Magmatic Replacement Research Articles

ВІКОВИЙ ІНТЕРВАЛ ФОРМУВАННЯ ГРАНІТОЇДІВ УКРАЇНСЬКОГО ЩИТА

The paper discusses Berdychiv type granitoids that have always stirred up interest among researchers of the Ukrainian Shield. These rocks include minerals that are uncharacteristic of granites - cordierite, sillimanite, andalusite - and are closely related to rocks of the supposed substratum. At the same time, there still remain questions about the genetic nature of Berdychiv granites. Despite the fact that most researchers treat these granites as intrusive (anatectic) formations, there are other concepts according to which these granites are considered to be the products of metasomatic transformation (magmatic replacement) of primarily argillaceous and marly series of substratum. The Stryzhavka open pit, which is located in the Upper Bug region of the Ukrainian Shield displays differently cataclised porphyry-like plagiogranitoids (predominantly, garnet-bearing varieties in its southwestern area, locally rock-forming potassic feldspar varieties), and garnet-bearing leucocratic granites similar to those of Berdychiv type granites. The monazites of biotite-garnet granodiorite and biotite-garnet granite were dated by uranium-lead (U-Pb) isotopic dating method. The monazite of granodioritеs is dated 2049,3±3,5 million years based on the upper intersection of concordia with discordia. Significantly lower values of age (about 15 million years younger) for the monazite selected from granite, approximating 2035,1±1,9 million years were established. We assume it to be caused by prolonged crystallization of granitic melt and rather early crystallization of more basiс granodioritic magma in comparison with granitic one. Thus, the duration of the granitoid formation uncovered in the Stryzhavka open pit exceeds 15 million years, which correlates accurately with the estimates for occurrence duration of the granitic melts, which gave rise to various granites of the Ukrainian Shield.

Processes of Replacement by Melt at Interaction between Refractory Materials and Industrially Produced Melts

In a number of industries (ferrous and nonferrous metallurgy, glass-making and silicate-producing technologies), interaction between refractory materials with melts results in sequences of reaction zonation (reaction columns) that show all principal features of diffusion-controlled metasomatic zoning. However, in contrast to the latter, reaction melt is generated together with crystalline phases in the rear zones of the columns. This melt is neither mechanically displaced melt that affects the refractory materials, nor produced by melting. The process generating this melt is most adequately defined as replacement by melt. The principal characteristics of the zoning are discussed below with reference to the corrosion of chromite–periclase refractory materials with melted slag in nickel-producing metallurgy. Similarities between the relations observed under different conditions and in different systems and the evolutionary dynamics of the process, specifics of melt generation and changes in its composition in the zones are demonstrated below with the use of data on other technologies and their experimental modeling. The mechanism of melt replacement is applicable to describing natural reaction processes of magma interaction with host rocks (magmatic replacement), with the following unobvious implications. (1) It is reasonable to expect that the minerals of the rocks should host melt inclusions. (2) It is reasonable to expect that certain minerals should be found in two distinct populations: (i) those in equilibrium with melt in the reaction column and (ii) those crystallizing from the cooling melt. (3) Two or more zones of the column can consist of the same minerals, but their proportions should be different. (4) Plastic deformations in the rear zones of the column (magmatic replacement) should be associated with brittle ones in the pristine host rocks and frontal (metasomatic) zones. (5) In contrast to the rocks of metasomatic columns, the material of magmatic-replacement zones can flow through fractures cutting across the host metasomatic rocks and thereby intersect the outer metasomatic zones.

Conditions of formation of the birushin monzonite intrusion, Sikhote Alin

The Birushin monzonite intrusion constitutes the core of a subvolcanic massif composed of crystalloclastic trachyandesites and pyroxene dioritic porphyries. The monzonites are saturated in xenoliths, which form a continuous series from micrograined pyroxene hornfelses and pyroxene microdiorites to medium-grained monzodiorites and monzonites. Xenoliths of different composition and texture occur together within limited areas, thus providing the impression of their transportation from different sources. At the same time, some xenoliths are multiphase and consist of three or more zones of different composition. The micrograined melanocratic xenoliths are characterized by trachytoid alignment of plagioclase and amphibole grains. Numerous measurements have revealed a persistent orientation of trachytic textures. Based on structural, chemical, and mineral data, it has been concluded that the core monzonites were formed by magmatic replacement in situ of the host crystalloclastic trachyandesites and dioritic porphyries.

