Intergranular Texture Research Articles

Classification and generation of the enclaves in Karapınar-Karacadağ volcanic rocks (Central Anatolia)

Karapınar-Karacadağ Volcanic Rocks (KKVR) have very complex magmatic history and outcropped the southwestern part of the Cappadocia Volcanic Province (Central Anatolia). Here we present the petrography and whole-rock chemistry of the enclave-bearing rocks to constrain their source and evolution history. These petrographic observation and geochemical data reveal that the enclaves in the Karapınar-Karacadağ volcanic rocks are magma mixing/mingling enclaves (MME), magma segregation enclaves (MSE), and xenoliths. Here we discriminated these enclaves into eight different types according to their mineral composition and textural features. The magma mixing/mingling enclaves (Type 1, 7, 8) are the mixing products of coeval more felsic and mafic magmas. They show hypocrystalline porphyritic, holocrystalline granular, and intergranular textures, and rich in mafic minerals, and have characteristic petrographic features such as quenched amphibole, bladed biotite, ocelli-quartz, sieved and cellular plagioclases. In andesites they range from basalt to andesite in composition. However, in basalts, they are in basaltic composition. The magma segregation enclaves (Type 2, 4, 5, 6), which are observed in almost all the KKVRs, are cognate xenolith because of plucking from the different parts of the magma chamber. They are holocrystalline and granular in texture. The magma segregation enclaves contained in the andesitic host rocks are hornblende gabbro and pyroxene gabbro in composition, whereas in the basaltic host rocks they are dunite, lherzolite, and basalt in composition. The xenolithic enclaves (Type 3) are observed in the basalts as quartz, plagioclase, biotite, and amphibole xenocrysts. Major oxides and trace element data of the studied rocks indicate that the MSEs are more primitive than their host rocks, and all of the enclaves (MME-MSE) are in accordance with their hosts. According to petrographic observations and geochemical data we propose that fractional crystallization, magma mixing and assimilation processes have a key role in the evolution of the KKVRs and their enclaves.

A grain texture model to investigate effects of grain shape and orientation on macro-mechanical behavior of crystalline rock

Grain texture characteristics play vital roles in modulating the anisotropy, strength, and ductility of crystalline rock . Grain textures (shape, size, orientation and heterogeneity) are a key parameter controlling response but are challenging to incorporate in a quantitative manner. We propose and incorporate a grain texture model (GTM) into granular mechanics modeling by the distinct element method (DEM). This model seeds then grows individual non-spherical grain clusters that honor the granular and intergranular textures of granites . The model is applied to “virtually sinter” Lac du Bonnet granite and represent the pre-through post-peak evolution of damage and deformation. The model is able to capture all significant features of this macroscopic mechanical evolution of damage through failure. With an increase in confining pressures, a transition from brittle to ductile response occurs in the post peak stage and the fitted Hoek-Brown strength envelopes for peak strength and crack-initiation stress in these numerical experiments are consistent with physical observations. An increase in aspect ratio of individual grains slightly increases the UCS/TS ratio, whereas an increase in dip angle decreases the UCS/TS ratio. Parametric studies varying the proportion of minerals and the form of contact groups with various grain shapes and orientations allow the systematic evaluation of controls of grain texture on the evolving macroscale strength and deformability . An increase in grain aspect ratio increases the proportion of inter-grain contacts. A change in dip of the grain long-axis does not change the balance of either the mineral groups or the contact groups but significantly influences the number of inter-grain tensile cracks and the resulting UCS/TS ratio.

A Reference Section Through Fast‐Spread Lower Oceanic Crust, Wadi Gideah, Samail Ophiolite (Sultanate of Oman): Insights From Crystallographic Preferred Orientations

AbstractWe established a 5,000 m thick profile through the paleo lower oceanic crust of the Samail ophiolite (Sultanate of Oman, Wadi Gideah), in order to investigate accretion processes beneath fast‐spreading mid‐ocean ridges. The Samail ophiolite is regarded as best on‐land analog for fast‐spread oceanic crust, and Wadi Gideah allows sampling of the entire lower crust along the Wadi bed. Here, we provide microstructural constraints to lower crustal accretion beneath fast‐spreading mid‐ocean ridges, which reveal changing microstructures and fabrics with depth. Grain size coarsening occurs from the foliated to the layered gabbro section. A ∼350 m thick zone of gabbroic rocks from the profile top, interpreted as frozen fillings of the axial melt lens and defined as varitextured gabbros, shows intergranular textures without crystallographic preferred orientations. Rocks from the varitextured/foliated gabbro transition (∼500 m thick) and from the upper foliated gabbros (∼600 m) are foliated and lineated. The lineation is absent or very weak in the underlying ∼800 m thick lower foliated gabbros. In layered gabbros, the fabric gradually strengthens and becomes more lineated down section with local scattering at small spatial scales. This implies distinct accretion mechanisms in the deep and shallow plutonic crust. For the layered and lower foliated gabbros, our data suggest in‐situ crystallizing individual magma reservoirs. Dominant lineation in the upper foliated gabbros and the varitextured/foliated gabbro transition suggests vertically transported crystal‐laden melts or mushes being consistent with subsiding crystal mushes from the axial melt lens, and/or crystallizing upward migrating melt expelled from the crystal mush beneath.

