Geochemistry of the Emeishan flood basalts at Yangliuping, Sichuan, SW China: implications for sulfide segregation
Geochemistry of the Emeishan flood basalts at Yangliuping, Sichuan, SW China: implications for sulfide segregation
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DOI：
10.1007/s00410-006-0094-3
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发表时间：
2006-06
影响因子：
3.5
通讯作者：
Xie‐Yan Song;Mei‐Fu Zhou;R. Keays;Z. Cao;M. Sun;L. Qi
中科院分区：
文献类型：
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作者：
Xie‐Yan Song;Mei‐Fu Zhou;R. Keays;Z. Cao;M. Sun;L. Qi
峨眉山玄武岩省北方杨柳坪地区发育一套厚达1,000 m的玄武岩序列，包括拉斑玄武岩的下、中单元和拉斑玄武岩和亚碱性玄武岩的上单元。玄武岩含SiO2 42- 55wt%，MgO 4.1- 8.3wt%。大部分岩浆的Gd/Yb > 2.0，Zr/Nb < 12，Nd（260 Ma）值为+2.5 ~+4.7。大多数玄武岩中铂族元素（PGE）亏损程度很低，含Pt 8-19 ppb，Pd 7-27 ppb。然而，一个显着比例的中间单元玄武岩强烈贫铂族元素与一些样品的浓度低于检测限。它们具有极高的Zr/Nb比值（高达14.5）和较低的Nd（260 Ma）值（+3.21 ~+0.65），具有广泛的下地壳混染特征。该单元的一些样品具有高Ni/Pd（3，965 - 61，198）和低Pd/Cr（410，000 - 3，930，000）比值，表明硫化物偏析和PGE在喷发前耗尽。原生岩浆是S-不饱和的，来自上地幔不同深度的部分熔融。早期和晚期阶段的岩浆，所代表的下部和上部的单元，经历AFC过程，诱导温和的S-饱和和PGE亏损的一些玄武岩，而由中间单元所代表的岩浆经历了更广泛的地壳污染，导致更强的S-饱和，在大多数情况下，显着的PGE亏损。
A well-developed, 1,000 m thick basaltic sequence in the Yangliuping region, northern part of the Emeishan basalt province, includes the Lower and Middle Units of tholeiitic basalts and an Upper Unit of both tholeiites and subalkalic basalts. The basalts contain 42–55 wt% SiO2and 4.1–8.3 wt% MgO. Most of these lavas have Gd/Yb > 2.0, Zr/Nb < 12, and ɛNd(260 Ma)values from +2.5 to +4.7. The platinum-group elements (PGE) are very mildly depleted in most of the basalts which contain 8–19 ppb Pt and 7–27 ppb Pd. However, a significant proportion of the Middle Unit basalts are strongly depleted in PGE with some samples having concentrations lower than detection limits. They have extremely high Zr/Nb ratios (up to 14.5) and low ɛNd(260 Ma)values (+3.21 to +0.65), features of extensive lower crustal contamination. Some samples in this unit have high Ni/Pd (3,965–61,198) and low Pd/Cr (410,000–3,930,000) ratios, indicating sulfide segregation and PGE depletion prior to eruption. The primary magmas were S-undersaturated and derived from partial melting at variable depths in the upper mantle. The early and late stage magmas, as represented by the Lower and Upper Units, underwent AFC processes which induced mild S-saturation and PGE depletion in some of the basalts, whereas the magmas represented by the Middle Unit experienced more extensive crustal contamination resulting in stronger S-saturation and in most cases significant PGE depletion.