Rocks Of Intermediate Composition Research Articles

Early Famenian Trachyandesite Magmatism of the Pripyat Paleorift, East European Platform: U–Pb Age and Petrology

On the basis of the mineralogical-petrographic and geochemical study of igneous rocks of intermediate composition of the Pripyat complex of the Late Devonian Pripyat-Dnieper magmatism area, two subphases of the Pripyat magmatism phase have been definished: late Fransian (Chernin or Skolodino-Chernin) with potassium specification and early Famennian (Turov-Drozdov (Yelets)) with potassium-sodium specification. For the first time, the U–Pb age of titanite from trachyandesites of the parametric borehole Pribor was obtained for rocks of Pripyat-Dnieper magmatism area at 364 ± 1 Ma. These data confirm the Famennian age of the Yelets subphase of the Pripyat phase of magmatism and make it possible to determine the duration of the magmatic activity of this structure at least 14 Ma taking into account the isotope dating of the earliest phases of magmatism in the Pripyat-Dnieper-Donetsk Paleorift Structure as 388 ± 12 Ma, 383.3 ± 3.8, 384.7 ± 3.9 and 383.6 ± 4.4 Ma in the Azov Region and 381 ± 2 Ma in the Pripyat-Dnieper magmatism area.

The Cretaceous of the Cameroon Atlantic Basin (Central Africa): sediment provenance, correlation, paleoenvironment and paleogeographic evolution of the Eastern Proto-Atlantic margin (Central Gondwana)

An integration of geochemistry, mineralogical and palynological data is used to depict the sediment provenance and distribution, the sedimentary environment and the paleogeographic evolution of the Cretaceous Atlantic Basin of Cameroon. The basin is located along the African western coast, in the heart of the Guinea Gulf, and includes the Campo, Douala, and Rio-del-Rey sub-basins from South to North. The sediments of the Campo sub-basin are derived from rocks of intermediate composition, those of the Douala sub-basin from intermediate to mafic rocks in the southern and felsic rocks in the northern parts, and those of the Rio-del-Rey sub-basin from felsic rocks with a contribution of rocks of intermediate composition. The paleoweathering index and Index of Chemical Variability values for Campo, Douala and Rio-del-Rey sub-basins range from 71.26 to 76.88 and 0.98 to 2.12, 67.15 to 99.39 and 0.37 to 0.90, and 73.17 to 92.90 and 0.44 to 10.03, respectively, indicating sub-maturity, high maturity and sub- to high maturity deposits. Al2O3 vs. V and Al2O3 vs. P2O5 plots, and palynologic data such as marine (e.g. dinoflagellates and microforaminifera) and continental (e.g. Classopollis sp., Ephedripites sp., Botryococcus sp.) species suggest a deposition in more or less deep lacustrine (Campo sub-basin), fluvio-deltaic to shallow marine (Douala sub-basin), and marginal to shallow marine (Rio-del-Rey sub-basin) environments. The paleoweathering index values, the clay minerals (kaolinite, illite and chlorite/vermiculite) distributions, and the palynological data point to arid to semi-arid climate in the Campo sub-basin and semi-arid to periodically humid conditions in the Douala and Rio-del-Rey sub-basins. The tectonic setting diagram suggests collision tectonic events that are inconsistent with the classic geological history of the Atlantic Basin. The situation is in fact related to Precambrian events that affected the basement rocks from which the studied sediments were derived, and probably to the compressional tectonics during the evolution of the South Atlantic Ocean. Although these sub-basins share the same geological history, there are some differences, such as the degree of sediment maturity, the diagenetic effects, the lithology of rock sources, the paleoenvironments of deposition and their fluctuation, depending on the moment the tectonic of Atlantic opening reached them (northward progression of the W-E extension) and the northern shearing along the southern area of the Benue through axis (Rio-del-Rey). The geological history of the Cameroon Atlantic Basin is similar to those of Gabonese and Nigerian basins, along the African western coast, and of those of the Brazilian basins along the eastern coast (Santos Basin to Pernambuco-Paraiba Basin).

РОЛЬ ДРЕВНЕГО КАРСТА В ФОРМИРОВАНИИ МЕСТОРОЖДЕНИЯ УРАНА (ГОРНАЯ ШОРИЯ)

Marmorized limestones of the Upper Proterozoic are widely viewed as host rocks of the Labysh deposit. The formation of uranium ores in carbonate rocks does not correspond to existing сoncepts of the uranium ore process geochemistry. A detailed study of lode ores of the deposit showed that karst cavities were originally filled with limestone fragments of different sizes and finely-dispersed kaolinite. Further, an input of thermal alkaline solutions led to the leaching of earth silicon of metamorphogenic limestone silicates and formation of “silicinate–tar” veins. In fact, the rocks hosting mineralization are morphologically veined sedimentary wellpermeable rocks with geochemical properties approximating to hydrothermally altered rocks of intermediate composition.

