（三）四房坝与硅质岩互层变质火山岩年龄测定结果和同位素地球化学特征
推荐回答（1个）
1.Sm-Nd年龄同位素数据和Nd同位素地球化学特征
对12个采自略阳三岔偏桥沟斜长花岗岩体之南的变质火山岩样品进行了Sm-Nd年龄同位素分析。分析结果列于表3-5-24。由表3-5-24和图3-5-39可以看出，四房坝一带与硅质岩互层变质火山岩有两类：一类包括样品 Q98222-1 和样品 Q98226，147Sm/144Nd比值和143Nd/144Nd比值大于球粒陨石值（0.1967，0.512638）。其147Sm/144Nd值分别为0.2236和0.2277，143Nd/144Nd值为0.513221和0.513226。与元素地球化学特征轻稀土相对重稀土强烈亏损特征完全一致（图3-5-33）；另一类样品147Sm/144Nd值由0.1139～0.1318，143Nd/144Nd值由0.511796～0.512258，与元素地球化学特征也十分对应（图3-5-33）。前者与庄科变质镁铁质火山岩类似，后者既不像庄科变质镁铁质火山岩，也不像三岔偏桥沟斜长花岗岩之北变质安山质岛弧火山岩（图3-5-40）。四房坝变质火山岩样品分散，不形成等时线（图3-5-39）。Nd模式年龄：轻稀土亏损型岩石（类似庄科变质镁铁质火山岩）样品tDM827～1092Ma，平均值960±132（σ）Ma；轻稀土富集型岩石样品tDM1656～2355Ma，平均值1904±183（σ）Ma。四房坝轻稀土富集型火山岩Nd模式年龄明显不同于偏桥沟斜长花岗岩之北变质安山质火山岩的Nd模式年龄，它们的tDM值明显偏低，平均值1158±30（σ）Ma。
表3-5-24 陕西省略阳县三岔偏桥沟南四房坝变质火山岩Sm-Nd年龄同位素分析结果
图3-5-39 陕西省略阳县三岔偏桥沟南四房坝变质镁铁质火山岩143Nd/144Nd-147Sm/144Nd图
2.Rb-Sr年龄同位素数据和Sr同位素地球化学特征
同Sm-Nd年龄测定样品，对12个采自偏桥沟南四房坝一带的变质镁铁质火山岩样品进行了Rb-Sr年龄同位素分析。结果列于表3-5-25，按样品Rb含量可把样品分成两类：一类包括样品Q98222-1和Q98226，具有轻稀土相对重稀土亏损稀土分布模式，Rb含量低（3.42×10-6～5.342×10-6），Sr含量相对较高，87Rb/86Sr比值低，由0.09227～0.1897，相应87Sr/86Sr比值≤0.707506，与庄科变质镁铁质火山岩Rb-Sr同位素系统十分类似（表3-5-18）；另一类包括Q98222-2～Q98222-10，Q98225等10个样品，具有轻稀土相对重稀土富集稀土分布模式，Rb含量高（13.78×10-6～127.1×10-6），Sr相对较低（43.76×10-6～202.6×10-6），87Rb/86Sr比值除样品Q98222-4为0.4064外，其余样品均大于1.3，87Sr/86Sr比值高，除样品Q98222-3为0.707938外，其余样品均大于0.713，最高值达0.774706，既不像庄科变质镁铁质火山岩Rb-Sr同位素系统，也不像三岔偏桥沟斜长花岗岩之北的变质安山质火山岩Rb-Sr同位素系统（表3-5-3、表3-5-18，图3-5-41）。样品分散，不形成等时线。变质火山岩原岩形成过程中可能受到过富Rb地壳物质严重混染。
图3-5-40 陕西省略阳县三岔偏桥沟南四房坝变质火山岩、庄科变质镁铁质火山岩和三岔偏桥沟变质安山质火山岩Sm-Nd同位素系统比较图
四房坝轻稀土亏损样品与庄科变质镁铁质火山岩类似，四房坝轻稀土富集型样品既不像庄科变质镁铁质火山岩，也不像同一条沟北部出露的变质安山质岛弧火山岩
表3-5-25 陕西省略阳县三岔偏桥沟南四方坝变质火山岩Rb-Sr年龄同位素分析结果
图3-5-41 陕西省略阳县三岔偏桥沟南四房坝变质火山岩、庄科变质镁铁质火山岩和三岔变质安山质火山岩Rb-Sr同位素系统比较图
3.变质镁铁质火山岩40Ar/39Ar坪年龄
对采自偏桥沟南四房坝的变质镁铁质火山岩样品Q98222-10进行了40Ar/39Ar坪年龄测定，结果列于表3-5-26，坪年龄226.9±1.8（2σ）Ma（图3-5-42）。明显表明，其年龄是岩石形成后遭受热事件时间。年龄采样位置与偏桥沟斜长花岗岩出露位置相差不远，其40Ar/39Ar坪年龄相差确很大，这反映它们现在位置可能不是最初形成时的位置。现在位置是后期构造作用造成的。
表3-5-26 陕西省略阳县三岔偏桥沟南四房坝变质镁铁质火山岩40Ar/39Ar阶段升温年龄测定结果
注：F*放射成因40Ar/39Ar之比。样品：Q98222-10全岩，J=0.014000。
图3-5-42 陕西省略阳县三岔偏桥沟南四方坝变质镁铁质火山岩40Ar/39Ar坪年龄
4.Pb同位素地球化学特征
偏桥沟南四房坝变质镁铁质火山岩Pb同位素分析结果列于表3-5-27。样品Pb同位素组成分布范围宽。与Nd同位素分析结果类似，出露这一地区的镁铁质火山岩的Pb同位素组成可划分为两类：一类由样品Q98222-1和Q98226组成，样品轻稀土相对重稀土强烈亏损，Pb同位素值相对较低，与庄科变质镁铁质岩类似，206Pb/204Pb 17.03026～17.06688，207Pb/204Pb 15.3904～15.53329，208Pb/204Pb 36.7380～37.09304；另一类，轻稀土相对重稀土富集，Pb同位素明显较上类型岩石富放射成因Pb，206Pb/204Pb 17.57346～18.93330，207Pb/204Pb 15.4517～15.62258，208Pb/204Pb 37.6871～39.00110。
表3-5-27 陕西省略阳县三岔偏桥沟南四房坝变质镁铁质火山岩Pb同位素分析结果
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