TY - JOUR
T1 - Preservation of evidence for prograde metamorphism in ultrahigh-temperature, high-pressure kyanite-bearing granulites, South Harris, Scotland
AU - Hollis, J.A.
AU - Harley, S.L.
AU - White, R.W.
AU - Clarke, G.L.
PY - 2006/4
Y1 - 2006/4
N2 - Mineralogical and mineral chemical evidence for prograde metamorphism is rarely preserved in rocks that have reached ultrahigh-temperature (UHT) conditions (>900 °C) because high diffusion and reaction rates erase evidence for earlier assemblages. The UHT, high-pressure (HP) metasedimentary rocks of the Leverburgh belt of South Harris, Scotland, are unusual in that evidence for the prograde history is preserved, despite having reached temperatures of ∼955 °C or more. Two lithologies from the belt are investigated here and quantitatively modelled in the system NaO-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O: a garnet-kyanite-K-feldspar-quartz gneiss (X
Mg = 37, A/AFM = 0.41), and anorthopyroxene-garnet-kyanite-K-feldspar quartzite (XMg = 89 A/AFM = 0.68). The garnet-kyanite gneiss contains garnet porphyroblasts that grew on the prograde path, and captured inclusion assemblages of biotite, sillimanite, plagioclase and quartz (<790 °C, <9.5 kbar). These porphyroblasts preserve spectacular calcium zonation features with an early growth pattern overgrown by high-Ca rims formed during high-P metamorphism in the kyanite stability field. In contrast, Fe-Mg zonation in the same garnet porphyroblasts reflects retrograde re-equilibration, as a result of the relatively faster diffusivity of these ions. Peak P-T are constrained by the occurrence of coexisting orthopyroxene and aluminosilicate in the quartzite. Orthopyroxene porphyroblasts [y(opx) = 0.17-0.22] contain sillimanite inclusions, indicative of maximum conditions of 955 ± 45 °C at 10.0 ± 1.5 kbar. Subsequently, orthopyroxene, kyanite, K-feldspar and quartz developed in equilibrated textures, constraining the maximum pressure conditions to 12.5 ± 0.8 kbar at 905 ± 25 °C. P-T-X modelling reveals that the mineral assemblage orthopyroxene-kyanite-quartz is compositionally restricted to rocks of X
Mg > 84, consistent with its very rare occurrence in nature. The preservation of unusual high P-T mineral assemblages and chemical disequilibrium features in these UHT HP rocks is attributed to a rapid tectonometamorphic cycle involving arc subduction and terminating in exhumation.
AB - Mineralogical and mineral chemical evidence for prograde metamorphism is rarely preserved in rocks that have reached ultrahigh-temperature (UHT) conditions (>900 °C) because high diffusion and reaction rates erase evidence for earlier assemblages. The UHT, high-pressure (HP) metasedimentary rocks of the Leverburgh belt of South Harris, Scotland, are unusual in that evidence for the prograde history is preserved, despite having reached temperatures of ∼955 °C or more. Two lithologies from the belt are investigated here and quantitatively modelled in the system NaO-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O: a garnet-kyanite-K-feldspar-quartz gneiss (X
Mg = 37, A/AFM = 0.41), and anorthopyroxene-garnet-kyanite-K-feldspar quartzite (XMg = 89 A/AFM = 0.68). The garnet-kyanite gneiss contains garnet porphyroblasts that grew on the prograde path, and captured inclusion assemblages of biotite, sillimanite, plagioclase and quartz (<790 °C, <9.5 kbar). These porphyroblasts preserve spectacular calcium zonation features with an early growth pattern overgrown by high-Ca rims formed during high-P metamorphism in the kyanite stability field. In contrast, Fe-Mg zonation in the same garnet porphyroblasts reflects retrograde re-equilibration, as a result of the relatively faster diffusivity of these ions. Peak P-T are constrained by the occurrence of coexisting orthopyroxene and aluminosilicate in the quartzite. Orthopyroxene porphyroblasts [y(opx) = 0.17-0.22] contain sillimanite inclusions, indicative of maximum conditions of 955 ± 45 °C at 10.0 ± 1.5 kbar. Subsequently, orthopyroxene, kyanite, K-feldspar and quartz developed in equilibrated textures, constraining the maximum pressure conditions to 12.5 ± 0.8 kbar at 905 ± 25 °C. P-T-X modelling reveals that the mineral assemblage orthopyroxene-kyanite-quartz is compositionally restricted to rocks of X
Mg > 84, consistent with its very rare occurrence in nature. The preservation of unusual high P-T mineral assemblages and chemical disequilibrium features in these UHT HP rocks is attributed to a rapid tectonometamorphic cycle involving arc subduction and terminating in exhumation.
KW - Garnet zonation
KW - Kyanite-orthopyroxene-quartz
KW - NCKFMASH
KW - P-T-X modelling
KW - Pseudosection
UR - http://www.scopus.com/inward/record.url?scp=33644803473&partnerID=8YFLogxK
U2 - 10.1111/j.1525-1314.2006.00636.x
DO - 10.1111/j.1525-1314.2006.00636.x
M3 - Article
SN - 0263-4929
VL - 24
SP - 263
EP - 279
JO - Journal of Metamorphic Geology
JF - Journal of Metamorphic Geology
IS - 3
ER -