Pressure-Temperature Estimates of Eclogites in the North Qaidam Ultrahigh-Pressure Metamorphic Belt in China

April 8th, 2011

The Tibetan plateau in Southwestern China is the result of multiple tectonic events. The region is composed of various terranes and tectonically altered zones, including the North Qaidam ultrahigh-pressure (UHP) metamorphic belt. The belt extends roughly 500 km NW–SE before being sinistrally offset by the Altyn Tagh fault and is found between the Qilian terrane and the North Qaidam basin. To the northeast of the Qilian terrane is the North China craton. The UHP belt formed through subduction of oceanic lithosphere, 560-495 Ma, and the resulting continental collision between the Qaidam and Qilian terranes, 456-397 Ma (Gehrels et al, 2003; Mattinson et al, 2007; Song et al, 2003; Zhiqin et al, 2006); however, the number of subduction zones and the subduction directions are unkown. Several models have been proposed to explain the formation of the North Qaidam UHP belt, including two endmember models: one of which states that the Qilain Ocean subducted to the southwest under the combined Qaidam-Qilian terranes (Gehrels et al., 2003; Mattinson et al, 2007), while the other states that the ocean was subducted to the north under the North China block (Zhiqin et al., 2006; Song et al, 2009ab).

Eclogites outcrop as blocks, boudins or layers throughout the North Qaidam metamorphic belt. Because eclogites are dry, they tend to record the peak pressures and temperatures that the region experienced as well as retrograde pressures and temperatures. Reconnaissance analysis of the eclogites in this region record three distinct stages of metamorphism: pre-peak metamorphism with pressures of 0.49-0.67 GPa at 410-490⁰C; peak eclogite facies metamorphism at 2.7-3.4 GPa and 610-700⁰ C, and a retrograde stage from <0.5 GPa and <650⁰C up to 1.3 GPa and 725⁰C (Zhang et al. 2009). To better constrain the tectonic history of the UHP belt, additional eclogites were analyzed petrographically and by electron probe microscopy. Eclogite sample 98-14B1 yields the mineral assemblage grt+cpx+bt+qtz, which demonstrates eclogite-facies metamorphis; however, the mineral assemblage does not allow for the estimation of peak pressure conditions. Garnets exhibit distinct reaction rims signifying retrogression and using grt-cpx thermometer (Krogh, 2000), I found temperatures from 634-675⁰C at pressures of 5-13 kbar; this agrees with the third, retrogressed, stage metamorphism as defined by Zhang et al (2009).

Chelsea Nissen, ’12
Horseshoe Bay, TX
Majors: Geology, Chemistry

Emily Walsh
Cornell College

Sponsor: Emily Walsh

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