Rocks Of Granitic Composition Research Articles

Role of seafloor production versus continental basalt weathering in Middle to Late Ordovician seawater 87Sr/86Sr and climate

The global climate of the Ordovician Period (486.9 to 443.1 Ma) is characterized by cooling that culminated in the Hirnantian glaciation. Chemical weathering of Ca- and Mg-bearing silicate minerals and the subsequent trapping of carbon in marine carbonates act as a sink for atmospheric CO2 on multi-million-year time scales, with basaltic rocks consuming CO2 at a greater rate than rocks of granitic composition. The oceanic Sr isotope ratio (87Sr/86Sr) can act as a geochemical proxy for the relative proportion of basaltic versus granitic weathering. Oxygen isotopes (δ18O) act as a proxy for paleotemperature and ice volume, providing a useful complement to 87Sr/86Sr in studies of ancient climate. Previous studies have reported stepwise cooling (increasing δ18O) during the Middle to Late Ordovician. Combined with Sr and C cycle models, this has led to the hypothesis that continental silicate weathering of mafic material drove Ordovician cooling (e.g., the Taconic Orogeny). However, Sr and C cycle models have not accounted for an apparent rise in sea level and seafloor production in the Middle Ordovician (Darriwilian), which would increase the hydrothermal Sr flux as well as degassing along continental volcanic arcs. Furthermore, some Ordovician studies contain temporal uncertainty between 87Sr/86Sr and δ18O curves if they are not based on paired analyses, which can obscure the relationship between silicate weathering and cooling. Here, we present new paired 87Sr/86Sr and δ18O data from conodont apatite and integrate this with both a deterministic (forward) and stochastic (reverse) modeling approach to argue that increased hydrothermal weathering played a role in driving marine 87Sr/86Sr, specifically an inflection occurring in the Pygoda serra conodont zone of the mid-Darriwilian Stage (∼ 460.9 Ma ± 1 My). This 87Sr/86Sr inflection is accompanied by an increase in δ18O, consistent with climate cooling. Clarifying the role of seafloor production for marine 87Sr/86Sr and the implications for Ordovician cooling allows for a more nuanced understanding of the factors that drive multi-million-year shifts in climate.

The Alapaevsk-Sukhoi Log porphyry copper zone, Middle Urals: The U-Pb age of productive magmatism

The Alapaevsk-Sukhoi Log zone about 100 km long and 3–10 km wide with numerous porphyry copper occurrences and small deposits is located in the eastern part of the East Ural volcanic megazone, Middle Urals. The long evolution of productive magmatism (according to the U-Pb (SHRIMP-II and LA-ICP-MS age of zircon) and its rejuvenation are established from the north and the south (Ma): from 411 ± 3 (Yalunina Gora pluton, town of Alapaevsk) to 404–406 ± 3 (Altynai-Artemovsk pluton, town of Artemovsk) and to 397 ± 4 (porphyric rhyodacites of the Shata area, town of Sukhoi Log). The K2O content in productive quartz diorites is 0.4–0.7, 0.8–1.2, and 0.4–0.7 wt %, respectively. The Mo-Cu porphyry occurrences are abundant in the Altynai-Artemovsk area. The granitic rocks of the quartz dioritic East Artemovsk pluton, which was recently found, are sericitized and contain significant sulfide mineralization. The structural position, age (365 ± 39 Ma, Rb-Sr errochron), composition of granitic rocks, and its mineralization are similar to those of the large (1.7 Mt Cu) Mikheevka deposit (U-Pb SHRIMP-II age of zircon is 356 ± 6 Ma) in the South Urals.

