The Very-low-grade Metamorphism And It's Deformation Of The Triassic In The Songpan-Garze Orogenic Belt

Songpan-Garze Orogenic zone lies between Tarim -Qaidam block,Qiangtang block and Yangtze block. It appears an 'inversed triangle'. The main body of this zone: the west is Yidun island belt and separated from Qiangtang block;the north is Animaqing mélange belt;the east and south is Longmenshan-Yanyuan foreland thrust belt. Xu Zhiqin considered it 'Geology Bermuda'. In this region Triassic stratum distributes very widely, and its area exceeds 100 thousand km2. The most zone develops very-low-grade metamorphism and obvious transformation. The very-low-grade metamorphism intervenes between sedimentary and metamorphism,and it is short of new metamorphosed balanced mineral combination. It is difficult to find mineral index of temperature and stress in this process. To construct elaborate P-T-t diagram. Therefore it becomes a nodus in geological process study. Since 1960s, Weaver, Kubler, Martin Frey,and Doug Robison have put forward compartmentalization projection in the international. In country, Bi xianmei, Suo shutian, etc. can only estimate the temperature and stress interval of this metamorphism and centigram the upper and lower limit by mensurating the crystallinity and other maturity degree parameters of illite, mica, and chlorite, etc. in slate. For giving an explaintion of strong transmogrification but feebleness metamorphism of and their corresponding relation of the Triassic stratum in Songpan-Ganzi area and opening out the temperature and stress condition, this paper summarized study production of predecessors, and put forward 5 conclusions about Songpan-Ganzi area Triassic main lithology(slate, sandrock,and blastopsammitic) through field geological investigation , actual measurement section plane study and combining indoor slice identification , electron probing and fluid enclosure testing analysis:1.Field outcrop strongly transmogrification, and often developed same cleavages(S1) erection or approximately erection 'B'fold and symmetry similituded fold. 2.The metamorphosed mineral combination is simple, and the typical metamorphosed mineral combination is Al+Chl+Ser(Mu)+Cc+Q, etc. The metamorphosed mineral develops mainly along the cleavages of the territorial cleavages faces(S1), bearing territorial dynamic metamorphism character and belonging to the lawsonite-albite-chlorite face of slightly metamorphism. 3.Territorial metamorphism P-T change focus on T:180～200oC, P:1～2kbar(<0.2Gpa).4.The age of the metamorphism is 175～159Ma; 5.It is the result of lithosphere gravity adjust and stretch function after Indo-chinese epoch impinging orogen in geotectonic events.