Granodioritic Rocks Research Articles

Geochemistry and fluid evolution of the peralkaline rare-metal granite, Gabal Gharib, Eastern Desert of Egypt

The Neoproterozoic pluton of Gabal Gharib granite Eastern Desert of Egypt is intruded in subduction-related calc-alkaline granitic rocks of granodiorite to adamellite composition. A zone of metasomatized granite was developed along the contacts at the expense of the calc-alkaline granite. The granite of Gabal Gharib is hypersolvus, composed mainly of orthoclase-microperthite, quartz, and interstitial arfvedsonite. Fluorite, zircon, ilmenite, allanite, and astrophyllite are the main accessories. Pegmatite pods as well as miarolitic cavities (mineral-lined cavities) are common and ranging in size from a few millimeters to 50 cm. Rare-metal minerals such as columbite, cassiterite, and fluorite have been identified from the miarolitic cavities. Geochemical studies revealed that Gabal Gharib granite is a highly fractionated granite, homogeneous in composition, with high contents of SiO2, and alkalis, high Ga/Al, and Fe/Mg ratios, and low concentrations of Al, Mg, and CaO relative to granodiorite–adamellite country rocks. Gabal Gharib granite is metaluminous to peralkaline with ASI (0.94–1.07). Trace element characteristics of Gabal Gharib granite include abundances of Rb, Nb, Ta, Sn, Th, U, Y, Ga, Zn, rare earth elements (REEs, except Eu), and F, and depletion in Sr, and Ba relative to granodiorite–adamellite country rocks. It has the geochemical characteristic of anorogenic A-type granite. The uniform trends of differentiation, normal REE distribution patterns, and low calculated tetrad effects of REE ( 600°C were also reported. Petrographic, geochemical, and fluid inclusion studies constrain that the peralkaline anorogenic granite of Gabal Gharib was derived from highly evolved magma probably originated by fractional crystallization of mantle source.

Shear-hosted base metal mineralisation at the Dana Peaks, Murchison Mountains, Fiordland, New Zealand

Darran Suite dioritic, tonalitic and granodioritic plutonic rocks and schistose Loch Burn Formation volcaniclastic rocks in the central Murchison Mountains at the Dana Peaks have been affected by widespread biotite-sericite-chlorite-albite-quartz-pyrite±carbonate±epidote/clinozoisite±titanite/rutile±actinolite alteration. More intense, paler coloured sericite-albite-quartz-pyrite±carbonate alteration is concentrated along orange weathered shear zones. Alteration assemblages are transitional between those commonly referred to as propylitic, potassic and phyllic. Altered rocks contain anomalous concentrations of copper, lead, zinc and silver over an area of c. 2.5×6 km. Metal concentrations 2–5 times those typical of Darran Suite plutonic rocks and the Loch Burn Formation are commonly associated with more extensive weak to moderate intensity alteration. Higher metal grades up to c. 0.5% copper, 1% zinc, 1.3% lead and 30 ppm silver are concentrated in or adjacent to the 1–5 m wide, more intensely altered shear zones which contain entrained lenses of pyritised country rock, breccias and quartz±K-feldspar±chlorite±carbonate±hematite±tourmaline veins. Some mineralised rocks also contain traces of tungsten (2–7 ppm), arsenic (<5–35 ppm) and tellurium (0.2–5.4 ppm). Most samples lack detectable molybdenum (<3 ppm), gold (<0.004 ppm) or bismuth (<0.2 ppm), with atypical higher values (40, 0.03 and 50 ppm, respectively) generally restricted to the most intensely altered and/or deformed rocks. The mineralised rocks show a close spatial and temporal relationship with several narrow ductile shear zones that probably developed in the Early Cretaceous between c. 128 and 110 Ma. Mineralised shear zones form minor splays off larger shear zones that are part of a major intra-arc fault system, active along or near the boundary between inboard and outboard parts of the Median Batholith at this time. Traces of similar lead mineralisation are present at the head of the Mid Burn c. 8 km to the northeast along strike on the same regional-scale fault system as the Dana Peaks locality. Fluids and metals are likely to have been derived from metamorphism of the Loch Burn Formation and/or adjacent plutonic rocks. A magmatic hydrothermal origin is considered unlikely as the metal assemblage lacks Mo, Bi, Au, Sn and W and the adjacent plutonic rocks probably crystallised several million years or more before alteration and mineralisation.