Archean to Paleoproterozoic evolution of the Crixás greenstone belt, Central Brazil: Insights from two contrasting assemblies of metaigneous rocks

The Archean-Paleoproterozoic Crixás greenstone belt is one of the most important components of the Crixás-Goiás Domain, the oldest crustal segment of the Central Brazil Goiás Massif. This gold-bearing greenstone comprises amphibolite facies ultramafic-mafic metavolcanic rocks overlaid by a low-grade metasedimentary succession associated with rocks of intermediate composition. We present new integrated geochemical and isotopic data of the metaigneous rocks to characterize the stages of magmatism and delineate the petrogenetic processes and tectonic environment of the different units. The data give important insights into the geological evolution of the Crixás greenstone belt during the Archean and Paleoproterozoic, including that the Crixás greenstone belt consists of genetically and temporal unrelated rocks that are spatially associated in a structural framework marked by stacking and imbrication. Two lithological assemblies were described: i) a Mesoarchean komatiite-tholeiite association formed by heterogeneous mantle plume melting in an oceanic plateau setting, and ii) a Rhyacian intraoceanic arc association. These units are allochthonous and override an Archean crustal basement dominated by granites and gneisses of mostly TTG composition. Chemical composition of the komatiites is similar to the Barberton-type Al-depleted komatiites, whilst the basalts consist of high-Mg tholeiites with flat chondrite-normalized REE patterns and immobile trace element systematics that resemble Phanerozoic oceanic plateau lavas. Eruption age of the ultramafic and mafic lavas is roughly estimated at ~3.0 Ga, based on previously published SmNd and PbPb isotopic studies, whereas whole-rock ReOs isotopic data of samples of komatiite and basalt presented here yielded a 2609 ± 65 Ma isochron age. This younger age may reflect a Neoarchean overprinting event associated with metasomatic Re-addition recorded in the lower ultramafic-mafic metavolcanic rocks, possibly related to the late stages of the cratonization of the Crixás-Goiás Domain during the emplacement of the youngest Archean granites of the region (~2.70 Ga). Newly described andesites and diorites possess a subduction zone fingerprint. LA-ICP-MS UPb zircon dating of an andesite sample provided an upper intercept Concordia age of 2172.2 ± 12.7 Ma, constraining the age of the intermediate magmatism. Therefore, these rocks are correlated with the Rhyacian syn-orogenic poorly-sorted graywackes and carbonaceous phyllites of the greenstone's upper stratigraphy. Andesites and diorites have homogeneous Paleoproterozoic two-stage Nd model ages of 2.28 to 2.26 Ga and positive initial εNd values of +2.34 to +2.69, indicating a juvenile nature for the arc magmatism. ReOs geochronology of gold-related arsenopyrite from a metasedimentary rock-hosted massive sulfide provided an age of 2137 ± 11 Ma, suggesting that the mineralization is also coeval to the Rhyacian collisional stage.

Geochemical systematics and U–Pb zircon age of the Vulvara anorthosite massif, Lapland granulite belt, Baltic shield: magmatic sources and metamorphic alteration of the rocks

ABSTRACT The Vulvara anorthosites occur among the rocks of intermediate composition, in contrast to most Lapland granulite belt (LGB) anorthosites, associated with mafic granulites. The U-Pb, Sm-Nd and REE geochemical data were used to compare Vulvara with other LGB anorthosites. The intrusion of the Vulvara massif occurred in the interval of 2100–1965 Ma. The first metamorphic processing took place 1947 ± 11 Ma ago under granulite facies. Postgranulite transformations occurred 1900–1850 Ma ago under amphibolite facies; earlier (1896 ± 15 Ma) and later (1852 ± 17 Ma, 1846 ± 16 Ma) thermal events were established. The Vulvara anorthosites, in comparison with other LGB anorthosites, are distinguished by a more acidic composition of plagioclase, a higher F number of rocks and mafic minerals, a higher content of SiO2, Na2O, K2O, Ba, Sr, Zr, Nb, Rb, and a lower content of CaO. The Sm-Nd and REE geochemistry suggest that there are three groups of LGB anorthosites, distinguished both by age and magma source.

Geochemical and mineralogical evidence of an offset in the Andean arc recorded in the Upper Cretaceous marine deposits of the Austral-Magallanes basin, Argentina

The Austral-Magallanes Basin covers the southernmost region of Patagonia, South America. South of Lago Argentino, the Calafate Formation documents part of the foreland stage of the evolution of this basin. Here, sandstones from the Calafate Formation were characterized using petrographical, X-ray diffraction (XRD) and geochemical data in order to establish their tectonic setting and sediment source areas. Based on our results, three intervals (lower, middle, and upper) were distinguished according to their composition. The lower and upper intervals are composed of felsic volcanic lithics, abundant quartz and orthoclase. A shift in the composition was observed in the middle interval, with an increase in the modal abundance of plagioclase as well as other accessory minerals such as pyroxene. In addition, XRD data reveal that the middle interval is dominated by smectite in the clay-fraction, whereas the lower and upper intervals present mixed-layer chlorite-smectite and chlorite assemblages. Similar differences are observed using major and trace elements: data from the lower and upper intervals point to source rocks of intermediate to acid composition, while the middle interval had source rocks of intermediate composition and a lower degree of weathering. With regard to the provenance studies, the three intervals have a strong affinity to intermediate compositions of the Tobífera Formation (El Quemado Complex) and with the diorite/andesite of the Patagonian Batholith, which implies an exclusive contribution from the western edge of the basin. However, the shift in the composition towards the middle interval indicates a sediment supply from the juvenile Andean arc, suggesting contemporaneous magmatic activity during the deposition of the Calafate Formation sandstones. This peak of magmatic activity may have been caused by an eastward migration of the arc, driven by a switch in the tectonic setting south of 48°S.

