Abstract The Oskarshamn-Jönköping Belt in southeastern Sweden is a geographically well-defined area comprising calc-alkaline intrusions and volcanic rocks together with units of coarse-grained clastic metasedimentary rocks. Deviating from the general composition is the Fröderyd Group with basalts of MORB character. The belt is surrounded by the 1.81-1.77 Ga Transscandinavian Igneous Belt. A conglomerate clast from the central part of the Oskarshamn-Jönköping Belt yielded a U—Pb zircon age of 1829±8. This age confirms the c. 1.83-1.82 Ga formation age of the belt. Sm—Nd whole rock analyses of various rock types throughout the Oskarshamn-Jönköping Belt show that no substantially older (>100 m.y.) continental material contributed to these rocks. Furthermore, the high positive εNd values of many of the analysed rocks points to the depleted mantle as the main component of their source. The Sm—Nd analyses also suggest that while some of the felsic units were formed by a high degree of magmatic fractionation from a mantle derived melt others were formed as the result of remobilisation of older Svecofennian crust. The new U—Pb data together with previously published ages imply that the formation of the Oskarshamn-Jönköping Belt was a rather quick process, possibly completed in c. 10 m.y. The most likely model of genesis for the area, based on the new U—Pb and Sm—Nd data, is formation at a continental margin subduction zone to the present south-west of a slightly older Svecofennian continent. In such a model the Fröderyd Group represents either a fore-arc setting or a back-arc rift.

dVolcanic and sedimentary rocks of the Vastervik and Valdemarsvik groups along the southern margin of the 1.90-1.85 Ga south Svecofennian crustal province in Sweden have been migmatised and intruded by granitoids and gabbros during extension (deformation phase D-1), at a depth of at least 10 km. Peak metamorphic conditions were reached around 1825 Ma. Extension switched to compression before cooling. The partially molten middle crust facilitated shortening and thickening by isoclinal and sheath folds on scales from a decimetre to 10 km (D-2). A pervasive mylonitic S-2 foliation developed. These structures were refolded into eastward plunging crustal scale upright folds with E(SE)-W(NW) trending axial planes (D-3). D-2-D-3 deformation structures are colinear and developed during top-to-the-WNW slip along S-2. Locally, zones of high ductile strain in garnet-bearing migmatite assemblages record considerable vertical displacement (from 850 to 300 MPa), possibly in nappe roots. Some major D-2 sheath folds are embraced by D-3- like, but NNW-trending megafolds, (D-4), apparently resulting from shortening around the former. D-2-D-4 deformations are interpreted to be broadly coeval and related to transpressive crustal shortening against the northern Svecofennian continental back-stop.After cooling to high greenschist-facies conditions, early structures underwent renewed transpression in north-south direction, expressed in localized, retrogressive shear zones (phase D-5). Much D-5 strain was accommodated by the 10 km wide, NW-SE striking, right-lateral Loftahammar-Linkoping Deformation Zone (LLDZ), operative around 1800-1780 Ma. Regional strain partitioning caused transverse shortening in 15-20 km wide border zones of the LLDZ. Later overprints (D-6) involve narrow, subparallel, NW-SE striking and steeply NE dipping low greenschist-facies mylonite zones, some of which merge into the regional, polyphase Asbro-Norrkoping Deformation Zone. D-6 was transpressive. oblique, left-lateral, NE-block-up slip suggests a W(SW)-E(NE) orientation of sigma (1).Peak metamorphism and partial melting in the supracrustal rocks was approximately coeval with greater than or equal to 1800-1835 Ma old subduction-related magmatism in the Oskarshamn-Jonkoping Belt (OJB), 100 km to the south-west. Seismic reflection profiling off the Baltic coast (BABEL line B) suggests northward polarity of subduction beneath the OJB and a back-arc environment of the Vastervik and Valdemarsvik groups between the OJB arc and the margin of the > 1850 Ma old Svecofennian orogen. We suggest that D-1-D-5 deformation was caused by intracontinental back-arc extension and subsequent closure of the back-are basin by oblique accretion of the OJB marginal arc onto the northern continent. Accretion involved northward indentation by midcrustal wedges, one of which coincides with and possibly caused the nucleation of the D-5 stage LLDZ.Ar-40/Ar-39 data from hornblende and white mica identify a prolonged cooling history of the area (1810-1490 Ma), influenced by the nearby, late- and postorogenic Transscandinavian Igneous Belt and, possibly, by largely hidden, c. 1530 Ma old anorogenic rapakivi-type intrusions. (C) 2001 Elsevier Science B.V. All rights reserved.

