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Tom Floden - One of the best experts on this subject based on the ideXlab platform.
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Rapakivi-related basement structures in the Baltic Sea area; a regional approach
GFF, 2000Co-Authors: Vaino Puura, Tom FlodenAbstract:Abstract The largest massifs of Rapakivi granites and related rocks in the western part of the East European Craton are located within the junction area of the Baltic Sea and the Gulfs of Bothnia, Finland and Riga, and along the Gulf of Finland. The Rapakivi massifs are of Palaeoproterozoic to Mesoproterozoic (Subjotnian) age, ranging from about 1.67 to 1.5 Ga. In the framework of the Palaeoproterozoic basement tectonics, all major and minor occurrences of Rapakivi magmatism are areally limited to within the boundaries of the Svecofennian juvenile crustal domain and its frontiers. The most voluminous of the intrusive bodies are located within the originally thickest (55–65 km) primary part of Svecofennia, where they are coupled with crustal thinning to 40–45 km and mafic underplating anomalies. Only minor Rapakivi bodies have intruded the originally thinner (40–45 km) peripheral Palaeoproterozoic crust. The largest igneous subprovinces, namely the Wiborg and Riga-Aland Rapakivi massifs, are furthermore co...
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Rapakivi granite anorthosite magmatism a way of thinning and stabilisation of the svecofennian crust baltic sea basin
Tectonophysics, 1999Co-Authors: Vaino Puura, Tom FlodenAbstract:In the Palaeoproterozoic, a 55–80 km thick layer of crust was formed in the Baltic Sea region during the Svecofennian orogeny at 1.9–1.8 Ga. Today, the remaining crustal thickness is 50–65 km. In the marginal parts of the 1,000,000 km2 Svecofennian Domain, the Moho depth reached 40–75 km, which exceeds the 40 km crustal thickness of the southwestern edge of the Karelian Archaean Domain to which Svecofennia was accreted. The 1.65–1.50 Ga Rapakivi magmatism of the Fennoscandian Province was limited to the Svecofennian Domain. The Rapakivi igneous structures are confined to areally isolated subprovinces, which each have distinct 20–60 Ma long age spans of formation. The petrologic sequences of the subprovinces are alike, although similar petrological events occur at different times in the various subprovinces. The internal structure of a subprovince generally consists of a main igneous polyphase unit in a central position, with smaller felsic intrusions, as well as mafic dike swarms, spread over the peripheries of the subprovince. The Rapakivi magmatism started in juvenile crust which was in a late stage of erosional thinning, 150–300 Ma after the period of maximal thickening. The most extensive Rapakivi igneous activities were associated with crustal thinning down to 45–50 km. As a result, the thinner marginal parts of Svecofennia and the large interior Rapakivi subprovinces were of similar thicknesses as the crust. The primary thickness of the original crust was maintained only in areas void of Rapakivi magmatism. No major events destroyed the Svecofennian and Rapakivi-related crustal structures subsequent to emplacement. Thus, it can be concluded that the extensive Rapakivi igneous activity substantially thinned and stabilised the overthickened portions of the Svecofennian crust.
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Rapakivi-granite–anorthosite magmatism — a way of thinning and stabilisation of the Svecofennian crust, Baltic Sea Basin
Tectonophysics, 1999Co-Authors: Vaino Puura, Tom FlodenAbstract:Abstract In the Palaeoproterozoic, a 55–80 km thick layer of crust was formed in the Baltic Sea region during the Svecofennian orogeny at 1.9–1.8 Ga. Today, the remaining crustal thickness is 50–65 km. In the marginal parts of the 1,000,000 km2 Svecofennian Domain, the Moho depth reached 40–75 km, which exceeds the 40 km crustal thickness of the southwestern edge of the Karelian Archaean Domain to which Svecofennia was accreted. The 1.65–1.50 Ga Rapakivi magmatism of the Fennoscandian Province was limited to the Svecofennian Domain. The Rapakivi igneous structures are confined to areally isolated subprovinces, which each have distinct 20–60 Ma long age spans of formation. The petrologic sequences of the subprovinces are alike, although similar petrological events occur at different times in the various subprovinces. The internal structure of a subprovince generally consists of a main igneous polyphase unit in a central position, with smaller felsic intrusions, as well as mafic dike swarms, spread over the peripheries of the subprovince. The Rapakivi magmatism started in juvenile crust which was in a late stage of erosional thinning, 150–300 Ma after the period of maximal thickening. The most extensive Rapakivi igneous activities were associated with crustal thinning down to 45–50 km. As a result, the thinner marginal parts of Svecofennia and the large interior Rapakivi subprovinces were of similar thicknesses as the crust. The primary thickness of the original crust was maintained only in areas void of Rapakivi magmatism. No major events destroyed the Svecofennian and Rapakivi-related crustal structures subsequent to emplacement. Thus, it can be concluded that the extensive Rapakivi igneous activity substantially thinned and stabilised the overthickened portions of the Svecofennian crust.