Petrogenesis of the Early-Triassic quartz-monzodiorite dykes from Central Jebilet (Moroccan Meseta): Trace element and Nd-Sr isotope constraints on magma sources, and inferences on their geodynamic context

A dyke swarm composed of hypabyssal rocks of intermediate composition cross-cuts Variscan granites and Palaeozoic metasedimentary country-rocks of the Jebilet range. Based on K-Ar isotope dating on kaersutite phenocrysts, these dykes are Early Triassic in age (ca. 240 ± 10 Ma).Due to more or less pervasive alteration, the primary mineralogical assemblages are often difficult to unravel. Plagioclase and amphibole are ubiquitous, but the extent of biotite, for instance, is subject to discussion. Two distinctive types of rocks are clearly present according to their mineralogical composition: most samples are characterized by a light-brown to greenish hornblende, while some samples are characterized by the presence of kaersutite, suggestive of an alkaline affinity.Incompatible trace element data, restricted to those elements which are considered to be relatively immobile during alteration processes, allow two groups of samples to be distinguished. The largest one displays distinctive negative Ti and Nb anomalies similar to those typical for magmas extracted from mantle sources, which interacted with hydrous fluids and silicate melts, and were thus enriched in Th and LREE relative to the HFSE. However, Nd and Sr data preclude late stage crustal contamination as a suitable explanation of these features in all but the most evolved samples. The LREE enrichment combined with relatively radiogenic Nd isotope signatures are interpreted to reflect the characteristics of source materials, which evolved in a rather primitive, ensimatic setting.The second group comprises only three samples but is of major interest as it does not show any clear Nb negative anomaly and its Th/Nb systematics suggests that crustal components were not involved in their genesis. This group is interpreted as having been produced through partial melting of mantle sources which did not suffer hydrous metasomatism nor recycling of old crustal components, but were enriched in a within-plate setting by low-degree mantle melts.Based on these data, the following tentative geological scenario is proposed. Lithospheric stretching and decompression would have triggered partial melting at relatively deep level of the lithosphere, and produced magmas with within-plate, alkaline-affinities. The ascent of these hot mantle melts would have triggered partial melting of fertile, hydrated domains of Late Proterozoic age present in the overlying mantle and/or lower crust, thereby generating the volumetrically predominant group of samples with « calc-alkaline » geochemical features, namely Nb and Ti negative anomalies. The two types of magmas likely interacted to produce hybrid liquids with variably deep Nb anomalies but broadly constant εNd values, as a result of the weak difference of their isotope signatures.These magmas are much younger (ca. 240 ± 10 Ma) than the last Variscan granites dated at ca. 300 Ma. On the other hand, they largely predate the tholeiitic continental flood basalts of the Central Atlantic Magmatic Province (CAMP) emplaced at ca. 200 Ma, and were thus emplaced amidst the 100 Ma time span between the end of the Variscan orogeny and the lithospheric break-up that led to the formation of the Atlantic Ocean. Therefore, the Jebilet dykes neither represent the ultimate manifestation of the Variscan orogeny nor a premise of the Atlantic opening. Instead, they document a particular event within a protracted intra-continental rifting history spanning the whole Permo-Triassic period. In general terms, this distensive event might be viewed as a local consequence of the dextral mega-shear which separated Europe from North Africa in Late Palaeozoic times. In this scope, the studied dykes could be described as a set of tension gashes, resulting from a local change in stress regime. Their formation was accompanied by a small pulse of intraplate magmatism, possibly because favorable fertile materials occurred locally at depth. At a larger scale, the whole dyke swarm cross-cutting the Variscan basement in Morocco is indicative of an aborted rifting event, largely predating the successful break-up of Pangea and the early opening of the Central Atlantic Ocean.

Petrogenesis of the Darvazeh mafic-intermediate intrusive bodies, Qorveh, Sanandaj-Sirjanzone, Iran

