Petrofabrics and seismic properties of garnet peridotite from the UHP Sulu terrane (China): Implications for olivine deformation mechanism in a cold and dry subducting continental slab

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• 作者：Zhiqin Xu ; Qin Wang ; Shaocheng Ji ; Jing Chen ; Lingsen Zeng ; Jingsui Yang ; Fangyuan Chen ; Fenghua Liang and Hans-Rudolf Wenk
• 关键词：Olivine ; Lattice-preferred orientation ; Water content ; Subducted continental slab ; Ultrahigh-pressure metamorphism ; Sulu terrane
• 刊名：Tectonophysics
• 出版年：2006
• 出版时间：18 July 2006
• 年：2006
• 卷：421
• 期：1-2
• 页码：111-127
• 全文大小：1310 K

文摘

Lattice-preferred orientations (LPO) of olivine, diopside, enstatite and garnet from the Zhimafang garnet peridotite body in the Sulu ultrahigh-pressure (UHP) metamorphic terrane (China) were measured using the electron backscatter diffraction (EBSD) technique. The peridotite was captured from a mantle wedge immediately adjacent the subducted Yangtze slab and then experienced the UHP metamorphism at 750–950 °C and 4–7 GPa. The olivine LPO is characterized by the [001] axis close to the stretching lineation and the (100) plane subparallel to the foliation, indicating the prevailing of (100) [001] slip. Enstatite LPO displays the dominance of (100) [001] slip. Diopside developed complex LPO patterns that are difficult to explain using a single slip system of (100) [001]. Garnet is almost randomly oriented due to its low volume fractions, cubic symmetry and the presence of numerous slip systems. Calculated seismic properties of the peridotite yield a maximum P-wave velocity normal to the foliation and a minimum along the foliation, with anisotropy up to 8 % in strongly sheared samples. The S-wave velocity pattern is complex but the fast polarization plane generally normal to the foliation. The inferred shear sense from the olivine LPO is top-to-SE, in contrary to exhumation-induced top-to-NW thrusting recorded in the quartz LPO, implying that the olivine LPO formed at early UHP metamorphic conditions. The olivine crystals have relatively low water contents (141–475 H/10⁶ Si), indicating a fluid-deficient environment for the LPO formation. The present study suggests that a combination of low temperature and UHP plays a much more important role than the water content to promote the activation of (100) [001] slip in olivine.