鹰峰环斑花岗岩地球化学特征及其构造意义

Geochemical Characteristics and Tectonic Significance of Rapakivi Granites in Yingfeng

柴达木北缘鹰峰环斑花岗岩出露于柴达木地块与南祁连地体之间的柴北缘造山带,是我国发现的又一元古宙环斑 花岗岩体.初步研究表明,鹰峰环斑花岗岩是具环斑结构和A-型花岗岩特征的典型的元古宙环斑花岗岩体,且属于A1亚 型,岩浆组合具双峰式特征.环斑结构主要由几个钾长石斑晶颗粒形成聚斑,中心有一斜长石内核,斑晶表面具不均匀高岭 土化,条纹构造明显且有规律,基质由细粒-微粒的石英组成,有明显重结晶及定向构造;岩石化学组成以高钾为特征,A/ NKC<1,A/NK>1,属准铝质;微量元素组成上富集Ba、U、Th、Ce、Hf、Sm,亏损Sr、Ta、Nb、Zr、Y,Rb/Sr(0.17～0.6)和 Rb/Ba(0.03～0.24)很低,岩石分异演化程度不高;稀土元素: REE、Ce、Zr含量高,Ga含量高达25×10-6以上,远远高出 其他类型花岗岩,但Eu(0.75～4.3)×10-6轻度亏损,属轻稀土富集型.通过对微量元素和稀土元素的地球化学行为分析, 鹰峰环斑花岗岩是发生在板内的一种岩浆作用,是下地壳的麻粒岩受底侵或拆沉作用地幔上涌影响,发生部分熔融,然后 经过分异演化形成了碱性的“干”岩浆,并在后碰撞的区域拉伸构造环境下侵位.同时伴随温度的降低,钠质的斜长石从钾 长石中出溶,并迁移到钾长石的边沿,形成了具环斑结构的A1型花岗岩.

Yingfeng rapakivi granite, outcropping in the north of the Qaidam basin orogenic belt between the Qaidam plate and South Qilian, is one of the Properozoic rapakivi granites found in China. This research shows that Yingfeng rapakivi granite is a typical Proterozoic rapakivi granite that has rapakivi texture and features of A-type granite, and belongs to A1 subtype, and the magma assemblage has two-apex characteristics. Rapakivi texture is mainly a polymer composed of several K-feldspar porphyritic crystals, with one plagioclase nuclear in it. The surface of the porphyritic crystal is kaolinized unevenly, its stripped structure is obvious and has orderliness. The matrix is composed of fine-particulate quartz and has obvious recrystallization and orientation textures. Its petrochemistry is characterized by high K, A/NKC1, belonging to quasi-aluminous rock. It is rich in minor elements Ba, U, Th, Ce, Hf, Sm, depleted in Sr, Ta, Nb, Zr, Y, Rb/Sr (0.17-0.6), and its Rb/Ba (0.03-0.24) ratio is very low. The differentiation of the rock is not high. The ∑REE, Ce, Zr contents are high, and the content of Ga can reach over 25×10-6, far higher than that of other granite types. Its Eu (0.75×10-6 - 4.3×10-6) is depleted slightly, belonging to LREE enrichment type. Through the analysis of the geochemistry of minor elements and REE, Yingfeng rapakivi granite is a kind of magmatism produced in the plate. It is the dry magma that is formed by the granulite, which is influenced by the uprush of the mantle of base intrusion or delamination, and then melted and then differentiated, and invaded in the post-collision area at the regional extentional tectonic environment. At the same time, with the decrease of temperature, Na-plagioclase is separated from the K-feldspar and transferred to the brim of the K-feldspar to form the ring porphyritic A1 granite.