Feldspar Phenocrysts Research Articles

Formation of Li-Rb-Cs greisen-type deposit in Zhengchong, Jiuyishan district, South China: Constraints from whole-rock and mineral geochemistry

The Jiuyishan complex located at Hunan Province, Jiuyishan district, South China, is composed of five granitic plutons (Xuehuading, Jinjiling, Pangxiemu, Shaziling and Xishan). The Zhengchong Li-Rb-Cs deposit is a greisen-type deposit hosted by the Jinjiling granite, adjacent to the Pangxiemu granitic pluton. Based on textures, three types of greisen may be distinguished at Zhengchong: medium-grained massive greisen, feldspar-phyric greisen and quartz-phyric greisen. Detailed studies on field occurrence, petrographic observations, whole-rock and mineral geochemistry reveals that the Jinjiling granites represent the protoliths of the greisen mineralization and not the Pangxiemu granites. Metasomatic alteration of different phases of the Jinjiling granites formed at least three greisen types mentioned above. The medium-grained greisen and quartz-phyric greisen are totally altered, but the feldspar-phyric greisen are relatively less altered with residual feldspar phenocrysts. We also propose three main indications for ore deposit exploration in this area: future mineral exploration in the district should focus on the periphery of the Pangxiemu pluton, particularly along structures, where greisen alteration has been developed in places.

A tuffaceous volcaniclastic turbidite bed of Early Miocene age in the Temburong Formation of Labuan, North‐West Borneo and its implications for the Proto‐South China Sea subduction in the Burdigalian

AbstractA 2 m thick bed of smectite‐rich clay within a sequence of distal deep water turbidites and marine mudstones of the Temburong Formation on the island of Labuan, Sabah, North‐West Borneo, is an altered, re‐sedimented pyroclastic tuff which is geochemically of rhyolite‐dacite composition. The tuff bed was deposited in deep water, although probably <1,000 m, by traction currents and retains relic siliceous shards, feldspar phenocrysts and biotite crystals but was dominated by ash‐grade material. The tuff is interpreted as a turbidite deposit and most probably records long distance sub‐marine transport down the depositional slope from an active volcanic system in the hinterland. It has a zircon U–Pb weighted mean age of 19.6 ± 0.1 Ma whilst combined forminiferal and nannofossil ages from the enclosing marine mudstones range from 21.5 to 19.4 Ma; together these indicate an early Miocene age for the Temburong Formation. The tuff is the result of a very short‐lived volcanic arc magmatic episode and was either related to similar aged volcanics of the Sintang Suite in central Borneo or a product of the Proto‐South China Sea subduction beneath the Cagayan arc north‐east of Borneo. The age indicates that deep water sedimentation in North‐West Borneo continued into the Burdigalian suggesting the subduction trench of the Proto‐South China Sea was active into the early Miocene, beneath the depocentre of the Temburong turbidite fan. By contrast, sandstones representative of the typical marine Temburong Formation yield abundant zircons which are dominately Cretaceous in age indicating an important switch in source provenance during the Early Miocene from the Crocker Formation that is dominated by Permian‐Triassic zircons. This difference in zircon populations likely reflects an important tectonic event in the early Miocene that resulted in uplift of central Borneo to supply sediment to the Sabah Trough whilst the input of material derived from the western terrains diminished. The Temburong Formation was sourced by reworking of uplifted Rajang Group, Sapulut or Trusmadi formations.

Understanding mafic-felsic magma interactions in a subvolcanic magma chamber using rapakivi feldspar: A case study from the Bathani volcano-sedimentary sequence, eastern India

The Ghansura Rhyolite Dome of the Bathani volcano-sedimentary sequence in eastern India originated from a subvolcanic felsic magma chamber that was intruded by volatile-rich basaltic magma during its evolution leading to the formation of a porphyritic andesite. The porphyritic andesite consists of rapakivi feldspars, which are characterized by phenocrysts of alkali feldspar mantled by plagioclase rims. Results presented in this work suggest that intimate mixing of the mafic and felsic magmas produced a homogeneous hybrid magma of intermediate composition. The mixing of the hot volatile-rich mafic magma with the relatively colder felsic magma halted undercooling in the subvolcanic felsic system and produced a hybrid magma rich in volatiles. Under such conditions, selective crystals in the hybrid magma underwent textural coarsening or Ostwald ripening. Rapid crystallization of anhydrous phases, like feldspars, increased the melt water content in the hybrid magma. Eventually, volatile saturation in the hybrid magma was reached that led to the sudden release of volatiles. The sudden release of volatiles or devolatilization event led to resorption of alkali feldspar phenocrysts and stabilizing plagioclase, some of which precipitated around the resorbed phenocrysts to produce rapakivi feldspars.

