Xenoliths Of Rocks Research Articles

Episodic Archean crustal accretion in the North China Craton: Insights from integrated zircon U-Pb-Hf-O isotopes of the Southern Jilin Complex, northeast China

Recent advances in high-precision in-situ analysis of zircon U-Pb-Hf-O isotopes have significantly improved our approach to studying the origin and evolution of Archean crust. This study presents high-precision in-situ zircon U-Pb-Hf-O isotopes for the Southern Jilin Complex (SJC) on the northeastern margin of the North China Craton (NCC) and explores the potential of such an integrated approach in understanding the Archean crust evolution and its tectonic implications. The SJC consists mainly of Archean granitoid gneisses hosting minor supracrustal rock xenoliths. Zircon U-Pb data from representative lithologies reveal three episodes of magmatism at ~2.78, ~2.69 and ~2.55 Ga, respectively. The former two granitoid magmatic events are recognized for the first time in NE NCC, while the latest one is widespread over the NCC. All metamorphic zircon domains document similar metamorphic ages of 2.51–2.48 Ga, consistent with the widespread metamorphism of ~2.50 Ga in the NCC. Most concordant magmatic zircons preserve “mantle-like” O isotopic signatures, indicating the magma sources were not contaminated by large amounts of high δ18O supracrustal sediments. Hf depleted mantle model ages of magmatic zircons display one predominant peak at 2.9–2.8 Ga and two subordinate peaks at 3.3–3.2 Ga and 2.7–2.6 Ga, indicating possible episodic crust-mantle differentiation in Southern Jilin. Integrated zircon U-Pb-O-Hf data from the SJC suggest that magma sources were mainly derived from multiple episodes of reworking of juvenile or ancient crust with mantle component addition, while recycling of sediments was trivial during the Neoarchean in this region. The spatial–temporal relations of magmatism, lack of supracrustal addition to magma sources, and other geological evidence from the NCC are in favor of a bottom-up geodynamic regime possibly related to plume activities, arguing against a widespread top-down mechanism involving modern-style subduction in the NCC during the Neoarchean.

Petrological features of picrobasaltic melts on Lanzarote, Canary Islands

The paper discusses issues related to the petrology of picrobasaltic melts forming lava flows on Lanzarote, Canary Islands. During ascientific and educational expedition on the aforementioned island, lava flows of picro basalt of the third phase of the 1731–1732 eruption were studied and tested. Preference was given to the well-preserved streams in the west and south of the island thatare accessible for direct study. Volcanogenic-sedimentary associations were identified in the studied outcropsand described, the structural and textural features of the flows were characterized, and samples with mantle rock xenoliths were taken for petrological reconstructions. New geochemical data onrocks and minerals are presented. It was established that the temperature of the initial melt was 1100–1180°C. Large harzburgitephenocrysts in picro basalt are xenocrysts formed as a result of melting and disintegration of mantle xenoliths.

Geothermobarometry of igneous rocks from Afonso Cláudio Intrusive Complex (Espírito Santo state, Southeastern Brazil), Araçuaí-West Congo Orogen: Further evidence for deep emplacement levels

The Afonso Cláudio Intrusive Complex (ACIC) is an inversely zoned pluton that intruded the Araçuaí-West Congo Orogen (AWCO) during the orogenic collapse, which lasted from 530 to 480 Ma. It is hosted in orthogneisses of the Pre-collisional Supersuite and paragneisses from Nova Venécia Complex and is mostly constituted by two off-centered mafic cores, composed by monzogabbro and monzodiorite, surrounded by quartz monzonite hills. Mingling and mixing zones were mapped between these rocks, where jotunite and quartz mangerite were locally found. In this study, we present a petrographic, mineral chemistry and geothermobarometric study concerning the most abundant ACIC rocks. These rocks showed emplacement conditions corresponding to middle and lower crust, which are similar to crustal levels estimated for its main host rocks. In addition, the ACIC rocks enclose many host rocks xenoliths, with some of them showing melting evidences. These evidences suggest that the ACIC host rocks were one of the main sources of the ACIC felsic magmas, which contaminated mantle mafic magmas during ACIC evolution. Due to the similarities between the AWCO post-collisional rocks, present work concludes that post-collisional plutons intruded between the middle and lower crust levels and were contaminated by its host rocks during its evolution and cooling, contributing also to the felsic magma sources.