Geochemical and isotopic studies of the Lady of the Lake Intrusion and associated tobacco root Batholith: Constraints on the genetic relation between Cretaceous mafic and silicic magmatism in Southwestern Montana

Small volumes of alkalic mafic intrusions are spatially associated with Cretaceous to Early Tertiary granodioritic to granitic intrusions in the batholithic province of southwestern Montana. The mafic rocks generally occur near the contacts of the Boulder, Pioneer, and Tobacco Root Batholiths with country rocks, but their genetic relation with the batholiths is uncertain. The Lady of the Lake Intrusion is a small layered body composed of melagabbro and gabbro that occurs along the south-central margin of the Tobacco Root Batholith near its contact with Archean country rocks. A diorite unit, spatially distinct from the granodiorite/quartz monzonite of the Batholith intrudes the gabbroic rocks of the Lady of the Lake Intrusion. Zircon crystals from the melagabbro and diorite units give U–Pb ages that are very similar to that of the Tobacco Root Batholith at 74.88±0.17 Ma and 76.24±0.08 Ma, respectively. Mineral chemistry, whole rock major and trace element compositions, and oxygen and sulfur isotope ratios have been utilized to evaluate the genetic relation between the Lady of the Lake Intrusion, the diorite, and the Tobacco Root Batholith. No significant variation in the composition of clinopyroxene is observed in different rock units of the Lady of the Lake Intrusion. Minor olivine with Fo64 in the melagabbro unit is interpreted to represent early crystallization in the base of the intrusion. Whole rock major and trace element compositions, as well as results from modeling using the MELTS program, are consistent with the premise that the diorite was produced by fractional crystallization of the same magma that was parental to the gabbros of the Lady of the Lake Intrusion. Both whole rock chemistry and oxygen isotopes support the interpretation that the parental magma was an uncontaminated mantle-derived basaltic magma. In contrast, trace element and oxygen isotopes indicate that the quartz monzonitic and granodioritic rocks of the Tobacco Root Batholith and the gabbroic rocks of the Lady of the Lake Intrusion are not related to the same parental magma. Elevated d18O values of quartz (9–10‰) and feldspar (8–9‰) from the Batholith are consistent with an origin involving partial melting of Archean and Proterozoic country rocks that was initiated by heat supplied by basaltic magma, similar to that which was parental to the Lady of the Lake Intrusion. d34S values of the melagabbro range from 0.1 to 0.7‰, and support the premise that the mantle-derived parental magma was uncontaminated. In contrast, d34S values of the gabbro and diorite units are elevated (0.6 to 6.5‰), and indicate that assimilation of country rock S occurred, primarily via volatile transfer. Sulfur contents of the Lady of the Lake Intrusion are low (<1000 ppm) and no evidence suggests that S was transferred to crustal melts that produced the Tobacco Root Batholith.

Structure and geochemistry of the adakitic Horoz granitoid, Bolkar Mountains, south-central Turkey, and its tectonomagmatic evolution

High-Al granitic and granodioritic rocks of the 55 Ma Horoz pluton in the Bolkar Mountains of southern Turkey provide important new constraints on the early Cenozoic evolution of the eastern Mediterranean region. The ENE–WSW-trending, sill-like pluton is intrusive into late Palaeozoic–early Mesozoic metamorphic rocks of the Central Tauride block, and is unconformably overlain by Plio-Pleistocene alluvial deposits. The metaluminous to peraluminous granitoids have high-K calc-alkaline to high-K shoshonitic compositions, and show enrichment in large ion lithophile and depletion in high-field strength elements relative to ocean ridge granite. Their high Al2O3 contents (15.9–20.06 wt%) and low SiO2, MgO, and Mg numbers are consistent with adakitic compositions. These geochemical features, coupled with low Sr/Y and La/Yb ratios and trace-element patterns, suggest that the Horoz magmas were produced in part by partial melting of a subduction-metasomatized mantle. The high-Al adakitic and calc-alkaline compositions are consistent with partial melting of a hydrated lithospheric mantle and an amphibolitic–eclogitic mafic lower crust that was triggered by delamination-induced asthenospheric upwelling. We propose that, following Palaeocene continental collision between the Tauride and Central Anatolian Crystalline Complex, the inferred lithospheric delamination was a result of foundering of the overthickened orogenic root. Asthenospheric upwelling beneath the young orogenic belt thermally weakened the crust, and caused uplift and tectonic extension leading to core complex formation (Nigde massif), development of an extensional volcanic province (Cappadocia), and tectonic collapse of the Central Tauride block (Bolkar Mountains). The shallow-level Horoz pluton was unroofed by ∼23 Ma as a combined result of crustal uplift and erosion throughout the Palaeogene.

