Nd-isotopic composition of Phanerozoic sediments in the Inner Zone of Southwest Japan Arc: implications on provenance characteristics and contribution to formation of mature island arc system

The Nd-isotopic data on sedimentary and metamorphic rocks of SW Japan Arc allow their discrimination into five different depleted mantle (Tdm) model age clusters, 2.6-2.45 Ga, 2.3-2.05 Ga, 1.9-1.55 Ga, 1.45-1.25 Ga, 1.2-0.85 Ga. The 2.6-2.45 Ga and 1.9-1.55 Ga model ages are also coincident with U-Pb inherited zircon ages of the above two epochs as well as the major magmatic activity in the Sino-Korean Craton (SKC). The 2.3-2.05 Ga model ages can be considered as the initial formation ages for the precursors of sedimentary rocks. The Nd-isotopic data suggest that the Hida Belt was most likely formed as a part of the SKC. The mantle underlying the Ryoke Belt had continental lithospheric signature during Triassic-Jurassic period. The 1.9-1.55 Ga model ages, especially 1.8 Ga~, can be associated with the formation of this belt. The source material for the sedimentary rocks occurring in the accretionary terrane of northeastern areas in the SW Japan Arc was probably in and around the SKC of the Ryoke Belt itself. The sedimentary rocks occurring in southwestern areas of the Arc were mainly composed of materials derived from a relatively younger source (1.45-0.85 Ga).

mentary rocks and/or their originated metamorphic rocks are presumed by Nd model ages with respect to the depleted mantle (DM) 5)-7) as described below. We estimate the initial formation ages of precursors for Paleozoic to Mesozoic coarse-grained sedimentary rocks occurring in the Inner Zone. This paper presents the new Nd-isotopic data and interpretation of model ages for understanding the evolution of the SW Japan Arc, whose data and interpretation can be also applied to understand similar matured arc systems. Nd ratios are 0.1966 and 0.512638 respectively. However, recently Nd model age (TDM) with respect to the DM have been used instead of CHUR. Several different numerical values as to the present 147 Sm/ 144 Nd and 143 Nd/ 144 Nd ratios of the DM have been used owing to the difference of its formation model. 8) In this study, the TDMs were calculated using following parameters for DM (= 0 Ma) and decay constant of 147 Sm; 143 Nd/ 144 Nd = 0.513150, 147 Sm/ 144 Nd = 0.2136, λ 147 Sm = 6.54 × 10 -12 y -1 . Following is a brief explanation on how to calculate the TDM.
Igneous rocks were formed from mantle-and/or lower crust-derived magmas (X Ga, Fig. 2). The igneous rocks formed on the earth's surface were directly affected by weathering and erosion. Some other igneous rocks consolidated in the deep crust are exposed at earth's surface through upheaval and then affected by weathering and erosion. Detrital grains originated from the weathered igneous rocks were mainly transported by streams and deposited at the sea, lake bottoms and so on, and hardened as sedimentary rocks through cementation and compaction (Y Ga). Some sedimentary rocks were transformed into metamorphic rocks by metamorphism (Z Ga). Passing through these geologic processes, Sm/Nd (and 147 Sm/ 144 Nd) ratios of the igneous rocks and sedimentary rocks (hereafter, including metamorphic rocks) would not be largely modified, which is a significant and fundamental assumption dealing with Nd model age. Actually, even if the Sm/Nd ratio is slightly changed, this change is impossible to be confirmed. Because the complete precursors (igneous rocks) are hardly identified on the field since they have been transformed into the equivalent sedimentary rocks. In other words, the precursors were lost from the Earth.
The Nd ratio) is largely different from real formation period (X Ga) of its precursor. In order to avoid large difference between X' Ga and X Ga, the age is generally applied to the rocks with low 147 Sm/ 144 Nd ratio less than 0.14. 6) Accordingly, this ratio is the lower the better for the calculation of the TDM. Furthermore, the rocks with high present 143 Nd/ 144 Nd ratios have not been used because of decline of significance as to the TDM. In this paper, the TDMs for the samples with present 143 Nd/ 144 Nd ratio less than 0.5125 and 147 Sm/ 144 Nd ratio less than 0.13 were calculated. Sampling and Nd-isotopic analysis. Representative argillaceous to psammitic sedimentary rocks and their originated metamorphic rocks were sampled and analyzed ( Fig. 1). Their Sm-and Nd-isotopic ratios and TDMs are presented in Table I and Fig. 3. Previously published Sm-and Nd-isotopic data 9)-17) have also been considered while calculating TDM. The extraction procedures for Sm and Nd from rock powders are following Kagami et al. (1989). 18 Nd, given in Table II and Fig. 3 were recorrected using following standard samples and their values; LaJolla (0.511858 19) ), BCR-1 (0.512638 18) ), JB-1a (0.512784 18) ), JNdi-1 (0.512115 19) ).

