HR: 1330h
AN: V22D-0621    [PDF]
TI: Exsolution Textures and Zircon Geochemistry, North Qaidam UHP Terrane, NW China
AU: * Mattinson, C G
EM: cgm@pangea.stanford.edu
AF: Dept. of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115 United States
AU: Liou, J G
AF: Dept. of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115 United States
AU: Jones, R E
AF: Dept. of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115 United States
AU: Wooden, J L
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Wu, C
AF: Chinese Academy of Geological Sciences, Institute of Geology, Beijing, 100037 China
AU: Yang, J
AF: Chinese Academy of Geological Sciences, Institute of Geology, Beijing, 100037 China
AB: Eclogite, peridotite and pyroxenite ($\pm$ garnet) enclosed in amphibolite-facies felsic gneisses of the North Qaidam Mountains preserve mineral exsolution textures probably formed during exhumation from ultra-high pressure (UHP) conditions. Garnet in eclogite, garnetite, and clinopyroxenite contains rutile rods aligned in multiple crystallographically controlled directions. Clinopyroxenites contain 4--6~mm diopside porphyroblasts in a 0.5--1~mm matrix of diopside, chlorite, amphibole, and Cr-magnetite. These porphyroblasts contain rods and plates of chromite or Cr-magnetite (18--22 wt.% Cr$_2$O$_3$, poorer in iron than matrix magnetite), ilmenite rods, chlorite plates, and sparse phlogopite. X-ray microdiffraction of exsolved chromite in one grain indicates the crystallographic orientation $(101)_{Chr}//(0\overline{1}0)_{Di}$, $(111)_{Chr}//(\overline{131})_{Di}$, $(131)_{Chr}//(\overline{111})_{Di}$. One clinopyroxenite contains diopside with exsolved Cr-spinel and magnesiokatophorite, and diopside from another sample contains stubby rutile prisms. These exsolution features are similar to those reported from other UHP terranes and kimberlites, and are consistent with the increased solubility of Ti in garnet, and K, Ti, and Si ($\pm$ OH) in clinopyroxene indicated by UHP experiments. Cathodoluminescence (CL) and SHRIMP-RG Th, U, and REE analysis of zircons from one amphibolitized eclogite reveals CL dark cores (probably magmatic) with high Th/U, moderate Th, U, and high REE concentrations, and a moderate Eu anomaly. Very bright rims (metamorphic) contain very low Th/U, Th and U concentrations, and much lower REE concentrations. SHRIMP U/Pb ages previously reported from eclogite are 443--496~Ma. Zircon cores (detrital) from two paragneisses contain high Th/U, moderate to high Th and U concentrations, and prominent Ce and Eu anomalies. Rims (metamorphic) contain very low Th/U and Th, high U, and lower, flatter REE patterns; Ce and Eu anomalies are small or absent. Zircons (magmatic) from three orthogneisses lack distinct rims, contain moderate to high Th/U, Th, U, and REE concentrations, and have small to moderate Ce anomalies, and moderate to very large Eu anomalies. SHRIMP U/Pb ages previously reported from orthogneiss are 932--1011~Ma.
DE: 3625 Descriptive mineralogy
DE: 3660 Metamorphic petrology
DE: 3670 Minor and trace element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting