Metamorphic garnet pyroxenite from the 540–600 m main borehole of the Chinese Continental Scientific Drilling (CCSD) project

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• 作者：Lingsen Zeng ; Fenghua Liang ; Zhenyu Chen ; Fulai Liu ; Zhiqin Xu
• 关键词：Sulu– ; Dabie UHP belt ; Eclogite ; Garnet pyroxenite ; CCSD ; Nb/Ta– ; Zr/Hf systematics
• 刊名：Tectonophysics
• 出版年：2009
• 出版时间：25 September 2009
• 年：2009
• 卷：475
• 期：2
• 页码：396-412
• 全文大小：1306 K

文摘

Compared to other mafic and ultramafic rocks from the CCSD main borehole as well as from the outcrops, rocks from the 540–600 m section is extraordinary in terms of its geophysical as well as geochemical properties. It consists of > 70 % hematite–ilmenite garnet pyroxenite (HI-GPX) and < 30 % intercalated rutile garnet pyroxenite (R-GPX). Whole-rock geochemical data show that HI-GPXs have: (1) relatively high V with an average of 606 ppm, but lower Nb and Ta; (2) highest TFeO, Fe₂O₃/FeO ratio, and highly variable but strong positive Eu anomalies with Eu/Eu up to 2.9; (3) anomalously high V/Sc ratios ranging from 8.39 to 43.23, average 15.03; and (4) high amounts of hematite–ilmenite solid solutions with a very fine intergrowth structure down to nanometer scale. V/Sc ratios in the CCSD garnet pyroxenites are correlated negatively with MgO, but positively with Fe₂O₃/FeO ratios. Both suites of pyroxenites have similar rare earth elements and high field strength elements geochemistry. These features demonstrate that these pyroxenites were formed from metamorphism of high-Fe and/or -Ti gabbroic cumulates. This can account not only for low high field strength elements (HFSE) and rare earth elements (REE) but also low Nb/Ta and Zr/Hf ratios in these rocks. Seemingly negative correlation between Nb/Ta and Zr/Hf in the CCSD metabasites, not significantly affected by UHP metamorphism, is also consistent with the silicate differentiation trend in a basaltic magma chamber.