苏鲁造山带加厚地壳的时空分布，熔融和去山根过程的地球化学研究

Although the Sulu orogen and Dabie orogen belong to the same Triassic collisional orogen between the North China and South China Blocks, their post-collisional magmatisms have two major differeces: (1) The low Mg# adaketic rocks formed by partial melting of thickened crust in Sulu orogen were developed in Late-Jurassic, but those in Dabie orogen were developed in Early Cretaceous; (2) There is a silent period of 141-131 Ma for magmatism following partial melting of thickened crust in Sulu orogen, but there is not in Dabie orogen. The reasons for these differences remain unknown. In order to reveal these reasons, this project plans to study on the post-collisiona igneous rocks in the Sulu orogen includs in the following 3 respects: (1) To reveal the temporal and spatial distribution of low-Mg# adakitic rocks in Sulu orogen and their protolith tectonic affinities by a systematic geochronological and geochemical work, which may provide constraints on crustal structure of the Sulu orogen and its influnce on magmatism; （2）Geochronological and geochemical studies on the late Jurassic migmatite in Sulu orogen to reveal the relationship with the coeval granite intrusions in origin and their thermal source; (3) Mg isotope geochemical studies of the Cretaceous high Mg# adakitic rocks and mafic intrusions in Sulu orogen to reveal mantle-crust interaction during mountain-root removal process and whether this pocess was effected by Pacific oceanic subduction.

苏鲁造山带和大别造山带同为华北和华南陆块的三叠纪碰撞造山带，但它们的碰撞后岩浆作用有两个重要差异：（1）苏鲁造山带加厚地壳熔融产生的低镁埃达克质岩发育在晚侏罗世，而大别山发育在早白垩世；（2）苏鲁造山带在加厚地壳熔融之后，在141-131Ma期间存在一岩浆作用间歇期，而大别造山带没有。产生这些差异的原因尚属未知。为了查明造成这些差异的原因，本项目将对苏鲁造山带碰撞后岩浆岩做如下三方面研究：（1）通过综合的年代学和地球化学研究查明苏鲁造山带低Mg#埃达克质岩时空分布特征，及源区地壳构造归属，为苏鲁造山带地壳结构及对岩浆作用的影响提供制约；（2）苏鲁造山带晚侏罗世混合岩的年代学和地球化学研究，查明混合岩与同时代花岗岩侵入体是否存在成因联系及它们的热源；（3）苏鲁造山带白垩纪高镁埃达克质岩和基性侵入岩的镁同位素地球化学研究，揭示该造山带去山根过程壳-幔相互作用性质和是否受西太平洋板块俯冲的影响。

本项目对苏鲁造山带进行了系统的野外勘察和细致的采样工作，对苏鲁造山带碰撞后花岗岩体的代表性花岗岩样品进行了主微量元素、传统Sr-Nd-Pb-O同位素、非传统Mg同位素研究，同时对12个样品进行了锆石CAMECA U-Pb定年。主要取得了以下几方面成果：. 1.精确制约了苏鲁造山带侏罗纪岩浆活动时限为166.2Ma至156.7Ma，该时限与郯庐断裂左行走滑时间完全一致。鉴于该时期无幔源岩浆活动，侏罗纪岩浆事件可能为郯庐断裂带走滑引发的苏鲁超高压岩片向北大规模推覆过程伴随的地壳熔融事件。苏鲁造山带侏罗纪花岗岩具有高的硅铝含量，低的铁镁含量等，属淡色花岗岩，表明它们可能来自一个中酸性岩石源区。苏鲁造山带侏罗纪花岗岩具有与大别类似的高Sr/CaO特点，同时具有互相耦合的低Th、REE含量，正Eu异常以及La/Yb，这些特点指示它们的源区为贫斜长石含帘石的高压源区，从而证实加厚长英质陆壳在苏鲁造山带北部普遍存在。该侏罗纪淡色花岗岩与大别山低镁埃达克质花岗岩对比揭示了加厚基性/长英质地壳熔体的差异和判别标准。. 2.苏鲁侏罗纪花岗岩具有高放射性成因Sr、Pb同位素以及极轻（δ26Mg达-0.75‰）的Mg同位素，其Mg同位素与Pb同位素具有很好的相关性，表明这些同位素的变化应为二元混合成因。由于其Sr、Pb、Mg同位素均落在华南和华北表壳岩石的混合区域，混合的两个端元应该为华南和华北的中上地壳。这些同位素比值随样品距离华南-华北缝合线的距离增大逐渐趋向两个端元的组成表明这一二元混合反应的是陆壳叠置，即华南中上地壳俯冲在华北中上地壳之下，随距离缝合线远近二端元地壳对花岗岩源区的贡献比例逐渐变化。. 3. 苏鲁造山带胶东和胶北地区白垩纪花岗岩类主要在两个时期侵位，分别是郭家岭岩体的130Ma和以三佛山为代表的114-119Ma。郭家岭岩体中的铁镁质包体具有和郭家岭岩体一致的年龄和锆石Hf同位素，表明其是析离体。郭家岭岩体的高Mg#和高Sr特征表明其属于高Mg埃达克质岩，代表了白垩纪130Ma左右胶北地区加厚镁铁质下地壳拆沉事件。郭家岭岩体与同区域侏罗纪岩体的微量元素含量及Sr-Nd-Pb同位素具有亲缘性，反映了侏罗纪加厚长英质地壳部分熔融的残留相在白垩纪也被拆沉并卷入郭家岭花岗岩熔融事件。