北苏鲁超高压变质带内多成因类型的变花岗质岩石及其地质意义

The polygenetic meta-granitic rocks and their geological significance,within the North Sulu ultrahigh-pressure belt

变花岗质岩石是北苏鲁超高压变质带出露最广泛的岩石类型。本文通过锆石年代学和全岩地球化学的系统研究,查明该区变花岗质岩石具有成因类型多样性的特点,揭示其构造归属的复杂性,这对于解释扬子板块与华北板块之间的俯冲-碰撞-折返动力学过程具有重要科学意义。锆石U-Pb定年结果表明,区内的变花岗质岩石由新元古代(820～750Ma)花岗质片麻岩和新太古代-古元古代(2700～1870Ma)TTG-花岗质片麻岩共同组成,且二者经历了完全不同的构造演化历史。其中,新元古代花岗质片麻岩经历了晚三叠世(222～213Ma)的变质-深熔事件,而新太古代-古元古代的TTG-花岗质片麻岩则记录了晚古元古代(～1864Ma)的变质事件。全岩地球化学结果表明,新元古代花岗质片麻岩的原岩成分趋近于A型花岗岩,属于过铝质-准铝质的高钾钙碱性系列,主量元素(AlT_2O_3、MgO、FeO、TiO_2、CaO、P_2O_5)与SiO_2显示负相关性,具有轻、重稀土分异和负Eu异常特征,表明其源区有斜长石的残留。富集大离子亲石元素和强烈亏损高场强元素的特点,进一步表明它们可能是中-上地壳物质部分熔融的产物。新太古代-古元古代TTG-花岗质片麻岩的原岩成分主要落入TTG和I&S型花岗岩区域,属于低钾-钙碱性系列,主量元素与SiO_2相关性不明显,多数样品的稀土总量比新元古代花岗质片麻岩略低,轻、重稀土分异相对较弱,具有正Eu或无Eu异常,表明其源区无斜长石而可能有角闪石的残留。它们的地球化学属性与新元古代花岗质片麻岩的成因特点存在显著差异。古元古代花岗质片麻岩的原岩可能为与造山有关的I型花岗岩,来自于太古宙基底岩石的重熔与再造。上述综合研究结果显示,北苏鲁超高压变质带内新元古代花岗质片麻岩的原岩具有亲扬子板块的属性,与扬子板块北缘罗迪尼亚超大陆裂解存在密切成因关系。而新太古代-古元古代TTG-花岗质片麻岩与华北板块变质基底、胶-辽-吉古元古构造带具有明显亲缘性。由此可见,在传统北苏鲁超高压变质带内的威海-乳山一带,由具有扬子板块属性和具有华北板块属性的变质基底,沿北东-南西方向共同组成了一条北苏鲁混杂岩带。这一新的研究成果充分表明,华北板块东南缘的新太古代-古元古代的陆壳物质普遍卷入到三叠纪时期扬子板块与华北板块之间的相向(双向)俯冲-折返造山过程中。

By integrated studies of whole-rock geochemistry and zircon geochronology, the meta-granitic rocks, exposed extensively within the North Sulu ultrahigh-pressure （UHP） belt, can be subdivided into several polygenetic types which have different tectonic affinities. This is of great significance for the interpretation of the subduction-collision-exhumation process between the North China Block （NCB） and the Yangtze Block （YB）. Abundant zircon U-Pb age data reveal that the protoliths of some meta-granitic rocks formed at Neoproterozoic （820-750Ma）, and experienced a Late Triassic （222-213Ma） metamorphic and partial melting event. In contrast, the protoliths of some other meta-granitic rocks within the Sulu UHP belt formed at Neoarchean to Paleoproterozoic （2700-1870Ma）, and experienced a Late Paleoproterozoic （-1864Ma） metamorphic event. The whole-rock geochemistry show that the Neoproterozoic rocks are likely to be A-type, peraluminous to metaluminous and high potassium calcium alkaline granites, while the Neoarchean to Early Paleoproterozoic meta-granitic rocks show the characteristics of TTG or I-＆ S-type, peraluminous and low potassium-calcium alkaline granites. The Neoproterozoic rocks display positive correlations between most major elements with SiO2, obvious differentiation between light rare earth elements （LREE） and heavy rare earth elements （HREE） with negative Eu anomalies, enrichment in LILE and depletion in HFSE. It can be deduced that they might be the products of partial melting of the upper to middle crust, and there were some residuals of plagioclase in their protolith source region. However, the Neoarchean to Early Paleoproterozoic TTG-granitic gneisses exhibit lower total rare earth elements （ΣREE）, weak differentiation between LREE and HREE with positive or no Eu anomaly, indicating they might derive from a source region without residual of plagioclase. In addition, the Late Paleoproterozoic （-1870Ma） rock shows similar features with I-type granite and it might derive from the remelting and reworked of middle to lower crust. Thus, the Neoarchean to Paleoproterozoic TTG-granitic gneisses have distinctly disparate petrogenesis characteristics with the Neoproterozoic granitic gneisses. Thereinto, the Neoproterozoic granitic gneisses have the affinity of the basement of YB, related to the breakup of the supercontinent Rodinia, while the Neoarchean to Paleoproterozoic TTG-granitic gneisses are similar to the basement in the Jiao-Liao-Ji Paleoproterozoic orogen, relevant to the amalgamation and breakup of the supercontinent Columbia. Therefore, it can be concluded that the continental crust materials from the basements of both NCB and YB are spatially distributed in the Haiyangsuo-Wuji-Weihai （HWW） area, along the northern margin of the traditional North Sulu UHP belt, which formed a northeast-southwest spread mélange belt called as the North Sulu mélange belt. This new result indicates that some Neoarchean to Paleoproterozoic basements from the NCB were involved into the Triassic subduction-exhumation process between the YB and NCB, which powerfully supports a two-side collisional model rather than the model involving one-side subduction and exhumation as in previous studies.