松潘-甘孜地体中部晚三叠世安山质增生弧的确定及其意义  

Late Triassic andesitic accrtionary arc in the central Songpan-Ganzi terrane and its tectonic significance

作　　者：夏磊[1] 闫全人[1] 向忠金[2] 江文[1] 宋博[3] 陈辉明[2] 

机构地区：[1]中国科学院大学地球科学学院,北京100049 [2]中国地质科学院地质研究所,北京100037 [3]中国地质调查局西安地质调查中心,西安710054

出　　处：《岩石学报》2017年第2期579-604,共26页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(40472119;40872147);中国地质调查局地质调查项目(12120113067500;DD20160201-07)联合资助

摘　　要：本文研究揭示,巴颜喀拉-松潘甘孜地体(简称松潘-甘孜地体)中部三叠系复理杂岩中的安山岩块是增生弧岩浆活动的产物,具有高度不均一均匀的岩石组构、地球化学与同位素组成。特别是这些安山岩显示了与松潘-甘孜三叠系海相浊积岩具有高度的Sr-Nd同位素亲缘性,沿亏损地幔和三叠系海相浊积岩混熔曲线分布,且主要分布于三叠系海相浊积岩端元区。这表明该增生弧安山质岩浆主要由增生楔中的海相浊积岩基质部分熔融形成,并受到了增生楔中大洋岩石圈残片等超镁铁/铁镁质组分的不同程度混染。安山岩时代(226.8±1.9Ma^213.7±0.9Ma)以及同时期广泛的S型花岗岩侵入活动(228±2Ma^204±7Ma)揭示增生弧形成于晚三叠世Norian期。前人工作表明,位于松潘-甘孜地体东北隅的诺尔盖-松潘盆地是一个周缘前陆盆地。因此,松潘-甘孜地体并非单一构造成因的地质体,至少由二叠纪-三叠纪演化的古特提斯大洋和中三叠世拉丁期-晚三叠世诺列期周缘前陆盆地两部分构成。前者是松潘-甘孜地体的主体部分,由松潘-甘孜古特提斯洋在三叠纪时期快速收缩形成的增生楔杂岩组成,其上发育增生弧,局部残存古特提斯大洋盆地及被构造移置的洋壳残片;后者为西秦岭弧(248~234Ma)与扬子地块碰撞的产物。Detailed observations presented in this paper show that the andesite blocks in the Upper Triassic turbidites in the central Bayan Har-Songpan Ganzi terrane （Songpan-Ganzi terrane in short ） in the northern Tibet Plateau were the components of an accretionary arc based on the accretionary wedge. The andesites are highly inhomogeneous in the phenocryst and matrix textures, geochemical and isotopic compositions （e. g. A/CNK =0. 62 - 1.15, σ = 1.99 -2. 35, Mg# =38 -46, εNd（t） = -6. 32 - -2. 98） , which indicates that magma of the accretionary arc was derived from various sources. High affinities of Sr and Nd isotopes with Triassic submarine turbidites suggest that andesitic magma of the accretionary arc was predominantly derived from melting of the turbidite matrix of the accretionary wedge, and somewhat contaminated by the uhramifc/mafic components form the oceanic lithosphere. Zircon U-Pb analyses for the andesites, plus syschronous S-type granites （228 ± 2Ma -204 ± 7Ma）, indicate that the accretionary arc was occurred in the Norian of Late Triassic （226.8 ±1.9Ma - 213.7 ± 0. 9Ma）. Previous working showed that the northestern part of the Songpan- Ganzi terrane was actually a peripheral foreland basin. Consequently, the Songpan-Ganzi terrane was one integrated by at least two tectosomes, evolved paleo-Tethys ocean and a peripheral foreland basin. The former is the main part of the Songpan-Ganzi terrane and composed of subduction-related accretionary complexes followed with the rapid shortening of the Songpan-Ganzi paleo-Tethys ocean in the Triassic, on which developed the andesitic accretioanry are in the Norian. The latter was constructed by collision between the West Qingling arc （248 -234Ma） and the Yangtze block in the Ladinian to Norian.

关 键 词：松潘-甘孜地体 斑状安山岩 增生楔杂岩 增生弧 

分 类 号：P542[天文地球—构造地质学] P588.144[天文地球—地质学]