冀西北水泉沟正长岩杂岩体的成因

An Investigation into the Petrogenesis of the Shuiquangou Syenite Complex, Northwest of Hebei Province

水泉沟碱性正长岩杂岩体侵入于太古宇桑干群变质岩中，岩体主要由碱性长石正长岩、角闪碱长正长岩、正长岩、石英碱长正长岩等组成，属碱性岩系列，在岩石的石英颗粒中见有熔融包体的存在。岩石的稀土元素分布模式呈平滑的右倾模式，无明显的铕异常。岩石的Ｐｂ同位素组成为２０６Ｐｂ／２０４Ｐｂ＝１６．４５７～１８．２８６、２０７Ｐｂ／２０４Ｐｂ＝１５．２７０～１５．４７２、２０８Ｐｂ／２０４Ｐｂ＝３６．５３９～３７．３９３，石英的δ１８Ｏ值为８．０‰～８．５‰，（８７Ｓｒ／８６Ｓｒ）ｉ≈０．７０５，岩石的ｔＤＭ为１．６～１．９Ｇａ，εＮｄ（ｔ）为－７．４５～－１３．１，明显大于区域基底的εＮｄ（ｔ）值（－２０±）。以上结果表明，岩体为上地幔和下地壳物质部分熔融的产物。岩体角闪石３９Ａｒ－４０Ａｒ年龄３２７Ｍａ。

The Shuiquanguo syenite complex which intruded into the Archean Sanggan Group metamorphic rocks consists of alkali syenite,hornblende alkali syenite, syenite, qaurtz alkali syenite, etc., it belongs to alkali rock series. Some of the quartz grains from the complex contain melt inclusions. The chondrite normalized REE patterns of the rocks show smooth right declined lines without apparent Eu anomaly. The isotopic composition of the rocks is 206 Pb/ 204 Pb=16.457￣18.286, 207 Pb/ 204 Pb=15.270￣15.472, 208 Pb/ 204 Pb=36.539￣37.393; δ 18 O qaurtz =8.0‰￣8.5‰; ( 87 Sr/ 86 Sr) i≈0.705; t DM =1.6￣1.9Ga, ε Nd ( t )= -7.45￣-13.1, the ε [HT5”SS] Nd ( t ) value of the rocks is larger than that of the regional basement(-20±). It can be deduced that the complex came from the partial melting of the lower crust and some of the material originally from the upper mantle.The alkali syenite complex was formed in the Hercinian period, with the 40 Ar 39 Ar age of hornblende being 327.4±9Ma.