COLLISION OF THE NORTH CHINA AND YANGTSE BLOCKS AND FORMATION OF COESITE-BEARING ECLOGITES - TIMING AND PROCESSES

Various types of eclogite, including coesite-bearing varieties formed under high-P-high-T conditions (up to 27 kbar, 700-800-degrees-C), and glaucophane schist occur in the Dabie Mountains and the Su-Lu terrane , eastern China. Dating of these high-pressure rocks by the Sm-Nd mine ral isochron and Ar-40/Ar-39 method suggests that the occurrence of th e high-pressure metamorphism was during the early Triassic. LREE-enric hed chondrite-normalized patterns for type-II ecologites and low initi al epsilon(Nd) of -14 to -3 and the Sr-87/Sr-86-values (0.706-0.710) f or various types of eclogites suggest that their protoliths were mainl y derived from Precambrian island arc or intraplate basalts in the bas ement of the Yangtse Block and the enriched pyroxenite layer in alpine peridotite. The P-T-t path of eclogite from the southern Dabie Mounta ins suggests that the uplift history of eclogite in the Dabie Mountain s can be subdivided into two stages: (1) fast uplift driven by thrust during continental-continental collision and deep subduction (at 221 M a) of the continental crust; (2) later gentle uplift with rise of the Dabie Mountains in the late Jurassic and Cretaceous (at 134 Ma). It is proposed that the collision between the North China Block and Yangtse Block began in the late Permian or early Triassic with a north-dippin g subduction zone. This was followed by subduction of the continental crust of the Yangtse Block under the North China Block during the Tria ssic. These two continental blocks were welded into a single tectonic unit in the late Jurassic or early Cretaceous. Different cooling histo ries for a coexisting gneiss and eclogite pair from Shima area suggest that the eclogite and their country rocks are not always coherent to each other. Some of them may have been juxtaposed through tectonic pro cesses from different levels.