Petrology and geochemistry of the Lattan Mountain magmatic rocks in the Sanandaj–Sirjan Zone, west of Iran

Igneous rocks are distributed in the Lattan Mountain area, the center part of Sanandaj–Sirjan Zone, west Iran. Based on the mineralogy and geochemical composition, these are subvolcanic, volcanic, and plutonic rocks. It includes basalts, andesitic basalt, and andesite, with porphyritic to microlithic porphyry and vitrophyric textures as well as dolerite, diorite, gabbro, and microdiorite with a varied granular, microgranular, intersertal, and intergranular textures. The chemical compositions indicated to calc-alkaline to transitional nature, enrichment in LIL elements (Rb, Ba, Th, U, and Pb), and depletion in Ti and Zr, as it is evident in spider diagrams normalized to a primitive mantle. In addition, samples have enrichments of LREE relative to HREE. The Rb-Sr whole-rock isochron of these magmatic rocks shows an age of 152 ± 14 Ma (late Jurassic). The initial 87Sr/86Sr (0.7047 to 0.7051) and 143Nd/144Nd ratios range from (0.512534 to 0.512710) and eNdt = − 0.1 to + 2.2 that indicated to BSE composition. Our results suggest that the magmas for the magmatic rocks were derived from metasomatized enriched MORB-like sources. These rocks were formed in an island arc setting during subduction and closure of the Neotethys oceanic lithosphere beneath the Iran microplate about 152 Ma.

Platinum group element geochemistry and whole‐rock systematics of the Vempalle sills, Cuddapah Basin, India: Implications on sulphur saturation history, mantle processes, and tectonic setting

The intracratonic Cuddapah Basin of southern peninsular India has multiple phases of magmatism exposed as mafic intrusives and extrusives in the Vempalle and Tadpatri formations. We report platinum group element geochemistry along with whole‐rock systematics of the mafic sills from the Vempalle Formation to understand their sulphur saturation history, source characteristics, and tectonic setting. These sills are medium‐ to fine‐grained dolerites with plagioclase and pyroxenes as essential minerals exhibiting sub‐ophitic to intergranular texture. Geochemically, they are characterized by moderate MgO (6.3–8.5 wt.%), Ni (55–105 ppm), Cr (20–130 ppm), and ∑PGE (22–61 ppb) contents and exhibit transitional to calc‐alkaline in nature. Primitive mantle‐ and chondrite‐normalized trace and rare earth element patterns display relative enrichment of LILE–LREE over HFSE, suggesting plume‐related intraplate magmatism with contributions from subcontinental lithospheric mantle (SCLM), lithosphere–asthenosphere interaction, crustal contamination, and fractional crystallization. Their REE relationship indicates the generation of parent magma from a heterogeneous source marked by 10–25% polybaric partial melting within spinel and garnet lherzolite domain. Platinum group elements (PGE) geochemistry reveals their sulphur‐undersaturated nature, and sulphide fractionation played a key role in the distribution of PGEs, which is evidenced by their enriched character, moderate degrees of partial melting, and crustal contamination. Further, the negative platinum anomalies in these sills indicate its removal during fractional crystallization, and the enrichment of Pd over Pt endorses the decoupling nature of these elements prior to their emplacement. Diagnostic trace element signatures and their relationship corroborate a riftogenic intraplate tectonic regime for the emplacement of the Vempalle sills through an ocean–continent transition zone.

Evidence for multiple 4.0–3.7 Ga impact events within the Apollo 16 collection

AbstractIn a histogram of lunar impact ages from the Apollo 16 site, there is a spike circa 3.9 Ga that has been interpreted to represent either a large number of nearly synchronous events or an abundance of samples that were affected slightly differently by the event that produced the Imbrium basin. To further scrutinize those age relationships, we extracted six centimeter‐sized clasts of impact melt from ancient regolith breccia 60016 and performed petrological and geochronological (40Ar‐39Ar) analyses. Three clasts have similar poikilitic textures, while others have porphyritic, aphanitic, or intergranular textures. Compositions and abundances of relict minerals are different in all six clasts and variously imply Mg‐suite and ferroan anorthosite target sequences. Estimated bulk compositions of four clasts are similar to previously defined group 1 Apollo 16 impact melt rocks, while the other two have higher Al2O3and lower FeO+MgO compositions. All six clasts have similar K2O and P2O5concentrations, which could have been derived from aKREEP‐bearing component among target sequences. Eighteen40Ar/39Ar analyses of the six clasts produced an age range from 3823 ± 75 to 4000 ± 23 Ma, consistent with estimates for the proposed late heavy bombardment. Four clasts have multiple temperature steps that define plateau ages. These ages are distinct, so they cannot be explained by a single impact event, such as the one that produced the Imbrium impact basin. The conclusion that these represent distinct ages remains after considering the possibility of artifacts in defining plateaus.

