Chalmers intrusion - age and crustal setting
Helena Kiel

Earth Sciences Centre
Göteborg university
B302 2001



Abstract
Thesis documents a mafic intrusion at Chalmers Technical University campus in Göteborg, 
southwest Sweden. Analysis and interpretation of textures, thermobarometry, zircon dating, 
and geochemistry are used to determine the age and crustal setting of the intrusion. It consists 
of a mostly fine-grained gabbro with relict magmatic textures. The edges are amphibolitic or 
mafic injection migmatites. The country-rock gneiss is mostly granitic. Granitic diapir-like 
structures intruding the gabbro have been interpreted as contact-metamorphic melt of country 
rock back-intruding the mafic magma. Metasomatising fluids from these diapirs triggered 
garnet growth and hornblende alteration at contacts at a late stage of magmatism, when 
minerals had crystallised but were still in diffusive communication with each other. Previous 
studies in southwest Sweden make use of granitic back-veins to date mafic intrusions. Zircon 
in country rock melted by the heat of the intrusion might experience new growth at this time. 
Thermobarometric calculations were made with TWEEQU from SEM-EDS mineral analyses. 
Results show equilibrium temperature and pressure of 800°C and 10kbar. Zircon and titanite 
U-Pb analyses were done with the NORDSIM ion-probe in Stockholm. Zircon cores and rims 
all show a concordant age of 1332 ± 7 Ma, the age of the intrusion, at which time xenocryst 
cores were reset. Titanites give an age of 1004 ± 25 Ma with older components pointing 
toward the formation age. Whole-rock analyses were made with SEM-EDS and laser ablation 
ICPMS. Tectonic discrimination using geochemistry is inconclusive but suggests a back-arc 
origin. The Chalmers intrusion was emplaced in subduction-generated country rock at an 
anorogenic stage of southwest Swedish crust formation. Subsequent Sveconorwegian 
metamorphism of lower grade affected titanites but did not cause re-equilibration of the 
geothermobarometric minerals or lead-loss in zircon. 

Keywords: SW Sweden, Sveconorwegian, thermobarometry, ion probe, zircon dating