Geology and metamorphic evolution of the Indian Head Range, a Grenvillian inlier in West Newfoundland

Sung, Gee-Woong (1992) Geology and metamorphic evolution of the Indian Head Range, a Grenvillian inlier in West Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Indian Head Range (IHR), which forms an inlier in the western foreland of the Appalachian orogeny in Newfoundland, is mainly underlain by an upper amphibolite facies to granulite facies metaplutonic complex. Granulite facies units have annealed, high strain fabrics and include metanorthosite, meta-lherzolite, metagabbro and noritic, dioritic, granitic and politic gneiss. They predate a less deformed, amphibolite facies, foliated hornblende granodiorite. -- The IHR shows two distinct sets of ductile deformation fabric : (1) a relatively high strain group comprising gneissic to migmatitic rocks; and (2) a relatively lower strain group comprising foliated granitoids. This grouping is compatible with a subdivision made on the basis of grade of metamorphism, with the higher strain group possessing granulite facies assemblage and the lower strain group being characterized by amphibolite facies assemblages. Considering the relative ages of the units, it appears that two metamorphic events occurred in the IHR. -- Rare aluminous-magnesian mineral assemblages, indicated by the phases sapphirine and kornerupine, occur in the politic gneiss, which forms km-scale inclusions in metagabbro and screens between metagabbro and other gneissic plutonites. -- Assemblages in the politic gneiss are subdivided into three groups. (1) Quartz-bearing assemblages consist of qtz-opx-crd-sil-bio-pla-ilm/mt ± spl ± krn ± gnt ± gdr ± (crn). Sapphirine-bearing assemblages are subdivided into an Al-rich assemblage (2) spr-krn-opx-crd-sil-bio-pla-ilm/mt ± spl ± rt, and al Al-poor assemblage (3) spr-opx-bio-pla-ilm/mt ± spl ± rt. The sub-assemblage spr-qtz has not been observed. Reaction textures suggest the following metamorphic history. In quartz-bearing rocks, an early crd-spl-high Al opx (8.5-9.5 wt% Al₂O₃) assemblage implies T ≈ 800°C and P ≈ 8-10 kbars. In quartz-free rocks, the reaction crd-spl = spr=opx took place under similar P-T conditions. Kornerupine replacement by the reaction krn = spr-crd-low Al opx (5-7 wt% Al₂O₃) implies much lower pressures (5-6 kbars) at similar T (750 - 800°C), indicating a quasi-isothermal synmetamorphic decompression event from 8-10 to 5-6 kbars. [Please see thesis for remainder of abstract.]

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