Cretaceous ultramafic volcanism in the Sredinnyi Range, Kamchatka

Late Mesozoic metapicrites and metapicrobasalts occur in all apoterrigenous rock sequences in Kamchatka’s Sredinnyi crystalline massif, forming sheets, flows, and sills ranging between a few and 100 m or greater in thickness. Metapicrites possess geochemical parameters that are intermediate between mid-oceanic ridge tholeiites and island-arc tholeiites, which is typical of volcanic rock complexes in marginal basins. The host rocks are terrigenous rock sequences of the Kikhchik series and its metamorphosed analogues, viz., the Kolpakova, Kamchatka, and Malka series, which were formed in a common continental-margin sedimentary basin during the late Mesozoic sedimentation cycle with the same source, which was a province in the eastern margin of the Asian continent. The extension of continental crust in the Cretaceous sedimentary basin and intersections by faults of the same age as that of the Okhotsk-Chukotka volcanogenic belt, combined to produce basite and ultrabasite volcanism. Emplacement of mantle material (the ascent of mantle plumages) accompanied by deep hydrogen-rich fluids into the basement of the crust from volcanogenic terrigenous deposits of the marginal basin produced considerable temperature increases in the crust, as well as magmatic replacement of volcanogenic sedimentary rock sequences with the subsequent generation of magma chambers and ascent of acid volcanic and granitoid rocks into the upper crust.

Geochemistry of granitization and magmatic replacement of basic volcanic rocks in the contact aureole of the Yurchik gabbronorite intrusion, Ganal Range, Kamchatka

Geological and geochemical data indicate that the formation of the granulite-like rocks in the contact aureole of the Yurchik gabbronorite intrusion of the Ganal Range, Kamchatka, was caused by the contact metamorphism, metasomatism, and local melting of the initial volcanosedimentary rocks of the Vakhtalka Sequence of the Ganal Group. The temperature in the inner part of the aureole reached 700–800°C and caused the transformation of the basic volcanic rocks of the sequence into two pyroxene-plagioclase, clinopyroxene-amphibole-plagioclase, and amphibole-plagioclase hornfelses, while sedimentary rocks were converted into garnet-biotite ± cordierite hornfelses. The hornfelsed basic volcanic rocks were locally subjected to metasomatic alteration and magmatic replacement with formation of biotite-orthopyroxene-plagioclase metasomatic bodies containing biotite-orthopyroxene-plagioclase ± garnet veinlets and aggregates. During these processes, sedimentary interlayers were converted into garnet enderbites at 700–800°C and 3.2–4.8 kbar. The comparison of the chemical composition of basic volcanic rocks of the Vakhtalka Sequence and their transformation products indicates that the metasomatic alteration and magmatic replacement correspond to siliceous-alkaline metasomatism (granitization) and cause subsequent and uneven influx of SiO2, Al2O3, Na2O, K2O, Rb, Ba, Zr, Nb, and Cl and removal of Fe, Mg, Mn, Ca, Cr, Co, Ti, Y, and S. REE data on basic metavolcanic rocks, hornfelses, and metasomatites suggest that the processes of hornfelsation, metasomatism, and magmatic replacement of the initial volcanic rocks were accompanied by significant increase in LREE and slight decrease in HREE. The Sr and Nd isotope study of the rocks in the aureole showed that the initial basic volcanic rocks of the Vakhtalka Sequence are isotopically close to both mature island arc tholeiites and mid-ocean ridge basalts. The metasomatic alteration and magmatic replacement of volcanic rocks in the aureole lead to the decrease of 143Nd/144Nd and increase of 87Sr/86Sr approximately parallel to mantle array. Pb isotopic ratios in the studied rocks become more radiogenic from initial metavolcanic rocks to metasomatites.