The Bergslagen ore province forms part of the Svecofennian domain of the Baltic Shield. The initial epsilon(Nd)-values of least-altered and albitized granite samples of the 1.88-Ga Bergslagen Older Granite suite vary between - 1.2 and + 2.5. The granite samples of the 1.78-Ga Fellingsbro-Malingsbo suite vary between - 0.3 and + 7.7. The Rb-Sr system of the Older Granite suite yields an errorchron of 1.70 +/- 0.01 Ga, with an initial Sr-87/Sr-86 ratio of 0.7053, indicating tectonothermal disturbance, perhaps due to the intrusion of younger granites of the Transscandinavian Igneous Belt.The initial Nd isotopic composition of the 1.88-Ga Older Granite suite can be interpreted in two extreme end-member models: (a) interaction between depleted mantle-derived melts and Archaean crustal material at 1.88 Ga; and (b) remelting of Early Proterozoic (pre-1.88 Ga) crust. Previous studies in the Baltic Shield have advocated the former model, and discarded the latter, because of absence of 2.0-2.4-Ga crust. Likewise, in the case of western Bergslagen, no Archaean crust is present. It is hence argued that the Nd isotopic data may also be interpreted in terms of generation of the Older Granite suite through remelting of a crustal precursor. In this case the 2.1-2.0-Ga depleted mantle model ages of the Older Granite suite could give an indication of the time that such a precursor was extracted from the depleted mantle.

The 1.88 Ga old within-plate continental tholeiites belong to the mainly metavolcanic, bimodal Bergslagen Supracrustal Series (BSS) in the Svecofennian domain of the Baltic shield. The BSS evolved in an intracontinental rift. Despite only moderate lithospheric extension (beta < 2) the degree of mantle melting was relatively high (tholeiites instead of alkali-basalts), possibly due to the thermal state of the convecting upper mantle at the time, ca. 150-degrees-C hotter than at present and to a relatively low solidus temperature of the lithospheric mantle source. There are no indications for a mantle-plume origin of these continental basalts.The tholeiites are the (near-)surface expression of a 10-20 km thick magmatic crustal underplate that probably underlies most of the SW Svecofennian domain and caused the Svecofennian crust to be three-layered, thick, but yet isostatically stable. Underplating strengthened the lithosphere sufficiently to control the time of cratonization.Geodynamic models for the origin of the BSS consistent with our interpretations are gravitational collapse of an overthickened continental accreted terrane margin, or back-arc processes related to a marginal volcanic arc 250 km further south. A tentative reconstruction of relative plate motions during Svecofennian time indicates predominantly longitudinal motion of the plates in question and complies with the relatively slow latitudinal drift of the shield at the time.Recent ion-microprobe U-Pb ages of Proterozoic zircons in Svecofennian metasediments imply about 0.2 Ga of pre-Svecofennian crustal evolution, while the Nd model-ages of the most primitive metatholeiites may be indicative of initial mantle differentiation as early as 2.4 Ga BP, accompanying protocrust formation.

The Early Proterozoic Bergslagen Supracrustal Series (BSS) of western Bergslagen consists of a pile of felsic volcanites and intercalated carbonates, with in the upper part an alternating succession of volcaniclastics and basic flows. This pile is intruded by a bimodal sequence of granites and basic sills and dikes.Major- and trace-element data reveal the influence of several processes that have acted upon the basic magma. Crustal contamination or supra-subduction zone (SSZ) modification, fractional crystallization and potassic and sodic alteration due to (sub-)seafloor processes are recognized. Using geochemical and petrological criteria the basic rocks are divided in two least-altered (LA) and two alkali-altered (AA) sets. Geochemically, the least-altered basalts resemble Phanerozoic continental within-plate tholeiites and thus contradict the interpretation by many previous authors of Bergslagen as an active margin volcanic arc. Alkali-alteration leads to pseudo calc-alkaline or pseudo alkaline signatures. However, a SSZ-component, observable in part of the LA set, indicates a relation in space and time to a convergent margin.

The contact relationships between metabasalts (eclogites, glaucophanites, prasinites, etc.) and the enclosing mica schists in the Venezuelan Coast Ranges favour a common petrological history for both. Mineralogical disequilibria, such as replacement textures and mineral zoning in the metabasic assemblages, can all be related to a single metamorphic cycle. Relict deuteric/late magmatic hornblende epitaxially overgrown by the barroisitic amphibole of this metamorphic event shows that the latter has been the only regional metamorphic episode to have affected these rocks. The high-pressure character of the metamorphism is a logical consequence of overthrusting related to collision of the Aruba-Blanquilla island arc with the...

Recent experimental work on peridotites and basalts1–3 and a reappraisal of ophiolite complexes4 supports the concept that basalts are derived from picritic parents5–7. The occurrence of a density minimum in the liquid line of descent8–10 explains why mid-ocean ridge tholeiites fall in a restricted compositional range, and parental picrites can only be found in the exceptional case when eruption is not yet controlled by a steady-state magma chamber7,11. We present here a preliminary account of a natural example of such a case from the Caribbean.