Rb-Sr and U-Pb Dating of the Daguzhai and Luobuli Granitic Massifs in South China

Rb-Sr and U-Pb isotopic studies of the two contrasting granite types of the Dauzhai and Luobuli massifs in South China provide new constraints on the interpretation of isotopic age data for plutonic igneous rocks.A Rb-Sr internal isochron age of 146±7Ma for the Luobuli adamelligte is interpreted to represent the age of magma crystallization,whereas the whold-rock Rb-Sr isochron yields an older apparent age of 161±10Ma which is regarded as resulting from contamination processes affecting the petrogenesis of 161±10Ma which is regarded as resulting from contamination processes affecting the petrogenesis of this adamellite.In the Daguzhai granite the marked scatter of whole-rock Rb-Sr data in isochron diagram is ascribed to the open sstem behavior of Rb during postmagmatic autometasomatism.Uniformity of intital ^87Sr/^86Sr ratio in this granite is indicated in a plot of ^87 Sr versus ^86Sr.The autometasomatism has also affected zircon U-Pb system ,resulting in a spread of data along the Concordia.curve between 165 and 125Ma.This spread is regarded as indicating the duration the autometasomatism.

Zircon U-Pb ages for Wulian granites in northwest Sulu and their tectonic implications

The Wulian complex occurring in the north- western part of the Sulu orogen consists of metaigneous and metasedimentary rocks at greenschist-facies. Zircon U-Pb dating for three granite intrusions within it yields concor-dant ages of (672±4), (742±9) and (747±14) Ma, respec-tively. These ages indicate that the granites are the Neopro-terozoic intrusions in the northern margin of the Yangtze plate, and correspond to the magmatic complex at Luzhen-guan in the eastern part of the Beihuaiyang belt in the Dabie orogen. They were tectonic slices scraped off from the Yang-tze plate during Triassic subduction and thus belong to a part of the accretionary wedge of the Yangtze plate subduc-tion. The discovery of extensive Neoproterozoic magmatic rocks in the Wulian area suggests that the suture location between the North China and the Yangtze plates lies north of the Wulian complex.

Fluid activity during exhumation of deep-subducted continental plate

It is well known that a great deal of fluid wasreleased during subduction of oceanic crust, resulting in arcmagmatism, quartz veining and metamorphic mineralizationof syn-subduction. In contrast, the process of continentalsubduction is characterized by the relative lack of fluid andthus no arc magmatism has been found so far. During exhu-mation of deep-subducted continental crust, nevertheless,significant amounts of aqueous fluid became available fromthe decomposition of hydrous minerals, the decrepitation ofprimary fluid inclusions, and the exsolution of structuralhydroxyls. This kind of metamorphic fluid has recently at-tracted widespread interests and thus been one of the mostimportant targets in deciphering the geological processesconcerning metamorphism, magmatism and mineralizationin collisional orogens. A large number of studies involvingstable isotopes, fluid inclusions and petrological phase rela-tionships have been accomplished in past a few years withrespect to the mobility and amount of metamorphic fluid inUHP metamorphic rocks from the Dabie-Sulu orogenic belt.The results demonstrate that the fluid activity during theexhumation of deep-subducted continental crust has the fol-lowing effects: (1) amphibolite-facies retrogression due topervasive fluid flow; (2) formation of HP quartz veins withineclogites due to channelized fluid flow; and (3) partial melt-ing of overlying crustal rocks due to focused fluid flow, pro-ducing syn-exhumation magmatism within the orogenic belt.In particular, the aqueous fluid released by decompressionexsolution of hydroxyl from UHP minerals is characterizedby low salinity and is capable of resulting in pervasive andchannellized flow. Therefore, the intensive study of fluid ac- tivity during exhumation of UHP metamorphic rocks can notonly provide insight into geodynamic processes that occurred in continental collisional belts, but also shed light on under- standing of crust-mantle recycling and relevant magmatism in subduction zone setting.