The Darvazeh intrusion in the northern Sanandaj-Sirjan zone in the Alpine-Himalayan orogenic belt is composed of olivine gabbro, gabbroic diorite and granitoid units that intruded Jurassic metamorphic rocks. Field relations, textures, and whole rock geochemistry indicate that most of the mafic to intermediate composition rocks are not cumulates and instead represent liquid compositions. The tholeiitic (or ferroan) gabbros and magnesian gabbroic diorites evolved along liquid lines of descent and have different parental magmas. This conclusion is based on separate trends, often with overlapping MgO contents, for Si, Fe, Ti, Ni, Cr, Zn, Cu and Sr. The mineralogy (late versus early magnetite and contrasting proportions of magmatic amphibole) of the two suites are also slightly different as is the trace element patterns (for example Th, U, Ti and Sr). The Mg# of mafic-intermediate rocks ranges from 46 to 73, implying that the ultimate source for the most mafic rocks was in the mantle, not the crust. Flat HREE patterns, Sm/Yb ratios (<2) show that melting occurred in the shallow mantle, above the stability field of garnet. Small negative Nb-Ta-Ti anomalies and enrichments of Cs, Rb, Pb, Pb, and Sr on mantle-normalized trace element diagrams are consistent with a subduction zone source for both the tholeiitic gabbros and the magnesian gabbroic diorites. Hence, we conclude that the parental mafic magmas were generated by partial melting of spinel peridotite in the mantle wedge above a subducting slab of oceanic lithosphere. However, the composition of plagioclase-olivine pairs suggest the gabbroic magmas were not water-rich, and their high Fe/Mg and Ti/V ratios show they were relatively reduced compared to typical arc magmas. For the gabbros, melting may have been initiated by decompression in an intra-arc extensional environment caused by slab roll back or oblique subduction that partially separated the Sanandaj-Sirjan zone from the Central Iran block. Components from the subduction fluid are smaller than in the subsequent gabbroic diorites which have more typical arc compositions. These mafic magmas rose and fractionated on different paths controlled by water content and fO2, with the gabbros following reduced Fe-enrichment trends and the magnesian gabbroic diorites following more oxidized trends. The country rocks, plate reconstructions, and trace element systematics (including high Th/La ratios) show that the arc was underlain by continental crust that was assimilated into the mafic and intermediate magmas. Thus, the Darvazeh pluton reveals the complex nature of mafic intrusive rocks, which formed in a continental arc experiencing extension as the Neo-Tethys oceanic lithosphere subducted beneath the Central Iranian plate.

Geochemistry, Nd-Pb Isotopes, and Pb-Pb Ages of the Mesoproterozoic Pea Ridge Iron Oxide-Apatite–Rare Earth Element Deposit, Southeast Missouri

Iron oxide-apatite and iron oxide-copper-gold deposits occur within ~1.48 to 1.47 Ga volcanic rocks of the St. Francois Mountains terrane near a regional boundary separating crustal blocks having contrasting depleted-mantle Sm-Nd model ages (TDM). Major and trace element analyses and Nd and Pb isotope data were obtained to characterize the Pea Ridge deposit, improve identification of exploration targets, and better understand the regional distribution of mineralization with respect to crustal blocks. The Pea Ridge deposit is spatially associated with felsic volcanic rocks and plutons. Mafic to intermediate-composition rocks are volumetrically minor. Data for major element variations are commonly scattered and strongly suggest element mobility. Ratios of relatively immobile elements indicate that the felsic rocks are evolved subalkaline dacite and rhyolite; the mafic rocks are basalt to basaltic andesite. Granites and rhyolites display geochemical features typical of rocks produced by subduction. Rare earth element (REE) variations for the rhyolites are diagnostic of rocks affected by hydrothermal alteration and associated REE mineralization. The magnetite-rich rocks and REE-rich breccias show similar REE and mantle-normalized trace element patterns. Nd isotope compositions (age corrected) show that: (1) host rhyolites have ɛ Nd from 3.44 to 4.25 and TDM from 1.51 to 1.59 Ga; (2) magnetite ore and specular hematite rocks display ɛ Nd from 3.04 to 4.21 and TDM from 1.6 to 1.51 Ga, and ɛ Nd from 2.23 to 2.81, respectively; (3) REE-rich breccias have ɛ Nd from 3.04 to 4.11 and TDM from 1.6 to 1.51 Ga; and (4) mafic to intermediate-composition rocks range in ɛ Nd from 2.35 to 3.66 and in TDM from 1.66 to 1.56. The ɛ Nd values of the magnetite and specular hematite samples show that the REE mineralization is magmatic; no evidence exists for major overprinting by younger, crustal meteoric fluids, or by externally derived Nd. Host rocks, breccias, and magnetite ore shared a common origin from a similar source. Lead isotope ratios are diverse: (1) host rhyolite has 206Pb/204Pb from 24.261 to 50.091; (2) Pea Ridge and regional galenas have 206Pb/204Pb from 16.030 to 33.548; (3) REE-rich breccia, magnetite ore, and specular hematite rock are more radiogenic than galena; (4) REE-rich breccias have high 206Pb/204Pb (38.122–1277.61) compared to host rhyolites; and (5) REE-rich breccias are more radiogenic than magnetite ore and specular-hematite rock, having 206Pb/204Pb up to 230.65. Radiogenic 207Pb/206Pb age estimates suggest the following: (1) rhyolitic host rocks have ages of ~1.50 Ga, (2) magnetite ore is ~1.44 Ga, and (3) REE-rich breccias are ~1.48 Ga. These estimates are broadly consistent and genetically link the host rhyolite, REE-rich breccia, and magnetite ore as being contemporaneous. Alteration style and mineralogical or textural distinctions among the magnetite-rich rocks and REE-rich breccias do not correlate with different isotopic sources. In our model, magmatic fluids leached metals from the coeval felsic rocks (rhyolites), which provided the metal source reflected in the compositions of the REE-rich breccias and mineralized rocks. This model allows for the likelihood of contributions from other genetically related felsic and intermediate to more mafic rocks stored deeper in the crust. The deposit thus records an origin as a magmatic-hydrothermal system that was not affected by Nd and Pb remobilization processes, particularly if these processes also triggered mixing with externally sourced metal-bearing fluids. The Pea Ridge deposit was part of a single, widespread, homogeneous mixing system that produced a uniform isotopic composition, thus representing an excellent example of an igneous-dominated system that generated coeval magmatism and REE mineralization. Geochemical features suggest that components in the Pea Ridge deposit originated from sources in an orogenic margin. Basaltic magmatism produced by mantle decompression melting provided heat for extracting melts from the middle or lower crust. Continual addition of mafic magmas to the base of the subcontinental lithosphere, in a back-arc setting, remelted calc-alkaline rocks enriched in metals that were stored in the crust. The St. Francois Mountains terrane is adjacent to the regional TDM line (defined at a value of 1.55 Ga) that separates ~1600 Ma basement to the west, from younger basements to the east. Data for Pea Ridge straddle the TDM values proposed for the line. The Sm-Nd isotope system has been closed since formation of the deposit and the original igneous signatures have not been affected by cycles of alteration or superimposed mineralizing events. No evidence exists for externally derived Nd or Sm. The source region for metals within the Pea Ridge deposit had a moderate compositional variation and the REE-rich breccias and mineralized rocks are generally isotopically homogeneous. The Pea Ridge deposit thus constitutes a distinctive isotopic target for use as a model in identifying other mineralized systems that may share the same metal source in the St. Francois Mountains terrane and elsewhere in the eastern Granite-Rhyolite province.