Evidence for magma–evaporite interactions during the emplacement of the Central Atlantic Magmatic Province (CAMP) in Brazil

Mafic sills of the Central Atlantic Magmatic Province (CAMP) in Brazil are temporally linked to the end-Triassic extinction (ETE), one of the largest mass extinctions of the Phanerozoic. The sills were emplaced into the volatile-rich sedimentary rocks of the Amazonas and Solimões basins in northern Brazil. Here we present a new geochemical study of 26 dolerite samples from 6 deep boreholes in the Brazilian basins, including whole-rock major and trace elements, whole-rock Sr–Nd isotopes and detailed biotite mineral chemistry. The bulk of the dolerites are characterized by phenocrysts of clinopyroxene and plagioclase in subophitic to intergranular textures, Fe–Ti oxides, and rare olivine and orthopyroxene. A different mineralogical assemblage (microphenocrysts of alkali-feldspar, quartz, biotite and apatite) is found in small independent domains, localized within the framework of coarser plagioclase and clinopyroxene laths. These fine-grained evolved domains crystallized in late-stage, evolved melt pockets in the interstitial spaces between earlier crystallized coarser grained crystals. The majority of the studied dolerites are generally evolved tholeiitic basalts and basaltic andesites with low TiO2 concentrations (<2.0 wt.%). Four samples have high TiO2 concentrations (>2.0 wt.%), and are found in the eastern part of the Amazonas Basin. Whole-rock major and trace element and Sr–Nd isotope geochemistry of both low- and high-Ti sills is similar to that of previously published CAMP rocks from the two magma types. Low-Ti sills show enriched isotopic signatures (143Nd/144Nd201Ma 0.51215 to 0.51244; 87Sr/86Sr201Ma 0.70568 to 0.70756), coupled with crustal-like characteristics in the incompatible element patters (e.g. depletion in Nb and Ta). Unaltered high-Ti samples show more depleted isotopic signatures (143Nd/144Nd201Ma 0.51260 to 0.51262; 87Sr/86Sr201Ma 0.70363 to 0.70398). Low-Ti dolerites from both the Amazonas and Solimões basins contain biotite with extremely high Cl concentrations (up to 4.7 wt.%). We show that there is a strong correlation between host-rock lithology and Cl concentrations in biotite from the dolerites, and interpret this to reflect large-scale crustal contamination of the low-Ti magmas by halite-rich evaporites. Our findings support the hypothesis that sill-evaporite interactions increased volatile release during the emplacement of CAMP. This strengthens the case for active involvement of this LIP in the end-Triassic crisis, and suggests that the sub-volcanic part of a LIP can be of major importance in driving climate change and mass extinctions.

Chemical weathering in subtropical basalt-derived laterites: A mass balance interpretation (Misiones, NE Argentina)

Subtropical laterites are partially found in the Argentinian province of Misiones, covering the Jurassic-Cretaceous Serra Geral flood basalts (known as Posadas Formation in Argentina). Two hypotheses have been proposed to explain the origin of these laterites. The autochthonous theory attributes the origin of these deposits to the chemical weathering of the underlying tholeiitic basalts. However, during the last decade, an allochthonous “tropical loess” model was proposed. Several mineralogical/geochemical characteristics of a lateritic profile (~2 m thick, 26°9′58″S–54°35′3″W) are analyzed in this paper in order to constrain the nature of weathering processes in this region and the provenance of the lateritic layer. The basaltic bedrock at the sampling site exhibits intergranular texture and is mainly composed of clinopyroxene, plagioclase and opaque minerals (likely titanomagnetite and ilmenite). Altered glass and apatite were also identified in lower proportions. The X-ray diffraction of the clay-size fraction shows prevailing kaolinite and hematite in the most altered samples, complemented by amorphous Fe-Al (hydr)oxides. Serra Geral basalts-normalized spidergrams and mass balance calculations confirmed almost complete losses of all major oxides (i.e., ~50 g of material was removed by weathering from each 100 g of the original basalt) and depletions of some trace elements (e.g., Sc, V, Sr, Cr, Ni, Rb and U) in the uppermost and intermediate layers of the lateritic profile, reflecting the weathering of feldspars, clinopyroxenes, volcanic glass and apatite. In contrast, some elements are enriched in the topmost and intermediate layers, due to intrinsic immobility during weathering (e.g., Zr, Hf, Ta, Th) or adsorption onto clays (e.g., Cs). Rare earth elements (REEs) are depleted in the top layers and enriched in the middle section of the profile. The preferential leaching of REEs from the uppermost levels during weathering and its accumulation in the middle section can be caused by a pH increase in this level, the decomposition of organic matter and the consequent precipitation of REEs which exhibit an affinity to form aqueous organic complexes, or the presence of Fe and Mn (hydr)oxides in the intermediate level. Weathering intensity was appraised by means of different alteration indices, which showed the extreme leaching endured by the basaltic bedrock. The index of lateritization (IOL), which ranges between 35 and 55, and the accompanying SAF ternary plot (SiO2-Al2O3-Fe2O3), indicate an increasing weathering trend towards the profiles' top, reflecting the modest loss of SiO2 (relative to Al2O3 and Fe2O3) during kaolinitization. The geochemical approach used to constrain the origin of the lateritic mantle that partially covers Misiones province, clearly relates the analyzed laterites to the underlying tholeiitic basalts, supporting the autochthonous origin theory.

