| Summary: | 碩士 === 國立臺灣大學 === 地質學系 === 86 === Okinawa Trough is the only one backarc basin, which is still
in the rifting stage of
western Pacific. From results of complete petrographic, 39Ar-40Ar dating, and
geochemical analysis of the volcanic rocks dredged from the Middle Okinawa
Trough, we could recognize the age of the magmatism and the geochemical
characteristics of the melts. Besides, we could further discuss
the petrogenesis of
magmas and its relationship with extensional tectonic setting
in the central part of
Okinawa Trough.
The volcanic rocks dredged from the Middle Okinawa Trough show a bimodal
volcanism with basalt and rhyolite in dominance. 39Ar-40Ar
dating results suggest that
there are two episodes of magmatism. The early episode commenced from the early
Pleistocene, about 1.5 Ma, and extensive magmasim occurred in
the recent episode,
younger than 1 Ma.
Including the analytical result of major and trace element
and the Sr-Nd isotopic
composition, we could delineate two types of basalt. Type (a)
basalt has higher MgO
(MgO>7 wt.%) and lower REE concentration (LaN = 10.6-12.2) than
type (b) basalt.
Rhyolite also can be recognized to two types, type 1 and type
2. The REE pattern of
the type 1 rhyolite is higher than the type 2 rhyolite,
although they both have Eu
anomaly. From the Rayleigh''s fractionation modeling, type 1 rhyolite could be
formed by the process of small amount of crustal contamination
and large degree of
fractional crystallization of type (a) basalt. On the other
hand, type 2 rhyolite may be
the remelting product of basic lower crust constructed by the basaltic magma
underplating.
Besides the basalt dredged from Aguni graben which has more enriched Sr-Nd
isotopic composition (87Sr/86Sr = 0.704311-0.704540; 143Nd/144Nd = 0.512709-
0.512760), the basalt from Iheya graben (87Sr/86Sr =
0.703892-0.704044; 143Nd/144Nd =
0.512926-0.512827) is similar to those from the Western Pacific Marginal Basin
OIB-like basalts, and doesn''t change a lot in the 1.5 my. The
depletion of HFSE and
enrichment of LILE and Pb of basalts are the same with Ryukyu
volcanic rocks. This
suggest that mantle source region of Okinawa Trough is
metasomatised by the fluxing
which is produced by the subduction of Philippine Sea Plate.
The two component , E-
MORB type upwelling asthenospheric mantle and subduction component form the
mantle region in Middle Okinawa Trough and partial melt to produce the magma in
the extensional setting of central part.
The tectonics and magmatism of southwestern margin of Okinawa Trough is
relating to the "postcollision collapse "stage of Northern
Taiwan. According to the
geochemical study of Kuishantao, northeastern offshore of
Taiwan, andesitic volcanic
rocks, I suggest that it is one of the Ryukyu volcanic front,
and consistent with the
geophysical study, about 110 km depth above the Wadati-Beneoff Zone.
|
|---|