Geology and Geochemistry of Iroko-Granites Southwestern Nigeria: implication on provenance

The geology, geochemistry of Iroko-granites have been studied and reported in this paper. The study area has been described as of Archaean –Early Proterozoic terrain underlain by migmatite-gneiss-quartzite complex with supracustal rocks. Large number of granites which outcrops in Iroko-Ekiti represent a typical occurrence of granitoids sporadically distributed in the basement and are known to belong to the Older Granite suites, which are attributable to the Pan-African Orogeny(750± 150Ma). The rocks occur, mostly as flat and low lying within sparse vegetation. Structures common on and around the outcrops include quartz vein, veinlets, pegmatite dykes which trend North-south, discrete exfoliated surfaces and xenoliths of older rocks. This study reveals that the granites belong to calc-alkalic suites, demonstrate metaluminous nature, and exhibit characteristics of I-type granites. The granite is a distinctive type in that it is relatively highly potassic, has high FeO/(FeO + MgO) ratio, and high average Zr (299.75ppm) concentration with other high field strength elements. The trace elements study implicates pronounced fractional crystallization during evolution of the granites and thus petrogenetically discriminates as Syn-collision provenance.

Geochronology and geochemistry of exotic blocks of Cadomian crust from the salt diapirs of SE Zagros: the Chah-Banu example

ABSTRACT Cadomian calc-alkaline I-type and within-plate A-type igneous rocks are widespread in the crust of Iran where they are ascribed to a convergent margin associated with the southward subduction of Prototethyan oceanic lithosphere beneath N Gondwana. These rocks are found as unmetamorphosed magmatic rocks and their metamorphic equivalents (mafic to felsic gneisses) could have been generated in both Cadomian arcs and associated rear-arcs. Nearly all these exposures also contain metamorphosed metasediments, whereas Cadomian igneous rocks of central Iran are associated with thick sequences of unmetamorphosed terrigenous rocks. In the Zagros Fold-Thrust belt of S Iran, salt diapirs contain abundant xenoliths of Cadomian igneous and sedimentary rocks in association with evaporites, dolomites, carbonates, and banded iron-salt deposits. This paper presents new zircon U-Pb and geochemical-isotopic data from igneous clasts in the Chah-Banu salt diapir in SE Zagros. Petrographic and geochemical data indicate two different types of rock clasts; calc-alkaline, I-type dacites-rhyolites, and arc-related to E-MORB-OIB-like gabbros, basalts, and dolerites. New zircon U-Pb ages show that dacites formed at 538.2 ± 2.2 Ma, whereas gabbros show ages of 539.0 ± 1.8 Ma. Zircons from dacites have negative εHf(t) values of – 1.1 to – 8.3. In contrast, zircons from gabbros have higher εHf(t) values of +4.5 to +8.5, indicating crystallization from mantle-derived juvenile magmas. Bulk rock Nd-Sr isotopic data (e.g. ɛNd(t) = +0.3 to +4.0 and 87Sr/86Sr(i) = 0.7059 to 0.70848) for gabbros, dolerites, and basalts confirm that these rocks originated from a mantle source similar to enriched parts of the subcontinental lithospheric mantle, whereas dacites and rhyolites (with εNd(t) = −3.4 to −4.1 and 87Sr/86Sr(i) = 0.70806 to 0.70907) show strong interaction with, and/or re-melting of older continental crust. We suggest that the bimodal calc-alkaline and E-MORB-OIB-like magmatic rocks in salt diapirs as well as associated evaporites and sedimentary rocks formed in a retro-arc rifted basin behind the Cadomian magmatic arc.

Multi‐stage metamorphism by progressive accretion of continental blocks, example from the Western Hindu Kush

AbstractBasement rocks from the Western Hindu Kush preserve evidence of multiple metamorphic and magmatic events that occurred along the boundary between the Archean–Proterozoic Afghan Central and Afghan–Tajik Blocks. To verify the different metamorphic stages or events, mineral textures and phase equilibria in metamorphic basement rocks and their age relations to magmatic episodes have been investigated. Quartzofeldspathic gneiss and migmatite with lenses of amphibolite (with assumed Proterozoic age for their metamorphism) are intruded by the Triassic Hindu Kush granitoid batholith and small Cretaceous and Oligocene granite intrusions. The age of thermal overprint (210–170 Ma) by the Triassic batholith is confirmed by new monazite data. Both Triassic and Cretaceous granitoids and surrounding basement rocks underwent subsequent metamorphism up to epidote–amphibolite facies. The degree of this metamorphism increases southward at the contact to the Kabul Block, which under‐plates the Western Hindu Kush from the south. An early Miocene age was obtained by Pb–Th analyses in thorite and huttonite, which are close or slightly younger than the Oligocene granite in this area. The Cretaceous meta‐granodiorite near the border with the Kabul Block contains xenoliths of granulite facies rocks that could come from the Neoarchean granulite facies basement of the Kabul Block. The multi‐stage metamorphic and magmatic evolution classifies the Hindu Kush mountain belt as a long‐lived suture zone that was active since the early Palaeozoic. The results of this study support the interpretation about possible relations of the Afghan Central Blocks to the southern margin of Eurasia during the evolution of Para‐ and Neotethys.

Origin of zircon megacrysts in alkaline lavas (French Massif Central): Petrology and in situ U-Pb-Hf isotopes

In the south eastern Devès volcanic domain of the French Massif Central, we sampled red zircon megacrysts in, both, alluvium from a small stream and neighbouring mugearite lava. LA-ICP-MS U-Pb dating results yield similar ages for both suites of zircons at about 2.6 Ma. Superchondritic signatures of the Hf isotopes clearly indicate a depleted mantle source for these zircons.The mugearite lava also contains xenoliths of mantle (lherzolites) and lower crustal rocks (granulites). The chemical compositions of the minerals within the mugearite lava are similar to the mineral compositions found in these xenoliths (granulites) and granulite and peridotite xenoliths from other volcanic localities of the Massif Central. The mugearite lava also preserves Variscan zircon crystals dated at 285 Ma, originating from recycled lower continental crust.In spite of their mantle origin, as recorded by in situ Hf isotopes, the geochemistry suggests that these Pliocene zircons do not result from crystallization in the mantle or in mantle-derived mafic magmas. In order to explain this inconsistency, we propose that these zircon grains crystallized from felsic melts produced during the ultimate differentiation of mantle-derived mafic magmas in the mid or upper continental crust. Subsequently, a last basaltic batch forced its way to the surface, scavenging upper mantle and lower crust xenoliths as well as zircon xenocrysts formed in these felsic melts which partly dissolved during their ascent.Large amounts of Cenozoic zircon megacrysts are often associated with sapphires in alluvial deposits in many continental volcanic domains, not only in the French Massif Central but also worldwide. Consequently, a huge volume of felsic differentiates issued from mantle-derived basaltic magmas might be suspected in many alkaline volcanic domains at the global scale.

Sulfide segregation mechanism of magmatic Ni mineralization in western Beishan region, Xinjiang, NW China: Case study of the Hongshishan mafic–ultramafic complex

A series of early Permian mafic–ultramafic complexes associated with Ni-Cu mineralization are distributed in western Beishan region, Xinjiang, NW China. The Ni mineralization in this area consists of two sub-belts, which have the same magma conduit at depth, similar rock types and similar emplacement ages, but different mineralization types. The Ni mineralization in the north (Hongshishan) sub-belt consists of low-tenor disseminated sulfide, whereas that in the south (Pobei) sub-belt consists of disseminated sulfide and vein-type sulfide with high tenor, suggesting different sulfide segregation mechanisms. The Hongshishan complex consists of dunite, wehrlite, olivine clinopyroxenite, troctolite, olivine gabbro, and gabbro (from earlier to later). The mafic rocks occur along margins of the complex, and the mineralized ultramafic rocks are confined to the center. The dunite and wehrlite host significant amounts of disseminated sulfide. The parental magma of the Hongshishan complex may have originated from 17.0% partial melting of the depleted mantle. It is characterized by high temperature (1368 ℃), high Mg (picritic affinity, 14.52 wt% MgO), oxidation and platinum group element (PGE) depletion, as constrained by the major and trace elements, olivine and chromite mineralogy, and PGE contents. Two stages of sulfide segregation have been recognized. Early sulfide segregation at depth, which is indicated by elevated Cu/Pd ratios and sulfide inclusions in chromite, produces the PGE-depleted parental magma. Later in situ segregation results in disseminated sulfide mineralization. Model of the olivine crystallization and PGE content shows that 0.01%−0.015% of the sulfide is segregated in situ, with R = 100–500. Significant crustal contamination is suggested by xenoliths of wall rock as well as negative Nb, Ta, and Ti anomalies and positive Pb and Sr anomalies in intrusive rocks. The addition of crustal Si and decrease in FeO by fractional crystallization are the main factors triggering S-oversaturation and later in situ segregation. The early segregation at depth is more likely caused by the addition of crustal S, although the δ34S values of sulfides (-0.75‰ to 1.15‰) indicate the isotopic signature of mantle sulfur. For comparison, the development of vein-type sulfide mineralization in the Poyi deposit is ascribed to assimilation of S-bearing Archean rocks. Finally, a model of “early segregation at depth + later in situ segregation + multistage pulsation” is proposed for the Ni mineralization in this area.