Behaviour of thermal waters through granite rocks based on residence time and inorganic pattern

Summary Thermal waters are certainly a substantial asset of the Galicia region of Spain. They can be regarded as worth developing because of their human health implications and, if thermal tourism is promoted, their importance to the local economy. In this paper the chemistry of major and trace inorganic elements in about 45 thermal springs and wells discharging in the same hydrographical system are presented and discussed. For handling the results of all measurements, graphical representations of B/Li vs. SO 4 2 - /Cl− ratios, Hill–Piper diagram, discriminant analysis (DA) and principal component analysis (PCA) were employed. All this with the intention to classify, based on their inorganic pattern, both thermal springs and wells waters, but also waters circulating through adamellite and granodiorite rocks. The results of the hydrogeochemistry analysis showed three main water families: sulphated, chlorinated and bicarbonated waters. The results show also the presence of saline materials with chloride influence in the deeper aquifer, allowing its classification in deeper and younger/shallow waters.

Evidence for an early Archaean granite from Bastar craton, India

Abstract: Granitoids in the early Archaean are believed to be potassium-poor tonalite–trondhjemite–granodiorite rocks. Only after continental crust attained sufficient thickness did true (relatively potassium-rich) granites form. No record of true granite prior to 3.4 Ga is available. We report a 3.6 Ga true granite from the Archaean Bastar craton in India. In contrast to the typical early Archaean granitoids, which are commonly deformed into gneisses, this granite is relatively undeformed. The age and composition of the granite implies that continental crust of the Bastar craton attained sufficient thickness to permit intracrustal melting at 3.6 Ga. Supplementary material: Representative major element, trace element and REE composition of the Dalli-Rajhara granite samples and a summary of SHRIMP U-Pb zircon data for the granite sample D-9 are available at http://www.geolsoc.org.uk/SUP18337 .

Interactions between dioritic and granodioritic magmas in mingling zones: plagioclase record of mixing, mingling and subsolidus interactions in the Gęsiniec Intrusion, NE Bohemian Massif, SW Poland

Dioritic and granodioritic rocks coexist in the Gesiniec Intrusion in SW Poland showing typical relationships in many mafic–felsic mingling zones worldwide, such as dioritic syn-putonic dykes and microgranular enclaves within granodioritic host. Plagioclase zonation from granodioritic rocks suggests late stage mixing probably with dioritic magma, whereas no magma mixing is recorded in plagioclase from dioritic rocks. The diorites seem to show effects of interaction with evolved, leucocratic melts derived from granodiorite, not with the granodioritic melt itself. We conclude that the diorites’ compositions were modified after their emplacement within the granodioritic host, when the diorites were essentially solidified and injection of evolved melt from granodiorite did not involve marked modification of plagioclase composition. Compositional zoning patterns of plagioclase in diorites can be modeled by closed system fractional crystallization interrupted by resorption induced probably by decompression. Granodioritic plagioclase seems to be affected by the same resorption event. Plagioclase that crystallized in dioritic magma before the resorption does not record interaction between dioritic and granodioritic magmas, suggesting that both magmas evolved separately.