Determination of divalent iron content in igneous rocks of ultrabasic, basic and intermediate compositions by a wavelength-dispersive X-ray fluorescence spectrometric method

This paper proposes the quantitative wavelength-dispersive X-ray fluorescence (XRF) technique of divalent iron content determination in igneous rocks and reports specifics of this technique in terms of classifying rocks into ultrabasic, basic, intermediate and acid ones. Unlike the widely used technique of titration, the proposed XRF technique does not require a labor-consuming procedure for sample preparation. In the fields of FeKβ5 line, and FeKβ′ and FeKβ″ satellites relative intensities of XRF emission spectrum lines are dependent on the valence state of iron in minerals. The ratio FeKβ5/FeKβ1,3 is preferred as the analytical parameter. The XRF method allows divalent iron content (expressed as FeO oxide) over 1.5% to be measured in igneous rocks of ultrabasic and basic compositions by the common calibration equation with the error below 10rel. %, that is comparable with the error of titrimetric analysis. The samples of igneous rocks of intermediate composition and granodiorites should be analyzed using calibration equations obtained with certified reference materials corresponding in composition to the study samples. For the samples of alkaline rocks the errors of FeO content determination might be in excess of 50rel. %.

The north-western margin of the Neuquén Basin in the headwater region of the Maipo drainage, Chile

AbstractVolcanic and pyroclastic rocks of intermediate composition, dating from Kimmeridgian time with peak ages during both the early–middle Tithonian and the late Hauterivian, characterize several localities of the north-western margin of the Neuquén basin along the main Andean Range at 33–34°S. The latter is thought responsible for the desiccation of the basin during Aptian–Albian time. The thick Tithonian submarine lavas at the Volcán valley (33°30′S), on the other hand, coincide spatially with the only pass-way recognized at that time in the Chilean Coast Range. This coincidence may be related to an isolated relic rift-like structure that coexisted with the regional intra-arc extensional setting during Tithonian time. Continental-scale rifting in the southern Andes has been recognized only for the Late Triassic–Early Jurassic period.

THE DIVERSITY AND NEW TYPES OF ROCKS IN PENYZEVICHY CAREER (KOROSTEN PLUTON)

New types of ore-bearing (ilmenite, apatite) olivine gabbro and rocks of intermediate composition between the latter and ore ilmenite norites and alkaline syenite with rybekite are found in Penyzevychy career. The feature of Penyzevychy gabbroids is a low iron oxidation (absence of primary magnetite) that indicatins on their formation at a deeper level than the most deposits and occurrences of apatite-ilmenite-titanomagnetite in the northern, central and north-western part of Korosten pluton. Obviously, this is due to the similarity of Penyzevychy occurrence and Nosachiv deposit, located in the significantly eroded block of the Korsun-Novomyrgorod pluton.

