SOURCE OF ENCLAVES IN THE VEGA INTRUSIVE COMPLEX: IMPLICATIONS FOR TECTONIC SETTING AND EMPLACEMENT STYLE

The 475 Ma Vega intrusive complex in north-central Norway is elongate and zoned from east to west, comprising Ylvingen biotite granite, Vega granodiorite, and Fugleværet granodiorite, all of which crystallized from strongly peraluminous anatectic magmas. Vega granodiorite contains magmatic layering that is related to incremental emplacement as sheet-like bodies in amphibolite grade metasedimentary rocks that followed a Barrovian path into the kyanite and staurolite stability fields. Emplacement of Vega granodiorite was followed by the Ylvingen and Fugleværet units. Both the Vega and Fugleværet units are enclave bearing with metasedimentary, amphibolite, ultramafic, and mafic magmatic enclave types. Similar metasedimentary enclaves are sparsely distributed within the Ylvingen biotite granite.

Calc-silicate gneiss, marble, quartzite, calcareous schist and garnetiferous types have rock equivalents in the host rocks to the complex and occur in swarms sometimes surrounding septa or screens of similar composition that are up to 1.5 km long, but are lacking or occur in small amounts in the Paleozoic age Horta nappe, the interpreted source of rocks to the complex.

Biotite ± garnet schist, quartzofeldspathic gneiss, biotite quartz plagioclase gneiss, and migmatite enclaves do not have rock equivalents in the host rocks to the complex and do not occur in enclave swarms but are abundant in the Horta nappe. Some amphibolite enclaves have ε_Nd > +8 and MORB-like trace element characteristics, similar to disrupted dikes in the host rocks and in screens. Amphibolite enclaves with ε_Nd> +8 are also mingled in migmatitic rocks of the Paleozoic age Horta nappe which was partially melted at ~478 Ma.

Calc-silicate gneiss, marble, quartzite, and calcareous schist septa and xenolith swarms, the formation of magmatic layering, and the elongate E–W trend of the complex can be explained by magma emplacement during orogen normal extension. The amphibolite dikes and enclaves with MORB-like characteristics and orogen thickening indicate that the complex was emplaced following the closure of a back-arc basin. Regional extension following closure of a back-arc environment may have been initiated by slab roll-back which caused intraplating of mafic magmas and provided heat for anatexis of the Paleozoic age rocks of the Horta nappe.