GEMOC ARC National Key CentreW.L. Griffin1,2, X. Wang1,3, S.E. Jackson1, N.J. Pearson1, S.Y. O'Reilly1
and X. Xu1,3
1. GEMOC Macquarie, 2. CSIRO Exploration and Mining, 3. Department
of Geology, Nanjing University, Nanjing, China
Field relationships in the 115 Ma Pingtandao complex (Fujian Province,
SE China) show extensive mingling of mafic and felsic magmas, producing
pillow-like enclaves, and suggest formation of at least some intermediate
members of the suite (granodiorite, quartz diorite) through hybridisation
(Xu et al., 1999). Similar Sr (initial 87Sr/86Sr =0.7065) and Nd (_Nd =
-1.5) isotopic compositions imply that both the felsic and the mafic magmas
represent contain a significant crustal component, but Nd model ages (TDM
= 1Ga), significantly younger than known crust in the region (1.4-2.0 Ga),
suggest a juvenile component as well. To investigate the processes
of crust-mantle interaction, and specifically the timing of any mixing
of crustal and mantle components, we have carried out a detailed study
of the Pingtan rocks, using LAM-MC-ICPMS techniques to analyse Hf isotopes
in individual zircon grains.
Zircons (?200x50 µm) were separated from a granite, a granodiorite
enclave (pillow) in the granite, a quartz diorite enclave, a granodiorite
and a gabbro. The zircons in each rock show several growth stages,
recognised by studies of external and internal morphology in cathodoluminescence
images. Detailed EMP (Hf, Y, U, Th) and LAM-ICPMS (REE, U, Th, other trace
elements) analysis shows that changes in internal morphology or zoning
style are accompanied by changes in trace-element patterns, reflecting
changes in magma composition and/or physical conditions. These data
show that the zircons have acted as recorders of the evolution of the magmas,
and that this evolution included abrupt changes in composition.
Hf isotopes were analysed using a Merchantek LUV266 Nd:YAG laser probe,
attached to a Nu Plasma multi-collector ICPMS. Typical spot sizes
were 30-40 microns; typical internal precision of 176Hf/177Hf on 1-2 minute
ablations was ±0.00002-6 (2SE). Interferences of 176Lu and
176Yb on 176Hf were corrected by monitoring 175Lu and 172Yb; the accuracy
and precision to which these overlaps can be corrected are comparable to
the internal precision up to 176Yb/177Hf =0.3 (Griffin et al., 2000).
Time-resolved data collection commonly showed that distinct "stratigraphy"
in Yb/Hf, Hf content and 176Hf/177Hf was intersected by the beam during
drilling, making it possible to select parts of the signal representing
major zones.
Zircons from all samples show a major peak at 176Hf/177Hf =0.2828±0.0001
(_Hf =+2-+4, TDM model ages =600 Ma). This is a minimum age for the source
and may reflect the remelting of Proterozoic/Paleozoic lower crust. The
zircons of the granodiorites and gabbro are mainly of this type, but some
have more radiogenic Hf. The quartz diorite and the granite also
contain this type, as well as others with less radiogenic Hf (176Hf/177Hf
=0.2825-26; TDM = 1.0-1.2 Ga). Both rocks also contain zircons with
more radiogenic Hf (176Hf/177Hf =0.2829-0.2830). Individual grains are
typically zoned from less to more radiogenic Hf compositions, but some
are reversely zoned. Late-stage zircons in each sample generally
have Hf isotope compositions intermediate between the extremes. The
isotopic zoning patterns, and the presence of distinct isotopic populations
corresponding to recognised growth stages, cannot be explained by fractional
crystallisation or restite unmixing, but require the mixing of magmas with
disparate Hf-isotope compositions, derived from different sources and having
different compositions.
The trace element and Hf isotope data suggest that mixing of crustally-derived
magmas with a juvenile component, followed by crystallisation of zircon
from the mixed magma, has taken place several times during the evolution
of the Pingtandao complex. The repeated input of mantle-derived magmas
probably has important provided heat for the production of the granitoid
magmas; the isotopic data suggest that the mantle-derived magmas also have
contributed new material to the crust.
The use of zircon as a process recorder recovers information on the
evolution of magmas that is lost during the analysis of whole-rock samples.
The availability of rapid, low-cost in-situ microanalysis of trace-element
patterns and Hf-isotope compositions of zircons, coupled with the detailed
study of internal morphology, is about to bring major advances in our understanding
of magma genesis in crustal settings.
References:
Griffin, W.L., Pearson, N.J., Belousova, E., Jackson, S.E., O'Reilly,
S.Y., van Achterberg, E., and Shee, S.R. 2000. Geochim. Cosmochim. Acta
64, 133-147.
Xu, X., Dong, C., Li, W. and Zhou, X. 1999. Lithos 46, 299-315.