The Nemuro Belt is a tectonic belt in the Paleo‐Kuril Arc system, the NW Pacific region. The Paleo‐Kuril Arc has been interpreted as an intraoceanic arc system between the Izanagi and Pacific Plates during the Late Cretaceous, suggesting that the boundary between these plates was a trench. However, this study shows it was a volcanic arc that developed atop a continental margin. To determine the nature of this arc during the Late Cretaceous, U‐Pb ages of detrital zircons from the Nemuro and Urahoro Groups in the Paleo‐Kuril Arc were analyzed. Our results identify two distinct types of detrital zircon U‐Pb age distributions. The first (Type 1) is characterized by multimodal age distributions with peaks ranging from ca. 1.8 Ga to 78 Ma. The other (Type 2) exhibits a unimodal age distribution with a peak at 60‐52 Ma. These different age distributions indicate a provenance transition occurred between the Late Cretaceous and Paleogene. Precambrian zircons in Type 1 sandstones indicate that the Paleo‐Kuril Arc was part of a continental plate, presumably the Okhotsk Block, in NE Asia during the Late Cretaceous. In contrast, Type 2 sandstones were supplied only from the magmatic arc region that the Izanagi‐Pacific Ridge subduction could have activated. This provenance transition suggests the initially continental arc became separated from its continental source at the beginning of the Paleogene.

No abstract

Zircon is resistant to alteration over a wide range of geological environments, and isotopic ratios within the mineral provide constraints on ages and their parental magmas. Trace element compositions in zircon are also expected to reflect those of their parent magmas, and have a potential as essential indicators for their host rocks. Because most detrital zircons that accumulate at river mouths are derived primarily from granitoids, the classification of zircon within granitoids is potentially meaningful. This study employs the conventional classification scheme of granites (I-, S-, M-, and A-types). To clarify geochemical characteristics of zircons in A-type granites, trace element compositions of zircons extracted from the A-type Ashizuri granitoids were examined. Zircons from the Ashizuri granitoids commonly show enrichments of heavy rare earth elements and positive Ce anomalies, indicating that these zircons were igneous in origin. In addition, zircons in these A-type granites are characterized by enrichments of Nb, Y, Ta, Th, and U and strong negative Eu anomalies, which exhibit good positive correlations with those in their whole rocks. This fact indicates that these signatures in zircons reflect well those in their parental bodies and are useful in identifying zircons derived from A-type granite. Based on compilations of available data, zircons from A-type granites can be clearly discriminated from other-types of granites within Nb/Sr-Eu anomaly, U/Sr-Eu anomaly, Nb/Sr-U/Sr, and Nb/Sr-Ta/Sr cross-plots. All indices used in these diagrams were selected based on the geochemical features of both zircon and whole rock of A-type granites. Application of these discrimination diagrams to detrital zircons will likely provide further insights. For example, some Hadean detrital zircons plot in similar fields to A-type granites, implying the existence of A-type magmatism in the Earth's earliest history. K E Y W O R D SAshizuri, A-type granite, HFSE, REE, zircon | INTRODUCTIONZircons are often used as geochronological and geochemical indicators for constraining the nature of their host rocks. Recent developments of analytical methods enable us to extract information concerning the host rock from a single zircon grain. As one such example, contribution rates of mature sediments into granitoid magma through geologic time have been examined based on compilations of oxygen isotope ratios of zircons extracted from granites (Valley et al., 2005) and of detrital zircons in river mouth sands (Iizuka et al., 2013). Belousova, Griffin, O'Reilly, and Fisher (2002) examined trace element contents in zircons from a variety of rock types and proposed some discrimination diagrams, but found that most rocks are inseparable except for peculiar rocks (kimberlite, carbonatite, and nepheline-syenite). Basically, most detrital zircons that accumulate at river mouths are derived primarily from granitoids (Hawkesworth, Cawood, Kemp, Storey, & Dhuime, 2009), and their U-Pb age populations reflect the age variation of the hinterlan...

Zircon is one of the most important minerals in geochronologic research. Isotopic ratios and trace elements in zircons are expected to reflect those of their parent magmas. Many geochemical researchers have proposed various discrimination diagrams for zircon to indicate tectonic setting and to identify source rock. Because most detrital zircons accumulated at river mouths are derived primarily from granitoids, the classification of zircon within granitoids is potentially meaningful. In our research, we focused on sediment involvement during granitoid formation and tried to identify trace‐element compositions in zircon that are sensitive to variation in sediment incorporation. To accomplish this, we examined trace‐element compositions of both the granitoids and the included zircons in the Kofu granitic complex and the Tanzawa tonalitic plutons in Japan. Among the high‐field‐strength elements (Th, U, Ta, Nb, Hf, and rare earth elements), only Nb and Ta concentrations in the granitoids increased as the rate of sediment contribution increased. However, the zircon did not show such trends in Nb and Ta content. Zircon Y and P contents exhibited a positive correlation, indicating that xenotime substitution occurs to some extent. Because P exists as pentavalent ions in igneous systems, its presence likely affects the concentrations of pentads in zircon. When we divided the Nb and Ta contents by the P content, it became clear that zircon Nb/P and Ta/P ratios increase depending on sediment involvement. While some exceptions exist, we found that zircon Yb/Gd ratios also respond to sediment involvement. Our data further demonstrated that zircons in granitoids with significant sediment incorporation are characterized by low Ce/P contents, which is partly attributable to monazite crystallization before zircon saturation. This study demonstrates that combining these element ratios is useful for indicating sediment incorporation.

Isotopic ratio measurements for 235U/238U and 234U/238U ratios were made for a series of solid materials using laser ablation multiple collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). To extend the...