Sediment transport to the Laptev Sea (Siberian Arctic) during the Holocene — evidence from the heavy mineral composition of fluvial and marine sediments

Heavy mineral studies of East Siberian river sediments, Laptev Sea surface sediments, and a sediment core of the western Laptev Sea were carried out in order to reconstruct the pathways of modern and ancient sediment transport from the Siberian hinterland to the Laptev Sea. The modern heavy mineral distribution of Laptev Sea surface sediments reflects mainly the riverine input. While the eastern and central part of the Laptev Sea is dominated by amphibole, which is supplied by the Lena River, the western part is dominated by pyroxene imported from the Siberian Trap basalts by the Khatanga River. The distribution of garnet and opaque minerals is additionally influenced by hydrodynamic processes. As a consequence of their high density, these minerals are predominantly deposited close to the river mouths. Heavy mineral and sedimentological studies of a sediment core of the western Laptev Sea were applied to reconstruct the postglacial history of the shelf area during the last 11 ka. In the lowermost interval of the core (> c. 10 ka), high accumulation rates and a heavy mineral composition similar to that of the modern Khatanga river indicate fluvial conditions. Additionally, the high mica content in this interval may indicate meltwater inflow from the Byrranga mountains. Strong variations in accumulation rates, grain-size distribution, and heavy mineral composition are observed in the time interval between c. 10 and 6 ka, which represents the main transgression of the Laptev Sea shelf. During the uppermost interval (<6 ka), rather stable conditions similar to the modem situation prevailed.     

Petrology of Whiteschists and Associated Rocks at Mautia Hill (Tanzania): Fluid Infiltration during High-Grade Metamorphism?

Talc–kyanite schists (whiteschists), magnesiohornblende–kyanite–talc–quartzschists and enstatite–sapphirine–chlorite schistsoccur at Mautia Hill in the East African Orogen of Tanzania.They are associated with metapelites and garnet–clinopyroxene–quartzmetabasites. Geobarometry (GASP/GADS equilibria) applied tothe latter two rock types indicates a peak pressure of P = 10–11kbar. These results are confirmed by the high fO₂ assemblagehollandite–kyanite–quartz and late-stage manganianandalusite that contains up to 19·5 mol. % Mn₂SiO₅. Maximumtemperatures of T = 720°C are inferred from late-stage yoderite+ quartz. A clockwise P–T evolution is constrained byprograde kyanite inclusions in metapelitic garnet and late-stagereaction rims of cordierite between green yoderite and talcthat reflect conditions at least 3–4 kbar below the peakpressure. Oxidizing conditions are recorded throughout the metamorphichistory of the whiteschists and chlorite schists, as indicatedby the presence of haematite coexisting with pseudobrookiteand/or rutile. Increasing water activity near peak pressuresis thought to have led to the breakdown of the high-pressureassemblages (Tlc–Ky–Hem and Mg-Hbl–Ky–Hem)and the subsequent formation of certain uncommon minerals, e.g.yellow sapphirine, Mn–andalusite, green and purple yoderite,piemontite and boron-free kornerupine. The proposed increasein water activity is attributed to fluid infiltration resultingfrom the devolatilization of underlying sediments during metamorphism. KEY WORDS: fluid infiltration; high-pressure amphibolite facies; East African Orogen; Pan-African; whiteschist     

The Petrology of Basanite-Tephrite Intrusions in the Erongo Complex and Implications for a Plume Origin of Cretaceous Alkaline Complexes in Namibia

Basanite intrusions from the Early Cretaceous Erongo complex,Namibia, have compositions consistent with near-primary mantlemelts derived from a depth of at least 100 km. These rocks providea key reference for the mantle component(s) involved in breakup-relatedmagmatism in this region. Initial Sr–Nd–Pb isotoperatios of the Erongo basanites and associated tephrites andphonotephrites (⁸⁷Sr/⁸⁶Sr = 0·70425–0·70465;     

Petrology of Felsic Granulites, Metapelites, Metabasics, Ultramafics, and Metacarbonates from Southern Calabria (Italy): Prograde Metamorphism, Uplift and Cooling of a Former Lower Crust

A high-grade metamorphic terrane in the southern part of theCalabrian massif (South Italy) has been petrographically mappedand the dominant rock types petrologically investigated. Bothmethods of investigation have led to the recognition of a continuoussection through a former lower crust which is 7 km thick. Itslower part consists predominantly of metabasic rocks togetherwith minor felsic granulites, its upper part of metapeliteswith minor metabasic and metacarbonate rocks. The rocks experienced a common two-stage prograde metamorphicevolution in which the second stage occurred after the lastpenetrative deformation. The prograde metamorphism which, accordingto radiometric dates, ended in late Hercynian time, was of themedium-pressure type of Miyashiro (1961), and equilibrationoccurred in the ‘medium-pressure granulite field’(characterized by the instability of olivine-plagioclase aswell as garnet-clinopyroxene-quartz). Estimates of the highest_P—_T conditions of prograde metamorphism give 7–8kb and approximately 800°C at the base, but 5–6 kband 650–700°C at the top of the section, at whichthe paragenesis staurolite-quartz indicates the transition tothe amphibolite facies. The existence of a metamorphic gradientin the lower crust section is demonstrated by the systematicchange in the compositions of ferro-magnesian minerals in divariantmetapelitic assemblages. The metamorphic evolution during the excavation history of theformer lower crust has been reconstructed using the numerousdisequilibrium reaction textures preserved in most rock types.The highest metamorphic conditions ended with a pressure decreaseof approximately 1.5 to 2 kb, which was followed by a periodof quasi-isobaric cooling in the middle crust. During this cooling,the stability field of the ‘high-pressure granulites’(garnet-clinopyroxene-quartz) was reached. The pressure decrease, which induced the end of the high-temperaturehistory of the lower crust, is interpreted as reflecting theerosion of the uppermost crustal levels as a response to overlappingof large crustal segments during the Hercynian orogeny. Consequently,the deduced P—T path of the upper, i.e. overthrust crustalsegment is thought to have been tectonically controlled.