Green core clinopyroxenes from Martin Vaz Archipelago Plio-Pleistocenic alkaline rocks, South Atlantic Ocean, Brazil: A magma mixing and polybaric crystallization record

Martin Vaz Archipelago is located at the parallel 20°S, 1200 km away from the Brazilian coastline and is composed of Plio-Pleistocenic alkaline rocks that represent the last volcanic event of the Vitória-Trindade Ridge. The Archipelago is divided into three main units: Pico das Gaivotas Unit (i) is made up of nosean-phonolite and phono-tephrite domes and tephri-phonolite dykes with clinopyroxene, nosean and potassic feldspar phenocrysts, and scarce kaersutite anhedral phenocrysts; the Bandeira Unit (ii), in turn, comprises melanephelinitic and basanitic lava flows with olivine and clinopyroxene (diopside) phenocrysts; and lastly, the Basal Unit (iii) is composed of pyroclastic deposits with tuffs, lapilli-tuffs and lapillite. The clinopyroxene phenocrysts from Pico das Gaivotas and Bandeira Units have anhedral green cores with sieved and poikilitic textures and show reaction rims composed of opaque minerals, indicating disequilibrium and compositional changes in the liquid. This study provides mineral chemistry data indicating that clinopyroxene rims are more primitive (Mg# 57.46–84.12) and enriched in Al–Ti–Fe3+-Ca compared to the Fe-rich green-cores, probably related with a more primitive Ca-rich magma input in the magmatic chamber after the green-core crystallization from a different liquid. Olivine crystals from Bandeira Unit show resorption features and no compositional affinity with the analyzed liquid, indicating that these crystals have a xenocrystic origin from mantle peridotitic source. Clinopyroxene geothermobarometry data suggest different pressure and temperature conditions for cores and rims during crystallization, reflecting a polybaric evolutionary history. Pico das Gaivotas Unit clinopyroxene phenocrysts suggest a particularly higher-pressure environment, and a complex evolutionary history.

Lithostratigraphy of the Mesoproterozoic Donkieboud Granodiorite (Komsberg Suite), South Africa and Namibia

Abstract The Donkieboud Granodiorite pluton forms an extensive intrusion across the border region between South Africa and southeast Namibia. The mesocratic grey, weakly to moderately K-feldspar porphyritic biotite ± hornblende ± orthopyroxene granodiorite represents the most extensive member of the late- to post-tectonic Komsberg Suite (~1 125 to 1 105 Ma) which intruded as sheet-like bodies into the older high grade paragneisses and orthogneisses (~1 230 to 1 140 Ma) of the Kakamas Domain of the Mesoproterozoic Namaqua-Natal Province. The Donkieboud Granodiorite comprises three main textural variations namely:a porphyritic to weakly porphyritic, relatively undeformed rock with randomly orientated ovoid and twinned feldspar phenocrysts;a weakly- to well-foliated gneiss with between 3 to 10% feldspar phenocrysts set in a medium-grained matrix anda patchy metamorphic charnockite variety. Large inclusions of the strongly foliated Twakputs (~1 210 Ma) and the Witwater (~1 140 Ma) garnetiferous granite gneisses occur within the Donkieboud Granodiorite and mafic xenoliths are common. The Donkieboud Granodiorite is variably deformed ranging from unfoliated to being gneissic. The foliation developed during its intrusion into an existing but waning regional stress field with the strain increasing towards the contacts with the surrounding country rocks. Subsequent km-scale open folding resulted in the reorientation of the gneissic foliation and locally, intense reworking of the fabrics along the margins of the folds. In places, the Donkieboud unit is crosscut by discrete mylonitic shears with a west to northwest trend. U-Pb zircon dating of the Donkieboud Granodiorite samples yielded concordia ages of between 1 118 and 1 107 Ma. Overall the Donkieboud Granodiorite has an intermediate to felsic composition (mean SiO2: 63.9 ± 2.2 wt.%) and is strongly metaluminous. This, together with its biotite-hornblende ± orthopyroxene mineral assemblage and the abundance of mafic xenoliths, suggests it is an I-type granitoid, with the source magma produced by partial melting of older igneous rocks that had not undergone any significant amount of chemical weathering. The εNd values of -1.15 and -0.11 and TDM values of 1 615 and 1 505 Ma are typical of the Komsberg Suite and indicate a significant contribution of older crustal material to the magma of the Donkieboud pluton.

