Takeru Yanagi

Lower crustal erosion induced by mantle diapiric upwelling: Constraints from sedimentary basin formation followed by voluminous basalt volcanism in northwest Kyushu, Japan

Abstract A sequence of geological events, beginning with basement subsidence to form a shallow-water sedimentary basin and subsequent voluminous basalt volcanism and uplift of land, has been observed in the back-arc region of northwestern Kyushu, Japan. The basin consists of a succession of marine and non-marine sediments with a total thickness of 1000–1500 m which range in age from 43 to 10 Ma. The basalt volcanism commenced at 10 Ma and continued until 1 Ma. Uplift started at around 30 Ma and continued after the cessation of the basalt volcanism. These geological phenomena may be explained by the convective coupling between the ductile lower crust and upper mantle induced by mantle diapiric upwelling. Thus, surface subsidence leading to sedimentary basin formation is attributed to lower crustal erosion by mantle diapiric upwelling. For an earth model with lower crustal and upper mantle viscosities of 10 19 –10 20 Pa s, 5 km of lower crust can be eroded 10–20 Myr after the start of convective coupling between the ductile lower crust and upper mantle, compatible with the period estimated by observations. In this process, the melt due to adiabatic mantle diapiric upwelling accumulates beneath the lower crust. The accumulation of low density melt in the space originally occupied by mantle material causes crustal uplift. When the stress state became extensional, as inferred from the extension of Okinawa Trough during the middle to late Miocene [1], the melt filling the eroded lower crustal area may have reached the surface, leading to voluminous basalt volcanism.

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.

Magma evolution observed in the Matsuura basalts in northwest Kyushu, Japan : An example of high-pressure open system fractional crystallization in a refilled magma chamber near the crust-mantle boundary

Abstract Formation of an extensive shallow water sedimentary basin before the voluminous Matsuura basalt volcanism in the back arc volcanic belt in northwest Kyushu, Japan, seems to indicate the lower crust erosion due to the upwelling of the mantle, which results in the decompression melting of mantle to form basaltic magma. The Matsuura basalts, 10 to 1 Ma, range in composition from low alkali tholeiitic basalt through alkali olivine basalt to basaltic andesite. Their compositions show a wide variation in MgO from 11.6 wt.% to 2.8 wt.%. Their chemical characteristics are: (1) evolution across the alkali–subalkali boundary, (2) crescent form of the basalt distribution on a SiO2 vs. MgO diagram, (3) enrichment in normative plagioclase component and a diminution in normative clinopyroxene component with the magma evolution, (4) abrupt termination of this plagioclase-enrichment trend at MgO=3.3 wt.%, (5) marked enrichment in SiO2 in aphyric basalts with MgO

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.

Mass transfer and reaction paths in alteration zones around carbonate veins in the Nishisonogi Metamorphic Rocks, southwest Japan

Abstract This paper describes local mass transfer and fluid-rock interaction in epidote-blueschist subfacies basic schist around retrograde dolomite + calcite veins in the Nishisonogi Metamorphic Rocks, southwest Japan. The veins have alteration zones on both sides. The mineralogical and chemical changes during alteration were documented along two sample traverses normal to the veins. Approaching the veins, the peak metamorphic assemblage (winchite + epidote + chlorite + calcite + albite + quartz) has been modified by progressive breakdown of amphibole, epidote, and calcite to illite and dolomite. Mass-balance calculations using the isocon method reveal that CO2 and K were added to the basic schists, and Ca was removed during alteration. The mass balance does not hold for these components between the vein + alteration zone and protolith as a whole. These findings suggest that the vein formation and metasomatism represent a process driven by fluid flow and longdistance element transport. Possible reaction paths between the vein-fluids and basic schists were estimated from the progressive change of mineral assemblages in the alteration zones. The phase relations on ionic activity diagrams indicate an increase in log(CO2 and decreases in log(aCa2+/σCa2+aH2+) and log(aMg2+/σMg2+aH2+) with progress of the alteration. Although a change in log(aK+/σK+aH2+) is poorly constrained on the activity diagrams, the mass balance suggests a concentration gradient of K across the alteration zones. Hence, the reaction paths and mass balance calculations suggest that a combination of the concentration gradients of CO2, Ca, Mg, and K produced by the vein-fluids drove the alteration process.

Genesis of continental crust under island arc conditions

Abstract In the Japan context, the following characteristics of arc volcanic rocks are important for crustal growth: (1) intimate association of volcanic rocks of tholeiitic and calc-alkaline series; (2) repeated eruption of basaltic and dacitic lavas in a volcanic succession; (3) gradual change in chemistry from tholeiitic to calc-alkaline volcanic rocks; (4) crystallization differentiation which includes the crystallization of plagioclase along with other mafic minerals; (5) marked enrichment of K2O associated with slight depletion of compatible elements; (6) an evolution limit represented by, for instance, Rb Sr ratios of 0.23–0.28 and SiO2 contents of 60–66 wt.% and (7) repetitive magma mixing. Batch fractionation in refilled magma chambers best accounts for these characteristics, transforming mantlederived magmas through calc-alkaline magmas to magmas of composition very close to the bulk composition of the upper continental crust. Systems of periodically refilled magma chambers are general in the Japanese arcs. Differentiates and accumulated crystals may form the compositional stratification of the continental crust.