Petrology of the north-western part of the Holyrood granite batholith

Barning, Kwasi (1965) Petrology of the north-western part of the Holyrood granite batholith. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Holyrood granite batholith is a dome-shaped igneous body, approximately 40 miles in length and 8 miles in width forming the backbone of the Avalon Peninsula. Its western part intrudes folded volcanic rocks of pre-Cambrian age, the Harbour Main Group. Its eastern boundary is defined by a fault. Petrographic studies of the north-western part of the batholith have indicated that it is biotite granite which can be divided into two main types: -- 1. A coarse-grained variety containing numerous inclusions, low in biotite and relatively little altered. -- 2. A medium-grained type with only few xenoliths, a higher biotite content and greater degree of alteration. -- Intrusive contacts between the granite and tuffs of the Harbour Main Group can be seen clearly in the field, but no intrusive relationships with the sediments of the Conception Group were observed in the area studied. However, basal conglomerates of the Conception Group contain cobbles of pink Holyrood granite indicating a pre-Conception age for the granite. No definite lineation or foliation appear in the outcrops, indicating that the granitic pluton is post-kinematic. Its contacts with the country rocks are very sharp without any pronounced mineralogical changes. Studies of rocks within the contact zone adjacent to the granite indicate that there is little evidence of contact (thermal) metamorphism accompanying the intrusion. The most striking effect of the granite on the older rocks has been the introduction of silica, water and/or carbon dioxide and to a minor extent the addition of lime. -- Parts of the country rocks were incorporated in the granitic magma during emplacement; by assimilation various granodioritic and dioritic hybrid rocks have been formed. The granite was subjected to considerable stress during and after crystallization. Several pegmatite bodies associated with the granite, some containing giant quartz crystals, are poor in minerals containing volatile elements. This indicates that either the granitic residum lacked such hyperfusible constituents, or they may have escaped from a shallow intrusion through a porous or fractured roof.

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