GEOLOGICAL SOCIETY OF THE PHILIPPINES

Ophiolitic Chromitite Petrogenesis


 

Rogel A. Santos, Ph.D.

Mines and Geosciences Bureau, DENR
School of Earth and Materials Science and Engineering, Mapua Tech



 

Abstract


 

The origin of ophiolitic chromitite has been a subject of controversy in mantle geochemistry. Although the concept of melt-rock interaction has been gaining popular support, the process of multi-stage melting could not be readily disregarded. In an attempt to delineate the process/es of chromitite formation, major oxides and Re-Os isotopic compositions of chromitites and host peridotites have been determined. Individual analyses of mineral chemistry of chromites, its solid inclusions and silicate hosts were done through EPMA whereas the Re-Os isotopic contents were extracted through isotope dilution-Carius tube digestion technique and were then analyzed through N-TIMS. The samples considered were taken from the ophiolite complexes of Palawan (POC) and Dinagat (DOC).


 

Two types of chromitite occurrences were observed in POC: the schlieren or layered-type in the upper level dunite of the ophiolite stratigraphy and the massive podiform type deep within the harzburgite section. The former is relatively more aluminian (Cr# = 0.38 - 0.62) than the latter (Cr# = 0.72 - 0.88). The relative frequency of solid inlusions (mainly diopside and pargasite with minor phlogopites and several varieties of sulfides and alloys of Ni and PGEs) is higher on schlieren type. In DOC the observed chromitite occurrences are of the schlieren/layered type and with the predominance of chromian chromitites (Cr# = 0.68 - 0.86) though pockets of aluminian chromitites (Cr# = 0.60 - 0.64) have been delineated.


 

More than fifty (50) samples (ultramafic host and chromitite samples) from the two complexes were analyzed for the Re-Os isotope determination. Among chromitites, the more aluminian and schlieren variety yield 187Os/ 188Os ratios of supra-chondritic values (0.1302-0.1570; 50Ma) while the podiform and massive chromitites yields sub-chondritic ratios (0.1237-0.1245; 600Ma). These results imply that in the formation of aluminian chromitites an introduction of radiogenic Os in involved while in the massive generally chromium –rich chromitites a long-term lowered 187Re/188Os is implied. The harzburgite section in both complexes yields sub-chondritic 187Os/188Os ratios (0.1197 – 0.1239; 250Ma) except on samples proximal to zones of schlieren chromitites (0.1238 - 0.1348) or pyroxenite dikes (0.1730).


 

Based on these geochemical systematics, chromitite petrogenesis can be deduced as follows: (a) the schlieren chromitites in POC manifest formation in a fluid-rich (having elevated 187Os values) environment; (b) the podiform chromitites imply formation through melt focusing in a relatively “dry” environment; (c) harzburgite formation might be coeval with podiform chromitite formation but the isotopic deviation from chondritic values in some samples is suggestive of modification by some later events (e.g., transition to subduction process);(d) chromitites even from a single ophiolitic complex can be formed possibly by at least two petrogenetic events – a multiple melting process for the pods and a melt-rock interaction process for the layered chromitites.

 

 
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