Assimilation Of Country Rocks Research Articles

Dynamics of differentiation in magma reservoirs

In large magma chambers, gradients of temperature and composition develop due to cooling and to fractional crystallization. Unstable density differences lead to differential motions between melt and crystals, and a major goal is to explain how this might result in chemical differentiation of magma. Arriving at a full description of the physics of crystallizing magma chambers is a challenge because of the large number of processes potentially involved, the many coupled variables, and the different geometrical shapes. Furthermore, perturbations are caused by the reinjection of melt from a deep source, eruption to the Earth's surface, and the assimilation of country rock. Physical models of increasing complexity have been developed with emphasis on three fundamental approaches. One is, given that large gradients in temperature and composition may occur, to specify how to apply thermodynamic constraints so that coexisting liquid and solid compositions may be calculated. The second is to leave the differentiation trend as the solution to be found, i.e., to specify how cooling occurs and to predict the evolution of the composition of the residual liquid and of the solid forming. The third is to simplify the physics so that the effects of coupled heat and mass transfer may be studied with a reduced set of variables. The complex shapes of magma chambers imply that boundary layers develop with density gradients at various angles to gravity, leading to various convective flows and profiles qf liquid stratification. Early studies were mainly concerned with describing fluid flow in the liquid interior of large reservoirs, due to gradients developed at the margins. More recent work has focused on the internal structure and flow field of boundary layers and in particular on the gradients of solid fraction and interstitial melt composition which develop within them. Crystal settling may occur in a surprisingly diverse range of regimes and may lead to intermittent deposition events even with small crystal concentrations. Incorporating thermodynamic constraints in the study of the dynamics of settling has only just begun. Many dynamical phenomena have been found using theoretical arguments, laboratory experiments on analog systems, and numerical calculations on simplified chemical systems. However, they have seldom been applied to natural silicate melts whose phase diagrams and important physical properties such as thermal conductivity and chemical diffusion coefficients remain poorly known. There is a gap between model predictions and observations, as many models are designed to explain large‐scale features and many observations deal with the local texture and mineral assemblages of the rocks. This review stresses the relevance to the geological problem of the work carried out in parallel in other disciplines, such as physics, fluid dynamics, and metallurgy.

A Strontium and Neodymium Isotopic Investigation of the Fongen--Hyllingen Layered Intrusion, Norway

The Fongen—Hyllingen Intrusion, situated ∽60 km SE of Trondheim, Norway, is a synorogenic layered mafic intrusion of Caledonian age 426-2+8⁠. The intrusion is divided into four evolutionary stages based on cryptic variations: Stage I—a basal reversal; Stage II—unchanged mineral chemistry or slight normal evolution; Stage III—a gradual regression; Stage IV— a strong normal fractionation trend Magma replenishment dominated during most of the crystallization, i.e. during Stages I, II and III Replenishing magma was more dense than resident, evolved magma, and continuing influx eventually caused a compositionally stratified magma column to form. Cryptic lateral variation is an important feature in the southern part of the complex and formed by in situ crystallization from a stratified magma along an inclined floor, where modal layering formed parallel to the crystallization front. Initial Sr- and Nd-isotopic ratios in the cumulates vary as a result of assimilation of country rock and subsequent mixing between uncontaminated, replenishing magma and contaminated, resident magma. The parental magma had a moderately depleted isotope composition, relative to Bulk Earth, with εNd=5·84 and Sri=0·70308, whereas the main contaminant was a partial melt of metapelitic country rock with εNd=-8·74 and Sri=0·7195 (Sri is the initial 87Sr/86Sr). Sri in the analysed cumulate whole-rock samples ranges from 0·70308 to 0·70535 and initial εNd ranges from. 1·58 to 5·84. There is a strong correlation between mineralogical composition and isotopic trends in most of the cumulates: the most primitive samples are the least contaminated, as reflected by relatively high ed and low Sr,, and more evolved samples have progressively lower eNi and higher Sry A gradual regression of several hundred metres thickness characterizes Stage III; stratigraphically upwards mineral compositions become more primitive and isotope compositions more depleted (higher εNd and lower Sri), implying a process of. progressive mixing-in of replenishing, primitive and uncontaminated magma. Magma influx in Stage III took place by fountaining, whereas magma addition was more tranquil in the earlier stages. The fountaining influx entrained resident, relatively evolved and contaminated magma, resulting in a hybrid magma which ponded at the floor. During prolonged magma addition with concomitant crystallization, the lowermost magma layer was replaced by progressively more primitive hybrid magma, creating a gradual regression in the crystallizing cumulate sequence. A detailed two-dimensional study reveals lateral variations in mineral compositions both at the base and top of Stage III, whereas lateral variations in Sr- and Nd-isotopic compositions are present at the top, but not at the base. This implies that the lowest crystallizing part of the magma column was essentially isotopically homogeneous, but compositionally stratified, before influx in Stage III. Isotopic gradients in the magma were strong close to the roof, where most of the assimilation occurred, and decreased downwards, merging into isotopically homogeneous magma. This stratified system was destroyed by turbulent mixing between replenishing and resident magma during fountaining influx in Stage III, and a new stratification was established with both an isotopic and a compositional gradient. After the final influx, crystallization continued in an essentially closed system, in which the remaining magma column eventually became homogenized, as magma layers mixed when their densities converged owing to release of buoyant, residual liquid during fractional crystallization.