Relationship between syndeformational partial melting and crustal-scale magmatism and tectonism across the Wet Mountains, central Colorado

In the Wet Mountains of central Colorado, we document evidence for increasing metamorphic grade and associated higher amounts of partial melting along a transect from northwest to southeast. Field observations of structural orientation and style, qualitative assessment of strain intensity, analysis of metamorphic mineral assemblages, and macroscopic identification of leucosomes and migmatites are complemented by the use of melt microstructures to carefully document the presence and locations of former partial melt and to identify melt-producing reactions. In the northwest Wet Mountains, migmatitic foliation is moderately well developed, and partial melting occurred via granite wet melting and muscovite-dehydration melting, with rare melt pseudomorphs remaining. At Dawson Mountain in the central part of the range, inferred former melt channels are preserved along grain and subgrain boundaries, deformation appears more intense, and anatexis occurred through biotite-dehydration melting. Farthest to the south, the highest intensity of strain is inferred, with very closely spaced foliations, abundant dynamic recrystallization, and local mylonitization occurring in rocks of granitic composition, and partial melting occurring via granite wet melting. Metapelitic rocks experienced biotite-dehydration melting and contain garnet with Mn-rich rims and Mn-poor cores mantled by plagioclase, decussate biotite, and quartz, textures indicating back-reaction between melt and garnet. These textures indicate there was abundant melt within these highest-grade rocks and extensive melt-rock interaction. Throughout the Wet Mountains, deformation is concentrated in areas where melt-producing reactions occurred, and melt appears to be localized along deformation-related features, suggesting a correlation between partial melting and deformation. The northern Wet Mountains contain few plutons, whereas the central and southern portions of the Wet Mountains contain more pervasive dikes and sills and may contain more former melt as a result of both higher metamorphic grade and widespread thermal insulation. The Wet Mountains represent an exhumed section of formerly molten middle crust located at the transition between upper and lower crust and provide insight into processes ongoing at depth in modern orogenic belts. The microstructures indicative of former partial melt, textures associated with melt-rock interaction, and melting reactions we have identified in the Wet Mountains will greatly facilitate the recognition of other such exhumed sections.

Geologic and biogeochemical role of crustose aquatic lichens in Lake Baikal

Based on materials collected in 2003–2007, data on aquatic lichens of the genus Verrucaria from the rocky littoral zone of Lake Baikal are discussed. The maximum density of their occurrence was observed at a depth of 1.5 m; 95–100% of rock fragments recovered from depths of 1.5–2.2 m are encrusted by 24–43% with the thalli of Verrucaria spp. It was found that the lichens contribute actively to the physical and physicochemical weathering of their rock substrate and prefer to reside on the rocks of granitic composition. The chemical composition of aquatic verrucaria is dominated by the same elements that are most abundant in the rocks (Ca > K > Fe > Al > Mg > P > S > Na > Mn > Sr > Ba), and, in most cases, the characteristic element ratios of the rock compositions are preserved in the composition of the thalli of Verrucaria spp. Compared with the elemental composition of the near-bottom water layer, the lichens most extensively concentrate the elements that slowly migrate in water: Gd > Sm > Pr > Nd > Al > La > Dy > Tb > Y > Lu > Ce > Yb > Be > Tm > Co > Nb > Mn > Zn. Compared with the composition of rocks, the mineral composition of the thalli of Verrucaria spp. is enriched by a factor of 100–10 in Hg > As > P > Zn > Li > S > U > Mo > Se > Cd > Ca > Tl > Sr > Pb > Be.

Two stages of granite formation in the Sredinny Range, Kamchatka: Tectonic and geodynamic setting of granitic rocks

The newly formed continental crust in southern Kamchatka was created as a result of the Eocene collision of the Cretaceous-Paleocene Achaivayam-Valagin island arc and the northeastern Asian margin. Widespread migmatization and granite formation accompanied this process in the Sredinny Range of Kam- chatka. The tectonic setting and composition of granitic rocks in the Malka Uplift of the Sredinny Range are characterized in detail, and the U-Pb (SHRIMP) zircon ages are discussed. Two main stages of granite forma- tion—Campanian (80-78 Ma ago) and Eocene (52 ± 2 Ma ago) have been established. It may be suggested that granite formation in the Campanian was related to the partial melting of the accretionary wedge due to its under- plating by mafic material or to plunging of the oceanic ridge beneath the accretionary wedge. The Eocene gra- nitic rocks were formed owing to the collision of the Achaivayam-Valagin ensimatic island arc with the Kam- chatka margin of Eurasia. In southern Kamchatka (Malka Uplift of the Sredinny Range), the arc-continent col- lision started 55-53 Ma ago. As a result, the island-arc complexes were thrust over terrigenous sequences of the continental margin. The thickness of the allochthon was sufficient to plunge the autochthon to a considerable depth. The autochthon and the lower portion of the allochthon underwent high-grade metamorphism followed by partial melting and emplacement of granitic magma 52 ± 2 Ma ago. The anomalously rapid heating of the crust was probably caused by the ascent of asthenospheric magma initiated by slab breakoff, while the Eurasian Plate plunged beneath the Achaivayam-Valagin arc.