Composition, sources, and genesis of granitoids in the Irtysh Complex, Eastern Kazakhstan

Intrusions of the Irtysh Complex are spatially restricted to the regional Irtysh Shear Zone (ISZ) and are hosted in blocks of high-grade metamorphic rocks (Kurchum, Predgornenskii, Sogra, and others) in the greenschist matrix of the ISZ. The massifs consist of contrasting rock series from gabbro to plagiogranite and granite at strongly subordinate amounts of diorite and the practical absence of rocks of intermediate composition (tonalite and granodiorite). The complex was produced in the Early Carboniferous, simultaneously with the onset of the origin of the ISZ itself. The granitoids composing the complex affiliate with diverse petrochemical series (from subaluminous plagiogranite of the andesite series to granite of the calc-alkaline series) and contain similar REE and HFSE concentrations [total REE = 103–163 ppm (La/Yb) n = 3.59–5.44, Zr (200–273 ppm), Nb (7.6–10.6 ppm), Hf (6.1–7.6 ppm), and Ta (0.68–1.19 ppm)] but are different in concentrations in LILE [Rb (3–9 and 121–221 ppm), Sr (213–375 and 77–148 ppm), and Ba (67–140 and 240–369 ppm)] and isotopic composition of Nd (ɛNd(T) from +5.3 in the plagiogranite to −1.2 in the granite) and O (δ18O from +9.4 in the plagiogranite to +14.5 in the granite). Data on the geochemistry and isotopic composition of metamorphic rocks of the Kurchum block and numerical geochemical simulations indicate that the granitoids were generated via the melting of a heterogeneous crustal source, which consisted of upper crustal metapelites and metabasites of the oceanic basement of the blocks of high-grade metamorphic rocks. The differences in the chemical and isotopic compositions of the granitoids were predetermined by the mixing of variable proportions of granitoid magmas derived from metapelite and metabasite sources.

The enigmatic sources of I-type granites: The peritectic connexion

There is little dispute about the sources of peraluminous, crustally evolved, S-type, granitic magmas. These are derived through partial melting of metasedimentary rocks that had a significant fraction of Al-rich clays in their protoliths. However, the origins of I-type magmas are, and always have been, in dispute. From isotope geochemistry, we know that I-types are not generally produced through fractionation of normal, juvenile, mantle-derived, mafic magmas. In addition, we can demonstrate that the chemical diversity among most I-type series is not primarily due to magma mixing. Thus, we start from the premise that most I-type magmas are dominantly crustal in origin, as reflected in their O isotope ratios. Experimental work on a range of potential hornblende- and/or biotite-bearing source rocks, as well as studies of felsic I-type rocks, indicate that the parent felsic melts for I-type magmas are mildly peraluminous. However, the rocks themselves are commonly metaluminous, especially at the more mafic end of the compositional spectrum. Chemically and mineralogically, the best explanation for this is that peraluminous melt left the I-type source terranes with entrained peritectic clinopyroxene. In detail, the chemistry of most I-type series is controlled by differential entrainment of this pyroxene, together with peritectic plagioclase, ilmenite/titanomagnetite and restitic apatite and zircon. So, what sorts of sources partially melt to produce the peraluminous I-type melts with peritectic clinopyroxene and ilmenite, while imparting a distinctly crustal isotope signature to the magmas? From experimental and theoretical perspectives (and with the exception of the uniquely Archean tonalite–trondhjemite–granodiorite [TTG] series), the best candidates for I-type protoliths are not mafic igneous rocks but arc volcanic rocks of intermediate composition (dacites to andesites), and possibly some relatively mafic granodiorites and tonalites, rich in biotite and hornblende. Thus, the S–I dichotomy in granite typology is unlikely to reflect simple sedimentary versus igneous sources, but rather the nature of the peritectic minerals entrained by the ascending granitic melts. There should be granitic rocks transitional between S- and I-type, depending on the balance between clay-rich and clay-poor rocks in the protolith. The fact that equivocal or transitional types appear to be uncommon is telling us that the packages of rocks that give rise to S- and I-type magmas are generally spatially separated from each other, and may also be separate in terms of their ages and tectonic environments. Additionally, melting of biotite + sillimanite assemblages in evolved metapelitic sources will occur significantly earlier during a crustal heating cycle than will that of the hornblende + biotite assemblages that will predominate in I-type sources. Thus, even where interlayered sources do exist, S- and I-type magmas may be produced as temporally separate batches.

Comendite-bearing subduction-related volcanic associations in the Khan-Bogd area, southern Mongolia: Geochemical data