THE PRINCIPAL GEOLOGICAL FEATURES OF THE MOOIHOEK PLATINIFEROUS DUNITE PIPE, EASTERN LIMB OF THE BUSHVELD COMPLEX, AND SIMILARITIES WITH REPLACED MERENSKY REEF AT THE AMANDELBULT MINE, SOUTH AFRICA

THE PRINCIPAL GEOLOGICAL FEATURES OF THE MOOIHOEK PLATINIFEROUS DUNITE PIPE, EASTERN LIMB OF THE BUSHVELD COMPLEX, AND SIMILARITIES WITH REPLACED MERENSKY REEF AT THE AMANDELBULT MINE, SOUTH AFRICA

Granitization and magmatic replacement in the contact aureole of the Yurchik gabbronorite massif (Ganal Ridge, Kamchatka)

It is shown that formation of high-temperature granulite-like rocks in the contact aureole of the Yurchik gabbronorite intrusion of the Ganal Ridge in East Kamchatka was caused by contact metamorphism, metasomatism, and local melting of the primary sedimentary-volcanogenic rocks of the Vakhtalkinskaya Sequence of the Ganal Group. The temperature in the inner part of the aureole reached 700–800°C and caused transformation of basic rocks into two-pyroxene-plagioclase, clinopyroxene-amphibole-plagioclase, and amphibole-plagioclase rocks, while sedimentary rocks were replaced by garnet-biotite and garnet-cordierite-biotite hornfelses. Locally, basic volcanic hornfelses were subjected to metasomatic alteration with the formation of bodies of biotite-orthopyroxene-plagioclase metasomatites. In the zones of the most intense fluid filtration, the metasomatites experienced local magmatic replacement resulting in the formation of biotite-orthopyroxene-plagioclase ± garnet migmatite veinlets and patches. Bodies of garnet enderbites were formed after sedimentary interlayers at temperatures of 700–800°C and a lithostatic pressure of 3.2–4.8 kbar. The comparison of the chemical composition of the Vakhtalkinskaya basic volcanics and the products of their transformation indicates that, in terms of chemistry, the metasomatic alterations and magmatic replacement correspond to siliceous-alkaline metasomatism (granitization) causing a subsequent and uneven influx of Si, Al, Na, K, Rb, Ba, Zr, Nb, and Cl and removal of Fe, Mg, Mn, Ca, and some trace elements (Cr, Co, Ti, Y, and S). The processes of metamorphism and metasomatism were presumably provoked by highly mineralized mantle fluids that were filtered through magmatic channels that served as pathways for gabbroid magma.

Rock material from the Weddell Sea Floor of the South Ocean

This work is devoted to the results of the joint Russian-German geodynamic research carried out in the Weddell Sea and West Antarctica during cruise ANT-XXII/3 of the R/V Polarstern in 2005. The study of rock samples collected from the sea floor showed that a heterogeneous structure of the Weddell Sea was formed by spatiotemporal combination of the destruction of continental crust, progressive thalassogenesis (oceanization-taphrogenesis), and rifting, as opposed to a spreading origin. High postconsolidation mobility during the destruction stage led to the areal dismembering and high permeability of the continental crust, as well as to tectonomagmatic activation. The main mechanism of reworking of the continental crust is recognized to be the magmatic replacement by basic-ultrabasic mantle material with formation of a secondary oceanic crust and preservation of relics of the continental crust. The Earth’s endogenous activity was driven by transmagmatic fluid flows, which ascended from the melted core and caused transformation of the Earth’s crust and mantle.

