Yoshiaki Tainosho

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.

Pan-African Alkali Granitoids from the Sør Rondane Mountains, East Antarctica

Abstract Alkali granitoids (500-550 Ma) representing a prominent Pan-African magmatic event are widely distributed in the Sor Rondane Mountains, Dronning Maud Land, East Antarctica. Geochemically, they are granitic to syenitic in composition and show an alkaline affinity of A-type granites. They are characterized by high K 2 O+Na 2 O (7-13 wt%) and K 2 O/Na 2 O (1-2), low to intermediate Mg#, wide ranges of SiO 2 (45-78 wt%), Sr (20-6500 ppm) and Ba (40-13000 ppm) and have Nb and Ti depletion in the primitive mantle normalized diagram. The granitoids are subdivided into Group I granites, Group II granites, Lunckeryggen Syenitic Complex and Mefjell Plutonic Complex. The Group I granites have higher Mg#, Sr/Ba, Sr/Y, (La/Yb) N and LREE/HREE, lower A/CNK, SREE and initial 87 Sr/ 87 Sr ratios and lack Eu anomalies compared to those with negative Eu anomalies in the Group II granites. The syenitic rocks from the Mefjell Plutonic Complex are higher in alkali, Ga, Zr, Ba, and have lower Mg#, Rb, Sr, Nb, Y, F and LREE/HREE with positive Eu anomaly, whereas the granites from the Mefjell Plutonic Complex have high LREE/HREE ratios with negative Eu anomaly. The Lunckeryggen syenitic rocks have intermediate Mg#, higher K 2 O, P 2 O 5 , TiO 2 , Fe 2 O 3 /FeO, Ba, Sr/Y and LREE/HREE ratios with lack of Eu anomalies and are lower in Al 2 O 3 , Ga, Y, Nb and Rb/Sr ratios. Based on chemical characteristics combined with isotopic data, we suggest that the Lunckeryggen syenitic body and Group I granitic bodies may be derived from the mantle-derived hot basic magma by fractional crystallization with minor assimilation. We also suggest that the Group II granites may be derived from assimilation with crustal rocks to varing degrees and then fractional crystallization in higher crustal levels (ACF model). The Mefjell Plutonic Complex seems to be derived from a heterogenetic magma source compared with other granitoids from the Sor Rondane Mountains. The syenitic rocks in the Mefjell Plutonic complex have a unique source (iron-enriched) and have a chemical affinity with the charnockites in Gjelsvikjella and western Muhlig-Hofmannfjella, but not like the Yamato syenites in adjacent areas.