Genesis of Diamondiferous Rocks from Upper-Mantle Xenoliths in Kimberlite

The conditions of genesis of diamondiferous ultrabasic and basic rocks from xenoliths in kimberlite were studied by combining the data from analytical investigations of their mineral phases and experimental results of the study of melting relations in the diamond-forming mineral systems of the upper mantle. The compositions of minerals in some samples of metasomatized diamondiferous eclogite associated with diamond-free eclogite from kimberlite of the Udachnaya pipe (Yakutia) were studied for the first time. The new results obtained in addition to the literature data were applied for generalization of estimates of genetically important characteristics of the chemical compositions of garnets, Ca-clinopyroxenes, and omphacites from diamond-bearing peridotite, pyroxenite, and eclogite. As a result, it was found that quite “fresh” minerals of diamondiferous rocks have typomorphic differences from the same minerals of diamond-free upper-mantle rocks. At the same time, it is significant that the compositions of minerals from diamondiferous rocks and paragenetic inclusions in diamonds are identical. These peculiarities of mineralogy of diamondiferous rocks are genetically significant; based on the mantle–carbonatite theory of the origin of diamond and associated mineral phases, this provides support for the same physicochemical origin of diamonds, minerals of diamondiferous rocks, and paragenetic inclusions in diamonds. Finally, the following genetic conclusions are made. (1) Completely miscible silicate (±oxide)–carbonate melts with dissolved carbon are the parental medium in petrogenesis of diamondiferous ultrabasic and basic rocks. (2) The physicochemically consistent formation of diamondiferous rocks and paragenetic inclusions of peridotitic and eclogitic minerals in diamonds occurred in the common diamond-forming chambers/reservoirs of parental melts; diamond-free peridotite, pyroxenite, and eclogite were the host mantle rocks for such chambers. (3) The origin of continuous series of diamondiferous peridotite–pyroxenite–eclogite rocks is controlled by the fractional ultrabasic–basic evolution of parental melts with exhaustion of olivine and orthorhombic pyroxene via the peritectic reactions. (4) Ascending flows of kimberlite magmas destroyed the parental chambers and captured diamonds with inclusions, individual minerals, their intergrowths, diamondiferous ultrabasic and basic rocks; at the entrance and exit from the chambers, they captured differentiated diamond-free host rocks of the mantle as well. (5) With further ascent from the mantle to the Earth’s crust, the material of diamond-forming chambers and diamond-free mantle was mixed in convecting kimberlite magma and was transported from the mantle to cumulative crustal chambers. (6) Kimberlite magmas were gradually solidified in stationary cumulative chambers with the release of highly compressed fluids; with an increase of pressure up to the critical values, they intruded into the rocks of the roof and ejected kimberlite with xenoliths of diamondiferous and mantle rocks to the surface with the formation of explosion pipes.

Petrografie xenolitů vyvřelých hornin v neovulkanitech z lomu Bučník u obce Komňa

Neovolcanites from the Uherský Brod area sporadically enclose cm to dm large xenoliths of plutonic rocks. This paper focuses on characterization of xenoliths from the Bučník hill which are enclosed in trachyandesites by using classic petrographic methods. Based on mineral composition and textural features of main rock-forming minerals, the studied xenoliths can be classified as fine- to coarse-grained pyroxenic-amphibolic gabbros or diorites, respectively. They consist mainly of intermediate polysynthetically twinned plagioclase (andesine to labradorite; An44–55) and yellow-brown pleochroic magnesiohastingsite to pargasite (XMg = 0.64–0.75; Si = 6.09–6.29 apfu; Ti = 0.30–0.42 apfu). Relicts of diopside (XMg = 0.70–0.80; Ti = 0.01–0.03 apfu; Na = 0.03–0.04 apfu), brown strongly pleochroic phlogopite leaflets (XMg = 0.58–0.65; Si = 5.51–5.60), apatite columns (predominantly fluorapatite; F = 0.43–0.77 apfu) and K-feldspar (Ab16–17Or82–84An00–01) grains are less frequent. Amphiboles sporadically enclose round or tabular inclusions of labradorite to bytownite (An64–72) and subhedral olivine (Fo64). Xenoliths are similarly to host trachyandesites affected by superimposed hydrothermal alteration. Secondary minerals represent acid plagioclase (An05–10), chlorites (clinochlore; XMg = 0.55–0.67; Si = 3.13–3.29 apfu), carbonates (calcite and siderite), Ti-minerals (ilmenite, rutile and titanite), sulphides (pyrite and chalcopyrite) and less frequently quartz. The genetic affinity of xenoliths and the host neovolcanites is evidenced by the similarity in mineral composition and whole-rock chemistry. Xenoliths can be interpreted either as material from deeper parts of the magma chamber, or more probably as crystal cumulates (i.e. equivalents of cumulate gabbros).

Calcium isotope sources and fractionation during melt-rock interaction in the lithospheric mantle: Evidence from pyroxenites, wehrlites, and eclogites

To investigate the effects of melt metasomatism on Ca isotope fractionation in the upper mantle, we report data for xenoliths of mantle rocks including composite peridotites (lherzolites and coexisting pyroxenite veins), wehrlites as well as eclogites and their minerals. The δ44/40Ca in clinopyroxenes (cpx) are ~0.3‰ lower than in coexisting garnets in eclogites from the Udachnaya kimberlite in the central Siberian craton, likely due to equilibrium isotope fractionation controlled by mineral structure. The δ44/40Ca in the pyroxenite veins (cumulates from mafic melts) and their host peridotites from Tariat in Mongolia range from 0.88‰ to 1.05‰ and are close to the Bulk Silicate Earth (BSE) value (0.95 ± 0.05‰). Calcium-Fe-rich low-orthopyroxene lherzolites and wehrlites (LW series) from Tok in the SE Siberian craton were formed by reaction of lherzolites and harzburgites with percolating silica-undersaturated melts. The δ44/40Ca (0.65‰–0.87‰) in the Tok peridotites are negatively correlated with Ca/Al, (Gd/Yb)N and positively with Mg# (indices of melt-rock reaction degrees). The δ44/40Ca ratios in the LW series rocks are lighter than the BSE estimate, and similar to δ44/40Ca in their host basalt and an olivine-cpx cumulate (0.70‰–0.76‰). These observations on the Tok suite can be explained by reaction of refractory peridotite protoliths with melts derived from mixed eclogite and peridotite sources. Overall, the δ44/40Ca variations in eclogites, wehrlites, and various rocks related to silicate melts in the mantle, demonstrate that melt metasomatism plays an important role in producing mantle Ca isotope heterogeneity. The data for coexisting garnet and cpx in eclogites outline a potential for garnet to affect Ca isotope inter-mineral and solid-melt fractionation in the mantle.

Alunite 40Ar/39Ar and Zircon U-Pb Constraints on the Magmatic-Hydrothermal History of the Zijinshan High-Sulfidation Epithermal Cu-Au Deposit and the Adjacent Luoboling Porphyry Cu-Mo Deposit, South China: Implications for Their Genetic Association

AbstractThe large Zijinshan high-sulfidation epithermal Cu-Au deposit, together with the adjacent Luoboling porphyry Cu-Mo deposit, constitutes a major porphyry-epithermal ore district in South China. Current debate centers on whether the Zijinshan and the adjacent Luoboling deposits are cogenetic or represent separate ore-forming events, which is a question of importance for exploration in the district. In this contribution, the magmatichydrothermal history of the relationship between Zijinshan and Luoboling is reconstructed based on new alunite 40Ar/39Ar ages from Zijinshan and zircon U-Pb ages of ore-related intrusions from both deposits. This study has been complemented by S isotope analysis on the dated alunite to assess their origin.Three types of coexisting alunite and sulfide assemblages exist at Zijinshan, namely, (1) alunite-quartz-covellite cemented breccias; (2) alunite-digenite veins and (3) banded alunite-pyrite veins. Their field occurrences and S isotope features suggest a magmatic-hydrothermal origin for alunite-quartz-covellite cemented breccias and alunite-digenite veins, whereas the origin of alunite-pyrite veins is likely to be related to magmatic steam. Given the intimate textural coexistence between sulfides and alunite, four undisturbed 40Ar/39Ar plateau ages obtained from alunite-quartz-covellite cemented breccia and alunite-digenite vein-type alunite define the timing of Zijinshan high-sulfidation mineralization from 102.86 ± 0.61 to 101.19 ± 0.60 Ma. These agree with bracketing zircon U-Pb ages of pre-ore dacite porphyry at 104.8 ± 0.9 Ma and 104.7 ± 0.5 Ma and a zircon U-Pb age of a post-ore granite porphyry dike at 99.5 ± 0.7 Ma. Combined with their field occurrences, the four alunite ages may imply episodic hydrothermal pulses and a possible time span of over 500 k.y. for the overall high-sulfidation mineralization. Two alunite samples from alunite-pyrite veins yield a slightly disturbed 40Ar/39Ar plateau age at 101.67 ± 0.61 Ma and an apparently undisturbed age at 99.91 ± 0.59 Ma, probably reflecting partial or complete thermal resetting related to the coeval granite porphyry dikes. At Luoboling, zircon U-Pb analysis yields an age of 133.6 ± 1.1 Ma for a dark, ore vein-bearing quartz-diorite porphyry sample and confirms the petrographic observation that they are xenoliths of early wall rocks for the porphyry mineralization. A granodiorite porphyry sample with abundant A-veins is interpreted as an intermineral porphyry phase, dated at 106.5 ± 1.4 Ma. This age is interpreted as the upper limit for porphyry Cu-Mo mineralization and agrees well with previously reported molybdenite Re-Os and biotite 40Ar/39Ar ages, which collectively suggest porphyry Cu-Mo mineralization and potassic alteration at Luoboling were formed within the interval between ca. 106 and 104 Ma.Taken together, the age of the Zijinshan high-sulfidation mineralization is substantially younger than the porphyry mineralization at Luoboling, and we conclude that there is no direct genetic link between the two deposits. However, the Zijinshan Cu-Au deposit is temporally associated with the deep porphyritic granodiorite, which gives zircon U-Pb ages from 102.1 ± 0.8 to 101.8 ± 0.5 Ma, overlapping with the alunite 40Ar/39Ar ages for the high-sulfidation mineralization. This finding has important implications for the ongoing exploration for the two mineralization types in the ore district and elsewhere.