Tectonothermal imprints in a suite of mafic dykes from the Chotanagpur Granite Gneissic complex (CGGC), Jharkhand, India: Evidence for late Tonian reworking of an early Tonian continental crust

Tectonothermal evolution of a set of mafic dykes in the felsic orthogneiss basement of the Chotanagpur Granite Gneissic Complex (CGGC) is presented. The gneissic basement rocks preserve high pressure granulite facies metamorphism (M2, ~800 °C and ~9 kbar) and develop pervasive planar fabric (S2) during a continent-continent collision event at 1000–950 Ma. Mafic dykes cut the S2 fabric of its basement it intrudes. Together with the S2, the mafic dykes have been deformed by three sets of folds viz. DkF1, DkF2 and DkF3. A conspicuous NS trending foliation defined by metamorphic amphibole is developed parallel to the axial plane of DkF1–2. The mafic dykes preserve relict igneous textures (e.g. intergranular texture) defined by clinopyroxene, orthopyroxene and plagioclase. Numerically computed phase diagram (pseudosection) with a representative bulk composition (with <10 vol% metamorphic minerals namely amphibole+ plagioclase rim + quartz) constrain the depth of emplacement of the mafic dykes (15–18 km). The geological features are consistent with rifting of the early Tonian basement of the CGGC and the then mantle beneath it. This lithospheric extension resulted in outpouring of basaltic magma that punctured the early Tonian basement rocks in the form of mafic dykes. Subsequent metamorphism that affected the mafic dykes and its basement culminated at ~700 °C and ~7.5 kbar (M3 metamorphism).The estimated physical conditions during the M3 metamorphic event suggest burial of the previously rifted basement possibly in a continent-continent collisional tectonic setting. The Th-U-total Pb ages of monazite grains from the host felsic orthogneiss date the M2 (ca. 964 Ma) and M3 (ca. 917 Ma) metamorphism. Based on commonality of geological and geochronological history among the CGGC, the Eastern Ghats Mobile Belt and Rayner complex (and its equivalent parts) in east Antarctica corroborates the view that Indo-Antarctic landmass were united and shared the similar geological histories during the entire Tonian period. During the Tonian period the Indo-Antarctic landmass witnessed two major phases of orogenesis (1000–950 Ma and < 920 Ma) that are punctuated by a phase of lithospheric extension manifested by the emplacement of mafic dykes.

Calc-alkaline magmatism associated with salt diapirs in the Shurab and Garmsar back-arc areas (Central Basin, Iran): magma genesis and tectonic implications

Medium- and high-K calc-alkaline magmatism of the Shurab (southeast Qom city) and Garmsar (northwest Garmsar city) areas occurred within the Lower Red Formation in the Central Basin behind the Urumieh-Dokthar Magmatic Arc. In terms of whole-rock geochemical analyses and in agreement with the petrographic features, all representative samples of the Shurab area are classified into three groups: group 1 with mainly intergranular texture comprises basalt/trachybasalt, while groups 2 and 3 with trachytic and porphyritic textures, respectively, have basaltic trachyandesite composition. The overall major constituents are plagioclase with composition in the range from An 49 Ab 23 to An 75 Ab 47 , clinopyroxene with composition in the range Wo 43-45 En 39-45 Fs 9-15 , and olivine with composition in the range from Fo 58 Fa 31 to Fo 67 Fa 40 . Minor minerals consist of opaque minerals and K-feldspar in the range Or 41-65 Ab 33-50 An 0.69-8 . The characteristic accessories are apatite and sphene. In the Garmsar area, rocks are seen as subvolcanic with mafic (basalt) and intermediate (trachybasalt/basaltic trachyandesite) compositions. The Garmsar area rocks represent intergranular, granular, ophitic, and subophitic textures. In these rocks, major mineral phases are plagioclase and clinopyroxene. The minor constituents are olivine, opaque minerals, amphibole, biotite, and quartz. Apatite is the most important accessory mineral. The rocks of both areas display REE patterns characterized by LREE-enriched and HREE-depleted segments typical of arc lavas. Primitive mantle-normalized trace element patterns for samples of both areas exhibit high ratios of strongly incompatible elements with similar bulk partition coefficients (e.g., Th/ Ta and Th/Ce), enrichment in large-ion lithophile elements (LILEs: Cs, Ba, Rb, Th) relative to the high field-strength elements (HFSEs: Ti, Hf, Zr, and REEs), and troughs for Nb, Ta, Ti, and Zr and peaks for Cs, Th, K, and Sr, all of which are indicators for subductionrelated magmatism. Subduction of the Neo-Tethys beneath the Eurasian margin led to upper mantle deformation and metasomatism. Once the Arabian plate collided with the Eurasian margin, subduction ended through a slab breakoff process, and thermal flux of asthenospheric origin uprising through the slab tear induced the thermal erosion of the mantle metasomatized during the previous subduction event and triggered its partial melting. Also, the late Eocene-early Oligocene collision of Eurasian with Arabian plates led to the subsidence and formation of faults and extensions in the Central Basin (i.e. the Shurab and Garmsar areas) such that eruption of medium- and high-K metasomatic magmatism along these faults and extensions caused postcollision volcanism in the Central Basin.