Petrology of plagiogranites of the Yenisei Batholith, Western Sayan

The tonalite-plagiogranite (tonalite-trondhjemite) association only occasionally occurs in the form of large granitoid bodies, such as the Yenisei Batholith (>500 km2 in area). The granitoids of the Yenisei Batholith belong to Na-rich tholeiitic rock series and differ from granitoids of the calc-alkaline series in having lower contents of alkalis and alumina (12–14 wt % Al2O3) and low contents of granitophile elements (Rb, Li, Cs, Be, Nb, Ta, and W), Cr, and Ni. The Cr/V (<0.10) and Rb/Sr (0.01–0.1) ratios of these rocks are at a minimum, and their K/Rb (600–1000) and Na/K (5–10) ratios are at a maximum compared to those of the rocks of the most widely spread granitoid batholiths. The plagiogranites typically have REE concentrations higher than those in oceanic plagiogranites and display weakly fractionated REE patterns (La/Yb = 1.4–3.4) with weak (or without) Eu anomalies. The lower initial Sr ratios of these rocks (0.704) and their relatively high concentrations of Pb, Zr, and B testify to the predominantly mantle provenance of their protolithic material. Geological and geochemical characteristics of the Yenisei pluton suggest that its genesis can be considered within the scope of the model of retrograde-type magmatic replacement and that the batholith was produces by the earliest granitization processes in the oceanic crust. The granitic melt was derived at low pressures (<5 kbar) and intermediate temperatures (∼700°C), at the inflow of an aqueous transmagmatic fluid into the magma-generating area and the subsequent fluid-magmatic differentiation. Considering the volumes and compositions of rocks composing the Yenisei Batholith, the latter can be attributed, similarly to other typical granitoid batholiths, to crustal plutons, which differ from both oceanic plagiogranites in ophiolitic belts and continental trondhjemites. The rocks can be regarded as an individual geochemical type of crustal plagiogranites.

The platiniferous dunite pipes in the eastern limb of the Bushveld Complex: Review and comparison with unmineralized discordant ultramafic bodies

The platiniferous dunite pipes of the Bushveld Complex are subvertical, cylindrical bodies (diameters of 180–500m) that crosscut the layered cumulates in the central sector of the eastern limb more or less at right angles. They are restricted to four localities. The Driekop and Mooihoek pipes are emplaced in the Upper Critical zone, the Onverwacht and Twyfelaar pipes occur in the Lower Critical and Lower zones, respectively. The discovery of the Mooihoek pipe by Hans Merensky and his Lydenburg Platinum Syndicate on 15th August 1924, the first occurrence of economic platinum mineralization to be located in the Bushveld Complex, is recounted. We also discuss details of the history of the evaluation and mining of the Driekop, Mooihoek and Onverwacht pipes, which are among the oldest platinum mines in the World, and review the comprehensive geological literature, including the original and very detailed descriptions by Percy Wagner. Discordant bodies are a well known feature of layered tholeiitic intrusions, but the four platiniferous pipes are unusual in that they are semi-concentrically zoned. The principal component is magnesian dunite, a fine-to-medium-grained assemblage of forsteritic olivine (Fo 85–80 ) with accessory Cr-spinel. The magnesian dunite forms comparatively deep-rooted, stock-like bodies. At Onverwacht and Twyfelaar, the stocks are rimmed by irregular bodies of magnesian harzburgite that is gradational into the pyroxenitic wallrocks, but at the noritic-hosted Driekop and Mooihoek pipes the stocks are rimmed by envelopes consisting of a younger discordant assemblage that is dominated by iron-rich wehrlite and iron-rich clinopyroxenite pegmatites. Both the magnesian dunite stocks and pegmatitic envelopes are typically barren; the PGE mineralization is concentrated within cylindrical, parsnip-shaped core-zones (maximum diameter 24m) wholly enclosed within the magnesian dunite stocks. The core-zones at Mooihoek and Onverwacht consist of hortonolite dunite and hortonolite wehrlite pegmatites (hortonolite is a moderately iron-rich olivine; average composition is Fo 50–44 ). The hortonolite dunite-wehrlite pegmatites are unusual, entirely ultramafic rocks (plagioclase is notably absent) that locally include metre-sized crystals of hornblende, phlogopite, and Ti-magnetite-ilmenite, the latter being derived from the alteration and dismembering of chromitite layers in the cumulate wallrocks. The core-zone at the Driekop pipe differs somewhat in that it consists of discrete mineralized pods and veins of iron-rich dunite and iron-rich wehrlite (average composition is Fo 73–72 ) intermingled with barren magnesian dunite. The mineralization at Twyfelaar is also associated with pods and veins of iron-rich dunite-wehrlite, but these are sporadically distributed throughout the magnesian dunite stock and do not constitute a mineable core-zone. There is no consensus as to the origin of the platiniferous pipes, but we suggest the spatial association of primitive and evolved ultramafic rocks within semi-concentrically zoned discordant bodies has not been adequately addressed. Comparisons with unmineralized discordant ultramafic bodies in the Bushveld Complex, specifically the magnesian dunite pipes and the iron-rich ultramafic pegmatite, may resolve this dilemma. Our preliminary conclusions suggest a two-stage magmatic hypothesis is applicable. The magnesian dunite is ascribed to flowage differentiation of new injections of primitive ultramafic magmas within vertical conduits, a hypothesis presented by Scoon and Mitchell (2004) to explain unmineralized magnesian dunite pipes in the central sector of the eastern limb. The core-zones and pegmatitic envelopes possibly formed due to magmatic replacement of either earlier formed magnesian dunite or layered noritic-anorthositic wallrocks; in both cases replacement was triggered by disequilibrium with downward draining iron-rich silicate melts in accordance with the hypothesis of Scoon and Mitchell (1994) for the iron-rich ultramafic pegmatite. Compositional differences between the core-zones and pegmatitic envelopes are ascribed either to preservation of arrested stages of replacement or localized derivation of the iron-rich silicate melts. The PGE were scavenged from reef-style deposits by the iron-rich silicate melts. These melts are only fertile in localities where the earlier-formed magnesian dunite pipes have provided the necessary heat to selectively melt PGE-bearing phases from layered cumulates located higher in the succession, or where the Merensky reef, for example, has been replaced by iron-rich ultramafic pegmatite in situ.

