Abstract
In this study, we report mesoscopic and microscopic structures in sheared rocks associated with the Wanni Complex/Highland Complex boundary shear zone (WHBSZ) which represents a deep part of a granulite-facies, crustal-scale ductile shear zone in the Proterozoic basement of Sri Lanka. Normal khondalitic mylonites, spinel-bearing khondalitic mylonites, semipelitic mylonites and metagranitoid mylonites (striped gneisses) found along the WHBSZ show mesoscopic to microscopic scale shear sense indicators such as S-C' fabrics, rotated garnet porphyroblasts, δ-type and σ-type asymmetric objects, asymmetric folds and sheath folds. These asymmetric mesoscopic to microscopic structures indicate that the dominant deformation along the WHBSZ is non-coaxial ductile flow and that the WC has been emplaced from NNW (present orientation) on top of the HC. The presence of mylonites and ultramylonites along the WHBSZ suggests severe grain size reduction during the shearing. All the mylonites studied along the boundary shear zone have developed a mylonitic foliation, defined by quartz plates and ribbons, elongated and recrystallized fine-grained aggregates of feldspar, and a very strong stretching lineation mostly defined by c-axis of sillimanite and stretched quartz and elongated aggregates of other minerals. All the mesoscopic and microscopic structures suggest that the WHBSZ has formed during the second phase of D2 non-coaxial deformation accompanying high-temperature (HT) to ultrahigh temperature (UHT) granulite facies meta-morphism related to the first collision between the WC and the HC. This study emphasizes that careful study of granulite facies mylonites may provide reliable shear sense indicators for understanding the nature and kinematics of deep crustal shear zones.
Highlights
Crustal-scale ductile shear zones are an important type of paleo movement zones and provide information about the nature and types of deformation at deep crustal levels associated with motions of lithospheric fragments (e.g. Passchier, 1988; Passchier and Coelho, 2006; Platt andBehr, 2011; Fossen et al, 2017)
The Wanni Complex/Highland Complex boundary shear zone (WHBSZ) in Sri Lanka is an excellent example for such deep crustal shear zones, along which two contrasting crustal blocks – namely, the Wanni Complex and the Highland Complex – are welded together (e.g. Kehelpannala, 1991, 1997, 2003), and provides ample evidences for deep crustal movements associated with plate collision during Gondwana assembly (e.g. Kehelpannala, 1997, 2003, 2004; Kröner et al, 2003)
It has been shown that the Wanni Complex/Highland Complex boundary is a suture zone (Kehelpannala, 1997, 2003, 2004; Kehelpannala and Ranaweera, 2007; Ranaweera, 2008), exposed as a crustal-scale ductile shear zone, that formed under high-temperature (HT) to ultrahigh temperature (UHT) granulite facies conditions (Kehelpannala, 2016a)
Summary
Crustal-scale ductile shear zones are an important type of paleo movement zones and provide information about the nature and types of deformation at deep crustal levels associated with motions of lithospheric fragments The two-collision model of Kehelpannala (2004) seems to explain the available data from all the lithotectonic units (see Touret et al, 2019), including the large volume of HC Paleoproterozoic metagranitoids (Baur et al, 1991; Kröner et al, 2017, 2018), the HC klippen occurring in the south-eastern part of the VC (Fig. 1; Kehelpannala 2016a), the Late NeoproterozoicCambrian metamorphic ages indicating multiple thermal events (Kehelpannala, 2016a and reference therein), and the clockwise P-T paths of the HC and WC (Raase and Schenk, 1994; Osanai et al, 2016b; Dharmapriya et al, 2017a; Touret et al, 2019 and references therein) Both the WC and HC have undergone polyphase ductile deformation before and during the Gondwana assembly (Berger and Jayasinghe, 1976; Yoshida et al, 1990; Voll and Kleinschrodt, 1991; Kehelpannala, 1991, 1997, 2003; Kriegsman, 1991, 1993, 1994; Kleinschrodt and Voll, 1994; Tani, 2002). This is confirmed by the presence of subgrains in some quartz grains (Figs. 7c,d) and sillimanite (Fig. 7e)
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