Initial tectonothermal evolution within the Scandinavian Caledonide Accretionary Prism: constraints from 40Ar/39Ar mineral ages within the Seve Nappe Complex, Sarek Mountains, Sweden
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In the Caledonide orogen of northern Sweden, the Seve Nappe Complex is dominated by rift facies sedimentary and mafic rocks derived from the Late Proterozoic Baltoscandian miogeocline and offshore‐continent–Iapetus transition. Metamorphic breaks and structural inversions characterize the nappe complex. Within the Sarek Mountains, the Sarektjåkkå Nappe is composed of c. 600‐Ma‐old dolerites with subordinate screens of sedimentary rocks. These lithological elements preserve parageneses which record contact metamorphism at shallow crustal levels. The Sarektjåkkå Nappe is situated between eclogite‐bearing nappes (Mikka and Tsäkkok nappes) which underwent high‐ P metamorphism at c. 500 Ma during westward subduction of the Baltoscandian margin. 40 Ar/ 39 Ar mineral ages of c. 520–500 Ma are recorded by hornblende within variably foliated amphibolite derived from mafic dyke protoliths within the Sarektjåkkå Nappe. Plateau ages of 500 Ma are displayed by muscovite within the basal thrust of the nappe and are consistent with metamorphic evidence which indicates that the nappe escaped crustal depression as a result of detachment at an early stage of subduction. Cooling ages recorded by hornblende from variably retrogressed eclogites in the entire region are in the range of c. 510–490 Ma and suggest that imbrication of the subducting miogeocline was followed by differential exhumation of the various imbricate sheets. Hornblende cooling ages of 470–460 Ma are recorded from massive dyke protoliths within the Sarektjåkkå Nappe. These are similar to ages reported from the Seve Nappe Complex in the central Scandinavian Caledonides. Probably these date imbrication and uplift related to Early Ordovician arrival of outboard terranes (e.g. island‐arc sequences represented by structurally lower horizons of the Köli Nappes). Metamorphic contrasts and the distinct grouping of mineral cooling ages suggest that the various Seve structural units are themselves internally imbricated, and were individually tectonically uplifted through argon closure temperatures during assembly of the Seve Nappe Complex. The cooling ages of 520–500 Ma recorded within Seve terranes and along terrane boundaries of the Sarek Mountains provide evidence of significant accretionary activity in the northern Scandinavian Caledonides in the Late Cambrian–Early Ordovician.Keywords:
Hornblende
Imbrication
Protolith
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Massif
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Phengite
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After many studies on geochronology, geochemistry and petrology of the eclogite from Bixiling have been done,its formation and metamorphic history are well known now. However,little work about elements and isotopes changes during the UHP metamorphism was conducted. The garnet Pb isotopes from Bxiling eclogites were studied by the stepwise-leaching technique in this paper. The Pb isotopic compositions obtained by different individual steps are various; However,the evolution trends of Pb isotopic compositions of different garnets and different progresses are almost consistent. The Pb-Pb isochron ages are about 3 billion years,which are different from the ages of protolith formation and UHP metamorphism obtained by precursors. Although the ages perhaps have no geologic significance,the results of stepwise-leaching may reflect that Pb isotopic compositions of the eclogite from Bixiling are mixture of materials from mantle and upper crust. In brief,the protolith of the eclogite was originated from mantle materials,and during rapid exhumation after the Triassic collison,the eclogites were matesomatized by the hydrous fluid with the upper crust Pb isotopic compositions.
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Isochron dating
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Abstract The Mitchell Dam Amphibolite is the southwesternmost of a series of amphibolite bodies in the Eastern Blue Ridge of central east Alabama. Several studies have produced various types of geochemical and geochronologic data for these amphibolites, but no systematic data on a continuous section has been available. Two test cores taken in the area where Mitchell Dam was constructed yielded a section about 138 ft thick but with 21.5 ft missing between the two cores. The lower 98 ft consist of massive hornblende-plagioclase amphibolite displaying some textural variations. The upper 32 ft consist of a metasedimentary unit that commonly shows some cataclasis. Metamorphic mineral assemblages in the rocks are typical of amphibolite facies metamorphism. Quartz, biotite, plagioclase, clinozoisite, and garnet are the common minerals in the metasedimentary units; in addition, green hornblende is added to the assemblage in metagraywackes. Hornblende and plagioclase dominate the amphibolite. In this study, 65 samples were taken at an average spacing of 1.7 ft (range from 0.5 to 7 ft) to build profiles to show chemical variation throughout the core and determine the protolith of the amphibolites. The amphibolite has bulk compositions characteristic of subalkaline basalts. The compositions follow a tholeiitic trend on an AFM diagram. Rare earth patterns are relatively flat, and high field strength elements are notably depleted, particularly zirconium. Whole rock and trace element geochemistry suggests a tholeiitic protolith formed over a subduction zone, most likely generated in the very early stages of subduction.
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Hornblende
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Eclogite was the most common metamorphic rock in orogenic belt, it contained abundant geodynamic information. According to the contact relationship of country rocks, the eclogite from 0~2 000 m in the main hole of the Chinese Continental Scientific Drill (CCSD) could be classified into three types: symbiotic eclogite gneiss;eclogite adjacent to ultrabasic rocks and symbiotic eclogite ultrabasic rocks. Compared to the geochemical differences with the three types of eclogites, the authors considered that the protoliths of these rocks were probably the basic-untrabasic complex evolved from the single magma chamber by cumulation and fractional crystallization. The main source materials of these protoliths were derived from the enriched mantle by partial melting in subduction zone of oceanic crust downthrusted to the mainland under a back-arc extensional magmatism.
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