AMPHIBOLE AND ASSOCIATED EXSOLUTIONS IN GARNET AND CLINOPYROXENE IN GARNET PERIDOTITE FROM THE NORTH QAIDAM UHP BELT, NW CHINA: IMPLICATIONS FOR LARGE QUANTITY OF HYDROXYL DEFECTS IN MANTLE WEDGE

A large garnet peridotite block occurs in quartzofeldspathic gneisses within the North Qaidam UHP metamorphic belt, the northern Tibet, NW China. Four different exsolution lamellae are recognized in porphyroblastic garnets of the garnet peridotite. These lamellae are (1) rutile, (2) orthopyroxene, (3) clinopyroxene, and (4) Na-rich (Na2O 4.07¨C6.69 wt%) amphiboles. All these exsolution lamellae occur in parallel to the isometric form {111} of garnet. The high abundances and regular distributions suggest that the rods of rutile, pyroxene and amphibole in the porphyroblastic garnets are generated by decompression-induced exsolution. Discovery of amphibole exsolutions in the porphyroblastic garnets has two-fold significance: (1) they all have high Na2O content, suggesting an Na-rich parental majoritic garnet forming at depths greater than 8 GPa; and (2) they are hydrous phases, suggesting the presence of significant amount (> 1000 ppm) of hydroxyl in the parental garnet host in the form of structural effects. Large amount of exsolved lamellae were also discovered in porphyroblastic clinopyroxenes in garnet peridotite and garnet pyroxenite. The hydro-phase amphiboles usually occur as the major exsolved rods in company with other exsolutions in different clinopyroxenes; this includes amphibole + quartz, amphibole + Cr-spinel °À orthopyroxene and amphibole + rutile. Based on the back-scattered electron (BSE) images, amphibole rods in some clinopyroxenes can reach up to 8.5 vol. % with about 3.8 vol. % quartz rods, and, 5 vol. % with 3.5 vol. % Cr-spinel, respectively. The high concentration of amphibole and quartz exsolutions suggest that the parental clinopyroxenes are supersilicic and/or rich in Cr, Al and hydroxyl (OH > 2000 ppm), which is believed to be formed in an environment of very high pressure and water-saturation, most probably the mantle with above the subduction zone. Discovery of various exsolution phases in garnet and clinopyroxene enable us to conclude that: (1) they originated and exhumed from depths greater than 240 km; and (2) It is possible that such majoritic garnet and supersilicic clinopyroxene could be important carriers that transport H2O into earth's deep interior during slab-induced down-going flow of mantle wedge materials.