Geochemistry Of Metabasites Research Articles

Geochemistry of metabasites from the North Shahrekord metamorphic complex, Sanandaj-Sirjan Zone: Geodynamic implications for the Pan-African basement in Iran

Metabasic rocks (amphibolites, garnet-amphibolites and eclogites) of the North Shahrekord Metamorphic Complex (NSMC) of the Sanandaj-Sirjan structural Zone in southwest Iran are part of a ductile shear zone forming lenses or blocks associated with orthogneiss, paragneiss and metagranite. Geochemical characteristics (immobile trace elements and Sr-Nd isotope ratios) of these metabasites reveal similar tholeiitic protoliths. The rocks are slightly altered during metamorphism but no evidence for sea floor alteration was found. Rare earth elements (REE) display relatively flat patterns or a slight enrichment of light REEs. The (La/Yb)cn ratios range from 1.42 to 4.91, the Nb/La ratio from 0.50 to 1.07, and the Sm/Nd ratio from 0.24 to 0.31 resembling to a continental back-arc basin. The (87Sr/86Sr)t isotopic ratios display a large range between 0.7041 and 0.7090, while 143Nd/144Nd(t) varies from 0.5120 to 0.5122 and between εNd(t) 2.5 and 7.2. Immobile trace element patterns and Sr-Nd isotopic characteristics reveal mixed sources of depleted mantle and enriched mantle type II with a contribution from a supra-subduction zone and some effects of crustal contamination. Therefore, the data indicates formation of metabasites in a supra-subduction zone setting during late Neoproterozoic to early Cambrian times. The generation and emplacement of OIB-like magmas in a back-arc environment is likely the result of rollback of oceanic subducting slab. The formation of NSMC Ediacaran to Cambrian basement rocks is approximately contemporary with other arc and back-arc basin basalts reported from Alborz, Central Iran and Turkey. These developed as a result of Proto-Tethys subduction beneath the Gondwana supercontinent during Ediacaran to early Cambrian times.

Petrology and whole-rock geochemistry of metabasites from borehole cores in the Na Zeleném and Pluhův Bor catchments in the Slavkov Forest, western Bohemia

Petrology and whole-rock geochemistry of metabasites from borehole cores in the Na Zeleném and Pluhův Bor catchments in the Slavkov Forest, western Bohemia

Geochemistry of metabasites of the Kolpakov Group of the Sredinny crystalline Massif in Kamchatka

Metabasites (amphibolites, garnet amphibolites, and basic crystalline schists) compose numerous sheeted bodies (often highly boudined) from a few to 100 meters thick in the plagiogneisses and migmatites of the Kolpakov Group. Chemically, they are reconstructed as basalts and picrites that were metamorphosed, as host terrigenous rocks, under the kyanite-sillimanite subfacies of the amphibolite facies (t = 620–650°C, Ps = 5.9–6.9 kbar). Metabasites are dominated by amphibolites and basic crystalline schists distributed throughout the entire section of the Kolpakov Group, whereas garnet amphibolites are more typical of the upper parts of the group, where they are intercalated with amphibolites, basic crystalline schists, plagiogneisses, and quartzites. Metaultrabasites (plagioclase-free amphibolites) occur much more rarely as small boudins up to few meters in size. According to U-Pb SHRIMP zircon dating, the plagiogneiss protolith age corresponds to the end of the Early-Late Cretaceous (90–100 Ma), which is similar in age to the weakly metamorphosed terrigenous deposits of the Kikhchik Group of the Sredinny Range. This allows us to consider the terrigenous rocks of these groups as isofacial sedimentary rocks. The same age (Early-Late Cretaceous boundary) was taken for protoliths of metabasites forming interbeds among metaterrigenous deposits of the Kolpakov Group. The interval of 100−90 Ma coincides with the beginning of the formation of the Okhotsk-Chukotka volcanogenic marginal-continental belt in East Asia. It is shown that the Kolpakov Group possesses the geochemical features of tholeiitic basalts of different geodynamic settings and comprises both typically island arc (low-Ti) and oceanic (moderate to high-Ti) tholeiites associated with ultrabasic volcanic rocks—picrites. Such a chemical peculiarity of basic rocks is typical of the marginal-continental extension zones (pull-apart basin) that were initiated on the sialic crust. It is obvious that similar geodynamic setting of the basite magmatism existed for the Sredinny Range of Kamchatka. The ascent of the mantle matter beneath the extension zone of the continental crust of the sedimentary basin and its intersection by faults that formed simultaneously with the Okhotsk-Chukotka volcanogenic belt served as the beginning of the basite volcanism in the sedimentary basin. They provided an intense fluid effect and a temperature increase in the crust with subsequent granitization and metamorphism of volcanogenic-terrigenous deposits and, finally, the development of the modern structure of the Sredinny Kamchatka Massif. The intense Late Cretaceous basite volcanism and associated granitoid magmatism in Kamchatka were presumably caused by the ascent of mantle plumes bearing hydrogen fluids.

Geochemistry of metabasites of Krasnaya Guba, Belomorian Mobile Belt

Geochemistry of metabasites of Krasnaya Guba, Belomorian Mobile Belt

Geochemistry of metabasites in the north of the Shahrekord, Sanandaj-Sirjan Zone, Iran

Metabasites are exposed in the north of Shahrekord that is a part of the structural zone of Sanandaj-Sirjan in south- western Iran. The metabasites are composed mainly of eclogites, amphibolites and garnet amphibolites. We have conducted a geochemical study of metabasites in order to determine their protolith nature and geodynamic setting. The whole rock chemistry indicates a basalt to basaltic andesite composition for the metabasites. Our geochemical investigation strongly suggests that the metabasites are derived from tholeiitic basalt and were emplaced in a MORB tectonic setting.

