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甘肃北山马鞍山北花岗岩为钙碱性岩浆系列,具有较高的SiO_2(>56%)、Al_2O_3(>13%)和较低的MgO(<3%)含量,Na_2O>K_2O;并且具有较高的Sr(>300×10~(-6))和低的Y(<18×10~(-6))、Yb(<1.9×10~(-6))含量;岩石富集LREE和大离子亲石元素(Rb、Ba、Th等),而亏损HREE和高场强元素(Nb、Ta、Yb等),Eu异常不明显或具弱的Eu负异常(δEu=0.55~0.85)。具岛弧花岗岩特征,也与俯冲洋壳熔融形成的埃达克岩相似。样品的Nb/Ta、Zr/Hf和Th/Ce比值分别为8.26~13.82、25.57~37.19和0.12~0.40。元素地球化学特征反映岩浆源区可能是受到地壳物质混染和来自消减残留板片析出流体或熔体交代双重改造的产物。结合区域资料和前人相关研究成果,推测其可能与本区晚二叠世—中三叠世的区域性挤压、快速的抬升及加厚地壳的重熔等事件的发生有关,这些事件的发生使得残留在地壳的洋壳或年轻的火山弧及增生楔物质的再循环而导致的地壳的侧向生长,将地温提高到源区的起始熔融温度以上,在榴辉岩相条件下部分熔融形成了原始埃达克岩浆,熔体在穿过地幔楔的过程中与地幔橄榄岩发生强烈的熔体—岩石反应,同时岩浆在上升过程中又受到地壳物质混染而形成。
The northern Ma’anshan granite in Beishan, Gansu is a series of calc-alkaline magmas with high contents of SiO 2 (> 56%), Al 2 O 3 (> 13%) and lower contents of MgO (<3%) and Na 2 O> K 2 O Sr (> 300 × 10 -6) and low Y (<18 × 10 -6) and Yb (<1.9 × 10 -6) (Rb, Ba, Th, etc.), Depleted HREE and high field strength elements (Nb, Ta and Yb), Eu anomalies are not obvious or weak Eu negative anomalies (δEu = 0.55 ~ 0.85). Island arc granite features, but also with the formation of subducted oceanic adakite similar. The Nb / Ta, Zr / Hf and Th / Ce ratios of the samples were 8.26-13.82, 25.57-37.19 and 0.12-0.40, respectively. The elemental geochemical characteristics of the magma source area may be caused by the crustal material contamination and the remodeling of the remnant fluid or melt from the residual slab. Based on the regional data and previous research results, it is speculated that these events may be related to the regional extrusion and rapid uplift of the Late Permian-Middle Triassic and the remelting of thickened crust in the area. The occurrence of these events Lateral growth of the crust occurs as a result of recirculation of the oceanic crust or young volcanic arcs and accretionary wedge material remaining in the crust, raising the ground temperature above the initial melting temperature of the source zone, under eclogite phase conditions The original Adak magma was formed by melting. The melt reacted strongly with the mantle peridotite through the mantle wedge. At the same time, the magma was mixed with crustal material during the ascending process.