A-type Granites Induced by a Breaking-off and Delamination of the Subducted Junggar Oceanic Plate, West Junggar, Northwest China

The A-type granites with highly positive ε Nd （t） values in the West Junggar, Central Asian Orogenic Belt（CAOB）, have long been perceived as a group formed under the same tectonic and geodynamic setting,magmatic sourceq and petrogenetic model. Geological evidence shows that these granites occurred at two different tectonic units related to the southeastern subduction of Junggar oceanic plate: the Hongshan and Karamay granites emplaced in the southeast of West Junggar in the Baogutu continental arc; whereas the Akebasitao and Miaoergou granites formed in the accretionary prism. Here the authors present new bulkrock geochemistry and Sr-Nd isotopes, zircon U-Pb ages and Hf-O isotopes data on these granites. The granites in the Baogutu continental arc and accretionary prism contain similar zircon ε Hf （t） values（+10.9 to +16.2） and bulk-rock geochemical characteristics（high SiO 2 and K 2 O contents, enriched LILEs（except Sr）, depleted Sr, Ta and Ti, and negative anomalies in Ce and Eu）. The Hongshan and Karamay granites in the Baogutu continental arc have older zircon U-Pb ages（315–305 Ma） and moderate 18 O enrichments(δ 18 O zircon =+6.41‰–+7.96‰); whereas the Akebasitao and Miaoergou granites in the accretionary prism have younger zircon U-Pb ages（305–301 Ma） with higher 18 O enrichments(δ 18 O zircon =+8.72‰–+9.89‰).The authors deduce that the elevated 18 O enrichments of the Akebasitao and Miaoergou granites were probably inherited from low-temperature altered oceanic crusts. The Akebasitao and Miaoergou granites were originated from partial melting of low-temperature altered oceanic crusts with juvenile oceanic sediments below the accretionary prism. The Hongshan and Karamay granites were mainly derived from partial melting of basaltic juvenile lower crust with mixtures of potentially chemical weathered ancient crustal residues and mantle basaltic melt（induced by hot intruding mantle basaltic magma at the bottom of the Baogutu continental arc）. On the other hand, the Miaoergou charnockite might be sourced from a deeper partial melting reservoir under the accretionary prism, consisting of the low-temperature altered oceanic crust, juvenile oceanic sediments, and mantle basaltic melt. These granites could be related to the asthenosphere’s counterflow and upwelling, caused by the break-off and delamination of the subducted oceanic plate beneath the accretionary prism Baogutu continental arc in a post-collisional tectonic setting.