Timing and geochemistry of potassic magmatism in the eastern part of the Svecofennian domain, NW Ladoga Lake Region, Russian Karelia
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Abstract:
The Puutsaari intrusion is a potassium-rich magmatic complex in the eastern part of the Svecofennian domain close to the Archaean border. The intrusion is generally undeformed in contrast to 1880 /1875 Ma-old country rock tonalitic migmatites and diatectites. The main rock types are: (1) mafic rocks of a gabbro /norite /diorite /quartz monzodiorite series; (2) quartz diorite /tonalite /granodiorite; and (3) coarse-grained microcline granite. The three rock-types intruded coevally forming a peculiar three-component mingling system. The mafic rocks, enriched in K, P, Ba, Sr and LREE, have marked shoshonitic affinities (K2O /1.97 /5.40, K2O/Na2O /0.6 /2.37). On a regional scale they demonstrate transitional geochemistry between less enriched syn-orogenic 1880 Ma-old gabbro /tonalite complexes and strongly enriched 1800 Ma post-collisional shoshonitic intrusions. The microcline granite as well as the tonalite / granodiorite rocks are geochemically similar to crustal anatectic granitoids of the NW Ladoga Lake area. The three rock groups do not form a single trend on Harker-type diagrams and are unlikely to be related by fractional crystallisation or mixing. Zircons from the Puutsaari microcline granite and from the mafic rock series have been dated by ion-microprobe (NORDSIM) at 1868.29/5.9 and 18699/7.7 Ma, respectively. Most zircons recovered from a granite sample had zoned or homogeneous cores and unzoned fractured rims. No statistically significant variation of zircon core and rim ages from the granite was established in the course of this study. Zircons from the mafic rock are unzoned. It is suggested that the mafic rocks at Puutsaari were derived from an enriched mantle shortly after the main Svecofennian collisional event and the roughly 1.88 Ga regional metamorphic culmination. The emplacement of the mafic melt caused anatectic melting of various crustal protoliths and produced coeval granitic and tonalitic compositions. # 2002 Elsevier Science B.V. All rights reserved.Keywords:
Diorite
Quartz monzonite
Titanite
Microcline
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Titanite
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Massif
Migmatite
Titanite
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Felsic
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The southernmost Sierra Nevada offers a view into the deep levels of the Mesozoic batholithic belt which constitutes much of the range to the north, and represents one of the major tectonic features of western North America. The main crystalline rocks of the study area are (1) the intrusive suite of Bear Valley, a middle Cretaceous tonalite batholith complex with coeval gabbroic intrusives, and (2) the gneiss complex of the Tehachapi Mountains, which consists of Early Cretaceous orthogneiss and subordinate paragneiss, with local domains having granulite facies metamorphic assemblages. The orthogneisses are dominantly tonalitic in composition, with significant layers of granodioritic to granitic and lesser dioritic to gabbroic gneiss. Quartz‐rich and psammitic metasedimentary rocks with subordinate marble constitute the main framework assemblage into which the plutonic rocks were emplaced. Field relations demonstrate assimilation of metasedimentary material into the orthogneiss and tonalite batholith magmas, and magma mixing between mafic, tonalitic, and granitic materials. Significant domains of both homogenization and inhomogenization are recognized isotopically within the mixed rocks. U/Pb zircon studies have resolved two major igneous suites and a third suite of postdeformational intrusives, all lying between 90 and 120 Ma. The first suite (gneiss complex of the Tehachapi Mountains) was emplaced at ∼115 Ma, and exhibits penetrative high‐temperature deformation developed at or near solidus conditions. A number of discordance patterns, along with the physical properties of the zircon, suggest minor inheritance of Proterozoic zircon and limited open system behavior in response to a major 100 Ma plutonic event. The 100 ± 3 Ma intrusive suite of Bear Valley crosscuts the older suite, but also exhibits significant synplutonic deformation. Mainly concordant zircon ages indicate the igneous crystallization age, but some discordances occur due to inheritance or entrainment of Proterozoic zircon. The high‐temperature deformation fabrics in these suites and within the metasedimentary framework rocks were crosscut by the granodiorite of Claraville (90 Ma) and pegmatite dikes (∼95 Ma). The granodiorite of Claraville shows strong inheritance of Proterozoic zircon and high initial 87 Sr/ 86 Sr and δ 18 O. Zircon populations from paragneiss and quartzite samples are dominated by Proterozoic detrital grains. Strontium and oxygen isotopic data on the zircon geochronology sample suite suggest simple twocomponent mixing of mantle‐derived gabbroic to tonalitic magmas with partial to complete melt products from the metasedimentary framework rocks. Sedimentary admixtures for some granitic rocks may be as high as 45%, but for the tonalitic batholithic complex are no higher than about 15%. Modeled values of 10–20% metasediment are typical for the orthogneisses. Initial 87 Sr/ 86 Sr correlates directly with δ 18 O, and generally correlates inversely with Sr content. Some subtle complexities in the Sr and O isotopic data suggest the involvement of a third cryptic component. Such a component could be early Phanerozoic ensimatic accretionary terranes that were structurally beneath the observed metasedimentary sequence, or altered oceanic crust and sediments introduced into the mantle magma source area by subduction. One of the initial aims of this study was to seek out remnants of Proterozoic sialic crystalline rocks within the gneiss complex of the Tehachapi Mountains. No such remnants were found, and our studies strongly suggest that sialic components within this link of the Mesozoic batholithic belt were introduced into mantle‐derived magraatic systems by anatexis of continent‐derived sedimentary rocks.
