Linus Brander

Brittle-plastic deformation in initially dry rocks at fluid-present conditions: transient behaviour of feldspar at mid-crustal levels

We present detailed microstructural and chemical analyses from an initially dry anorthositic rock deformed during wet amphibolite facies conditions. Three different domains representing the microstructural variation of the deformed samples are investigated in detail in terms of fracture morphology and mode, grain characteristics and chemistry of present phases. Results show transient deformational behaviour where a close interaction between brittle, plastic and fluid-assisted deformation mechanisms can be observed. Our analysis allows us to describe the succession, interrelationships and effects of active mechanisms with progressively increasing strain in three so-called stages. In Stage 1, initial fracturing along cleavage planes promoted fluid influx that caused fragmentation and chemical reactions, producing fine-grained mineral assemblages in the fractures. Deformation twins and dislocations developed in clast pieces due to stress relaxation. Passive rotation of conjugate fracture sets and interconnection of intracrystalline fractures formed micro-shear-zones, constituting Stage 2. Microstructures and grain relationships indicate the activity and fluctuation between fracturing, dissolution-precipitation creep, grain boundary sliding and locally dislocation creep, reflecting the transient behaviour of brittle and plastic deformation mechanisms. Further rotation and widening of fractures into overall foliation parallel shear-bands (Stage 3) promoted strain partitioning into these areas through increased fluid influx, influence of fluid-assisted grain boundary sliding, phase mixing and presence of weak phases such as white mica. We suggest that local differences in fluid availability, volume fraction of weak phases produced by fluid present metamorphic reactions coupled with volume increase and local variations in stress concentration induced transient brittle-plastic behaviour. The studied shear-zone represents an example of the transformation of a rigid dry rock to a soft wet rock during deformation through syntectonic fracturing.

Igneous and metamorphic geochronologic evolution of granitoids in the central Eastern Segment, southern Sweden

The Eastern Segment abutting the Transscandinavian Igneous Belt (TIB) mostly consists of rocks with overlapping igneous ages. In the Eastern Segment west of Lake Vättern, granitoids of clear TIB affinity exhibit strong deformational fabrics. This article presents U–Pb zircon ages from 21 samples spanning the border zone between these deformed TIB rocks in the east, and more thoroughly reworked rocks in the west. Magmatic ages fall in the range 1710–1660 million years, irrespective of the degree of deformation, confirming the overlapping crystallization ages between deformed TIB rocks and orthogneisses of the Eastern Segment. A common history is further supported by leucocratic rocks of similar ages. Prolonged orogenic (magmatic) activity is suggested by continued growth of zircon at 1.66–1.60 Ga. Six of the weakly gneissic rocks show zircons with cathodoluminescence-dark patches and embayments, possibly partly replacing metamict parts of older magmatic crystals, with 207Pb/206Pb ages dominantly between 1460 and 1400 million years, whereas three of the gneisses have zircon rims with calculated ages of 1440–1430 million years. Leucosome formation took place at 1443 ± 9 and 1437 ± 6 Ma. The minimum age of SE–NW folds was determined by an undeformed 1383 ± 4 million years crosscutting aplitic dike. Sveconorwegian zircon growth was not found in any of the samples from the studied area. To our knowledge, 1.46–1.40 Ga metamorphism affecting the U–Pb zircon system has not previously been reported this far northeast in the Eastern Segment. We suggest that the E–W- to SE–NW-trending deformation fabrics in our field area were produced during the Hallandian–Danopolonian orogeny and escaped later, penetrative Sveconorwegian reworking.

Tracing the 1271–1246 Ma Central Scandinavian Dolerite Group mafic magmatism in Fennoscandia: U–Pb baddeleyite and Hf isotope data on the Moslätt and Børgefjell dolerites