Superimposing Magma Chamber and Melting Regime Processes: How Easy is it to (Mis)Interpret the Liquid Products?

Choice of partial melting model. Less-thanperfect fractional melting processes (O'Hara 1993) yield liquid products which quite closely simulate products of equilibrium partial melting (EPM); a useful first approximation is that the partial melting event supplies a liquid which is an EPM product of the true source region composition. Choice of magma chamber process. Less-thanperfect fractional crystallisation processes (O'Hara 1993, O'Hara and Mathews 1981) in magma chambers yield liquid products which quite closely simulate those of equilibrium partial crystallisation (EPC) of the gross input to the magma chamber, including any materials assimilated from the roof and walls; a useful first approximation is that the partial crystallisation event supplies a liquid which is an EPC product (the equations are identical to EPM) of the average magma chamber input composition. Simplifying assumptions. In this formulation the space problem is ignored and contamination by assimilation of country rock or previously erupted lavas assumed to be non-existent. It is further assumed that the distribution coefficient, d, for each element remains constant throughout both processes and is the same in each process. A simple two-stage model The simplest twostage model of a 'real' system in which erupted

Magma influx and mixing in the Bjerkreim-Sokndal layered intrusion, South Norway: evidence from the boundary between two megacyclic units at Storeknuten

The Bjerkreim-Sokndal layered intrusion belongs to the Proterozoic anorthositic province in the Rogaland area of southern Norway. The northwestern part of the intrusion comprises a ca. 6 km-thick Layered Series made up of megacyclic units (MCU) arranged in a syncline; each megacyclic unit reflects the influx of fresh magma into the chamber. The boundary between megacyclic units III and IV has been studied in detail at Storeknuten on the southern flank of the syncline. The megacyclic units can be subdivided into a series of cumulate stratigraphic zones; the interval from the top of zone IIIe to the base of zone IVd is exposed in the Storeknuten area. Modally layered plagioclase-hypersthene-ilmenite-magnetite-augite-apatite cumulates belonging to zone IIIe are overlain by 30 m of massive plagioclase-rich rocks (commonly containing ilmenite and/or hypersthene) constituting zone IVa. The entry of cumulus olivine defines the base of zone IVb (dominantly plagioclase-olivine-ilmenite cumulates) which is about 100 m thick. Many of the olivines are partly or completely replaced by Ca-poor pyroxene/FeTi oxide symplectites. This massive leucotroctolitic zone is overlain by modally layered, laminated plagioclase-hypersthene-ilmenite cumulates of zone IVc. The successive entry of magnetite, apatite (accompanied by Ca-rich pyroxene) and inverted pigeonite defines zones IVd, e and f respectively. The entry of K-feldspar (accompanied by Fe-rich olivine) defines the base of a jotunitic transition zone which passes upwards into mangerites and quartz mangerites. There is a compositional regression through zone IVa. The upper part of zone IIIe has Ca-poor pyroxene with about En68, plagioclase with An44–48 and a Sr-isotope ratio of about 0.7062, while the base of zone IVb has olivine with Fo75 together with En78, An53 and 0.7050 respectively. Similar reversals are shown by the minor element compositions of plagioclase and FeTi oxides. Sr-isotope ratios increase systematically up through zone IVb (reaching 0.7058 in zone IVd) while An% and Sr in plagioclase and Ni and Cr in FeTi oxides decrease. Olivine compositions vary unsystematically and are believed to have changed their Fe:Mg ratios as a result of trapped liquid shift. The magma residing in the chamber when the influx at the base of megacyclic unit IV took place was compositionally zoned, and assimilation of gneissic country rock at the roof had resulted in the Sr-isotope ratio increasing up through the magma column. The new magma had a Sr-isotope ratio of about 0.7050 while the resident magma had a ratio of 0.7062 at the floor, increasing upwards. The new magma mixed with the basal layer(s) of the compositionally zoned resident magma and crystallization of this hybrid magma during influx and mixing produced the compositional regression in zone IVa. When magma influx ceased, olivine-bearing rocks began to crystallize at the base of zone IVb. The leucotroctolites at the base of this zone are the most primitive rocks in the entire intrusion. The systematic increase in Sr-isotope ratios up through zone IVb resulted from progressive mixing between new and resident magma. This mixing either took place during magma influx or by the progressive mixing of overlying resident magma layers during crystallization. Calculations based on geochemical modelling, the thickness of cumulate stratigraphy repeated and Sr-isotope ratios indicate that the new magma influx had a thickness of 350–500 m in the Storeknuten section and that the leucotroctolites of zone IVb represent about 20–30% crystallization of this influx.