Petrogenesis of Discordant Magnesian Dunite Pipes from the Central Sector of the Eastern Bushveld Complex with Emphasis on the Winnaarshoek Pipe and Disruption of the Merensky Reef

Discordant ultramafic bodies are a significant feature of the Bushveld Complex, South Africa. Many of these bodies form subvertical, cylindrical pipes oriented more or less at right angles to gently dipping layered rocks. Three main types of discordant ultramafic body have been identified: the (unmineralized) magnesian dunite pipes, the iron-rich ultramafic pegmatite, and the platiniferous dunite pipes. The magnesian dunite pipes, such as the Winnaarshoek occurrence, are most prominent in the central sector of the eastern limb. The Winnaarshoek pipe consists of magnesian dunite, which is a fine- to medium-grained assemblage of forsteritic olivine (Fo85‐83) with accessory Cr spinel, together with an extensive rim of clinopyroxenite pegmatite. We emphasize that the mineralogy and chemistry of the magnesian dunite pipes is quite different from those of the iron-rich ultramafic pegmatite (coarsely crystalline rocks composed largely of fayalitic olivine, augite, and FeTi oxides). The platiniferous dunite pipes constitute a third category of discordant ultramafic body because, in addition to Pt-rich core zones, they consist of both magnesian dunite and iron-rich ultramafic pegmatite. In determining the possibility of locating pipe-type platinum ores in other layered intrusions, the spatial association of primitive and differentiated lithologies within the Bushveld occurrences should be recognized. Mining operations exploiting layered reefs in the Bushveld Complex are adversely affected by discordant ultramafic bodies, and identification of the different types has important consequences. Both the unmineralized magnesian dunite and platiniferous dunite pipes may be associated with catastrophic downwarping of the layered cumulate wall rocks, whereas the discordant iron-rich ultramafic pegmatite bodies are considerably less disruptive as they exhibit evidence of passive volume-for-volume replacement. Ultramafic and mafic cumulate layers, including the Merensky reef, are cut out from the downwarped envelope associated with the Winnaarshoek pipe as well as from a juxtaposed megapothole structure. A transitional facies of the Merensky reef is developed on the margins of the megapothole and a localized thick reef facies occurs on the margins of the downwarped envelope. Field relationships demonstrate that the different types of discordant ultramafic body in the Bushveld Complex are unlikely to be consanguineous. The iron-rich ultramafic pegmatite is ascribed to magmatic replacement of layered cumulate wall rocks in response to the downward-draining of differentiated melts derived from within the intrusion, but the magnesian dunite pipes are a product of primary magmatic processes. Mineralogical and chemical similarities between the magnesian dunite pipes and layered olivine cumulates located in the lowermost part of the intrusion are unlikely to be coincidental. The olivine layers accumulated as a result of fractional crystallization of ultramafic magmas episodically injected into the intrusion as basal flows; the magnesian dunite pipes formed by flowage differentiation of similar magma batches within vertical conduits. The only liquidus phases during formation of the olivine layers and magnesian dunite pipes were forsteritic olivine and Cr spinel. Crystallization of orthopyroxene and plagioclase in the vertical conduits was suppressed as a high temperature was maintained by forced upward convection, such that the differentiated residue was ejected upward into the resident magma. Vertical conduits developed in response to syn-Bushveld tectonism that included diapirism, triggered by the anomalously high heat flux associated with the central sector of the eastern limb. The clinopyroxenite pegmatite rim, as well as the downwarping and absence of ultramafic and mafic layers proximal to the Winnaarshoek pipe, is attributed to selective partial melting of ferromagnesian components within noritic wall rocks. Facies changes on the margins of the downwarped structure, as well as within the megapothole, may indicate the nearly synchronous formation of all three features at Winnaarshoek: Merensky reef, megapothole, and magnesian dunite pipe.