Lithological and structural controls on the emplacement of a Neoarchean plutonic complex in the Carajás province, southeastern Amazonian craton (Brazil)

New geological and geochronological (Pb-Pb evaporation) data from the Vila Jussara suite, a representative of the Neoarchean granitoids of the Carajás province, southeastern Amazonian craton (Brazil), are discussed. The Vila Jussara suite crops out as a series of coalescing plutons elongated in the E-W direction. These plutons generally have sigmoidal shapes, and steep dips (75–85°) that follow the regional trend. The centermost areas of plutons are generally slightly deformed, while the marginal portions display mylonitic aspect and are delimited by sinistral shear zones belonging to the transcurrent system of the Itacaiúnas shear belt. These granitoids cover a large compositional spectrum, with four main lithotypes: (i) Seriated biotite-hornblende monzogranite; (ii) biotite-hornblende tonalite; (iii) porphyritic biotite monzogranite-granodiorite; and (iv) porphyritic hornblende-biotite monzogranite-granodiorite. The microstructural analysis of the studied rocks reveals: (i) microfractures in feldspar phenocrysts, filled with a quartz-feldspar matrix; (ii) preferred orientation of euhedral feldspar crystals in an igneous matrix; and (iii) evidence of moderate-to high-temperature of solid-state deformation (>500 °C). These microstructures and field observations point to a continuum of deformation, from the (sub) magmatic state down to sub-solidus conditions. Dating of three types of granitoids out of four gives ages of 2754 ± 2.2 Ma, 2752 ± 5.7 Ma and 2745 ± 3 Ma. The different granitoid varieties of the Vila Jussara suite were thus emplaced simultaneously. It is proposed that emplacement of the granitoids was controlled by shear zones reactivated during oblique collision and that plutons where constructed by multiple injections of magmas, generating extensive hybridization.

Mineralogical and Geochemical Study of the Zeolitized Volcaniclastic Rocks of Petrota region, Evros Prefecture, Northeastern Greece

The present work studies the tuffs associated with the volcanic area of the Paleogene Sheinovets caldera, located on the southeastern part of the Rhodope massif, in Bulgaria. Its purpose is to describe the mineralogical and geochemical composition of the zeolitized volcaniclastic deposits in the broader area of Petrota village, in the northwesternmost part of the Greek regional unit of Evros, in northeast Greece. The samples studied in this work were collected from seven (7) different locations, covering an area of almost 4 km in length and 2 km in width. Macroscopically, the samples display a greyish-green hue and they are widespread in the area of study. They often contain fragments of the crystalline metamorphic basement (mica-schists, phyllites, amphibolites, quartzites) and/or rhyolitic clasts. A rhyolitic outcrop of greyish-pink hue is observed in the Mavri Petra region, probably related to the Rupelian acid volcanism that occurred in the Sheinovets caldera. The mineralogy of the tuffs was studied under light polarizing microscope and using Scanning Electron Microscopy (SEM), and it was further confirmed by X-ray powder diffraction (XRPD) method. The initial matrix of the tuffs consisted predominantly of glass shards that are now partly or fully altered into zeolites and clay minerals, such as celadonite, displaying characteristic pseudomorphic structures. The dominant zeolite is clinoptilolite, while in some areas the presence of mordenite is also noticed. Feldspar phenocrysts are abundant, and they are represented by plagioclase and sanidine. Although quartz crystals can be observed under light-polarizing microscope solely in metamorphic fragments, the presence of silica polymorphs was also confirmed through SEM and XRPD analysis, with quartz and cristobalite prevailing. The mineralogical assemblage includes phenocrysts of biotite and in some cases amphiboles, while pyroxene, epidote, garnet, titanite, apatite, zircon, ilmenite, magnetite and rutile are additional minerals which have been identified locally in accessory quantities. Chemical analysis was carried out for major and trace elements, using the X-ray fluorescence (XRF) method and 4 Acid digestion ICP-MS analysis, respectively. The samples of Mavri Petra region, exhibit high concentrations in Cu, Pb, Mn, V, P and W. The analyzed concentrations of U in the area of Palaeokklisi are relatively high in comparison to the rest due to its proximity to a fault. Towards the northwestern study area, close to the Greek-Bulgarian border, the concentrations of Sr appear to be particularly high, probably because the area is closer to the volcanic centre.