Petrografia e química do Granito Aripuanã, sul do Cráton Amazônico

O termo Granito Aripuanã refere-se a um conjunto de corpos graníticos circunscritos não deformados, intrusivos em rochas da sequência metavulcanossedimentar do Grupo Roosevelt, reconhecidos na porção sul do Cráton Amazônico. O corpo granítico principal constitui um batólito ovalado com diâmetro menor que 15 km, que ocorre a norte da cidade de Aripuanã, no noroeste do estado do Mato Grosso. É constituído de hornblenda-biotita sieno a monzogranitos porfiríticos grossos, de coloração cinza ou vermelha e localmente com texturas rapakivi. Nas áreas marginais é observada uma redução de granulação das feições porfiríticas com predomínio da fácies equigranular fina, associada a resfriamento rápido. As fácies tardias comuns são constituídas de diques equigranulares de cor cinza, aplitos róseos e cinza e veios pegmatíticos. Ocorrem veios pós-magmáticos de quartzo, fluorita, muscovita em zonas de deformação rúptil, em que as rochas mostram intensa alteração hidrotermal. Esse hidrotermalismo afeta de forma heterogênea as rochas encaixantes e as rochas magmáticas, facilitando a circulação de fluidos e favorecendo a concentração de sulfetos de zinco disseminados, sempre relacionados ao fraturamento. Os granitos hospedam xenólitos de rochas paraderivadas encaixantes, de granodioritos, quartzo dioritos e rochas micáceas. Nas bordas, ocorrem veios e apófises graníticas, além de metamorfismo de contato nas rochas encaixantes. Os monzogranitos são peraluminosos e pertencem à série de alto potássio a shoshonítica, similares a granitos “ferroan”, apresentando assinatura de granito tipo A, pós-colisionais a anorogênicos e associado à evolução do Arco Magmático Jamari.

Emplacement and growth of alkaline dikes: Insights from the shonkinite dikes (Elchuru alkaline complex, SE India)

The Mesoproterozoic Elchuru alkaline complex in the Prakasam Alkaline Province (PAP), Eastern Ghats Belt, SE India is intruded by coeval shonkinite dikes. Magmas parental to both the host nepheline syenite and shonkinite dikes formed during ca. 1350 Ma continental-rift related magmatism. The shonkinite dikes show fine-to medium-grained equigranular to foliated textures with clinopyroxene, biotite, amphibole, K-feldspar perthite and nepheline as major mineral phases. The dikes grew by inflation and coalescence of dike segments along intrusive steps, anastomosis and subsequent coalescence, and thermal/mechanical erosion of the host rocks. A few dikes resulted by filling the fractures created by shearing. Inflation of intrusive steps between magma segments by Mode I opening is the major mechanism of shonkinite dike emplacement and growth. However, the shonkinite dikes also demonstrate the application of remote shear stress on the dike walls resulting in anastomosis, entrapment and segmentation of tabular host rock xenoliths, and in extreme cases brecciation of the host rocks under volatile-rich and low-viscous melt conditions. Fractal analysis of xenolith-bearing dike further illustrates that thermal erosion was facilitated and accentuated by mechanical breakdown of host rock xenoliths due to interactions of tensile and shear forces. Cross-cutting relationships between the shonkinite dikes suggest local deviation in the least compressive stress direction during dike emplacement. The shonkinite dikes of Elchuru demonstrate that spatially- and temporally-restricted high-volatile low-viscous alkaline magmas may have distinctly different styles of emplacement, as controlled by interaction of near field (magmatic) tensile stress and far field (tectonic) shear stress, in an evolving continental-rift setting.