Role of fluid mixing and wallrock sulfidation in gold mineralization at the Semna mine area, central Eastern Desert of Egypt: Evidence from hydrothermal alteration, fluid inclusions and stable isotope data

The Semna gold deposit is one of several vein-type gold occurrences in the central Eastern Desert of Egypt, where gold-bearing quartz veins are confined to shear zones close to the boundaries of small granitoid stocks. The Semna gold deposit is related to a series of sub-parallel quartz veins along steeply dipping WNW-trending shear zones, which cut through tectonized metagabbro and granodiorite rocks. The orebodies exhibit a complex structure of massive and brecciated quartz consistent with a change of the paleostress field from tensional to simple shear regimes along the pre-existing fault segments. Textural, structural and mineralogical evidence, including open space structures, quartz stockwork and alteration assemblages, constrain on vein development during an active fault system. The ore mineral assemblage includes pyrite, chalcopyrite, subordinate arsenopyrite, galena, sphalerite and gold. Hydrothermal chlorite, carbonate, pyrite, chalcopyrite and kaolinite are dominant in the altered metaggabro; whereas, quartz, sericite, pyrite, kaolinite and alunite characterize the granodiorite rocks in the alteration zones. Mixtures of alunite, vuggy silica and disseminated sulfides occupy the interstitial open spaces, common at fracture intersections. Partial recrystallization has rendered the brecciation and open space textures suggesting that the auriferous quartz veins were formed at moderately shallow depths in the transition zone between mesothermal and epithermal veins. Petrographic and microthermometric studies aided recognition of CO 2-rich, H 2O-rich and mixed H 2O–CO 2 fluid inclusions in the gold-bearing quartz veins. The H 2O–CO 2 inclusions are dominant over the other two types and are characterized by variable vapor: liquid ratios. These inclusions are interpreted as products of partial mixing of two immiscible carbonic and aqueous fluids. The generally light δ 34S of pyrite and chalcopyrite may suggest a magmatic source of sulfur. Spread in the final homogenization temperatures and bulk inclusion densities are likely due to trapping under pressure fluctuation through repeated fracture opening and sealing. Conditions of gold deposition are estimated on basis of the fluid inclusions and sulfur isotope data as 226–267 °C and 350–1100 bar, under conditions transitional between mesothermal and epithermal systems. The Semna gold deposit can be attributed to interplay of protracted volcanic activity (Dokhan Volcanics?), fluid mixing, wallrock sulfidation and a structural setting favoring gold deposition. Gold was transported as Au-bisulfide complexes under weak acid conditions concomitant with quartz–sericite–pyrite alteration, and precipitated through a decrease in gold solubility due to fluid cooling, mixing with meteoric waters and variations in pH and fO 2.

The geochemistry of the Sorvik granite—a TIB-1 granite

The Sörvik granite occurs as rounded massifs in the Revsund granite in central Sweden. The rock is medium-grained with rounded quartz crystals (‘drop quartz’) and occasional, sometimes albiterimmed megacrysts of K-feldspar. It is a faintly peraluminous I-type granite with some A-type affinities showing a limited chemical variation. The SiO2 content normally varies between 68 and 77 weight%. The rock is fairly high in Rb and rather low in Ta, Nb, Sr and Ti. REE patterns are weakly differentiated with moderate negative Eu-anomalies. Chemically, the Sörvik granite is more similar to rapakivi granite than to the neighbouring Revsund granite. εNd at the time of intrusion varies between −1.5 and −0.9. Zr-thermometry gives a minimum temperature around 800°C for the magma generation. The source rock is modelled as a granodioritic rock. Anhydrous melting due to biotite breakdown supplies insufficient amounts of water to meet a reasonable rheologically critical melt percentage. If the temperature estimate is correct, either low degrees of granitic melt might segregate from its place of origin, or some water was added during melting. A local, probably late-magmatic F-metasomatism has caused a Nb-Ta-U-Th anomaly resulting in specific Nb minerals.

Limited iron isotope variations in recent lateritic soils from Nsimi, Cameroon: Implications for the global Fe geochemical cycle

