PETROLOGY OF KAMBUR AND BORG IN ICELAND: TWO MID-MIOCENE SILICIC DOMES, COOL, WET CALC-ALKALINE CONTRASTS WITH HOT DRY THOLEIITIC MAGMATISM

As an oceanic island located where a mid-ocean spreading center coincides with an active hotspot, Iceland’s magmatism is predominantly basaltic, but silicic magmatism is also relatively abundant. In keeping with the setting, most silicic magmas are tholeiitic, and alkalic magmas are also common. Anomalous calc-alkaline silicic volcanic rocks have been reported at a single central volcano, Króksfjörður (Pedersen & Hald 1982;Jónasson et al 1992; Willbold et al 2009). We compare the petrology of two glassy, mid-Miocene silicic domes in the Westfjords – Kambur in Króksfjörður and Borg in the more typical central volcano Árnes (Jordan et al 2008, 2013) – that demonstrate the uniqueness of Króksfjörður’s calc-alkalic rocks. Borg rhyolites (~76 wt% SiO₂) have relatively sparse, anhydrous phenocryst assemblages (~10%, plag + cpx + opx), whereas Kambur rhyolites (~70 wt% SiO₂) are extremely phenocryst-rich (35-50%) with abundant hydrous minerals (plag + hbl + cpx; cummingtonite also reported [Pedersen & Hald 1982]). Borg has high Fe and low Mg (Mg# ~7) and very high HFSE and HREE and a large negative Eu anomaly (Nb 50 ppm, Zr 370 ppm, Yb 8 ppm, Eu/Eu* 0.5), whereas Kambur is relatively Mg-rich and depleted in HFSE and HREE with minimal Eu anomaly (Mg# 42, Nb 10 ppm, Zr 180 ppm, Yb 1.3 ppm, Eu/Eu* 0.95). Glasses from both domes are high-Si rhyolite (78% SiO2) but Kambur has higher CaO and Al₂O₃ and lower FeO. Zircon saturation temperatures based on whole rock analyses are ~810^o C for Borg, 710 ^o C for Kambur (Boehnke et al 2013); Kambur glass yields a saturation T of 705 ^o C. Kambur’s characteristics document much cooler, wetter conditions than is typical of Iceland’s silicic magmas and that petrogenesis of this magma paralleled in fundamental ways that in subduction zones. It remains to be explained how an Icelandic environment can yield conditions of magma generation that match in important ways those in subduction environments.