Anatectic reworking and differentiation of continental crust along the active margin of Gondwana: a zircon Hf–O perspective from West Antarctica

The Fosdick migmatite–granite complex of West Antarctica preserves evidence of two crustal differentiation events along a segment of the former active margin of Gondwana, one in the Devonian–Carboniferous and another in the Cretaceous. The Hf–O isotope composition of zircons from Devonian–Carboniferous granites is explained by mixing of material from two crustal sources represented by the high-grade metamorphosed equivalents of a Lower Palaeozoic turbidite sequence and a Devonian calc-alkaline plutonic suite, consistent with an interpretation that the Devonian–Carboniferous granites record crustal reworking without input from a more juvenile source. The Hf–O isotope composition of zircons from Cretaceous granites reflects those same two sources, together with a contribution from a more juvenile source that is most evident in the detachment-hosted, youngest granites. The relatively non-radiogenic εHf isotope characteristics of zircons from the Fosdick complex granites are similar those from the Permo-Triassic granites from the Antarctic Peninsula. However, the Fosdick complex granites contrast with coeval granites in other localities along and across the former active margin of Gondwana, including the Tasmanides of Australia and the Western Province of New Zealand, where the wider range of more radiogenic εHf values of zircon suggests that crustal growth through the addition of juvenile material plays a larger role in granite genesis. These new results highlight prominent arc-parallel and arc-normal variations in the mechanisms and timing of crustal reworking v. crustal growth along the former active margin of Gondwana.

南极西部福斯迪克混合岩-花岗岩杂岩（Fosdick migmatite–granite complex）保存了冈瓦纳古大陆古活动陆缘段上两期地壳分异作用的证据：一期发生于泥盆纪-石炭纪，另一期则为白垩纪。泥盆纪-石炭纪花岗岩中的锆石（zircon）Hf-O同位素组成，可通过两种地壳源区物质的混合得到解释——这两种源区分别为下古生代浊积岩序列的高级变质等效岩类，以及泥盆纪钙碱性侵入岩套，该结果支持“泥盆纪-石炭纪花岗岩记录了地壳再造过程，未受新生源物质加入影响”的解释。白垩纪花岗岩中的锆石Hf-O同位素组成则反映了上述两种源区，外加一种新生源物质的贡献，该贡献在拆离带赋存的最年轻花岗岩中表现最为显著。福斯迪克杂岩花岗岩中的锆石具有相对非放射的εHf同位素（εHf isotope）特征，与南极半岛二叠纪-三叠纪花岗岩的锆石εHf特征相似。然而，福斯迪克杂岩花岗岩与冈瓦纳古活动陆缘沿线及跨陆缘其他地区的同期花岗岩存在显著差异，包括澳大利亚塔斯曼造山带（Tasmanides）与新西兰西部省：在这些区域，锆石εHf值的分布范围更广且更具放射特征，表明通过新生物质添加实现的地壳生长在花岗岩成因中发挥了更重要的作用。本研究的新结果揭示，沿冈瓦纳古活动陆缘，地壳再造与地壳生长的机制及时间尺度存在显著的弧平行与弧垂直方向的变化。