Rare Earth Element Behaviour During Evolution and Alteration of the Dartmoor Granite, SW England

The distribution of rare earth elements (REE) within the compositionally zoned Dartmoor pluton is used to constrain models of granite evolution and to assess the effects of pervasive hydrothermal alteration on REE mobility. The main process of magma evolution was crystal fractionation of early plagioclase, biotite, and accessory minerals (apatite, monazite, zircon, and xenotime). Concentrations of REE (particularly LREE and Eu) and other elements (Fe2O3t, MgO, CaO, TiO2, Zr, Ba, and Sr) decrease strongly with evolution of the pluton from 71 to 74% SiO2. These trends, and the inward zoning of the pluton, are compatible with differentiation by crystal fractionation at the level of emplacement, a process that gave rise to a marginal cumulate granite (CGM) modified by country rock assimilation, a body of inner granite (PM), and a late-stage evolved granite (FG) that intruded the earlier types. REE modelling of the Dartmoor granite types by fractional crystallization of REE-enriched accessory minerals from a parent PM-granite shows that the FG-granite cannot have formed from a residual liquid left by crystallizati on of the CGM-granite. Two discrete stages of crystallization occurred; side-wall cumulate CGM-granite crystallization dominated by LREE-enriched monazite fractionation followed by a late-stage mobile residual FG-granite in which fractionation was dominated by HREE-enriched apatite and zircon. Modelling supports the idea that largescale assimilation of country rock was not the dominant process during Dartmoor granite evolution. Pervasive hydrothermal alteration locally affected all Dartmoor granite types, altering primary plagioclase, biotite, apatite, monazite, and, to a lesser extent, zircon and xenotime. During pervasive sericitization, chloritization, and tourmalinization, REE were mobilized over distances of centimetres only and redistributed into the secondary alteration products seridte, chlorite, tourmaline, allanite, and sphene. Whole-rock REE abundances were not affected

Petrogenetic relationships between pegmatite and granite based on geochemistry of muscovite in pegmatite wall zones, Black Hills, South Dakota, USA

