Inception, growth and collision of the Ordovician Macquarie Arc, Lachlan Orogen, Eastern Australia

The Ordovician intra-oceanic Macquarie Arc is preserved in a tectonostratigraphic terrane,
faulted to the west and east against coeval, quartz-rich turbidites of the Adaminaby Group,
within the Lachlan Orogen of eastern Australia. Debates exist concerning the
allochthoneity of the Macquarie Arc, the polarity of its related subduction and the nature
and exact timing of collision with Gondwana. These key problems are addressed by the
integrated application of field observations, petrography, zircon U-Pb-Hf isotopes and
whole rock geochemistry of key units within the Macquarie Arc stratigraphy. By these
approaches, it has been possible to answer (i) the timing and juvenility of the arc initiation,
(ii) the timing of arc-continent collision, and (iii) the allochthoneity and emplacement
mechanism of the Macquarie Arc onto the eastern edge of Gondwana. A major problem
in establishing this information is the low silica, mostly basaltic to andesitic character of
the Macquarie arc rocks, meaning low yields of small zircons and thereby hampering
radiometric dating. New zircon U-Pb-Hf results from the oldest portions of the Macquarie
Arc (Mitchell Formation) confirm that arc initiation was during the Early Ordovician at
~480 Ma, and support the juvenile intra-oceanic nature of the arc’s earliest stages with an
initial ɛHf signature of +12 to +13. Samples from the Middle Ordovician Fairbridge
Volcanics yield a mostly Ordovician zircon population with a youngest population at
~443 Ma, which have slightly decreased initial ɛHf values of +8 to +14. Older
Gondwanan inheritance is rare apart from a few Precambrian zircons. These results
indicate the arc started having continental influence during Middle-Late Ordovician. The
whole rock geochemistry shows a time-spatial variation of calc-alkaline rocks with highK (locally shoshonitic) to medium-K transition from Early to Middle-Late Ordovician,
similar to the modern analogues of Mariana and Fiji island arcs. These distinctly intraoceanic island arc characteristics and the lack of any observable stratigraphic
relationships between Macquarie Arc volcaniclastic units and the surrounding quartz
turbidites of the Adaminaby Group indicate that the arc may have been initiated well
outboard of eastern Gondwana via eastward subduction at high dip angle.
Collision of the arc with Gondwana should in theory result in deposition of units within
the trench region with a mixed arc and continent provenance. The Triangle Formation is
an enigmatic unit that has been assigned to the Macquarie Arc, but it has both arc and
continental features. Zircon U-Pb results show that the youngest detrital zircon age in the
Triangle Formation is 456 Ma which is a dominant age of the Macquarie Arc, as well as
ii
a large population of pre-Ordovician zircons similar to the Adaminaby Group. Late
Ordovician detrital zircons are absent from the Adaminaby Group which has a youngest
detrital zircon component of ~481 Ma. The Triangle Formation is likely a mixture of
sources similar to the Adaminaby Group and Macquarie Arc sources, occurring in a
trench-fill setting at the beginning of the arc-continent collision.
To reveal the emplacement mechanism of the Macquarie Arc and the deep crustal
structure, the post-collision Silurian intrusions of the Browns Creek Intrusive Complex
and the Davies Creek Granite were investigated. This is because they only intrude into
the Macquarie Arc and not the nearby coeval continent derived Adaminaby Group. The
zircon U-Pb ages indicate two igneous phases at 430-437 Ma and 420-426 Ma, and a
zircon recrystallisation phase at 395-396 Ma attributed to a hydrothermal event. Some
Precambrian zircon xenocrysts and magmatic zircon initial ɛHf values of -5.1 to +4.7
from these intrusions indicate the Macquarie Arc, whose rocks have only juvenile initial
ɛHf (+8 to +15) values, is an allochthon over Gondwanan continental basement, instead
of having a root deep to the mantle.
In summary, the Macquarie Arc was initiated far from the continent with no continental
contamination, most likely via outboard (eastward) subduction at high dip angle. The arc
started colliding with the eastern Gondwana during the Late Ordovician (~456 Ma),
indicated by the trench-fill sedimentary protoliths of the Triangle Formation. Preservation
of juvenile island arc on continental margins is aided by outboard subduction that results
in emplacement of the arc complex as a klippe in an upper plate position on top of the
passive margin sequence, instead of an autochthon extending deep to the mantle,
amalgamated with the continent through a back-arc closure. Modern analogues for similar
arc-continent collisions are observed in Oman and Taiwan where they are associated with
widespread deformation and orogenesis.
Therefore, the mechanism of continental growth along eastern Gondwana involved the
episodic addition of juvenile oceanic terranes via east-dipping subduction, and the detrital
zircon ages could record the process. However, the zircon signature from the oceanic arc
will always be muted, due to the zircon-poor mafic-intermediate igneous rocks that
dominate such arcs in the Phanerozoic. Trench-fill sediments sourced from both the
continental margin and the arriving allochthonous island arc potentially provide key
constraints to the nature and timing of these collisions. In addition, the zircon inheritance
studies of granites demonstrate how the deep crustal architecture of other proposed exotic
terranes can be deduced.