Application of zircon to magmatic investigations: I. Exploring effects of magmatic-tectonic interplay on silicic magma genesis in Iceland; II. Elucidating copper mineralization trends in a Mid-Jurassic magmatic system, Yerington, NV, USA

Abstract

Timing and duration of magmatism, involvement of fluid or assimilation of material, and geodynamic context are all important when assessing the processes by which silicic magmas are produced, erupted, or emplaced. This research relies heavily on in situ analyses of zircon, in combination with whole rock elemental and isotopic data, to investigate the causes and effects of silicic magmatism in two very different contexts.

First, in situ age and trace element analyses of zircon from ore-bearing porphyry dikes at Yerington Copper Mine, NV reveal a progressive drop in oxidation state over the ~1 Myr the dikes formed. Unlike previous studies which have found a correlation between bulk rock Cu content and oxidation state as measured by Eu and Ce anomalies in zircon from porphyry Cu systems, no correlation was found at Yerington. In addition, chemical abrasion treatment of zircon, which has been shown to improve age resolution, does not significantly change trace element or oxygen isotope compositions in zircon.

Second, this research uses in situ zircon U-Pb geochronology, trace element, and O and Hf isotope analyses in addition to whole rock elemental and isotopic data to evaluate the petrogenesis of multiple extinct silicic magmatic systems in Iceland and the role tectonism plays on their formation. Major findings are: 1) Involvement of hydrothermally altered, low-δ18O material prior to zircon crystallization in silicic systems in Iceland is ubiquitous; 2) A combination of partial melting of Icelandic crust and fractional crystallization of those melts and/or fresh mantle melts produce the vast majority of silicic units studied; magmas produced via pure partial melting or fractional crystallization are rare; 3) Rare calc-alkaline rocks at Króksfjörður volcano have distinctly different whole rock Pb, Nd, and Hf and in situ zircon Hf isotope compositions than coeval tholeiitic units, implying different petrogenetic mechanisms operated simultaneously for ~1 Myr; 4) Longevity of silicic magmatic systems in Iceland is strongly tied to rates of rifting.