Title

Golden Horn Batholith, Washington: Determining why sodic amphiboles occur in a peraluminous composition

Presenter Information

William Lochridge

Document Type

Oral Presentation

Location

SURC Ballroom C/D

Start Date

16-5-2013

End Date

16-5-2013

Abstract

The Golden Horn batholith is a zoned granitic pluton with a shallow emplacement of the magma chamber in the upper crust, but little is known about the geochemistry and petrography of the entire batholith. Sodic amphiboles usually occur in peralkaline melts but do occur in the peraluminous Golden Horn batholith. Whole rock geochemistry analyses of six alkali granite and granodiorite rocks using XRF (x-ray fluorescence) indicate negative trends for the major oxides versus SiO2 except for K2O. Samples from the top of the batholith that have very low concentrations of CaO are indicative of plagioclase fractionation while a spatially nearby sample does not indicate such fractionation meaning that the batholith is not homogeneous in composition. Moderate Al/(Ca+Na+K) ratios indicate peraluminous compositions and FeOtotal/(FeOtotal+MgO) ratios indicate magnesian compositions trending to ferroan compositions from the bottom to the top of the batholith. Petrographic analyses of fourteen thin sections show that samples from the top of the batholith contain sodic amphiboles (arfvedsonite and riebeckite) with little to no biotite. Hornblende and biotite abundances increase towards the bottom of the batholith, consistent with a more mafic composition. Late crystallizing accessory minerals such as apatite and zircon are seen as inclusions in quartz and feldspars. Quartz is also late crystallizing and seen as intergrowths in feldspars and surrounding mafic minerals. Mantling of albite around potassium feldspar is also seen throughout the batholith indicative of magma mixing event(s). The presence of sodic amphiboles in peraluminous compositions reflects the plagioclase fractionation that occurred in the batholith.

Poster Number

63

Faculty Mentor(s)

Chris Mattinson, Wendy Bohrson, Aaron Mayfield

Additional Mentoring Department

Geological Sciences

Additional Mentoring Department

Geological Sciences

Additional Mentoring Department

Geological Sciences

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May 16th, 8:20 AM May 16th, 10:50 AM

Golden Horn Batholith, Washington: Determining why sodic amphiboles occur in a peraluminous composition

SURC Ballroom C/D

The Golden Horn batholith is a zoned granitic pluton with a shallow emplacement of the magma chamber in the upper crust, but little is known about the geochemistry and petrography of the entire batholith. Sodic amphiboles usually occur in peralkaline melts but do occur in the peraluminous Golden Horn batholith. Whole rock geochemistry analyses of six alkali granite and granodiorite rocks using XRF (x-ray fluorescence) indicate negative trends for the major oxides versus SiO2 except for K2O. Samples from the top of the batholith that have very low concentrations of CaO are indicative of plagioclase fractionation while a spatially nearby sample does not indicate such fractionation meaning that the batholith is not homogeneous in composition. Moderate Al/(Ca+Na+K) ratios indicate peraluminous compositions and FeOtotal/(FeOtotal+MgO) ratios indicate magnesian compositions trending to ferroan compositions from the bottom to the top of the batholith. Petrographic analyses of fourteen thin sections show that samples from the top of the batholith contain sodic amphiboles (arfvedsonite and riebeckite) with little to no biotite. Hornblende and biotite abundances increase towards the bottom of the batholith, consistent with a more mafic composition. Late crystallizing accessory minerals such as apatite and zircon are seen as inclusions in quartz and feldspars. Quartz is also late crystallizing and seen as intergrowths in feldspars and surrounding mafic minerals. Mantling of albite around potassium feldspar is also seen throughout the batholith indicative of magma mixing event(s). The presence of sodic amphiboles in peraluminous compositions reflects the plagioclase fractionation that occurred in the batholith.