摘要:烟镇岩体是北秦岭燕山期酸性侵入岩之一，它的岩石成因及深部动力学过程研究可为区域构造演化提供新的约束。对烟镇岩体黑云母二长花岗岩样品YZ04进行了LA-ICP-MS锆石U-Pb定年，30个测点中有23个测点的同位素比值位于207Pb/235U-206Pb/238U一致线上，它们的年龄值形成了复杂锆石年龄谱，7个年龄组依次为2 421 Ma、1 821 Ma、1 063 Ma、942.6 Ma、817.6 Ma、～145.0 Ma和～131 Ma，区域上均存在与之对应的岩浆热事件。最年轻的一组14颗锆石的加权平均年龄131.3±0.6 Ma，表明烟镇岩体形成时代为早白垩世早期。烟镇岩体具有高硅、碱及贫镁、钙的特征，属于高钾钙碱性系列。烟镇岩体样品富集大离子亲石元素，亏损高场强元素Nb、Ta，在微量元素蛛网图上显示明显的Nb、Ta、P及Ti负异常。烟镇岩体的稀土配分模式具有右倾平滑、轻稀土富集及重稀土亏损及无明显负Eu异常等特征，（La/Yb）N范围为7.97～49.19，其LaN-（La/Yb）N成分投点趋势表明源区组成和部分熔融程度控制了烟镇岩体的成分变异。烟镇岩体具有高Sr、低Y的特征，属于埃达克岩，它是加厚大陆下地壳的部分熔融形成的，部分熔融源区的残余相矿物包括石榴石、金红石，且无（富钙）斜长石残留。烟镇岩体是～131 Ma区域岩石圈拆沉作用的产物，这也是形成锆石年龄谱的深部控制因素，华北克拉通燕山期岩石圈巨大减薄效应波及到北秦岭地区。
关键词: 烟镇岩体/
锆石定年/
年龄谱/
埃达克岩/
岩浆（流体）混合/
岩石圈拆沉/
北秦岭
Abstract:Yanzhen granitic body is one of Yanshanian acid intrusions in northern Qinling Mountains. It is significant to find new constrains on regional tectonic evolution based on petrogenesis and deep process of Yanzhen granitic body. The LA-ICP-MS dating sample YZ04 is biotite monzonitic granite, and the results of 23 zircon spots in total 30 spots are concordia with 207Pb/235U-206Pb/238U trendline. The U-Pb age of 23 spots are clustered in 2 421 Ma, 1 821 Ma, 1 063 Ma, 942.6 Ma, 817.6 Ma, ～145.0 Ma and～131 Ma. Zircon age spectrum of Yanzhen granitic body from Proterozoic to Early Cretaceous has formed, which recorded the corresponding regional magmatic events. The weighted average 206Pb/238U age of 14 zircon spots from 129.7±1.1 Ma to 133.9±1.5 Ma is 131.3±0.6 Ma（MSWD = 0.99）, and it is the formation age of Yanzhen granitic body. Yanzhen granitic body is characterized by higher SiO2 and alkali, and lower MgO and CaO. Compositional spots of Yanzhen granitic body fall into high-K calc-alkaline series in the SiO2-K2O diagram. Samples from Yanzhen granitic body are enriched in large ion lithophile elements and depleted in high field-strength elements, which has formed troughs of Nb, Ta, P and Ti in spider diagram. In chondrite-normalized REE distribution pattern diagram, Yanzhen granitic body samples have shown the characteristics of LREE enrichment and HREE depletion without obvious Eu negtive anomaly, whose （La/Yb）N ratios are from 7.97 to 49.19. The spots trend of Yanzhen granitic body in LaN-（La/Yb）N diagram has suggested that their compositional change is controlled mainly by source compositon and degree of partial melting. Samples of Yanzhen granitic body are higher Sr content and lower Y content, which has suggested that it belongs to adakite. Yanzhen granitic body originated from partial melting of the thickened lower crust, and its residual phases of partial melting source included garnet, rutile, and no plagioclase. Yanzhen granitic body is product of lithosphere delamination occurred in ～131 Ma, and it is deep process of forming Yanzhen zircon age spectrum. It suggests that lithosphere thinning of north China craton in Yanshanian has affects northern Qinling Mountains.
Key words:Yanzhen granitic body/
Zircon dating/
Age spectrum/
Adakite/
Magma（fluids）mixing/
Lithosphere delamination/
Northern Qinling Mountains
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