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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 ([email protected])
Arundhuti Ghatak, University of Rochester, Rochester, NY-14627 ([email protected])
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 SiO2 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 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.
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 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 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 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.
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 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.