Two New Boulders of Orbicular Rocks from the Skellefte District, Northern Sweden

Abstract

A large glacial boulder of quartz-diorite found at the village of Långträsk contains closely packed orbicules characterized by dark core fragments surrounded by numerous light and dark shells. The composition of the shells is related essentially to the relative proportions of amphibole/biotite and andesine plagioclase.

The shell structure of different orbicules is similar. Many orbicules are broken, or appear to have been plastically deformed. Fragments of orbicules have provided the core material for additional shell growth in some examples.

The quartz-diorite matrix represents a later differentiate. Orbicules have been altered by pegmatite-like precipitations of quartz in the cores and light shells immediately surrounding the cores. The plagioclase of unaltered shells is more anorthitic than matrix plagioclase. The zonal precipitation of biotite, titanite and epidote is related to this period.

Orbicular crystallization occurred after amphibolitic fragments were mixed into a dioritic magma during the early stages of crystallization of the magma. The main, part of the shell structure probably resulted from the precipitation of matrix material around the fragments. Diffusion between the shells and the core accounted for modifications of the original distribution. Amphibole was the dominant mafic phase to crystallize. The present matrix, orbicular quartz and most of the biotite are related to the rock's later history, after orbicular crystallization had ceased. The primary orbicule structure seems to correspond to the older granite in the area, i. e. Jörn granite, while the later migmatization is probably associated with the younger granites, i. e. Revsunds granite.

A smaller boulder of orbicular rock was found at Järvfallet. The orbicules are from 1.5 to 5.5 cm across. The cores show an inner concentration of granitic material surrounded by compact hornblende with a little biotite. Shells are alternatingly hornblende or feldspar rich. Quartz is found throughout the rock. The shell minerals are arranged radially, often with minerals of one shell penetrating another. The matrix, which is medium grained hornblende granite, is similar to Adak granite. It is suggested that the orbiculite boulder was derived from the Adak massif, NW of the find locality towards the source of the ice flow.