The controls of radioelement distribution in the Etive and Cairngorm granites : implications for heat production

Abstract

Radiometric, whole rock trace element and petrological studies are reported for two late Caledonian granite complexes from the Grampian Highlands, Scotland. These studies throw light on the magmatic history of the intrusions and. more particularly. on their radioelement geochemistry and heat production which is interpreted in a geothermal context. The Etive Complex is a multiphase intrusion, ranging from diorite to granite in composition, emplaced by a cauldron subsidence mechanism. Its complex magmatic history involved crystal fractionation, both in-situ and at depth, coupled with episodic magma mixing in a deep magma chamber. Radioelement contents (means for the whole complex; 12.7 ppm Th, 2.9 ppm U, 4.1% K20) increase with magmatic differentiation and are concentrically zoned in the N Cruachan and Starav units. Mass balance calculations, incorporating radiometric, whole rock trace element, fission track and accessory phase microprobe data, show that uranium and thorium contents were, initially, controlled by the crystallisation of apatite + zircon + sphene ± allanite and chevkinite. Later, thorite and monazite became important thorium-hosts. Locally, enhanced uranium levels in the Starav Granites followed expulsion and limited outward migration of uranium-rich residual fluids. The distribution of radioelements in surface samples suggests that heat production decreases with depth in some units. Similar studies have identified four units in the Cairngorm Granite; NE Granite - Porphyritic Granite - Microgranite - Main Granite. Radioelement contents increase with magmatic evolution from the N Granite to the Main Granite; 26.5 > 32.3 ppm Th, 4.3 > 10.1 ppm U, 4.6 > 4.7% K20 (mean values). Uranium and thorium contents were controlled, predominantly, by the crystallisation of apatite + zircon + sphene ± allanite in the NE Granite and of apatite + zircon + monazite + xenotime + Nb-Ta-oxides ± thorite ± uraninite in the Main Granite. Minor amounts of uranium reside in secondary sites in hydrothermally altered samples. Modelled surface heat flow anomalies are 5.8 mW m-2 and 23.0 mW m-2 for the Etive Complex and Cairngorm Granite respectively. Comparison of calculated and preliminary heat flow measurements in the Cairngorm granite indicates that at least 35% of the observed heat flow arises from radioactive sources in the granite and that background heat flow is low.