碧口地块北缘花岗岩脉地球化学特征、成因机制及与金成矿关系探讨

GEOCHEMICAL CHARACTERS,PETROGENESIS OF GRANITIC DIKES AND THEIR RELATIONSHIP WITH GOLD MINERALIZATION IN NORTHERN MARGIN OF BIKOU TERRANE

碧口地块夹持于秦岭造山带、松潘-甘孜造山带与扬子板块之间,伴随中国大陆印支期造山事件,在其内部及北缘产出有大量花岗岩类侵入体,其中北缘主要为花岗岩脉,由东向西可分为康县月照—琵琶寺、文县联合村—阳山等多个产出密集区,在密集区内先后发现了塘坝、水洞沟、阳山、联合村、甲勿池等多个金矿床,针对琵琶寺、阳山及甲勿池等地的花岗岩脉的地球化学研究表明,它们具有高的w(SiO2)67.65%~74.86%、w(Al2O3)13.68%~19.61%,A/CNK=1.11~5.46,稀土元素总量较低(24.22×10-6~97.26×10-6),LREE富集,HREE亏损,轻重稀土分馏明显(LaN/YbN=3.29~22.40),具有弱负铕异常(δEu=0.70~0.79),微量元素特征表现为大离子亲石元素(LILE)强富集和高场强元素(HFSE)相对亏损。对岩浆源区及成分进行判别发现岩浆源区为下覆的陆壳基底,成分以杂砂岩为主。综合分析认为碧口地块北缘花岗岩脉活动高峰为晚三叠世,属后碰撞构造环境,成因机制与碰撞造山过程中加厚地壳拆沉作用和幔源物质底侵作用密切相关,地壳增厚导致的减压增温和幔源热流的注入诱使中上地壳杂砂岩部分熔融形成花岗质岩浆上侵而形成;同时,碧口地块北缘在早侏罗世(190Ma^195Ma)发生大规模金成矿作用,晚于岩浆活动高峰期近20Ma,成矿流体与岩浆岩极可能共用一个通道,花岗岩脉密集发育区易于成矿流体的聚集从而形成诸如阳山世界级超大型金矿床。

Bikou terrane is bounded by Qinling orogenic belt to the north,Songpan-Ganzi oro- genic belt to the west,and Yangtze Block to the southeast. Accompanying with Indon-China oro- genic events in Chinese continent, a lot of granitic dikes intruded in Bikou terrane and its north margin. There are mainly granitic dikes in the north margin, divided into several concentrated are- as from east to west, such as Yuezhao-Pibasi in Kangxian, Lianhecun-Yangshan in Wenxian. In these areas, large to super large gold deposits were discovered one after another, for examples Tangba, Shuidonggou, Yangshan, Lianhecun, and Jiawuchi gold deposits. However, petrogenesis, tectonic setting and relationship with mineralization of these granitic dikes have been seldom stud- ied. In this paper,new data are presented on geochemical compositions of the granitic dikes in Pi- bashi,Yangshan and Jiawuchi in order to discuss petrogenesis,sourees of magma and tectonic set- ting of diagenesis of these dikes. Study shows that the SiO2 and Al2O3 of the granites range from 67.65% to 74.86% and 13.68% to 19.61% respectively, and the A/CNK varies from 1.11 to 5.4. Rare earth elements （REE） content of the granites is low （24.22× 10 6 -97.26×10-6）, with LaN/YbN ratios of 3.29=22.40. Light rare earth elements （LREE） are concentrated evident- ly and heavy rare earth elements （HREE） are slightly negative Eu anomalies （δEu=0.70-0.79 deficient. The REE patterns are characterized by 79）. Granites are also enriched in large ion litho- phile elements （IALES） and relatively depleted in high field strength elements （HFSES）. It is showed that the granitic dikes were likely originated from subducted continental basement with greywackes as major composition. On the basis of analytical data of geochronology of granites and gold deposits in this area,it is proposed that the granites in north margin of Bikou terrane belong to post-eollisional setting with a peak activity of late Triassic and closely related to delamination of thickening crust and underplating of mantle-derived material during collisional and orogenic processes. These dikes were derived from partial melting of greywackes in middle-upper crust as a result of decompression and increasing temperature of thickening crust and upwelling of mantle- derived heat flow. Simultaneously, large-scale gold mineralization occurred in north margin of Bikou terrane at early Jurassic （190 Ma-195 Ma）, 20 Ma younger than the peak age of magma activity. Ore-forming fluids and magmatie rocks may ascend along a common path, so the concen- trated districts of granitic dikes are favorable for gathering the ore-forming fluids, and conse- quently forming super large gold deposit.