Ophiolitic chromitites – partial melting or melt-rock interaction product?: The Re-Os isotope implications
1,4R.A.Santos, 2K. Suzuki, 1B. Takano, 2Y.Tatsumi,>3Y.Miyata and 3Y. Nozaki
1Dept. of Systems Sciences, University of Tokyo
2Institute for Geothermal Sciences, Kyoto University
3Ocean Research Institute, University of Tokyo
4Philippine Mines and Geosciences Bureau
ABSTRACT
Chromitite formation in ophiolite complexes has been a topic of debate in the field of petrochemistry. Earlier concepts considered multiple stage-partial melting and magma mixing as the main processes for its formation. With the advent of the melt-rock interaction theory, controversies on the unity of the Cr# (Cr/ Cr + Al + Fe) and Mg# (Mg/ Fe + Mg) of both the dunite rind and the enclosing harzburgite, the presence of dunitic envelope in both the Al-rich and Cr-rich chromitites, and the relative abundance of incompatible elements such as Ti and REEs in chromitites compared to the peridotite host have been resolved. Recent studies on the Re-Os isotopic signature of chromitites and of the peridotite (the dunite envelope and the adjacent harzburgite) hosts from two ophiolite complexes, the Palawan (POC) and the Dinagat ophiolite (DOC)complexes, yield results which are suggestive of multiple processes in chromitite formation even within an ophiolite body. Chromitite formation within a single ophiolitic complex may not even be coeval.
In POC massive podiform chromitites (Cr# - 0.78 to 0.84) mostly having orbicular textures, yield Re-Os ages of around 500Ma in contrast to the schlieren, more aluminian chromitites (Cr#- 0.20 to 0.69) which are around 60 Ma. The schlieren variety exhibits generally euhedral grains with relatively higher frequency of solid inclusions (amphiboles, phlogopites, pyroxenes, and various base metal sulfides and PGEs). The podiform chromitites on the other hand generally contain highly fractured grains probably due to post-depositional brittle fracturing. Within the complex these two chromitite groups are of contrasting distribution; the schlieren in the upper levels close to the dunite-gabbro interface whereas the podiform variety is deep within the tectonite harzburgite section.
In DOC, the chromitites are generally of schlieren (Cr#-0.68 to 0.78) variety occurring within a zone of about 20-30 meters thick and are mainly confined in the upper level dunite. Solid inclusions are similar to that of POC chromites but with the absence of phlogopites.
Consideration of the 187Os/188Os ratios of the chromitites and peridotites as against their Al2Owt%, the harzburgites are mainly confined in the depleted end of the Ronda ophiolite trend – a feature generally exhibited by depleted mantle materials. Using the same parameters, the contributing factors for chromitite precipitation could also be delineated; in DOC chromitite formation could be attributed to the fluid addition process whereas the melt addition process is mainly responsible for the schlieren chromitite of POC. The podiform chromitite plot of POC does not define a very distinct trend but apparently it covers the “enriched end” in the partial melting trend of the Ronda ophiolite.
The schlieren chromitites in general contains elevated 187Os/188Os with respect to present day chondritic ratio (0.1270) while the podiform chromitites are at sub-chondritic levels.
Taking into consideration the initial 187Os /188Os ratios of the chromitites as plotted against the reciprocal of their total Os contents, it could be inferred that podiform chromitites could have been formed mainly due to melt accumulation probably consequent to harzburgite formation whereas schlieren chromitite formation my have been due to melt –rock reaction of mantle wedge material with melts from a subducted slab.