Isotopic data and Nd model ages.
It is noteworthy that the TDMs obtained from certain belt (or district in the belt) of the terranes are not randomly scattered but cluster around restricted specific time [Vol. 80(B),  20) brackets as shown in the TDM frequency diagram (Fig. 3) and summary as shown in Table II Accretional terrane. We didn't analyze Sm and Nd isotopic compositions of rocks from the Maizuru Belt because these are mainly volcanic rocks and volcaniclas-tic sediments which were formed in oceanic and island arc settings. 28) The Sm-Nd and Rb-Sr whole rocks ages of Kamiaso conglomerates are 2.07 Ga 13) and 2.06-1.89 Ga, 29) respectively. The TDMs (2.6 Ga) of the rocks are coincident with one of the U-Pb inherited zircon ages. The TDMs in Ashio Belt cluster at 1.85-1.6 Ga except northern Ashio, northern Yamizo and southern Yamizo, which are coincident with typical Ryoke Belt and Kyogatake Complex (Kiso in the Mino Belt). The TDMs of southern Yamizo are completely coincident with those of the Miso-gawa Complex (Kiso).
Discussions. Initial Nd isotopic ratios of some igneous rocks of the SKC with activity ages from mid-Archean (ca. 3.6 Ga) to mid-Proterozoic (ca. 1.5 Ga) are plotted along the Nd isotope evolution line of DM 13),30) (refer to Fig. 2 31) These data suggest that the evolution line of DM is useful for interpreting the initial formation age of sedimentary rocks and their originated metamorphic rocks in the SKC. The paleogeographic configuration of such terranes (or belt or district) of the SW Japan Arc is not yet properly understood. As described above, the initial 143 Nd/ 144 Nd ratios of the rocks from Kamiaso conglomerates and Oki-Dogo Island are plotted on one of the Nd evolution lines of the SKC. Triassic to early Jurassic mafic volcanic rocks constituting the Ryoke Belt were formed from the continental lithospheric mantle. 32) These two observations have led to conclusion that some belts of SW Japan Arc were formed under a continental regime. Furthermore, the analyzed samples in this study also show an affinity with active and passive continental margins as seen in some discrimination diagrams. 33 Chubu district 2.1-1.8 12) 3.42, 24) 2.56, 24) 1.84, 24) 1.13 24) Ryoke Belt 1.9-0.85 Higo district 1.25-0.85 14), Table I 1 Mie district -Chubu district 1.9-1.55 9), 17)  Table I).

Accretional Terrane
(1) 2.6-2.45 Ga; metamorphic rocks from the Oki-Dogo Island (Hida Belt) and granitic conglomerates from the Kamiaso district (Mino Belt) belong to this age category. TDMs of 2.6-2.45 Ga, especially the older one, are probably initial formation ages of the precursors of sedimentary rocks because they are close to 2.8-2.6 Ga of SKC. 13),22),30) The Hida Belt is probably formed in and around the SKC as pointed out by Isozaki (1997) 4) and others.
(2) 2.3-2.05 Ga; sedimentary rocks from Kiso in the Chubu district (Mino Belt) and from the southern Yamizo and metamorphic rock from the Hida (Hida Belt) belong to this age category. Main Proterozoic igneous activities in the SKC have started at ca. 2.1Ga. 34), 35) The oldest inherited zircon CHIME ages that were given for the gneisses from southern Korea Peninsula are 2.15 Ga. 36) Considering this, though the TDMs of 2.3 Ga might indicate the formation age of precursors of sedimentary rocks, and to confirm this matter it is necessary to obtain more detailed age data such as zircon ages.