O. Eklund - One of the best experts on this subject based on the ideXlab platform.
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COMPOSITIONAL ZONING OF Rapakivi FELDSPARS AND COEXISTING QUARTZ PHENOCRYSTS
The Canadian Mineralogist, 2008Co-Authors: Axel H. E. Müller, O. Eklund, Reimar Seltmann, Bernd Kober, Teresa Jeffries, Andreas KronzAbstract:The compositional zoning of plagioclase-mantled K-feldspar, defining the Rapakivi texture, and of the associated quartz phenocrysts from the Paleozoic Land’s End (U.K.) and Altenberg–Frauenstein (Germany) granites, and the Proterozoic Hammarudda (Finland) granite porphyry, has been investigated by laser-ablation – inductively coupled plasma – mass spectrometry, electron-probe micro-analyses, cathodoluminescence and thermal ionization mass spectrometry in order to investigate the formation of the Rapakivi texture in two different eons. Recent analytical developments and the Ti-in-quartz geothermometer lead to interpretations of the trace-element zoning in quartz phenocrysts coexisting with Rapakivi feldspars. There is an approximate coincidence with Ba-rich growth zones in plagioclase-mantled K-feldspar and Ti-rich zones in coexisting quartz phenocrysts. Both types of zoning indicate increasing temperatures of crystallization. The formation of the plagioclase mantles seems to be related to quartz-resorption events. The inferred temperature of crystallization increased after marginal resorption of quartz phenocrysts by about 82°C in the Altenberg–Frauenstein magma and 44°C in the Hammarudda magma, on the basis of the Ti-in-quartz geothermometer. The temperature increase is correlated positively with the crystallization of plagioclase mantles on the K-feldspar. The quartz phenocryst in the Land’s End granite shows normal core-to-rim zoning of Ti (decreasing concentrations), indicating a gradual decrease in magma temperature. We contend that the increase in the quartz-crystallization temperature of >25°C after a resorption event is indicative for the interaction with mafic magma. Therefore, the interaction of a crystal-saturated granitic magma and a mafic magma is the driving force causing nucleation and crystallization of plagioclase on K-feldspar phenocrysts, even though the Pb isotope, Ba, Sr, and Rb zoning of the mantled K-feldspar phenocryst have not clearly recorded an interaction between granitic and mafic magmas. The frequency of Rapakivi feldspars in the rock correlates with the portion of mafic magma involved in the mingling and mixing process. Isothermal decompression during adiabatic magma ascent may have contributed to the plagioclase mantle formation in the case of the Altenberg–Frauenstein and Hammarudda granites. The rare Rapakivi feldspars in the Land’s End granite developed during an early stage of magmatic evolution; as a result, tracing the conditions of formation of the Rapakivi texture is speculative in that case.
