Fluid Histories During HP and UHP Metamorphism in Dabie Shan, China: Constraints from Trace Elements, Fluid Inclusions, and Stable Isotopes
by Yilin Xiao
Date of Examination:2001-01-23
Date of issue:2001-03-06
Advisor:Prof. Dr. Jochen Hoefs
Referee:Prof. Dr. Reiner Kree
Referee:Prof. Dr. Gerhard Wörner
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Description:Dissertation
Abstract
English
The Dabie-Sulu ultrahigh-pressure (UHP) metamorphic belt in East China is known to be the largest among recognized UHP terrains in the world. It represents deep parts of a collision zone between the North China and Yangtze cratons. Tectonically, the Dabie Shan could be subdivided into four metamorphic units: the North Huaiyuang metamorphic belt, the North Dabie Complex (NDC), the South Dabie UHP Terrain (SDT), and the Susong metamorphic belt. Recent geochronologic investigations revealed significantly different ages for the SDT and NDC. The main rocks of the NDC, characterized by ISr of 0.709-0.710 and eNd (T) of 15 to 20, were derived by partial melting of the lower-intermediate crust; by contrast, most gneisses from the SDT have higher eNd (T) values of 2 to 10. On the other hand, coesite-bearing eclogites are abundant in the SDT, whereas eclogitic rocks have not been undisputably identified in the NDC so far. The present study deals with the different metamorphic fluid evolution of the NDC and the SDT terrains. Major and trace elements analyses of both whole rock and minerals, microprobe data, oxygen isotope laser-probe measurements, hydrogen isotope data and detailed fluid inclusion studies on various minerals in key samples from both areas lead to the following conclusions: (1) Pressure and temperature calculations suggest different P-T paths for the rocks from the SDT and NDC: rocks in the SDT have metamorphic peak temperatures that are synchronous with the maximum pressure along a nearly isothermal decompression path during early uplift, whereas those from the NDC show the maximum pressure that is followed by a decompression path with substantial heating. On the other hand, the consistent trend that the rocks in the northern margin have been subjected to much higher P-T conditions than those in the south margin may indicate the subduction direction of the Yangtze craton. (2) The fluid history of metamorphic rocks from the SDT is different from that of the NDC. The fluid system of the UHP metamorphic rocks in the SDT evolved from highly concentrated Ca-rich brines (prograde metamorphism) towards NaCl-dominated solutions (peak metamorphism) and low salinity aqueous fluids during retrograde metamorphism. Although minor amounts of CO2 have been found in these fluids, CO2 appears to be not important during the metamorphic evolution of the rocks from the SDT. By contrast, the fluid phase of the rocks from the NDC was dominated by CO2-rich fluids, corresponding to granulite-facies. (3) Metamorphic rocks in the Dabie Shan area display a large range of d18O values, which probably resulted from variations of their protoliths and from heterogeneous fluid-rock interactions. The protoliths of the metamorphic rocks from the SDT have interacted with meteoric water on a regional scale prior to subduction, whereas the oxygen isotope compositions of the metamorphic rocks from the NDC show no obvious indications of such a water rock interaction. (4) Oxygen isotope mapping of garnet and fluid inclusion data indicate that the fluid phases in the rocks from both SDT and NDC, except for the cold eclogite from Lidu, were more or less closed during their peak metamorphism. Garnet from Lidu shows a systematic oxygen isotope zoning, which can be related to fluid infiltration during garnet growth, indicating that the eclogite was open to fluids during its peak metamorphism. From this point of view, the cold eclogite may have had a very different fluid evolution compared to the UHP eclogites in the SDT. (5) All investigated samples, except for the Lidu eclogite, have very low water activity during their peak metamorphism. The low water-activity during UHP metamorphism indicates that free-fluid phase is not ubiquitous; however, a free-fluid is present within a grain scale as evidenced from fluid inclusions.
Keywords: UHP metamorphism; Fluid inclusions; Stable isotopes; Trace elements; Dabie Shan