The compositions of large samples of granitic pegmatite wall zones have been determined for a suite of ten pegmatites of diverse geochemical character and degree of compositional evolution in the Keystone area of the Black Hills. Whole-rock compositions are strongly peraluminous, and they deviate substantially from the granite minimum composition in quartz-albite-orthoclase normalized components, showing considerably more scatter than Harney Peak Granite whole rocks. Wall-zone minerals are commonly coarsely segregated, leading to large modal variability among whole rocks. These features make whole-rock samples of wall zones unsuitable for the determination of initial pegmatite bulk compositions. Trace and minor element compositions of muscovite separates from the wall zones were thus determined to eliminate the effects of modal variability on trace element concentrations so that geochemical differences between pegmatites could be modeled. Estimates of initial pegmatite melt trace element concentrations range from 800–4000 ppm Rb, 100–1000 ppm Cs, 200–2000 ppm Li, and 1–50 ppm Ba. Trace element concentrations of muscovite from a given pegmatite generally cluster together, although several show considerable intra-pegmatite scatter, and there are large overlaps among different pegmatites. The geochemical characteristics of samples from the Etta pegmatite indicate mixing with and assimilation of country rocks. Exceptionally low Rb Cs ratios of muscovite from the Etta pegmatite are similar to those of muscovite from K-feldspar-rich assemblages of other pegmatites where the Rb concentration of melt may have been buffered by crystallizing assemblages that had bulk Rb distribution coefficients close to 1. The large degree of scatter of geochemical parameters among these pegmatites precludes derivation of the entire suite by either single-stage crystallization or partial melting processes, nor can the pegmatites be satisfactorily related by any fractionation trajectory from a single starting composition. Fractional crystallization is difficult to model due to an apparent geochemical decoupling that exists among several of these elements in different pegmatites, e.g., Rb from Cs, or Li from Rb/Cs, and nonsystematic variation of Ba concentrations. However, the magnitude of observed rare-element enrichments could be achieved by 75–90% fractional crystallization of melts that crystallized Harney Peak Granite in nearby exposures. The inter-pegmatite scatter and apparent geochemical decoupling that characterize the suite indicate an origin by partial melting of a heterogeneous source rock or rock sequence, followed by an intermediate episode of fractional crystallization, prior to final emplacement of the pegmatite melts. Additional compositional variations among samples from the wall zones of individual pegmatites were caused by processes during or after emplacement, including local interaction with host metamorphic rocks and fractionation during internal evolution of the pegmatites as they crystallized.

The Northeast Kingdom batholith, Vermont: magmatic evolution and geochemical constraints on the origin of Acadian granitic rocks

Five Devonian plutons (West Charleston, Echo Pond, Nulhegan, Derby, and Willoughby) that constitute the Northeast Kingdom batholith in Vermont show wide ranges in elemental abundances and ratios consistent with major crustal contributions during their evolution. The batholith consists of metaluminous quartz gabbro, diorite and quartz monzodiorite, peraluminous granodiorite and granite, and strongly peraluminous leucogranite. Contents of major elements vary systematically with increasingSiO 40) and have small negative Eu anomalies. The strongly peraluminous Willoughby leucogranite has unique trace-element abundances and ratios relative to the rest of the batholith, including low contents of Hf, Zr, Sr, and Ba, low values of K/Rb (80–164), Th/Ta (<9), Rb/Cs (7–40), K/Cs (0.1–0.5), Ce/Pb (0.5–4), high values of Rb/Sr (1–18) low to moderate REE contents and light-REE enriched patterns (with small negative Eu anomalies). Flat REE patterns (with large negative Eu anomalies) are found in a small, hydrothermally-altered area characterized by high abundances of Sn (up to 26 ppm), Rb (up to 670 ppm), Li (up to 310 ppm), Ta (up to 13.1 ppm), and U (up to 10 ppm). There is no single mixing trend, fractional crystallization assemblage, or assimilationscheme that accounts for all trace elementvariations from quartz gabbro to granite in the Northeast Kingdom batholith. The plutons originated by mixing mantle-derived components and crustal melts generated at different levels in the heterogeneous lithosphere in a continental collisional environment. Hybrid rocks in the batholith evolved by fractional crystallization and assimilation of country rocks (<50% by mass), and some of the leucogranitic rocks were subsequently disturbed by a mild hydrothermal event that resulted in the deposition of small amounts of sulfide minerals.