Geology of the Renard 2 pipe to 1000 m depth, Renard Mine, Québec, Canada: insights into Kimberley-type pyroclastic kimberlite emplacement

The Renard 2 pipe is currently the deepest-drilled and most extensively studied kimberlite body in the Renard cluster, central Quebec, Canada, forming the major component of the Mineral Resource of Stornoway Diamond Corporation’s Renard Mine. Renard 2 is infilled with two distinct kimberlite units that exhibit Kimberley-type pyroclastic kimberlite and related textures. Hypabyssal kimberlite also occurs as smaller cross-cutting sheets and irregular intrusions. The units are distinguished by their rock textures, groundmass mineral assemblages, olivine macrocryst size distributions and replacement products, mantle and country rock xenolith contents, whole rock geochemical signatures, bulk densities and diamond grades. These differences are interpreted to reflect different mantle ascent and near-surface emplacement processes and are here demonstrated to be vertically continuous from present surface to over 1000 m depth. The distinctive petrological features together with sharp, steep and cross-cutting internal contact relationships, show that each unit was formed from a separate batch of mantle-derived kimberlite magma, and was completely solidified before subsequent emplacement of the later unit. The mineralogy and textures of the ultra-fine-grained interclast matrix are consistent with those described at numerous Kimberley-type pyroclastic kimberlite localities around the world and are interpreted to reflect rapid primary crystallization during emplacement of separate kimberlite magmatic systems. The units of fractured and brecciated country rock surrounding the main kimberlite pipe contain kimberlite-derived material including carbonate providing evidence of subsurface brecciation. Together these data show that Renard 2 represents the deeper parts of a Kimberley-type pyroclastic kimberlite pipe system and demonstrates that their diagnostic features result from magmatic crystallisation during subsurface volcanic emplacement processes.

The Mesozoic and Palaeozoic granitoids of north-western New Guinea

A large portion of the Bird's Head Peninsula of NW New Guinea is an inlier that reveals the pre-Cenozoic geological history of the northern margin of eastern Gondwana. The peninsula is dominated by a regional basement high exposing Gondwanan (‘Australian’) Palaeozoic metasediments intruded by Palaeozoic and Mesozoic granitoids. Here, we present the first comprehensive study of these granitoids, including field and petrographic descriptions, bulk rock geochemistry, and U–Pb zircon age data. We further revise and update previous subdivisions of granitoids in the area. Most granitoids were emplaced as small to medium-scale intrusions during two episodes in the Devonian–Carboniferous and the Late Permian–Triassic, separated by a period of apparent magmatic quiescence. The oldest rocks went unrecognised until this study, likely due to the younger intrusive events resetting the K–Ar isotopic system used in previous studies. Most of the Palaeozoic and Mesozoic granitoids are peraluminous and in large parts derived from partial melts of the country rock. This is corroborated by local migmatites and country rock xenoliths. Although rare, metaluminous and mafic rocks show that partial melts of mantle-derived material played a minor role in granitoid petrogenesis, especially during the Permian–Triassic. The Devonian–Carboniferous granitoids and associated volcanics are locally restricted, whereas the Permian–Triassic intrusions are found across NW New Guinea and further afield. The latter were likely part of an extensive active continental margin above a subduction system spanning the length of what is now New Guinea and potentially extending southward through eastern Australia and Antarctica.

Time scales and length scales in magma flow pathways and the origin of magmatic Ni–Cu–PGE ore deposits

Ore forming processes involve the redistribution of heat, mass and momentum by a wide range of processes operating at different time and length scales. The fastest process at any given length scale tends to be the dominant control. Applying this principle to the array of physical processes that operate within magma flow pathways leads to some key insights into the origins of magmatic Ni–Cu–PGE sulfide ore deposits. A high proportion of mineralised systems, including those in the super-giant Noril'sk-Talnakh camp, are formed in small conduit intrusions where assimilation of country rock has played a major role. Evidence of this process is reflected in the common association of sulfides with vari-textured contaminated host rocks containing xenoliths in varying stages of assimilation. Direct incorporation of S-bearing country rock xenoliths is likely to be the dominant mechanism for generating sulfide liquids in this setting. However, the processes of melting or dissolving these xenoliths is relatively slow compared with magma flow rates and, depending on xenolith lithology and the composition of the carrier magma, slow compared with settling and accumulation rates. Chemical equilibration between sulfide droplets and silicate magma is slower still, as is the process of dissolving sulfide liquid into initially undersaturated silicate magmas. Much of the transport and deposition of sulfide in the carrier magmas may occur while sulfide is still incorporated in the xenoliths, accounting for the common association of magmatic sulfide-matrix ore breccias and contaminated “taxitic” host rocks. Effective upgrading of so-formed sulfide liquids would require repetitive recycling by processes such as re-entrainment, back flow or gravity flow operating over the lifetime of the magma transport system as a whole. In contrast to mafic-hosted systems, komatiite-hosted ores only rarely show an association with externally-derived xenoliths, an observation which is partially due to the predominant formation of ores in lava flows rather than deep-seated intrusions, but also to the much shorter timescales of key component systems in hotter, less viscous magmas. Nonetheless, multiple cycles of deposition and entrainment are necessary to account for the metal contents of komatiite-hosted sulfides. More generally, the time and length scale approach introduced here may be of value in understanding other igneous processes as well as non-magmatic mineral systems.

