Abstract

The Heilhornet Pluton is a medium-sized (c. 50 km2), sub-circular, discordant intrusion which penetrates the thrust zone separating the Helgeland Nappe Complex from paragneisses in the Vestranden region, north-central Norway. The intrusion consists of biotite ± hornblende granite and ranges in composition from 61.5 to 72.5 wt.% Si02 The initial Sr-ratio (c. 0.7070) shows that crustal sources have been involved in the petrogenesis of the granite. A U-Pb zircon age of 444 ± 11 Ma for the pluton indicates that juxtaposition of the nappe complex with metasedimentary rocks in Vestranden took place prior to or during the late Ordovician. A consequence of this is that the general correlation of the metasedimentary rocks in Vestranden with the Seve Nappes must be revised. Final eastward thrusting of the complex, together with a yet unknown part of the Vestranden supracrustal sequence, probably took place during the middle Silurian to early Devonian, Scandian, orogenic event.

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