Mid‐Neoproterozoic Tectonics of Northwestern India: Evidence of Stitching Pluton along 810 Ma Phulad Shear Zone

AbstractIn northwestern India, Phulad Shear Zone (PSZ, ~810 Ma) demarcates the boundary between South Delhi Fold Belt to the east and Marwar craton to the west. This shear zone has regional NE‐SW trend with small bends of N‐S orientations. PSZ is characterized by steep southeasterly dipping mylonitic foliation with steep oblique stretching lineation. The PSZ has developed in a ductile transpression with top‐to‐the‐NNW reverse sense of movement associated with a component of sinistral‐slip movement on horizontal section. The 200 by 6 km porphyritic Phulad granite occurs along and across the PSZ, and it is variably deformed. Phulad granite shows evidence of magmatic foliation with preservation of parallel alignment of euhedral feldspars phenocrysts and microgranitoid enclaves. The feldspar phenocrysts show simple twin interfaces parallel to the direction of elongation. This granite also shows development of solid‐state foliation parallel to this magmatic foliation. Detailed study of structural elements suggests that Phulad granite has formed during the regional deformation in the country rock shear zone prior to its complete crystallization. Our data indicate that the releasing bends of N‐S orientation within the PSZ have provided the space required for emplacement of Phulad granite in a transpressional regime. EPMA U‐Pb‐Th monazite and U‐Pb LA‐ICP‐MS zircon ages in Phulad granite indicate a magmatic age of 819.1 ± 4 and 818 ± 18 Ma, respectively. PSZ represents the suture along which Marwar craton accreted with remaining India to form the Greater India landmass. The present study suggests that Phulad granite acted as a stitching pluton during this suturing along PSZ at 810–820 Ma.

Formation of rapakivi feldspar in the Deer Isle Granite Complex, coastal Maine: in situ lead isotope and trace-element analysis

Rapakivi and alkali feldspar phenocrysts from the Devonian Deer Isle Granite Complex, Maine were investigated to see if magma mixing or isothermal decompression was responsible for their formation. Pb-isotope and trace-element analyses of alkali feldspar ovoids and their plagioclase inclusions were obtained by LA-ICP-MS to determine the characteristics of the initial magma; these were compared with the plagioclase mantles. Rapakivi grains show a decrease in Pb-isotopic values and an increase in Ba, La, and Ce concentrations from the ovoids and inclusions to the innermost plagioclase mantles adjacent to the ovoids. These variations, along with CL imagery of quartz phenocrysts, indicate an open system with compositional changes in the magma chamber occurring during rapakivi feldspar growth. Repeated episodes of magma mixing/replenishment by Ba and LREE enriched magmas with lower Pb-isotopic ratios (along with hybridized variations with the host magma) created localized dissolution of alkali feldspar and quartz. Alkali feldspar phenocrysts proximal to these zones of thermal perturbation were first resorbed and then mantled by plagioclase that records the more primitive signature of the more mafic melt. Pb-isotopic values and Ba, La, and Ce concentrations trend back to the initial magma composition at the mantle rims as the effects of the mixing event dissipated. For grains that were somewhat more distal to these zones of mixing, resorption of the alkali feldspar crystals did not occur, yet the fresh supply of Ba resulted in Ba-rich alkali feldspar mantles over Ba-poor alkali feldspar cores. Other alkali feldspar crystals were too far from the site of mixing; hence, they lack any evidence of open system crystallization. As crystallization of the chamber continued along solidification fronts, batches of cooler crystal-rich magmas settled en masse to the floor. Disaggregation of these batches during settling, and subsequent accumulation on the reservoir floor, brought grains with disparate crystallization histories together. Filter pressing of the cumulate pile flushed highly evolved melts/fluids out from interstitial pores to the high silica, upper portions of the chamber. The crystallization, transportation, and juxtaposition of disparate types of feldspar phenocrysts suggest that a relatively large and active magma chamber, periodically recharged by localized batches of melt, must have existed. In this case, it is clear that rapakivi mantled feldspars are the result of magma mixing; such interpretations may apply to many other occurrences as well.

