1:45 PM - 2:00 PM
[SGC50-01] Trace element composition in zircons from A-type granitoids in the Cape Ashizuri, SW Japan: Implications for new source rock index using zircon
Keywords:Zircon trace element composition, A-type granite, LA-ICP-MS, the Cape Ashizuri, SW Japan
To obtain primary features of trace element composition in zircons from A-type granite, we conducted in-situ LA-ICP-MS trace element analysis for 47 zircon grains from 4 samples of rapakivi granite, Qtz-syenite and melanocratic syenite in the Ashizuri complex. These alkalic granitoids in Ashizuri complex have characteristics of A-type granite (Loiselle and Wones, 1979) and within-plate granite (Pearce et al., 1984), and is suggested to be formed dominantly by crystal fractionation process from OIB-type basaltic magma (Stein et al., 1996).
Zircons from the Ashizuri A-type granitoids show oscillatory zoning, homogeneous dark core, disturbed texture and patchy texture in Cathodoluminessence images. Based on the typical igneous REE patterns (HREE enrichment, negative Eu anomaly and positive Ce anomaly) and its similarity each other in zircons from rapakivi granite and Qtz-syenite, it is suggested that the trace element composition of the zircons reflect those of the host magma. On the other hand, some zircons in melanocratic syenite show large variations of REE pattern and include obvious inherited cores, which imply contamination of surrounding rocks to host magma. To clarify characteristics of zircons crystallized from A-type granitic magma, we only use data from the rapakivi granite and the Qtz-syenite for comparison with previous data from other type (S-, I-, M-type) granitic rocks.
The magmatic zircons from the Ashizuri A-type granitoids show large Eu negative anomalies (Eu/Eu* = <0.004 – 0.070), high Nb contents (2.85 – 236 ppm), relatively high Y (423 – 13520 ppm) and U contents (171 – 7908 ppm), and relatively low Sr contents (0.08 – 2.38 ppm). These trace element feature in the zircons probably reflect high HFSE, low Eu and Sr concentration in the host A-type granitic magma. The geochemical characteristics of the zircons from Ashizuri complex become more distinct in Eu/Eu*, Nb/Sr, U/Sr, Y/Sr closs-plot diagram, showing lower Eu/Eu*, higher Nb/Sr, U/Sr and Y/Sr ratio compared to those in zircons from other type granitic rocks. Previous trace element data in zircons from A-type granitoids in Delamerian Orogen, South Australia (Pankhurst et al. 2013) is also plotted on similar area to the Ashizuri A-type granitoids in Eu/Eu*vs. Nb diagram. Because high HFSE, low Eu and Sr contents are common feature in the A-type granitic rocks, these diagrams are probably useful for distinguish zircons crystallized in A-type granite from those in other granitic rocks.