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Recycling in subduction zones: Evidence from eclogites and blueschists of NW China

Lavis, Shaun 2005. Recycling in subduction zones: Evidence from eclogites and blueschists of NW China. PhD Thesis, Cardiff University.

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The geochemical evolution of subducting oceanic crust (including both mafic and sedimentary components) has important implications for our understanding of the origin of mantle heterogeneity and the geochemistry of volcanic-arc basalts (VAB). However, knowledge of the actual geochemical evolution of the subducting oceanic crust is largely based on indirect evidence, such as the composition of VAB and our understanding of the geochemical properties of key trace elements. Certain kinds of blueschist belt are thought to consist of exhumed portions of subducted oceanic crust. It is therefore possible to derive direct evidence of the geochemical evolution of subducted oceanic crust by studying the metamorphic rocks (i.e. greenschists, blueschists and eclogites) of blueschist belts. The bulk rock chemical analysis of greenschists, blueschists and eclogites from the western Tian Shan blueschist belt, Xingjiang Province, People's Republic of China (PRC), has enabled the geochemical effects of subduction zone metamorphism to be determined. Some eclogites from Tian Shan are thought to have undergone "ultra-high pressure metamorphism" (UHPM) at pressures >2.5GPa, and to have passed through the dehydration reactions associated with subduction zone processes. However, it is shown in this thesis that the eclogites have not been subjected to UHPM. Novel approaches have been developed to interpret the geochemistry of metamorphic rocks, including a new method for identifying "immobile elements" and a method for identifying fresh (i.e. un-metamorphosed) chemically-similar samples from large published datasets. Protoliths of meta-basaltic rocks from Tian Shan are shown largely to originate from seamounts, continental volcanic arcs and basalts associated with attenuation of continental crust. Comparison of the chemistry of the metamorphosed rocks with likely protolith compositions has not revealed any systematic changes associated with subduction zone metamorphism. To augment the information obtained from the Tian Shan rocks, samples were also analysed from the blueschist belts of the Qilian Mountains, Gansu/Qinhai Province, PRC. Protoliths of the meta-basaltic rocks were shown to be back-arc basin basalts and basalts associated with back-arc rifting. Comparison of the geochemistry between likely protolith and metamorphic rock compositions revealed no systematic changes. This indicates that no geochemical effects associated with subduction zone metamorphism. It is shown, by comparing the Loss on Ignition (LOI) of carbonate-bearing meta-basaltic rocks with the H2O content of H2O-saturated MORB, that the blueschists and eclogites have not significantly dehydrated. Thus, no mobile phase with the capacity to transport fluid-mobile elements was present during subduction zone metamorphism. The identification of no chemical changes associated with subduction zone metamorphism in this work is consistent with similar investigations that have recently been published. Ostensibly, such findings have important implications for our understanding of the origin of mantle heterogeneity and VAB compositions. However, the assumption that rocks from blueschist belts, such as those of the western Tian Shan blueschist belt, are analogous to actual deeply subducted oceanic crust is shown not to be valid. This may particularly be true with respect to differences between initial H2O content of blueschists and eclogites compared with actual subducting altered oceanic crust. Consequently, any interpretations of the geochemical evolution of such rocks should not be extended to cover the evolution of actual subducting oceanic crust.

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Earth and Environmental Sciences
ISBN: 9781303201547
Date of First Compliant Deposit: 30 March 2016
Last Modified: 12 Feb 2016 23:15

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