Fractal Analysis of In Situ Host Rock Nepheline Syenite Xenoliths in a Micro-Shonkinite Dyke (The Elchuru Alkaline Complex, SE India)

Formation of the fragments of the wall-rock during dyking is one of the important manifestations of instantaneous magmatic events. This process is well documented at shallower depths of Earth’s crust but not at deeper levels. In this paper the in situ xenoliths of host rock nepheline syenite within a micro-shonkinite dyke emplaced at mid-crustal depths is described and the fractal theory applied to evaluate origin of the xenoliths. The nepheline syenite xenoliths are angular to oval shaped and sub-millimetre to ~50 cm long. The xenoliths are matrix supported with clasts and matrix being in equal proportions. Partly detached wall-rock fragments indicate incipient xenolith formation, which suggested that the model fragmentation processes is solely due to dyke emplacement. Fractal analytical techniques including clast size distribution, boundary roughness fractal dimension and clast circularity was carried out. The fractal data suggests that hydraulic (tensile) fracturing is the main process of host rock brecciation. However, the clast size and shape are further affected by postfragmentation processes including shear and thermal fracturing, and chemical erosion. The study demonstrates that dyking in an isotropic medium produces fractal size distributions of host rock xenoliths; however, post-fragmentation processes modify original fractal size distributions.

The age of monzonites of the eastern part of the Korsun-Novomirgorod pluton (Ingul megablock of the USh)

The article presents the results of monzonites research of the eastern part of Korsun-Novomirgorod pluton. The pluton is situated in the northwestern part of the Ingul megablock. Besides basic rocks varieties — gabbro-anorthosite and rapakivi granites the rocks of monzonite composition of different genesis are widely presented here. Small bodies of monzonites form outputs among rapakivi granites of Shpola massif at the junction of tectonic structures of different rank. Monzonite is a hybrid rock which contains xenoliths of frame rock and possibly ancient dyke formations. Zircon is represented by several generations, which are distinguishable by optical studies, as well as by different U / Th ratio. Some grains contain a nuclei and zircon inclusions, appearance of which was determined by the process of contamination. All fractions obtained discordant zircon age values. Dating calculated by isotope ratio 207Pb / 206Pb has been taken as the most reliable. Due to the presence of relict zircons received isotopic age of the figures are overstated and do not reflect the formation of monzonite. As the maximum possible monzonite age we can take a minimum age, calculated by 207Pb / 206Pb — 1.740 Ma. The assumption about the formation of monzonite melt as a result of contamination of basic magma by ancient crustal rocks of acid composition has been given, which does not contradict the results of the study of the isotopic composition of strontium, neodymium, and geochronological data.

Mineralogy, geochemistry and geochronology of mafic magmatic enclaves and their significance in evolution of Nongpoh granitoids, Meghalaya, NE India

Abstract The Cambrian Nongpoh granitoids, intrusive into the Precambrian gneissic complex and metasediments of the Shillong Group, represent a major phase of granitic magmatism in the Shillong Plateau. The Nongpoh granitoids comprise diorite, granodiorite, porphyritic and grey granites. Porphyritic granite is the dominant lithology exposed in the Nongpoh granitoids, and contains three types of enclaves, viz. xenoliths of gneissic rocks, dark grey porphyry and biotite-rich microgranular enclaves. The mafic magmatic enclaves (MMEs) of various dimensions and shapes, including rounded, ellipsoidal, rectangular, angular to subangular, and stretched bodies, were produced by evolving nature and contrasting kinematics of interacting felsic and mafic magmas. The biotite-rich enclaves are due to the injection of syn-magmatic mafic dykes in felsic magma. The various textural assemblages and sub-linear variations between silica and major oxides, chemical mixing and element diffusion suggest that multistage magma hybridization was the key process during the evolution of the Nongpoh granitoids. The 501 Ma age obtained by chemical (U–Th–Pb) dating of monazite in MME ascertained the age of the magma hybridization event in the Nongpoh granitoids, which is equivalent to the igneous activity at 500 Ma during the amalgamation of Eastern Gondwana.