Kyanite formation and element fractionation in the high-al eclogites from the sulu UHP metamorphic terrane
Journal
Terrestrial, Atmospheric and Oceanic Sciences
Journal Volume
21
Journal Issue
2
Pages
277-298
Date Issued
2010
Author(s)
Li, Chia-Tzu
Liu, Yung-Hsin
Yang, Huai-Jen
Zhang, Jianxin
Shau, Yen-Hong
Iizuka, Yoshiyuki
Yu, Shu-Cheng
Abstract
Eclogite com po si tions are crit i cal pa ram e ters for un der stand ing chem i cal evo lu tion in the Earth, par tic u larly in subduction zones. A group of eclogites from the Sulu ul tra-high pres sure (UHP) meta mor phic terrane in east ern China shows un com mon petrographic and compositional fea tures. They are char ac ter ized by (1) zoisite porphyroblasts co ex ist ing with in clu sion-free, in clu sion-con tain ing, and net work kyan ite, (2) high Al 2 O 3 of 18.4 -29.2% with high MgO of 8.59 -11.3%, and (3) in ten sive HFSE-REE fractionations rep re sented by [Zr/Sm] ch and [Ti/Gd] ch ra tios of 0.1 -3.9 and 1.1 -9.0, re spec tively. High-pres sure flu ids played a ma jor role on de vel op ing these fea tures. Kyan ite shows two tex tural va ri et ies. Ky I is in clu sion-free and in ap par ent tex tural equi lib rium with gar net and omphacite, im ply ing for ma tion from plagioclase break down. Ky II in cludes kyan ite net works and porphyroblasts; the for mer oc curs mostly in gar net, whereas the lat ter en closes gar net and omphacite grains. Ky II were crys tal lized at the ex pense of gar net and omphacite dur ing eclogite-fluid in ter ac tion. The low [HREE] ch ratios of 1 -2 in di cate that the protoliths were arc cumulates. The well-de fined in verse Al 2 O 3 -SiO 2 and Al 2 O 3 -CaO cor re la tions are not the char ac ter is tics of ig ne ous pre cur sors; there fore, must re flect meta mor phic mod i fi ca tions. The com par i son to mafic cumulates shows that the low-Al sam ples are compositionally better rep re sen ta tives of protoliths. Mass bal ance cal cu la tions point to an ol iv ine gabbronoritic protolith. The in ten sive HFSE-REE frac tion ations re flect compositional dif fer ences be tween two sam ple groups. Group I sam ples have superchondritic HFSE-REE ra tios [(Nb/La, Zr/Sm, Ti/Gd) ch = 2 -7] with de ple tions in LREE. In con trast, group II sam ples show HFSE de ple tions and LREE en rich ments. These compositional dif fer ences were ex plained as re flect ing el e ment mo bil ity dur ing eclogite-fluid in ter ac tion. Re leased at tem per a tures > 750C, the high-pres sure flu ids were en riched in HFSE, REE, and LILE. Upon mi gra tion, the flu ids first pre cip i tated HFSE into the group I sam ples, then, LILE and LREE into the group II sam ples. This model is sup ported by the oc cur rence of zoisite porphyroblasts in the group II sam ples, and in ter sti tial zir con and clus ters of small rutile grains along an nealed frac tures in the group I sam ples. Our in ter pre ta tion for the ob served compositional vari a tions im plies lim ited el e ment mo bil ity dur ing subduction and exhumation of continental lithosphere, consistent with existing models, which proposed that the chemical flux to metasomatized mantle wedge is mainly from subducted sediments.
Type
journal article