DISCUSSION ON THE RELATIONSHIPS BETWEEN ECLOGITES AND THEIR ENCLOSING COUNTRY ROCKS IN THE ALTUN--NORTH QAIDAM UHPM BELT OF THE NORTHWESTERN CHINA: "TECTONIC EMPLACEMENT" OR "IN SITU METAMORPHISM" MODEL?

The Altun-North Qaidam area between the Qaidam block and Qilian blocks in northwest China has become the focus of recent attention because of the discovery of aboundant eclogites and the occurrence of UHP metamorphic evidences in both eclogites and their enclosing country rocks. The Altun metmarphic complex(Altun Group) and the North Qaidam metamorphic complex(Dakendaban group), previously regarded respectively as early Proterozoic metamorphic basement of the Tarim and Qaidam block, are mainly made up of felsic orthogneiss, paragneiss and amphibolite with minor eclogite, granulite,ultrabasic rock(garnet predotite) and marble, more recently, they have been considered as the results of Jingning period (0.9-1.0Ga) orogeny. Our geochemical and Sm-Nd isotopic data show the protoliths of the eclogites in the Altun-North Qaidam were derived from a long-term depleted mantle with an oceanic affinity, probably a "T" or"E" type MORB source, whereas their enclosed gneisses are similar to the gneisses of the Qilian block with Nd model age of 1.8-2.2Ga, indicating a continental affinity. Therefore, the geochemical and Sm-Nd isotopic data seem to favor tectonic emplacement relationship between eclogites and their country rocks. However, the field relationship indicating that there is not large scale tectonic displacement between eclogites and their country rocks, and petrographic data(e.g. coesite inclusions in country rocks) imply that eclogites and their country rocks experienced a common UHP metamorphism and exhumation ,which favor in situ relationship. The controversy cases can be explained as: the protoliths of the eclogites may derive from geotectonic setting with an oceanic basin rather than a large, mature, oceanic demain, probably a rifting setting. This implies that protolith of eclogite with tholeiitic composition (on the basic of zircon U-Pb dating. It is reasonable to assume that the fomation age of the protolith of eclogite is about 750-800Ma) emplaced into thinned Precambrian continental crust, and they experienced a common early Paleozoic HP-UHP metamorphism and following exhumation. Generally, although both model can not be reconciled completely with the observed phenomena, up to now, the "in situ" metamorphism model is seems more reasonable to explain the present field structure, petrological, geochemical and radiometric data.