Evolution of fluids responsible for iron skarn mineralisation: An example from the Vyhne-Klokoc deposit, Western Carpathians, Slovakia

Vyhne-Klokoc, the largest Fe-Skarnn deposit in the Western Carpathians, is related to the emplacement of a large granodiorite pluton in the central zone of a Neogene stratovolcano. Skarn mineralisation is developed in places where apophyses of the pluton intruded basement carbonates. Granodiorite in the apophyses grades into rocks of granitic composition, involving the replacement of mafic minerals and a concomitant decrease in Fe-content. Ca-magnetite exoskarns (not accompanied by endoskarns) developed in three paragenetic stages. Fluid inclusion (Fl) data for quartz in granodiorite suggest the existence of aqueous fluid immiscibility during the early hydrothermal stages. Three end-members of Fis were recognised, with a continuum between all three types. High salinity, liquid-rich, probably secondary Fls (29-68 wt % NaCl eq., Th 450 to 570'C, composed of NaCl+FeCl2+KCl) coexist with vapour-rich Fls with low but variable salt contents (+CO2). Probably late secondary Fls (1-25 wt % NaCl eq., composed mainly of NaCl+CaCl2, Th 188–283°C) form the other end-member type of Fls trapped in granodiorite quartz. Fls from skarn garnets show a large variation in salinity (4-23 wt % NaCl eq., composed of NaCl±FeCl2+CaCl2+ KCl+MgCl2) and Th (220–370°C), independent of the garnet types, probably reflecting variable amounts of magmatic fluids and low salinity meteoric waters. Fls in retrograde quartz, calcite and sphalerite show progressively more dilute (0-4 wt % NaCl eq, Th 215–380°C), probably dominantly meteoric fluids with evidence for boiling at shallow depth. Chlorite crystallisation temperatures, calculated from the chlorite geothermometer, are in good agreement with the Th data for Fls in associated skarn minerals. Compositional changes in the granodiorite apophyses are the result of subsolidus autometasomatic reactions of accumulated saline magmatic fluid inside the apophyses with pre-existing mafic mineral phases. Reactions add the iron to the fluid -the potential source for magnetite skarn. Later mixing with dilute, cooler probably meteoric waters had the effect of decreasing the salinity and density of the equilibrated magmatic fluid, making it more buoyant and capable of moving out from the apophyses into the country rocks, causing metasomatic reactions and precipitating magnetite. An overlap exists between the FI microthermometry data from primary Fls in garnets and late secondary Fls in the granodiorite quartz indicating the same sources of the hydrothermal fluids - probably mixtures of magmatic and meteoric waters. Based on fluid inclusion, geological, petrological and mineralogical data, an integrated fluid evolution model involving magmatic and meteoric fluids is developed to explain the geological and fluid controls on Fe-skarn mineralization associated with granodiorite intrusions. Die Evolution von Fluiden bei Fe-skarn Mineralisation: Ein Beispiel von der Lagerstatte Vyhne-Klokoc, West-Karpathen, Slowakei.

