摘要
Garnet amphibolite is one of the common metabasic rocks exposed in collisional orogenic belt,the metamorphic evolution of which is associated closely with orogenic processes.The Yaganbuyang garnet amphibolites occur as blocks hosted by massive granitic gneiss,and consist mainly of hornblende,garnet,clinopyroxene,plagioclase,biotite,quartz with minor rutile/ilmenite and phengitic muscovite.These garnet amphibolites were interpreted to have experienced decompression-dominated evolution that can be divided into three generations(M1,M2,M3),based on the petrographic observations and phase equilibria modeling calculated by THERMOCALC.The assemblage of the first generation(M1)is inferred to possibly be dominated by garnet+omphacite+rutile+phengite+quartz,which is modeled to be roughly stable at P>25 kbar and T>800℃.The second generation(M2)is characterized by the local symplectites of clinopyroxene+plagioclase produced from omphacite,indicating a near-isothermal decompression from~23.8 kbar/875℃to~10 kbar/852℃.The third generation(M3)is marked by the kelyphitic rims of plagioclase+hornblende around garnet and of hornblende+ilmenite around clinopyroxene,involving the late-stage retrogression from~9.8 kbar/848℃to~5.8 kbar/645℃.Zircon U-Pb dating yielded one group metamorphic age of c.500 Ma that is interpreted to represent the timing of the peak eclogite-facies metamorphism.A combination of petrography observation,phase modeling results and geochronology data suggests that the Yaganbuyang garnet amphibolites have once undergone eclogite-facies metamorphism by continental subduction rather than crustal thickening.Therefore,the Yaganbuyang area is an eastward extension part of the South Altyn HP-UHP metamorphic belt.
基金
supported by the National Natural Science Foundation of China(Nos.42030307,41972054,41430209,41672187,41902051)
the MOST Special Fund from the State Key Laboratory of Continental Dynamics,Xi’an,China。