Geochemistry of the granitoid rocks and their minerals from Lixa do Alvão-Alfarela de Jales-Tourencinho (Vila Pouca de Aguiar, northern Portugal)

Several types of Hercynian syntectonic peraluminous granites containing biotite and muscovite and post-tectonic sub-aluminous to peraluminous biotite granite with inclusions of biotite granodiorite occur in the area of Lixa do Alvão-Alfarela de Jales-Tourencinho. NbY, TaYb, Rb(Yb + Ta) and Rb(Y + Nb) diagrams of rocks, some variation diagrams of rocks and biotites, REE patterns of rocks and thermodynamic conditions of crystallization show distinction between these two groups. The oldest syntectonic granite (320 ± 12 Ma) is a fine-grained muscovite-biotite granite corresponding to the Erzgebirgian Hercynian event. The other syntectonic granites range from two-mica granite (309 ± 3 Ma, representing the Late Wichita Hercynian event) to muscovite granite with tourmaline. Variation diagrams of major and trace elements of rocks, biotites and muscovites show fractionation trends for the syntectonic granites ranging from two-mica granite to muscovite granite with tourmaline. The oldest granite does not belong to them. This granite and the least silicic samples of the two-mica granite correspond to two distinct magmas originated by partial melting of metasedimentary materials and emplaced at 3.5-3 kbar, 690°C, f O 2 10 −18–10 −17 bar at 320 ± 12 and 309 ± 3 Ma ago, respectively. Least-squares analysis of major elements and modelling of trace elements indicate that the most silicic samples of the two-mica granite were derived from its magma by fractional crystallization of plagioclase, quartz and biotite. However, the great influence of volatiles producing metasomatic effects prevented testing whether the most evolved later syntectonic granites were derived from the same magma by fractional crystallization. These granites were completely crystallized at ∼ 600-440°C and metasomatized at ∼ 400°C. The post-tectonic granite is also crust anatectic and its magma was emplaced at pressures of > 3.5 kbar, 710°C and f O 2 of 10 −16 bar. Its granodioritic inclusions are hybrid rocks and the result of assimilation of country rocks by the host granite magma. Inclusions and granite were completely crystallized at ∼ 700-470°C and metasomatized at 440°C.

Contamination of the Marcy Anorthosite Massif, Adirondack Mountains, NY: petrologic and isotopic evidence

Oxygen isotope analyses of 101 samples from the Marcy Anorthosite Massif (61 from this study, 40 from Taylor 1969), indicate that two major and distinct processes of crustal contamination have affected the massif. Ninety percent of the 93 samples with over 65% plagioclase are enriched in 18O by ∼ 2.6 ‰ relative to “normal” anorthosites or gabbros: the average δ18O for 83 enriched samples is 9.5 ‰ Depletions in 18O occur in 8% of the samples which have values ranging from 3.0 to 5.8 ‰ Only 2 of the samples fall within the normal magmatic range for anorthosites. Low δ18O values of 3.0 to 5.8 ‰ in the anorthosite occur only near contacts, and a gradient in δ18O occurs near the contact within the border zone of the massif. Low δ18O values in both the anorthosite and adjacent wollastonite skarns (with δ18O down to −1.3) were probably caused by isotopic exchange with heated meteoric water when the anorthosite intruded at shallow levels, prior to Grenvilleage (∼ 1.1 by) granulite facies metamorphism. The 18O-enrichment was ascribed to exchange between anorthosite and 18O-rich marble by means of a pervasive, H2O-CO2 fluid during the regional metamorphism by Taylor (1969). However, a number of lines of evidence argue against this hypothesis: 1) the preservation of premetamorphic low δ18O values in anorthosite from the border zone as well as preserved gradients in δ18O from a number of localities, 2) mass balance calculations of the amount of marble necessary to produce the 18O enrichment 3) metamorphic phase equilibria which buffer $$P_{{\text{CO}}_{\text{2}} } $$ and $$P_{{\text{H}}_{\text{2}} {\text{O}}} $$ to low values, and, 4) recent oxygen isotope analyses show homogeneity which indicates that magmatic oxygen isotope compositions have been preserved. We evaluated the importance of magmatic assimilation of country rock at the present level of intrusion as an alternative cause of the 18Oenrichment. Samples from 2 distinct lobes of the massif were analyzed: the NE lobe where xenoliths of metasedimentary country rock are common, and the NW lobe, where xenoliths are scarce and the country rock is dominantly granitic. The mean values of δ18O for these two lobes are 9.6 in the NE and 9.3 in the NW. Thus, magmatic assimilation at the present level of exposure probably had only localized and relatively minor effects on the oxygen isotope composition of the massif. This conclusion is supported by Rb/Sr data: variations in Rb content and (87Sr/ 86Sr)i show that such crustal contamination is localized, generally occurring only in samples near the border zone. All of the available results suggest that the 18O-enrichment is a magmatic feature, acquired prior to intrusion at the present level of exposure.

