Magmatic Evolution and Ascent History of the Aries Micaceous Kimberlite, Central Kimberley Basin, Western Australia: Evidence from Zoned Phlogopite Phenocrysts, and UV Laser 40Ar/39Ar Analysis of Phlogopite–Biotite

Abstract

The Neoproterozoic Aries kimberlite was emplaced in the central Kimberley Basin, Western Australia, as a N-NNE-trending series of three diatremes infilled by lithic-rich kimberlite breccias. The breccias are intruded by hypabyssal macrocrystic phlogopite kimberlite dykes that exhibit differentiation to a minor, high-Na-Si, olivine-phlogopite-richterite kimberlite, and late-stage macrocrystic serpentine-diopside ultramafic dykes. Mineralogical and geochemical evidence suggests that the high-Na-Si, olivinephlogopite-richterite kimberlite was derived from the macrocrystic phlogopite kimberlite as a residual liquid following extended phlogopite crystallization and the assimilation of country rock sandstone, and that the macrocrystic serpentine-diopside ultramafic dykes formed as mafic cumulates from a macrocrystic phlogopite kimberlite. Chemical zonation of phlogopite-biotite phenocrysts indicates a complex magmatic history for the Aries kimberlite, with the early inheritance of a range of high-Ti phlogopite-biotite xenocrysts from metasomatized mantle lithologies, followed by the crystallization of a population of high-Cr phlogopite phenocrysts within the spinel facies lithospheric mantle. A further one to two phlogopite-biotite overgrowth rims of distinct composition formed on the phlogopite phenocrysts at higher levels during ascent to the surface. Ultraviolet laser 40 Ar/ 39 Ar dating of mica grain rims yielded a kimberlite eruption age of 815. 4 ± 4. 3 Ma (95% confidence). 40 Ar/ 39 Ar laser profiling of one high-Ti phlogopite-biotite macrocryst revealed a radiogenic 40 Ar diffusive loss profile, from which a kimberlite magma ascent duration from the spinel facies lithospheric mantle was estimated (assuming an average kimberlite magma temperature of 1000 C), yielding a value of $0. 23-2. 32 days for the north extension lobe of the Aries kimberlite.

Neoproterozoic reworking of the Palaeoproterozoic Capricorn Orogen of Western Australia and implications for the amalgamation of Rodinia

“…1) comprises high grade metamorphic rocks of the Northampton and Mullingarra complexes in which granulite and amphibolite facies metamorphism has been dated at 1079 and 1058 Ma, respectively (Bruguier et al 1999;Fitzsimons 2003). Zircon provenance data from paragneiss units within these two complexes indicate the Mesoproterozoic Albany -Fraser Orogen as a likely sediment source and, combined with the limited extent of high-grade metamorphism, has been used to suggest that the Northampton and Mullingarra Complexes are allochtonous and are derived from much further south (Fitzsimons 2003). Mafic dykes that are emplaced both within the Northampton Complex and the Yilgarn Craton, indicate that tectonic juxtaposition of the two complexes with the West Australian Craton must have taken place prior to c. 750 Ma (Wingate & Giddings 2000).…”

“…Mafic dykes that are emplaced both within the Northampton Complex and the Yilgarn Craton, indicate that tectonic juxtaposition of the two complexes with the West Australian Craton must have taken place prior to c. 750 Ma (Wingate & Giddings 2000). Consequently, there is some evidence for tectonism within the Pinjarra Orogen between 1080 and 750 Ma (Fitzsimons 2003). From the geometry of the 750 Ma mafic dykes and brittleductile northerly trending dextral shear zones that post-date their emplacement (Byrne & Harris 1993), Fitzsimons (2003 argues that dextral transpressional within the Pinjarra Orogen took place around 750 Ma.…”

“…Consequently, there is some evidence for tectonism within the Pinjarra Orogen between 1080 and 750 Ma (Fitzsimons 2003). From the geometry of the 750 Ma mafic dykes and brittleductile northerly trending dextral shear zones that post-date their emplacement (Byrne & Harris 1993), Fitzsimons (2003 argues that dextral transpressional within the Pinjarra Orogen took place around 750 Ma. Although broadly consistent with this model, the Ar data presented here indicate much earlier tectonic activity along this western margin of the West Australian Craton.…”

“…Despite this, the presence of Mesoproterozoic and Neoproterozoic rocks of the Pinjarra Orogen ( Fig. 1) attests to an eventful geological evolution of the western margin of the craton (Myers 1990;Fitzsimons 2003) following its amalgamation in the Palaeoproterozoic era.…”

“…Biotite age standard Tinto B, with a K -Ar age of 409 Ma (Rex & Guise 1995) was placed at 5 mm intervals throughout the aluminium package to monitor the neutron flux gradient. Tinto B is a standard that has seen widespread use in the literature (Kelley et al 1994;Sherlock & Kelley 2002;Downes et al 2006). Correction factors are as follows: ( 36 Ar/ 37 Ar)Ca ¼ 0.000255, ( 39 Ar/ 37 Ar)Ca ¼ 0.00065 and ( 40 Ar/ 39 Ar)K ¼ 0.0015. Corrections for ( 38 Ar/ 39 Ar)K and ( 38 Cl/ 39 Ar)K were not undertaken because of the Proterozoic age and low Cl characteristics of the samples and the short amount of time between irradiation and the time of analyses.…”