Goiás Groups may represent either a blanket of euxinic shelf prograding onto deep water facies or allochtonous slices of outer shelf type metasediments transported during late Archean thrusting.

COBERTURAS METASSEDIMENTARES DO PROTEROZÓICO MÉDIO: OS GRUPOS ARAÍ E PARANOÁ NA REGIÃO DE NIQUELÂNDIA - COLINAS, GOIÁS

In the Niquelândia-Colinas, Goias, Middle Proterozoic Arai Group nonconformably overlies granite-gneiss basement and remains of the possibly Early Proterozoic Ticunzal Formation. The Arai rocks comprise the lower Arraias Formation of quartzite, feldspathic quartzite, and metaconglomerate, and the upper Trairas Formation of carbonatic metapelite, calcschist, chlorite schist, and marble lenses, bearing greenschist facies mineral parageneses. Proeminent deformation structures are the domal shape outlined by the structural high of basement rocks and the large 5-20° S20W plunging asymmetric folds. The latter have not been recognized within the younger Paranoa Group metasediments which unconformably overly the Arai Group. The Paranoa Group is divided into two sequences. The lower is made of five psamitic and pelitic units, with the discontinuous Sao Miguel metaconglomerate at the base. The upper sequence comprises four mostly pelitic and carbonatic units, with less important psamitic layers. The rocks have been metamorphosed to the biotite zone of the greenschist facies. Deformation is represented by an earlier phase of tight to isoclinal folding with axial-plane schistosity, possibly coeval with large nearly N-S trending thrusts verging towards the east. These structures are refolded by asymmetric to reverse chevron folds with north-south axial-plane crenulation cleavage. The latest phases are of SW plunging open folds, and large vertical folds with sub-horizontal E- W trending axes. The lower psamo-pelitic sequence correlates with the unmetamorphosed Paranoa Group outcropping near Alto Paraiso de Goias, to the east of the study area. The dominantly pelitic and carbonatic upper sequence is equivalent to the Minacu Formation described further to the northwest. Stratigraphic relations between these sequences and their natural division in easily recognized units allow their characterization as groups of formations conferring supergroup rank to the Paranoa.

Titanian chondrodite and clinohumite in marbles from the �tztal crystalline basement

Marble lenses within the polymetamorphic Otztal crystalline complex frequently contain pockets of clinohumite and chondrodite and their titanium-rich varieties. Chondrodites contain up to 9.1 wt% TiO2 and clinohumites about half that amount. The maximum amounts of Ti in both minerals do not exeed values of 0.4 in the formula. XTi and XF are correlated by a slope of −1, indicating the efficiency of the TiO2Mg−1F−2 substitution at constant OH (Evans andTrommsdorff, 1983). Clinohumites sometimes contain lamellae which have nonstoichiometric cation ratios reflecting a composition between clinohumite and olivine. In many cases Ti-rich clinohumites and chondrodites coexist with Mg-ilmenite. Ti-rich clinohumites and chondrodites often break down to a symplectite of Ti-poor clinohumite (or olivine) and Mg-ilmenite. Parts of the marbles, which are barren of Mg-ilmenite contain F-rich clinohumites. The latter do not show any signs of breakdown features. We conclude therefore that fluorine stabilizes the humite group minerals to a wide range of metamorphic conditions. Ti-rich humite-group minerals are not stable till amphibolite facies conditions, and below F-rich humite-group minerals and olivine are the more stable phases in a H2O-rich, SiO2-poor carbonate system.

Scheelite bearing calc-silicate gneisses in the Provence crystalline basement (Var, France)

In the Tanneron massif (north of Cannes) two series of gneisses occur in a basement-cover relationship. The gneiss basement series includes ortho-and paragneisses. The cover series is geochemically related to a shale-greywake sequence with a thick acid volcanic intercalation (leptynite). Swarms of calc-silicate lenses occur sporadically along with marble lenses in the cover and without associated marble in the basement. Scheelite enrichment in the calc-silicate is frequent. The absence of a black shale type environment, the lack of nearby granite, the development of peri-anatectic features (quartzo-feldspathic veins ranging from aplite to pegmatite, local fusion giving birth to small granitoid bodies) and the geochemical characteristics (enrichment in Be, Ta and Eu) suggest that the tungsten mineralization has a peri-anatectic origin.

