| Summary: | The study area is situated within the South-West Swedish Province (SWSP), which forms part of the Precambrian Baltic Shield. The SWSP is separated from the Svecofennian Province, in the east, by a major suture, the Protogene Zone, and a major belt of sialic igneous rocks, the Trans-Scandinavian Igneous Belt. The SWSP is generally considered to have had a different petrogenetic history from the neighbouring Svecofennian Province, at least for part of its Precambrian development. The SWSP is transected by a major zone of mylonitization which divides it into Northern and Southern Segments, which differ in petrology and geological development. The whole of the SWSP show overprinting by the effects of the younger Sveconorwegian Orogeny.Research was concentrated to the immediate north and south of the dividing Mylonite Zone, close to the town of Varberg. The southern rock unit is known locally as the Varberg Formation and the northern rock unit as the Bua Formation. Outcrop density along the coastal strip is good to fair.It was believed that examination and interpretation of the metabasic rocks of the region might prove valuable in establishing the petrogenetic development of the region. This research has discriminated three distinct groups of metabasic rocks; differeing in their field characteristics, petrologies, petrochemistries, mineral chemistries and petrogeneses. These are referred to as the metabasic rocks of Groups I, II and III respectively.The metabasic rocks of Group I have had their origin in basic igneous activity which occurred prior to, or during, the early orogenic activity which established the fundamental character of the Varberg Formation rocks; the Varberg Orogenic Event. The geochemical data suggest that these metabasic rocks are derived from a suite of basic igneous rocks, probably largely minor intrusions, which relate to a parent magma which was tholeiitic in type. Orogenic intrusion is indicated, perhaps in an island arc environment. These basic intrusive rocks appear, from their field relationships with their country rocks, to have undergone the same range of deformations and metamorphisms as their hosts.The metabasic rocks of Group II have had their origin in basic igneous activity which post-dates the Varberg Orogenic Event, during a period of anorogenic magmatism (the Varberg Thermal Event). The geochemical characters of these rocks indicate their development from a tholeiitic parent magma in a continental rift environment. Protometamorphic development of garnet amphibolites synchronous with emplacement of these intrusions is indicated by their mineralogies and petrographies. They were subsequently in part metamorphosed to garnet pyribolites, on reversion to orogenic conditions (Sveconorwegian Orogeny). The rocks of Group II are largely concentrated within the Varberg Formation, with impounding against the discontinuity separating the Sua and Varberg Formations. There was, however, some leakage into the Bua Formation. Acid plutonism also occurred with, and was genetically related to, the basic magma influx during the Varberg Thermal event. The granitic products were also largely impounded at the BuaNarberg interface, but with some transgression. Both the basic and the acid igneous units were involved in thrust reworking of the interface zone (the Mylonite Zone), with local retrogression to amphibolite facies.The metabasic rocks of Group III are unrelated to either the Group I or Group II magmatic episodes and occur only within the Bua Formation. They are divisible into two sub-groups; Group iliA, with chemical affinity with low-K tholeiites of the island arc environment, and Group IIIB, comparable with continental rift basalts. The Group iliA rocks include metapyroxenite; brecciated and intruded by scapolite gabbro. This juxtaposition of deep level accumulate characters and high level, gas-charged brecciation can probably be linked with major tectonic translation, perhaps related to the Hallandian Orogeny. The metamorphism of the basic rocks is linked with, and forms an integral part of, the petrogenetic history of the Bua Formation.The igneous and metamorphic histories revealed from this study of the metabasic rocks of the Varberg region provide useful strictures in any interpretation of the geological development of the South West Swedish Province. The separate development of the Southern and Northern Segments is confirmed. The early development of the Varberg Formation seems also distinct from its now neighbouringmSvecofennian Province. The following petrogenetic development model is proposed, which allows for the fresh information obtained from the region:1) Development of the crustal segment represented by the Varberg formation in an early orogeny affecting the Group I igneous rocks (The Varberg Orogeny).2) Separate development of the Sua Formation sequences.3) Conjunction of the Sua and Varberg Formations during orogenic activity (The Hallandian Orogeny; c. 1600 my), such that the Bua Formation metamorphic rocks form a cover sequence to the Varberg Formation. This was preceded by the docking of the deeply-eroded, Varberg Formation, continental block against the Svecofennian Province (c. 1750 my).4) A period of crustal extension, petrological underplating, magmatic intrusion, crustal heating, volatile migration and acid plutonism affecting the composite crustal segment(The Varberg Thermal Event; c. 1400 my).5) Renewed orogeny (The Sveconorwegian Orogeny; c. 1000-900 my).
|
|---|
