NOVEL SHOCK MICROSTRUCTURES IN ACCESSORY MINERALS FROM THE HAUGHTON DOME, DEVON ISLAND, NUNAVUT, CANADA
This study presents high resolution electron backscatter diffraction (EBSD) microstructural data for zircon and monazite from historic samples of the basement gneiss, in which bulk shock pressures have been previously constrained based on major phases [4, 6]. Shocked zircon and monazite grains have been investigated from shock stage 1b (sample 72110), 2 (7273) and 3 (7192, Dig-9) [4, 6]. At lower shock levels zircon displays planar microstructures consistent with mechanical shock {112} twin formation [7] and deviatoric transformation to the high pressure polymorph reidite [8]. Zircon grains from shock stage three show a more chaotic microstructure with multiple orientations of tightly spaced sets of reidite that are variably recrystallized to zircon neoblasts. Monazite from lower shock stages contains a number of mechanical twin orientations that are indicative of shock deformation [9]. At higher shock pressures a lath like structure of interlocking twin orientations has been identified. This microstructure is suggestive of a reversion transformation from a high pressure polymorph [10] and is the first evidence for the transformation of monazite during shock.
References: [1] Osinski et al. (2005) MAPS, 40, 1759–1776, [2] Young et al. (2013) GRL, 40, 3836–3840, [3] Redeker & Stöffler (1988) MAPS, 23, 185–196, [4] Scharer & Deutsch (1990) GCA, 54, 3435–3447, [5] Singleton et al. (2015) GSASP 518, 135–148, [6] Metzler et al. (1988) MAPS 23, 197–207, [7] Erickson et al. (2013) Am Min 98, 59–65, [8] Erickson et al. (2017) CMP 176, 6, [9] Erickson et al. (2016) Geology 44, 635–638. [10] Delaey (1991) Pha. Trans Mat. 5, 585–684.