Trace Element Geochemistry including the HFSE in Magnetites of Calc-Alkaline Plutons: the Tanzawa Complex of the Izu-Bonin-Mariana Arc and the Ladakh Batholith Complex, NW Himalaya

In this study we attempt to contribute to the understanding of a prominent feature, namely the Nb-Ta depletion, in arc magmatic trace element geochemistry. Traditionally, this depletion is explained by residual mantle-wedge phases with Nb and Ta affinities, such as titaniferous ilmenite, rutile or titanite, or by an amphibole. Here we propose a mechanism long advocated to explain the calc-alkaline trend (Bowen vs. Fenner) in MgO-FeO (total Fe)-(Na2O + K2O) ternary diagram by early crystallization and separation of magnetite in subduction zone magmas associated with high oxygen fugacity environments. In support of our hypothesis we provide high-precision multiple trace element data, including the high field strength elements (HFSE), in separated magnetites and mafic mineral phases from mafic magmatic enclaves associated with tonalite suites of two different magmatic arcs - the Tanzawa Complex of the Izu-Tanzawa Collision Zone in Japan, and from the Ladakh Batholith Complex of NW Himalayas. The Tanzawa Complex is composed of diverse rock suites with SiO2 varying from 43 - 75 percent, ranging from hornblende gabbro through tonalite to leuco-tonalite. The geochemical characteristics of low K- tholeiites, enrichment of large ion lithophile elements (LILE), and depletion of HFSE in rocks of this plutonic complex are similar to those observed in the volcanic rocks of the IBM arc. The Ladakh batholith Complex is one of the granitic belts exposed north of the Indus-Tsangpo suture zone in Ladakh, representing calc-alkaline plutonism related to the subduction of the Neotethys floor in Late Cretaceous. This batholith comprises predominantly I-type granites with whole rock delta delta 18O values of 5.7-7.4 per mil, without major contribution from continental crustal material. We analyzed 22 trace elements by ICP-MS, including Nb-Ta-Hf-Zr, in separated magnetites from five gabbros of the Tanzawa tonalite-gabbro complex and from three tonalitic gabbros of the Ladakh batholith. In NMORB normalized plots the trace element patterns of all the magnetites analyzed show enrichment in Nb, Ta, Pb, Sr (5X NMORB) and Zr (2X NMORB) with characteristically high Nb/Ta and Zr/Hf ratios. In contrast, the patterns show anomalously low La, Ce, Pr, Nd, Sm and Hf concentrations (less than 0.1 NMORB). It is noteworthy that in the normalized trace element plot, all the magnetites show high Nb/Ta ratios in contrast with high Ta/Nb ratios observed in typical arc magmas. These data support our hypothesis that magmatic crystallization of Fe-Ti oxides under high oxygen fugacity conditions during initial crystallization and formation of the Izu-Bonin and Ladakh-type arc batholiths may be the primary cause of depletion of the HFSE in later magmatic differentiates of less mafic and more felsic granitic arc rocks.