北秦岭饭铺闪长岩的岩石成因及其构造意义

Petrogenesis and tectonic implications of the Fanpu diorite in the northern Qinling orogenic belt

饭铺闪长岩的岩石成因模型可为深入认识北秦岭古生代中性岩浆作用的活动规律及深部过程提供进一步的限定。饭铺岩体两件样品的锆石U-Pb同位素年龄值分别为422.1±5.5 Ma和424.2±5.2 Ma,表明其形成于晚志留世。其锆石的^(176)Hf/^(177)Hf比值介于0.282560~0.282676,ε_(Hf)(t)值介于-8.48~14.45,T_(DM2)为1938~488 Ma。地球化学分析结果显示,饭铺岩体属钙碱性系列,均具有富硅、碱、钙和镁,Mg^(#)值介于41.7~57.4。在微量元素蛛网图中,饭铺岩体具有Nb、Ta、Ti异常。饭铺岩体的稀土总量为117×10^(-6)~171×10^(-6),(La/Yb)_(N)比值范围是6.71~18.1,δEu值介于0.82~0.95,表现为轻稀土元素富集、重稀土元素亏损的配分特征,未显示明显的负Eu异常。饭铺岩体形成于岛弧构造环境,岩浆起源于下地壳,可能受到少量幔源物质的混合,推测是商丹洋向北深俯冲作用的产物。

The petrogenesis of the Fanpu diorite can provide new constraints on the activity and deep process of the Paleozoic intermediate magmatism in the North Qinling orogenic belt.Two samples from the Fanpu pluton are dated at 422.1±5.5 Ma and 424.2±5.2 Ma,respectively,indicating that they formed in the Late Silurian.^(176)Hf/^(177)Hf ratios of zircon grains of the Fanpu diorite range from 0.282272 to 0.282918,ε_(Hf)(t)values are between -8.48 and 14.45,and T_(DM2) are between 1938 Ma and 488 Ma.The Fanpu diorite belongs to the calc-alkaline series,which are enriched in silicon,alkali,calcium,and magnesium,with Mg^(#)values ranging from 41.7 to 57.4.In the spider diagram of trace elements,the Fanpu diorite has shown the depletion of Nb,Ta and Ti.Total REE contents of the Fanpu diorite are between 117×10^(-6) and 171×10^(-6),and ratios of(La/Yb)_(N) range from 6.71 to 18.1.The δEu values are between 0.82 and 0.95.Their REE distribution patterns are characterized by light REE enrichment and heavy REE depletion,without obvious negative Eu anomaly.Magma of the Fanpu diorite might originate from the lower crust and underwent crust-mantle interaction in an island arc tectonic environment,which was related to the northward subduction of the Shangdan Ocean.