Geochemistry of basaltic flows from a basalt ring structure of the Serra Geral formation at Água Vermelha dam, Triângulo Mineiro, Brazil: implications for the magmatic evolution of the Paraná-Etendeka Province

The Serra Geral Formation belongs to the Parana-Etendeka Magmatic Province (PEMP) and its geochemical and petrographic characteristics are not homogeneous. Many studies segment this group into six basaltic and two rhyolitic magma-types. It is believed that its extrusion occurred through crustal fissures in the Cretaceous, but some authors described the presence of conduits in the shape of basaltic ring structures (BRS) in the Agua Vermelha region in the North of the province. The BRS rocks, based on textures and structures, were divided into four groups—central flow, basal flow, main ring dyke and lava flow—with a very similar petrography, composed of plagioclase (labradorite-bytownite), clinopyroxene (augite) and oxide (titanomagnetite) with intergranular texture. The whole-rock analyses of the basal and lava flows allow classifying them as tholeiitic basalts of the Paranapanema magma-type. Geochemical data interpretation suggests an enriched magma source, with low degree of partial melting, high depth of melt generation and without significant crustal contamination. The BRS experienced fractional crystallization on shallow magma chamber, influenced by successive new injections from different parental magmas which would be responsible for the pulses of effusion and explosion. Thus, the singularities of the BRS of Agua Vermelha are important to comprehend the evolution of the PEMP.

Characterization and utilization potential of basalt rock from East-Lampung district

The aim of this research was to study the petrography and chemical properties of basalt rock from East Lampung district, Lampung province. Petrography analysis was performed using a polarization microscope, and analysis of chemical composition using X-RF method. From the analysis of basalt rock samples, the mineral composition consists of pyroxene, plagioclase, olivine, and opaque minerals. Basic mass of basalt rock samples is, composed of plagioclase and pyroxene with subhedral-anhedral shape, forming intergranular texture, and uniform distribution. Mineral plagioclase is colorless and blade shape, transformed into opaque minerals with a size of <0.2 mm, whereas pyroxene present among the blades of plagioclase, with a greenish tint looked and a size of <0.006 mm. Mineral opaque has a rectangular shape to irregular, with a size of <0.16 mm. The chemical composition of basalt rock samples, consisting of 37.76-59.64 SiO2; 10.10-20.93 Fe2O3; 11.77-14.32 Al2O3; 5.57-14.75 CaO; 5.37-9.15 MgO; 1.40-3.34 Na2O. From the calculation, obtained the value of acidity ratio (Ma) = 3.81. With these values, indicate that the basalt rock from East Lampung district has the potential to be utilized as stone wool fiber.

Mafic subvolcanic intrusions and their petrologic relation with the volcanism in the south hinge Torres Syncline, Paraná-Etendeka Igneous Province, southern Brazil

The hypabyssal intrusions investigated in this study are located in the east-central region of the state of Rio Grande do Sul, in the south hinge of the Torres Syncline. The intrusions comprise twenty-four dikes and ten sills, intruding in ponded pahoehoe, compound pahoehoe, rubbly and acidic lava flows of the south sub-Province of the Paraná Igneous Province and the sedimentary rocks of the Botucatu, Pirambóia, Santa Maria and Rio do Rasto Formations, on the edge of the Paraná Basin. The intrusive dikes in the flows have preferred NNW-SSE direction and the intrusive dikes in the sedimentary rocks have preferred NE-SW direction. Regarding the morphology, the dikes were separated into two different groups: symmetrical and asymmetrical. The small variation in facies is characterized by fine to aphanitic equigranular rocks. The rocks were divided into two types: Silica Supersaturated Tholeiite (SST) – dikes and sills consisting of plagioclase and clinopyroxene as essential minerals, with some olivine and felsic mesostasis, predominant intergranular texture and subordinate subophitic texture; and Silica Saturated Olivine Tholeiite (SSOT) – dikes consisting mainly of plagioclase, clinopyroxene and olivine, and predominant ophitic texture. The major and trace element geochemistry allows classifying these hypabyssal bodies as basalts (SSOT), basaltic andesites and trachyandesites (TSS) of tholeiitic affinity. The mineral chemistry data and the REE behavior, combined with the LILE and HFSE patterns, similar to the flows and low-Ti basic intrusions of southern Brazil and northwestern Namibia allow suggesting that these dikes and sills were part of a feeder system of the magmatism in the Paraná-Etendeka Igneous Province. The preferred direction of the intrusive dikes in the sedimentary rocks of the Paraná Basin coincides with tectonic-magmatic lineaments related to extensional processes and faulting systems that served as vents for dike swarms parallel to the Brazilian coast, with the same direction as the Namibia coast dike swarm. This suggests that these dikes were part of the triple junction system related to the opening of the South Atlantic Ocean. The preferred directions of the intrusive dikes in the lava flows are similar to the directions of the Ponta Grossa and Rio Grande Arcs and the Torres Syncline. They may have been a part of, or been caused by one or more geotectonic cycles that originated these structures. The emplacement process of the asymmetric dikes suggests they were enclosed under the hydraulic fracture model, since they do not follow a pre-existing fracture filling pattern. The emplacement of the sills conforms to the weakness zones of the sedimentary units. Regarding the intrusive dikes in the flows, divided by lithofacies associations, also taking into account the geochemical and petrographic similarities, it is observed that these dikes are part of a supply system of the basic lava flows, stratigraphically positioned above the host lava flows.

