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RESEARCH PRODUCT

Geometric aspects of synkinematic granite intrusion into a ductile shear zone — an example from the Yunmengshan core complex, northern China

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The Cretaceous Yungmengshan core complex in northern China contains a large syntectonic granodiorite batholith that intrudes a slightly older diorite intrusion. A major gently dipping ductile decollement shear zone is developed along the contact of the diorite and granodiorite. The shear zone is invaded by a large volume of granitic and pegmatite veins associated with the main granodiorite batholith during activity of the shear zone under high-grade metamorphic conditions. Progressively older veins are more strongly deformed into tight cylindrical fold structures rotated into parallelism with the lineation and foliation in the shear zone. Parallelism of veins to the foliation is partly due to this rotation, but also to foliation-parallel injection of younger syntectonic pegmatite veins. Several small-scale structures have been recognized that allow distinction of solid-state deformation of veins. Granite veins do not extend much above the ductile shear zone that seems to act as a lid and an effective depository to intruding granite veins from the underlying batholith. There was considerable volume increase in the footwall and lower part of the shear zone by vein intrusion. Gently dipping ductile shear zones are a common feature of many continental metamorphic core complexes throughout the world, for example in the Basin and Range (Reynolds & Lister 1987; Fletcher & Bartley 1994; Foster & Fanning 1997; Foster et al. 2001), Algeria (Caby et al. 2001), New Guinea (Baldwin et al. 1993) and in the Aegean Sea (Lister et al. 1984; Walcott & White 1998; Pe-Piper et al. 2002). The shear zones are thought to accommodate most of the crustal-scale extension that is typical for the development of such core complexes. Core complexes are also commonly associated with the intrusion of granitoid plutons and associated rhyolitic volcanism (e.g. Caby et al. 2001; Foster et al. 2001; Pe-Piper et al. 2002). Many granitoid intrusions in core complexes can be shown to have intruded during active deformation of the ductile shear zones of the complex (Caby et al. 2001; Foster et al. 2001; Pe-Piper et al. 2002). In such cases, parts of a pluton may cut mylonitic rocks, while other parts are mylonitized with the same orientation of mylonitic structures as in the truncated older parts, and this is used as evidence of syntectonic intrusion. In the Yunmengshan of northern China, a Cretaceous core complex developed in Archaean basement and Proterozoic metasediments at the northern rim of the North China Craton (Fig. 1) (Davis et al. 1996). The core complex is centred on a granodiorite batholith, which is truncated at the northern, eastern and southern sides by brittle and ductile shear-zone segments. The segments dip gently away from the granodiorite batholith towards the north, east and south (Figs 1 and 2). Mapping by Davis et al. (1996) has shown that, although these shearzone segments are of different metamorphic grade and presumably of different age in different parts of the pluton, they share a common orientation of stretching lineations. They are therefore inferred to have formed as part of one tectonic shear-zone system that acted as a major decollement of the core complex. Its present variation in dip direction and dip is probably due to folding over the developing core complex, a feature also observed in many other complexes (e.g. Fletcher & Bartley 1994). Along the NE side of the core complex, the Yunmengshan granodiorite is bordered by metadioritic rocks and the contact is affected by a ductile shear zone, the Sihetang shear zone discussed in this paper (Fig. 2). The metadiorites are intrusive into Proterozoic metasediments (Fig. 2) (Miyun metadiorite; Davis et al. 1996), From: BRUHN, D. & BURLINI, L. (eds) 2005. High-Strain Zones: Structure and Physical Properties. Geological Society, London, Special Publications, 245, 65–80. 0305-8719/05/$15.00 # The Geological Society of London 2005. but the intrusive contact is rarely exposed and is marked by a Mesozoic brittle thrust fault in most places (Fig. 2). This fault post-dates intrusion of the diorite and granodiorite, and seems to postdate and cut the Sihetang ductile shear zone. The thrust and the Sihetang shear zone are cut in the SE by the brittle Hefangkou normal fault (Fig. 1). The Sihetang shear zone is an amphibolite facies ductile shear zone with a thickness of 20–50 m (Fig. 2) (Davis et al. 1996). The shear zone is centred on the contact between the metadiorite and the granodiorite, and has strongly mylonitized the igneous rocks. Despite the presence of the shear zone, it is clear that the contact between the granite and granodiorite of the Yunmengshan batholith and the Miyun metadiorite was intrusive: xenoliths of metadiorite occur in the granite, and granite veins intrude the metadiorite. The metadiorite therefore predates the granitic phases. Dating of both units has given concordant U–Pb zircon ages of 159 + 2 Ma for the Miyun metadiorite and ages between 151 and 127 Ma for the Yunmengshan granodiorite (Davis et al. 1996). Although the main body of the granodiorite is deformed by the Sihetang shear zone, there is a large volume of granite dykes that show different stages of deformation, and a decrease in deformation intensity with a decrease in relative age within the shear zone, based on cross-cutting relations. This suggests that the shear zone was active while intrusive material was added from lower levels of the granodiorite pluton. The Sihetang shear zone is not affected by late brittle deformation and forms a flat ‘roof’ to the batholith. It is only invaded by minor veins, although these represent a considerable volume of material. Some work has been published on synkinematic intrusion of granite veins into ductile shear zones (Zurbriggen et al. 1998; Pawley et al. 2002), but three-dimensional (3D) control on geometry is limited in these studies. The excellent outcrop condition and the sharp colour contrast between the metadiorite and the leucocratic veins in the Sihetang shear zone Proterozoic metasediments