THE VARIED PERSONALITIES OF SILICIC ICELANDIC INTRUSIONS: A 10 MY RECORD OF SILICIC MAGMA PETROGENESIS IN THE SHALLOW CRUST PRESERVED BY ZIRCON

Silicic rocks make up ~10% of Iceland’s crust, the largest concentration in a modern oceanic setting. Silicic volcanic rocks have been studied extensively, but intrusive rocks, which provide a unique perspective on the construction and evolution of silicic magma systems and dynamic processes within them, have received significantly less attention. Focusing especially on zircon, we have investigated 9 intrusions with varying characteristics (e.g. size, proportions of and relations between mafic and silicic rocks) that span 10 My of Icelandic geologic history: Sandfell (11.8 Ma), Reyðarártindur (7.3 Ma), Viðidalsfjall (6.8 Ma), Austurhorn (6.5 Ma), Slaufrudalur (6.4 Ma), Snæfellsness-Knörr (4.9 Ma), Vesturhorn (4.2 Ma), Flyðrur (3.9 Ma), and Lýsuskarð (1.4 Ma).

Trace element compositions of zircons from Icelandic intrusions form a broad but coherent geochemical array consistent with that of zircons from Icelandic volcanoes: Hf is relatively low (typically <13,000 ppm); U/Yb, Gd/Yb, and Ti are high (>0.1, >0.04, 12 ppm median, respectively); and Th/U increases from ~0.3 to ~1 as Th and U concentration rise. Zircon in situ ε_Hf for all intrusions is between +10 and +16. Notably, low zircon δ¹⁸O is nearly universal (< +4.5 ‰) regardless of the style of intrusion, relative fraction of silicic material, or whole rock composition (diorites to granophyres). Intrusive zircon δ¹⁸O generally extends to lower values than in extrusive zircons; in some cases to extreme lows (< -5 ‰). Median zircon δ¹⁸O for individual intrusions ranges from +4.5 to -0.7 ‰, with 7 of 9 intrusions between +3.6 and +2.4 ‰.

Heterogeneous δ¹⁸O suggests recycling of hydrothermally (meteoric)-altered crust is important in the genesis of silicic magmas in the Icelandic crust. A wider range of values in intrusive (vs. extrusive) zircons may reflect complex, prolonged, open-system histories experienced by intrusive magmas relative to their extrusive counterparts, as they are assembled from geochemically diverse magma batches. In the case of Vesturhorn, which is dominantly composed of mafic rocks, a higher median δ¹⁸O of +4.5 ‰ likely indicates fractional crystallization of typical Icelandic basalt producing silicic magma, but even here low δ¹⁸O values require the incorporation of some altered crust.