Zircon chronochemistry of the Mesa Falls Tuff (Yellowstone, USA): progressive remelting of a felsic source by underplated mafic magma
J. MICHAEL PALIN
Department of Geology, University of Otago, Dunedin, New Zealand (michael.palin@otago.ac.nz)
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(1) Regional setting and geochronology of the Mesa Falls Tuff. |
Zircon (ZrSiO4) contains a wealth of petrogenetic information for intermediate to felsic igneous rocks. Excimer (193 nm) laser ablation (ELA) provides access to these data via highly reproducible in situ (≥15 micrometer spot) sampling for analysis by ICP-MS. Rapid scanning over a wide mass range by quadrupole ICP-MS allows concurrent measurement of major and trace elements with U-Th-Pb isotopes. Integration of spatially resolved age and compositional data forms the basis of zircon chronochemistry.
In order to understand better the temporal and chemical evolution of felsic magma bodies in the upper crust, the zircon chronochemistry of large-volume ignimbrites of the Yellowstone volcanic field is being examined. Three populations of zircons can be distinguished in the 280 km3 Mesa Falls Tuff (MFT) based on combined age and trace element criteria. Population 1 is oldest (1350 ± 30 ka) and has extremely high U with very negative Eu anomalies. Population 2 has intermediate age (1300 ± 30 ka) and trace element characteristics. Population 3 is youngest (1190 ± 70 ka) and has the lowest U with the smallest negative Eu anomalies. Smooth trace element variations within and between the 3 populations provide evidence for zircon growth from progressively higher degree partial melts of a felsic source (feldspar + zircon residue) culminated by mixing with primitive melt. These and published data are consistent with remelting of plutonic residues and altered (18O-depleted) caldera-fill of the Huckleberry Ridge Tuff (2000 ka, 2500 km3) by underplated mafic magma 160 ± 100 kyr prior to eruption of the MFT.
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(2) Ash fall layers and unwelded base of Mesa Falls ignimbrite with pumice blocks. |
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(3) Mesa Falls ignimbrite contains at least 3 kinds of zircon based on trace elements. |
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(4) Mesa Falls Tuff zircon U-Pb ages reveal an extended magmatic history prior to eruption. |
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(5) Mesa Falls magma appears to have been generated by progressive melting of older felsic igneous rocks (Huckleberry Tuff and intrusive equivalents) by influx of basaltic magma. |
