苏鲁芝麻坊超镁铁岩的岩石成因: 大陆科学钻探卫星孔(CCSD-PP1)岩心研究

杨经绥; 李天福; 张仲明; 陈松永; 张儒媛

苏鲁芝麻坊超镁铁岩的岩石成因: 大陆科学钻探卫星孔(CCSD-PP1)岩心研究

1.
中国地质科学院地质研究所大陆动力学实验室,北京 100037
2.
美国斯坦福大学地质与环境科学系,加利福尼亚州 94305

基金项目:

中国大陆科学钻探“973”项目 2003CB716500

详细信息

作者简介:
杨经绥(1950-), 男, 研究员, 岩石学专业.E-mail: yangjsui@ccsd.org.cn

中图分类号: P588.12
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出版历程
- 收稿日期: 2006-06-02
- 刊出日期: 2006-07-25

Petrogenesis of the Zhimafang Ultramafic Body in the Sulu Ultrahigh Pressure Metamorphic Belt: CCSD-PP1 Core Study

1.
Key Laboratory for Continental Dynamics, Chinese Academy of Geological Sciences, Beijing 100037, China
2.
Department of Geological and Environmental Sciences, Stan ford University CA94305, USA

摘要

摘要: 苏鲁超高压变质带(UHP)中的芝麻坊超基性岩体是一个不寻常的地幔岩块,由高度亏损的橄榄岩组成,因地幔交代作用而富集REE和LILE.岩体由交替出现的石榴石橄榄岩和不含石榴石橄榄岩层组成.2种类型的岩石具有许多共同的成分特征,并在相同的温压p-T条件下形成.岩石记录了数期地质事件,但没有进变质作用的证据.平衡矿物对计算出的p-T条件为6~7GPa.发生在中元古代的部分熔融的差异造成了互层状的2种岩石类型.橄榄岩中锆石220Ma的U—Pb同位素年龄记录了扬子板块和北中国板块碰撞,但此年龄也许并不代表岩石UHP变质峰期的年龄,而可能记录了俯冲带中地幔楔的碎块从深部折返时的年龄.我们认为石榴石可能不是由于俯冲变质作用形成,有可能是岩石从地幔深部上升过程中从斜方辉石中出溶而成。
- 石榴石橄榄岩 /
- 苏鲁 /
- CCSD /
- 超高压变质作用 /
- 地幔
Abstract: The Zhimafang ultramafie body in the Sulu ultrahigh pressure (UHP) metamorphic belt is a block of unusual mantle composed of highly depleted peridotite,which has been enriched in REE and LILE by metasomatism. It consists of interlayered garnet peridotite and garnet-free peridotite,which have many common compositional features and formed under the same p-T conditions. The rocks record several geological events; however,evidence of progressive metamorphism has not been confirmed. Calculated p-T conditions from equilibrium mineral pairs suggest pressures of 6-7 GPa. Partial melting of the sequence in the Mesoproterozic was probably responsible for the interlayering of the two rock types. AU-Pb zircon age of 220 Ma records the collision between Yangtze and North China blocks,but this age may not represent the time of peak UHP metamorphism. More likely,it records exhumation of the block from the deep portion of the subduction zone. The available data suggest that the garnet was exsolved from enstatite as rocks rose from the deep mantle.
- garnet peridotite /
- Sulu /
- CCSD /
- UHP metamorphism /
- mantle

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图 1 南苏鲁地区简化地质图(a); 芝麻坊超镁铁岩体和中国大陆科学钻探CCSD-PP1钻孔位置(b); PP1钻孔主要岩性单元的简化柱状图(c); 石榴石橄榄岩的岩心照片(d)

Fig. 1. A simplified geological map of the southern Sulu region(a); profile of the Zhimafang ultramafic body and location of the PP1 drillhole of the Chinese Continental Scientific Drilling(CCSD)(b); a simplified column of the main li- thological units of the PP1(c); and a photograph of the garnet peridotite core(d)

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图 2 PP1钻孔超镁铁岩石柱状图(a); 成分剖面中SiO₂、Al₂O₃、CaO和K₂O含量随岩石类型的变化(b—e)

Fig. 2. Column of the ultramafic section in the PP1 drill hole(a); SiO₂, Al₂O₃, CaO and K₂O content variation with rock types in compositional profiles(b-e)

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图 3 石榴石橄榄岩和不含石榴石橄榄岩的显微照片

a.石榴石方辉橄榄岩(C21-134-55):等粒结构, 石榴石在空间上与Opx伴生, 甚至很小的颗粒(见箭头); b.石榴石方辉橄榄岩(C34-151- 74): 石榴石变斑晶中橄榄石包体; c.石榴石方辉橄榄岩(C27-144-63): 小颗粒的Grt、Opx和Ol作为粗粒Ol之间的空隙充填物; d.石榴石方辉橄榄岩(C25-141-60): 空间伴生的Grt、Opx和铬铁矿; e.纯橄岩(C31-147-68): 无形变的等粒结构; f.方辉橄榄岩(C37-154-78): 粗粒Ol和Opx的嵌晶结构和弱定向; g.纯橄岩(C36-153-77): 粗粒Ol间充填的小Ol; h.石榴石金云母脉(C28-145-65): 变斑晶石榴石和金云母片是岩石中仅有的矿物, 石榴石有蚀变边