The Timbavati Gabbro of the Kruger National Park

The structure, geochemistry and isotope geochemistry of the Timbavati Gabbro, a suite of basic, intrusive sills located within and outside the Kruger National Park, are discussed. The available information is integrated into a single genetic model for the Timbavati intrusions @ this model involves melting of upper mantle material and accumulation of the magma in an intermediate magma chamber from which batches of magma escaped at various times to form the different phases of the Timbavati Gabbro. Both fractional crystallisation and assimilation of country rock played a part in the geochemical evolution of the Timbavati Gabbro. The age of the Timbavati Gabbro appears to be quite young, predating the rocks of the Karoo Sequence by a relatively short time.

A SISTEMÁTICA Rb/Sr EM ROCHAS GRANITÓIDES: CONSIDERAÇÕES INTERPRETATIVAS, LIMITAÇÕES E EXEMPLOS BRASILEIROS

A literature review about the Rb/Sr systematics in undertaken, as applied to granitoid rocks, poiting out potentials and interpretation limitations. Ages derived from Rb/Sr isochrons pinpoint either crystallization or post-crystallization homogenization events. Isochronic Sr/ 86 Sr initial ratios are a clue to the Sr evolution stage at the site of magma generation, and can evaluate the relative crustal-mantle contributions. Cogenetic granitoids, generated from isotopically homogeneous sources, show identical initial ratios: isochronic ages should pinpoint the true emplacement age, since magma cooling is generally thought to be a fast geologic process. Neverthless, supposed cogenetic granitoid often present real variations in initial ratios, a behavior which prcbably can be explained by several petrogenetic processes (e.g., mixing of magmas, crystal separation from magma mushes during protracted crystallization, assimilation of country rocks). Some of the cited mechanisms can be simulated using several Rb/Sr variation diagrams. Provided data dispersion is not linked to systematic errors of various kinds, it is clear thar anomalous Rb/Sr data reflect the influence of the cited genetic mechanisms, and may thus become valuable tool for petrogenetic interpretations. Rb/Sr data of selected Brazilian granitoid massifs show significant spread; a brief discussion of these and of the corresponding geological setting indicate that many discrepancies arise from analytical and/or interpretation errors, although it is clear that several ones are actually due to the influence of complications in the petrogenetic process, such as pointed out above.

The Formation of Chrysoberyl in Metamorphosed Pegmatites

The Formation of Chrysoberyl in Metamorphosed Pegmatites

The effects of assimilation of country rocks by magmas on 18O/ 16O and 87Sr/ 86Sr systematics in igneous rocks

Examples of positive correlations between initial 87Sr/ 86Sr and δ 18O have now been shown to be very common in igneous rock series. These data in general require some type of mixing of mantle-derived igneous rocks with high- 18O, high- 87Sr crustal metamorphic rocks that once resided on or near the Earth's surface, such as sedimentary rocks or hydrothermally altered volcanic rocks. Mixing that involves assimilation of country rocks by magmas, however, is not a simple two-end-member process; heat balance requires appreciable crystallization of cumulates. In such cases, the isotopic compositions may strongly reflect this open-system behavior and indicate the process of assimilation, whereas the major element chemical compositions of the contaminated magmas will be largely controlled by crystal-melt equilibria and crystallization paths fixed by multicomponent cotectics. A variety of oxygen and strontium isotope “mixing” curves were therefore calculated for this process of combined assimilation-fractional crystallization. The positions and characteristics of the resultant curves on δ 18O- 87Sr/ 86Sr diagrams markedly diverge from simple two end-member mixing relationships. Based on the above, model calculations can be crudely fitted to two igneous rock suites (Adamello and Roccamonfina in Italy), but the shapes of the calculated curves appear to rule out magmatic assimilation as an explanation for most δ 18O- 87Sr/ 86Sr correlations discovered so far, including all of those involving calc-alkaline granitic batholiths and andesitic volcanic rocks. The isotopic relationships in such magma types must be inherited from their source regions, presumably reflecting patterns that existed in the parent rocks (or magmas) prior to or during melting.

