GEMOC ARC National Key Centre

INTEGRATED IN-SITU U-PB, HF-ISOTOPE AND TRACE-ELEMENT ANALYSIS OF DETRITAL ZIRCONS: EVENT SIGNATURES IN THE PUR-BANERA BELT, RAJASTAN
 

W.L. Griffin1,2, S.E. Jackson1, E. A. Belousova1 and S. Walters3

1. GEMOC Macquarie
2. CSIRO Exploration and Mining
3. GeoDiscovery Group

U-Pb dating of individual zircon grains from sedimentary rocks to produce age spectra is a powerful tool for analysing the age distribution of igneous activity within the source area. However, the application of the technique to terrane analysis, basin analysis and mineral exploration has been limited by the high cost of ion microprobe analysis. This barrier has now been lowered significantly by the application of laser-ablation ICPMS techniques to obtain precise in-situ U-Pb ages on single grains (typical precision (2sd) on ²⁰⁶Pb/²³⁸U ages of ±20-30 Ma). The multi-collector ICPMS with laser-ablation microprobe allows rapid high-precision analysis of Hf-isotope compositions (±1 ε_{Hf) in the same grains, which provides information on the juvenile vs recycled nature of the source material of the zircon-bearing igneous rocks. Trace-element data obtained by EMP analysis (Hf, U, Th, Y) and as a byproduct of the Hf-isotope analysis (Yb, Lu) can be compared with our large database to constrain the composition of the source rock for each zircon grain. The power of this integrated approach is demonstrated in this preliminary report.}
 
 

_{Zircon samples were taken from three drainages across the mineralised Pur-Banera Belt, Rajastan, India. 142 grains (100-300 µm) were examined by cathodoluminescence/BSE imaging and analysed by EMP. 74 of these were selected for U-Pb (LAM-ICPMS) and Hf-isotope (MC-ICPMS) analysis. The U-Pb spectra (Fig. 1) reflect an extended mid-Proterozoic magmatic/metamorphic event (1900-1400 Ma), similar to that known in parts of Australia. Hf isotope compositions are bimodal, reflecting both the addition of juvenile material (}ε_{Hf =+ 9 to +14) and recycling of older crust (}ε_{Hf = -5 to -10) (Fig. 2).}
 
 

_{From 1900-1800 Ma, juvenile additions to the crust were dominant, while remelting of Paleoproterozoic crust (TDM = 2250-2350) began ca 1700-1800 Ma. During the period 1700-1500 Ma, juvenile material was progressively mixed with recycled material, with the latter dominating. Igneous rocks produced from 1400-1500 Ma were derived mainly by recycling of older crustal components. Trace-element data (Fig. 3) indicate a relatively narrow range of intermediate to mildly felsic compositions, with little evidence for the presence of highly evolved granitoids.}
 
 

Figure 1: Cumulative-frequency curves showing distribution of 206Pb/238U ages on zircons of three samples.

 

 

Figure 2: Epsilon ó age plot for analysed zircons.
 
 

Figure 3. Y and U concentrations (EMP) of analysed zircons, overlain on fields representing compositions of zircons from different rock types (E.A. Belousova, in prep.)

  LAM-ICPMS techniques can produce, rapidly and at relatively low cost, zircon age spectra with detail equivalent to that obtained by ion-probe analysis. However, these age spectra in themselves carry limited information about the actual processes associated with different time intervals. The integration of in-situ Hf-isotope and trace-element analysis on the same grains adds significant information on the types and sources of magmatic rocks, which enhances interpretation of the age spectra. This integrated approach results in "event signatures" that will be useful in correlation studies, and in basic research on crustal evolution.