Alvar Soesoo

Divergent double subduction: Tectonic and petrologic consequences

Divergent double subduction, involving removal of an oceanic basin from both sides and collision, has distinct fingerprints of magmatism and significant implications for the tectono-magmatic evolution of orogenic belts. The complete evolution of a divergent double subduction system involves four stages: (1) initial interaction between the two divergent subduction zones on both sides of a single oceanic plate; (2) closure of the oceanic basin when the two overriding plates meet, followed by (3) detachment of the oceanic lithosphere from the overlying crust and sedimentary sections; (4) sinking and disappearance of the oceanic slab into the mantle. Detachment of the oceanic slab leads to intense decompressional melting of mantle and triggers melting of accretionary-wedge strata and lower crust, resulting in large-scale magmatic and volcanic activity. The main consequences of a divergent double subduction system are that it produces opposed thrust systems, extensive long-lived granitoid magmatism with mantle isotopic signature, and volcanism that evolves toward bimodalism. This model can be applied to the western half of the Paleozoic Lachlan fold belt (southeastern Australia) to explain the wide-scale Silurian to early Carboniferous granitoid magmatism, its spatial and temporal relationships, and late bimodal magmatism. Closure of the oceanic basin is thought to have occurred during the Early to Middle Devonian. Detachment of the oceanic slab led to felsic to intermediate-composition magmatism. Bimodal volcanism involving local basaltic flows is a reflection of subsequent sinking of the slab.

A multivariate statistical analysis of clinopyroxene composition: Empirical coordinates for the crystallisation PT‐estimations

Abstract In spite of several studies attempting to relate the chemical composition of clinopyroxenes to the nature of their host rock or depth of crystallisation, there is no empirical background to enable compilation of both pressure and temperature conditions of CPX crystallisation. On the basis of experimental data and employing multivariate statistics, the empirical PT‐space was developed. This PT‐space allows distinction of CPXs crystallising from basaltic magmas over a large range of temperature (1100–1300°C) and pressure (0‐>20 kbar). In low‐pressure crystallisation (P<2 kbar), the Ti/Mg ratio in magma can be related to the clinopyroxene composition enabling primitive and evolved basaltic magmas to be distinguished in spite of coincidence in the En‐Wo‐Fs values of clinopyroxenes. These diagrams are particularly useful in studies of rocks and xenoliths in the oceanic floor environment, since the stratigraphical sequence of samples and crystallisation parameters, including a nature of possible parent...

Layered intrusions and traffic jams

A wide range of explanations has been proposed for the origin of repetitive layering in mafic-ultramafic and in (per)alkaline intrusions. Here we propose that the interaction of mineral grains that sink and float in the crystallizing magma is an alternative mechanism that can explain many of the features of layered intrusions, without the need to invoke extrinsic factors. Similar to traffic jams on a motorway, small perturbations in crystal density develop that impede further ascent or descent of buoyant or heavy minerals, respectively. These “traffic jams” separate layers of magma from the rest of the magma chamber. The magma in the individual layers further evolves as a largely independent subsystem, with gravitational sorting organizing the mineral distribution within each layer. Layering can develop in the intermediate range between full mineral separation in low-viscosity or slowly cooling magma chambers and homogeneous crystallization in high-viscosity or fast-cooling chambers. This self-organization mechanism provides a novel explanation for the formation of rhythmic layering in low-viscosity magmas, for example in the Ilimaussaq igneous complex in southwest Greenland.

The ca. 1.8 Ga mantle plume related magmatism of the central part of the Ukrainian shield

Abstract: Palaeoproterozoic (ca. 1.8 Ga) mafic and ultramafic dykes are widely distributed within the whole Sarmatian segment of the East-European craton. This paper focuses on new geochronological, geochemical and isotope data obtained for mafic and ultramafic dykes of the Ingul terrain. Geochronological data available for these dykes indicate ages around 1800 Ma. We provide a new U–Pb zircon age of 1810 ± 15 Ma obtained for a dolerite dyke in the Kirovograd area. Geochemical and petrographical data allow identification of three groups of dykes: (1) kimberlites, (2) high-Mg# subalkaline rocks (picrite, camptonite, subalkaline dolerite etc.) and (3) tholeiite dolerite. Rocks of these groups were probably derived from different sources. εNd1800 values of studied rocks vary from 0.7 to 2.8. The highest values were obtained for mantle xenoliths and their kimberlite host (εNd1800 = 2.5–2.8). Rb–Sr data yield a regression age of 1729 ± 20 Ma with an initial 87Sr/86Sr = 0.70366 ± 41 (MSWD = 10.8). The whole-rock lead isotope data scatter, but data for sub-groups of samples can tentatively be fitted to parallel 1.8 Ga isochrons. The geochemical data indicate rocks to have formed by partial melting and the degree of melting is thought to be a function of formation depth, the latter ranging from sub-lithospheric to lower-crustal levels; we assume that melting was caused by a mantle plume. Dyking in the Ingul terrain was closely associated in time and space with metasomatic albitites that host numerous economic U deposits.

Depositional framework of the East Baltic Tremadocian black shale revisited

This article presents a centimetre- to micrometre-scale study of sedimentary fabrics from Lower Ordovician metalliferous black shale from the Baltic palaeobasin. Two sections of the Türisalu Fm. NW and NE Estonia were analysed with light microscopy and scanning electron microscopy. This rock unit is characterised by mostly thin bedding ( < 10 mm), common occurrence of minor erosional features, and a large variety of sedimentary fabrics, including graded, cross-laminated and massive fabrics. Based on this, we suggest that dynamic sedimentation events, rather than commonly assumed slow net sedimentation, may be the dominant mechanism behind the accumulation of these beds. The storm-related near-bottom flows and the bed-load transport of mud particles were likely common distribution agents of organic-rich mud. The mud (re)distribution, mainly via near-bottom flows and controlled by flat seafloor topography and general clastic starvation, might explain the present lateral distribution and diachronous character of the Türisalu Fm. Documented traces of microbial mat growth and siliceous sponges in the NW Estonia indicate that in more sheltered settings, biogenic factors played a vital role in developing primary mud characteristics. The geochemical palaeoredox proxies, and high trace metal and organic matter content suggest that mud sedimentation could occur under anoxic conditions. The observed sedimentary fabrics and traces of bioturbation, however, favour prevailing oscillating redox conditions in the lower water column. The recorded heterogeneity of microfabrics indicates that dynamic transport and intermittent deposition together with biogenic factors likely forced the development of an array of unique (bio)geochemical microenvironments for syngenetic trace element sequestration.