Initiation and 35 Myr Duration of S-Type Granitic Magmatism in an Accretionary Orogen

Granites in circum-pacific igneous provinces can often be
divided into 'S-' and 'I'-types on the basis of elemental and
isotopic characteristics [e.g. 1]. This division reflects a
dominance of sedimentary or igneous source components, and
dating of granites allows tracking of the relative contributions
to growing batholiths in accretionary orogens. In the Lachlan
and New England Orogens of Eastern Australia, a progression
from S-type through I-type to A-type granites occurs
repeatedly over ~100 Myr periods. S-type intrusion occurs
during the first major extensional event following major
compression, while later I-type intrusion reflects a diminished
sedimentary source and increasing mantle contribution. The
timing of S- and I-type granites is therefore related to the
tectonic evolution of orogens [2].
The S-type phase of intrusion in the New England
accretionary orogen (~300-280 Ma) records an environment
switching from compression to extension and back to
compression as part of this process. We have undertaken U-Pb
dating of zircons extracted from small (~backarc) gabbroic
and dioritic bodies (Bakers Creek Supersuite), recalculated the
ages of S-type granites (Hillgrove Supersuite), and compiled
chronological data for the New England Orogen in order to
investigate the establishment of magmatism in this area. We
find a progression through: (1) diverse magmatism involving
gabbroic, dioritic, S- and I-type during establishment of
magmatism (305-295 Ma); (2) a major pulse of S-type
magmatism (295-285 Ma), ending with; (3) I-type and HREEdepleted
(deep crustal) magmatism around the time of the
Hunter-Bowen orogenic event (~285-266 Ma).
Magmatism occurs over a ~35 Myr period in the region of
most voluminous intrusion, but timing varies along-strike and
may reflect assymetric rifting of the orogen. Together, these
data time the heating, melting, depletion, and exhaustion of
the S-type magma source.
[1] Kemp, A. I. S. & Hawkesworth, C. J., 2003. Treatise
on Geochemistry 3, 349-410. [2] Collins, W. J. & Richards, S.
W., 2008. Geology 36, 559-562.