Petrogenesis Of The Latetriassic-Early Eocene Granitoids In The Tengchong Block

Through the study of petrography,geochemistry of major elements,trace elements and REEs,zirconLA-ICP-MS U-Pb chronology,and Hf isotopes of the Lianghe,Banwa,Longling,Menglian and Mengnong intrusions of the Tengchong block,this paper discussed the granite petrogenesis and the geodynamics mechanism.Based on the previous researches,this paper achieved the following results.?1?The rock type of the Late Triassic Lianghe intrusion is syenite granite with some biotite and muscovite.The rocks have intrusive age of approximately 218 Ma.The contents of SiO₂ are from70.28%to 74.49%,the values of A/CNK are from 1 to1.12,and the molecular content of normative mineralcorundum are from 1.06%to1.92%.The intrusion belongs to peraluminous high-potassium calc-alkaline series,and is inferred to be S-type syenite.The?_Hf?t?values are from-10.74 to-4.14,indicating that the Lianghe intrusion is the product of ancient curst sediment melting and formed by the magmatic-thermal events during the closure of the Paleo-Tethys Ocean and the open of the Mid-Tethys Ocean.?2?The Early Cretaceous intrusions consist of ferric and Magnesian series.The Banwa intrusion is dominated by alkali-feldspar granite with intrusive age of approximately 130 Ma,SiO₂ contents of 75.75%-77.71%,including minerals of fluorite,potash feldspar,and quartz.The A/CNK values are from 0.91 to 1.01,and the molecular content of normative mineral?corundum?are less than 1%.The temperature calculated from Ti-in-zircon geothermometers is 661?,and the?_Hf?t?values of most samples are less than-5,which shows that the melting is dominated by crust source material in the magma formation.The Menglian intrusion belongs to magnesian series rock,and the Longling intrusion belongs to ferric-magnesian rock.The Menglian intrusion is composed of granodiorite and monzogranite,with the intrusive age of 114-125Ma,SiO₂ contents of 67.66%-70.71%.The accessory minerals involve hornblende,and the?_Hf?t?values are-3.0 to-0.7.The rock type of the Longling intrusion is syenite granite,with the formation age of approximately116Ma,SiO2 contents of 69.57%-71.69%,and the?Hf?t?values are from-3.2 to-1.6.The ?Hf?t?values of Menglian and Longling intrusions have an evolutionary trend to positive value,and the Menglian intrusion have large amount of dark micro-inclusions.These studies have shown that the mantle material contribute to the intrusion formation.The Early Cretaceous intrusions are products of southward subduction,withdraw and break-off of lithospheric plate of the Bangonghu-Nujiang Ocean.?3?The rock type of Eocene Mengnong intrusion is syenite-monzogranite-granodiorite,with the intrusion age of 53 Ma and with the gabbro age of 52 Ma.These show that the ferric and magnesian magma is formed from partial melting of subducted mantle wedge caused by high temperature.Therefore,the Mengnong intrusion is the product of magma upward juvenile ferric-magnesian magma when Neo-Tethys Ocean subducted eastward,and the intrusion is I-type granite.?4?The felsic magmatic activities are closely related to metallization in the Tengchong block.The granites show metallogenic specialization characteristics in age,rock type,and so on.Most granites related to the Sn,W-Sn ore deposits belong to highly fractionated granites with high differentiation index values,while the granites related to Cu-Sn,Cu-W,and W ore deposits mainly belong to undifferentiated granites.