Geochemistry and genesis of granitoid rocks from Southern Sri Lanka

Pyroxene granulites of intermediate to felsic composition comprise a major part of the Precambrian terrain exposed in southern Sri Lanka. Chemical characteristics and field relations reveal that there are two closely associated groups of granitoids both occurring as discrete layers or deformed bodies. Granitic-adamellitic rocks are more common and can be distinguished from tonalitic-trondhjemitic rocks. Granitoids of these two groups and minor metabasalt and metadiorite/meta-andesite have been subjected to high-grade metamorphism and were mapped as charnockite or charnockitic rocks. Rocks of granitic composition are mostly meta-aluminous and, in the AFM diagram, fall in the tholeiitic field. These rocks are mainly A-type granites characterized by high K2O/Na2O, FeO/MgO and high Zr, Nb and LREE contents, most belonging to the relatively Ce- and Y-enriched A2-type. Tonalitic rocks are meta-aluminous to per-aluminous and display a calc-alkaline trend. These rocks are classified as I-type, although some of them have very low K2O and REE contents.

Lithospheric velocity structures under the southern New England Orogen: Evidence for underplating at the Tasman Sea margin

Wide‐angle reflection and refraction seismic studies indicate that two velocity models are appropriate for the structure of the crust and upper mantle under the New England Batholith, eastern Australia. Under the southern part of the batholith crustal velocities increase gradually from 5.5–6.03 km/s at the surface to 6.45 km/s near a distinct Moho at 34–35 km depth, suggesting that rocks of granitic composition make up a large part of the crust. Under the northern part there is a (? mafic) sill‐like feature at 21–24 km depth with a velocity maximum of 6.7 km/s. The crustal velocities are significantly less than those of the Lachlan and Thomson Orogens underlying the Bowen‐Gunnedah‐Sydney Basin system just to the west of the New England Orogen (typically > 6.3 km/s at > 10 km depth) and of terranes in the northern New England Orogen. In areas with similar granite geochemistry and inferred to have similar evolutionary process (for example the Peninsular Ranges Batholith, southwestern USA), there are again c...

Petrographic and microthermometrical studies of emeralds in the ‘Garimpo’ of Capoeirana, Nova Era, Minas Gerais State, Brazil

Petrographic investigations in the area of the Capoeirana emerald deposit, Minas Gerais State, revealed two main lithostructural units. The first unit is comprised of gneissic rocks of granitic composition belonging to the basement complex, and the second is composed of a highly weathered metasedimentary-metavolcanic sequence represented by metapelitic schists, amphibolites, schists derived from ultramafic rocks, and quartzites. Quartz and pegmatoid veins appear near the contacts between the gneissic rocks and the mineralization metasedimentary-metavolcanic sequence. The emerald mineralization is dominantly concentrated within the intercalations of meta-ultramafic schists near the contact of the pegmatoid veins. Microthermometric studies of the fluid inclusions of the emerald grains indicate that crystallization occurred in the pressure and temperature ranges of 2000 to 2750 bar and 450 to 650 °C, respectively. These data suggest that the mineralizing solutions have had a late hydrothermal-pneumatolytic origin characterized by low pressures, suggestive of the paragenesis talc + tremolite + carbonate + biotite-phlogopite + chlorite of the emerald wall rocks.

Relationship between Shear Zones and Sr Isotope Compositions of Granitic Rocks in the Hida Belt, Japan

The dextral shear zones in the Hida belt, central Japan, show close temporal and spatial relationships with some of the Late Triassic to Early Jurassic Funatsu granitic intrusions. The Sr isotopic compositions of intrusions associated with the shear zones suggest large crustal contributions to the magma chemistries. In contrast, granitic intrusions not accompanied by shear zones show less crustal signatures. Various models could possibly explain the genetic relationship between shear zones and granitic magmas with crustal isotope characteristics, though it is not clear which model is most applicable. It is suggested that contrasting characteristics in the two types of intrusions reflect different stress conditions. The granitic rocks without large crustal contributions are likely to be the products of extensional stress conditions. Those with a larger amount of crustal contamination seem to be generated in compressional stress situations that accompanied the development of ductile shear zones along the co...