Lithostratigraphy of the Mesoproterozoic Windpoort Granite (Spektakel Suite), western Namaqualand, South Africa

AbstractThe Windpoort Granite is a porphyritic, leucocratic granite belonging to the Spektakel Suite, a group of late- to post-tectonic granites intruded into the orthogneisses and supracrustal metamorphic rocks in western Namaqualand. Like other granites of this type, it is devoid of penetrative tectonic foliation, at most displaying a magmatic foliation parallel to the boundaries of the intrusion. Its main characteristic setting it apart from other Spektakel Suite granites in western Namaqualand is its tightly packed arrangement of small stubby alkali feldspar phenocrysts. Its U-Pb LA-ICPMS age of 1087 ± 11 Ma agrees with the age of other Spektakel Suite granites showing similar field relationships. It classifies geochemically as a highly potassic monzogranite with more evolved compositions than all other plutons of the Spektakel Suite.

Carboniferous 40Ar/39Ar Age of Rare-Metal Enriched Rhyolites and Ignimbrites in the Sakmara Allochthon (Southern Urals)

The Sakmara Allochthon (Southern Urals) comprises a widely distributed folded stratum of rhyolites and ignimbrites, which are characterized by Nb–Zr–REE geochemical specification. This stratum overlies unconformably folded Paleozoic complexes, including Devonian ones. Rhyolites contain potassium feldspar and quartz phenocrysts, as well as granite xenoliths. According to geochemical features, volcaniс rocks were formed under the intraplate setting; in terms of the chemical composition, they belong to the A-type granites. The 40Ar/39Ar age of the felsic rock groundmass of rhyolites is 303 ± 2 Ma. The feldspar phenocrysts yielded ages of 306 ± 3 and 337 ± 3 Ma. The first estimate, as well as the matrix age, is associated with the formation of the volcanic complex, while the second value is probably associated with the age of xenogenic material. The complex of rare-metal enriched rhyolites characterizes the stage of the terminal Late Carboniferous rift magmatism in the rear part of the active continental margin. This complex formed prior to collision and thrusting of ophiolitic allochthons.

Reservoir characteristics in the Cretaceous volcanic rocks of Songliao Basin, China: A case of dynamics and evolution of the volcano-porosity and diagenesis

To explain the strong spatial heterogeneity of volcanic reservoirs porosity in the Songliao Basin and provide new ideas for predicting good volcanic reservoirs in other similar basins, the relationship between the pore evolution process and lithology of volcanic reservoirs has been described in this article. With the description and interpretation of core, thin section, scanning electron microscope, and the results of mercury injection experiment, this article clarifies the lithology, pore types, and pore structure features of the volcanic reservoirs in the Songliao Basin. The rocks of volcanic reservoirs in study area contain pyroclastic rock and volcanic lavas. The most common lithologies are rhyolite, volcanic breccia, and volcanic tuff. The pore size, morphology, and structure vary greatly between these three lithologies, the reason of which we think is the different volcanic eruption process as well as rock composition and its structure. The digenetic evolution of rhyolite includes gas dissipation of magmatic condensation; vesicles fulfilling by hydrothermal fluid; kaolinization and sericitization of feldspar phenocrysts; carbonation, devitrification, and recrystallization of felsic matrix; and finally, the dissolution of feldspar phenocrysts and felsic matrix. As for volcanic breccia, it usually go through the compaction, quartz and calcite filling the original pores between volcanic breccias, and dissolution of mineral debris together with tuff matrix. Similar with the rhyolite, volcanic tuff also undergoes the carbonation and kaolinization of felsic matrix, the dissolution of feldspar and felsic matrix, and compaction. Due to these comprehensive processes, a comprehensive analysis of volcanic rock lithology, which can indicate lithology distribution vertically and horizontally, is very necessary during volcanic reservoirs evaluation and prediction. These detailed analyses will help explorers to find potential reservoirs by distinguishing the diagenetic evolution and pore characteristic of volcanic reservoirs.

