Geochemistry Of Metasediments Research Articles

Detrital zircon geochronology and geochemistry of metasediments from the Vorontsovka terrane: implications for microcontinent tectonics

ABSTRACTThe Vorontsovka terrane (VT) is an important component of the East Sarmatian Orogen (ESO) which divides the Precambrian cores of the Sarmatian and Volgo-Uralia segments of the East European Craton (EEC). The tectonic framework of the VT remains controversial due to poor constraints from geochemical and geochronological studies. In this article we present detrital zircon U–Pb ages and geochemical features of the Precambrian meta-sedimentary rocks from the VT, which occur interlayered with calc-silicate rocks and metabasites. Most of the zircons from metasediments possess oscillatory zoning and high Th/U ratios (>0.2), indicating magmatic provenance. Their 207Pb/206Pb ages cluster around 2093 ± 7, 2126 ± 7, 2158 ± 12, 2189 ± 16, and 2210 ± 31 Ma, correlating with the ages of magmatic zircon cores from the surrounding igneous suites, and reflecting a single tectono-magmatic cycle (~2200–2100 Ma) in the source area. Age of the youngest detrital zircon grain from the metasedimentary rocks and the cores of zircon grains from igneous suites show 207Pb/206Pb ages at 2094 and 2106 Ma, respectively. Together with the largest age clusters of 2126 ± 7 and 2158 ± 12 Ma of the magmatic cores of the detrital zircons, the timing of sedimentation is inferred as ~ 2100–2170 Ma.The metapelites display strong rare earth element fractionation with variable Eu anomalies ((La/Yb)N = 7.0–14.5, Eu/Eu* = 0.49–1.23). In contrast, the calc-silicate rocks and metabasites lack Eu anomalies ((La/Yb)N = 5.2–11.5, Eu/Eu* = 0.87–1.00). The large-ion lithophile (LILE) and high field strength element (HFSE) concentrations of most samples are comparable with those of the upper continental crust (UCC). The rocks possess negative anomalies of Th, Nb, Sr, and Zr relative to UCC. Their high Index of Compositional Variability (0.85–1.32, up to 1.8 in metabasites) and relatively low Chemical Index of Alteration (46.1–70.4) indicate that the metapelitic sediments were immature to weakly immature and probably underwent minor chemical weathering. The protoliths of the metabasites are interpreted as interlayered volcano-sedimentary and pyroclastic material. Relict clastic textures of the VT rocks, their geochemical features, and the grain morphology of detrital zircons suggest that the sediments were derived from intermediate and felsic provenances, which were most likely deposited in an environment with active volcanism. We envisage an active continental margin setting in the southwestern part of the Volgo-Uralia segment of the EEC related to the assembly of the Palaeoproterozoic Columbia supercontinent. Combined with recent data from surrounding terranes of the ESO, our results suggest that the VT represents an accretionary prism along a continental arc within the Sarmatia and Volgo-Uralia oceanic realm in the Palaeoproterozoic.

Monazite geochronology and geochemistry of meta-sediments in the Narryer Gneiss Complex, Western Australia: constraints on the tectonothermal history and provenance

Mt. Narryer and Jack Hills meta-sedimentary rocks in the Narryer Gneiss Complex of the Yilgarn Craton, Western Australia are of particular importance because they yield Hadean detrital zircons. To better understand the tectonothermal history and provenance of these ancient sediments, we have integrated backscattered scanning electron images, in situ U–Pb isotopic and geochemical data for monazites from the meta-sediments. The data indicate multiple periods of metamorphic monazite growth in the Mt. Narryer meta-sediments during tectonothermal events, including metamorphism at ~3.3–3.2 and 2.7–2.6 Ga. These results set a new minimum age of 3.2 Ga for deposition of the Mt. Narryer sediments, previously constrained between 3.28 and ~2.7 Ga. Despite the significant metamorphic monazite growth, a relatively high proportion of detrital monazite survives in a Fe- and Mn-rich sample. This is likely because the high Fe and Mn bulk composition resulted in the efficient shielding of early formed monazite by garnet. In the Jack Hills meta-sediments, metamorphic monazite growth was minor, suggesting the absence of high-grade metamorphism in the sequence. The detrital monazites provide evidence for the derivation of Mt. Narryer sediments from ca. 3.6 and 3.3 Ga granites, likely corresponding to Meeberrie and Dugel granitic gneisses in the Narryer Gneiss Complex. No monazites older than 3.65 Ga have been identified, implying either that the source rocks of >3.65 Ga detrital zircons in the sediments contained little monazite, or that >3.65 Ga detrital minerals had experienced significant metamorphic events or prolonged sedimentary recycling, resulting in the complete dissolution or recrystallization of monazite.

Geochemistry of metasediments from the phyllite–quartzite group, Iberian Pyrite Belt: Provenance, source-area weathering and geotectonic implications

Geochemistry of metasediments from the phyllite–quartzite group, Iberian Pyrite Belt: Provenance, source-area weathering and geotectonic implications

Geochemistry of metasediments from the Precambrian Hapschan Series, eastern Anabar Shield, Siberia

The Precambrian Hapschan Series in the Anabar Shield in northern Siberia is composed of garnet paragneisses (pelitic graywackes), marbles, calciphyres, and calc-silicate rocks (carbonates and carbonate-rich clastic sediments), and a minor amount of enderbite and mafic granulite. Trace element distributions in the paragneisses (especially REE patterns and low Cr and Ni) suggest felsic sources for the graywacke protoliths. With exception of Mn, Pb, and Sr trace elements in Hapschan carbonates and calc-silicates are housed chiefly in chiefly silicate contaminant phases. When normalized to Al 2O 3 to partially compensate for trace elements contained in Al-silicates, most trace elements are depleted in Hapschan carbonates relative to average Phanerozoic marine limestone (PML). Rb, Cr, Ni and Ti are strikingly depleted in Hapschan as well as other Precambrian high-grade carbonates relative to PML. The element depletions in Hapschan marbles may be explained by some combination of loss during granulite-facies metamorphism or by removal in another depository such as BIF. Geologic and geochemical data are consistent with deposition of the Hapschan Series in a cratonic or passive continental margin setting, during the Early Proterozoic. Basement to this series may be the high-grade rocks of the Daldyn and Upper Anabar Series from which a zircon U ion probe age of 3.3 Ga has been reported.

Geochemistry of metasediments in the Hercynian and pre-Hercynian crust of the Schwarzwald, the Vosges and Northern Switzerland

Several units in the pre-Hercynian gneiss basement and Hercynian schists of the Schwarzwald (Southwestern Germany and Northern Switzerland) and the Vosges (France) were characterized geochemically. The samples are attributed to possible sedimentary protoliths by means of their main constituents. Groups of analyses belonging to the defined protoliths are then compared by means of statistical procedures (the Mann-Whitney test and multiple discriminant analysis) which reveal significant differences so that every area is characterized by a specific pattern of major and trace elements. Furthermore, correlations between some units of the Schwarzwald and the Vosges are possible. The gneisses of Ste. Marie-aux-Mines (Vosges) can be closely correlated with those of the Central Schwarzwald, whereas there is a distinct contrast between gneisses of the Central Schwarzwald and gneisses of the Southern Schwarzwald. The schists of Baden-Baden (Schwarzwald) most probably have their counterpart in the schists of Ville in the Vosges.