Abstract Two compositionally different volcanic cycles are documented by geological mapping, lithofacies and petrographic analyses of Jurassic volcanic rocks in the northeastern sector of the Deseado Massif, constituting the western counterpart of the Karoo-Ferrar LIPs in Patagonia. The first cycle is of intermediate composition and is represented by andesitic lava flows, phenoandesitic moderate-grade ignimbrites and volcaniclastic deposits. These rocks constitute a previously unknown large outcrop (~100 km2) of the Bajo Pobre Formation. The second cycle (Chon Aike Formation) is acidic in composition and constituted by several pyroclastic deposits (ignimbrites and co-ignimbrite lag breccias), lavic bodies (rhyolitic lava domes, dykes, and coulees) and rhyolitic pyroclastic dykes. Structural measurements collected in the studied area suggest an extensional tectonic control on the emplacement of, at least, the Chon Aike Formation rocks. We document two major NE-striking (SE-dipping) normal faults, which constitute direct field evidence of the extensional tectonic control on the Jurassic Chon Aike Silicic LIP in the Deseado Massif at the time when southern Gondwana broke up.

中文翻译：

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Deseado 地块东北部 Chon Aike 硅质唇缘的侏罗纪火山作用

摘要 Deseado 地块东北部侏罗纪火山岩的地质绘图、岩相和岩相分析记录了两种成分不同的火山旋回，这些火山岩构成了巴塔哥尼亚 Karoo-Ferrar LIP 的西部对应物。第一个旋回具有中等成分，以安山岩熔岩流、斑安山岩中品位冰凝灰岩和火山碎屑沉积物为代表。这些岩石构成了 Bajo Pobre 组以前未知的大型露头（约 100 平方公里）。第二个旋回（Chon Aike 组）的组成为酸性，由几个火山碎屑沉积物（火成岩和共火成岩滞后角砾岩）、熔岩体（流纹质熔岩穹顶、岩脉和古沟）和流纹质火山碎屑岩脉构成。在研究区域收集的结构测量表明，至少对 Chon Aike 组岩石的就位存在伸展构造控制。我们记录了两条主要的 NE 走向（SE 倾角）正断层，它们构成了冈瓦纳南部断裂时 Deseado 地块侏罗纪 Chon Aike 硅质 LIP 受到伸展构造控制的直接现场证据。