Abstract

We report new experimental data on the composition of magmatic amphiboles synthesised from a variety of granite (sensu lato) bulk compositions at near-solidus temperatures and pressures of 0.8–10 kbar. The total aluminium content (Al^(tot)) of the synthetic calcic amphiboles varies systematically with pressure (P), although the relationship is nonlinear at low pressures (<2.5 kbar). At higher pressures, the relationship resembles that of other experimental studies, which suggests of a general relationship between Al^(tot) and P that is relatively insensitive to bulk composition. We have developed a new Al-in-hornblende geobarometer that is applicable to granitic rocks with the low-variance mineral assemblage: amphibole + plagioclase (An_(15–80)) + biotite + quartz + alkali feldspar + ilmenite/titanite + magnetite + apatite. Amphibole analyses should be taken from the rims of grains, in contact with plagioclase and in apparent textural equilibrium with the rest of the mineral assemblage at temperatures close to the haplogranite solidus (725 ± 75 °C), as determined from amphibole–plagioclase thermometry. Mean amphibole rim compositions that meet these criteria can then be used to calculate P (in kbar) from Al^(tot) (in atoms per formula unit, apfu) according to the expression: P (kbar)=0.5+0.331(8)×Al^(tot) +0.995(4)×(Al^(tot))^2. This expression recovers equilibration pressures of our calibrant dataset, comprising both new and published experimental and natural data, to within ±16 % relative uncertainty. An uncertainty of 10 % relative for a typical Al^(tot) value of 1.5 apfu translates to an uncertainty in pressure estimate of 0.5 kbar, or 15 % relative. Thus the accuracy of the barometer expression is comparable to the precision with which near-solidus amphibole rim composition can be characterised.

Highlights

  • Plutons of broadly granitic composition are widespread in the continental crust and play an important role in the evolution of orogenic terranes

  • The aim of this study is to address the above concerns regarding the extant Al-in-hornblende barometers by performing phase equilibrium experiments on a range of lower Al2O3 starting materials, at pressures of 10 down to 0.8 kbar and over a range of near-solidus temperatures

  • In both low-pressure apparatuses, heating was by external furnace; temperature was measured by a K-type thermocouple inserted into a well adjacent to the sample in the CSV or resting on the pressure vessel in the TZM, but calibrated for the offset with the sample temperature

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Summary

Introduction

Plutons of broadly granitic (or “granitoid”) composition are widespread in the continental crust and play an important role in the evolution of orogenic terranes. Dated plutons of known emplacement depth can be used as ‘crustal nails’ to track complex crustal exhumation during orogenesis (Anderson 1996; Ague and Brimhall 1988b). Granitoid plutons have close associations with hydrothermal ore deposits, notably porphyry copper deposits (PCDs) that are the world’s primary source of Cu and Mo (Sillitoe 2010). Granitoid bodies are predominantly found in the upper continental crust (2–20 km), with mineralised porphyry stocks occupying the shallow limit of this range (≤4 km). Providing an accurate quantitative constraint on the emplacement depth of individual intrusions, at the shallow crustal depths that typify many ore-related plutons, has proven difficult (Anderson et al 2008).

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