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 mineral isochron and 40Ar/39Ar method suggests that the occurrence of the high-pressure metamorphism was during the early Triassic. LREE-enriched chondrite-normalized patterns for type-II ecologites and low initial epsilon(Nd) of -14 to -3 and the Sr-87/Sr-86-values (0.706-0.710) for various types of eclogites suggest that their protoliths were mainly derived from Precambrian island arc or intraplate basalts in the basement of the Yangtse Block and the enriched pyroxenite layer in alpine peridotite. The P-T-t path of eclogite from the southern Dabie Mountains suggests that the uplift history of eclogite in the Dabie Mountains can be subdivided into two stages: (1) fast uplift driven by thrust during continental-continental collision and deep subduction (at 221 Ma) 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-dipping subduction zone. This was followed by subduction of the continental crust of the Yangtse Block under the North China Block during the Triassic. These two continental blocks were welded into a single tectonic unit in the late Jurassic or early Cretaceous. Different cooling histories 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 processes from different levels.