Geochemistry and Thermo-barometry of Mafic Igneous Rocks of the Almabulagh Complex (NW Iran)

The Almabulagh complex is located in the Sanandaj-Sirjan structural zone, NW of Hamadan. This complex consists of low-grade metamorphic rocks and two intrusions of felsic and mafic rocks. In this research, we studied the mafic intrusion. The intrusion has ophitic, subophitic and inter-granular textures and is mainly composed of plagioclase, clinopyroxene, amphibole and minor amounts of K-feldspar, quartz and opaque minerals. Secondary minerals include epidote, chlorite and amphiboles formed in the rim of clinopyroxene. Clinopyroxene is of augite type; amphibole is tschermakite; plagioclase is in the range of andesine-labradorite; K-feldspar is anorthoclase; and the opaque mineral is ilmenite. According to the bulk rock geochemistry and mineral chemistry of clinopyroxene, the mafic intrusion of the Almabulagh complex has been crystallized from a calc-alkaline magma and is related to subduction zone. In spider diagrams Nb displays a distinctive negative anomalies compared to the primitive mantle, which suggest a possible subduction-related magmatism. Emplacement temperature and pressure of mafic rocks measured by pyroxene-amphibole thermometer and amphibole barometer indicate temperature and pressure ranges of 750 to 900 °C and 6 to 7 kbar, respectively. Based on Sm/Yb and La/Sm ratios, the parental melt of the mafic rocks of the Almabulagh complex derived from partial melting of garnet–spinel lherzolite. According to the bulk rock geochemistry and mineral chemistry of the mafic rocks, they formed in a subduction area during Late Jurassic-Early Cretaceous before the Neo-Tethys closure in Sanandaj-Sirjan zone.

Corpos hipabissais correlacionados à Formação Serra Geral na região do Cerro do Coronel, RS: geologia e petrologia

The Serra Geral Formation is characterized by an expressive volume of tholeiitic basic lava flows and a subordinate presence of acid volcanites. Intrusive bodies (dikes and sills) are correlated to this unit, and, along with volcanic deposits, constitute the Parana Magmatic Province. This research is about the geological and petrological investigation of basic-intermediate intrusions of the Cerro Coronel region, southeast of the city of Pantano Grande, RS. These hypabyssal bodies are stratigraphically linked to the Serra Geral Formation and form an alignment with NW-SE orientation. They are intrusive in sedimentary rocks of Irati and Rio Bonito Formations. All intrusives present columnar disjunctions and are affected by strong NE and NW fracturing. The small facies variation is predominantly constituted by fine equigranular rocks and, rarely, porphyritic terms. Intergranular texture is a very common feature, but subophitic texture is present too. Primary mineralogy is characterized by plagioclase, augite, opaque minerals and, scarcely, olivine. Apatite and quartz occur as accessory minerals. Mesostasis of felsic composition occurs as the final stage of crystallization. Geochemical data of major and trace elements allow the classification of the rocks as basaltic andesite with tholeiitic affinity, whose evolution occurred probably by fractioned crystallization mechanisms, involving mainly plagioclase and augite. The characteristics of the major, trace, and rare earth elements are compatible with those presented for magmatism related to large continental tholeiitic provinces such as Deccan Traps, Paraná-Etendeka, Columbia River, Siberian Traps and Karoo. These rocks have concentrations below 2% of TiO2, showing a tendency to Esmeralda magma-type of the Paraná Magmatic Province.

