The Characteristics And Origin Of The Cretaceous Granitoids In Luocang Area,Tibet

The Gangdese Zone refers to the tectonic-magmatic belt between the Bangong-Nujiang Suture Zone?BNSZ?and The Yarlung-Zangbo Suture Zone?YZSZ?,which contains about 80%of the magmatic rocks in Tibet Autonomous Region,and the granitoid exposed is about 110,000 square kilometers,accounting for about half of magmatic rock in The Gangdese Zone.In this paper,the Cretaceous granitoid in the Luocang area,the west of Gangdese,is considered as the object of study.This paper research on petrology,geochemistry,zircon U-Pb chronology and other aspects about the Cretaceous granitoid to reveal the diagenetic age,petrogenesis,source types of the magma,tectonic setting and the relationship between the evolution of the Gangdise tectonic-magmatic belt and the rock.The study shows that the mode of occurrence of the Cretaceous granitoid in the Luocang area is mainly stock.The Cretaceous granitiod rock in the area was divided into four invasive sequences,corresponding to four rock units:the Early Cretaceous fine-grained granodiorite???¹K₁?,the Early Cretaceous coarse-grained granodiorite???²K₁?,the Late Cretaceous monzogranite???K₂?and the Late Cretaceous granodiorite-porphyry????K₂?,according to the characteristics of rock forming,petrology and the zircon U-Pb chronology.The Early Cretaceous granitoid belongs to the rock associations of high-K calc-alkaline and quasi-aluminous series,and its major elements characteristics shows the rock is I-type granitoid.The Late Cretaceous granitoid is peraluminous and belong to the rock associations of high-K calc-alkaline or shoshonite series.The geochemical characteristics indicats the closely connection between its magma source and the crust.The characteristics of the rare earth element?REE?in the Early Cretaceous granitoid shows that the rock haves high total content of REEs,and its total content changes a little,REEs fractionate evidently,enrichment of LREE,and weak negative Eu anomaly.Comparing with the Early Cretaceous granitoid,the total content of REE in Late Cretaceous granitoid is higher,and it changes a lot.The fractionation between HREE and LREE is more obvious,with more LREE enriched.The inclination degree of REE distribution pattern curve is greater,with weak or no Eu anomaly.The result of trace element analysis shows that the Cretaceous granitoid in Luocang area enriches in Rb,Th,K,Zr,Hf and other large ion lithophile?LILE?elements,and deplete in Ba,U,Ta,Nb,Sr,P,Ti and other high field strength elements?HFSE?.Nb,Ta,Ti in the Cretaceous granitoid are obviously deleted,it shows that the rock may belong to island-arc granitoid.And the anomalies of various other elements indicate that the the rock is mixing source of the crust and mantle.The Early Cretaceous granitoid in Luocang area is I-type granitoid,which originated from the crust-mantle mixed source in an island-arc environment.The Late Cretaceous granitoid is also I-type granitoid,but the contribution of crustal material in its magma formation is great.Combined with regional tectonic evolution,the Cretaceous granitoid in research area most likely formed in the tectonic setting of the Neo-Tethys northward subduction.The Cretaceous granitoid in research area confirmed the mechanism of the Yarlung Zangbo Neo-Tethys Ocean northward subduction was exist existed in Early Cretaceous,and the functioning range of the mechanism could involve the south of fault uplift zone in the Gangdise tectonic belt.The time which the mechanism started to act is earlier than 126.3Ma,and the mechanism was still working in Late Cretaceous.The subduction was still woking in about 84.7Ma,it illustrates that the time of collision between Asian and Indian continent is later than 84Ma.