GEOCHRONOLOGICAL EVIDENCE FROM DISCORDANT PLUTONS FOR A LATE PROTEROZOIC OROGEN IN THE CALEDONIDES OF FINNMARK, NORTHERN NORWAY

Rb-Sr whole-rock, Sm-Nd mineral and U-Pb zircon dates are reported for minor granites and Seiland Igneous Province gabbros in the Kalak Nappe complex (Middle Allochthon), northern Norwegian Caledonides. The Litlefjord (Rb-Sr WR 813 +/- 62 Ma, MSWD 2.7; U-Pb zircon 804 +/- 19 Ma, MSWD 5.5) and Repvag (Rb-Sr WR 851 +/- 130 Ma, MSWD 5.1) granites and the Hasvik gabbro (Sm-Nd WR/minerals 700 +/- 33 Ma, MSWD 0.7) cut previously-deformed metasediments of the Soroy Succession while the Kvalfjord (Sm-Nd WR/minerals 612 +/- 33 Ma, MSWD 0.1) and Storvik (Sm-Nd WR/minerals 604 +/- 44 Ma, MSWD 0.2) gabbros cut 'basement' paragneisses. The main (D1-D2) deformation affecting the Soroy Succession in the Kalak Nappe complex, which was previously attributed to the early Ordovician Finnmarkian phase of the Caledonian orogeny, took place before c. 800 Ma in the pre-Caledonian 'Porsanger Orogeny'. Early 'Soroy' plutons of the Seiland Igneous Province were apparently emplaced during this event. However, the later, more voluminous 'Stjernoy' intrusions are c. 700-520 Ma old and post-date the Porsanger orogeny. These pre-Caledonian intrusions must be inter-, rather than syn-orogenic as previously claimed. This obviates the problem of accommodating the voluminous Seiland magmatism during orogenic compression and is consistent with an early Caledonian rift origin for these rocks, probably related to the opening of Iapetus.