Norite and charnockites from the Venda Nova Pluton, SE Brazil: Intensive parameters and some petrogenetic constraints

The Venda Nova Pluton (VNP) is a zoned ring structure emplaced in the southern portion of the Neoproterozoic Araçuai Belt, in Espírito Santo, Brazil. It is a slightly westward tilted cylinder-like intrusion, with an almost circular horizontal section. In the center of this structure, an off-centered gabbro-noritic core, surrounded by syeno-monzonitic rocks, intrudes an outer ring of charnockites and norite. These envelop the syeno-monzonitic and gabbro-noritic center, as a narrow discontinuous belt. While, in the core intrusion, mingling and mixing processes are widespread and well documented in the literature, in the outer ring, the norite and charnockite layers show predominantly homogeneous and isotropic internal structures. Nevertheless, smaller interaction zones between charnockites and norite denote a comparatively more restricted mingling process. The norite is a fine-grained rock with hypidiomorphic granular to intergranular texture. The charnockites are medium-grained and made up of: (a) orthopyroxene-tonalite, (b) orthopyroxene-quartz-diorite, and (c) orthopyroxene-granodiorite with hypidiomorphic granular to porphyritic textures. In all lithotypes both ortho- and clinopyroxene are replaced by hornblende and biotite. Two contrasting compositional sequences have been recognized, based on whole rock geochemistry: (1) a basic, with tholeiitic affinities (norite) and, (2) an intermediate, medium-K calc-alkaline, comprising the charnockites. Estimated crystallization temperatures, which have been calculated from micro-probe analysis of pyroxenes, range from 915 ± 25 °C to 960 ± 50 °C. Re-equilibration temperature (ilmenite-magnetite calibration) is around 600 ± 50 °C. This indicates oxygen fugacities four order of magnitude below the FMQ-buffer and a reduced environment. Coeval pressure conditions estimated from the Al-content in hornblende range from 5.5 ± 0.6kbar. Data obtained for the norite point toward an evolution from the partial melting of an anhydrous tholeiitic mantle magma. The charnockites may correspond to the differentiation of a calc-alkaline parental magma. The later could be the hybrid product from the contamination and mixing between the anhydrous tholeiitic magma with other deeper crustal sources. Further the magmatic system evolved through fractionation of orthopyroxene, clinopyroxene, plagioclase and ilmenite. Our results support the hypothesis for the evolution of the Venda Nova Pluton through a mantle-crust delamination process probably related to the collapse of the Araçuai orogeny.

Alkaline lamprophyric province of Central Iran

AbstractPaleozoic lamprophyres exhibit good exposures in the western part of the Central–East Iranian microcontinent. These rocks crop out as volcanoes, dykes, and plugs. The constituent minerals are amphibole, clinopyroxene, plagioclase, K‐feldspar, olivine, Cr‐spinel, titanite, biotite, and ilmenite. The main textures in volcanic lamprophyres are porphyritic, trachytic, microlithic, and variolitic, whereas in dykes and plugs, intergranular texture is common. These lamprophyres are regionally metamorphosed in some areas. Petrographical and geochemical characteristics of the studied rocks suggest that they are classified as alkaline lamprophyres and camptonites. They are enriched in alkalis (Na2O + K2O), large ion lithophile elements, and light rare earth elements, and the features of trace element concentrations are similar to those of within‐plate basalts. This study suggests that the lamprophyres were derived from different degrees of partial melting of metasomatized amphibole‐bearing spinel lherzolite. Subduction of Paleo‐Tethys oceanic crust from the Early to late Paleozoic resulted in enrichment in fluids in the mantle, and lamprophyric magmatism occurred along the minor and major faults. This limited but typical lamprophyric magmatism in a broad area of Central Iran suggests that, in spite of the long length of the Paleozoic (∼250 my), it was a relatively calm era from the viewpoint of magmatism in Central Iran.

Petrochemistry and tectonic setting of mafic volcanic rocks in the Chon Daen–Wang Pong area, Phetchabun, Thailand

AbstractThe mafic volcanic rocks and hypabyssal rocks in the Chon Dean‐Wang Pong area are possibly the southern extension of the western Loei Volcanic Sub‐belt, Northeast Thailand. They are least‐altered, and might have been formed in Permian–Triassic times. The rocks are commonly porphyritic, with different amounts of plagioclase, clinopyroxene, orthopyroxene, amphibole, Fe–Ti oxide, unknown mafic mineral, and apatite phenocrysts or microphenocrysts, and are uncommonly seriate textured. The groundmass mainly shows an intergranular texture, with occasionally hyalophitic, intersertal and ophitic–subophitic textures. The groundmass constituents have the same minerals as the phenocrysts or microphenocrysts and may contain altered glass. The groundmass plagioclase laths may show a preferred orientation. Chemically, the studied rock samples can be separated into three magmatic groups: Group I, Group II, and Group III. These magmatic groups are different in values for Ti/Zr ratios. The averaged Ti/Zr values for Group I, Group II, and Group III rocks are 83 ± 6, 46 ± 12, and 29 ± 5, respectively. In addition, the Group I rocks have higher P/Zr, but lower Zr/Nb relative to Group II and Group III rocks. The Group I and Group II rocks comprise tholeiitic andesite–basalt and microdiorite–microgabbro, while the Group III rocks are calc‐alkalic andesite and microdiorite. According to the magmatic affinities and the negative Nb anomalies on normal mid‐oceanic ridge basalt (N‐MORB) normalized multi‐element plot, arc‐related lavas are persuasive. The similarity between the studied lavas and the Quaternary lavas from the northern Kyukyu Arc, in terms of chondrite‐normalized rare earth element (REE) patterns and N‐MORB normalized multi‐element patterns, leads to a conclusion that the mafic volcanic rocks and hypabyssal rocks in the Chon Daen–Wang Pong area have been formed in a volcanic arc environment.

