Discovery Of Hornblende Gabbro In Sangueda,Zetang,Tibet: Evidence Of Closure Time Of The Yarlung Zangbo

The collision between India and Eurasia Plates resulted in closure of Neo Tethys,and formation of the Yarlung Zangbo Suture Zone,which extends from east to west for approximate 2000 km in southern Tibetan Plateau.The ophiolite outcrop in Zetang is the largest outcrop in the east of Yarlung Zangbo Suture Zone,and it's intruded in by hornblende gabbro(consisted of grained,porphyritic,megacryst,cumulate hornblende gabbro and amphibolite)in Sangueda.Previous studies on hornblende gabbro in Sangueda named it spilite or basalt for ignoring geographical connections,but in this paper,it's proved to be intrusive rock after thorough investigation.As shown in the TAS diagram,most projection points of hornblende gabbro fall into the gabbro area,which is a kind of basic rock.It can be distinguished from the Huck diagram that hornblende is carried into rock mass by other magma,and there's a huge unbalance between hornblende and magma.Different rock samples in the rock mass indicate similar distributions of trace elements,which are typical for island arc magmatic rocks in the environment of plate subduction: large ion lithophile elements,such as Cs,Rb,Th,K and Ba,and La are enriched,high field strength elements,such as Ta,Nb,Ti and Th are depleted,and Sr is positively anomalous.Chondrite normalized REE patterns are consistent,indicating that they have characteristics of cognate magmatic series.Studies on the closure time of Neo Tethys haven't produced a well-recognized conclusion,and findings of hornblende gabbro in Sangueda in this paper can be used to infer when India and Eurasian Plates started and finished collision.Hornblende gabbro intrudes in between serpentine peridotite(passive epicontinent)and andesite(active epicontinental volcano-sedimentary formation),indicating that it was formed after the collision;meanwhile,it's rich in volatile matter,a typical characteristic of arc magma,so that it was formed in the plate subduction environment before collision.Combining these two contradictory results,hornblende gabbro in Sangueda was formed at the early stage of the collision,when Neo Tethys just closed.Analysis with Zircon U-Pb dating revealed that the age of hornblende gabbro in Sangueda and the time of Neo Tethys' closure is 47.6±2.2Ma.Previous studies have found that ophiolite outcrop in Zetang was formed in the plate subduction environment,and the age of Zircon U-Pb is 154.9Ma±2.0Ma,so the plate subduction lasted for at least 110 Ma,and so did the closure of Neo Tethys.The lower limit of the Neo Tethys ocean scale is about 3470 km,based on the expansion rate of the North Atlantic Ocean,which is about 34 mm per year.