Kyoichi Ishizaka

Setouchi high-Mg andesites revisited : geochemical evidence for melting of subducting sediments

Abstract In order to evaluate the mechanism of production of unusual high-Mg andesite (HMA) magmas, Pb–Nd–Sr isotopic compositions were determined for HMAs and basalts from the Miocene Setouchi volcanic belt in the SW Japan arc. The isotopic compositions of Setouchi rocks form mixing lines between local oceanic sediments and Japan Sea backarc basin basalts, suggesting a significant contribution of the subducting sediment component to the HMA magma generation. Mixing calculations using compositions of an inferred original mantle and local oceanic sediments suggest that a sediment-derived melt, neither an H 2 O-rich fluid nor an amphibolite/eclogite-derived melt, could have been produced first and served as a plausible metasomatic agent for the HMA magma source. The unusual tectonic setting, including subduction of a newly-borne hence hot plate, may be responsible for melting of subducting sediments.

Origin of high magnesian andesites in the Setouchi volcanic belt, southwest Japan, I. Petrographical and chemical characteristics.

High-magnesian andesites of middle Miocene age occur in southwest Japan, forming an obvious volcanic belt. These andesites have low FeO*/MgO ratios (0.546–0.931), and are rich in Ni (101–312 ppm), Co (30.0–45.1 ppm), and Cr (208–756 ppm). They are relatively aphyric (phenocrysts <10 vol.%), and the phenocrysts of magnesian olivine (∼Fo88) are in equilibrium with the host high-magnesian andesite magmas on the basis of the Fe-Mg exchange partitioning. These features suggest that the high-magnesian andesites are not differentiated or accumulative; they appear to represent primary andesites generated in the upper mantle. These southwest Japanese high-magnesian andesites are rich in incompatible elements, and show light rare earth enrichment relative to boninites, suggesting that the former is derived from a less depleted mantle source than the latter.

Asthenospheric injection and back-arc opening: Isotopic evidence from northeast Japan☆

Nd and Sr isotopic compositions were determined for the Tertiary volcanics from the back-arc side of the NE Japan arc. Sr isotopes show a linear trend through time from an enriched signature (87Sr86Sr = 0.705437) to a depleted signature (87Sr86Sr = 0.70270). In a complementary fashion, Nd isotopes start at low value (ϵNd = −0.80) and show a gradual increase (ϵNd=8.3) with decreasing age. Isotopic change of Nd and Sr from the enriched signature to the depleted one is synchronous with the opening of the Japan Sea at ∼ 15 Ma. This synchronism indicates that the opening of the Japan Sea was initiated by the injection of the asthenosphere. During the pre-opening stage, the mantle wedge was composed of a two-layered structure: the sub-continental lithosphere and the underlying asthenosphere. The volcanics of this stage characterized by an enriched isotopic signature were derived from a source with a higher proportion of sub-continental lithosphere. The sub-continental lithosphere of the back-arc side was thinned by the injection of the depleted asthenosphere, which accelerated the growth of the MORB source within the mantle wedge of the back-arc side and resulted in magma generation with the depleted isotopic signature of Nd and Sr at the post-opening stage.

Secular variation of magma source compositions beneath the Northeast Japan arc

Abstract Sr and Nd isotopic compositions were determined for Tertiary volcanic rocks with ages of 22, 16 and 8 Ma along the volcanic front in the NE Japan arc in order to examine the secular variation of magma source compositions in the mantle wedge. These rocks show no through-time variation in both Sr and Nd isotope ratios (0.70412–0.704759 and 2.09–4.12 ∈-units, respectively). It follows that the subduction components do not increasingly pollute the magma source region during the continuous subduction of oceanic lithosphere. The degree of pollution by the subduction components is controlled by the amount of clinopyroxene and aluminous phases in mantle wedge peridotites, because H 2 O which causes partial melting of the mantle wedge beneath a volcanic front is supplied by the breakdown of both amphibole and chlorite in the dragged hydrated peridotite formed by the addition of slab-derived fluid beneath the fore-arc region.