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The origin of Rapakivi texture by sub-isothermal decompression
Precambrian Research, 1999Co-Authors: O. EklundAbstract:Abstract The Rapakivi texture (plagioclase-mantled K-feldspar ovoids, rounded quartz megacrysts and euhedral plagioclase megacrysts in a more fine-grained granitic matrix) has been studied in five Proterozoic (1.64–1.55 Ga) anorogenic Rapakivi granite batholiths in the Fennoscandian Shield with emphasis on mineral stability and inherited cores. K-feldspar ovoids and rounded quartz megacrysts with deep embayments consist principally of a core zone and an outer melted and recrystallized zone. When the K-feldspar ovoids are mantled by plagioclase, the outermost part of the mantle is homogeneous, but towards the K-feldspar, a skeletal texture develops. Intensive parameters obtained from different textural positions show that mineral inclusions in cores of the K-feldspar ovoids and quartz megacrysts were formed at low T (∼680–720°C) and high P (5–6 kbar) conditions, while inclusions in the periphery of ovoids and the plagioclase mantles display high T (∼780°C) and intermediate to low P (3.5–1 kbar). The lowest P is comparable to that during solidification of the matrix. The total water equivalent of the volatile content in the magma has been calculated as ∼2.5%. The amount of solids in the magma at ∼1 kbar has been estimated as ∼40%. Theoretical calculations and experimental data for mineral stabilities in granitic systems suggest that the texture formed when a crystal-saturated (Kfsp+Qtz+Pl ∼60%) and volatile-undersaturated A-type granite magma was transported under approximately constant temperature (760–780°C) from the lower-middle crust (5–6 kbar) to upper crustal levels. According to phase stabilities in the eutectoid granite system, quartz and K-feldspar were resorbed but plagioclase remained stable and precipitated during this sub-isothermal rise of magma. Textural (presence of disequilibrium textures) and mineralogical (presence of different mineral assemblages, including relics) evidence of a sub-isothermal rise of the Rapakivi magmas is better preserved in subvolcanic and contact varieties of Rapakivi granites than in the more deep-seated Rapakivi granites formed by slow cooling.
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a flow foliated ignimbrite related to the aland Rapakivi granite in sw finland
Terra Nova, 1996Co-Authors: O. Eklund, S. Fröjdö, Alexey Shebanov, Kari Ylikyyny, Ulf B. AnderssonAbstract:A flow-foliated felsic ignimbrite constitutes the uppermost lithological unit of the 1.58 Gyr anorogenic magmatic rocks in SW Finland. The ignimbrite is derived from an explosive eruption of hot (≅ 950 °C) phenocryst-bearing A-type (Rapakivi-type granite magma. The ignimbrite is close in composition to subvolcanic Rapakivi granites that occur in the margins of the kand Rapakivi batholith. The subvolcanic granites crystallized under a pressure of ≅ 1 kbar and at temperatures of about 650–700 °C. However, both major and rare earth elements show that the ignimbrite- forming magma was more fractionated than the magma forming the subvolcanic varieties. Supported by evidence of mafic-felsic magma mingling, it is suggested that injection of hot mafic magma into a shallow magma chamber produced the high temperature of the ignimbrite-forming magma. This injection increased the magmatic and the volatile pressure that caused the eruption of the dry felsic magma.
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A flow‐foliated ignimbrite related to the Åland Rapakivi granite in SW Finland
Terra Nova, 1996Co-Authors: O. Eklund, S. Fröjdö, Alexey Shebanov, Kari Yli-kyyny, Ulf B. AnderssonAbstract:A flow-foliated felsic ignimbrite constitutes the uppermost lithological unit of the 1.58 Gyr anorogenic magmatic rocks in SW Finland. The ignimbrite is derived from an explosive eruption of hot (≅ 950 °C) phenocryst-bearing A-type (Rapakivi-type granite magma. The ignimbrite is close in composition to subvolcanic Rapakivi granites that occur in the margins of the kand Rapakivi batholith. The subvolcanic granites crystallized under a pressure of ≅ 1 kbar and at temperatures of about 650–700 °C. However, both major and rare earth elements show that the ignimbrite- forming magma was more fractionated than the magma forming the subvolcanic varieties. Supported by evidence of mafic-felsic magma mingling, it is suggested that injection of hot mafic magma into a shallow magma chamber produced the high temperature of the ignimbrite-forming magma. This injection increased the magmatic and the volatile pressure that caused the eruption of the dry felsic magma.