Intimate Correlation in Composition between Granitic Rocks and Their Country Rocks in Japan

Granitic rocks in Japan have an intimate correlation with the country rocks which they intrude. The granitic rocks which intrude basic rocks are mainly quartz diorite and tonalite, whereas those which intrude acidic rocks are mainly biotite granite or hornblende-biotite granodiorite. This relation suggests that the granitic magmas changed their compositions by assimilation of country rocks. $$K_{2}O/Na_{2}O$$ ratios of granitic rocks support the assimilation hypothesis as origin of major granitic rocks in Japan.

The application of radioelement studies in solving petrological problems of the Precambrian intrusive Homme granite in the Flekkefjord area, south Norway

A Th- and U-enriched granite has been found within a sequence of high-grade metamorphic gneisses and migmatites in the Flekkefjord area bordering the S Rogaland anorthosite province in SW Norway, where the country rocks and the other intrusions all have very low contents of these elements. The major-element distribution within the Homme granite suggests a certain magmatic differentiation, although a K- and Si-rich zone in connection with the discordant northern contact is supposed to be due to assimilation of felsic country rocks. This model was tested by studying the radioelements which exhibit the lowest values exactly in the northern K- and Si-rich zone, thus supporting the model of assimilation, while the alkali enrichment in the southernmost part is connected with high contents of radioelements, thus suggesting a rest magmatic stage.

Country-rock assimilation and the genesis of Cu-Ni sulfides in the Water Hen Intrusion, Duluth Complex, Minnesota

The Water Hen intrusion is a small (2 miles long, 1 mile wide) body of layered rocks which has been intruded into the basal portion of the Duluth Complex, near Hoyt Lakes, Minnesota. Cyclical units of mineral-graded layers are well developed in the intrusion and range from dunites at their bases through troctolites to anorthosites at their tops. Contaminated igneous rocks and associated footwall inclusions are found predominantly in the lower third of the intrusion. The contaminated zones contain cordierite, orthopyroxene, aluminous spinel, graphite, and sulfides.Cu-Ni sulfide mineralization occurs mainly in a basal dunite unit with lesser amounts found in contaminated dunites and peridotites associated with mineral-graded units higher in the intrusion. Pyrrhotite, chalcopyrite, cubanite, and pentlandite are the essential sulfide minerals.Sulfur isotopic values determined on sulfides in metasedimentary footwall rocks give delta 34 S values of around +18 per mil while those sulfides associated with contaminated rocks in the intrusion give delta 34 S values of about +15 per mil. Sulfides associated with the basal dunite, not obviously contaminated, give delta 34 S values of about +11 per mil.The occurrence of footwall inclusions and contaminated rocks in the intrusion taken together with sulfur isotopic evidence suggests that part and perhaps a large portion of the sulfur in the intrusion was derived from the metasedimentary footwall rocks.

Clinopyroxenes and biotites from the Centre III igneous complex, Ardnamurchan, Argyllshire

SummaryMajor- and trace-element analyses of the clinopyroxenes and biotites from the ring-intrusions of Centre III in the Tertiary igneous complex of Ardnamurchan are presented. It is shown that there are systematic differences in mineral chemistry between the eucrites, the gabbros, and the dolerites, and in particular the clinopyroxenes show an iron enrichment trend that is similar to that found in fractionated tholeiitic intrusions. The minerals from the intcrmediate rocks, the tonalite and quartz monzonite in the centre of the complex, do not show the further iron enrichment expected if these rocks had formed by continued fractionation of basic magma. On the basis of the chemistry of the clinopyroxenes and biotites, the intermediate rocks are interpreted as hybrids formed by the remelting and assimilation of country rocks into basic magma.

Oxygen isotopic study of the Cretaceous granitic rocks in Japan

The (δ18O values of nine Cretaceous granitic rocks from the low P/T type regional metamorphic zone of Japan are +10.0 to +13.2‰ relative to SMOW, while ten Cretaceous granitic rocks from the non-metamorphic zone are +7.9 to +9.8‰. The 18O-enrichment in the former rocks is mainly attributed to oxygen isotopic exchange between the granitic magma and the surrounding metamorphic rocks during regional metamorphism. The assimilation of 18O-rich country rocks is also possible in the cases such as gneissose granite and migmatite.

