Abstract
The quartz-K-feldspar-cemented breccia of Berglia-Glassberget in the Lierne municipality in central Norway forms an ellipsoid structure 250 m × 500 m in size. The hydrothermal breccia is barren in terms of economic commodities but famous among mineral collectors for being a large and rich site of crystal quartz of various colours and habits. Despite being a famous collector site, the mineralization is rather unique in respect to its geological setting. It occurs within Late Palaeoproterozoic metarhyolites of the Lower Allochthon of the Norwegian Caledonides regionally isolated from any other contemporaneous hydrothermal or magmatic event. In order to understand better the formation of the Berglia-Glassberget breccia, the chemistry, fluid inclusion petrography and age of the breccia cement were determined. Structural features indicate that the Berglia-Glassberget is a fault-related, fluid-assisted, hydraulic breccia which formed by single pulse stress released by a seismic event. 40Ar-39Ar dating of K-feldspar cement revealed a middle Triassic age (240.3 ± 0.4 Ma) for this event. The influx into the fault zone of an aqueous CO2-bearing fluid triggered the sudden fault movement. The high percentage of open space in the breccia fractures with cavities up 3 m × 3 m × 4 m in size, fluid inclusion microthermometry, and trace element chemistry of quartz suggests that the breccia was formed at depths between 4 and 0.5 km (1.1 to 0.1 kbar). The origin of the breccia-cementing, CO2-bearing Na-HCO3-SO4 fluid may have been predominantly of metamorphic origin due to decarbonation reactions (T > 200 °C) of limestones of the underlying Olden Nappe. The decarbonation reactions were initiated by deeply derived, hot fluids channelled to sub-surface levels by a major fault zone, implying that the breccia is situated on a deep-seated structure. Regionally, the Berglia-Glassberget occurs at a supposed triple junction of long-lived fault zones belonging to the Møre-Trøndelag, Lærdal-Gjende and the Kollstraumen fault complexes. These fault systems and the associated Berglia-Glassberget earthquake are the expression of rifting and faulting in northern Europe during the middle/late Triassic.
Highlights
Breccias are fragmented rocks which are commonly found in the highest, most fluid-saturated part of the crust, where brittle deformation is dominant e.g., [1,2]
On the total alkali oxide vs. silica (TAS) diagram (Figure 6A), the unaltered metarhyolites fall in the rhyolite field, but from sampling over a larger area some of the felsic rocks were found to fall in the trachyte field [23]
The structural characteristics described above breccia classify formed the Berglia-Glassberget genetically as fault-related, fluid-assisted hydraulic by a single pulse mineralization stress typically genetically as fault-related, fluid-assisted hydraulic breccia formed by a singleformed pulse stress for upper crust levels e.g., [1,36]
Summary
Breccias are fragmented rocks which are commonly found in the highest, most fluid-saturated part of the crust, where brittle deformation is dominant e.g., [1,2] They occur across a wide range of settings: sedimentary breccia, impact breccia, fault breccia (gouge, cataclasite, pseudotachylite), hydrothermal breccia, hydrothermal-magmatic breccia, and purely magmatic breccia. Genetic aspects like the degree of involvement of magmatic processes and the depth of formation may provide additional information to define hydrothermal and hydrothermal-magmatic breccias, in particular. Classifications of these breccia types commonly include both partly genetic and purely descriptive nomenclature, as discussed by [3,12,13]
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