The vertical section of volcanic rocks in the Khan-Bogd Late Paleozoic depression, southern Mongolia, in the belt of southern Mongolian Hercynides contains comendites. The basement of the depression is made up of Devonian ophiolites (older than 362 Ma) overlain by volcanic associations of an active continental margin (ACM) (dated at 330 Ma) and a bimodal association (dated at approximately 290 Ma), which is subdivided into a lower unit (BLU), dacites of the intermediate layer (IL), and a bimodal association of the upper unit (BUU). The volcanic associations of the Devonian and ACM are calc-alkaline and poor in TiO2. The BLU rocks have higher alkalinity and TiO2 concentrations and show a transition from the tholeiitic to calc-alkaline series in the course of differentiation with the origin of comendites and trachyrhyolites, including those with adakite characteristics. The IL dacites are analogues of calc-alkaline magmas of the ACM type. The BUU volcanic association is composed of tholeiite basalts with moderate Ti concentrations (of the MORB type), comendites, and trachyrhyolites, with a compositional gap at rocks of intermediate composition. The variations in the canonical ratios of incompatible trace elements and petrochemical parameters of the Khan-Bogd volcanic rocks show that their parental magmas were derived mostly from a source of basalts of the arc type (IAB) with the addition of variable proportions of a source of the MORB type. The greatest role of the latter is identified in the magmas of the bimodal association. BLU and BUU are separated by IL, a fact testifying that the bimodal volcanism occurred simultaneously with normal dacite continental-marginal volcanism. Although the geodynamic environments in which volcanic rocks were formed somewhat varied during the development of the Khan-Bogd depression, a subduction environment remained predominant, and the volcanic rocks were derived from an IAB-type source. The subduction volcanic associations produced thereby are differentiated and vary in composition from basites to dacite and rhyolite, which could be formed at the assimilation of continental crustal material (CC). Conceivably, the bimodal volcanic association was generated when the subduction zone was approached by a mid-oceanic ridge, whose material could be added in appreciable amounts to the subduction sources. The volcanic evolution of the Khan-Bogd depression shows an evolution of geodynamic environments and the composition of the volcanic rocks generally resembling those in the western margin of North America in the Cenozoic. The acid BLU and BUU rocks were most probably generated by different mechanisms. The BLU comendites and trachyrhyolites were likely formed by the crystallization differentiation of an arc basite magma of elevated alkalinity. The acid BUU rocks resulted from the anatexis of basites of this association, particularly spilitized ones (as well as any other basites) and the subsequent crystallization differentiation of the anatectic magmas.

Crustal structure beneath the Montserrat region of the Lesser Antilles island arc

We use receiver function analysis to estimate the first‐order subsurface seismic structure in the vicinity of the island of Montserrat, located in the northern Lesser Antilles arc and home of the active Soufrière Hills Volcano. Near‐surface complexity in the island structure inhibits our ability to resolve lateral variations in seismic structure, so we average the signals from eight stations in two directions to produce a first‐order regional seismic velocity model. Although we are unable to resolve a precise sharpness of the crust‐mantle transition, we can limit its thickness to less than ∼4 km. Lateral variations in shallow structure complicate a receiver function based estimate of Poisson's ratio, but the data indicate that in both directions sampled by the receiver functions, the lower crustal Poisson's ratio is greater than 0.27 and more likely in the range 0.29–0.30. We estimate a mean crustal thickness of ∼30 km, and the observations suggest that the crust may be slightly thinner northwest of the island (∼26–30 km) than it is to the south (∼30–34 km). The high P wave speed and Poisson's ratio indicate a generally mafic lower crust, with rocks of intermediate composition not precluded in the upper part of the lower crust. We did not find evidence for a thick high‐speed lower crust (&gt;7.4 km/s) as has been inferred in some other arcs.

Andalusite and Na- and Li-rich cordierite in the La Costa pluton, Sierras Pampeanas, Argentina: textural and chemical evidence for a magmatic origin

The La Costa pluton in the Sierra de Velasco (NW Argentina) consists of S-type granitoids that can be grouped into three igneous facies: the alkali-rich Santa Cruz facies (SCF, SiO2 ~67 wt%) distinguished by the presence of andalusite and Na- and Li-rich cordierite (Na2O = 1.55–1.77 wt% and Li2O = 0.14–0.66 wt%), the Anillaco facies (SiO2 ~74 wt%) with a significant proportion of Mn-rich garnet, and the Anjullón facies (SiO2 ~75 wt%) with abundant albitic plagioclase. The petrography, mineral chemistry and whole-rock geochemistry of the SCF are compatible with magmatic crystallization of Na- and Li-rich cordierite, andalusite and muscovite from the peraluminous magma under moderate P–T conditions (~1.9 kbar and ca. 735°C). The high Li content of cordierite in the SCF is unusual for granitic rocks of intermediate composition.

Age and petrogenesis of volcanic and intrusive rocks in the Sulphur Spring Range, central Nevada: Comparisons with ore-associated Eocene magma systems in the Great Basin