A Triassic large igneous province in the Pontides, northern Turkey: geochemical data for its tectonic setting

A strongly deformed and metamorphosed Triassic oceanic seamount(s) and plateau succession extends as an east–west belt for 1100 km along the Pontides of northern Turkey. This succession, known widely as the Nilüfer unit, consists mainly of metabasic lava and tuff–marble–phyllite association including tectonic slices of ultramafic rock and gabbro. According to the conodont findings the unit formed during the Early to Mid-Triassic, and the isotopic age data indicate that it underwent high-pressure greenschist facies metamorphism during the latest Triassic period. The metavolcanic rocks form over 80% of the sequence. The Nilüfer unit covers an area of 120,000 km2, with the volume of mafic lava estimated as 2×105km3. Such a huge volcanic pile has erupted rapidly in a relatively short period during the Early to Mid-Triassic (approx. 10 Ma). Hypotheses for the origin of the Nilüfer unit include a ‘seamount’, ‘intra-arc and/or fore-arc basin’, ‘oceanic plateau’, and ‘Early Triassic rift’. The geochemistry of metabasites and that of relict magmatic clinopyroxenes indicate that there are two main mafic rock groups in the Nilüfer unit displaying tholeiitic and alkaline affinities. No metabasite and clinopyroxene sample display typical orogenic basalt affinity or a subduction signature. Geochemical data obtained in this study are consistent with the derivation of the metabasites from the topmost extrusive layers of an oceanic plateau (LIP) together with the volcanic rocks of seamount(s).

Geochemistry of metabasites from NE Sardinia, Italy: nature of the protoliths, magmatic trend, and geotectonic setting

The metabasites of Montiggiu Nieddu occur as a lenticular body in the Hercynian migmatite complex of NE Sardinia and consist of two major lithological associations: ultramafic amphibolites and plagioclase-banded amphibolites, which are genetically related by processes of cumulate differentiation of an original tholeiitic magma.

Geochemical evidence used to test alternative plate tectonic models for pre-Upper Jurassic (Palaeotethyan) units in the Central Pontides, N Turkey

East–west trending, inferred Palaeotethyan subduction–accretion complexes form much of the pre-Upper Jurassic basement of the Central Pontides. A number of specific tectonic units are present that include variably metamorphosed basic igneous rocks, as dismembered ophiolites, coherent stratigraphical successions, ordered thrust sheets, and blocks in melange. About 200 samples from a c. 200 km long N–S traverse were analysed by a high-precision X-ray fluorescence technique, supplemented by electron microprobe analysis. The basic igneous rocks have not experienced significant chemical mobility after formation and are thus useful for discrimination of tectonic settings of formation using well known binary and ternary plots and mid-ocean ridge basalt (MORB)-normalized ‘spidergrams’. From N to S the main results are that basalts of the Kure Complex were erupted in a subduction-influenced tectonic setting, favouring interpretation as a dismembered ophiolite formed in a marginal basin along the S Eurasian margin. Thick, mainly volcanic, successions of the Cangaldag Complex show a subduction-influenced calc-alkaline composition, above a basement varying from MORB/island arc tholeiite type to high-Mg (boninitic) type, with an inferred origin as an oceanic arc above forearc basement. Meta-basites from the adjacent Bayam Melange indicate a supra-subduction zone origin, possibly as a dismembered forearc ophiolite (possibly related to the Cangaldag). A large overriding thrust sheet of mainly ultramafic rocks, the Elekdag, is interpreted as a supra-subduction zone ophiolite, based on depleted chrome spinel composition (analysed by microprobe). Eclogitic blocks in structurally underlying melange are of mid-ocean ridge (MOR) type. Further south, a northward-dipping, inferred subduction–accretion complex, the Domuzdag–Saraycikdag Complex, includes MOR-type meta-basites in the north, and more ‘enriched’ compositions in the south, of within-plate origin. Lastly, the geochemistry of meta-basites in the south, within the Kargi Complex, suggests a plume-MOR to within-plate type eruptive setting, without continental crustal influence. The geochemical data mainly support a northward rather than southward subduction model for the Palaeotethyan assembly of the Central Pontides, particularly since the Kure ophiolitic basalts in the north exhibit a subduction-influenced chemical signature, in contrast to all the units in the south which lack any identifiable subduction component. Comparisons with published geochemical data from the Karakaya Complex in the SW Pontides reveal similar units, interpreted as accreted MORB lithosphere and oceanic seamounts, but emphasize that the Cangaldag arc is presently the only known Palaeotethyan subduction arc-related unit in Turkey. Copyright © 1999 John Wiley & Sons, Ltd.

Geochemistry of metabasites from the Nevado-Filabride complex, betic cordilleras, Spain: Relics of a dismembered ophiolitic sequence

Major- and trace-element distribution, including REE data, in the metabasites of the Nevado-Filabride Complex (Betic Cordilleras, Spain) suggests that the whole mafic-ultramafic association represents a dismembered ophiolitic sequence. All the analyzed samples have been affected by sea-floor alteration. Yet only the fine-grained rocks have suffered some significant mobilization of the LREE. Life the other Western Mediterranean ophiolites, this sequence is related to the opening of the Liguro-Piemontese basin during Mesozoic times. However, the metabasites from Betic Cordilleras differ from the ophiolitic basalts by higher values of the more incompatible over less incompatible element ratios. The entire variation of these ratios in the Liguro-Piemontese basin is related to a dynamic melting process coupled with a diapiric uprise of the mantle source from the stability field of garnet peridotites. possibly up to the spinel-plagioclase facies transition. In this process, the ophiolites of the Betic Cordilleras would record the deepest stages and the lowest degrees of melting. This feature is related to the peculiar location of these ophiolites at the western end of the Liguro-Piemontese basin.