Aeromagnetometry and aerogammaspectrometry integrated with U-Pb zircon geochronology of northern Bossoroca ophiolite, Brasiliano Orogen

Abstract Age delimitation integrated with aeromagnetometric and aerogammaspectrometric survey advances the understanding of ophiolite evolution in the Brasiliano Orogen. We focused on the Bossoroca ophiolite, because oceanic crustal and mantle rocks contain zircon in metasomatic chloritite. A metadiorite and a metavolcanoclastic rock were also studied to delimit relationship between ophiolite and island-arc infrastructure and superstructure. Zircon crystals were dated by laser ablation inductively coupled plasma emission spectroscopy. Ages of zircon from Campestre metavolcanoclastic rock are 920-840 (peak 842) Ma, Bossoroca chloritite 900-800 (peak 868 Ma) and Capivaras metadiorite 698 Ma. Ages 920-800 Ma correspond to processes in the oceanic crust, whereas 698 Ma was a late magmatic intrusion (Capivaras metadiorite) into the island-arc infrastructure. Aeromagnetometric and aerogammaspectrometric data delimit the occurrence and structure of the ophiolite. These are major multiproxy markers of geotectonic processes early in the Brasiliano Orogen.【摘要】 将航磁测量（aeromagnetometric survey）与航空伽马能谱测量（aerogammaspectrometric survey）相结合开展年龄厘定工作，深化了对巴西利亚造山带（Brasiliano Orogen）内蛇绿岩（ophiolite）演化的认知。本次研究聚焦于博索罗卡蛇绿岩（Bossoroca ophiolite），因其洋壳与地幔岩中赋存着交代蚀变绿泥岩（metasomatic chloritite）包裹的锆石。此外还对一套变闪长岩（metadiorite）与变火山碎屑岩（metavolcanoclastic rock）开展研究，以厘清蛇绿岩与岛弧基底及盖层之间的成因关联。采用激光剥蚀电感耦合等离子体发射光谱法（laser ablation inductively coupled plasma emission spectroscopy）对锆石晶体进行定年分析，结果显示：Campestre变火山碎屑岩的锆石年龄区间为920~840 Ma（峰值842 Ma），博索罗卡绿泥岩的锆石年龄区间为900~800 Ma（峰值868 Ma），卡皮瓦拉斯变闪长岩的锆石定年结果为698 Ma。其中920~800 Ma的年龄区间对应洋壳形成与演化过程，而698 Ma则代表后期岩浆侵入岛弧基底的事件（即卡皮瓦拉斯变闪长岩的形成时代）。航磁与航空伽马能谱测量数据明确了该蛇绿岩的产出范围与构造特征，上述成果均为巴西利亚造山带早期大地构造过程的关键多代用指标。