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
Field relationships in the 115 Ma Pingtan complex in coastal 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 (?150x50 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 studies by EMP (Hf, Y, U, Th) and LAM-ICPMS (REE, U, Th,
other trace elements) show 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. It is clear
that the zircons have acted as recorders of the evolution of the magmas,
and that this evolution included abrupt changes in composition.
Hf isotope measurements were done 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 on 1-2 minute
ablations was 176Hf/177Hf of ±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 given above (Griffin et al., 2000).
Time-resolved data collection commonly showed that distinct "stratigraphy"
in Yb/Hf, Hf intensity 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±0.00005
( _Hf =+4, TDM model ages =600 Ma). This is a minimum age for the source
and may reflect the remelting of Proterozoic lower crust with low Lu/Hf
. 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
=0.2826-27; TDM = 0.8-1.0 Ga). Both rocks also contain zircons with
more radiogenic Hf (76Hf/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 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
resorption, but require the mixing of magmas with disparate Hf-isotope
compositions, derived from different sources. Correlations between
trace element patterns and Hf isotope compositions indicate that these
magmas also were different in composition.
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 Pingtan complex. The repeated input of mantle-derived magmas probably
has been important in providing heat for the production of the granitic
complexes, but 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.