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四顶黑山岩基是甘肃北山北带规模较大、岩石类型丰富的复式岩基之一,由四顶黑山岩体(早期)、双沟山岩体(中期)和马鞍山北岩体(晚期)组成。同位素测年表明,四顶黑山岩体侵位时代为早石炭世(327~351Ma),双沟山岩体形成于中—晚二叠世(263.5Ma±4.7Ma),而马鞍山北岩体为中三叠世(237.8±4.3Ma)。四顶黑山岩体为一基性—超基性杂岩体,由辉长岩、角闪辉长岩和少量橄榄岩组成,岩石富Mg、Fe,LREE富集,Eu呈正异常,富集相容元素而亏损不相容元素,可能代表了晚古生代底侵到加厚地壳底部的玄武质岩浆。双沟山岩体主要由花岗闪长岩、石英闪长岩组成;而马鞍山北岩体主要由花岗闪长岩和斑状二长花岗岩组成。双沟山岩体和马鞍山北岩体均具有埃达克岩类似的岩石地球化学特征:SiO_2>56%(平均分别为65.94%和66.88%),Al_2O_3≥16%(平均分别为15.77%和15.10%)和MgO<3%(平均分别为1.79%和1.61%),微量元素Sr>400×10~(-6)(平均分别为418×10~(-6)和499×10~(-6));Yb<1.9×10~(-6)(平均分别为1.26×10~(-6)和1.17×10~(-6));Y<18×10~(-6)(平均分别为10.39×10~(-6)和11.44×10~(-6)),LREE富集和Eu为弱负异常—正异常,相对亏损高场强元素(Nb、Hf和Ti等)。结合岩体产出的构造位置及区域地质演化,认为双沟山岩可能是在中—晚二叠世,研究区处于造山后伸展环境,地幔物质上涌,底侵于地壳底部,导致玄武质下地壳发生部分熔融形成,而马鞍山北岩体可能与古特提斯洋板块于晚二叠世到三叠纪向北俯冲诱发了本区陆内俯冲有关。
The four Montenegro rock basins are one of the large-scale and rock-rich complex rock basins in the northern part of Beishan Mountain, Gansu Province. They are composed of four Montenegro rock masses (early stage), Shuangdoushan rock mass (middle stage) and Ma’anshan north rock mass (late stage) composition. Isotope dating shows that the emplacement age of the four Montenegro plutons is Early Carboniferous (327 ~ 351Ma), that of Shuangdoushan pluton is middle-late Permian (263.5Ma ± 4.7Ma), and that of the Ma’anshan pluton is Middle Triassic (237.8 ± 4.3Ma). The four Montenegro plutons are a basic-ultramafic complex, composed of gabbro, hornblende gabbros and a small amount of peridotites. The rocks are enriched in Mg, Fe, and LREE. Eu is positively anomalous and rich in phases Content of elements and loss of incompatible elements may represent basaltic magmas of late Paleozoic uplifts to thickened crust. The Shuangdoushan rock mass is mainly composed of granodiorite and quartz diorite, while the Ma’anshan northern rock mass is mainly composed of granodiorite and porphyritic monzonite. Both the Shuangtianshan and Ma’anshan northern rocks have similar geochemical characteristics of adakites: SiO 2> 56% (average 65.94% and 66.88%, respectively) and Al 2 O 3 ≥ 16% (average 15.77% and 15.10 respectively %) And MgO <3% (average 1.79% and 1.61% respectively), trace elements Sr> 400 × 10 -6 (average 418 × 10 -6 and 499 × 10 -6, respectively) ); Yb <1.9 × 10 -6 (average 1.26 × 10 -6 and 1.17 × 10 -6, respectively); Y <18 × 10 -6 10.39 × 10 -6 and 11.44 × 10 -6), LREE enrichment and Eu are weak anomaly-positive anomaly, and relatively depleted high-field-strength elements (Nb, Hf and Ti, etc.). Based on the tectonic setting of the rock mass and the regional geological evolution, it is considered that the Shuangdou rock may be in the middle-late Permian, the research area is in the post-orogenic extensional environment, the mantle material is upwelling and the bottom of the crust is invaded, leading to basaltic The lower crust is partially melted and the Ma’anshan northern pluton may be related to the intracontinental subduction induced by the subduction of the Gêttiisian plate in the Late Permian to Triassic.