Lithium isotope fractionation during incongruent melting: Constraints from post-collisional leucogranite and residual enclaves from Bengbu Uplift, China
Lithium isotope fractionation during incongruent melting: Constraints from post-collisional leucogranite and residual enclaves from Bengbu Uplift, China
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不同成分熔融过程中的锂同位素分馏：来自中国蚌埠隆起碰撞后淡色花岗岩和残余包体的约束
DOI：
10.1016/j.chemgeo.2016.06.004
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发表时间：
2016-11
期刊：
影响因子：
3.9
通讯作者：
Casey, John F.
中科院分区：
文献类型：
--
作者：
Gao, Yongjun;Xiao, Yilin;Gu, Hai-ou;Casey, John F.
Lithium (Li) elemental and isotopic compositions of the Jurassic Jingshan leucogranites, including garnet-rich mafic enclaves and wall rock Wuhe gneisses from the southeast margin of North China Craton (NCC) were investigated to understand the behavior of Li isotopes during post-collisional magmatism. The Jingshan leucogranites have distinct U-shape REE patterns with Y and REE concentrations significantly lower yet Sr/Y ratios higher than their presumed source rocks, i.e., the Dabie-Sulu gneisses. Trace element modeling of REE and Sr/Y suggests these elemental signatures of the Jingshan leucogranites can be consistently explained by a fluid-present crustal incongruent partial melting: Bt + Qz + Pl + H2O = Grt + melt, leaving mainly Grt + Bt with minor allanite in the residuum. The mafic enclaves show identical Sr-Nd isotopic compositions with their host leucogranites, contrasting with the Wuhe gneiss and the exposed regional lower crust. The garnet-rich mafic enclaves are thus interpreted as entrained residual phases formed by this incongruent partial melting.The Jingshan leucogranites show relatively high δ7Li values (+ 4.0‰ to + 9.0‰) and low Li concentrations (4.7–11.3 ppm) in comparison to published data for worldwide granites. In contrast, the residual enclaves show low δ7Li values (as low as + 0.6‰) and high Li concentrations (as high as 118 ppm). Garnet separated from residual enclaves is characterized by a narrow range of low δ7Li values (− 1.5‰ to − 0.1‰) with high Li concentrations from 32.9 to 81.7 ppm. By contrast, coexisting quartz shows relatively high δ7Li values (+ 15.0‰ to + 16.6‰) with very low Li concentrations (~ 1 ppm). Biotite from both leucogranite and residual enclaves shows high Li concentrations (195–382 ppm) and relatively heavy Li isotope compositions (+ 3.2‰ to + 7.5‰). The Li elemental and isotopic signatures of the residual enclaves can be modeled as a Grt-Bt rich residuum mixed with leucogranite melt in various proportions. This work indicates that the Li isotopic compositions for magmatic rocks that are derived from anatexis of mid to lower crustal gneisses may not be a faithful source indicator as commonly suggested.
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影响因子：
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