|
Hans Vrijmoed, University of Oslo (Norway)
Yuri Podladchikov, University of Oslo (Norway)
Torgeir Andersen, University of Oslo (Norway)
Fernando Corfu, University of Oslo (Norway)
|
|
The previously reported fine-grained 'Fe-Ti type' garnet olivine websterite is located in the northern part of the well known ultra-high pressure (UHP) area of the Western Gneiss Complex (WGC) in Norway. The body is cut by a conjugate set of metasomatic fractures filled dominantly with coarse-grained garnet-phlogopite-websterite and garnetite. Standard thermobarometric techniques based on electron microprobe analyses yield pressure (P) and temperatures (T) estimates around 4.0 GPa, and 800°C for the olivine-bearing body and 5.5 GPa, and 800°C for the websterite consistent with UHP conditions. A zircon in garnetite in the fractures has a concordant U-Pb age of 403.8 ± 2.8 Ma. Polyphase inclusions including microdiamond, coupled with 87Sr/86Sr ratios in clinopyroxene and whole rock ranging from 0.73 to 0.74 suggest that the olivine-websterite was infiltrated by melts/fluids from the host rock gneiss. A zircon grain with concordant U-Pb age of 397.8 ± 1.4 Ma formed in one of the leucosomes in the gneiss.
The WGC is well known for its occurrences of HP to UHP rocks, mainly found as eclogite boudins and lenses and more rarely within felsic gneisses. Present observations document a regional metamorphic gradient increasing towards the NW, and structures in the field can account for the exhumation of the (U)HP rocks from ∼2.5 to 3 GPa. Assuming lithostatic pressures the Svartberget body must have come from a burial depth of at least 150 km. However there is a lack of observable structures in the field to explain exhumation from extreme UHP conditions (5.5 GPa or more) to normal HP-UHP conditions (2.5-3GPa), which are common pressures calculated from eclogites in western parts of the WGC. Because of the regional and mostly coherent metamorphic gradient across the WGC terrain it is difficult to account for local extreme pressure excursions such as documented from within the Svartberget peridotite.
We introduce here a conceptual model to explain the main features of the Svartberget body. The Svartberget body is part of an (ultra-) mafic intrusion and is enclosed by paragneisses, which themselves are enclosed in the main basement orthogneisses. During Scandian collision, the rocks are buried and consequently heated. At a certain point melting commences in the paragneiss, which then will expand due to the volume-increasing melt-reaction. The surrounding orthogneisses do not melt to the same extent because of different compositions and hence melting temperatures. The result is an increase in pressure in the paragneissic unit to extreme UHP conditions, where the included Svartberget (ultra-) mafic body fractures and fluids/melts from the paragneisses infiltrate to form a metasomatic column. In the final stage, the surrounding orthogneiss breaks releasing the extreme UHP in the paragneiss unit. Continued exhumation from 'normal' (U)HP takes place on structures documented by direct observations from the region.
|
