TRACKING MAGMA MIXING: IN-SITU HF-ISOTOPE ANALYSIS OF ZIRCONS

W.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.