The onset of post-collisional magmatism in the Macururé Domain, Sergipano Orogenic System: The Glória Norte Stock

In the northern-central portion of the Sergipano Orogenic System there is an expressive Neoproterozoic granitic magmatism with high-K calc-alkaline and shoshonitic affinities. The Glória Norte Stock (GNS, 45 km2) is the most important representative of the shoshonitic magmatism in one the domains of the Sergipano System, the Macururé. The contacts of the stock with the host metasedimentary rocks are discordant and steep, with generation of amphibolite facies hornfels. The GNS is made up of predominantly porphyritic quartz-monzonite and monzogranite. It shows a magmatic flow foliation defined by oriented mafic enclaves and feldspar phenocrysts, without evidence for solid state regional deformation. Mafic microgranular enclaves (MME) are abundant and present different sizes and shapes. Minette and biotite diopside cumulate enclaves are also present. Coexistence between two different magmas is indicated by crystal corrosion and dissolution textures, compositional zoning of feldspar and presence of clusters of mafic minerals. Grain size decrease towards the rims of the MME indicates fast cooling of small drops of mafic magma, due to temperature contrast with the felsic magma. The monzonites and granites of the GNS have shoshonitic affinity, and the enclaves are related to ultrapotassic suites (MgO > 3%, K2O > 3%). LREE are enriched as compared to HREE, and there are remarkable negative anomalies of Ta, Nb, Ti, P, Sr and Eu, mostly in the enclaves. The MME have been probably formed from a mantellic magma with shoshonitic affinity. The observed evolution from MME to quartz-monzonites and monzogranites is essentially linked to a process of fractional crystallization. The relations between Ta/Yb and Th/Yb ratios suggest enriched mantle as a possible source of this magmatism. The relative enrichment in Rb, Th, Ce and Sm indicates that magma was generated in post-collisional events. The U-PbSHRIMP age of 588 ± 5 Ma in zircon crystals indicates that the emplacement of the GNS represents a post-collisional magmatism, marking the end of collisional processes in the Macururé Domain.

Petrogenesis of adakites from the Sheyda volcano, NW Iran

The Sheyda volcano study area, located in the Hamedan-Tabriz volcanic belt of western Iran, is composed of intermediate to acidic volcanic rocks of Miocene age and is partially covered by Plio-Quaternary sedimentary rocks. The igneous rocks have primarily microlithic, porphyrytic and glomeroporphytic textures and less frequently, trachytic texture involving phenocrysts of plagioclase feldspar and hornblende with minor alkali feldspar, quartz and biotite. The Sheyda high-potassium, calc-alkaline trachydacites and dacites have been called "adakites" on the basis of their high SiO2 (67.5–51.7 wt %), Al2O3 (18–15.6 wt %), Sr/Y (67–49) and La/Yb (28–23), and low Mg# (average 41.5). Rare earth elements geochemistry data, demonstrate that these rocks do not originate as melts of subducted oceanic crust. Rare earth element abundance variations and the absence of a significant positive relationship between Sr/Y and La/Yb ratios and SiO2 and MgO contents indicate that fractional crystallization or partial melting can played role in their genesis. The existence of the subducted oceanic slab beneath the studied area and the thin continental crust as well as thinning in the lithosphere suggested that these adakites originated from melting metasomatized continental lithosphere that could be garnet or clinopyroxene bearing amphibolite in composition.

CARBONIFEROUS 40Ar/39Ar AGE OF THE RARE METAL-ENRICHED RHYOLITES AND IGNIMBRITES IN THE SAKMARA ALLOCHTHON OF THE SOUTHERN URALS, THEIR GEOCHEMICAL FEATURES AND GEODYNAMIC SETTING

In the structure of the Sakmara allochthon of the Southern Uralssequence with rhyolites and ignimbrites locally occures. They have Nb-Zr-REE geochemical specialization. This sequence unconformably overlays folded Paleozoic complexes, including the Devonian ones. Rhyolite contains K-feldspar and quartz phenocrysts, K-feldspar glomeroporphyrites and granite xenolith. Geochemical features of the rhyolites show intraplate-originated affinities and A-type granite composition.40Ar/39Ar age of the felsitic matrix of the rhyolites of 303±2 Ma defines the age of the volcanic complex origin. For feldspar phenocrysts the age of 306±3 Ma and 337±3 Ma is obtained. The first value coincides to the matrix age and connected with formation of the volcanic complex. The second value belongs, apparently, to xenogenic material. Obtained age values reflect the evolution of Carboniferous active continental margin magmatism, widespread in different structural zones of the Urals. Rare-metal rhyolites characterize the final late Carboniferous intraplate (rift-related) back-arc magmatism at the active continental margin. Volcanism preceded to a collision-related ophiolitic thrust nappes emplacement.

