Oceanic ridge metamorphism of the East Taiwan Ophiolite |
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| Authors: | J G Liou W G Ernst |
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| Institution: | (1) Department of Geology, School of Earth Sciences, Stanford University, 94305 Stanford, California, USA;(2) Department of Earth and Space Sciences, Institute of Geophysics and Planetary Physics, University of California, 90024 Los Angeles, California, USA |
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| Abstract: | Brecciated mafic+ultramafic plutonic rocks of the East Taiwan Ophiolite occur as detritus and slide blocks in the Pliocene
Lichi Mélange. These plutonic rocks have been subjected to two stages of post-magmatic recrystallization: (I) pre-brecciation
ridge-type metamorphism attended by high-grade greenschist and rare amphibolite facies physical conditions; and (II) later
off-axis metamorphism under zeolite to lowest greenschist facies conditions that postdated brecciation, submarine talus accumulation
and deposition of associated pelagic sediments. The effects of the earlier ridge metamorphism are the main concern of this
paper.
(I) Dominant antigorite together with chlorite and talc in some ultramafics suggests that these rocks recrystallized at T>350 ° C. The primary compositions of gabbroic calcic plagioclase have been modified from An 45–70 to An 13–38, and the igneous
clinopyroxenes and hornblendes partly replaced by actinolite+chlorite. Stable mineral assemblages in the metagabbros are thus
∼oligoclase+actinolite+chlorite±very rare epidote+sphene, and intermediate plagioclase +actinolite+chlorite+sphene. Amphibolites
are less common and consist of more calcic plagioclase (An 25–49)+hornblende. The presence of assemblages transitional between
greenschist and amphibolite facies for basaltic compositions is suggestive of very low-pressure thermal metamorphism such
as would be appropriate to the crustal portions of an oceanic spreading center.
(II) The occurrence of vein albite+actinolite+ chlorite near the base of the brecciated plutonic sequence and vein prehnite+laumontite
in the upper part suggests that the brecciated plutonic rocks were later feebly retrograded under conditions of the greenschist
and zeolite facies respectively-probably some distance removed from the thermal regime of a mid-oceanic ridge.
The East Taiwan Ophiolite probably represents the western termination of the Philippine Sea lithospheric plate. Portions of
this oceanic crust and underlying mantle were incorporated in the Lichi Mélange of the Coastal Range of eastern Taiwan as
a consequence of antithetic faulting and erosion. This process evidently accompanied east-directed underflow of the Asiatic
(South China Sea) plate. |
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