Laterites are oxidized Fe-rich soils covering one third of the continents and are drained by half of the continental waters. They therefore represent a key component of the iron geochemical cycle at the Earth's surface, yet no iron isotope study has been conducted on recent laterites so far. Building on previous integrated morpho-pedological studies of soils located in an equatorial rainforest, Southern Cameroon, we have undertaken a mineralogical, elemental and isotopic study of iron in two lateritic profiles. One is a borehole 3620 cm deep going to the parent granodioritic rock, located at the top of a hill, whereas the other is 675 cm deep and is located downhill. Mössbauer spectroscopy reveals that typically ≥ 96% of the soil's iron is held in nanocrystalline hematite and goethite. Iron isotope measurements performed by plasma source mass spectrometry show that Fe isotopic equilibrium was rarely reached between these iron oxide and hydroxide despite their small size. Overall, it is found that most samples display iron isotope signatures very close to the mean crustal value, with a maximum range of 0.2‰ in δ 57Fe. Given that lateritic soils evolve over millions of years, show large variations in Fe concentrations and mineralogical abundances, this range is surprisingly small when compared to other Fe isotope studies of soils from different climatological contexts, that can easily show δ 57Fe ranges in excess of 1‰ at the bulk sample scale. The most likely explanation of this finding is that despite notable vertical and lateral Fe mobility in the studied laterites from Cameroon, as computed using the open-system mass fraction transport function, τ Fe,w, iron remained mostly in the oxidized form as shown by Mössbauer pectroscopy. This probably results from the strong bioturbation and pedoturbation of these lateritic soils that lead them to remain porous over a large thickness and facilitated the flow of oxygenated waters from the surface down to the saprolite horizon. This study therefore reveals that soils that remained as an open system for iron do not necessarily show large Fe isotopic variations. It is suggested that the Fe drained from such lateritic soils should have δ 57Fe values within ~ 0.1‰ of that of the continental crust. This is in contrast with previous soil studies, notably from sites located at higher latitudes, that imply that these soils should release isotopically more variable Fe to surface waters. Such possible contrasted isotopic signatures from different surface waters will likely lead to an isotopically heterogeneous ocean given the short residence time of Fe in seawater.

Porphyry Cu-Mo deposits in the eastern Xing’an-Mongolian Orogenic Belt: Mineralization ages and their geodynamic implications

The eastern Xing’an-Mongolian (Xing-Meng) Orogenic Belt (XMOB) is one of the important areas of porphyry copper (Cu)-molybdenum (Mo) deposits in China. However, studies on the exact ages of mineralization and their geodynamic significance are very limited. In this study, granodioritic rocks from the Duobaoshan Cu deposit and Daheishan Mo deposit were selected to make zircon SHRIMP U-Pb analyses in order to constrain their mineralization ages. Geochronological data indicate that two episodes of mineralization took place in the Duobaoshan Cu deposits. The granodiorite related to the Duobaoshan porphyry Cu deposit was formed in the Early Paleozoic with zircon U-Pb age of 485±8 Ma, whereas the granodiorites related to the Sankuanggou skarn-type Cu deposit were emplaced in the Jurassic with zircon U-Pb ages of 176±3 and 177±3 Ma. In the Daheishan area of Jilin Province, the emplacement age of the granodiorite porphyry related to the porphyry Mo deposit was dated at 170±3 Ma, and the unmineralized monzogranite at 178±3 Ma. Therefore, two episodes of Cu-Mo mineralization were developed in the eastern XMOB, at ~485 Ma and ~175 Ma, respectively. Based on the geological history and spatial-temporal distribution of the granitoids in northeastern (NE) China, it is proposed that the Duobaoshan Cu deposit was related to the collision of the Xing’an and Erguna blocks in the Early Paleozoic, and the Sankuanggou Cu and Daheishan Mo deposits were related to subduction of the Paleo-Pacific plate during the Jurassic.

Geochemistry and geochronology of high-grade rocks from the Grove Mountains, East Antarctica: Evidence for an Early Neoproterozoic basement metamorphosed during a single Late Neoproterozoic/Cambrian tectonic cycle

