Geochemical significance of a garnet lherzolite from the Dahongshan kimberlite, Yangtze Craton, southern China
Geochemical significance of a garnet lherzolite from the Dahongshan kimberlite, Yangtze Craton, southern China
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DOI：
10.2343/geochemj.35.315
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发表时间：
2001-10
影响因子：
0.8
通讯作者：
Hong‐fu Zhang;M. Sun;F. Lu;Xin-hua Zhou;Mei‐Fu Zhou;Yongshun Liu;Guohui Zhang
中科院分区：
文献类型：
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作者：
Hong‐fu Zhang;M. Sun;F. Lu;Xin-hua Zhou;Mei‐Fu Zhou;Yongshun Liu;Guohui Zhang
湖北大洪山金伯利岩区位于元古宙扬子板块北方边缘。金伯利岩在古生代爆发（约。326 Ma），其中一些与贫瘠的钾镁煌斑岩有关。从该金伯利岩区采集到的异常新鲜的石榴石二辉橄榄岩为直接研究扬子板块下古生代岩石圈地幔的性质提供了机会。二辉橄榄岩的主要元素组成（2.66wt%Al2O3，2.82wt%CaO和90.9Mg#）中等亏损，橄榄石中镁橄榄石含量约为90.5%。全岩[（La/Sm）N = 13，（Eu/Yb）N = 0.48]和单斜辉石[（La/Yb）N = 61]的稀土元素特征表明，二辉橄榄岩至少经历了亏损-富集两个过程。亏损过程来自低程度的部分熔融，表现为相对较高的NiO含量（0.50%）的橄榄石和PPGE富集地幔标准化的PGE模式的全岩。微量元素和Sr-Nd同位素数据表明，金伯利岩的LREE和LILE富集是由于熔体渗透作用，而熔体渗透作用发生在金伯利岩喷发之前。Rb-Sr和Sm-Nd数据约束的年龄约为510 Ma，这是远大于金伯利岩喷发的年龄，但与含金刚石钾镁煌斑岩的喷发时间的中央的岩石。稳定的Gt-Opx-Cpx矿物组合给出的温压估计值为1042°C和34 kbar，对应于110 km深度，完全在石墨稳定性范围内。由此推断的二辉橄榄岩的地温远高于典型的大洋地温（40 mW/m2），但仍低于大洋地温。因此，我们的研究和以前的数据表明，扬子克拉通下的岩石圈地幔在古生代比典型的老克拉通下的岩石圈地幔，即，卡普瓦尔、西伯利亚和华北。
The Dahongshan kimberlite field in Hubei Province, China, is situated in the northern margin of the Proterozoic Yangtze Craton. The kimberlites erupted in the Paleozoic (ca. 326 Ma), some of which are associated with barren lamproites. An unusually fresh garnet lherzolite collected from this kimberlite field provides an opportunity to directly study the nature of the Paleozoic lithospheric mantle beneath the Yangtze Craton. The lherzolite is moderately depleted in major element compositions (2.66wt% Al2O3, 2.82wt% CaO, and 90.9 Mg#), with a forsterite content in olivine around 90.5%. The rare earth element characteristics of the whole rock [(La/Sm)N = 13 and (Eu/Yb)N = 0.48] and of the constituent minerals such as clinopyroxene [(La/Yb)N = 61] reveal that at least two processes were recorded in the lherzolite: depletion followed by enrichment. Depletion process was derived from low degree of partial melting, as demonstrated by relatively high NiO contents (0.50%) in olivines and PPGE-enriched mantle-normalized PGE pattern of the whole rock. The enrichments in LREE and LILE are attributed to melt infiltration that must be an event prior to the kimberlite eruption as constrained by trace element and Sr-Nd isotopic data. The Rb-Sr and Sm-Nd data constrained an age of about 510 Ma, which is much older than the age of kimberlite eruption, but consistent with the eruption time of diamond-bearing lamproites on the centre of the Craton. The stable Gt-Opx-Cpx mineral assemblage gives a T-P estimation of 1042°C and 34 kbar, corresponding to 110 km depth, well within the graphite stability field. The geotherm inferred from this lherzolite was much higher than the typical cratonic geotherm (40 mW/m2), but still lower than the oceanic geotherm. Thus, our study and previous data demonstrate that the lithospheric mantle beneath the Yangtze craton was less depleted and was hotter in Paleozoic than the lithospheric mantle beneath typical old Cratons, i.e., the Kaapvaal, the Siberia, and the North China.