科研之友
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摘要
The once deeply buried conglomerates of the Solund basin contain up to 30% basaltic clasts together with up to 20% altered peridotite clasts, both derived from the adjacent Solund-Stavfjord Ophiolite Complex (SSOC). The basaltic clasts range in size from 2 to 30 cm across and display centimeter-thick hematite reddened zones. Four types which form an alteration-related evolutionary sequence can be distinguished. Type 1 consists of a green core with a 1-2 cm thick red zone along its rim. Type 2 shows, in addition, a perturbation of the red zone into the center of the clasts. In Type 3, numerous compartments with green cores surrounded by red zones build up the clast. Type 4 displays diffuse red zones and patches typically with a circular form that resembles Liesegang's structures. This evolutionary sequence relates to a progressive deformation evident from fractures, faults and shear zones. Hematite reddened zones form along clast boundaries and along fractures that are oriented at high angle to the boundaries of the clasts. The boundary-parallel zones migrate towards the center of the clasts and the fracture-parallel zones migrate away from the fractures. These fracturing and deformation are coupled with chemical and mineralogical changes responsible of the changes in color of the alteration zones. Compared to the basaltic rocks of the SSOC, the basaltic clasts of the Solund basin contain higher contents of K2O, volatiles (LOI) and MgO, lower contents of CaO and are more oxidized with an average Fe3+/(Fe3+ + Fe2+) ratio of 0.43 (+/-0.10) as compared to 0.30 (+/-0.07) in the SSOC basalts. The replacement of magmatic clinopyroxene by chlorite induces the decrease in CaO. The replacement of ilmenite by titanite provides iron to form hematite in the red zones. The red zones progressively replace the green core and are, at their external boundary, in turn replaced by Mg-riched green alteration zones characterized by a high content of zoned amphibole. A decrease in CaO and an increase in MgO towards the rim of the basaltic clasts is contrary to the chemical evolution of the peridotite clasts, where the MgO is depleted and the CaO increases with increasing degree of alteration. This suggests that clasts of different lithologies exchanged components during the alteration process. To explain these observations, we use a model including internal stresses through a diffusion-reaction process and external stresses through sediment loading and tectonic forces. The mineralogy of the altered basaltic clasts (chlorite, epidote, albite, actinolite) corresponds to low-grade metamorphic conditions in accordance with or slightly higher than the peak temperature of 230-320 degrees C previously calculated for this basin. The study shows that the final compaction of multicomponent clast systems takes place through an interaction between mineral reactions, metasomatism and deformation, which differs from the compaction of monocomponent systems that occurs dominantly through pressure solution.
- 出版日期2012-9-5