(3) 1.9-1.55 Ga; the belts or districts in the belt belonging to this age category are, Ryoke Belt (except for Shishijima, Mie), Mino Belt (Chubu), Hida Belt (some of metamorphic rocks), Ashio Belt (except for Yamizo). Two samples from the Renge and Tanba Belts fall under the same age category. As described above, U-Pb inherited zircon ages from the Ryoke metamorphic and igneous rocks are 1.95-1.8 Ga. These zircon ages are coincident with the older (1.8 Ga~) TDMs ages from the Ryoke Belt. In that respect, it is notable that the U-Pb zircon ages of 1.95-1.8 Ga are also recognized in other belts of SW Japan Arc. One of the Proterozoic igneous activities of the SKC took place around 2.0 Ga and 1.7 Ga 34) and 2.0-1.6 Ga. 37) The age category (1.9-1.55 Ga), especially older (1.8 Ga~) ages, probably indicates initial formation age of the precursors of the sedimentary rocks of the Ryoke Belt as well as the other belts of this age category. Triassic to early Jurassic mantle underlying the Ryoke Belt had continental lithospheric signatures as described above. The fundamental formation age of this belt is probably close to 1.9-1.55 Ga (or 1.8 Ga~) and it was formed in and around the SKC. The sources of sedimentary rocks occurring in the accretionary terrane (Mino and Ashio Belts, excluding Yamizo) of northeastern areas in the SW Japan Arc are probably in and around the SKC or Ryoke Belt itself. The Hida metamorphic rocks belong to age categories (2) and (3) above, however, some rocks from this belt plot close to the 0.5 Ga line as shown in Fig. 3. These data suggest Nd ratios relationship for Paleozoic to Mesozoic sedimentary rocks from SW Japan Arc. Used Sm and Nd isotopic data are given in Table I, published data (see Table II). Fields of Yangtze Craton and 3.6-3.2 Ga igneous rocks were cited from Chen and Yang (2000). 7) Straight lines indicating each age are explained by following example. That is, that some metamorphic rocks were formed at relatively young age. Accordingly, the Hida metamorphic rocks have probably been formed from several rocks (protoliths) with various ages ranging from Proterozoic to early Paleozoic.
(4) 1.45-1.25 Ga; the terranes belonging to this age category are mainly in the Renge, Suo and Chizu Belts. One of the middle Proterozoic igneous activities in the SKC is ca. 1.4 Ga. 34) (5) 1.20-0.85 Ga; this age category is mainly defined by the rocks from Higo district that is generally accepted as the western extension of the Ryoke Belt (Fig. 1). However, the TDMs of Higo are quite different from those of the typical Ryoke Belt, which implies that the former is not a part of the latter as pointed out by Osanai et al. (1996) 38) and others. According to CHIME age using zircon, 39) their inherited Proterozoic ages are scattered between 1.9 Ga and 0.8 Ga and most of them are concentrated between 1.4 Ga and 0.8 Ga. The rocks from the Chizu Belt belong to both age categories (4) and (5).
Some sedimentary rocks collected from the districts belonging to (4) and (5) contain detrital zircons with old ages (ca. 1.9 Ga) as described above. If all sedimentary rocks consist of mixtures of several different components with various ages as well as detrital zircons, the obtained TDMs are of little value. However, one of the middle to late Proterozoic igneous activities in the SKC took place at ca. 1.4 Ga and 1.0-0.7 Ga. 34) Furthermore, based on the high of 143 Nd/ 144 Nd ratios (> 0.5122, Fig. 4) in addition to normal 147 Sm/ 144 Nd ratios (0.13-0.10) for the sedimentary rocks belonging to age categories (4) and (5), their source materials have been considered to be formed at relatively young age. Though the TDMs of Akiyoshi Belt belonging to accretionary zone couldn't be obtained because of high 143 Nd/ 144 Nd and 147 Sm/ 144 Nd ratios, they plot between 0.5 Ga and 1.0 Ga lines (Fig. 4). This data imply that the precursors of sedimentary rocks are young and did not have a long history. Thus, even if the sedimentary rocks were formed from the materials with various ages, most contributing materials of the sedimentary rocks should have younger ages. It is needless to say that this matter should be confirmed using other techniques such as U-Pb zircon method.
Considering the ages of mid-to late Proterozoic orogeny and Sm-Nd isotopic data, the ages from 1.45 Ga to 0.85 Ga (age categories (4) and (5)) defined by rocks collected from southwestern areas (Renge, Suo and Chizu Belts, Higo district in the Ryoke Belt, accretionary terrane including Akiyoshi and Tanba Belts) in the SW Japan Arc probably indicate the formation age of the precursors of the sedimentary rocks.