Caves in non-limestone rocks of Norway

The gneisses and schist bedrock in Bjugn, Afjord and Al counties of Norway are remarkably rich in caves and shelters. The caves are mainly developed along tectonic structures, primarily prominent fissure zones and nappes. Five modes of origin seem reasonable: 1. Tectonic movements and gravity sliding, producing displacement of hillsides. 2. Frost shattering. 3. Wave abrasion during stages of high sea-levels. 4. Preferential weathering in brecciated zones. 5. ‘Karstic’ dissolution of small, isolated marble lenses within the gneisses. In many cases, two or more of the processes appear to have worked together. Abundant speleothem growth suggests that calcareous zones exist in the rocks. The caves reach a horizontal extent of up to 200 m and vertical depths of 55 m. These much neglected cave types arc valuable shelters and habitats for animals, as well as paleontologically important pit-fall traps. Some of the caves and shelters are also potential archeological localities.

Petrology of chlorite-spinel marbles from NW Spitsbergen (Svalbard)

Small dolomite marble lenses and bands occur in the vast Caledonian migmatite and gneiss area of NW Spitsbergen (Svalbard archipelago). The fine-banded marbles contain numerous assemblages of minerals: calcite, dolomite, olivine, clinohumite, diopside, amphibole, chlorite, spiner and phologopite. The coexistence of calcite + dolomite + olivine + chlorite + spinel over the entire area indicates metamorphic temperatures of 600 to 680° at an estimated pressure of 4 kilobars. A temperature of near 600°C for the peak of metamorphism is suggested by mineral assemblages at the southernmost locality, Jäderinfjellet. Calcite-dolomite geothermometry indicated 595°C at the same locality. The spatial distribution of the marble assemblages suggests that metamorphism occurred under nearly isothermal conditions over an area of at least 25 by 30 kilometres.

Dedolomitized Marble Lenses in Shear Zone Tectonites, Medicine Bow Mountains, Wyoming

Lenses of dedolomitized marble occur interspersed with Precambrian cataclastic rocks of the northeast-trending Mullen Creek-Nash Fork shear zone in the east-central portion of the Medicine Bow Mountains of southeast Wyoming. Eight lenses of tremolite marble (dolomitic) and one small body of serpentinized forsteritic marble (calcic) were observed. The Mullen Creek-Nash Fork shear zone separates a thick sequence of low grade metasedimentary rocks, including two locally siliceous dolomitic marble (metadolomite) units of the Nash Fork Formation, from a higher grade felsic and amphibolitic complex. The marble lenses that occur within the shear zone are considered to be fault slices of the dolomitic units that were dedolomitized during mylonitization. The formation of tremolite and forsterite in the lenses was probably facilitated by the presence of a water-rich fluid in the shear zones. Minor occurrences of talc-dolomite-calcite-quartz and tremolite-dolomite-calcite-quartz assemblages in the main sequence of Nash Fork Formation metadolomite are probably also a function of reactions in zones locally enriched in fluids. The presence of assemblages of Tc + Dol + Cc + Q and Tr + Dol + Cc + Q without the intermediate assemblage Tc + Tr + Dol + Cc + Q in the Nash Fork Formation and Tr + Dol + Cc and Fo + Cc + Dol without Tr + Fo + Cc + Dol in shear zone marble slices suggests that each set of reactions may have been controlled by different fluid paths. The stable talc and forsterite assemblages apparently formed under conditions of higher than the tremolite assemblages. Parent dolomite in the marble slices, although recrystallized, only partly decomposed to form tremolite; the tremolite-dolomite ratio increasing with increasing silica content. Under local conditions of higher tremolite apparently reacted with much of the excess dolomite to form forsteritic olivine (). Both tremolite and olivine have since been partially altered to talc and antigorite respectively (most pseudomorphically). This alteration represents a secondary or late retrogressive event superposed upon the upper greenschist or lowermost almandine amphibolite grade dedolomitization process that accompanied the development of the mylonites.