Relationship Between SEM-Cathodoluminescence and Trace Element Chemistry of Quartz in Granitic Igneous Rocks of the Oslo Continental Rift

Numerous granitic intrusions were emplaced in the Oslo area during the main rifting episode (275-240 Ma). The Drammen biotite-granite and Eikeren-Skrim alkali-feldspar granite (ekerite) represent two of the most evolved rock types related to this period of plutonic activity.This study documents the textural and chemical features of the different generations of quartz (Qz) present in the two granites, in relation with the igneous and hydrothermal processes at the origin of their formation.Both granites suffered pervasive alteration by magmatic and hydrothermal (sub-solidus) fluids. Contrary to other minerals, primary igneous quartz is well preserved and revealed by SEM-cathodoluminescence (SEM-CL) images. Several generations of secondary quartz are also recognisable and clearly related to late magmatic and/or subsolidus fluid infiltrations.Four quartz types can be categorised by SEM-CL luminescence and texture:Type 1: Bright primary quartz frequently showing oscillatory growth zoning from core to rim, in the form of alternating bright and dark ring-liked sectors reflecting compositional variations of the crystallising melt.Type 2: Light grey up to hundreds μm wide diffuse alteration zones, which follow grain boundaries and open cracks cutting through grains of Qz type 1.Type 3: Usually darker than Qz type 2. Contrary to Qz type 2, not necessarily directly associated with macroscopic fracturing features. Qz type 3 is featured by irregular patterns cutting through Qz type 1 and 2 and seems to be the result of dissolution/recrystallisation processes, possibly through a network of micro cracks related to the major fluid channelling fractures.Type 4: Narrow cracks and patches of black Qz crosscutting all the other types.EPMA in situ analysis of the different quartz generations confirm that the intensity of luminescence of quartz is strongly positively correlated to the presence of specific trace elements in quartz like Ti and Al. Hence the different quartz types mentioned before can be distinguished by levels of luminescence and trace element chemistry. Qz type 1 contains about 250 (200-350) ppm Ti and 130 (80-160) ppm Al, whereas Qz type 2 and 3 have overlapping concentrations averaging 60 (10-120) ppm Ti and 90 (50-130) ppm Al. Qz type 4 was not analysed due to the narrowness of the structures. These results document that quartz alteration by percolating fluids has a purifying effect, leaching Ti and Al out of the quartz by diffusion (Qz type 2)and dissolution/recrystallisation (Qz type 3).SEM-CL textures and chemistry of quartz help to understand the complex magmatic and hydrothermal history of these granites, whose original magmatic picture is strongly overprinted and blurred by pervasive fluid-related alteration features.