EXPLODING XENOLITHS AND THE ABSENCE OF ELEPHANTS GRAVEYARDS IN GRANITE BATHOLITHS

Biotite monzogranite near the eastern margin of the late Devonian Sout h Mountain Batholith at Portuguese Cove, Nova Scotia, surrounds a larg e (20 m x 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 Gr oup. The xenolith has two distinct types of surfaces against the monzo granite: (i) one rounded, diffuse, and apparently old surface that had prolonged contact with the granite magma; and (ii) two straight, shar p, and apparently new surfaces that had more recently come into contac t 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 monzogra nite to pegmatite to coarse quartz-sericite, cut the xenolith parallel to bedding producing a cumulative volume expansion of similar to 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 inj ection occurred in response to cracking of the xenolith by thermal str ess fracturing following its incorporation into the granitic magma. Th ermal modelling shows that stresses are highest in the pelite layers p erpendicular to bedding leading to fracture parallel to bedding. We al so consider that stored elastic strain energy and release of free aque ous 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 t his process may explain the absence of elephants' graveyards (accumula tions of large stoped blocks) on the floors of granite batholiths, and high degrees of country rock contamination in some granite batholiths . (C) 1998 Elsevier Science Ltd. All rights reserved.