Dmitry Konopelko

1.8 Ga Svecofennian post-collisional shoshonitic magmatism in the Fennoscandian shield

Abstract At least 14 small (1–11 km across) 1.8 Ga Svecofennian post-collisional bimodal intrusions occur in southern Finland and Russian Karelia in a 600-km-long belt from the Aland Islands to the NW Lake Ladoga region. The rocks range from ultramafic, calc-alkaline, apatite-rich potassium lamprophyres to peraluminous HiBaSr granites, and form a shoshonitic series with K 2 O+Na 2 O>5%, K 2 O/Na 2 O>0.5, Al 2 O 3 >9% over a wide spectrum of SiO 2 (32–78%). Although strongly enriched in all rocks, the LILE Ba and Sr and the LREE generally define a decreasing trend with increasing SiO 2 . Depletion is noted for HFSE Ti, Nb and Ta. Available isotopic data show overlapping values for lamprophyres and granites within separate intrusions and a cogenetic origin is thus not precluded. Initial magmas (Mg#>65) in this shoshonitic association are considered to be generated in an enriched lithospheric mantle during post-collisional uplift some 30 Ma after the regional Svecofennian metamorphic peak. However, prior to the melting episode, the lithospheric mantle was affected by carbonatite metasomatism; more extensively in the east than in the west. The melts generated in the more carbonate-rich mantle are extremely enriched in P 2 O 5 ∼4%, F∼12,000 ppm, LILE: Ba∼9000 ppm, Sr∼7000 ppm, LREE: La∼600 ppm and Ce∼1000 ppm. The parental magma underwent 55–60% fractionation of biotite+clinopyroxene+apatite+magnetite+sphene whereupon intermediate varieties were produced. After further fractionation, 60–80%, of K-feldspar+amphibole+plagioclase±(minor magnetite, sphene and apatite), leucosyenites and quartz-monzonites were formed. In the west, where the source was less affected by carbonatite metasomatism, calc-alkaline lamprophyres (vogesites, minettes and spessartites) and equivalent plutonic rocks (monzonites) were formed. Removal of about 50% of biotite, amphibole, plagioclase, magnetite, apatite and sphene produced peraluminous HiBaSr granites. The impact of crustal assimilation is considered to be low. At about 1.8 Ga, the post-collisional shoshonitic magmatism brought juvenile material, particularly enriched in alkalis, LILE, LREE and F, into the crust. Although areally restricted, the regional distribution of the post-collisional intrusions may indicate that larger volumes of 1.8 Ga juvenile material resides in unexposed parts of the crust.

Geodynamics of late Paleozoic magmatism in the Tien Shan and its framework

The Devonian-Permian history of magmatic activity in the Tien Shan and its framework has been considered using new isotopic datings. It has been shown that the intensity of magmatism and composition of igneous rocks are controlled by interaction of the local thermal upper mantle state (plumes) and dynamics of the lithosphere on a broader regional scale (plate motion). The Kazakhstan paleocontinent, which partly included the present-day Tien Shan and Kyzylkum, was formed in the Late Ordovician-Early Silurian as a result of amalgamation of ancient continental masses and island arcs. In the Early Devonian, heating of the mantle resulted in the within-plate basaltic volcanism in the southern framework of the Kazakhstan paleocontinent (Turkestan paleoocean) and development of suprasubduction magmatism over an extensive area at its margin. In the Middle-Late Devonian, the margins of the Turkestan paleoocean were passive; the area of within-plate oceanic magmatism shifted eastward, and the active margin was retained at the junction with the Balkhash-Junggar paleoocean. A new period of active magmatism was induced by an overall shortening of the region under the settings of plate convergence. The process started in the Early Carboniferous at the Junggar-Balkhash margin of the Kazakhstan paleocontinent and the southern (Paleotethian) margin of the Karakum-Tajik paleocontinent. In the Late Carboniferous, magmatism developed along the northern boundary of the Turkestan paleoocean, which was closing between them. The disappearance of deepwater oceanic basins by the end of the Carboniferous was accompanied by collisional granitic magmatism, which inherited the paleolocations of subduction zones.Postcollision magmatism fell in the Early Permian with a peak at 280 Ma ago. In contrast to Late Carboniferous granitic rocks, the localization of Early Permian granitoids is more independent of collision sutures. The magmatism of this time comprises: (1) continuation of the suprasubduction process (I-granites, etc.) with transition to the bimodal type in the Tien Shan segment of the Kazakhstan paleocontinent that formed; (2) superposition of A-granites on the outer Hercynides and foredeep at the margin of the Tarim paleocontinent (Kokshaal-Halyktau) and emplacement of various granitoids (I, S, and A types, up to alkali syenite) in the linear Kyzylkum-Alay Orogen; and (3) within-plate basalts and alkaline intrusions in the Tarim paleocontinent. Synchronism of the maximum manifestation and atypical combination of igneous rock associations with spreading of magmatism over the foreland can be readily explained by the effect of the Tarim plume on the lithosphere. Having reached maximum intensity by the Early Permian, this plume could have imparted a more distinct thermal expression to collision. The localization of granitoids in the upper crust was controlled by postcollision regional strike-slip faults and antiforms at the last stage of Paleozoic convergence.