High-resolution electron microscopy observations of the microstructure of a rapidly solidified Mg–9.0 wt% Al–1.0 wt% Zn–4.0 wt% Sn alloy

The present study examines the microstructure of rapidly solidified ribbons of an Mg–9.0 wt% Al–1.0 wt% Zn–4.0 wt% Sn alloy. XRD and transmission electron microscopy (TEM), including high-resolution transmission electron microscopy (HRTEM), confirmed that the rapidly solidified ribbons consist of α-Mg, γ-Mg17Al12 and β-Mg2Sn phases. The predominant fraction of the γ-Mg17Al12 phase reveals intergranular texture morphologies with the Burgers OR. The minor fraction of the γ phase exists in short-lath morphology with a typical grain size of 30 nm, and its orientation relationship with the matrix is also the Burgers orientation relationship. The β-Mg2Sn particle reveals spherical morphology with the typical grain size of 40 nm, and its orientation relationship with the matrix is in the form of , .

The basaltic volcanic rocks in the Tuyon Basin, NW China: Petrogenesis and tectonic implications

The Tuyon basin is located near the crossing of the Tien Shan orogenic belt, the Western Kunlun fold system and the TarimAlashan blocks. The crossing of two lithospheric scale fault, the Talas-Ferghana dextral strike-slip fault and the NE extended South Tien Shan strike-slip fault, is just at the southern margin of the Tuyon basin. Therefore, many geoscientists all over the world have been attracted to petrogenesis of the Cenozoic basaltic volcanic rocks in the Tuyon basin and their tectonic implications. According to previous researches, the basalts emplaced within the Tuyon basin are of two units, the lower basalts and the upper basalts, aged 120~110 Ma and 70.4~36.6 Ma respectively. All these basalts are classified as alkali series, enriched in potassium, and considered as a result of a small plume probably rooted at a shallow level in the asthenosphere. Our new works on the igneous rocks in the Tuyon basin suggest that the so-called lower basalts are preserved as a series of cone dikes, which are emplaced in Jurassic-Lower Cretaceous sedimentary rocks; the upper basalts are formed as a series of volcanic eruptions, including calderas, ring dikes, necks and volcanic cones. For example, there are three volcanic cones in the line between the Kuvt cone and Bilebluk cone. We have discovered basaltic intrusive units in Jurassic sandstone and conglomerate strata. The basaltic extrusive units have the same occurrence with the early Cretaceous sediments, but uncomfortably overlain by the Paleogene sedimentary rocks. Therefore, the Tuyon basin is a compound volcanic basin, including series of volcanic eruptions with lavas and subvolcanic rocks, rather than it has two-layer structure consisted of two igneous units. Our investigations indicate that the basaltic rocks are mainly of dark gray, massive structure and porphyritic texture. The basaltic extrusive rocks are developed as columnar joint structure, vesicular structure and amygdaloidal structure. Most of volcanic rocks within Tuyon basin contain phenocrysts such as olivine, clinopyroxene and plagioclase, and occasionally phlogopite, amphibole and apatite. Olivine belongs to chrysolite circled by fine-grained aggregate of magnetite and talc. Most of olivine phenocrysts have been altered into iddingsite and talc. The clinopyroxene mainly belongs to augite, which characterize hourglass structure and zoned texture. The plagioclase phenocrysts have more content of anorthite. They are mainly andesine and occasional labradorite, but rare in zoned texture. The matrix minerals are clinopyroxene, plagioclase, magnetite and volcanic glass, and mainly formed tholeiitic texture, occasionally intergranular texture and vitrophyric. Therefore, the Cenozoic volcanic rocks in the Tuyon basin show characteristics of alkali series, and suggest that the region may was at extensional condition. In the Arhesaiordy volcanic-sedimentary sequences there are five basaltic sills intruded into the Jurassic sedimentary deposits. Many megacrysts have been found in the lowest basaltic layer, such as augite, phlogopite, amphibole and anorthoclasite. From the same layer we have collected lots of the deep-seated xenoliths including peridotite, granulite, pyroxenite, gneiss and so on. In the other four layers, the number of megacrysts and deep-seated xenoliths has been decreased quickly. There are also lots of deep-seated xenoliths in the Tasgeler caldera. Therefore, basaltic magma has the characteristics of rapid ascent. Consequently, the basaltic volcanic rocks in the Tuyon basin are formed direct from primary magma, which is rare affected by fractional crystallization.