Geochemistry of peridotite xenoliths in Early Cretaceous high-Mg# diorites from the Central Orogenic Block of the North China Craton: The nature of Mesozoic lithospheric mantle and constraints on lithospheric thinning
Geochemistry of peridotite xenoliths in Early Cretaceous high-Mg# diorites from the Central Orogenic Block of the North China Craton: The nature of Mesozoic lithospheric mantle and constraints on lithospheric thinning
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DOI：
10.1016/j.chemgeo.2009.12.006
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发表时间：
2010-02
期刊：
影响因子：
3.9
通讯作者：
Wenliang Xu;De‐Bin Yang;Shan Gao;F. Pei;Yang Yu
中科院分区：
文献类型：
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作者：
Wenliang Xu;De‐Bin Yang;Shan Gao;F. Pei;Yang Yu
华北克拉通是太古代克拉通失去其厚厚的岩石圈龙骨的最好例子。尽管太古代龙骨的移除通常被认为发生在东部地块的中新生代和中央造山地块的古元古代，但移除的确切时间和面积范围存在争议，部分原因是缺乏对性质的了解。关于中新生代岩石圈地幔。本文报道了华北造山带福山早白垩世高镁闪长岩中稀有橄榄岩捕虏体的矿物学和地球化学资料。这些捕虏体提供了深入了解岩石圈地幔的性质下中央NCC在中生代。橄榄岩捕虏体以尖晶石方辉橄榄岩和贫单斜辉石的尖晶石二辉橄榄岩为主，少量含铬纯橄榄岩。方辉橄榄岩和贫Cpx二辉橄榄岩的镁橄榄石平均含量为92.3，贫CaO（0.59-1.06wt.%）和0.15- 1.47wt%的Al 2 O 3。这些特征与太古宙岩石圈地幔的特征相似，表明早白垩世华北克拉通中央造山地块下存在太古宙岩石圈地幔。纯橄榄岩占捕虏体总量的0.5%，其特征是相对较低的Mg#（90.1）和存在浸染状铬铁矿（Cr#=73-85）。纯橄榄岩中的橄榄石具有较高的Ca（320- 770 ppm）和Ti（18- 29 ppm）浓度以及低Ni丰度（2000- 2690 ppm）与方辉橄榄岩和贫Cpx二辉橄榄岩相比（Ca=40-80ppm; Ti=0.23-8.1ppm; Ni=2970- 3440 ppm），表明纯橄榄岩是太古代岩石圈与硅质熔体相互作用的产物。方辉橄榄岩中次生金云母和角闪石的存在，贫Cpx的二辉橄榄岩以及片理化纯橄榄岩中脉状斜方辉石的存在与轻稀土元素富集相耦合。这些交代矿物与低Ca/Al（5-18）、LaN/YbN（0.41-2.99）和高Ti/Eu（526-1474）的单斜辉石一起，表明太古代岩石圈地幔曾被含挥发分的硅酸盐熔体覆盖。方辉橄榄岩和贫Cpx二辉橄榄岩捕虏体的Rb-Sr等时线年龄（111± 23 Ma）表明，交代叠加作用发生在早白垩世，与寄主岩浆侵位时间和华北克拉通大规模岩浆活动时间相吻合。这一剖面的中生代岩石圈地幔与NCC东部的同期岩石圈地幔明显不同，表明NCC岩石圈的破坏仅限于东部，而太古代地幔则保存在NCC中部。
The North China Craton is the best example of an Archean craton that has lost its thick lithospheric keel. Although removal of the Archean keel is generally considered to have occurred in the Meosozoic–Cenozoic in the Eastern Block and Paleoproterozoic in the Central Orogenic Block, the exact timing and areal extent of the removal is debated, partly due to lack of knowledge about the nature of the Meosozoic lithospheric mantle. Here we report mineralogical and geochemical data on rare peridotite xenoliths from the Early Cretaceous high-Mg# diorites from Fushan in the Trans-North China Orogen (TNCO) of the North China Craton (NCC). These xenoliths provide insights into the nature of the lithospheric mantle underlying the central NCC during the Mesozoic. The peridotite xenoliths are dominated by spinel harzburgite and clinopyroxene (Cpx)-poor spinel lherzolite, with minor chromite-bearing dunite. The harzburgite and Cpx-poor lherzolite have average forsterite contents of 92.3 and are depleted in CaO (0.59–1.06wt.%) and Al2O3(0.15–1.47wt.%). These features are similar to those of Archean cratonic lithospheric mantle, suggesting the presence of Archean cratonic mantle beneath the Central Orogenic Block of the NCC in the Early Cretaceous. Dunites make up ∼5% of the xenolith population and are characterized by relatively low Mg# (90.1) and the presence of disseminated chromite (Cr#=73–85). The olivines in the dunites have high Ca (320–770ppm) and Ti (18–29ppm) concentrations as well as low Ni abundances (2000–2690ppm) compared to those from harzburgite and Cpx-poor lherzolite (Ca=40–80ppm; Ti=0.23–8.1ppm; Ni=2970–3440ppm), suggesting that the dunites were produced through an interaction between the Archean lithosphere and siliceous melts. The presence of secondary phlogopite and amphibole in the harzburgite and Cpx-poor lherzolites as well as veined orthopyroxene in the foliated dunite is coupled with light rare earth element enrichments. Together with low Ca/Al (5–18) and LaN/YbNratios (0.41–2.99) and high Ti/Eu ratios (526–1474) of clinopyroxenes, these metasomatic minerals indicate that the Archean lithospheric mantle had been overprinted by a volatile-bearing silicate melt. The Rb–Sr isochron age (111±23Ma) of the harzburgite and Cpx-poor lherzolite xenoliths shows that the metasomatic overprinting occurred in the Early Cretaceous, in agreement near the time of the emplacement of the host magma and coeval voluminous igneous activities in the NCC. This section of Mesozoic lithospheric mantle is markedly different from the contemporaneous lithospheric mantle in the eastern NCC, implying that the destruction of the NCC lithosphere was confined to the eastern NCC and that the Archean mantle was preserved in the central NCC.