Presentation Information
[SMP31-P13]Incipient slab melting inferred from a garnet amphibolite–trondhjemite association in the Paleozoic subduction complex of the Wakasa area, Tottori Prefecture, SW Japan
*Mizuki Takahashi 1, Shunsuke Endo2, Atsushi Kamei2, Simon Richard Wallis3 (1.Graduate school of Natural Science and Technology, Shimane University , 2.Department of Earth Sciences, Faculty of Science and Engineering, Shimane University, 3.Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo)
Keywords:
Slab melting,Garnet amphibolite,Trondhjemite,High-temperature subduction
Partial melting of basaltic oceanic crust in subduction zones is closely linked to the formation of Archean TTG suites and Phanerozoic adakites, and thus plays a key role in continental crust evolution. To constrain the incipient stage of slab melting at relatively low temperatures (<750 oC), natural rock records preserved in subduction complexes are essential. Garnet amphibolite associated with tonalite-trondhjemite has been reported from several subduction complexes worldwide and interpreted as evidence for partial melting of subducted oceanic crust (e.g., Lazaro et al., 2011). In this study, we investigate the petrogenesis of garnet amphibolite and trondhjemite from the Wakasa area, Tottori Prefecture, using P-T estimates for garnet amphibolite, whole-rock major and trace element geochemistry of both lithologies, and zircon U-Pb geochronology of trondhjemite.
In the Wakasa region, ultramafic rocks of the Oeyama Ophiolite, one of the oldest subduction-related orogens in the Circum-Pacific region, are exposed (Kimura and Hayasaka, 2019). Foliated serpentinite occurs along the base of the ultramafic body, and garnet amphibolite and trondhjemite form boudinaged layers within pelitic schist interlayered with the serpentinite.
The garnet amphibolite mainly consists of brown amphibole and porphyroblastic garnet. Garnet preserves prograde compositional zoning and contains abundant inclusions of quartz, clinozoisite, plagioclase, rutile, and zircon. The shape preferred orientation of matrix amphibole defines a planar-linear fabric. Some samples are pervasively crosscut by leucocratic domains composed of plagioclase, quartz, muscovite, and randomly oriented euhedral zoisite. Garnet enclosed in those domains is replaced by plagioclase + quartz and locally shows atoll textures. Trondhjemite shares a similar mineral assemblage with the leucocratic domains, including coarse euhedral zoisite, but is markedly coarser-grained and mylonitized. Locally, mylonitized trondhjemite is crosscut by undeformed quartz-albite veins, indicating later-stage fluid activity.
Peak P-T conditions of the garnet amphibolite were estimated using quartz-in-garnet Raman geobarometry and Zr-in-rutile geothermometry. The highest residual pressures retained by quartz inclusions (0.30-0.33 GPa), combined with maximum Zr contents in rutile, yield peak conditions of ~700 oC and ~1.5 GPa, consistent with the stability field of magmatic zoisite.
Trace element patterns of the garnet amphibolite resemble MORB, except for enrichments in large-ion lithophile elements (alkalis, Ba, Pb, and Sr), indicating a basaltic oceanic crustal protolith. Trondhjemite lacking later veins shows incompatible trace element concentrations and patterns nearly identical to those of trondhjemite interpreted as products of incipient slab melting (~750 oC, 1.5 GPa) in subduction complexes (Lazaro et al., 2011).
The estimated peak P-T conditions are too low for dehydration melting of subducted oceanic crust, but exceed the H2O-saturated solidus of basaltic systems. This suggests that flux melting occurred in response to infiltration of aqueous fluids released during dehydration of serpentinized slab mantle.
Zircons from the trondhjemite (with later veins) yield a weighted mean 238U-206Pb age of 340 Ma (n = 26, MSWD = 1.2). Except for one statistically excluded grain at ~400 Ma (Th/U = 0.31), all zircons display very low Th/U ratios (0.00-0.03) and oscillatory zoning. These results suggest that the trondhjemite was initially generated by slab melting prior to 400 Ma and subsequently overprinted by fluid-driven metasomatism during the Renge metamorphic event at ~340 Ma.
References:
Kimura and Hayasaka (2019) Lithos 342-343, 345-360
Lazaro et al. (2011) Lithos 126, 341-354
In the Wakasa region, ultramafic rocks of the Oeyama Ophiolite, one of the oldest subduction-related orogens in the Circum-Pacific region, are exposed (Kimura and Hayasaka, 2019). Foliated serpentinite occurs along the base of the ultramafic body, and garnet amphibolite and trondhjemite form boudinaged layers within pelitic schist interlayered with the serpentinite.
The garnet amphibolite mainly consists of brown amphibole and porphyroblastic garnet. Garnet preserves prograde compositional zoning and contains abundant inclusions of quartz, clinozoisite, plagioclase, rutile, and zircon. The shape preferred orientation of matrix amphibole defines a planar-linear fabric. Some samples are pervasively crosscut by leucocratic domains composed of plagioclase, quartz, muscovite, and randomly oriented euhedral zoisite. Garnet enclosed in those domains is replaced by plagioclase + quartz and locally shows atoll textures. Trondhjemite shares a similar mineral assemblage with the leucocratic domains, including coarse euhedral zoisite, but is markedly coarser-grained and mylonitized. Locally, mylonitized trondhjemite is crosscut by undeformed quartz-albite veins, indicating later-stage fluid activity.
Peak P-T conditions of the garnet amphibolite were estimated using quartz-in-garnet Raman geobarometry and Zr-in-rutile geothermometry. The highest residual pressures retained by quartz inclusions (0.30-0.33 GPa), combined with maximum Zr contents in rutile, yield peak conditions of ~700 oC and ~1.5 GPa, consistent with the stability field of magmatic zoisite.
Trace element patterns of the garnet amphibolite resemble MORB, except for enrichments in large-ion lithophile elements (alkalis, Ba, Pb, and Sr), indicating a basaltic oceanic crustal protolith. Trondhjemite lacking later veins shows incompatible trace element concentrations and patterns nearly identical to those of trondhjemite interpreted as products of incipient slab melting (~750 oC, 1.5 GPa) in subduction complexes (Lazaro et al., 2011).
The estimated peak P-T conditions are too low for dehydration melting of subducted oceanic crust, but exceed the H2O-saturated solidus of basaltic systems. This suggests that flux melting occurred in response to infiltration of aqueous fluids released during dehydration of serpentinized slab mantle.
Zircons from the trondhjemite (with later veins) yield a weighted mean 238U-206Pb age of 340 Ma (n = 26, MSWD = 1.2). Except for one statistically excluded grain at ~400 Ma (Th/U = 0.31), all zircons display very low Th/U ratios (0.00-0.03) and oscillatory zoning. These results suggest that the trondhjemite was initially generated by slab melting prior to 400 Ma and subsequently overprinted by fluid-driven metasomatism during the Renge metamorphic event at ~340 Ma.
References:
Kimura and Hayasaka (2019) Lithos 342-343, 345-360
Lazaro et al. (2011) Lithos 126, 341-354