Back to Abstract Index
Paper Number: 86
Evidence
for hybridization in the Tynong Province granitoids, Lachlan Orogen,
Eastern Australia
K.R. Regmia, R.F. Weinbergb, I.A.
Nichollsb,, R. Maasc
aDepartment
of Geology, Faculty of Science, University of Namibia, Windhoek,
Namibia, kregmi@unam.na
bSchool of Earth, Atmosphere and Environment, Monash
University, Clayton, Melbourne, VIC 3800, Australia
cSchool
of Earth Sciences, University of Melbourne, VIC 3010, Australia
Abstract
Granitoids of the Tynong Province, Lachlan Orogen, Australia show
evidence for mingling and mixing between mantle- and crustal-derived
magmas, such as microgranitoid enclaves (MMEs), larger dioritic bodies,
like many other plutons of this Orogen. This paper explores the extent
of hybridization of these rocks by means of petrography, major and trace
element geochemistry, whole rock Rb-Sr and Sm-Nd isotope data, as well
as Hf isotopes in zircons, focusing on samples of the Toorongo and
Tynong plutons. At outcrop scale, there are widespread rounded,
pillow-shaped mafic-intermediate enclaves, commonly with concave-convex
irregular boundaries and xenocrysts of quartz and K-feldspars derived
from the surrounding felsic rock. These enclaves show textural features
characteristic of hybridization; especially: rapakivi and antirapakivi
textures, quartz and feldspar ocelli, and acicular apatite. Harker
diagrams of major and trace elements typically define clear to diffuse
linear trends,most compatible with for magma mixing, probably combined
with other processes such as fractional crystallization.
Zircons yielded a weighted average crystallization age of 375 ± 2 Ma
( LA-ICP-MS; 2σ) and εHf(t) varying between -1.1 and +7.8 for
the Toorongo pluton, and 368 ± 7 Ma and 353 ± 6 Ma ( LA-ICP-MS; 2σ) and
εHf(t) varying between +4.3 and +8.8, and -0.8 to +9.3 for
two samples of the Tynong pluton. Hafnium isotopes indicate a relatively
small crustal contribution in the origin of zircons in these rocks, and
the higher average εHf(t) values for zircons of the Tynong
pluton (+6.9 and +4.3) compared to that of the Toorongo pluton (+3.9),
suggests that the Tynong pluton has a less significant crustal input. In
general, rocks of the Tynong Province have εNd and Sr(i), varying from
close to chondrite values to crust-like, radiogenic values. However,
taken as a whole they do not define any clear trend between εNd or
87Sr/86Sr values and silica content, with more
felsic rocks of some of individual plutons having less evolved
87Sr/86Sr isotopic signatures than more mafic
rocks, contrary to expectations from simple mixing of magmas from mantle
and crustal sources. These results suggest that the magmas may have
undergone more than one phase of hybridization: juvenile mantle-derived
magmas mixed with crustal magmas to varying extents, and subsequent
interactions between the hybrids led to local inversion of expected
isotopic trends with silica.
Keywords: multiple hybridisation, magma mixing, Hf isotopes,
zircon, Tynong Province, Lachlan Orogen