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Magma mixing, the petrogenetic link between anorthositic suites and Rapakivi granites, Åland,SW Finland
Mineralogy and Petrology, 1994Co-Authors: O. Eklund, S. Fröjdö, B. LindbergAbstract:Der Rapakivi Batholit von Åland besteht aus verschiedenen Graniten, die in ihrer Textur und Zusammensetzung das Feld von quarzporphyritischen über Rapakivigranite mit K-Feldspat-Ovoiden (Wiborgite und Pyterlite) und aplitischen Graniten abdecken. Eine mafische magmatische Serie von Dolerit-Gängen, Noriten, Anorthositen und Monzodioriten ist mit diesen Batholiten eng verbunden. Die erste größere Intrusivphase des Åland, Rapakivi Batholiten besteht aus quarzporphyritischem Hornblende Rapakivi. Dieses Gestein enhält kleine Amöboide, mafische Enklaven, Labradorit Megakristalle, Quarzocelli, Xenolithe mit Amphibolrändern und unregelmäßige Aggregate von granophyrischem Granit. Diese Produkte von Ungleichgewichts-Bedingungen gehen auf die Mischung zwischen basaltischen und granitischen Magmen zurück. Geochemische Modelle zeigen, daß der quarzporphyritische Hornblende-Rapakivi eine Mischung von 15%. eisenreichen Monzodiorit (mafisches Endglied) und 85% Quarz-Feldspatporphyr (felsisches Endglied) ist. Der Monzodiorit stammt von einer Norit-Anorthosit-Monzodiorit Serie. Der QuarzFeldspat-Porphyr entstand durch teilweise Aufschmelzung des Nebengesteines, die durch Intrusionen heißen basischen Magmas verursacht wurden. Strukturelle, texturelle und geochemische Daten zeigen, daß Magmamischung ein wichtiger petrogenetischer Prozeß der Bildung der frühesten Rapakivi-Granit-Intrusionen im Åland, Batholith waren. Petrographische Hinweise auf Magmamischung können auch in der größten Intrusion des Batholiths, dem Wiborg Rapakivi Granit, gefunden werden. Wegen des hohen Anteils felsischer Komponenten ist es schwierig, das Magmamixing in diesen Gesteinen chemisch zu quantifizieren. Zirkon- und Apatitfraktionierungs-Trends weisen jedoch darauf hin, daß auch die WiborgitRapakivis Komponenten aus einer gemischten Quelle enthalten. The Åland, Rapakivi batholith consists of several granites that differ texturally and mineralogically from quartz-porphyritic varieties to Rapakivi varieties with K-feldspar ovoids (wiborgites and pyterlites) and aplitic granites. Closely associated with the batholith there is a mafc magmatic series of dolerite dykes, norites, anorthosites and monzodiorites. The earliest major intrusive phase of the Åland, Rapakivi batholith consists of quartzporphyritic hornblende Rapakivi. This rock contains small amoeboidal mafc enclaves, labradorite megacrysts, quartz ocelli, amphibole-mantled xenoliths and irregular clots of granophyric granite. These disequilibrium features are products of mixing between basaltic and granitic magmas. Geochemical modelling indicates that the quartzporphyritic hornblende Rapakivi is a mixture of 15% hi-Fe monzodiorite (mafic endmember) and 85% quartz-feldspar porphyry (felsic end-member). The monzodiorite is derived from a norite-anorthosite-monzodiorite series. The quartz-feldspar porphyry is produced by partial melting of the country rock caused by intrusions of hot basic magma. Structural, textural and geochemical features suggest that magma mixing was an important petrogenetic process in the formation of the earliest Rapakivi granite intrusions in the Åland, Rapakivi batholith. Petrographic evidence of magma mixing can also be found in the major intrusion of the batholith, the wiborgite Rapakivi granites. Chemically the mixing is difficult to specify in these rocks because of a high proportion of felsic component. Zircon and apatite fractionation trends, however, indicate that the wiborgite Rapakivis also contain components from a mixed source.
Vaino Puura - One of the best experts on this subject based on the ideXlab platform.
