Geochronology and Geochemistry of the Shahewan and Caoping Granites in SouthQinling and Their Significance
-
摘要: 对南秦岭沙河湾和曹坪岩体花岗岩进行了地球化学、锆石U-Pb年龄和锆石Lu-Hf同位素分析.地球化学分析显示,两个岩体富碱、富镁,属于高钾钙碱性准铝质I型花岗岩.轻、重稀土分馏明显,富集轻稀土元素和大离子亲石元素Rb、Ba、Sr等,亏损重稀土元素和高场强元素Nb、Ta、Ti、Y和Yb等,大部分样品的Eu元素显示无异常或弱正异常(δEu=1.00~1.12),仅有一个样品Eu元素为中等负异常(δEu=0.66).锆石U-Pb年龄分析显示,沙河湾岩体和曹坪岩体的岩浆结晶年龄分别为214~207 Ma和215~212 Ma;锆石Hf同位素分析显示沙河湾岩体的εHf(t)值为-1.38~1.81,二阶段模式年龄(tDM2)为1.30~1.11 Ga;曹坪岩体的εHf(t)值为-1.0~3.1,tDM2为1.29~1.03 Ga.沙河湾和曹坪岩体中的绝大部分样品具有相似的岩石学、形成时代、地球化学及锆石Lu-Hf同位素特征,显示两岩体具有同源性.研究表明,两个岩体的母岩浆来源于中元古代地壳与地幔物质的混合,形成于后碰撞阶段发生拆沉作用的伸展阶段.指示勉略洋在晚三叠世之前已完全闭合,扬子板块和华北板块碰撞造山运动已达到后期Abstract: This study presents the granites from the Shahewan and Caoping plutons in South Qinling area, including their geochemical, geochronological, and zircon Lu-Hf isotopic compositions. Based on the geochemical analyses, the two plutons exhibit high alkali and MgO contents and belong to metaluminous I-type granite of high-K calc-alkaline series. The samples show evident fractionation of light rare earth elements (LREE) and heavy rare earth elements (HREE), as the samples are enriched in LREE and large ion lithophile elements (LILE, e.g. Rb, Ba and Sr) and relatively depleted in HREE and high-field-strength elements (HFSE, e.g., Nb, Ta, Ti, Y, and Yb). Most of the samples exhibit slightly or no positive Euanomalies except for one sample showing moderate negative anomaly. LA-ICP-MS U-Pb zircon age analyses indicate that the magmatic crystallization age of the Shahewan pluton is 214-207 Ma while the Caoping pluton is 215-212 Ma. Zircon Hf isotopic analyses show that the εHf(t) values are between -1.38 and +1.81 and the two-stage model ages (tDM2) are 1.30-1.11 Ga for the Shahewan pluton. Correspondingly, the εHf(t) values are -1.0 to +3.1 and tDM2 values of 1.29-1.03 Ga for the Caoping pluton. Nearly all the samples in the Shahewan and Caoping plutons display similar characteristics of petrology, geochronology, geochemistry and zircon Lu-Hf isotopes, indicating homologous evolution. It is concluded that the parental magma of the two plutons is a mixture of Mesoproterozoic crust and mantle and the plutons formed during the extensional delamination stage of the post-collisional stage. The Mianlue Ocean closed completely before the Late Triassic, at which time the collisional orogeny between the Yangtze and North China Blocks was in the late stage.
-
Key words:
- geochemistry /
- U-Pb zircon ages /
- Hf isotopes /
- Shahewan and Caoping plutons /
- South Qinling.
点击查看大图
计量
- 文章访问数: 1614
- HTML全文浏览量: 84
- PDF下载量: 124
- 被引次数: 0