plutons. In addition, minor dike-like intrusion of a gra- nitic rock is distributed ...... rhyolite (Crecraft et al., 1981; Norman et al., 1992; Gerber et al., 1995; Miller and ...
Geochemical Journal, Vol. 47, pp. 167 to 183, 2013
Petrogenesis of the ridge subduction-related granitoids from the Taitao Peninsula, Chile Triple Junction Area YOSHIAKI KON ,1* TSUYOSHI KOMIYA,2 RYO ANMA,3 TAKAFUMI H IRATA,4 TAKAZO SHIBUYA,5 S HINJI YAMAMOTO2 and SHIGENORI MARUYAMA6 1
Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8567, Japan 2 Department of Earth Science and Astronomy, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan 3 Graduate School of Life and Environmental Sciences, University of Tsukuba, Ten-no dai 1-1-1, Tsukuba 305-8572, Japan 4 Laboratory for Planetary Sciences, Kyoto University, Kitashirakawa Oiwake-cho, Kyoto 606-8502, Japan 5 Precambrian Ecosystem Laboratory (PEL), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan 6 Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan (Received February 17, 2012; Accepted February 5, 2013) Geochemical compositions are reported for Late Miocene to Pliocene granitoids from the Taitao Peninsula near the Chile ridge subduction zone. Major element compositions of Taitao granitoids show a resemblance with those of TTG suites. However, trace element compositions are characterized by low Sr (50–300 ppm), moderately high Y (10–45 ppm) and Yb concentrations (1–5 ppm), and low Sr/Y and La/Yb ratios compared with those of typical adakites, which are presumably produced by melting of young and hot oceanic crust under eclogite to garnet amphibolite conditions. Instead, trace element composition of the Taitao granitoids resembles that of typical calc–alkaline arc magmas. Based on trace element compositions, together with tectonic constraints, we infer that the Taitao granitoids were generated by partial melting of the subducted oceanic crust in garnet-free amphibolite conditions at depths shallower than 30 km. Our results indicate that slab-melting-related magmas do not necessarily show a HREE-depleted signature, which was used as evidence for slab-melting for granitic rocks of the TTG suites. Keywords: granite, ridge-subduction, slab-melting, Taitao, adakite, TTG
subducted oceanic crust or lower mafic continental crust at garnet amphibolite to eclogite conditions (e.g., Martin, 1994, Foley, 2008). In contrast, modern granitoids, which do not show strong HREE depletion, are regarded as derived from partial melting of the metasomatized mantle wedge (e.g., Futa and Stern, 1988; Rogers and Hawkesworth, 1989) or melting of garnet-free lower mafic continental crust (e.g., Nakajima and Arima, 1998). Even in the Phanerozoic, some andesites and dacites have shown chemical compositions resembling that of the Archean TTG (e.g., Defant and Drummond, 1990). Socalled adakites, which are typically produced in hot subduction zones, are regarded as products attributable to the partial melting of young oceanic crust (e.g., Drummond and Defant, 1990). Furthermore, previous works have emphasized that young (72.1 wt%), low MgO (