Zircon U–Pb and Wolframite Sm–Nd Dating of the Bayinsukhtu Tungsten Deposit in Southern Mongolia and its Geological Significance

AbstractThe Bayinsukhtu tungsten deposit is a newly discovered quartz‐vein tungsten deposit in the Xing'an–Mongolia Orogenic Belt (XMOB) in southern Mongolia, hosted by the Bayinsukhtu granite porphyry. The granite porphyry is located mainly south of the study area, over 3 km2. The rock consists of quartz and feldspar phenocrysts in a fine‐grained matrix, also mainly composed of feldspar and quartz. The granite porphyry samples demonstrate high SiO2 and high alkalinity. All samples also straddle the high‐potassium calc‐alkaline series. In a plot of the molar ratios of A/NK versus A/CNK, the granites are metaluminous. The chondrite‐normalized REE patterns are characterized by large negative Eu anomalies and fractionated LREEs. The U–Pb age of zircons from the granite porphyry is 298.8 ± 1.8 Ma, and the Sm–Nd age of the five wolframite samples from the tungsten deposit is 303 ± 19 Ma. The cooling age of the granite porphyry and tungsten mineralization is within the error of measurement and is of the Late Carboniferous age. Geological and geochronological evidence shows that the tungsten mineralization and the granite porphyry at Bayinsukhtu are genetically closely related and that they are results of Carboniferous magmatism. Their tectonic setting is related to the accretion of the Central Asian Orogenic Belt during the late Paleozoic era.

Petrology of Metaluminous A-Type Rhyolite Discovered from Hadjer el Hamis Volcanoes (Lake Chad Basin)

Metaluminous (P.I. > 1) rhyolite from Hadjer el Hamis consisted of quartz, alkali feldspar, clinopyroxene (hedenbergite), amphibole (F-arfvedsonite) and oxides-hydroxides (ilmenite, magnetite, limonite) phenocrysts is characterized by the negative Eu, Ba, P, Sr and Ti anomalies. This metaluminous rhyolite and the early discovered peralkaline rhyolites in Hadjer el Hamis volcanoes derive likely from the same source, according to their coexistence on the same sector and their similar Zr/Nb ratios. The causes of magma heterogeneity are likely linked to varying amounts of extraction of an earlier melt phase or tectonic juxtaposition or a sudden increasing of fO2 in silicic magmas, triggered from a hydrothermal process, associated with F- and alkali-bearing fluids influx, which promoted the enrichment of Na in the hedenbergite rims and the crystallization of arfvedsonite.

New petrographic and geochemical tracers for recognizing the provenance quarry of trachyte of the Euganean Hills, northeastern Italy

AbstractTrachyte of the Euganean Hills is a subvolcanic porphyritic rock historically used as carving and building stone in northern and central Italy–primarily from Roman times onward–with the first evidence of its use dating back to prehistory. The numerous quarries and very similar trachyte varieties, as well as the widespread use of this stone, create several problems in defining its provenance for archaeological and historical materials. New petrographic and geochemical tracers for recognizing the provenance quarry of Euganean trachyte are presented here, providing a comprehensive reference database for archaeometric studies. The petrographic markers principally include quantitative data on mineralogical composition and textural features of phenocrysts and groundmass, determined by image analysis of chemical maps acquired by micro X‐ray fluorescence and scanning electron microscope‐energy‐dispersive X‐ray spectroscopy; particular use has been made of data on the abundance and grain‐size distribution of feldspar phenocrysts, phenocrysts‐groundmass ratio, content of SiO2 phases in the groundmass, and the arrangement and grain‐size of microlites in the matrix. The geochemical tracers involve composition of bulk rock and phenocrysts, determined by X‐ray fluorescence and laser ablation inductively coupled plasma mass spectrometry, respectively; quarry recognition can be achieved using plots built from concentrations of major and trace elements, with mineral‐scale chemistry being the most effective and precise discriminant property, especially in the case of biotite and, secondarily, augite, kaersutite, and magnetite.

