The “ultramafic” Intrusion’s within the peridotite from Härbergsdalen : Petrological characterization of infiltration record within the lithospheric mantle

Abstract: The mantle constitutes ~80% of Earths volume, however, due to its location deep beneath Earth’s surface the mantle is inaccessible to study in situ. Despite this, we still have information about its composition thanks to seismic studies which haven proven the mantle to be in generally homogenous and dominated by a rock called peridotite. However, mantle rocks such as peridotites sometimes, due to seismic activities, end up at Earth’s surface, for example as orogenic peridotite.The studied lithologies representing pyroxenitic and amphibolitic veins that were sampled near Härbergsdalen, located in northern Jämtland and within the Seve Nappe Complex (SNC). The SNC is interpreted as the outermost western margin of the paleocontinent of Baltica and part of the Middle Allochthon, one of four allochthons that together forms the Scandinavian Caledonides, an allochthon is a large sheet of rock that’s been displaced from its original position. During the Caledonian orogeny the western parts of Baltica was subducted under the paleo continent of Laurentia. Today ultramafic bodies are commonly occurring along the Seve Nappe, represented by peridotites, sometimes intruded by variety of veins. Such veining may shed light on P-T conditions and tectonic setting of the Earth’s mantle fragments and helps to understand the geological history of the Scandinavian Caledonides.Through applications of petrographic microscope (textural observations) and an electron microprobe (chemical analysis of minerals composition), collected samples from two different veins within orogenic peridotite outcrop on the western slopes of Aretstjakke mountain in Härbergsdalen were studied. The veins could easy be distinguished from each other as one almost black vein centered by red core and a second zonated, coarser sample. Results show that the veins don’t share a common history, one of the veins has a Garnet-bearing clinopyroxenitic core surrounded by zonated amphiboles. These amphiboles have an actinolite core and a rim of magnesio-hornblende, but also fully altered magnesio-hornblende grains. This shift in composition from relatively low P-T conditions in the actinolite core to more moderate P-T conditions in the magnesio-hornblende rim indicates progression into an environment deeper down into the Earths lithosphere. The other studied vein had amphiboles with pargasitic core surrounded by tremolite rim. Indicating retrogression through a drop in P-T conditions. In this second vein, there was also no signs of garnets, instead spinel’s where a common feature.These results indicate the veins were formed during different events, however further studies have to be made to correlate and understand all the processes recorded within the Aretstjakke peridotite outcrop. Further on, this outcrop is the third ever within the Seve Nappe Complex where garnet-bearing clinopyroxenite has been found, an interesting find worth more studies to put in context.