New Insights into the Origin of Marginal Reversals from the Koitelainen Layered Intrusion, Fennoscandian Shield, Finland (Invited)
Abstract
A puzzling feature of mafic-ultramafic intrusions is the occurrence of marginal reversals in which mineral crystallization sequences and compositional trends are both distinctly opposite to those predicted by liquidus phase equilibria. Such marginal reversals are almost universally developed in magmatic bodies, irrespective of their age, geographical location, size, form, and the composition of parental magmas, strongly indicating that some fundamental process(es) are involved in their genesis. To gain insights into this long-standing problem we have recently studied in detail an approximately 200 m thick body of fine- to medium-grained pigeonite gabbro that occurs at the base of the 3 km thick mafic-ultramafic Koitelainen Layered Intrusion (KLI). It is chilled against the underlying basement gneisses but reveals a non-chilled upper contact with the overlying orthopyroxenites of the KLI. The uppermost rocks of the body show well-developed plagioclase lamination. There is a sharp textural and compositional break between the body and overlying orthopyroxenites. The body is unique in showing remarkably systematic reverse fractionation trends from the base to the very top. These are exemplified by a significant upward increase in whole-rock Mg# from about 30% to 80%, and in normative An from about 20% to 70%. Especially noteworthy is the upward dramatic depletion in all incompatible trace elements. For instance, La reveals a 250-fold decrease from 27.5 ppm to 0.11 ppm and Zr shows a 340-fold decrease from about 170 ppm to 0.5 ppm. This is accompanied by a systematic upward decrease in ratios of highly incompatible elements (e.g. Zr/Y from 9 to 1; La/Yb from 20 to 1). The body is interpreted as a specific marginal reversal that has developed by in situ crystallization of the leading edge of the fractionated magma in a slowly opening fissure. The reverse compositional trends are attributed to the emplacement of increasingly more primitive magma that crystallized in progressively less supercooled conditions associated with increasing distance from the cold country rocks. The effective removal of the evolved liquid boundary layer from growing crystals by continuous magma flow provided the inward formation of progressively more adcumulate rocks. The development of a marginal reversal has been, however, aborted by a rapid emplacement of a large volume of more primitive magma parental to the overlying 3 km thick rock sequence of KLI. Apart from a sharp upper contact, the new magma caused significant textural and compositional reconstitution of the uppermost gabbroic rocks of the marginal reversal. This was accomplished through their partial melting, compaction and recrystallization that resulted in a considerable decrease in mineral grain size, an appearance of plagioclase lamination, and ultra-depletion in highly incompatible elements. Thus, a specific marginal reversal of the KLI acquired its anomalous textural and compositional features due to formation by a combined operation of several magmatic and post-magmatic processes.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.V14A..03L
- Keywords:
-
- 1036 GEOCHEMISTRY / Magma chamber processes;
- 1749 HISTORY OF GEOPHYSICS / Volcanology;
- geochemistry;
- and petrology;
- 3640 MINERALOGY AND PETROLOGY / Igneous petrology;
- 3643 MINERALOGY AND PETROLOGY / Layered magma chambers