Issue 1: February 1996

Abstract

A strontium isotopic investigation of the Bjerkreim-Sokndal layered intrusion, southwest Norway

FM. Nielsen¹, IH. Campbell², M. McCulloch² and JR. Wilson ¹Statoil's Research Centre, Postuttak, N-7004, Trondheim, Norway, ²Research School of Earth Sciences, Australian National University, GPO Box 4, 2601 Canberra, A.C.T., Australia, ³Geologisk Institut, Aarhus Universitet, 8000 Aarhus C, Denmark and To whom correspondence should be addressed
ABSTRACT
The 230 km² Proterozoic Bjerkreim-Sokndal layered intrusion has a monzonoritic bulk composition and comprises a 6 km thick, broadly leuconoritic Layered Series (LS) overlain by unlayerd mangerite and quartz mangerite. In the Bjerkreim lobe the LS comprises six megacyclic units (MCU 0, IA, IB, II, III and IV) in a syncline. This lobe is surrounded by migmatitic gneisses; the roof to the intrusion is missing. The mg-number opx, An % and whole-rock initial ⁸⁷Sr/⁸⁶Sr isotope ratios (Sr0) display broadly parallel trends through MCUs II and III, and into MCU IV, with decreasing mg-number (75-58) and An % (50-40) accompanying increasing Sr0 (0.7050-0.7085). This correlation breaks down in the upper part of MCU IV and mangerite; the quartz mangerite has Sr0 values of ~0.7085. Abrupt reversals occur across the MCU boundaries.

The LS crystallized on the floor of a periodically replenished magma chamber that was continually assimilating country rock gneisses. Strong compositional zoning of the magma developed as a result of repeated replenishments with relatively dense, promitive ferrobasaltic magma (Sr0=0.7049) along the floor and the development of extensive buoyant roof melts. Assimilation took place on a massive scale (up to ~50%) in the upper parts of the chamber.

KEY WORDS. assimilation; fractional crystallization; hybrid magma; layered intrusion; Sr-isotopes

Pages: 171 - 193

General Information

This page is maintained by OUP admin

Last updated 24 Apr 96