Constraining genesis and geotectonic setting of metavolcanic complexes: a multidisciplinary study of the Devonian Vrbno Group (Hrubý Jeseník Mts., Czech Republic)
Vojtěch JanoušekJaroslav AichlerPavel HanžlAxel GerdesVojtěch ErbanVladimír ŽáčekVratislav PecinaMarta PudilováKristýna HrdličkováPetr MixaEliška Žáčková
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Felsic
Anatexis
Underplating
Continental arc
Massif
Back-arc basin
Anatexis
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Migmatite
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The Neoarchaean high amphibolite facies Fuping Group consists mainly of meta arkosic leuco leptite, semipelitic biotite leptite gneiss, hornblendic rocks, marbles and calcsilicate rocks, which have been partly subjected to differential anatexis. In the Xiaohuilonggou section, northwest of the Xiaojue town , the hornblendic rocks occur in a relatively high proportion and can be divided into two types. One is thick bedded amphibolite, consisting of plagioclase, hornblende, and locally small amounts of quartz, garnet and biotite, with some accessory minerals such as zircon and apatite. In which, there are some felsic patches of various shapes and scales as the products of the initial stage of anatexis. The anarectic derivatives are low in REE and HFS element contents and somewhat high in LREE/HREE ratio compared with their parent rock. The main reason is that in the anatectic process the accessory minerals , especially zircon, behaved as restitic crystals. The other is bedded or boudinage like amphibolite interbedded with biotite leptite gneiss. It is composed mainly of plagioclase and hornblende, without or only with very little accessory minerals. So being different from what mentioned above, the newly formed felsic derivatives are higher in REE contents and lower in LREE/HREE ratio than the amphibolite. In the both cases, the felsic derivatives are obviously lower in LREE/HREE ratio than the typical Archaean TTG rocks because of that the anatexis of the Fuping Group happened under middle low pressure conditions. It is also observed that the thick bedded amphibolite and its anatectic product are quite different in Nd isotopic compositions, showing the preservation of isotopic disequilibrium. Some felsic materials nearby the bedded amphibolite, being very high in LREE/HREE ratio and quite different in Nd isotopic composition from the amphibolite, are considered to be formed by the anatexis of the biotite leptite gneiss.
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ABSTRACT A combined petrological, geochronological and geochemical study was carried out on felsic veins and their host rocks from the North Qaidam ultrahigh‐pressure (UHP) metamorphic terrane in northern Tibet. The results provide insights into partial melting of deeply subducted continental crust during exhumation. Partial melting is petrograpically recognized in metagranite, metapelite and metabasite. Migmatized gneisses, including metagranite and metapelite, contain microstructures such as granitic aggregates with varying outlines, small dihedral angles at mineral junctions and feldspar with magmatic habits, indicating the former presence of felsic melts. Partial melts were also present in metabasite that occurs as retrograde eclogite. Felsic veins in both the eclogites and gneisses exhibit typical melt crystalline textures such as large euhedral feldspar grains with straight crystal faces, indicating vein crystallization from anatectic melts. The Sr–Nd isotope compositions of felsic veins inside gneisses suggest melt derivation from anatexis of host gneisses themselves, but those inside metabasites suggest melt derivation from hybrid sources. Felsic veins inside gneisses exhibit lithochemical compositions similar to experimental melts on the An–Ab–Or diagram. In trace element distribution diagrams, they exhibit parallel patterns to their host rocks, but with lower element contents and slightly positive Eu and Sr anomalies. The geochemistry of these felsic veins is controlled by minerals that would decompose and survive, respectively, during anatexis. Felsic veins inside metabasites are rich either in quartz or in plagioclase with low normative orthoclase. In either case, they have low trace element contents, with significantly positive Eu and Sr anomalies in plagioclase‐rich veins. Combined with cumulate structures in some veins, these felsic veins are interpreted to crystallize from anatectic melts of different origins with the effect of crystal fractionation. Nevertheless, felsic veins in different lithologies exhibit roughly consistent patterns of trace element distribution, with variable enrichment of LILE and LREE but depletion of HFSE and HREE. There are also higher contents of trace elements in veins hosted by gneisses than veins hosted by metabasites. Anatectic zircon domains from felsic veins and migmatized gneisses exhibit consistent U–Pb ages of c . 420 Ma, significantly younger than the peak UHP eclogite facies metamorphic event at c . 450–435 Ma. Combining the petrological observations with local P–T paths and experimentally constrained melting curves, it is inferred that anatexis of UHP gneisses was caused by muscovite breakdown while anatexis of UHP metabasites was caused by fluid influx. These UHP metagranite, metapelite and metabasite underwent simultaneous anatexis during the exhumation, giving rise to anatectic melts with different compositions in various elements but similar patterns in trace element distribution.