Discordant Iron-Rich Ultramafic Pegmatites in the Bushveld Complex and their Relationship to Iron-Rich Intercumulus and Residual Liquids

The iron-rich ultramafic pegmatites comprise a suite of coarse-grained rocks that form discordant bodies within the layered sequence of the Bushveld Complex. These pegmatites, which are considerably more abundant than is generally realized, provide evidence for the differentiation of iron-rich residual melts. The pegmatites are composed largely of iron-rich olivine and clinopyroxene, together with Ti-magnetite and ilmenite. Feldspar is characteristically absent, but paradoxically the pegmatites preferentially replace anorthositic cumulates. Two subgroups are recognized, olivine-clinopyroxene pegmatite and Fe-Ti oxides pegmatite. With increased stratigraphic height the pegmatites become richer in Fe-Ti oxides. Thus, olivine-clinopyroxene pegmatite is prevalent in the Upper Critical and Lower Main Zones, whereas Fe-Ti oxide pegmatite is restricted to the Upper Main and Upper Zones. Zoned pegmatite, with a core of Fe-Ti oxide pegmatite, is transitional between the two subgroups. New whole-rock and electron microprobe analyses of olivine-clinopyroxene pegmatite from the Upper Critical and Lower Main Zones provide convincing evidence that their composition is directly related to height. Cryptic compositional variations are analogous to those displayed by the layered cumulates, but for a given height the pegmatites are always more evolved. Compositions of clinopyroxene in the pegmatites reflect a near-linear relationship with height, whereas cumulus pyroxenes display upward iron-enrichment trends complicated by replenishment and reaction with trapped intercumulus liquid. The olivine-clinopyroxene pegmatite formed by magmatic replacement of earlier-formed cumulates in response to infiltration of iron-rich melts. Suitably differentiated melts comprised intercumulus and residual liquids derived from thick anorthosite layers. The absence of feldspar, although not fully understood, is attributed to an immiscible relationship between dense, iron-rich melts and light, silica-alkali-rich liquids. The latter infiltrated upward to be reincorporated into the resident magma. The iron-rich melts, however, drained down into the crystallizing cumulate pile. Channelling along early-formed fractures and joints was significant, locally resulting in huge pipe-like bodies of pegmatite. The iron-rich melts became increasingly differentiated with height, partly in response to the fractional crystallization of more evolved cumulates. The olivine-clinopyroxene pegmatites are related to infiltration of Fe-Ti oxide-rich silicate melt, whereas Fe-Ti oxide pegmatite is ascribed to Fe-Ti oxide liquid, as originally argued by Bateman (1951). The Bushveld Complex followed the Fenner trend of almost uninterrupted iron enrichment. Evidence of pronounced iron enrichment is, however, manifested in the discordant iron-rich ultramafic pegmatites several thousands of metres below the height at which iron-rich cumulates are observed.

Magmatic replacement of carbonate bodies involving formation of nepheline syenites and other alkalic rocks, with example of Dezhnev massif

In a nepheline syenite massif produced by an infiltrational replacement of limestones by a magmatic granitizing solution, the four recognizable nearly concentric zones (from granite up to and including nepheline syenite), as well as the type and the sequence of the general and the zonal parageneses of minerals, are functionally related to the progressively rising chemical potentials of the alkalies, from the core to the periphery of the massif, in a demonstrable harmony with the phase rule and with the Korzhinskiy's principle of the acid-base interactions. — V.P. Sokoloff