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Rapakivi-related basement structures in the Baltic Sea area; a regional approach
GFF, 2000Co-Authors: Vaino Puura, Tom FlodenAbstract:Abstract The largest massifs of Rapakivi granites and related rocks in the western part of the East European Craton are located within the junction area of the Baltic Sea and the Gulfs of Bothnia, Finland and Riga, and along the Gulf of Finland. The Rapakivi massifs are of Palaeoproterozoic to Mesoproterozoic (Subjotnian) age, ranging from about 1.67 to 1.5 Ga. In the framework of the Palaeoproterozoic basement tectonics, all major and minor occurrences of Rapakivi magmatism are areally limited to within the boundaries of the Svecofennian juvenile crustal domain and its frontiers. The most voluminous of the intrusive bodies are located within the originally thickest (55–65 km) primary part of Svecofennia, where they are coupled with crustal thinning to 40–45 km and mafic underplating anomalies. Only minor Rapakivi bodies have intruded the originally thinner (40–45 km) peripheral Palaeoproterozoic crust. The largest igneous subprovinces, namely the Wiborg and Riga-Aland Rapakivi massifs, are furthermore co...
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Rapakivi granite anorthosite magmatism a way of thinning and stabilisation of the svecofennian crust baltic sea basin
Tectonophysics, 1999Co-Authors: Vaino Puura, Tom FlodenAbstract:In the Palaeoproterozoic, a 55–80 km thick layer of crust was formed in the Baltic Sea region during the Svecofennian orogeny at 1.9–1.8 Ga. Today, the remaining crustal thickness is 50–65 km. In the marginal parts of the 1,000,000 km2 Svecofennian Domain, the Moho depth reached 40–75 km, which exceeds the 40 km crustal thickness of the southwestern edge of the Karelian Archaean Domain to which Svecofennia was accreted. The 1.65–1.50 Ga Rapakivi magmatism of the Fennoscandian Province was limited to the Svecofennian Domain. The Rapakivi igneous structures are confined to areally isolated subprovinces, which each have distinct 20–60 Ma long age spans of formation. The petrologic sequences of the subprovinces are alike, although similar petrological events occur at different times in the various subprovinces. The internal structure of a subprovince generally consists of a main igneous polyphase unit in a central position, with smaller felsic intrusions, as well as mafic dike swarms, spread over the peripheries of the subprovince. The Rapakivi magmatism started in juvenile crust which was in a late stage of erosional thinning, 150–300 Ma after the period of maximal thickening. The most extensive Rapakivi igneous activities were associated with crustal thinning down to 45–50 km. As a result, the thinner marginal parts of Svecofennia and the large interior Rapakivi subprovinces were of similar thicknesses as the crust. The primary thickness of the original crust was maintained only in areas void of Rapakivi magmatism. No major events destroyed the Svecofennian and Rapakivi-related crustal structures subsequent to emplacement. Thus, it can be concluded that the extensive Rapakivi igneous activity substantially thinned and stabilised the overthickened portions of the Svecofennian crust.
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Rapakivi-granite–anorthosite magmatism — a way of thinning and stabilisation of the Svecofennian crust, Baltic Sea Basin
Tectonophysics, 1999Co-Authors: Vaino Puura, Tom FlodenAbstract:Abstract In the Palaeoproterozoic, a 55–80 km thick layer of crust was formed in the Baltic Sea region during the Svecofennian orogeny at 1.9–1.8 Ga. Today, the remaining crustal thickness is 50–65 km. In the marginal parts of the 1,000,000 km2 Svecofennian Domain, the Moho depth reached 40–75 km, which exceeds the 40 km crustal thickness of the southwestern edge of the Karelian Archaean Domain to which Svecofennia was accreted. The 1.65–1.50 Ga Rapakivi magmatism of the Fennoscandian Province was limited to the Svecofennian Domain. The Rapakivi igneous structures are confined to areally isolated subprovinces, which each have distinct 20–60 Ma long age spans of formation. The petrologic sequences of the subprovinces are alike, although similar petrological events occur at different times in the various subprovinces. The internal structure of a subprovince generally consists of a main igneous polyphase unit in a central position, with smaller felsic intrusions, as well as mafic dike swarms, spread over the peripheries of the subprovince. The Rapakivi magmatism started in juvenile crust which was in a late stage of erosional thinning, 150–300 Ma after the period of maximal thickening. The most extensive Rapakivi igneous activities were associated with crustal thinning down to 45–50 km. As a result, the thinner marginal parts of Svecofennia and the large interior Rapakivi subprovinces were of similar thicknesses as the crust. The primary thickness of the original crust was maintained only in areas void of Rapakivi magmatism. No major events destroyed the Svecofennian and Rapakivi-related crustal structures subsequent to emplacement. Thus, it can be concluded that the extensive Rapakivi igneous activity substantially thinned and stabilised the overthickened portions of the Svecofennian crust.
