Origin of Middle Cambrian and Late Silurian potassic granitoids from the western Kunlun orogen, northwest China: a magmatic response to the Proto-Tethys evolution

摘要

This paper presents new SHRIMP zircon U-Pb chronology, major and trace element, and Sr-Nd-Hf isoto-pic data of two Early Paleozoic granitic phitons (Yierba and North Kudi) from the western Kunlun orogen, in attempt to further constrain the Proto-Tethys evolution. SHRIMP zircon U-Pb dating shows that the Yierba pluton was emplaced in the Middle Cambrian (513±7 Ma) and the North Kudi pluton was emplaced in the Late Silurian (420.6=6.3 Ma). The Yierba pluton consists of quartz monzodiorite, quartz monzonite and granodiorite. These granitoids are metaluminous and potassic, with initial ~(87)Sr/~(86)Sr ratios of 0.7072-0.7096, ε_(Nd) (T) of-0.2 to -1.6 and ε_(Hf) (T) (in-situ zircon) of-1.2. Elemental and isotopic data suggest that they were formed by partial melting of subducted sediments, with subsequent melts interacting with the overlying mantle wedge in an oceanic island arc setting in response to the intra-oceanic subduction of Proto-Tethys. The North Kudi pluton consists of syenogranite and alkali-feldspar granite. These granites are metaluminous to weakly peraluminous and potassic. They show an affinity of A_1 subtype granite, with initial ~(87)Sr/~(86)Sr ratios of 0.7077-0.7101, ε_(Nd) (T) of -3.5 to -4.0 and ε_(Hf) (T) (in-situ zircon) of-3.9. Elemental and isotopic data suggest that they were formed by partial melting of the Precambrian metamorphic basement at a shallow depth (<30 km) during the post-orogenic regime caused by Proto-Tethyan oceanic slab break-off. Our new data suggest that the subduction of the Proto-Tethyan oceanic crust was as early as Middle Cambrian (~513 Ma) and the final closure of Proto-Tethys was not later than Late Silurian (~421 Ma), most probably in Middle Silurian.