Geochemical Characterization of the Magmatic Stratigraphy of the Kalgoorlie and Black Flag Groups – Ora Banda to Kambalda Region.Vol. Report 226.

Abstract

A new dataset of ~2800 high-quality whole-rock geochemical analyses of subvolcanic, volcanic, and volcaniclastic rocks from the Kalgoorlie Group is used to construct chemostratigraphic barcodes within the broader Ora Banda – Kambalda region, encompassing most of the previously defined domains of the Kalgoorlie Terrane as well as the Bulong complex area of the Kurnalpi Terrane. Twenty-six distinct mafic compositional units are identified based on combinations of major and trace element variation diagrams, and in particular, on bivariate plots utilizing strongly incompatible trace elements. These distinctions highlight the controls that various fundamental igneous processes and source compositional variations exerted on evolving magma compositions. There are broad trends in compositional change common to the chemostratigraphy in most areas throughout the region. As described in numerous previous studies, the three commonly identified regional stratigraphic levels (basaltic lower, komatiitic/basaltic middle, and basaltic upper) correspond to changes in tectonomagmatic processes, and in particular, a general increase in the degree of mantle partial melting, an increase in the amount and/or effect of crustal contamination, and an increase in overall magmatic compositional diversity.According to the new barcode data, individual established lithostratigraphic units, typically of formation level, seldom comprise a single geochemical unit. Most stratigraphic formations comprise at least two distinct geochemical units. Most of these units occur at multiple stratigraphic levels, and in most regional stratigraphic columns. Thus, the individual greenstone stratigraphic formations comprise overlapping flow fields, each representing the products of a discrete eruptive event and/or eruptive centres that tapped genetically unrelated magma sources. This potentially makes some stratigraphic formation boundaries somewhat arbitrary reflections of changing source eruptions. Although complex, this diversity ensures that, in most cases, the position of stratigraphically unknown geochemical samples can be uniquely established as long as enough samples are taken over a continuous stratigraphic interval that also incorporates the overlying and underlying units.However, there are rare cases where local mafic geochemical associations appear anomalous with respect to the surrounding associations. For example, in the Kundana region to the west of the Zuleika Shear Zone and in an area close to Londonderry Siding, a stratigraphy characteristic of the Ora Banda Domain, rather than of the Coolgardie Domain, is intersected in drillcore and in outcrop. The reasons for this remain elusive.Because of the greater continuity of low-viscosity flows that characterize its basalt-dominated stratigraphy, the application of chemostratigraphic barcoding is more reliable or appropriate for the Kalgoorlie Group than for the overlying Black Flag Group. Geochemical characteristics and relationships within the felsic-dominated Black Flag Group, nevertheless, provide broad stratigraphic information and significant insight into crustal architecture and regional geological evolution.The regional distribution of the mafic chemostratigraphic associations is distinctly asymmetric, in many cases ignoring previously established domain boundaries. However, a broadly north-trending structure incorporating segments of (from south to north) the Zuleika Shear Zone, Abattoir Fault, and the Bardoc Shear Zone (ZAB) does appear to separate the ‘western domains’ (Coolgardie and Ora Banda) from the ‘eastern domains’ (Kambalda, Boorara, Parker, and the Bulong complex). These domain groups have distinctive chemostratigraphic variations that probably reflect a fundamental crustal architectural control and potentially provide some basis for reassessing the present domain boundaries. In particular, the mafic units in the western domains probably reflect melting of a more depleted source in thicker lithosphere. The Ora Banda Domain is clearly a part of the western domains throughout evolution of the lower and middle greenstone stratigraphy, but it also appears transitional to the eastern domains in terms of its diversity of fractionated layered sill-derived magmatism in its upper stratigraphy. The chemostratigraphic differences between the western and eastern domains extend to the felsic units of the Black Flag Group. The subvolcanic, volcanic, and volcaniclastic rocks of the Black Flag Group have the composition of sanukitoids, which are likely derived from a metasomatically enriched lithospheric mantle source. They show a wide range in La/Th ratios, which broadly correlates with crystallization age, source composition, and location. Low La/Th sanukitoid is typically older (>c. 2678 Ma), derived from a source with less radiogenic Nd (i.e. is more crustal) and is more common west of the ZAB, whereas high La/Th sanukitoid is typically younger (<c. 2675 Ma), has more mantle-like isotopic compositions and is virtually restricted to east of the ZAB. The simplest explanation for the regional compositional variations in the mafic and felsic rocks is that the ZAB is the exposed trace of a structure separating compositionally contrasting translithospheric domains. Northern and southern extensions of that trace should be entirely identifiable based on geochemical and isotopic data