O. Tapani Rämö - One of the best experts on this subject based on the ideXlab platform.
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The Suursaari conglomerate (SE Fennoscandian shield; Russia)—Indication of cratonic conditions and rapid reworking of quartz arenitic cover at the outset of the emplacement of the Rapakivi granites at ca. 1.65Ga
Precambrian Research, 2013Co-Authors: Jussi Pokki, O. Tapani Rämö, Jarmo Kohonen, Tom AndersenAbstract:Abstract Ultramature, quartz arenitic conglomerate of the Suursaari Island, Gulf of Finland, Russia is one of the oldest known post-Svecofennian sedimentary rocks on the Fennoscandian shield and provides an important sedimentary record from the period between the collapse and resultant stabilization of the Svecofennian orogen (at ca. 1.8 Ga) and the emplacement of the first anorogenic Rapakivi granites (at ca. 1.65 Ga). The Suursaari conglomerate is considered a remnant from a previously unknown depositional stage involving formation of an ultramature mid-Proterozoic sedimentary cover in purely cratonic conditions. U–Pb detrital zircon ages from the conglomerate peak at 1.88 Ga and 1.90 Ga implying Svecofennian synorogenic granitoids as the dominant source. Svecofennian metasedimentary rocks are the most likely source for the observed Paleoproterozoic (2.05–1.90 Ga) and Archean (3.3–2.5 Ga) zircon ages. Moreover, ca. 7% (ten grains) of the zircons recovered from the quartz arenitic matrix of the Suursaari conglomerate are of Rapakivi age (ca. 1.65 Ga). These constrain a maximum depositional age for the conglomerate and imply that the Svecofennian orogen had reached continental peneplane prior to the Rapakivi stage. The Suursaari conglomerate is thus one of the earliest marks of the initiation of a major intracratonic rifting stage. Rapid recycling of lithified sediment resulting in the deposition of the conglomerate was probably the result of accelerated erosion caused by block movements at the Rapakivi stage (incipient, aborted rifting). The current exposure of the Rapakivi plutons suggests rejuvenation of the topography of the peneplane at ca. 1.65 Ga and significant localized erosion of the Svecofennian roof of the Rapakivi plutons. This eventually led to enhanced erosion that tapped lithologic units beneath the ultramature sedimentary cover and to the deposition of the immature Mesoproterozoic ‘Jotnian’ arkosic redbeds, preserved in fault-bounded blocks in the vicinity of the Rapakivi granite intrusions. The Suursaari conglomerate is compositionally comparable to the quartzites of the extensive cratonic cover sequences of midcontinental North America but it was not deposited in a subsiding, transient extensional basin setting.
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Miocene Rapakivi granites in the southern Death Valley region, California, USA
Earth-Science Reviews, 2005Co-Authors: James P. Calzia, O. Tapani RämöAbstract:Abstract Rapakivi granites in the southern Death Valley region, California, include the 12.4-Ma granite of Kingston Peak, the ca. 10.6-Ma Little Chief stock, and the 9.8-Ma Shoshone pluton. All of these granitic rocks are texturally zoned from a porphyritic rim facies, characterized by Rapakivi textures and miarolitic cavities, to an equigranular aplite core. These granites crystallized from anhydrous and peraluminous to metaluminous magmas that were more oxidized and less alkalic than type Rapakivi granites from southern Finland. Chemical and isotope (Nd–Sr–Pb) data suggest that Rapakivi granites of the southern Death Valley region were derived by partial melting of lower crustal rocks (possibly including Mesozoic plutonic component) with some mantle input as well; they were emplaced at shallow crustal levels (4 km) in an actively extending orogen.
