PETROGENESIS OF GRANITIC INTRUSIONS IN THE UPPER ALLOCHTON OF WEST NORWAY: HYDROUS ANATEXIS OF METASEDIMENTS DURING LATE-CALEDONIAN OPHIOLITE OBDUCTION

In the upper allochton of the west Norwegian Caledonides minor granitic sheets and one granitic complex (The Sogneskollen granitic complex) were intruded at ca 425Ma during obduction of the ca 443Ma Solund-Stavfjord Ophiolite Complex (SSOC). The granites generally have high Ba, Sr and Na₂O, very low Y values (<7ppm), negative eNd_420Ma values (ca. -1 to -10) and ⁸⁷Sr/⁸⁶Sr_420Ma ratios of 0.70437-0.70705. The REE patterns are strongly HREE depleted and La/Yb_n varies from <1 to ca 100. Most samples display small positive Eu anomalies, but those that are most fractionated have small negative anomalies.

It is suggested that the SGC, and other similar granitic sheets in the area, most probably formed by melting of sediments in the presence of a H₂O-bearing volatile phase during obduction of the ophiolite complex. The quartz-feldspathic source sediments most probably contained detritus from long-term Rb-depleted lower continental crust and from an island arc. The sediments were deposited in large amounts in the marginal basin now represented by the SSOC. During obduction the sediments were buried beneath the hot overriding SSOC and the associated island-arc and yielded granitic magmas during partial melting.

The granites are geochemically similar, and have the same age, as granites in the Lindås nappe further to the south and in Scotland. It is speculated that the granites have a similar petrogenesis and that their distribution in fact map the extent of the late Caledonian marginal basin now represented by the Solund-Stavfjord Ophiolite Complex.