Geochemical changes accompanying mylonitisation of granite at the base of the Helgeland Nappe Complex, Nord-Trøndelag, central Norway

• Main Authors: Roberts, David, Nissen, August L.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2006
• Subjects: GEOKJEMI; MYLONITT; GRANITT; BERGGRUNNSGEOLOGI; Harran; Helgeland; Høylandet; Kollstraumen; Kongsmoen; Norway
• Online Access: https://hdl.handle.net/11250/2674171

A progressive conversion of megacrystic granite into mylonites at the base of the Helgeland Nappe Complex in the Caledonides of central Norway is reflected in microstructural, textural and modal changes. These mylonites record a considerable, regional,Scandian thrust deformation followed by a late-Scandian phase of major extension. The chemical changes that accompanied the structural and mineralogical conversion are also quite significant. Major oxides generally show wt.% losses (e.g.,Al2O3down >12% and CaO 50%) except for SiO2 which gains 10%,partly reflecting a twofold increase in modal quartz. Of the trace elements, Sr and Ba fall markedly, up to 60%, a feature that reflects their affinity for the modally diminishing feldspars (65 vol.% in granite to <20 vol.% in mylonites). An application of normalised mylonite\/protolith ratio plots for selected oxides indicates that this regionally important mylonitic shear zone is of the volume-gain\/isovolume variety. Such volume-gain systems, commonly involving fluid-enhanced silicification, are more typical of extensional shear situations. These results conform well with the regional structural picture wherein the major, extensional, late-Scandian, Kollstraumen detachment zone occurs in the footwall to the mylonitic base of the Helgeland Nappe Complex; and extensional structures overprint the thrust fabric in the mylonitised granite. 56205