Geochemical and Nd–Hf isotopic constraints on the origin of the ~ 1.74-Ga Damiao anorthosite complex, North China Craton
Geochemical and Nd–Hf isotopic constraints on the origin of the ~ 1.74-Ga Damiao anorthosite complex, North China Craton
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DOI：
10.1016/j.lithos.2009.07.002
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发表时间：
2009-12
期刊：
影响因子：
3.5
通讯作者：
T. Zhao;W. Chen;Mei‐Fu Zhou
中科院分区：
文献类型：
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作者：
T. Zhao;W. Chen;Mei‐Fu Zhou
大庙杂岩体由斜长岩（85%）、苏长岩（10%）、镁橄榄石（4%）和少量的长长英岩（<1%）组成，均被辉长岩脉和铁闪长岩脉切割。该杂岩体含丰富的Fe-Ti-P氧化物矿石，侵位序列为斜长岩、苏长岩→ Fe-Ti-P氧化物矿石→锰铁矿。杂岩中所有不同的岩性都具有相似的轻稀土元素（LREE）富集模式和Nd同位素组成，εNd（t）值大多在−5.0至−4.0之间。锆石的平均εHf（t）值为镁橄榄石的−4.7和苏长岩的−5.9。斜长石和单斜辉石显示出从斜长岩和苏长岩到Fe-Ti-P氧化物矿石再到锰铁矿的连续组合范围，表明这些岩性是由共同的母岩浆分异形成的。大庙杂岩中辉长岩脉以高Al 2 O 3（14.5- 17.1wt.%）为特征，Sr（~1000 ppm）和Mg#s [100 Mg/（Mg+ Fe 2+）]=56-73），类似于北美Harp Lake和拉勒米斜长岩杂岩的高铝辉长岩母体。大庙杂岩的高铝辉长岩脉可能反映了杂岩形成的母熔体，而铁闪长岩脉可能代表高铝辉长岩岩浆结晶后的残余熔体。高铝辉长岩具有不相容元素比值（Zr/Nb=14-24; La/Nb=3.6-4.8），与附近的同生、幔源、基性岩脉群和火山岩明显不同，而与下地壳相似。它们的锆石εHf（t）和全岩εNd（t）组成沿着华北太古代~ 2.5Ga岩石的演化线，表明大庙杂岩起源于古老的下地壳。要形成下地壳的高铝辉长岩岩浆，需要异常高的温度和高度熔融（>75%）。我们认为，地壳岩石在华北板块合并过程中被拖入上地幔，当时的环境温度高（12 kbar时>1271 °C），足以引起大规模熔融，形成一个深源岩浆房。熔体经过多压结晶作用，形成斜长岩岩浆，并在中地壳深度注入形成大庙杂岩。
The ~1.74-Ga Damiao complex in the North China Craton consists of anorthosite (85%), norite (10%), mangerite (4%) and minor troctolite (<1%), all of which are cut by gabbroic and ferrodioritic dikes. The complex hosts abundant Fe–Ti–P oxide ores and has an emplacement sequence from anorthosite and norite to Fe–Ti–P oxide ores to mangerite. All of the different lithologies in the complex have similar light rare earth element (LREE)-enriched patterns and Nd isotopic compositions with εNd(t) values mostly within the range of −5.0 to −4.0. Average εHf(t) values of zircon are −4.7 for mangerite and −5.9 for norite. Plagioclase and clinopyroxene show a continuous range of composition from anorthosite and norite to Fe–Ti–P oxide ores to mangerite, suggesting that these lithologies formed by differentiation from a common parental magma. Gabbroic dikes in the Damiao complex are characterized by high Al2O3(14.5–17.1 wt.%), Sr (~1000 ppm) and Mg#s [100 Mg/(Mg+Fe2+)]=56–73), similar to the high-Al gabbros parental to the Harp Lake and Laramie anorthosite complexes in North America. The high-Al gabbroic dikes of the Damiao complex may reflect the parental melt from which the complex formed, whereas the ferrodioritic dikes may represent residual melts after anorthosite crystallization from the high-Al gabbroic magma. The high-Al gabbros have incompatible element ratios (Zr/Nb=14–24; La/Nb=3.6–4.8), which differ significantly from those of nearby coeval, mantle-derived, mafic dike swarms and volcanic rocks but are similar to those of the lower crust. Their zircon εHf(t) and whole-rock εNd(t) compositions plot along the evolution lines of the ~2.5 Ga Archean rocks in the North China Craton, suggesting that the Damiao complex was derived from an ancient lower crust. Exceptionally high temperatures and high-degrees of melting (>75%) would have been required to form the parental high-Al gabbroic magma from the lower crust. We propose that the crustal rocks were dragged into the upper mantle during amalgamation of the North China Craton, where the ambient temperature was high enough (>1271 °C at 12 kbar) to cause extensive melting, producing a deep-seated magma chamber. The melt then evolved through polybaric crystallization, producing anorthositic magmas with abundant suspended plagioclase that were finally injected at mid-crustal depth to form the Damiao complex.