新疆西天山阿拉套山东部孔吾萨依 A 型花岗岩成岩年代、地球化学特 征及成因

The Kongwusayi granite body is located on the east side of the southern slope of the Alataw Mountain in the western Tianshan Mountains of Xinjiang. The rock body is mainly composed of alkali-feldspar granite, coarse-grained potassium-feldspar granite, and fine-grained potassium-feldspar granite. The weighted average age value of 206Pb/238U for the alkali-feldspar granite is 293.1 ± 3.6 Ma, and the age of the fine-grained potassium-feldspar granite is 293 ± 3.7 Ma. The three types of granite have similar geochemical characteristics and belong to the peraluminous - weakly peraluminous high-K calc-alkaline series of granite. From the alkali-feldspar granite to the coarse-grained potassium-feldspar granite and then to the fine-grained potassium-feldspar granite, the degree of Eu negative anomaly is getting higher and higher, and the depletion of elements such as Ba, Sr, P, Ti and the enrichment degree of Rb, Th, U, etc. gradually increase, indicating that there is a crystallization differentiation trend among the magmas of the three. The alkali-feldspar granite, coarse-grained potassium-feldspar granite, and fine-grained potassium-feldspar granite are all rich in Ga (the 104Ga/Al ratio is 2.6 - 3.5), have a high Fe* value (=FeOT/(FeOT + MgO), 0.88 - 0.97) and have a relatively high TiO2/MgO ratio (0.51 - 1.25), showing the characteristics of typical A-type granite. The A-type granite in the study area has a high zircon saturation temperature (853.86 - 931.56 °C) and a low zircon Ce(Ⅳ)/Ce(Ⅲ) ratio (1.9 - 77), suggesting that the magma of this A-type granite was formed under high temperature and low oxygen fugacity conditions. The three types of granite have a relatively high Y content (43.3×10−6 - 106×10−6), a relatively high εNd(t) (+3.0 - +5.2) and a relatively low (87Sr/86Sr)i value (0.7027 - 0.7069), indicating that the Kongwusayi A-type granite may be derived from the partial melting of the basaltic crust. However, compared with the Carboniferous basalt in the western Tianshan Mountains, the εNd(t) value of the Kongwusayi granite is lower, while the (87Sr/86Sr)i value is higher. Therefore, there should be the addition of ancient crustal materials in the magma source area of the Kongwusayi granite.

孔吾萨依花岗岩体位于新疆西天山阿 拉 套 山 南 坡 东 侧，岩 体 主 要 由 碱 长 花 岗 岩、粗 粒 钾 长 花 岗 岩、细粒钾长花岗岩组成。碱长花岗岩的２０６Ｐｂ／２３８Ｕ 加权平均年龄值为２９３．１±３．６Ｍａ，细粒钾长花岗岩的年龄为２９３±３．７Ｍａ。三类花岗岩具相似的地球化学特征，属准铝质—弱过铝质的高钾钙碱性系列花岗岩。从碱长花岗岩到粗粒钾长花岗岩再到细粒钾长花岗岩，其 Ｅｕ负异常程度越来越高，元素Ｂａ、Ｓｒ、Ｐ、Ｔｉ等的亏损及 Ｒｂ、Ｔｈ、Ｕ 等的富集程度逐步增高，显示三者的岩浆之间具有结晶分异趋势。碱长花岗岩、粗 粒 钾 长 花 岗 岩 及 细 粒 钾 长 花 岗 岩 均 富Ｇａ（１０４Ｇａ／Ａｌ比值２．６～３．５）、高 Ｆｅ＊ 值（＝ＦｅＯＴ／（ＦｅＯＴ＋ＭｇＯ），０．８８～０．９７）和具较高的 ＴｉＯ２／ＭｇＯ 比值（０．５１～１．２５），显示出典型 Ａ 型花岗岩的特征。研究区 Ａ 型花岗岩具有高的锆石饱和温度（８５３．８６～９３１．５６℃）和低的锆石 Ｃｅ（Ⅳ）／Ｃｅ（Ⅲ）比值（１．９～７７），暗示该 Ａ 型花岗岩岩 浆 形 成 于 高 温 和 低 氧 逸 度 条 件 下。三 类 花 岗 岩 具 较 高的 Ｙ 含量（４３．３×１０－６～１０６×１０－６）、较高的εＮｄ（ｔ）（＋３．０～＋５．２）和较低的（８７Ｓｒ／８６Ｓｒ）ｉ 值（０．７０２７～０．７０６９），表明孔吾萨依 Ａ 型花岗岩 可 能 源 于 玄 武 质 地 壳 的 部 分 熔 融。但 与 西 天 山 石 炭 纪 玄 武 岩 相 比，孔 吾 萨 依 花 岗 岩 的εＮｄ（ｔ）值较低，而（８７Ｓｒ／８６Ｓｒ）ｉ值较高，因此孔吾萨依花岗岩的岩浆源区应有古老地壳物质的加入。