(一)庄科变质镁铁质火山岩产出地质概况和岩石元素地球化学特征

2025-03-16 23:30:34
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回答1:

庄科变质镁铁质火山岩大面积出露于陕西省略阳县庄科一带,南西与三岔变质镁铁质-安山质火山岩相邻,在三岔偏桥沟南四房坝一带也见类似岩性出露。岩石常呈厚层块状,边缘绿片岩化强烈。有较强碳酸盐化。强烈折曲。夹薄层灰白色变质泥质岩石。

表3-5-16 陕西省略阳庄科变质镁铁质火山岩主、微量元素组成(wB

续表

岩石主、微量元素组成列于表3-5-16。主元素显拉斑玄武岩特征(图3-5-21)。CaO/Al2O3≈0.72,Al2O3/TiO2≈11.8,CaO/TiO2≈8.5,与洋中脊玄武岩类似(Melson et al.,1976;Sun and Nesbitt,1977、1978;Condie,1989)。轻稀土相对重稀土强烈亏损(图3-5-22),La为球粒陨石4.3~6.6倍,Yb为8.6~15.2倍,(La/Sm)N0.37~0.63,(Gd/Yb)N0.87~1.02,无Eu异常。相对于N-MORB,Rb、Ba、Th相对其他微量元素富集,Sr、K、Ce、P、Zr、Hf、Sm、Ti、Y、Yb、Sc与N-MORB类似(图3-5-23)。具有大的Nb负异常。在Ti-Zr-Y、Nb-Zr-Y和Ti-Zr-Sr图中样品分别落入洋底板玄武岩,低钾拉斑玄武岩,或大洋中脊拉斑玄武岩范围(图3-5-24~图3-5-26)。

图3-5-21 陕西省略阳县庄科变质镁铁质火山岩AFM图

图3-5-22 陕西省略阳县庄科变质镁铁质火山岩稀土分布模式

图3-5-23 陕西省略阳县庄科变质镁铁质火山岩N-MORB标准化微量元素分布模式

图3-5-24 陕西省略阳县庄科变质镁铁质火山岩Nb-Zr-Y图

图中符号意义见图3-3-5说明

图3-5-25 陕西省略阳县庄科变质镁铁质火山岩Ti-Zr-Sr图

图中符号意义见图3-3-5说明

图3-5-26 陕西省略阳县庄科变质镁铁质火山岩Ti-Zr-Y图

图中符号意义见图3-3-5说明

变质镁铁质火山岩中可见灰白色薄层状变质泥质岩石夹层,其Sm≈3.772/10-6,Nd≈19.805/10-6,Sm/Nd≈0.19,143Nd144Nd≈0.511747,Rb≈143/10-6,Sr≈71/10-6,Rb/Sr≈2.01,87Sr/86Sr≈0.723775,与变质镁铁质火山岩截然不同(表3-5-17、表3-5-18)。

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