Zircon U-Pb Dating, Geochemical Characteristics of the Gneissic Granite from North Qinling Terrain in Tongbai Area and their Geological Significance
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摘要: 对桐柏地区北秦岭地体中侵入秦岭岩群的2个片麻状花岗岩体进行了锆石年代学和岩石地球化学研究。新庄岩体的主要岩性为片麻状白云母二长花岗岩，保水寺岩体的主要岩性为片麻状二长花岗岩，两者均具糜棱岩化。LA-MC-ICP-MS锆石U-Pb测年结果（429±3） Ma （MSWD=0.12，n=9）和（445±3） Ma （MSWD=1.02，n=16）分别代表其形成年龄，均形成于志留纪。两者均具有高硅、高铝、低镁、低铁的特点，属于高钾钙碱性系列；稀土元素总量较低，LREE相对富集，HREE相对亏损，Eu负异常中等；富集大离子亲石元素Rb、Ba、U，亏损高场强元素Nb、Ta及P、Ti。新庄片麻状花岗岩为强过铝质S型花岗岩；保水寺片麻状花岗岩为弱过铝质I型花岗岩，重稀土元素Y、Yb、Lu明显亏损。根据有关岩体的形成年龄及与围岩的接触关系，限定了桐柏地区秦岭岩群麻粒岩相变质时限为498~488 Ma，角闪岩相退变质时限在488~428 Ma，区域糜棱岩化作用发生在428 Ma之后。保水寺岩体形成于岛弧环境，其形成与古特提斯洋（商丹洋）的向北俯冲有关；新庄岩体形成于碰撞环境，其形成与桐柏地区早古生代末发生的弧陆碰撞事件有关。Abstract: The zircon geochronology and geochemistry of two gneissic granite masses intruded into Qinling group-complex in North Qinling terrain, Tongbai area have been studied in this paper. The main lithology of Xinzhuang granite mass is gneissic talc monzonitic granite, while the ones of Baoshuisi granite mass is monzonitic granite. Both of them have experienced mylonitization. The LA-MC-ICP-MS Zircon U-Pb dating has obtained the ages of (429±3) Ma (MSWD=0.12, n=9) and (445±3) Ma (MSWD=1.02, n=16), which represent the emplacement ages of these two rock mass, that is they were formed in Silurian period. The geochemistry analysis show that both granites are characterized by high Si and Al, low Mg and Fe, and they belong to high-K calc-alkaline series. Their REEs are relatively low, with relatively enriched LREE, depleted HREE and medium-negative Eu anomaly. In addition, these rocks are enriched in LILE (such as Rb, Ba, U), but depleted in HFSE (such as Nb, Ta), P and Ti. The Xinzhuang gneissic granite belongs to strongly peraluminous S-type granite, while the Baoshuisi gneissic granite belongs to weak peraluminous I type granite, with obviously depleted HREE (Y, Yb, Lu). Based on the formation age of related rock mass and the contact relation with wall rock, it's limited that the granulite-facies metamorphism in Qinling group-complex took place at ca. 498~488 Ma, the amphibolite-facies retrogressive metamorphism occurred at ca. 488~428 Ma, and the regional mylonitization take place after 428 Ma. The Baoshuisi granite mass was formed in island arc environment, which is related to the northward subduction of Paleo-Tethys Ocean (Shangdan Ocean). The Xinzhuang granite mass was formed in impact environment, which is related to the arc-continent collision event at the end of the Early Paleozoic in Tongbai area.
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Key words:
- Tongbai area /
- North Qinling terrain /
- gneissic granite /
- zircon U-Pb dating /
- oceanic subduction
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