The North Qaidam Ophiolite provides insight into the mechanisms that formed the rocks of the Tibetan Plateau before its uplift through the Cenozoic India-Eurasia collision. The ophiolite, juxtaposed and folded with an ultrahigh-pressure (UHP) terrane across the Luiliangshan Detachment Zone, separates the two major terranes in the region, the Qaidam and Qilian blocks. Two endmember models exist for the creation of the North Qaidam Ophiolite: (1) the closing of a single ocean through the collision of small continental masses with the Eurasian continent above a single north-facing subduction zone, and (2) the closing of several oceans through the collision of larger continental blocks, such as the Qaidam and Qilian blocks, through multiple north-and south-facing subduction zones before collision of the entire area with the Eurasian continent. Because the former model predicts the closing of a single ocean, its ophiolite should record the geochemical signatures of a mid-ocean ridge basalt (MORB) environment. The multiple ocean closure of the latter model should demonstrate geochemical signatures in the ophiolite, which evidence a supra-subduction zone (SSZ) environment such as a backarc, intra-arc, or forearc basin.
Previous geochemical research records evidence of both MORB and SSZ environments in the North Qaidam Ophiolite. Zircons from the ophiolite show MORB signatures, whereas whole rock geochemical analyses of the ophiolitic rocks reveal signatures of an SSZ environment. To determine which model best describes the creation of the North Qaidam Ophiolite, we analyzed the petrography and whole rock geochemistry of 10 ophiolitic rocks. Petrographic analysis indicated pyroxene crystallization prior to plagioclase crystallization, which suggests an SSZ environment; whereas major element oxides revealed signatures of MORB environments. Trace element analysis revealed a smaller ratio of the incompatable immobile element Y to the compatible element Cr, and less V than Ti, which indicate an SSZ environment as well. We concluded that the North Qaidam Ophiolite was formed in a backarc basin because of the presence of both MORB and SSZ signatures, which best matches the model of multiple ocean closure.
Natashia Pierce, ’11
Cedar Rapids, IA
Albion College, Michigan
Sponsor: Emily Walsh