Existence of andesitic primary magma: An example from southwest Japan

A high-magnesian andesite (SiO2 58.50%, MgO 9.47%) occurs at Teraga-Ike in southwest Japan. It belongs to the Setouchi volcanic rocks of middle Miocene age and carries olivine and bronzite as phenocrysts (4.2 and 1.4 modal percent, respectively). This andesite is characterized by Mg-values as high as 75, suggesting that it may be a primary andesite. Olivine phenocrysts (Fo87–91) are in equilibrium with the groundmass (= liquid) on the basis of Fe-Mg exchange partitioning between olivine and liquid, and they have high NiO contents (up to 0.45%). Chromite inclusions in olivine and rarely bronzite have high Cr2O3 contents (max. 54.87%). These features strongly suggest that the Teraga-Ike andesite keeps the chemical composition of the primary magma generated in the upper mantle, and therefore verify the existence of primary andesite magmas.

Magnesian andesite and basalt from Shodo-Shima Island, southwest Japan, and their bearing on the genesis of calc-alkaline andesites

Abstract Calc-alkaline andesites and olivine tholeiitic basalts are widely distributed on Shodo-Shima island, southwest Japan. The Fo content of olivine phenocrysts in the andesite is higher than in the basalt. The primary magma of the andesite, estimated on the basis of the olivine fractional crystallization model, is not basaltic but andesitic. The basalt contains both chromite and titanomagnetite as inclusions in olivine phenocrysts, while only chromite appears in the andesite. The Cr content of chromite in the andesite is higher than in the basalt. These facts again indicate that the andesite cannot be a fractionation product of the basalt, and that andesitic and basaltic primary magmas were generated independently.

K, Rb and Sr abundances and Sr isotopic composition of the Tanzawa granitic and associated gabbroic rocks, Japan: Low-potash island arc plutonic complex

The granitic and associated gabbroic rocks of the Tanzawa plutonic complex of Miocene age occurring in the northern part of the Izu-Bonin arc are characterized by low abundances of K (229–6790 ppm) and Rb (0.414–12.1 ppm), low K 2 O/Na 2 O ratios (0.037–0.21), moderately high K/Rb ratios (541–630), low Rb/Sr ratios (0.00137–0.0579) and low initial 87 Sr/ 86 Sr ratios (0.70332–0.70372). This indicates that acid to intermediate plutonic rocks with these geochemical characteristics also occur in island arc environments besides mid-oceanic ridge environments. They represent, together with associated gabbroic rocks, a low-potash island arc plutonic complex and are expected to occur beneath young island arcs, although now unexposed. The Tanzawa plutonic complex may have been formed by differentiation of low-K calc-alkaline magma.

Batch fractionation model for the evolution of volcanic rocks in an island arc: an example from Central Japan

Abstract Analyses of fifty-one rock samples from three stratovolcanoes in Central Japan revealed that K and Rb contents vary in a saw-toothed fashion with the growth of these volcanoes. Peaks and valleys of the saw-toothed variation pattern of Rb (and also K) increase at first and then gradually converge on constant values. This variation trend is also shown by the Rb/Sr ratio. The convergent Rb/Sr ratio (0.23–0.24) at the peaks coincides with recent estimates of the average value for continental crust. These geochemical features are well explained by the batch fractionation model. In this model, the magma reservoir lying at the top of the mantle is periodically supplied with a batch of parental magma, while the magma in it undergoes continuous crystallization and the cumulate is continuously removed by the divergent movement of the mantle. This model, working under physical conditions in the crust-mantle structure of an island arc, not only accounts for the above geochemical features, but also gives insight into the genesis of the calc-alkaline rock series and of the continental crust.

Occurrence of oceanic plagiogranites in the older tectonic zone, Southwest Japan

Abstract K, Rb and Sr concentrations and Sr isotopic compositions were determined for the Dai granitic rocks of trondhjemitic composition occurring in a serpentinite mass in the Nagato tectonic zone formed in the Late Paleozoic era, and for the granitic rocks of quartz dioritic composition recently dredged from the seamount of the Kyushu-Palao Ridge. Both granitic rocks are characterized by low abundances of K and Rb, low K 2 O/Na 2 O ratios, high K/Rb ratios, low Rb/Sr ratios and low initial 87 Sr/ 86 Sr ratios. These characteristics suggest that strong similarities may exist between the Dai granitic rocks and the dredged granitic rocks, and that the Dai granitic rocks may be classified as oceanic plagiogranite. These oceanic plagiogranites may plausibly represent single-stage mantle-derived granites, possibly from the suboceanic mantle.