Development of inverted metamorphic isograds in the western metamorphic belt, Juneau, Alaska

Abstract

An inverted metamorphic gradient is preserved in the western metamorphic belt near Juneau, Alaska. The western metamorphic belt is part of the Coast plutonic-metamorphic complex of western Canada and southeastern Alaska that developed as a result of tectonic overlap and/or compressional thickening of crustal rocks during collision of the Alexander and Stikine terranes. Detailed mapping of pelitic single-mineral isograds, systematic changes in mineral assemblages, and silicate geothermometry indicate that thermal peak metamorphic conditions increase structurally upward over a distance of about 8 km. Peak temperatures of metamorphism increase progressively from about 530 "C for the garnet zone to about 705 "C for the upper kyanite-biotite zone. Silicate geobarometry suggests that the thermal peak metamorphism occurred under pressures of 9-11 kbar. The metamorphic isograds are in general parallel to the tonalite sill that is regionally continuous along the east side of the western metamorphic belt, although truncation of the isograds north of Juneau indicates that the sill intrusion continued after the isograds were established. Our preferred interpretation of the cause of the inverted gradient is that it formed during compression of a thickened wedge of relatively wet and cool rocks in response to heat flow associated with the formation and emplacement of the tonalite sill magma. Garnet rim compositions and widespread growth of chlorite suggest partial re-equilibration of the schists under pressures of 5-6 kbar during uplift in response to final emplacement and crystallization of the tonalite sill. The combined results of this study with previous studies elsewhere in the western metamorphic belt indicate that high-T/high-P metamorphism associated with the collision of the Alexander and Stikine terranes was a long-lived event, extending from about 98 Ma to about 67 Ma.

Terrane assembly and structural relationships in the eastern Prince Rupert Quadrangle, British Columbia

“…We attribute both the regional metamorphism and the inverted metamorphic gradient in the Prince Rupert transect to thrust emplacement of hotter rocks above the Prince Rupert shear zone over the cooler rocks to the west. To the north, in Alaska, staurolite-kyanite-grade metamorphism along the eastern side of the belt has been ascribed (1) to contact metamorphism around 100-90 Ma plutons (Cook and Crawford, 1994;Stowell and Inman, 1991;Stowell and Crawford, this volume), and (2) to emplacement of the foliated tonalite plutons of the Great Tonalite Sill of Brew (1988) (Himmelberg et al, 1991(Himmelberg et al, , 1994. Neither of these mechanisms can be invoked for the Prince Rupert area.…”

Geology of western Ullsfjord, North Norway, with emphasis on the development of an inverted metamorphic gradient at the top of the Lyngen Nappe Complex

“…(1) downheating from the emplacement of a hot upper plate (Wagner & Srogi 1987;Duebendorfer 1988;Hubbard 1989); (2) emplacement of hot magmatic bodies and subsequent contact metamorphism of the rocks below the intrusion (Hubbard & Harrison 1989;Himmelberg et al 1991); and (3) inversion of isotherms within a subduction zone (Peacock 1987(Peacock , 1988Peacock & Norris 1989). England & Molnar (1993) and Ruppel & Hodges (1994) discuss how dissipative shear heating along thrust faults may contribute to the formation of synmetamorphic, inverted metamorphic gradients.…”

“…The development of synmetamorphic, inverted metamorphic gradients is a topic of much discussion (Graham & England 1976;Crawford et al 1987;Peacock 1987;Duebendorfer 1988;Hubbard 1989;Himmelberg et al 1991;Jain & Manickavasagam 1993) and the mechanisms by which they form are not well understood. and Krogh et al (1990) suggest insequence stacking of the nappes and production of the metamorphic inversion at the top of the Balsfjord Group by downheating from the overlying Tromsø Nappe Complex.…”

Orogenesis, high-T thermal events, and gold vein formation within metamorphic rocks of the Alaskan Cordillera

“…A belt of tonalitic plutons were intruded approximately 5 km east of the megalineament between 68-61 Ma (Barker et al, 1986;Wood et al, 1991). The tonalities are believed to have been the primary source of heat for development of an inverted metamorphic gradient that extends westward as far as the megalineament (Himmelberg et al, 1991). Extensive Eocene magmatism forming part of the Coast batholith took place from about 56--48Ma (Barker and Arth, 1990;Snee, unpub, data), approximately 20-50 km east of the gold belt.…”

“…Auriferous veins show a strong spatial association with relatively competent igneous bodies of varied composition; these rocks are, however, many tens of millions of years older than the veining. The veins are also spatially associated with greenschist-facies rocks of an inverted metamorphic gradient of up to 8 km in thickness (Himmelberg et al, 1991). (Goldfarb et al 1991b).…”