RODINGITES DERIVED FROM ECLOGITES OF OPHIOLITES AND ITS ASSOCIATED SERPENTINITE AT CHANGAWUZI, WESTERN TIANSHAN, CHINA

The Southwestern Tianshan ophiolitic mélange extends along the NE-SW axis of the tectonic ridges and faults of the Tianshan Mountain Range. The ophiolitic rocks and sequence is poorly preserved. Serpentinized ultramafic rocks occur together with interlayered south-dipping blueschists or greenschists in Changawuzi ophiolitic sequence, which suggest representing relics of a Silurian oceanic crust. Rodingites included in the serpentinite complex were formed at the expense of eclogite enclosed in the ultramafic rocks of Changawuzi ophiolites. The rodingites contain a mineral assemblage of grossular, diopside, prehnite, chlorite, and relict omphacite and Fe-Mg-Al garnets. The relict early mylonitic serpentines can be clearly recognized and distinguished from microtexture, and they exhibit nanometer size crystals with deformed lattice pattern under the observation of HRTEM. The mylonitic texture of the serpentinite is considered to originate from early, i.e. ocean-floor serpentinization and related to later subduction-related high-pressure metamorphism. The secondary serpentinization is evident by the relict olivines and pervasive well-developed brucite veins, which suggest an overprint texture on the HP eclogitic facies metamorphism. HRTEM also reveals a clear antigorite crystal structure for the late elongated serpentines, and indicates a consistent PT forming conditions with that estimated from rodingites during rodingitization. The presented data indicate that the formation of the rodingites result from a phase of secondary serpentinization during exhumation of the subducted oceanic plate. The rodingitization started at 370~410 °C/6.5~8.5 kbar, while pervasive rodingitization took place under the condition of 200~350°C/4~6 kbar. The established PT path shows a retrograde track from eclogites to rodingites. We, therefore, conclude that the process of rodingitization could also take place in the subduction zone during exhumation in addition to its more common occurrence under ocean-floor metamorphic conditions.