New SHRIMP U-Pb zircon ages from the Stavely region, western Victoria: July 2014-June 2016

This Record presents 20 new zircon U-Pb geochronological ages, obtained via Sensitive
High-Resolution Ion Microprobe (SHRIMP), from three meta-sedimentary, eight volcanic-hypabyssal
(subvolcanic), and nine plutonic samples from the Stavely Project area within the Grampians-Stavely
Zone of the Delamerian Orogen in western Victoria (Table 1.1). The U-Pb age data presented here
builds on earlier work of Lewis et al. (2015).
The timing of emplacement of compositionally diverse igneous rocks associated with the Mount
Stavely Volcanic Complex is not well understood (Buckland, 1987). Previous work suggests that some
of the plutonic rocks share geochemical characteristics with the volcanic succession, whereas others
are compositionally distinct (Taylor et al., 2014). This study was undertaken to better understand the
chronology of felsic-intermediate magmatism in the Grampians-Stavely Zone.
Cambrian meta-sedimentary rocks yield detrital zircon age spectra dominated by zircons with
Neoproterozoic dates, with the youngest detrital zircon populations providing maximum depositional
ages of 560.9 Â± 4.2 Ma, 552 Â± 8 Ma, and 538 Â± 23 Ma. In general, spectra consist of a dominant peak
at c. 580-555 Ma, and subordinate peaks at c. 1185-950 Ma, c. 1760 Ma, c. 2030-2005 Ma, c.
2165 Ma, c. 2300-2295 Ma, c. 2655-2545 Ma, c. 3260-3235 Ma. These turbiditic meta-sedimentary
rocks are entirely free of Stavely Arc-derived zircon grains (c. 510-500 Ma). A minimum age for
deposition of the sedimentary rocks is provided by magmatic crystallisation ages of c. 510-500 Ma
from the igneous rocks that intrude the succession.
A fault-slice of quartz dolerite from the Boonawah Volcanic Belt to the west of the main Mount Stavely
Belt (Schofield et al., 2015a, fig. 1.2), yielded a magmatic crystallisation age of 509.8 Â± 2.5 Ma, and is
distinguishably older than many of the other igneous rocks analysed in the Stavely region. The quartz
dolerite is interpreted to represent an early phase of Cambrian magmatism, which was subsequently
thrust-faulted upward relative to the surrounding, slightly younger, volcanic belts. The age of quartz
dolerite (509.8 Â± 2.5 Ma) is indistinguishable from the fault-slice of granodiorite at 511.3 Â± 2.9 Ma
dated by Lewis et al. (2015).
Three volcanic rocks from the Mount Stavely Volcanic Complex yielded magmatic crystallisation ages
of c. 504-502 Ma, which coincide with the emplacement of five dacitic porphyries-c. 506-500 Ma-
two from this study and three from Lewis et al. (2015), Re-Os molybdenite model ages derived from
porphyry-hosted mineralisation (c. 510-500 Ma; Norman, 2015), and emplacement of four
granodioritic and two dioritic rocks of the Bushy Creek Igneous Complex at c. 502-498 Ma (this
Record) demonstrating that igneous activity, porphyry emplacement, and mineralisation in the Mount
Stavely Belt were contemporaneous. These data indicate that the Stavely Arc developed along the
eastern margin of the Delamerian Craton prior to and during the onset of the Delamerian Orogeny at
c. 500 Ma. Igneous rocks associated with the Stavely Arc were emplaced during a period of rapid (c.
10 Ma) volcanic arc development.
Two Devonian plutons-the Mafeking Granodiorite and Mirranatwa Granite-yielded magmatic
crystallisation ages of c. 404 Ma and c. 403 Ma, respectively, and attest to renewed magmatism in the
Grampians-Stavely Zone approximately 100 Ma after development of the Stavely Arc.

Tags: Earth Sciences, GA Publication, Published_External
Organization: Geoscience Australia Data