Timing and geochemistry of potassic magmatism in the eastern part of the Svecofennian domain, NW Ladoga Lake Region, Russian Karelia

Abstract The Puutsaari intrusion is a potassium-rich magmatic complex in the eastern part of the Svecofennian domain close to the Archaean border. The intrusion is generally undeformed in contrast to 1880–1875 Ma-old country rock tonalitic migmatites and diatectites. The main rock types are: (1) mafic rocks of a gabbro–norite–diorite–quartz monzodiorite series; (2) quartz diorite–tonalite–granodiorite; and (3) coarse-grained microcline granite. The three rock-types intruded coevally forming a peculiar three-component mingling system. The mafic rocks, enriched in K, P, Ba, Sr and LREE, have marked shoshonitic affinities (K2O=1.97–5.40, K2O/Na2O=0.6–2.37). On a regional scale they demonstrate transitional geochemistry between less enriched syn-orogenic 1880 Ma-old gabbro–tonalite complexes and strongly enriched 1800 Ma post-collisional shoshonitic intrusions. The microcline granite as well as the tonalite–granodiorite rocks are geochemically similar to crustal anatectic granitoids of the NW Ladoga Lake area. The three rock groups do not form a single trend on Harker-type diagrams and are unlikely to be related by fractional crystallisation or mixing. Zircons from the Puutsaari microcline granite and from the mafic rock series have been dated by ion-microprobe (NORDSIM) at 1868.2±5.9 and 1869±7.7 Ma, respectively. Most zircons recovered from a granite sample had zoned or homogeneous cores and unzoned fractured rims. No statistically significant variation of zircon core and rim ages from the granite was established in the course of this study. Zircons from the mafic rock are unzoned. It is suggested that the mafic rocks at Puutsaari were derived from an enriched mantle shortly after the main Svecofennian collisional event and the roughly 1.88 Ga regional metamorphic culmination. The emplacement of the mafic melt caused anatectic melting of various crustal protoliths and produced coeval granitic and tonalitic compositions.

Nd isotope variation across the Archaean-Proterozoic boundary in the North Ladoga Area, Russian Karelia

Abstract In order to investigate the boundary between Archaean crust of the Karelian Craton and Paleoproterozoic crust of the Svecofennian Orogen in the area north and west of the lake Ladoga (the North Ladoga Area) 24 samples of mostly granitoid rocks, collected along the 100 km long profile across the inferred suture, were analysed for their Nd isotopic composition. Ten previously published Nd data were incorporated in the dataset. It was established that gneisses from so-called “mantled domes” north of the suture have very low εNd values and represent reworked Archaean basement. North of the Kirjavalahti Dome, the presence of Archaean basement under Kalevian sedimentary cover is registered by abundant Archaean cores in zircons, revealed by a SHRIMP study, and by low εNd values in the rocks of the 1874±13 Ma-old Alattu dyke complex. The transition from Archaean to Proterozoic crust is registered by a shift from very low to Bulk-Earth-type εNd values and occurs within a 10-20 km-wide zone north of the inferred suture. The rocks of the Svecofennian Orogen south of the suture are characterized by relatively uniform initial Nd isotopic composition (average εNd value at 1880-1860 Ma +0.7, n = 17). In terms of Nd isotopic composition, this domain, the Lahdenpohja Domain, is comparable to the Svecofennian terranes to the west: the Central Finland Granitoid Complex and the Accretionary Arc Complex of Southern Finland. The Primitive Arc Complex of Central Finland has, in contrast, significantly more juvenile Nd isotopic composition.

Lead sources in ore deposits and magmatic rocks of the Tien Shan and Chinese Altay

The Altaid orogen consists of Paleozoic subduction-accretion complexes and magmatic arcs as well as narrow Precambrian basement slivers which were accreted, consolidated and then deformed during Paleozoic collisions and subsequent Alpine-Himalayan deformations between the East European craton in the West, the Siberian craton in the East, and the Alai-Tarim and Karakum microcontinents in the South. The Altaids are the site of abundant plutonism and host some of the largest gold deposits in the world, especially of the orogenic gold type. Over 100 new lead isotope data show that each one of the Altaid domains investigated is characterized by distinct lead isotope signatures and that there is an W-E Pb isotope gradient suggesting a progressive transition from a continental crust environment in the west (Western Tien Shan) to an almost 100% juvenile (mantle-derived) crust environment in the east (Chinese Altay). Our data indicate also the locally extensive presence of old continental crust at the base of the Tien Shan east of the Talas-Farghona fault but not west of it. The lead isotope signatures of the ore deposits follow closely those of the magmatic and basement rocks of the host domains suggesting that no unique reservoir has been responsible for the gold concentration in this orogen.