Ultrahigh-pressure (UHP) metamorphic rocks, represented by coesite- or its pseudomorph-bearing eclogites, have been found mainly from continent-continent collision orogenic belts, and garnet peridotite bodies are also known to occur in such UHP belts. The UHP eclogites and garnet peridotite bodies/layers/lenses are commonly enclosed within metamorphic rocks derived from continent crustal materials composed by moderate to low pressure metamorphic minerals, although they should have been located under deep mantle depths (>50 km). Therefore, elucidation of juxtaposition processes between the mantle material and the host crustal material is one of main subjects for the petrology in the UHP belts. Delineating of pressure-temperature (P-T) paths of these UHP garnet peridotite bodies can give us indispensable constraints to clarify the juxtaposition process of mantle and crustal materials in the continent-collision settings and the exhumation processes of deeply subducted rocks with higher density than the crustal rocks. In this paper, we summarize the commonly used methods to determine P-T histories of the UHP garnet peridotite bodies (i.e., geo-thermometer and barometer) and discuss P-T paths of UHP rocks in collision type orogenic belts, and their tectonic significance.
Podiform chromitites have been interpreted as cumulates formed via melt/harzburgite reaction and subsequent melt mixing within the upper mantle. Recent finding of ultrahigh-pressure (UHP) minerals such as diamond from some podiform chromitites has, however, seriously required us to reconsider the whole framework of podifrom chromitite genesis. The UHP podiform chromitites are characterized by presence of frequent silicate lamellae in chromian spinel and only PGE (platinum-group elements) alloys as platinum-group minerals. The UHP chromitites were possibly formed by deep recycling of the ordinary low-pressure podiform chromitites. Diamonds could be formed by reduction of CO2 metasomatically supplied to chromian spinel in advance. Silicate lamellae in chromian spinel of UHP chromitites were possibly derived from primary silicate inclusions (pyroxenes, pargasite and Na phlogopite), commonly found in the low-pressure chromitite. They were decomposed/partially melted and resolved in high-pressure chromian spinel during compression/heating during downward transportation, and were exsolved as silicate lamellae on decompression/cooling during uprising. PGE sulfides commonly found in the low-pressure chromitite may have been decomposed to PGE alloys and sulfur-rich melt/fluid that were removed outside. We should systematically re-examine all podiform chromitites that have been ever documented, but our preliminary examination indicates that both concordant and discordant chromitites from Oman ophiolite are of low-pressure origin, containing primary pargasite inclusions in chromian spinel. More thorough characterization of the UHP chromitite may enable us to place constraints on the style of mantle convection that provided the MORB source possibly comprising the UHP chromitite. Possible presence of ringwoodite as one of UHP minerals in chromitite may favor the two-layer mantle convection for supplying the source mantle that formed the oceanic lithosphere.
The Oman ophiolite is a remnant of Neo-Tethyan oceanic lithosphere that has been modified by arc-related magmatism during oceanic thrusting prior to the obduction to the Arabian continent. To understand the formation of oceanic mantle lithosphere at a spreading ridge and subsequent modification at an incipient subduction zone a km-scale mineral chemical mapping of the mantle section was conducted in the Fizh and Salahi blocks in the northern Oman ophiolite. The range of spinel Cr#[=Cr/(Cr+Al) atomic ratio] in harzburgites becomes wider from the region of paleo-ridge segment center to the margin. In the northern part, where a paleo-ridge segment margin was inferred, refractory harzburgites with spinel Cr# greater than 0.7 are abundant and are distributed as a band (3 km in width) extending from the basal thrust to the Moho. Such high refractory harzburgites are associated with thick dunite bands in which spinel Cr# is also high (greater than 0.7). Dunites in the Fizh and Salahi mantle sections have spinel with Cr# ranging from 0.45 to 0.8 and tend to have higher spinel Cr# than the harzburgites. Moreover, the dunites with high Cr# spinel (greater than 0.7) are abundant in the basal part of the mantle section. The distribution of refractory harzburgite and dunite in the northern Oman ophiolite can be modeled as follows. During oceanic thrusting the Oman ophiolite was displaced above an incipient subduction zone. The fluid released from metamorphic sole due to thermal metamorphism of altered oceanic crust infiltrated into the mantle section. Dunite channels may have acted as pathways for fluid infiltration from the base of the ophiolite, which was instrumental for flux melting of wallrock harzburgite. The presence of the high refractory harzburgites in the northern Fizh mantle section implies that the infiltration of fluid from the base of ophiolite was extensive in the ridge segment boundary region.
The understanding of olivine fabric has dramatically been progressed during the last~10 years by both natural and experimental studies along with the major technological improvement. Crystal-preferred orientations (CPO) are the expression of crystallographic fabrics of grains within the rock with respect to the structural frame (X-, Y- and Z-axes). CPO patterns can be interpreted based on the plane of plastic flow and the flow direction. Olivine fabrics, which are olivine CPO patterns, have been classified into five types: A, B, C, D and E types by a series of experimental studies. An additional AG type has also been proposed in recognition of its common occurrence in nature. New results have already required major modifications to the geodynamic interpretation of the upper mantle, although some uncertainties still remain regarding the olivine fabrics and their development.