Episodic deformation and reactions in mylonitic high-grade metamorphic granulites from Dronning Maud Land, Antarctica

Ane K Engvik,Cornelia Mertens,Claudia A Trepmann

doi:10.1016/j.jsg.2020.104196

Abstract

Microfabrics of mylonitic orthopyroxene-bearing quartzofeldspatic granulites from central Dronning Maud Land, Antarctica, have been investigated to evaluate the deformation behaviour at lower crustal conditions. The microfabric is characterized by a fine-grained matrix of dispersed feldspar, quartz and orthopyroxene, which contains monophase elongate quartz and feldspar aggregates, so-called ribbons, and elongate monocrystalline orthopyroxene ribbon grains. The observed crystallographic preferred orientation (CPO) of orthopyroxene ribbon grains that contain clinopyroxene exsolution lamellae in (100) and kink bands in equidimensional porphyroclasts indicate (100)[001] dislocation glide. Fine-grained matrix orthopyroxene formed by deformation of porphyroclasts and growth in strain shadows during diffusional creep, indicating a separate stage of deformation and reaction. Although dislocation creep of coarse ribbon quartz is indicated by sutured grain boundaries at high angle to the ribbon boundary and few subgrains, the weak CPO is rather unspecific. Fine-grained matrix quartz shows no indication of dislocation creep. We suggest that epitactic growth during quartz-producing mineral reactions played a major role for ribbon formation during diffusional creep. Our study demonstrates the importance of episodic deformation and metamorphic reactions in lower continental crust with transient high stresses allowing for dislocation glide of orthopyroxene enclosed in a fine-grained polyphase matrix undergoing diffusional creep on long term.

Highlights

Granulites, orthopyroxene- and feldspar-dominated high-tempera ture metamorphic rocks, are well-known from Precambrian continental areas and form large parts of the lower continental crust underlying orogenic belts (Jackson et al, 2004; Brown, 2007)
- Serrated phase boundaries of orthopyroxene porphyroclasts, the rounded slightly elongate shape of matrix grains in polyphase strain shadows and generally dominating phase boundaries (Figs. 4, 6 and 10) indicate an influence of metamorphic reactions associated with subgrain rotation and cataclasis forming dispersed grains in the polyphase matrix during diffusional creep. - Quartz in ribbons shows evidence of dislocation creep by sutured grain boundaries at high angle to the ribbon boundary and few subgrains
The crystallographic preferred orientation (CPO) is weak and quartz in the matrix is not deformed by dislocation creep (Figs. 5, 7 and 8)

Summary

Introduction

Granulites, orthopyroxene- and feldspar-dominated high-tempera ture metamorphic rocks, are well-known from Precambrian continental areas and form large parts of the lower continental crust underlying orogenic belts (Jackson et al, 2004; Brown, 2007). For an understanding of the development of orogens, knowledge of the rheology of granulites at varying metamorphic and tectonic conditions is essential (e.g., Maierovaet al., 2017). Observa tions from exhumed metamorphic quartzofeldspatic granulitic rocks, which record grain-scale mechanical and chemical transformations at depths, are indispensable, and microstructural studies are rare (Martelat et al, 1999; Hanmer, 2000; Franek et al, 2011; Maierovaet al., 2017). Little is known on the deformation behaviour of orthopyroxene in quartzofeldspatic granulites (e.g., Hanmer, 2000). Jin et al, 2001; Zhang et al, 2006; Orzol et al, 2006; Moghadam et al, 2010), the knowledge on deformation mechanisms of orthopyroxene at high-grade metamorphic conditions is scarce Abundant studies on clinopyroxene deformation behaviour at high-pressure conditions have been published from natural samples (e.g. van Roermund and Boland, 1981; Philippot et al, 1992; Godard and Van Roermund, 1995; van der Klauw et al, 1997; Stockhert and Renner, 1998; Aoya, 2001; Piepenbreier and Stockhert, 2001; Terry and Heidelbach, 2006; Engvik et al, 2007; Wang et al, 2010) and experi ments (e.g. Jin et al, 2001; Zhang et al, 2006; Orzol et al, 2006; Moghadam et al, 2010), the knowledge on deformation mechanisms of orthopyroxene at high-grade metamorphic conditions is scarce

Methods

Discussion

Conclusion