HR: 1340h
AN: T23D-1637    [Abstracts]
TI: Locating the boundary of the North and South Lhasa Block: Eclogites within the Gandese belt, Southern Tibet
AU: * Zeng, L
EM: changting1970@yahoo.com
AF: Institute of Geology, Chinese Academy of Geological Sciences, Baiwanzhuang St. 26, Beijing, 100037, China
AU: Liu, J
EM: earofwood@yahoo.com
AF: Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Shuangqing Rd. 18, Beijing, 100085, China
AU: Xie, K
AF: Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Shuangqing Rd. 18, Beijing, 100085, China
AB: A nearly east-west trending eclogite belt, several hundred meters wide and tens of miles long, occur from Sumdo to Jiaxing, East of Lhasa. Field, petrography, and geochemical observations show that: (1) though affected by the late stage of fluid/magma infiltration, most of eclogites within this belt have preserved N-MORB type trace element geochemistry; (2) thin layers of eclogite are sandwiched between quartzite layers, which mimic the sheet-dike structure of a typical ophiolite; (3) fresh eclogites consist of garnet (Gt), omphacite (Omph), epidote (Epi), amphibole (Am), Phengite (Phen), rutile (Rt), and minor apatite (Ap), zircon (Zr), and iron oxides; (4) highly retrograde eclogites consist of garnet, amphibole, plagioclase (Pl), paragonite (Para), quartz (Q), rutile, and minor apatite and zircon. Relict omphacites only can be seen in the core of large hornblende or as inclusions within garnet. Two types of garnet occur in these eclogites: garnets with a lot of inclusions do not show any chemical zoning, whereas garnets free of inclusions show distinctive zoning. Such zonings are characterized by sharp increase in Fe, decrease in Mg, but nearly constant Ca from core to rime, which suggest a nearly isobaric heating during their exhumation. Therefore, emplacement of the Mesozoic Gandese granitic batholith might have been responsible for the latest exhumation of the Sumdo eclogite. Omphacites in the matrix also show distinct zoning with Na₂O decrease from 7.0 to 4.0 wt% toward their rims. However, omphacite cores have similar compositions to those of tiny garnet-hosted inclusions. Preliminary data indicate that metamorphic reactions in these eclogites are represented by prograde Gt-1 + Amph + Epi + Q + Rt + Ap + Zr, Gt-1 + Amph + Omph-1 + Epi + Rt + Ap + Zr, and peak Gt + Omph-1 + Phen + Rt + Ap + Zr, and retrograde Gt-1 + Omph-2 + Amph + Epi + Phen + Ap + Zr, and Gt-2 + Amph + Epi + Para. The nature of the Mesozoic-Early Cenozoic granotoids within the Lhasa Block show substantial N-S variations, suggesting a fundamental difference in the lithospheric structure across the Lhasa Block. This eclogite belt may represent such a plate boundary of the North and South Lhasa Block.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 3625 Petrography, microstructures, and textures
DE: 3660 Metamorphic petrology
DE: 8100 TECTONOPHYSICS
DE: 8150 Plate boundary: general (3040)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting