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

1
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_at_earth.roches
ter.edu) Arundhuti Ghatak, University of
Rochester, Rochester, NY-14627 (arun_at_earth.rochest
er.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 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 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 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 lt2)
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 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.