Exploding xenoliths and the absence of ‘elephants’ graveyards’ in granite batholiths

Biotite monzogranite near the eastern margin of the late Devonian South Mountain Batholith at Portuguese Cove, Nova Scotia, surrounds a large (20 m×25 m), anisotropic (decimetre-scale interbedded psammite and semipelite) xenolith of the metamorphic (regional greenschist/thermal cordierite hornfels) country rocks of the Cambro-Ordovician Meguma Group. The xenolith has two distinct types of surfaces against the monzogranite: (i) one rounded, diffuse, and apparently old surface that had prolonged contact with the granite magma; and (ii) two straight, sharp, and apparently new surfaces that had more recently come into contact with the granite magma. More than 375 thin sheets (width <0.1 cm, n=224; 0.1–0.5 cm, n=66; 0.5–1.0 cm, n=51; 1.0–5.0 cm, n=30; 5.0–10.0 cm, n=3; >10 cm, n=2), ranging in composition from monzogranite to pegmatite to coarse quartz–sericite, cut the xenolith parallel to bedding producing a cumulative volume expansion of ∼14% in the xenolith. Aspect ratios in some of the thin sheets exceed 1000:1. Several sheets cut the xenolith perpendicular to bedding producing a volume expansion of <4%. We postulate that this pattern of sheet injection occurred in response to cracking of the xenolith by thermal stress fracturing following its incorporation into the granitic magma. Thermal modelling shows that stresses are highest in the pelite layers perpendicular to bedding leading to fracture parallel to bedding. We also consider that stored elastic strain energy and release of free aqueous vapour at the tips of propagating fractures may contribute to the disintegration of the xenolith. If explosive disintegration of large, anisotropic, passively stoped blocks of country rock is common, then this process may explain the absence of elephants’ graveyards (accumulations of large stoped blocks) on the floors of granite batholiths, and high degrees of country rock contamination in some granite batholiths.