Between 1271 and 1246 Ma, dolerite dykes and sills of the Central Scandinavian Dolerite Group intruded into the Fennoscandian Shield during three distinct magmatic pulses. They are distributed around five large magmatic complexes extending from Sweden to western Finland and record large-scale intracratonic tensional stress. Coeval plutonism is observed in the westernmost terrane of the Sveconorwegian orogen in southern Norway, but differs in the sense of a bimodal character and uncertain Fennoscandian ancestry of the host terrane. We report a U–Pb baddeleyite age of 1269 ± 12 Ma for a gabbronoritic member of an E-trending set of dykes, called the Moslatt Dolerites, near Lake Vattern in southern Sweden, much farther to the south than any previously known Central Scandinavian Dolerite Group rock. A similar age of approximately 1275 Ma is obtained for a meta-dolerite sheet in the Borgefjell basement window in the Scandinavian Caledonides in Mid-Norway. The initial epsilon-Hf values for these two dykes are +3.9 and +10.1, respectively, and correspond to the range of values for other occurrences of the Central Scandinavian Dolerite Group (+4.7 to +10.3). They add to the evidence that the Central Scandinavian Dolerite Group is characterized by more positive epsilon values (depleted source) than other mafic Proterozoic suites in Fennoscandia. These results extend the distribution of c . 1270–1245 Ma mafic magmatism in Fennoscandia, particularly when accounting for significant Caledonian shortening. The Central Scandinavian Dolerite Group and coeval bimodal magmatism in S Norway may represent distal magmatic events related to a Mesoproterozoic subduction along the western margin of Fennoscandia rather than hotspot (mantle plume) activity as previously suggested.

Age, tectonic setting and petrogenesis of the Habo Volcanic Suite: Evidence for an active continental margin setting for the Transscandinavian Igneous Belt

Abstract The Habo Volcanic Suite is situated in the southwestern part of the Smaland-Varmland batholiths of the Transscandinavian Igneous Belt (TIB), Sweden. It comprises mainly intermediate to mafic pyroclastic and redeposited volcanic rocks and is distinct from other, more felsic, volcanic rocks of the Smaland-Varmland batholiths. The Habo Volcanic Suite has been intruded by a TIB-granite, here dated at 1660±9 Ma. U-Pb ages of zircon from the Habo Volcanic Suite fall into two groups; 1795±14 Ma for domains with Th/U >0.2 and 1694±7 Ma for rims with Th/U <0.2. The 1795±14 Ma age is interpreted as the crystallization age of the Habo Volcanic Suite, linking it to other Smaland volcanic rocks, whereas the 1694±7 Ma rims are interpreted to reflect a thermal event related to intrusions of younger TIB rocks. These interpretations are supported by LA-ICPMS REE analyses which show flatter REE patterns and Th/U <0.2 for metamorphic zircon; and positive Ce anomalies for magmatic zircon. Two geochemically different...

The Plat Sjambok Anorthosite and its tonalitic country rocks: Mesoproterozoic pre-tectonic intrusions in the Kaaien Terrane, Namaqua–Natal Province, southern Africa

The Plat Sjambok Anorthosite crops out near Prieska Copper Mines in the Namaqua–Natal Province of southern Africa. It is a massif-type anorthosite, previously regarded as a late-tectonic intrusion and part of the ca. 1100 Ma bimodal Keimoes Suite. Our new ion probe U–Pb zircon data show that the Plat Sjambok massif intruded at 1259 ± 5 Ma, before the 1220 Ma Namaqua collision events and is thus approximately 150 million years older than the Keimoes Suite. Despite the proximity to Prieska Mines, the anorthosite is located in the Kaaien Terrane close to the Brakbos Fault, which is the boundary with the Areachap Terrane in which Prieska Mines is situated. We dated the Nelspoortjie Tonalite, the main country rock of the Plat Sjambok Anorthosite, by laser ablation ICPMS at 1273 ± 13 Ma. Both intrusions thus originated concurrently with the 1286–1241 Ma volcanic rocks of the Areachap Group, which developed in a subduction-related arc setting, prior to its collision with the Kaaien Terrane and Kaapvaal Craton. Metamorphic zircon rims in the Plat Sjambok Anorthosite give an age of 1122 ± 7 Ma, a time that corresponds to a quiet period in the Areachap Terrane. We propose a tectonic model in which formation of the Nelspoortjie Tonalite and Plat Sjambok Anorthosite was driven by intrusions from the mantle into a back-arc related tensional environment within the Kaaien Terrane, possibly situated above an Archaean crustal tongue. This led to heating in a thickened crustal setting in which the tonalite originated as a partial melt of amphibolite. The anorthosite then formed as a mixture of mantle-derived gabbro and Archaean crustal rocks, which explains the 2100–2600 Ma zircon–Hf crustal residence ages and the Sm–Nd trend towards an old crustal source. The anorthosite and its country rocks were only juxtaposed with the Prieska Copper Mining District by late-tectonic uplift and transpressional movements on the Brakbos Fault towards the end of the Namaqua tectogenesis.