安徽铜陵矿集区石英闪长岩—花岗闪长岩成因的Sr-Nd-Hf-O同位素制约

安徽铜陵矿集区是我国长江中下游构造—岩浆—成矿带中一个重要的铜金多金属成矿区，区内广泛发育晚中生代以石英闪长岩和花岗闪长岩为主的中酸性闪长质侵入岩体，且其与成矿关系密切。关于铜陵矿集区侵入岩的成因，前人已有大量研究成果发表，但仍存有一定争议。本文在广泛收集前人研究资料和成果的基础上，选择区内铜官山石英闪长岩体和新屋里、胡村花岗闪长岩体开展全岩Sr-Nd同位素和锆石原位Hf-O同位素分析，以进一步探讨其成因机制。研究表明，石英闪长岩和花岗闪长岩的结构构造和矿物组成特征相似，全岩主量元素、微量元素和稀土元素组成特征也基本一致，均属高钾钙碱性岩石系列，并具有埃达克质岩石特征。石英闪长岩I_Sr值为0.707 2～0.709 0，ε_Nd（t）值为-13.0～-8.3，锆石ε_Hf（t）值为-23.5～-8.7，δ¹⁸O值为6.5‰～7.4‰；花岗闪长岩ISr值0.707 7～0.709 8，ε_Nd（t）值为-13.6～-8.5，锆石ε_Hf（t）值为-19.0～-7.4，δ¹⁸O值为6.7‰～8.1‰。石英闪长岩和花岗闪长岩Sr-Nd同位素组成特征和锆石Hf-O同位素组成特征亦基本一致，指示它们具有相同的成岩物质来源。结合区域地质背景和构造演化，本文认为，铜陵矿集区石英闪长岩和花岗闪长岩为富集岩石圈地幔岩浆与壳源埃达克质岩浆混合并上升侵位所形成，富集岩石圈地幔岩浆并非起源于古太平洋俯冲洋壳或俯冲洋壳析出流体交代的富集岩石圈地幔，而是新元古代华夏洋俯冲析出流体交代扬子板块岩石圈地幔所形成，壳源埃达克质岩浆为新元古代华夏板块与扬子板块碰撞之后形成的新生地壳熔融而成。铜陵矿集区晚中生代大规模岩浆作用的诱因是大陆板内构造背景下富集岩石圈地幔部分熔融岩浆上涌加热以及受古太平洋板块挤压和地壳加厚再转向拉张应力下的减压熔融。

Petrogenesis of quartz diorite and granodiorite in the Tongling ore district,Anhui Province: Sr-Nd-Hf-O isotopic constraints

Tongling ore district in Anhui Province is one of the important Cu-Au ore-concentration districts in the Middle-Lower Reaches of Yangtze River structural-magmatic-metallogenetic belt. Late Mesozoic intermediate-acid diorite intrusive rocks dominated by quartz diorite and granodiorite are widely developed in this area, and are closely related to mineralization. A large number of previous studies have been published on the genesis of intrusive rocks in the Tongling ore district, but there are still some controversies. Based on widespread collection of previous research data and results, the quartz diorite from Tongguanshan and the granodiorite from Xinwuli and Hucun in this area are selected to carry out the Sr-Nd isotopes and in situ zircon Hf-O isotopes in order to discuss its petrogenesis. The results show that the structure and mineral composition of quartz diorite and granodiorite are similar, and the major elements, trace elements and rare earth elements are basically the same. They belong to the high-K calc-alkaline rocks and have the characteristics of adakite-like rocks. I_Sr values of quartz diorite are ranged from 0.707 2 to 0.709 0, ε_Nd（t） = -13.0～-8.3, ε_Hf（t） values of the contain zircons are ranged from -23.5 to -8.7, δ¹⁸O = 6.5‰～7.4‰. ISr values of granodiorite are ranged from 0.707 7 to 0.709 8,ε_Nd（t） = -13.6～-8.5,ε_Hf（t） = -19.0～-7.4, δ18O = 6.7‰～8.1‰. The similar geochemical characteristics of Sr-Nd-Hf-O isotopes for quartz diorite and granodiorite indicate that they have the same source of diagenetic material. In addition, quartz diorite and granodiorite in the area contain a large number of inherited zircons of Neoproterozoic ages （mainly 1.0～0.8 Ga）, indicating that the diagenetic material contains a large number of ancient crustal materials. Combining with the regional geological background and tectonic evolution, this paper holds that quartz diorite and granodiorite in the Tongling ore district were formed by the mixing and upward emplacement of enriched lithospheric mantle partial melting magma and crustal adakitic-like magma, and the crustal adakitic-like magma was formed by the melting of the juvenile crust formed after the collision of Neoproterozoic cathaysia block and Yangtze plate. The Late Mesozoic large-scale magmatism in the Tongling ore district was attributed to upwelling and heating of the enriched lithospheric mantle partial melting magma and decompression melting of the thickened crust controlled by the stress changes from the subduction of the Paleo-Pacific plate. And the geotectonic background of the magmatic activities maybe inside of the continental plate.