Abstract —Alkaline and subalkaline granites of the Keivy megablock belong to the potassic alkali-calcic ferroan rocks enriched in large-ion lithophile, high-field strength and rare-earth elements. Based on these criteria, they correspond to A 2 -type granites (Eby, 1992). The granites have elevated Y/Nb (1.9–2.5) and Yb/Ta (3.4–3.7) ratios typical of crustally derived granites. The initial 176 Hf/ 177 Hf ratios in the central parts of zircon crystals vary within 0.281004–0.281175 at εHf(T) ranging from –2.89 to 3.79. It is supposed that the underplating of high-temperature mafic melts caused melting of metasomatically altered lower-crustal rocks and formation of palingenetic lower-crustal melts, which during subsequent ascent in the upper crust experienced fractional crystallization to more siliceous subalkaline and alkaline melts.

中文翻译：

---

科拉半岛 Keivy Megablock 新太古代碱性花岗岩的 Lu-Hf 同位素-地球化学锆石系统学和成因

摘要——Keivy 巨型块体碱性和亚碱性花岗岩属于钾碱钙铁质岩，富含大离子亲石体、高场强和稀土元素。基于这些标准，它们对应于 A 2 型花岗岩（Eby，1992）。花岗岩具有升高的 Y/Nb (1.9–2.5) 和 Yb/Ta (3.4–3.7) 比率，这是典型的地壳衍生花岗岩。在 εHf(T) 范围从 –2.89 到 3.79 时，锆石晶体中心部分的初始 176 Hf/177 Hf 比率在 0.281004–0.281175 之间变化。推测高温基性熔体的底镀引起交代蚀变的下地壳岩石熔融并形成古生代下地壳熔体，在随后的上地壳上升过程中经历了分步结晶，形成更多的硅质亚碱性和碱性熔体。