Using amphibolite chemistry to assist structural and stratigraphic interpretation at Broken Hill, Australia. Issue 1 (2nd January 2021)

• Record Type: Journal Article
• Authors: Stevens, B. P. J.
• Is Part Of: Australian journal of earth sciences. Volume 68:Issue 1(2021)
• Page Start: 51
• Page End: 82
• Publication Date: 2021-01-02
• Subjects: Broken Hill -- amphibolite -- fractionation -- Proterozoic -- sills -- younging -- cross-section -- structure; Earth sciences -- Australia -- Periodicals; Earth sciences -- Periodicals; Geology -- Australia -- Periodicals; Geology -- Periodicals; 559.405
• DOI: 10.1080/08120099.2020.1757508
• Languages: English
• ISSNs: 0812-0099

Abstract

Abstract: Multiple samples taken across the strike of conformable syn-depositional amphibolite (meta-dolerite) bodies in the Willyama Supergroup, Broken Hill, showed that all exhibit chemical variation, mostly consistent with igneous fractionation. The fractionation trends are characterised by very significant reduction in MgO, Ni and Cr from base to top of individual sills, increase in Zr, and less regular variation in Fe and Ti contents. This asymmetric chemical zonation can be used to determine the original 'way up' in amphibolite sills as thin as 12 m. Given that the amphibolite sills were emplaced prior to folding of the enclosing sequence, the way up direction of the sills is also the younging direction of the enclosing metasediments, making this a useful tool in stratigraphic and structural studies of folded rocks. Outcropping conformable amphibolite bodies were selected for across-strike sampling in variably well-interpreted stratigraphic positions. Despite uncertainties related to overly wide-spaced sampling and possible internal structural complexity, in most cases the fractionation trend agreed with the prior stratigraphic interpretation, providing an independent stratigraphic 'way up' or 'younging' direction and demonstrating the potential usefulness of the technique. Subsequent sampling and analysis of drill core through amphibolite sills in the Broken Hill mine area tested alternative interpretations of the enclosing stratigraphy and structure. Fractionation patterns in three intersections of the amphibolite immediately west of the orebody clearly show younging towards the orebody. Intersections of the thicker amphibolite immediately east of the orebody reveal complex and conflicting fractionation patterns indicating internal structural complexity, consistent with this amphibolite being involved in a major thrust zone. The results in the mine area are in agreement with those of a previous study based on graded-bedding observations in the enclosing metasediments. The identification of igneous fractionation patterns in the amphibolites confirms the interpretation that the amphibolite bodies were in fact sills rather than lavas. The extremely high iron contents of some of the amphibolite sills are the result of extreme fractionation of a tholeiitic magma, not metasomatism. KEY POINTS: Most Broken Hill amphibolites retain relatively simple igneous fractionation patterns. The fractionation patterns can indicate the stratigraphic younging direction. The stratigraphy west of the Broken Hill orebody youngs eastward towards the orebody. The amphibolite east of the orebody shows complex deformation.