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Book Chapter

Quartz inclusions in garnet: Time capsules of early mountain building

By
Kyle T. Ashley
Kyle T. Ashley
Department of Geological Sciences, Virginia Tech, 4044 Derring Hall (0420), Blacksburg, Virginia 24061, USA
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Richard D. Law
Richard D. Law
Department of Geological Sciences, Virginia Tech, 4044 Derring Hall (0420), Blacksburg, Virginia 24061, USA
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Robert J. Bodnar
Robert J. Bodnar
Department of Geological Sciences, Virginia Tech, 4044 Derring Hall (0420), Blacksburg, Virginia 24061, USA
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Kenneth A. Eriksson
Kenneth A. Eriksson
Department of Geological Sciences, Virginia Tech, 4044 Derring Hall (0420), Blacksburg, Virginia 24061, USA
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Published:
September 11, 2017
Publication history
27 January 2017

ABSTRACT

Much of the early prograde history in metamorphic rocks is lost due to overprinting at near-peak conditions or through retrograde modification during exhumation. Fortunately, inclusions encapsulated in rigid porphyroblasts may preserve a record of early burial conditions. Quartz inclusions in garnet porphyroblasts from the Strafford Dome, eastern Vermont, have homogeneous Ti concentrations ([Ti]) that differ from matrix quartz, which retains a history of Si-liberating metamorphic reactions and fluid influx. We applied growth-composition models to evaluate potential processes associated with Ti partitioning in quartz before encapsulation in garnet, including a model for constant-volume growth of quartz due to mineral dissolution-transfer processes and growth as a result of Si-liberating diagenetic and metamorphic reactions. Because these processes typically occur at low temperatures, quartz with exceedingly low [Ti] (<<1 ppm) would be formed and cannot account for the homogeneous Ti distribution at concentrations between 2.5 and 5 ppm observed in the sample. This suggests that chemical reequilibration through dynamic recrystallization must have taken place prior to encapsulation in garnet. Analysis of fluid and graphite inclusions with Raman spectroscopy in different microstructural settings allowed the characterization of fluid composition and temperature of microstructure development early in the prograde history. The findings from this study exemplify the utility of garnet hosts to shield inclusion minerals from chemical modification and recrystallization during later events. As such, they provide a window into the early stages of orogenesis and provide insights concerning the mechanisms controlling equilibration of quartz.

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Contents

GSA Memoirs

Linkages and Feedbacks in Orogenic Systems

Geological Society of America
Volume
213
ISBN electronic:
9780813782133
Publication date:
September 11, 2017

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