Fig. 3. Microphotographs of garnet peridotite and garnet-free peridotite

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图 4 K₂O、Al₂O₃、MgO、NiO、FeO、Cr₂O₃、SiO₂与MgO+< FeO>的变异图(a—h), 石榴石橄榄岩较不含石榴石橄榄岩有相对高的Al₂O₃、CaO、SiO₂和低的MgO以及含量相同的Cr₂O₃和NiO; 2种岩石均显示球粒状陨石标准化LREE富集模式, 但石榴石橄榄岩的REE丰度要高于不含石榴石橄榄岩(i, j)

Fig. 4. K₂O, Al₂O₃, MgO, NiO, FeO, Cr₂O₃, SiO₂vs. MgO+< FeO> diagrams(a-h)clearly separate Grt-peridotite from GF-peridotite by higher Al₂O₃, CaO and SiO₂, and lower MgO, and similar contents of Cr₂O₃and NiO; both show chondrite-normalized LREE enriched patterns, but the REE abundance of Grt-peridotite are higher than that of GF-peridotite(i, j)

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图 5 石榴石橄榄岩中的橄榄石比不含石榴石橄榄岩显示较宽区域的Fo和较高的SiO₂含量

Fig. 5. Olivine in Grt-peridotite shows wider range of Fo and higher SiO₂contents than GF-peridotite

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图 6 石榴石橄榄岩和不含石榴石橄榄岩中透辉石和顽火辉石显示相同的宽度大的成分区间; Na₂O、Al₂O₃和Cr₂O₃呈正相关, 但与MgO和CaO呈负相关

Fig. 6. Diopsite and enstatite in Grt-and GF-peridotite show a similar and wide range in composition; Na₂O shows a posi- tive relation to Al₂O₃, Cr₂O₃and a negative relation to MgO and CaO

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图 7 铬铁矿在2种橄榄岩中均显示高100 Cr/(Cr+Al), 石榴石橄榄岩比不含石榴石橄榄岩有相对高的100 Mg/(Mg+Fe²⁺), 但两者都落在已知构造区外

Fig. 7. Chromite in both Grt- and GF-peridotite shows high 100 Cr/(Cr+Al), the former has relatively high 100 Mg/ (Mg+Fe²⁺)than the latter, and both are plotted away from tectonically known region

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图 8 石榴石橄榄岩中变斑晶石榴石(Lg-Grt)和等粒石榴石(Sm-Grt)的组分是变化的, Sm-Grt比Lg-Grt有相对高的Cr₂O₃和相对低的Al₂O₃、MgO、SiO₂含量

Fig. 8. Compositions of porphyroblastic garnet(Lg-Grt) and equigranular garnet(Sm-Grt)in Grt-peridotite are varied, the Sm-Grt have relatively higher contents of Cr₂O₃and lower contents of Al₂O₃, MgO and SiO₂than Lg-Grt

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图 9 芝麻坊超镁铁岩体中石榴石橄榄岩和不含石榴石橄榄岩的p-T条件计算

左上角字母是参考文献中作者; 数字分别是获得的温度(℃)和压力(105 kPa); 详细计算结果参见表 6

Fig. 9. Calculated p-T condition for Grt-and GF-peridotite from the Zhimafang ultramafic body

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图 10 芝麻坊橄榄岩全岩MgO-FeO关系(Boyd et al., 2004)

熔融曲线表示在不同压力下被熔融成分的百分比(10%的间隔), 以GPa为单位(从1~7 GPa; Walter, 1998)

Fig. 10. Bulk rock MgO-FeO relations for the Zhimafang peridotite

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表 1 CCSD-PP1钻孔岩心的岩性单元

Table 1. Lithological units in the PP1 drill hole

表 2 芝麻坊橄榄岩中橄榄石的代表性化学成分

Table 2. Representatives of olivine in the Zhimafang peridotite

表 3 芝麻坊超镁铁岩体中石榴石橄榄岩和不含石榴石橄榄岩中代表性的辉石化学成分

Table 3. Representatives of pyroxenes in Grt- and GF peridotite from the Zhimafang ultramafic body

表 4 芝麻坊石榴石橄榄岩中代表性石榴石的化学成分

Table 4. Representatives of garnet composition from the garnet peridotite in Zhimafang

表 5 芝麻坊橄榄岩中代表性铬铁矿成分分析

Table 5. Chromite compositions in the Zhimafang peridotite

表 6 CCSD-PP1石榴石橄榄岩的变质温压条件计算

Table 6. Metamorphic p-T conditions of PP1 garnet peridotites

表 7 计算得出的理论上斜方辉石的成分

Table 7. Weight percent analyses of theoretical orthopyroxenes

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