Chemical heterogeneities of Caledonian (?) pseudotachylites in the Eidsfjord Anorthosite, north Norway

Plattner, Ulrike; Markl, Gregor and Sherlock, Sarah (2003). Chemical heterogeneities of Caledonian (?) pseudotachylites in the Eidsfjord Anorthosite, north Norway. Contributions to Mineralogy and Petrology, 145(3) pp. 316–338.

DOI: https://doi.org/10.1007/s00410-003-0455-0

Abstract

The 1.8–1.7 Ga Eidsfjord Anorthosite Complex on Langøy, Vesterålen, north Norway is thrust over monzonitic gneisses in a 100-m-wide fault zone. The anorthosite contains abundant pseudotachylite which also overprint ductilely formed (ultra)mylonites in the monzonitic gneisses. 40Ar– 39Ar dating of the pseudotachylites indicates that this deformational event took place during the end of the Caledonian orogeny. Pseudotachylite formation by in-situ melting is corroborated by a detailed petrological investigation of chemical and textural criteria. Identity of host-rock mineral compositions and modes with pseudotachylite clasts and calculated modes confirm the common notion that transport over more than a few cm did not occur during pseudotachylite formation, because the characteristics of the host rocks are inherited by the pseudotachylite. Pseudotachylite compositions lie in the range of XRF whole-rock analyses of deformed and undeformed host rocks, demonstrating that no material was added or lost during pseudotachylitisation. The pseudotachylites show strong chemical heterogeneity on a scale of tens of micrometers. This compositional variation can be described by an approximately binary mixture of plagioclase with a homogenised mafic component, comprising orthopyroxene, clinopyroxene and Fe–Ti oxides. Although the pseudotachylite matrix is not homogenised and locally shows schlieren textures, the mafic phase in the anorthositic rocks must have homogenised very well before incorporation into the pseudotachylite. Different viscosities and mixing properties for the Al- and the Fe-rich parts of the pseudotachylite melt are inferred.