The Grove Mountains of East Antarctica are an inland continuation of the Prydz Belt. The high-grade metamorphic complex in this area is composed of felsic orthogneisses and mafic granulites, with minor paragneisses and calc-silicate rocks. U–Pb zircon analyses using SHRIMP and LA-ICP-MS techniques reveal that the protoliths of mafic granulites and felsic orthogneisses were emplaced during a short time interval of ca. 920–910 Ma. Mafic granulites can be divided into low- and high-Ti groups. They have initial ɛ Nd values [ ɛ Nd( T)] ranging from +0.8 to −1.9. TiO 2 is positively correlated with FeO t/MgO and La/Nb ratios, whereas it shows a negative correlation with ɛ Nd( T) values, indicating that the petrogenesis of their protoliths involved partial melting of a weakly enriched subcontinental lithospheric mantle and fractional crystallization of the magma accompanied by minor crustal contamination. Felsic orthogneisses have an affinity of A 2-type granites, characterized by enrichment in REE, Y, Zr, Th and Ga and high Ga/Al and Y/Nb ratios. Most of them have ɛ Nd( T) values of −0.7 to −3.5 and Nd depleted mantle model ages ( T DM) of 1.76–1.65 Ga, and a few have low ɛ Nd( T) values of −10.4 to −10.6 and old T DM of 2.46–2.27 Ga, reflecting a heterogeneity in their source region. Their protoliths were probably produced by high-temperature partial melting of tonalitic–granodioritic rocks triggered by the underplating of mantle-derived mafic magma during post-orogenic extension. U–Pb analyses also reveal a metamorphic age of ca. 2050 Ma from detrital zircons in a paragneiss, suggesting that the sedimentary materials of the paragneiss may have come from an as yet undiscovered Early Paleoproterozoic orogen of unknown provenance. Voluminous mafic–felsic intrusives and a small amount of sedimentary rocks constitute an Early Neoproterozoic basement of the Grove Mountains. Subsequently, this basement experienced only a single Late Neoproterozoic/Cambrian metamorphic cycle at ca. 550–535 Ma. The available data indicate that the Prydz Belt is a collage of multiple basement terranes and each of them has a distinct tectonic evolution. This supports the suggestion that the Prydz Belt may represent a Late Neoproterozoic/Cambrian collisional orogen that resulted in the final phase of the Gondwana assembly.

Paleoweathering conditions of Upper Carboniferous siliciclastic rocks of SW Hungary

The clay mineralogical and chemical compositions of Upper Carboniferous siliciclastic rocks from the western flank of the Villány Mountains (Téseny Sandstone Formation) have been investigated to determine paleoweathering conditions, as well as to appraise the influence of the post-depositional processes upon source rock signature. The clay-mineral assemblage of the samples consists predominantly of illite±muscovite, suggesting a potassium metasomatism in the Téseny clastics. Therefore the use of the Chemical Index of Alteration (CIA), which provides a consistent quantitative framework for examining weathering, leads to erroneous conclusions without correction for K-metasomatism. When considered in Al2O3-CaO* + Na2O-K2O (A-CN-K) compositional space, orthogneiss and igneous rock clasts selected from the Téseny conglomerate reflect two different weathering trends; one (including orthogneiss, quartz diorite, and andesite samples) shows an ideal trend observed for granodioritic rocks, and the other (including aplite, rhyodacite, and rhyolite samples) follows a trend from a slightly more K-feldspar-rich fresh rock composition than that of average granite. Intermediate to intense chemical weathering of the source areas is indicated by premetasomatized CIA values of 77–84 for the samples from borehole Siklósbodony-1, suggesting that these rocks have gained about 6–7% K2O (in A-CN-K space) during metasomatism.

Paleoproterozoic granitoids in the basement of Bangladesh: A piece of the Indian shield or an exotic fragment of the Gondwana jigsaw?

We present the first precise age for Precambrian basement rocks in Bangladesh. These lie within the Dinajpur block, located between the Indian Craton to the southwest and the Shillong Massif to the east. There are no surface outcrops and the rocks were intersected by drill holes. They consist of a suite of tonalitic and granodioritic rocks, variously deformed to granitic gneiss and intruded by younger monzogranite. A tonalite obtained at a depth of 227.48 m in drill hole BH-2 at Maddhapara, in northwestern Bangladesh, records a SHRIMP zircon 207Pb/ 206Pb magmatic age of 1722 ± 6 Ma. Paleoproterozoic rocks with similar magmatic ages are unknown in the adjacent Indian blocks of the Chotanagpur Plateau (Indian Craton) or Shillong Massif. This lack of comparable ages may be due to the paucity of precise radiometric ages from the Indian terrains or, more likely, because there are real age differences, with the buried rocks at Maddhapara representing a separate and discrete microcontinental fragment (the Dinajpur block) that was trapped by the northward migration of India during Gondwana dispersal.

