Magma Mixing Recorded In Gangdese Magmatic Belt And Its Implications For Continental Crust Growth

There are two types of subduction zone on Earth,which are crust-suturing subduction zone?CSSZ?where collisional orogens have built eventually,and ocean-margin subduction zone?OMSZ?where accretionary orogens have ultimately formed,respectively?Scholl and von Huene,2009?.Accretionary orogens are characterized by significant crust growth,whereas collisional orogens are traditionally regarded as sites related to crustal reworking with limited or without crust growth.The Himalayan-Tibetan orogen is a typical continent-continent collisional orogen on the world.Before the Indo-Asia collision,Neo-tethyan Ocean and the active continental margin on the Lhasa terrane consistitute a typical crust-suturing subduction zone.The¹600 km-long Gangdese magmatic belt on the south of the Lhasa terrane records the tectonic evolution from the subduction of Neo-Tethyan ocean to the Indo-Asia collision,and intriguinglly,both of the subduction-and collision-related intrusive rocks show predominately high and positive?_Nd?t?and?_Hf?t?values,which indicate a close relation to continental crust growth.Therefore,conducting research on the Gangdese igneous rocks can provide us important knowledge of continental growth in continent-continent collisional orogen?or crust-suturing subduction zone?.In this study,we focus on the Dongga dioritic plotons and the Ringqênzêintrusive complex in the central part of the Gangdese belt,a segment between Xaitongmoin to Xigaze.We present detailed field observation,petrology,new zircon U–Pb dating results and in situ Hf or O isotopic data,new whole-rock geochemistry and Sr-Nd isotopic data for these rocks and also have discussed the petrogenesis of these rocks and the implications for crust growth.We draw the following conclusions:?1?Zircon U-Pb dating reveals that the Dongga diorites were crystallized at176.5±1.2 Ma.The abundant 191.3±1.2 Ma inherited zircons in the dated sample imply that crustal materials were involved in the magma source.These diorites have high Mg^#values,depleted(⁸⁷Sr/⁸⁶Sr)_i ratios?0.703570⁰.703595?,?_Nd?t?values?+6.1⁺6.5?and zircon?_Hf?t?values?+12.0⁺14.7?,and relatively high ¹⁴⁷Sm/¹⁴⁴Nd ratios,indicating non-negligible mantle contributions.In petrography,acicular apatite crystals surrounding mafic clots in the Dongga diorites,together with the abundant mafic enclaves in the surrounding coeval granitoids imply a magma mixing origin for the diorites.Our geochemical simulation also confirmed that the Dongga diorites were formed by mixing of crustal and mantle-derived magmas.In addition,the petrogenesis of magma mixing supports vertical continental crust growth during the subduction of the Neo-tethyan ocean during the Early Jurassic.?2?Zircon U-Pb dating yields Eocene ages of 50.2±0.5 Ma,46.4±0.5 Ma,50.6±0.6 Ma and 50.4±0.5 Ma for the quartz diorite,hornblende granodiorite,host pyroxene-bearing granodiorite and its dioritic enclave,respectively.Reverse zoning of plagioclase?spike zone?,pyroxene and plagioclase relicts exist in both of the dioritic enclaves and the granitoids,as well as the mixing trends on the plots of P₂O₅ versus SiO₂ and trace elements,indicating a petrogenesis of magmatic mixing.The relatively low SiO₂ contents?53.43-56.23 wt.%?,high Mg^#values?48-56?and high An value of plagioclase further suggest that precursor magmas of the enclaves are mantle-derived.The dioritic enclaves and granitoids have indistinguishable(⁸⁷Sr/⁸⁶Sr)_i ratios?0.70460⁰.70480?but distinct?_Nd?t?values?+1.8 to+3.0 for the dioritic enclaves,and+0.2 to+0.6 for the granitoids,respectively?.The dioritic enclave show similar zircon?_Hf?t?and?¹⁸O to the granitoids(dioritic enclaves:?_Hf?t?=+3.7-+8.5,?¹⁸O=4.56-7.00‰,granitoids:?_Hf?t?=+1.2-+8.5,?¹⁸O=5.59-7.21‰).Binary mixing calculations based on isotopes show that mantle contributions are more than 55%for these samples.In addition,we compiled 59 Sr isotopic data,73 Nd isotopic data,29zircon O isotopic data,over 2000 zircon Hf isotopic and age data,and 80 Ti-in-zircon temperature data from Gangdese belt.Based on these data,we argue that the the majority of the syn-collisional Gangdese granitoids were also generated by magma mixing processes with mantle contributions more than 30%.Therefore,the extensive and significant mantle contributions in the magma of the collision-related Gangdese granitoids demonstrate considerable continental crust growth during the syn-collisional period.Furthermore,our data combined with previously published data suggest that the ca.50 Ma samples have highest Ti-in-zircon temperatures?up to900??,whereas other samples show lower temperatures mostly below 750?.Because the Gangdese granitoids with unusual high Ti-in-zircon temperatures had been produced dominantly at a very short time?⁵0 Ma?and were related to input of mantle-derived materials,we suggest that a slab breakoff was most likely responsible for the formation of the high-temperature magmas and the continental crust growth at the syn-collisional period in collisional orogens.Consequently,we argue that continental crust growth in collisional orogens has probably far been underestimated.