Geochemistry Of Eclogites From North Qaidam And Its Dynamic Significance

Geochemistry of eclogites from Shaliuhe, Yukahe, Shitieshan and Shenglikou area of North Qaidam, West China reveals that the majority of their protoliths are geochemically distinguished with mafic igneous rocks of oceanic affinity, including IAT, OIB and MORE. These rocks expirenced assimilation of continental crust to variant extents, and the occasional involvement of igneous rocks of continental affinity could not be excluded.This work firstly reprot tetrade effect of REE in eclogitic rocks, indicating the presence of active volatile- bearing fluid-rock interactions under open system in subductional process. Previously observed connection between tetrade effect and systemic low Nb/Ta, Zr/Hf, Y/Ho ratios also presents. However, the Nb/Ta ratios lower than chondrite value (~17) in eclogites are inconsistent with previously proposed Nb/Ta high in subducted oceanic crust to complement the mass balance of Nb and Ta in the evolution of continental crust.Anomalous enrichment of heavy REE are observed in some of samples from Xitieshan, which may resulted from partial melting with garnet as extraction residue during subduction.Comparation on published age-dating shows the ultra-high pressure maramorphism in North Qaidam occurred during 450~500Ma beyond errors and systemiclly antedated the regional continent- continent collision at 440Ma.Based on above-mentioned results and interpretations, this work hereby propose 2 different models on the early paleozoic evolution in North Qaidam, arc-continent collision and ophiolite-subduction. During arc-continent collision, protoliths of eclogites were scratched off from oceanic island-arc and oceanic islands and carried downwards by subducting continental slab. They expirence HPM or UHPM together in matle depth then exhumed buoyantly, and tectonicly extruded probably with the assistance of normal faults in crust level. The ophiolite-subduction is firstly proposed by this work, which supposes part of protoliths of present eclogites form North Qaidam was formerly ophiolite obduted onto continental edge and involved into the subduction together with continental slab. It is worth noting that only if the earlier formation ages of ophiolite are positively recognized, the ophiolite-subduction scenario could be confirmed, otherwise this may lead to ambiguity for oceanic eclogites cropped out with typical crustal rocks like grantic gneiss and marbles.The arc-continent collision and ophiolite-subduction are both related to continental suduction and probably UHP metamorphism, and neither of them has a position inMaruyama's (1996) classification, as suggests that the UHPM are not uniquely assocated with continent-continent collision.A combination of regional geology and geochemistry indicates the Cambrian and Ordovician evolution in the south of central Asian zone is charactelized by complex collages between continental blocks with island arcs under the compressional background perhaps related to the final convergence within Gondwanaland, the so-called late Pan-african movement. This may provide an explanation for the relatively mass occurance of UHPM belts within central Asian zone and adjacent Qinling-Qilian-Kunlun zones.With the constraints from published paleopmagnetic data on motion rates of continental blocks prior to their collision, this work firstly proposed the ultra-fast subduction rate could assist the underthrust of low-density continental crust into mantle depth to some extents. The ultra-fast motion is cofirmed before collision between North China and Yantze, Eurasia and India, and may also be appliable to that related to Kokchtav UHPM event. The significance of ultra-fast motion in UHPM formation lies in that the slab pull could not be the unique mechanism to drive continental crust to subduct.