Widespread base- and precious-metal anomalies, oxidized sulfi de veins, silicifi ed calcareous shales and carbonates, and altered porphyry intrusions occur in the northeastern Sulphur Spring Range, Nevada, 80 km south of important gold deposits in the Carlin trend. The small historic mines and prospects in the area are spatially and perhaps genetically related to a suite of variably altered dikes, small lava fl ows, silicic domes, and related pyroclastic rocks. New major- and trace-element data and U-Pb zircon ages show that the East Sulphur Spring volcanic suite is Eo-Oligocene in age (36‐31 Ma) and ranges in composition from high MgO- basaltic andesite to peraluminous rhyolite. The major- and trace-element compositions of the volcanic rocks are characteristic of continental margin subduction zone magmas and form a high-K, calc-alkaline suite with low Fe/Mg ratios. In addition, the rocks have negative Nb and Ti anomalies and elevated Ba, K, and Pb on normalized trace-element diagrams. Crustal melting is indicated by the eruption of a peraluminous garnet-bearing ignimbrite and as a component in hybridized andesite. The nature of this suite and its potential for mineralization is elucidated via comparisons to other Eocene age volcanic rocks associated with much larger gold and copper deposits in the Great Basin. The East Sulphur Spring suite is more similar to Eocene igneous rocks found along and near the Carlin trend than it is to those erupted while the Bingham porphyry copper deposit developed 300 km farther to the east. For example, the East Sulphur Spring suite and the Eocene magmatic rocks along the Carlin trend are less alkaline than the Bingham suite and lack its unusual enrichment of Cr, Ni, and Ba in intermediate composition rocks (58‐68 wt% silica). Nonetheless, the Bingham and East Sulphur Spring volcanic suites both preserve evidence of mixing that created intermediate compositions. For example, an andesite has obvious mineral disequilibrium with plagioclase, biotite, clinopyroxene, orthopyroxene, olivine, and amphibole coexisting with extensively resorbed megacrysts of quartz, K-feldspar, and garnet—indicative of mixing basaltic andesite or andesite and largely crystallized garnet-bearing rhyolite. On the other hand, we found no evidence for mixing with a mafi c alkaline magma like that in the Bingham Canyon magma-ore system. We conclude that: (1) an unusual tectonic setting prevailed during the Eocene and Oligocene of the western United States that promoted the production of oxidized mafi c magma in an arclike setting, but far inland as a result of the rollback of the Farallon slab; (2) the mafi c magmas intruded or erupted separately, or mixed with more silicic magma generated by fractional crystallization and assimilation of crustal materials; and (3) these mafi c magmas may have delivered signifi cant amounts of sulfur and chalcophile metals to upper crustal magma chambers and eventually to Paleogene ore deposits in the eastern Great Basin.

Evidence of Magma Mixing in the ‘Daly Gap’ of Alkaline Suites: a Case Study from the Enclaves of Pantelleria (Italy)

The island of Pantelleria consists of trachytes, pantellerites and minor mildly alkaline basalts. Rocks of intermediate composition (falling in the so-called ‘Daly Gap’) such as mugearites, benmoreites and mafic trachytes occur only in the form of enclaves in trachytes and pantellerites inside the main caldera of the island (Caldera ‘Cinque Denti’), which collapsed during the ‘Green Tuff’ ignimbrite eruption at ∼50 ka. The enclaves include volcanic, subvolcanic and intrusive rock types. The enclaves in host trachyte contain traces of glass; devitrified glass occurs within enclaves in host pantellerites. Minerals in the enclaves show regular compositional variations with whole-rock silica content. Glass present in the medium-grained samples is interpreted to be the result of incipient melting. The major and trace element compositions of the enclaves show regular and linear variations between an evolved mafic magma (hawaiite) and a felsic end-member similar to the ‘Green Tuff’ trachyte. Fractional crystallization modelling of compatible and incompatible trace elements (V, Ni, Zr, La, Sm, Lu, Nb, Y, Th) does not reproduce the observed trends. Rocks of intermediate composition within the ‘Daly Gap’ can be explained only by magma mixing between an already differentiated mafic magma (hawaiite) and an anorthoclase-rich trachytic melt in the lower and higher parts, respectively, of a stratified magmatic chamber. Medium-grained enclaves are interpreted as the result of fragmentation of solidified mixing layers in the roof of the magma chamber during the eruption of the ‘Green Tuff’, when the collapse of the caldera took place. Diffusion calculations suggest a residence time of <5 days for the enclaves in their host magmas.

Lithologic mapping of the Mordor, NT, Australia ultramafic complex by using the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER)

Spectral measurements made in the Mordor Pound, NT, Australia study area using the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), in the laboratory and in situ show dominantly Al-OH and ferric-iron VNIR–SWIR absorption features in felsic rock spectra and ferrous-iron and Fe,Mg-OH features in the mafic–ultramafic rock spectra. ASTER ratio images, matched-filter, and spectral-angle mapper processing (SAM) were evaluated for mapping the lithologies. Matched-filter processing in which VNIR + SWIR image spectra were used for reference resulted in 4 felsic classes and 4 mafic–ultramafic classes based on Al-OH or Fe,Mg-OH absorption features and, in some, subtle reflectance differences related to differential weathering and vegetation. These results were similar to those obtained by match-filter analysis of HyMap data from a previous study, but the units were more clearly demarcated in the HyMap image. ASTER TIR spectral emittance data and laboratory emissivity measurements document a wide wavelength range of Si-O spectral features, which reflect the lithological diversity of the Mordor ultramafic complex and adjacent rocks. SAM processing of the spectral emittance data distinguished 2 classes representing the mafic–ultramafic rocks and 4 classes comprising the quartzose to intermediate composition rocks. Utilization of the complementary attributes of the spectral reflectance and spectral emittance data resulted in discrimination of 4 mafic–ultramafic categories; 3 categories of alluvial–colluvial deposits; and a significantly more completely mapped quartzite unit than could be accomplished by using either data set alone.