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Tracking magmatic processes through Zr/Hf ratios in rocks and Hf and Ti zoning in zircons: An example from the Spirit Mountain batholith, Nevada

Published online by Cambridge University Press:  05 July 2018

L. Lowery Claiborne*
Affiliation:
Earth and Environmental Sciences, Vanderbilt University, SC 5717 Science & Engineering Bldg., Stevenson Center Dr., Nashville, TN, USA
C. F. Miller
Affiliation:
Earth and Environmental Sciences, Vanderbilt University, SC 5717 Science & Engineering Bldg., Stevenson Center Dr., Nashville, TN, USA
B. A. Walker
Affiliation:
US Geological Survey, USGS-Stanford Micro-isotopic Analytical Center, Stanford University, Green Building, 367 Panama St., Stanford, CA, USA
J. L. Wooden
Affiliation:
US Geological Survey, USGS-Stanford Micro-isotopic Analytical Center, Stanford University, Green Building, 367 Panama St., Stanford, CA, USA
F. K. Mazdab
Affiliation:
US Geological Survey, USGS-Stanford Micro-isotopic Analytical Center, Stanford University, Green Building, 367 Panama St., Stanford, CA, USA
F. Bea
Affiliation:
Department of Mineralogy and Petrology, Campus Fuentenueva, University of Granada, 18002 Granada, Spain

Abstract

Zirconium and Hf are nearly identical geochemically, and therefore most of the crust maintains near-chondritic Zr/Hf ratios of ∼35–40. By contrast, many high-silica rhyolites and granites have anomalously low Zr/Hf (15–30). As zircon is the primary reservoir for both Zr and Hf and preferentially incorporates Zr, crystallization of zircon controls Zr/Hf, imprinting low Zr/Hf on coexisting melt. Thus, low Zr/Hf is a unique fingerprint of effective magmatic fractionation in the crust. Age and compositional zonation in zircons themselves provide a record of the thermal and compositional histories of magmatic systems. High Hf (low Zr/Hf) in zircon zones demonstrates growth from fractionated melt, and Ti provides an estimate of temperature of crystallization (T TiZ) (Watson and Harrison, 2005). Whole-rock Zr/Hf and zircon zonation in the Spirit Mountain batholith, Nevada, document repeated fractionation and thermal fluctuations. Ratios of Zr/Hf are ∼307–40 for cumulates and 18–30 for high-SiO2 granites. In zircons, Hf (and U) are inversely correlated with Ti, and concentrations indicate large fluctuations in melt composition and T TiZ (>100°C) for individual zircons. Such variations are consistent with field relations and ion-probe zircon geochronology that indicate a >1 million year history of repeated replenishment, fractionation, and extraction of melt from crystal mush to form the low Zr/Hf high-SiO2 zone.

Information

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2006

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