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Radiogenic isotopes of the Estonian and Latvian Rapakivi granite suites: new data from the concealed Precambrian of the East European Craton
Precambrian Research, 1996Co-Authors: O. Tapani Rämö, Hannu Huhma, Juho KirsAbstract:Abstract The Precambrian crystalline bedrock of the Baltic countries is covered under Phanerozoic sedimentary rocks that flank the Fennoscandian Shield in the south. The covered bedrock consists mainly of Palaeoproterozoic medium- to high-grade metamorphic rocks and unmetamorphosed Rapakivi granites and related mafic rocks (mainly gabbros and anorthosites). Our UPb zircon data show that small Rapakivi granite plutons in Estonia are 1630 Ma old and felsic and mafic rocks from the Riga batholith of Latvia and westernmost Estonia are 1580 Ma old. The Estonian plutons have ϵNd (1630 Ma) values ranging from −0.5 to −2.5. The mafic and felsic rocks of the Riga batholith have ϵNd (1580 Ma) values between +0.3 and −0.6, except for a pervasively altered silicic volcanic rock on the northern flank of the batholith with an ϵNd value of −4.6. Initial 87 Sr 86 Sr ratios of the mafic rocks are of the order of 0.7036 to 0.7037 and conform to the evolution of average subcontinental mantle. The Pb isotopic compositions of the felsic and mafic rocks (including the low-ϵNd prophyry) are relatively radiogenic with single-stage μ-values of the order of 8.2. The isotopic characteristics of the Estonian and Latvian Rapakivi granites are similar to those of the classic Rapakivi granites of southern Finland. Our data suggest that the felsic rocks of the Estonian and Latvian Rapakivi suites were derived from Palaeoproterozoic (near-chondritic Nd, relatively high-U Pb ) protoliths. The data also imply that the lower crust and upper mantle in this area are devoid of a major Archaean component and that the lithosphere may become more juvenile southward from the Fennoscandian Shield.
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1700 Ma Shachang complex, northeast China: Proterozoic Rapakivi granite not associated with Paleoproterozoic orogenic crust
Geology, 1995Co-Authors: O. Tapani Rämö, I. Haapala, Matti VaasjokiAbstract:Proterozoic Rapakivi granites are found on all continents and are characteristically associated with Paleoproterozoic crustal domains that predate the granites by
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New U-Pb ages from the Wiborg Rapakivi area: constraints on the temporal evolution of the Rapakivi granite-anorthosite-diabase dyke association of southeastern Finland
Precambrian Research, 1991Co-Authors: Matti Vaasjoki, O. Tapani Rämö, Matti SakkoAbstract:Abstract New U-Pb data on zircons, monazites, and baddeleyite suggest that the Wiborg Rapakivi batholith and associated mafic rocks in southeastern Finland were emplaced mainly between 1650 and 1625 Ma. The earliest anorogenic magmatism related to the intrusion of the Rapakivi granites was the emplacement of some diabase dykes about 1665 Ma ago, while the youngest porphyries intruded the Rapakivi granites at 1615 Ma. The process involved three peaks of diabase activity at 1665, 1645, and 1635 Ma and two major granite events at 1640±5 and 1630±5 Ma, the former of which comprises also the intrusion of minor gabbroic-anorthosite bodies. On the whole, the result was the emplacement of at least 105 km3 of rock material over a period of 50 Ma. Within southern Finland, the Rapakivi magmatism continued until 1540 Ma by emplacement of the West Finnish intrusions, which combined are as extensive as the Wiborg area. Globally, the Proterozoic Rapakivi event probably represents the largest pulse of intracratonic magmatism which occurred during geological history and may be a consequence of rapid growth of continental masses in the Early Proterozoic.
Ulf B. Andersson - One of the best experts on this subject based on the ideXlab platform.