The distribition and diagnostic features of deformed plutonic rocks in two terranes of the Namaque mobile belt along the Orange (Gariep) River, South Africa

Two periods of deformation have been documented by detailed mapping in a strip of 3000 km 2 along the Orange (Gariep) River. The mapping has enabled field discrimination and regional correlation of well-exposed plutonic tectonites that volumetrically dominate the western part of the Namaqua mobile belt. The associated metasedimentary and metavolcanic successions that supplied tectonostratigraphic keys for correlation of rock units, were described in previous publications. Deformation took place during an orogenesis that may be termed the Orange River Event (ORO, ~2.0 to ~1.6 Ga, with associated calcalkaline magmatism) and the later Namaqua Event (NMO, ~1.2 to ~1.1 Ga, with associated granitic magmatism). The ORO magmatism includes the Vioolsdrif Suite, represented by outcrops of deformed leucogranite, adamellite, granodiorite and mafic rocks that were later intruded by small mafic complexes. The products of the NMO magmatism include various members correlated with the Little Namaqualand Suite (the Kabis, Pipeline, Swartmodder, Koeipoort, Mission, Coboop and Eendoorn gneisses). Textural properties of the metaplutonic rocks vary from very coarsely porphyroblastic ( e.g. Eendoorn gneiss, megacrysts >10 cm) to fine even-grained (e.g. some Kabis gneiss). The megacrysts, invariably composite on microscopic scale, are deformed relicts of phenocrysts. Variation in strain is well illustrated by megacrysts that grade from nearly equidimensional to flattened, visible as pencil-shaped streaks on exposed surfaces. Several gneisses that appear to be unrelated because of different textural properties, merely record variation in strain intensity ( e.g. the little deformed granodioritic occurrences at Vioolsdrif are coeval with severely deformed rocks at Pella, 150 km furthur to the east). Detailed structural-stratigraphic mapping and petrographic study helped to prevent unnecessary proliferation of names. Compressional tectonics were responsible for the general style of both orogeneses reflecting the effects of progressive ductile shearing rather than consecutive discrete deformational events. ORO intrusives belong to specific tectonic terranes and tend to be batholithic (in the Pofadder terrane). The grade of metamorphism was greenschist to amphibolite. The rapid tectonism of the subsequent NMO cycle led to terrane accretion and to granulite/upper amphibolite grade metamorphism. Associated granitic intrusives are not terrane-specific. The predominant sheet-like to phacolithic habits reflect emplacement during horizontal tectonism.

Lithological interpretation of crustal composition in the Fennoscandian Shield with seismic velocity data

In this study, we report the results of an investigation of lithological interpretation of the crust in the central Fennoscandian Shield (in Finland) using seismic wide-angle velocity models and laboratory measurements on P- and S-wave velocities of different rock types. The velocities adopted from wide-angle velocity models were compared with laboratory velocities of different rock types corrected for the crustal PT conditions in the study area. The wide-angle velocity models indicate that the P-wave velocity does not only increase step-wise at boundaries of major crustal layers, but there is also gradual increase of velocity within the layers. On the other hand, the laboratory measurements of velocities indicate that no single rock type is able to provide the gradual downward increasing trends. Thus, there must be gradual vertical changes in rock composition. The downward increase of velocities indicates that the composition of the crust becomes gradually more mafic with increasing depth. We have calculated vertical velocity profiles for a range of possible crustal lithological compositions. The Finnish crustal velocity profiles require a more mafic composition than an average global continental model would suggest. For instance, on the SVEKA'81 transect, the calculated models suggest that the crustal velocity profiles can be simulated with rock type mixtures where the upper crust consists of felsic gneisses and granitic–granodioritic rocks with a minor contribution of amphibolite and diabase. In the middle crust, the amphibolite proportion increases. The lower crust consists of tonalitic gneiss, mafic garnet granulite, hornblendite, pyroxenite and minor mafic eclogite. Assuming that these rock types are present in sufficiently extensive and thick layers, they would also have sufficiently high acoustic reflection coefficients for generating the generally well-developed reflectivity in the crust in the central part of the shield. Density profiles calculated from the lithological models suggest that there is practically no density contrast at Moho in areas of the high-velocity lower crust. Comparison of reflectors from FIRE-1 and FIRE-3 transects and the velocity model from SVEKA'81 wide-angle transect indicated that the reflectors correlate with velocity layering, but the three-dimensional structures of the crust complicate such comparisons.

