1. Figure 1.Geological map of the Tanzawa plutonic complex (modified from Kawate and Arima, 1998). We analyzed magnetite and other mineral fractions in gabbros from the Doshi (north) and Ohtana (south) stage-1 gabbros of this Miocene tonalitic to gabbroic complex, similar to volcanic rocks of the IBM arc. This plutonic complex is considered as exposed cross-section of oceanic island arc crust. Trace Element Geochemistry of magnetites and other mineral fractions in Gabbros, Tanzawa Complex (IBM) and in Ladakh Granodiorites, NW Himalayas. Asish R. Basu, University of Rochester, Rochester, NY-14627 (abasu@earth.rochester.edu) Arundhuti Ghatak, University of Rochester, Rochester, NY-14627 (arun@earth.rochester.edu) Abstract – In this study we will address the issue of Nb-Ta depletion in arc rocks and propose that early magmatic crystallization of Fe-Ti oxides under high oxygen fugacity conditions during the initial crystallization and formation of Izu-Bonin type arc is the primary cause of depletion of high-field-strength elements such as Nb and Ta. To document this proposition we will provide Nb and Ta concentration data in Fe-Ti oxides that formed during the initial crystallization of the arc-magma, such as in the deeper part of the Izu-Bonin-Mariana arc as exposed in the Tanzawa Plutonic Complex (TPC) as a result of tectonic collision between the IBM and the Honshu arcs. This complex is composed of quite diverse rock types with SiO 2 varying from 43 to 75 wt %. These rocks range 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 high field-strength elements (HFSE) in rocks of this plutonic complex are similar to those observed in the volcanic rocks of the IBM arc. We also provide multiple trace element data in separated magnetite minerals from the base of the Ladakh batholithic complex that formed between 50-60 Ma in an intra-oceanic arc north of the advancing Indian lithosphere before its collision with Tibet. These magnetites show Nb-Ta enrichment, similar to the magnetites of the Tanzawa Complex in Japan. Figure 2. The Trans-Himalayan Plutonic Belt extending from the Kohistan batholith in the west to Lhasa in Tibet and beyond in the east, including the 50-60 Ma old Ladakh batholithic complex. We analyzed magnetite mineral separates in mafic-rich basal members of this arc complex that formed as a result of subduction of the Indian plate to the north. Figure 3. Multiple trace element concentrations of whole rock, dark fraction (hornblende +magnetite +zircon + sphene), light fraction (feldspar + apatite + quartz), and magnetite fraction (magnetite [95%] + feldspar [<2%]) normalized to N-MORB for Gabbros of the Tanzawa Plutonic Complex. Elements are arranged according to varying incompatibility (Sun and McDonough, 1989; Tatsumi and Eggins, 1995). Notice Nb-Ta enrichment in the magnetite fraction. Figure 4. Similar to DAK-6, this gabbro from Doshi also shows Nb-Ta enrichment in the magnetite fraction as well as in the dark mineral fraction. Dark mineral fraction consists mostly of hornblende with pyroxene and some plagioclase impurity. Figure 6. Another Doshi gabbroic sample shows strong Nb-Ta enrichment in magnetite fraction. The light mineral fraction contains plagioclase, apatite and sphene. The dark mineral fraction consists of hornblende and pyroxene. Figure 5. This gabbro from the Ohtana intrusion shows characteristic Nb-Ta depletion in the whole rock as well as in the light mineral fraction. However, notice Nb-Ta positive anomaly in the magnetite and the dark mineral fraction consisting of pyroxene and hornblende. Figure 7. Spider plot in magnetite-separates from the lower part of the Ladakh batholithic complex. Notice Nb-Ta enrichment in all three samples although Zr and Hf show lower concentration in the magnetites. The analyzed magnetites contain some impurities of feldspar, that possibly cause Sr and Pb spikes. REFERENCES S. Kawate and M. Arima, The Island Arc 7, 342-358, 1998. N. Srimal, PhD Thesis, University of Rochester, 1986. ACKNOWLEDGEMENT We are grateful to Dr. M. Arima of Yokohama National University for providing the gabbroic samples of the Tanzawa Complex. We are also grateful to Dr. N. Srimal of Florida International University for providing the Ladakh batholith samples. Dr. R. Hannigan of Arkansas State University provided considerable help in the ICPMS analyses of the magnetite samples. This work is partially supported by a grant from the NSF-EAR.