Shigeru Iizumi

Spatial variations of Sr and Nd isotope ratios of Cretaceous-Paleogene granitoid rocks, Southwest Japan Arc

Cretaceous-Paleogene granitoid rocks and contemporaneous volcanic rocks are widely distributed in the Inner Zone of Southwest Japan. This intense intermediate to felsic magmatism is considered to have taken place on the eastern margin of the Eurasian Continent, before the Southwest Japan Arc drifted away from the continent in the middle Miocene, resulting in the opening of the Japan Sea. The granitoid rocks show regional variations in terms of their radiometric age, petrography, Sr, Nd and O isotope ratios. Based on Sr and Nd isotope ratios, granitoid rocks can be divided into three zones (South, Transitional and North) between the Median Tectonic Line and the Japan Sea. Granitoid rocks and associated gabbros of the North Zone have low initial Sr isotope ratios (0.7048 to 0.7068) and high initial εNd values (+3 to-2.2), whereas granitoid rocks and gabbros from the South Zone have high initial Sr isotope ratios (0.7070 to 0.7088) and low initial εNd values (-3.0to-8.0). Most granitoid rocks from the Transitional Zone have Sr and Nd isotope ratios that lie between those of the North and South Zones, although there is some overlap. Contamination of magmas by upper crust cannot explain this geographical variation in Sr and Nd isotopes. Instead, the regional variation is attributed to compositionally different, magma sources (probably upper mantle and lower crust), beneath the North and South Zones. This is supported by the Sr and Nd isotopic ratios of upper mantle and lower crustal xenoliths included in Cenozoic volcanic rocks in the North and South Zones. These ratios are similar to those of the granitoid rocks in the respective zones. It is suggested that a micro-continent or island arc consisting of continental crust was underthrust beneath the South Zone before or during the Cretaceous, resulting in compositionally distinct sources for granitoid rocks of the North and South Zones. The large variation observed in Sr and Nd isotope ratios for Transitional Zone granitoid rocks is explained by variable proportions of the two different crustal and upper mantle components.

SmNd, RbSr and ThUPb zircon ages of syn- and post-tectonic granitoids from the Axum area of northern Ethiopia

Abstract Sm Nd, Rb Sr and Th U Pb zircon ages for four syn-tectonic and two post-tectonic granitoids from the Axum area of northern Ethiopia are determined. Two of the syn-tectonic granitoids (the Azeho and Deset) are intrusions into structurally southeast facing, predominantly tholeiitic arc metavolcanics and associated metasediments situated west of the central steep zone in the area. The other two syn-tectonic granitoids (the Chila and Rama) are intrusions into structurally northwest facing metasediments and calc-alkaline metavolcanics at the eastern part of the steep zone. One of the post-tectonic granites (the Sibta Granite) occurs west of the central steep zone and the second (the Shire Granite) cuts the central steep zone. Preliminary geochemical data of all the granitoids show that they are enriched in large ion lithophile elements, depleted in high field strength elements and have 1-type characteristics, similar to calc-alkaline granitoids in subduction-related volcanic arc setting. The geochronological data indicate three discrete intrusive events: ∼800 Ma to the east of the central steep zone, ∼750 Ma to the west and a post-tectonic intrusion at around 550 Ma. The ∼-750 and ∼800 Ma ages of the intrusives are interpreted as minimum ages of arc magmatism in the respective blocks across the central steep zone, and the ∼550 Ma age of the post-tectonic granites records the final magmatic event in the region. The contrast of age across the central steep zone emphasises that this zone is a major structural element that might have played a significant role during the accretion of structurally and lithologically contrasting tectonostratigraphical blocks.

Mantle sources and magma–crust interactions in volcanic rocks from the northern Kenya rift: geochemical evidence

Abstract Miocene to Quaternary mafic (SiO2 54 wt.%) lavas from Samburu Hills, northern Kenya rift, exhibit significant variations in major and trace element compositions, and Sr–Nd isotopic ratios. Bimodal lavas were erupted in the early and middle Miocene (20–15 Ma and 15–10 Ma) and Pliocene (4.1–3.6 Ma), whereas only mafic lavas were produced in the upper Miocene (7.5–5.3 Ma) and Quaternary (2.0–0.1 Ma). Incompatible element concentrations (Zr, Nb, Y, Rb, and K) vary widely in the Miocene mafic lavas, but are depleted in Pliocene and Quaternary equivalents. In the Miocene and Pliocene mafic lavas, initial 87Sr/86Sr ratios vary from 0.70315 to 0.703808, and initial 143Nd/144Nd ratios vary from 0.51254 to 0.51288. 143Nd/144Nd ratios of the Quaternary lavas are similar (0.512724–0.512873), but 87Sr/86Sr ratios are higher (0.70432–0.70468). On a Nd- vs. Sr-isotopic diagram, the Quaternary lavas plot near bulk earth, whereas all other mafic lavas lie within the depleted mantle quadrant and at the left end of the East African Carbonatite Line (EACL). The geochemical variations of the mafic lavas originated from heterogeneous mantle sources produced by interaction of plume and lithospheric mantle, and subsequently modified by variable degrees of partial melting. The evolved lavas are divisible into evolved high-alkali (Na2O+K2O>9 wt.%) early Miocene latites, trachyphonolites, and middle Miocene and Pliocene trachytes; less evolved low-alkali (Na2O+K2O

Rb - Sr Whole Rock-Mineral Isochron Ages of Plutonic Rocks from the Wanni Complex, Sri Lanka

Abstract Granitoid plutons in different East Gondwana fragments give evidence for vigorous felsic magmatism during the Pan-African period (800 to 500 Ma). The Sri Lankan basement, which is mainly composed of Proterozoic high-grade metamorphic rocks, was intruded by a few late- to post-tectonic syenitic and granitic plutons. Reliable geochronological data for these plutons are few, and some of the available data are inconsistent with the ages of the surrounding metamorphic country rocks. This report presents five Rb-Sr whole-rock-mineral isochron (WRMI) ages and initial Sr isotope ratios of three granitoid plutons from the Wanni Complex, Sri Lanka, namely the Ambagaspitiya, Tonigala and Kotadeniya Granites. Two samples from the Ambagaspitiya Granite yielded ages of 520±5 and 502±15 Ma, with initial Sr ratios of 0.7103±0.0003 and 0.7125±0.0009 respectively, whereas ages of 467±27 and 497±11 Ma and initial Sr ratios of 0.7070±0.0004 and 0.7085±0.0008 were determined for two Tonigala Granite plutons. A single sample from the Kotadeniya Granite gave an age of 533±19 Ma and an initial Sr ratio of 0.7202±0.0018. These ages are consistent with the geological relations between the granitoids and their country rocks, which underwent granulite facies metamorphism between 650 and 550 Ma, and also with available U-Pb zircon ages (∼550 Ma). The present study gives improved chronological correlation of the felsic magmatism in East Gondwana fragments in Sri Lanka, India, Madagascar, Antarctica and Western Australia. Each of the Sri Lankan plutons exhibits different initial Sr isotope ratios, indicating that their magmas were derived from distinct source materials.