Speaker – Drew Coleman, UNC-Chapel Hill |

Seminar Title – Missed Connection: Ignimbrite Seeking Plutonic Relationship |

Abstract – Recent advances in geochronology allow us to explore magma systems with great precision. As a result, plutons are now generally recognized to be slowly accumulated bodies that, despite being texturally homogeneous, never existed as large, dominantly liquid magmas. This new understanding requires significant rethinking of some of the most basic concepts in igneous petrology. Most importantly, plutons cannot represent crystal cumulates left behind after extraction of rhyolitic liquids during ignimbrite eruptions. Detailed geochronology in plutonic systems associated with caldera-forming eruptions reveals two new insights: 1) the plutonic rocks preserved in these systems are out of sync with the ignimbrite eruptions, and 2) economic mineralization (Mo, F, Pb and Zn, and to a lesser extent Au and Ag) is associated with plutons that accumulated within a few hundred thousand years after ignimbrite eruption, suggesting a genetic link between the two. Recognizing that plutons do not represent the frozen remains of huge fractionating, convecting magma bodies, raises the question of how petrologists have been fooled into thinking they were for so long. Part of the answer is likely that plutonic rock textures reflect the impact of millions of years under amphibolite- to greenschist-facies metamorphic conditions during pluton assembly. Thus, mapped pluton contacts reflect the thermal history, rather than the magmatic assembly history, of the body of rock.