The Igaliko nepheline syenite complex. General description

The Igaliko Nepheline Syenite Complex is in the Julianehåb District of Southern Greenland, at about 61°N and 45°W. The syenites and related rocks cover an area of about 450 square kilometres in the country between Tunugdliarfik and Igaliko Fjord on the west, and the Inland lce to the east. The Complex belongs to the group of Precambrian igneous intrusions, comprising the Gardar 19neous Province, exposed in the country around Ivigtut and Julianehåb. The Complex comprises four distinct intrusive centres, termed, in order of decreasing age, the Motzfeldt Centre, the North Qoroq Centre, the South Qoroq Centre and the Igdlerfigssalik Centre. With these are associated a number of small satellitic syenites which generally pre-date the rocks of the main centres of activity and numerous alkali trachyte and Big Feldspar Dykes, which, for the most part, belong to the regional Mid-Gardar swarms. The satellitic syenites and the three earlier syenite centres are earlier than the Mid-Gardar dykes swarms. Three members of the Igdlerfigssalik Centre are also earlier than the dykes but the four late intrusions in this centre cut the majority of the dykes. Each centre is made up of several intrusive members; including satellitic intrusions, the Complex consists of 28 separate units as well as several small dykelike bodies of syenogabbro and alkali gabbro. Within each centre the individual intrusions have arcuate, steep-sided outcrops with discordant, intrusive relations towards earlier members although, occasionally, there is evidence that petrographically and texturally distinct syenites were intruded at only short intervals thus possibly representing rapid pulses of magma from the same source. The mineralogy of the syenites is usually simple: perthitic alkali feldspar, nepheline and alkali pyroxene are the dominant minerals; these are accompanied by fayalitic olivine, alkali amphibole, biotite, magnetite-rich opaques, analcite, natrolite and apatite. Pegmatites are not numerous, but the Complex does contain the celebrated Narssârssuk pegmatite which lies within the outer member of the later group of syenites in the Igdlerfigssalik Centre. Except where locally contaminated by assimilation of country rocks, the Igaliko syenites are all nepheline bearing; the commonest type is foyaite. The quartz syenite – alkali gabbro intrusive of Klokken, about 5 km south of the Complex, is not regarded as a satellite. Igneous layering and mineral lamination are common internal structures in the syenites. These generally define concentric, inward-dipping structures within individual intrusions; the frequency with which the well-developed internal structures in one syenite may be sharply truncated by a later syenite indicates that, in some instances, an appreciable amount of time must have elapsed between successive intrusions. The structures found in the Igaliko syenites are identical with those described from other layered alkaline, basic and ultrabasic intrusions. 19neous activity within the Complex began at least as early as the Mid-Gardar. Syenites of the earliest centre intrude sediments, agglomerates and lavas belonging to the Eriksfjord Formation and also cut a dolerite dyke which may belong to the relatively early Gardar dyke swarm. The early Østfjordsdal Syenite (pre-South Qôroq Centre) cuts a small swarm of nepheline porphyry dykes. The dykes of the Mid-Gardar swarms maintain their regional WSW-ENE strike throughout most of the Complex except near Qôroq where there is a marked swing into a more northerly direction. Two major sinistral faults, striking ESE and east-west, displace the earlier centres of the Complex and the dykes. The faults are members of a regional set developed throughout southern Greenland. Since no alkaline dyke cuts the younger of the two faults, it is considered likely that the 2 km sinistral transcurrent movement of this fault post-dates the late syenites in the Igdlerfigssalik Centre; the late syenites are cut by a number of alkaline dykes petrographically identical with those displaced by the sinistral faults. Thus, the latest syenites at Igaliko may be of slightly earlier date than the later members of the Ilímaussaq Intrusion and the Central Complex of Tugtutôq, although still belonging to the Late-Gardar group of intrusions.

Structure and stratigraphy of the manganese deposits of Visakhapatnam and Srikakulam districts, India

The deposits include bedded oxide ores which are considered primary and syngenetic with the enclosing paragneisses (khondalites), and secondary massive botryoidal ores and irregular replacement bodies generally associated with zones of strong structural disturbance. The manganiferous rocks occupy a definite stratigraphic position in the sequence of aluminous and calcareous sediments from which the khondalite formations of the Eastern Ghats were derived. Kodurites, once interpreted as igneous intrusives with which the manganese ores were considered genetically related, represent hybrid rock types formed as a result of assimilation of manganiferous country rock by granite.