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Evidence of crustal contamination of mafic rocks associated with Rapakivi rocks: an example from the Nordingrå complex, Central Sweden
Geological Magazine, 2001Co-Authors: Anders Lindh, Ulf B. Andersson, Thomas Lundqvist, Stefan ClaessonAbstract:Gabbro and leucogabbro are volumetrically important rocks in the Nordingra Rapakivi complex, East Central Sweden. Plagioclase, ortho- and clinopyroxenes, and olivine dominate the gab- bro. Perthitic orthoclase and quartz are interstitial in relation to the major minerals. The present work is based on 232 major-element and a large number of trace element analyses together with 15 whole rock Sm-Nd isotope analyses of the Nordingra gabbroic rocks. eNd(T) values are negative, -1.1 to -3.2; the most negative values come from the gabbro. Most rocks are enriched in iron, some extremely enriched; none represent primitive mantle melts. The range of Mg-numbers is the same in the gabbro and the leucogabbro. Plots of the Ni-content vs. the Mg-number are scattered, but there is a positive correlation between these two parameters. The primary mantle-normalized ratios between similar trace elements are normally strongly different from one. Values larger as well as smaller than one are found for the same ratio in different rocks. The rare earth elements are only weakly fractionated with small Eu anomalies, negative for the gabbros and positive for the leucogabbros. The primary magma of the Nordingra gabbro-anorthosite is thought to have been derived from a mildly depleted mantle source. Variations in the degree of partial melting of a reasonably homogeneous enriched mantle do not explain the observed chemical evolution. Crystal differentiation can account for some geochemical features, especially the Fe-enrichment. Crustal contamination is required by other characteristics as, for example, the negative eNd(T) values and the irregular and sometimes high primary-mantle normal- ized incompatible trace-element ratios. Al-rich relic material from the formation of the Rapakivi gran- ite melt is another source of assimilation. Most probably contaminants are heterogeneous, including undepleted crust (represented, for example, by early Svecofennian and Archaean granitoids), depleted crust (restitic after Rapakivi magma extraction), and to some degree the associated Rapakivi magma itself. Significant parts of this crust should be Archaean in age.
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a flow foliated ignimbrite related to the aland Rapakivi granite in sw finland
Terra Nova, 1996Co-Authors: O. Eklund, S. Fröjdö, Alexey Shebanov, Kari Ylikyyny, Ulf B. AnderssonAbstract:A flow-foliated felsic ignimbrite constitutes the uppermost lithological unit of the 1.58 Gyr anorogenic magmatic rocks in SW Finland. The ignimbrite is derived from an explosive eruption of hot (≅ 950 °C) phenocryst-bearing A-type (Rapakivi-type granite magma. The ignimbrite is close in composition to subvolcanic Rapakivi granites that occur in the margins of the kand Rapakivi batholith. The subvolcanic granites crystallized under a pressure of ≅ 1 kbar and at temperatures of about 650–700 °C. However, both major and rare earth elements show that the ignimbrite- forming magma was more fractionated than the magma forming the subvolcanic varieties. Supported by evidence of mafic-felsic magma mingling, it is suggested that injection of hot mafic magma into a shallow magma chamber produced the high temperature of the ignimbrite-forming magma. This injection increased the magmatic and the volatile pressure that caused the eruption of the dry felsic magma.
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A flow‐foliated ignimbrite related to the Åland Rapakivi granite in SW Finland
Terra Nova, 1996Co-Authors: O. Eklund, S. Fröjdö, Alexey Shebanov, Kari Yli-kyyny, Ulf B. AnderssonAbstract:A flow-foliated felsic ignimbrite constitutes the uppermost lithological unit of the 1.58 Gyr anorogenic magmatic rocks in SW Finland. The ignimbrite is derived from an explosive eruption of hot (≅ 950 °C) phenocryst-bearing A-type (Rapakivi-type granite magma. The ignimbrite is close in composition to subvolcanic Rapakivi granites that occur in the margins of the kand Rapakivi batholith. The subvolcanic granites crystallized under a pressure of ≅ 1 kbar and at temperatures of about 650–700 °C. However, both major and rare earth elements show that the ignimbrite- forming magma was more fractionated than the magma forming the subvolcanic varieties. Supported by evidence of mafic-felsic magma mingling, it is suggested that injection of hot mafic magma into a shallow magma chamber produced the high temperature of the ignimbrite-forming magma. This injection increased the magmatic and the volatile pressure that caused the eruption of the dry felsic magma.