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Abstract The Berere HTHP Complex belt in Maevatanana area of north–central Madagascar formed in the ∼ 2.5 Ga orogeny and underwent high temperature (up to 1050°C) and high pressure (up to 11.5 kbar) granulite facies metamorphism. Then a widespread anatexis took place and numerous widely distributed felsic leucosomes formed. The majority of these leucosomes are parallel to the schistosity of the complex or are present as stockworks, as thin layers, or as lenses at different scales in the host rocks. Here, we report new petrographic data, zircon LA‐ICP‐MS U‐Pb ages, and Lu–Hf isotopic data for felsic leucosomes within this complex. Anatexis, as identified by the petrological study of felsic leucosomes in the field and in thin sections, involved initial ternary feldspar exsolving to produce antiperthite and a quartz + plagioclase ± K‐feldspar + sericite mineral assemblage around feldspar grain boundaries. Dissolution is apparent along muscovite grain boundaries, and residual sericite is present around the margins of feldspar and quartz, all suggesting that anatexis was driven by reactions involving muscovite. Zircon U–Pb dating indicates that the felsic leucosomes within the complex formed at 2467–2369 Ma. The majority of samples have positive ∊ Hf ( t ) values, although a few have negative values, suggesting their formation from magmas predominantly sourced from the depleted mantle, possibly with the involvement of minor amounts of crustal materials. Two‐stage Hf model ages and ∊ Hf ( t ) values for these samples are consistent with those for gneisses of the basement, indicating that the felsic leucosomes were formed by the anatexis of gneisses and both of their protolith formed during the formation of continental crust in Meso‐Neoarchean (ca. 3.1–2.7 Ga). As such, the crystallization age of the felsic leucosome (∼2.4 Ga) represents the timing of regional anatexis and a change to post‐orogenic tectonism. And this anatexis is also corresponds to the thermal event in Dharwar craton in India which has a pronounced similar Precambrian geology with Madagascar, providing an important constraints on the correlation of the two continental fragments.
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Igneous differentiation
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According to the thermal structure of the lithosphere, the continental crust is located within the range of the MBL (mechanical boundary layer) and its heat transfer is controlled by thermal conductivity of the rocks in the crust. Therefore, based on the theory of thermal conductivity and the same thermophysical parameters as those obtained in previous studies, we calculated the thermal state of the crust underplated by basaltic magma. The result suggests that a felsic magma layer, thinner than 250 m, would be generated at 850℃ by the emplacement of a basalt magma sill (1200℃, 500 m thick) into continental crust during a very short period (≤ 2700 a). Geological and geochemical characteristics and isotopic chronological data of the bimodal volcanic rock association in the Changpu Formation of the Yutian Group in the Longnan-Xunwu area of Southern Jianxi suggest that the rhyolite of this association might be derived directly from partial melting of the upper crust caused by emplacement of quartz tholeiite magma. The Mesozoic continental crust in SE China (≤50 km in thickness) was located in the MBL and the amount and formation age of the felsic magma generated by the underplating of basaltic magma would be constrained by heat conductivity of the rocks in the crust. Granite and rhyolitic rocks dominate (90%) the Mesozoic igneous rock in SE China, while basaltic rocks are minor with their formation ages different from those of felsic rocks. Therefore, the formation of the extensive felsic igneous rocks in SE China may have not been caused by the underplating of basalt.
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