Geochronology, petrogenesis and tectonic implications of Jurassic granites in the Liaodong Peninsula, NE China

Mesozoic igneous rocks are widespread throughout eastern China, but precise geochronological and petrogenetic constraints were previously lacking. Ten samples from the Liaodong Peninsula in northeastern China were chosen for zircon U–Pb SHRIMP and laser ablation ICP-MS dating. The magmatic ages range from 179 ± 3 to 156 ± 3 Ma. Data compilation indicates that contemporaneous granitic magmatism is widespread throughout eastern China, establishing the Jurassic as an important period of igneous activity in eastern China. Petrographically, these granites can be divided into three groups that underwent a complex history of crystal fractionation. Two end-members of granodioritic and monzogranitic magma are identified. The granodioritic rocks, having lower ( 87Sr / 86Sr) i and higher ɛ Nd( t) values than those of the monzogranitic rocks, came from the partial melting of juvenile crust, whereas the monzogranitic rocks came from partial melting of the Precambrian basement. It is proposed that Pacific plate subduction resulted in crustal thickening and subsequent lithospheric delamination which resulted in the upwelling of asthenospheric mantle and formation of juvenile crust by underplating of mantle-derived magma in the lower crust. A subsequent underplating and heating event from the asthenosphere partially melted the overlying pre-existing underplated mafic rocks and ancient crust, leading to the formation of granodioritic and monzogranitic magmas. This thickening by subduction could have been a necessary precursor for limited delamination in the Jurassic and more extensive delamination in the Early Cretaceous.

Weathering and deterioration of rock properties of the Dabotap pagoda (World Cultural Heritage), Republic of Korea

The Bulkugsa Dabotap pagoda was built in AD 761, and designated as World Cultural Heritage by UNESCO in 1995. The ca. 270 blocks used in the construction of the pagoda consist mainly of white grey alkali granite with medium-grained equi-granular texture and small mialolitic cavities. Small quantities of biotite granite, granodiorite, gabbroic and tuffaceous rocks were also used. Some overlapping blocks are very damp. The pagoda body is inclined toward the northeast owing to differential loading. Fractures and artificial deterioration of the pagoda are not serious, but the surfaces of blocks show exfoliations that indicate granular decomposition of minerals. The supporting rocks and the round handrail of the octagonal second floor are too weathered to be treated and are being replaced. The roof stones on the first and second floor are seriously damaged because there is no drainage route for rainwater. The joints between blocks have fractures that cross each other; they have been further damaged by inserting concrete, cement mortar, rock fragments and iron plates. This has accelerated discoloration and fractures. Widespread colonization by algae, fungi, lichens and bryophytes has occurred on the margins and surfaces roof stones of the pagoda. Chemical treatment will be necessary to remove the vegetation cover.

Mineralogy and Geochemistry of an Epithermal Manganese District, Sierras Pampeanas, Argentina

The manganese mineralization studied in this paper represents the major concentration of this element in Argentina. It occurs in an area of 70 x 30 km, in the Sierras Pampeanas ranges, located in the provinces of Córdoba and Santiago del Estero. The analyzed area represents about 30% of the entire mineralized system. It is quite unusual because it is hosted in granodioritic and rhyodacitic rocks. Geochemical, petrological, and structural analyses demonstrate that these rocks are unrelated to the mineralization. The geometry and distribution of the veins are mainly controlled by faults, where the manganese is found mainly as fault-breccia cement. These faults are the result of a dextral shear system that produced Riedel-type structures related to N-S lineaments. Textural and structural analyses of the veins indicate four deformational events. Ore minerals precipitated during the first three events, whereas the gangue was mainly formed during the last episode. X-ray diffraction and microprobe studies show that Mn4+ oxides (hollandite, pyrolusite, ramsdellite, romanèchite, cryptomelane, and coronadite) associated with Fe3+ oxides (goethite and hematite) are the ore minerals. The gangue consists of calcite, opal, and lesser amounts of barite, fluorite, and clay minerals. The studied area represents shallow levels of an epithermal ore deposit; hence, it is possible to predict that these veins may continue to deeper levels.

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The errors in the chemical analyses of a granodiorite rock powder for 6 constituents (K 2O, Na2O, CaO,∑Fe2O3, MgO, and MnO) have been determined by replicate analyses of the whole rock and of a series of different sieved fractions. Sample inhomogeneity is indicated. This is confirmed by analyses of the fused rock powder. Fusion to a glass is therefore recommended for the preparation of geochemical rock standards.