Mixing of basaltic and andesitic magmas in the Bazman volcanic field of southeastern Iran as inferred from plagioclase zoning

AbstractLate Cenozoic basalts of the Bazman volcanic field, Makran volcanic arc of southern Iran, contain two types of plagioclase feldspar phenocrysts with significant textural and compositional differences. The most common type is rather homogeneous with only weak zoning and maximum An content of 83 mol.%. The less common type of phenocryst exhibits complex zoning and, other than rims, is close in composition and similar in texture to those of associated andesites. This type of plagioclase phenocryst is characterized by an engulfed core with oscillatory zoning, which is overgrown by sieve-textured, moderately zoned mantle, and a relatively narrow rim. In both rock types, the An content of the core is between 40 and 63 mol.% with abrupt fluctuations. No significant correlation between An content and MgO, FeO, SrO and BaO is apparent in the core of phenocrysts in basalts. Anorthite content of the core of phenocrysts in andesites inversely correlates with SrO and BaO. The mantle of plagioclase phenocrysts in both rock types is characterized by sharp increases of An (up to 41 mol.%), MgO, and FeO, in the contact with the core. Anorthite correlates positively with MgO and FeO in the mantle, but correlation between An and SrO and BaO is not evident. It is assumed that plagioclase phenocrysts originally crystallizing from the host andesitic magma were interrupted by mixing with a hotter, juvenile basaltic magma. The resulting changes in temperature, composition, and H2O content of the surrounding melt caused compositional zonation, and the development of resorption in the cores and sieve texture in the mantles. As the An contents of the rims of the phenocrysts resemble the average An content of the groundmass plagioclases in both rock types, it is thought that the two involved magmas gained their independent physical identity before the formation of compositionally-distinct rims of plagioclase phenocrysts.

울릉도 곰바위용결응회암 내 흑요암의 산출특징과 성인

이 연구의 목적은 울릉도 남서부 지역에 분포하는 흑요암의 화산학적, 광물학적 특징을 통하여 흑요암의 성인과 산출의미를 고찰하는 데 있다. 흑요암은 곰바위용결응회암 하부에서 국부적으로 발달하며, 다양하고 복잡한 조직과 유동띠가 관찰된다. 흑요암은 알칼리장석 반정과 응회암 및 조면암의 암편과 혼재하여 산출된다. 흑요암은 파동형, 렌즈상의 덩어리로 산출되거나 파쇄된 흑요암 입자들로 인해 얇은 층리로 발달한다. 진주상 균열, 둥근 또는 구형균열과 같은 냉각균열이 특징적으로 발달하는데, 이는 빠르게 냉각하여 규산질이 풍부한 비정질 상태의 흑요암이 형성되었음을 지시한다. 수화작용은 흑요암의 가장자리에서 상대적으로 현저하여 변질띠를 이룬다. 흑요암의 유리질 기질에는 자형의 새니딘과 아노소클레이스 같은 알칼리장석 반정 외에도, 투휘석, 흑운모, 티탄철석, 철산화물 등이 수반된다. 특히 산점상의 미세결정들은 대부분 알칼리장석류와 티탄철석으로 구성된다. EPMA 정량분석 결과, 흑요암의 유리 기질부는 조면암질 조성을 보이며, 철 함량은 약 3 wt.%로 높은 편이다. 본 흑요암은 용암돔이나 칼데라의 붕괴에 의하여 고온상태에서 역-화산재 흐름 퇴적물이 계곡부로 운반된 후, 차가운 지표면과 접촉하였으며 유동성을 띠는 가운데서 급랭하여 형성된 것으로 보인다. The purpose of this study is to provide the information on genesis of obsidian occurring in the southwestern part of Ulleung Island, Korea, and to discuss its implications for volcanic activity through volcanological and mineralogical properties of obsidian. Obsidian occurs locally at the lower part of the Gombawi welded tuff, showing various complex textures and flow banding. Though obsidian is mostly homogeneous, it is closely associated with alkali feldspar phenocrysts, reddish tuff, and greyish trachyte fragments. The obsidian occurs as wavy, lenticular blocks or lamination composed of fragments. Cooling fractures developed on obsidian glass are characterized by perlitic cracks, orbicular or spherical cracks, indicating that obsidian rapidly quenched to form an amorphous silica-rich phase. It is evident that hydration took place preferentially at the outer rim relative to the core of obsidian, forming alteration rinds. The glassy matrix of obsidian includes euhedral alkali feldspars, diopside, biotite, ilmenite, and iron oxides. Microlites in glassy obsidian are composed mainly of alkali feldspars and ilmenite. Quantitative analysis by EPMA on the obsidian glass part shows trachytic composition with high iron content of 3 wt.%. Accordingly, obsidian formed with complex textures under a rapid cooling condition on surface ground, with slight rheomorphism. Such results might be induced by collapse of lava dome or caldera, which produced the block-and-ash flow deposit and the transportation into valley while keeping high temperatures.