Collaborative Research: Are Plutons Assembled Over Long Periods of Time Through Amalgamation of Small Ephemeral Magma Chambers?

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Plutons are basic building blocks of the continental crust. They are widely regarded to reflect ancient magma chambers, yet their true relationship to magma chambers is obscure and controversial. Resolving the relationship bears on questions such as mechanisms of magma transport in the crust, the pluton "room" problem, relations between plutonism and regional deformation, the longevity of magmatic systems, why some magma erupts and some is trapped to form plutons, and relationships between magma budget, recharge, and eruption in igneous systems. Answers to these questions depend on precisely measuring the longevity and intrusive histories of plutonic systems. There are two end-member concepts of what plutons represent. At one end is the long-held notion that plutons represent large magma bodies (hundreds or thousands of cubic kilometers) emplaced in single events on a time scale of 10,000-1,000,000 years. Alternatively, plutons may grow incrementally by amalgamation of small episodic intrusions over durations in excess of 10 million years. In the latter view, large volumes of magma never were present during growth of the pluton and the pluton may be highly, perhaps cryptically, composite, and fundamentally diachronous.This project examines two nested plutonic complexes in the Sierra Nevada, the Tuolumne and Whitney Intrusive Suites. Each of these Late Cretaceous systems has been magnificently exposed by glaciation and is relatively undeformed. New data from the Tuolumne indicate that it was emplaced over roughly 10 million years, and at least one constituent intrusion (the Half Dome pluton) was emplaced over greater than 3 m.y. These results favor incremental emplacement and cryptically composite intrusions. However, the general applicability of the results is uncertain, and the history of the similar (but larger) Whitney system is poorly known. Using U/Pb geochronology, the lifetimes of each magmatic system, and whether individual plutons were emplaced in single batches or in many small increments is being determined. In the Half Dome pluton that requires a cryptically composite emplacement mechanism, unconventional mapping techniques (e.g., magnetic susceptibility) are being explored in an attempt to distinguish composite from noncomposite plutons, and thus to map in the field the internal structures of cryptically composite plutons.The intellectual merit of this proposal lies in testing prevailing hypotheses for pluton emplacement and crustal growth. If our hypothesis is correct then pluton emplacement processes are markedly different from the current paradigm. Assessing the rates and mechanisms of pluton emplacement bears on basic research questions spanning the disciplines of structural geology, petrology, tectonics, and geologic hazards. Additionally, analytical techniques at the UNC geochronology lab will be improved and new techniques for recognizing cryptic intrusive contacts in plutons are being established. Results from our work will be useful to scientists and naturalists at Yosemite, Sequoia, and Kings Canyon National Parks as they work to increase public understanding of science.