Geochemistry and magmatic history of eclogites and ultramafic rocks from the Chinese continental scientific drill hole: Subduction and ultrahigh-pressure metamorphism of lower crustal cumulates |
| |
| Authors: | Yongsheng Liu Keqing Zong Peter B Kelemen Shan Gao |
| |
| Institution: | 1. Dept. Applied Geology, Curtin University, GPO Box U1987, Bentley, WA 6845, Australia;2. School of Earth, Atmosphere and Environment, Monash University, Melbourne, Victoria 3800, Australia;3. University of Wyoming, Laramie, USA;1. Department of Earth and Environmental Sciences, University of Ottawa, Ottawa, ON K1N 6N5, Canada;2. Natural Resources Canada, Geological Survey of Canada, Ottawa, ON K1A 0E4, Canada;3. Department of Earth Sciences, University of New Brunswick, Fredericton, NB E3B 5A3, Canada;1. State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China;2. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi''an 710069, China;3. Department of Earth Sciences, Faculty of Earth and Life Sciences, VU University Amsterdam, de Boelelaan 1085, NL-1081HV Amsterdam, The Netherlands |
| |
| Abstract: | Three distinct groups of eclogites (low-Mg–Ti eclogites, high-Ti eclogites and Mg-rich eclogites) and ultramafic rocks from the depth interval of 100–680 m of the Chinese Continental Scientific Drill Hole were studied. The low Mg#s (= 100?molar Mg/(Mg + Fe)) (81–84%) and low Ni (1150–1220 ppm) and high Fe2O3total (13–15 wt.%) contents of ultramafic rocks suggest a cumulate origin. Mg-rich eclogites show middle and heavy REE enrichments, which could not be produced by metamorphic growth of garnet. Instead, if the rocks formed from a light REE enriched magma, there may be an igneous precursor for some garnets in their protolith. Alternatively, perhaps they formed from a light REE depleted magma without garnet. The high-Ti eclogites are characterized by unusually high Fe2O3total contents (up to 24.5 wt.%) and decoupling of high TiO2 from low Nb and Ta contents. These features cannot be produced by concentration of rutile during UHP metamorphism (even for samples with TiO2 > 4 wt.%) of high-Ti basalts, but could be attributed to crystal fractionation of titanomagnetite (for those with TiO2 < ～ 4 wt.%) or titanomagnetite + ilmenite (for those with TiO2 > ～ 4 wt.%). Thus, we suggest that protoliths of the high-Ti eclogites were titanomagnetite/ilmenite-rich gabbroic cumulates. As a whole, the low-Mg–Ti eclogites are geochemically complementary to the high-Ti eclogites, Mg-rich eclogites and ultramafic rocks, and could be metamorphic products of gabbroic/dioritic cumulates formed by high degree crystal fractionation. All these observations suggest that parental materials of the ultramafic rock-eclogite assemblage could represent a complete sequence of fractional crystallization of tholeiitic or picritic magmas at intermediate to high pressure, which were later carried to ultrahigh-pressure conditions during a continental collision event. |
| |
| Keywords: | |
| 本文献已被 ScienceDirect 等数据库收录！ |
|