日高変成帯主帯南端部に分布する花こう岩類の地質および化学組成 特に含きん青石花こう岩類について

日高変成帯主帯南端部に分布する花こう岩類の地質および化学組成 特に含きん青石花こう岩類について

Thermal diffusivity of igneous rocks at elevated pressure and temperature

Thermal diffusivity measurements of seven igneous rocks were made to temperatures of 400°C and pressures of 200 MPa. The measuring method was based on the concept of cylindrical symmetry and periodic heat pulses. The seven rocks measured were Westerly (Rhode Island) granite, Climax Stock (Nevada) quartz monzonite, Pomona (Washington) basalt, Atikokan (Ontario, Canada) granite, Creighton (Ontario, Canada) gabbro, East Bull Lake (Ontario, Canada) gabbro, and Stripa (Sweden) granite. The diffusivity of all the rocks showed a positive linear dependence on inverse temperature and, excluding the East Bull Lake gabbro, showed a linear dependence on quartz content. (Quartz content varied from 0 to 31% by volume.) Diffusivity in all cases rose or remained steady with increasing confining pressure. The pressure effect was strongest at lowest pressures and vanished by levels between 10 and 100 MPa, depending on rock type. The pressure effect (measured as a percentage change in diffusivity) is stronger in the four rocks of granitic composition than in the three of basaltic composition. Our results agree well with existing thermal diffusivity measurements at atmospheric pressure.

The Precambrian tectogenesis in the Bohemian Massif

The Cadomian and Variscan tectogeneses are two distinctive and easily distinguished cycles of a long-term geological process leading to the almost complete crustal consolidation of the Bohemian Massif. The internal parts of the massif are those of the main development of the Upper Proterozoic geosyncline and the intensive Cadomian metamorphism and plutonism. The Precambrian of the Bohemian Massif is divided into two principal regional chronostratigraphic units: the Moldanubian and the Brioverian; the age of the boundary between them is estimated at about 1000 Ma B. P. The Brioverian is subdivided into three units: the Lower, Middle and Upper. The Moldanubian is provisionally subdivided into two units: the Lower and the Upper. The maximum of the metamorphic activity of the Cadomian cycle falls approximately within the sedimentation interval of the Middle Brioverian, and that of the Variscan cycle in the Devonian. Compared with the Cadomian regional metamorphism which attained mostly amphibolite to granulite facies, the Variscan metamorphism generally did not exceed greenschist facies. The origin of the granitoid rocks of the massif is closely associated with the metamorphic processes of the two above-mentioned cycles. Cadomian granitoids are represented mostly by rocks of granodiorite-tonalite and durbachite types whereas the Variscan intrusives are dominated by rocks of granitic composition.

Chemical evolution of high-grade metamorphic rocks — Anatexis and remotion of material from granulite terrains

The geochemistry of acid, sub-acid metamorphic rocks of sedimentary origin from a sequence of increasing metamorphism of the Western Italian Alps has been studied. Analysis of bulk chemistries and of pairs of geochemically coherent elements (K—Rb, K—Pb, Na—Sr, Fe—Zn, etc.) indicates that metamorphism up to amphibolite facies is essentially an isochemical process even if anatexis is active, the products of initial melting recrystallizing together with unmelted solids. Rocks of granitic composition have been strongly depleted during granulite facies metamorphism in lithophile trace elements and probably in silicate fractions rich in alkalis. Such depletions have been correlated to extensive melting of pelitic rocks under water pressure condition determined by the progressive release of H 2O from hydrated phases involved in the melting. According to fractional melting models, differentiated low-viscosity fractions of silicate melts rich in water and granitophile elements have been removed and diffused outwards under a pressure gradient established by the increase in total volume of the rocks undergoing extensive anatexis. Most of the melt which has not migrated, because of the continuous loss of water and minimum-granite fractions, enhances progressively solidus conditions and recrystallizes within the unmelted solids giving granulitic parageneses. The intimate association between melted and unmelted materials causes granulites to present small-scale elemental and isotope disequilibria. Extensive melting may deeply modify the primary distribution of elements, favouring the attainment in granulites of magmatic-like trends.

Petrology of autoliths in granitic rocks

Homeogenie inclusions - autoliths - widely distributed in granitic rocks, are restricted to hypabyssal and mesabyasal massifs of the gabbro-granite, tonalite-granodiorite, and adamellite-granite composition. Several geological and petrological characteristics of autoliths (independence of their composition from that of the Intruded rocks, close similarity between their composition and the composition of granitic rocks in which they occur, isofacial relation between autolilhs and the enclosing granites, etc.) distinguish them from other kinds of inclusions and indicate that they are regular members of the granitic series. From detailed chemical and mineraloglcal studies of autoliths from different massifs of the USSR and from analysis of data in the literature, we conclude thai autoliths form in the process of differentiation of granitic magmas. —Authors.

Early Precambrian Rocks of Granitic Composition

The granitoids and the corresponding gneisses of the old sialic crust are not exclusively represented by rocks of tonalitic and granodioritic composition. Potassium-rich rocks of true granitic composition occur even in the oldest known complexes of the Earth's crust.The petrography of six samples (39 individual specimens) from the southwestern part of the Early Precambrian Amitsoq gneiss complex in the Godthaab area in Greenland, collected for dating purposes, and of 12 Early Precambrian granitoid pebbles from the basal Moodies conglomerate in South Africa shows that (a) half of the investigated samples of the Amitsoq gneiss, and (b) ten of the Moodies granitoid pebbles are rocks of true granitic composition. The rocks show no evidence of sedimentogenic origin.

A Minimal Test of Significance of Nonuniform Density of Data Points in a Three Variable Closed Array

An algorithm and computer program are described for making a conservative test of significance of clustering of data points in a three-variable closed array. Normative compositions of granitic rocks, projected to points in the albite–orthoclase–quartz triangle, have a clustering that is significant at the 99% level, if the proportions of these three minerals are the only variables.

Justification for Both Ionic and Thermal Reactions in Grenville Province Pelitic Rocks near Sudbury, Ontario, Canada

Metamorphic reactions related to isograds derived from aluminum silicate-bearing pelitic schists were studied in an area of Grenville Province adjacent to Southern Province rocks near Sudbury, Ontario. Progressing from the northwest to southeast of the area, the meaningful hand-drawn isograds are: (1) sillimanite first occurrence, (2) the last occurrence of staurolite when associated with the entire assemblage, (3) K-feldspar first occurrence, (4) staurolite last occurrence as inclusions in garnet, (5) muscovite last occurrence, and (6) kyanite last occurrence. Whole-rock chemical analysis of 14 representative pelitic schist hand specimens in the area were collected and used to show that metamorphic factors, and not chemical differences, were responsible for the metamorphic isograds. The entire area lies thermally above the melting of rocks of granitic composition. Breakdown curves of the minerals related to the isograds have been used to imply a gradient of 670 °C to 750 °C and 6.3 to 7.3 kilobars, across the area, but the equations for these breakdowns are not entirely substantiated by the modal abundance and textural data.To a first approximation, the rocks may be considered homochemical, but many deviations (due partly to metasomatic change) from this exist. The ionic breakdown of kyanite to muscovite has been shown and an explanation as to why muscovite selectively replaces kyanite and not sillimanite is given. The breakdown of muscovite at the higher grades has been inferred to form K-feldspar, but not sillimanite. Near the kyanite isograd, textures showing the thermal breakdown of kyanite (left over after the partial ionic breakdown of the mineral) to sillimanite are shown. The rocks must have had at least K and possibly Fe added metasomatically to account for the textures shown. From generalized modal abundance surfaces (trend surface analysis), general equations representing the difference in modal abundance of minerals across various isograds were determined and from these, specific equations explaining the breakdown of a particular mineral at its isograd were derived. The most significant of these reactions is the first staurolite isograd, where it is inferred to breakdown in the following way, in the area studied:[Formula: see text]The dissolved Al and Si forms the fibrolite (sillimanite) lenses common in adjacent pelitic rocks

Variations of modal composition of granitic rocks in some regions around the pacific

Granitic rocks from the Andes and from Japan, Kurile Islands, and Kamchatka have been investigated in their modal composition determined. Principal components analysis of the data reveals that there is three main factors influencing variation of modal composition of rocks that can be interpreted in forms of petrological processes. There is a similarity in results of the investigation of rocks from Japan, Kamchatka, and Andean regions.

On the origin of variations in the composition of granitic rocks of Chile and Bolivia

On the origin of variations in the composition of granitic rocks of Chile and Bolivia