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Porphyritic
Quartz monzonite
Felsic
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Abstract New sites of Mesoproterozoic granitoid magmatism in southern Sweden have been discovered by recent fieldwork and U-Pb zircon age determination. In the area around Stenshuvud, in eastern Skåne, granitic melts were intruded into country-rock gneisses of unknown age at c. 1450 Ma. The 1458plusmn;6 Ma Stenshuvud intrusion is composed of several rock varieties including quartz monzonite, tonalite, monzogranite, and late aplites. The Stenshuvud granitoids proper have glomeroporphyritic textures defined by monomineralic aggregations of feldspar or quartz and polymineralic aggregations of amphibole, biotite and magnetite. At 1442plusmn;9 Ma, the Tåghusa granites were intruded along the contact between the Stenshuvud granitoids and the country-rock gneisses. These granites have streaky appearances, which are due to the presence of short, sub-parallel mafic mineral aggregations. The subsequent intrusion of leucogranites as cross-cutting veins and small bodies was the last phase of the entire magmatic event. All these granitoids appear to be co-genetic and belong to a metaluminous to marginally peraluminous, ferro-potassic, high-K calc-alkaline to shoshonitic sequence. Trace elements indicate similar source materials for both principal intrusions. An εNd-value of -0.6 and a TDM model age of 1.85 Ga indicate involvement of older crustal materials in the generation of the melt(s). The studied granitoids feature both I- and A-type characteristics but are not typical of either type. While the Stenshuvud granitoids were intruded during NE-SW compressional stress that caused shearing and folding, the Tåghusa granites are post-compressional and show no solid-state deformation.
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Amphibole
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Hornblende
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Fractional crystallization (geology)
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Abstract Numerous calc-alkaline granitoid intrusions in the eastern Kunlun Orogen provide a valuable opportunity to constrain the evolution of the orogen. The age and genesis of these intrusions, however, remain poorly understood. The granitoid intrusions near the Balong region, eastern Kunlun Orogen, consist of granodiorite, diorite and syenogranite. The granodiorite contains crystallized segregations, abundant mafic microgranular enclaves (MMEs) and small quartz diorite stocks. In situ zircon U–Pb dating reveals that the granodiorites and quartz diorites were emplaced between 263 and 241 Ma, whereas the syenogranite was produced at c . 231 Ma. The granodiorite and quartz diorite have a calc-alkaline affinity and are metaluminous and Na-rich, with slightly enriched Sr–Nd isotope compositions. The granodiorite is characterized by fractionated REE patterns, whereas the quartz diorite displays a relatively flat REE pattern. The MMEs are consistent with the granodiorite in terms of incompatible elements and Sr–Nd isotope composition. Compared to the granodiorite and diorite, the syenogranite has higher SiO 2 , K, Rb, Th and Sr contents and a lower Rb/Sr ratio. The results presented here, when combined with regional geological data, indicate that the granodiorite and quartz diorite were derived from dehydration melting of mafic lower crustal rocks during the N-directed subduction of the Anyemaqen ocean lithosphere in Late Permian–Middle Triassic times, whereas the syenogranite was produced at a higher crustal level in a syn-collisional setting compared to the granodiorite.
Diorite
Petrogenesis
Geochronology
Adakite
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Anatexis
Leucogranite
Sillimanite
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