A ‘Three-Increase Model’ for the Origin of the Marginal Reversal of the Koitelainen Layered Intrusion, Finland

摘要

To gain insights into the long-standing problem of marginal reversals, we have studied in detail the ∼200 m thick marginal zone of the ∼3 km thick, mafic Koitelainen Layered Intrusion (KLI). This marginal zone consists of pigeonite gabbros that are chilled against gneisses of the underlying basement but reveals a non-chilled upper contact with orthopyroxenites of the overlying Layered Series. There is a sharp textural and compositional break between the rocks of the marginal zone and the Layered Series of the KLI. The marginal zone is remarkable in showing systematic reverse fractionation trends from the bottom to the very top. These are exemplified by a significant upward increase in whole-rock and pyroxene Mg-number along with normative and actual An content of plagioclase, as well as by an upward decrease in all incompatible components (e.g. TiO₂, Zr, Y, REE). This is accompanied by a systematic upward decrease in ratios of highly incompatible elements, such as Zr/Y and La/Yb and in initial whole-rock ε_Nd. The uppermost rocks of the marginal zone are very fine-grained and show well-developed plagioclase lamination and ultra-depletion in all incompatible elements (e.g. Zr = 0·5 ppm; Ce = 0·11 ppm). The marginal zone is interpreted as having developed by in situ crystallization of magma in a slowly opening fissure. The origin of reverse compositional trends is attributed to three principal factors, which we refer to as the ‘three- increase model’: (1) an increase in compositional primitivity of the magma that gradually filled the chamber; (2) an increase in the degree of chemical equilibrium among phases associated with increasing distance from the cold country rocks; (3) an increase in the proportion of cumulus minerals in response to more effective removal of evolved liquid from in situ growing crystals with distance from the intrusive contact. The marginal reversal of the KLI is distinct from typical reversals in layered intrusions in that its development was interrupted by the emplacement of a large volume of new magma that was parental to the overlying Layered Series. This new, hot magma caused significant textural and compositional reconstitution of the uppermost gabbroic rocks of the marginal reversal by their partial melting, compaction and recrystallization. This gave rise to a considerable decrease in grain size, appearance of plagioclase lamination, and ultra-depletion in highly incompatible elements. The anomalous nature of these rocks is thus a manifestation of their restitic rather than primary magmatic origin. The proposed ‘three-increase model’ may represent a general explanation for the origin of marginal reversals in many mafic sills and layered intrusions.