Abstract In ancient volcano-sedimentary successions, the reconstruction of the facies architecture may be difficult because of incomplete exposure, deformation, metamorphism and hydrothermal alteration. The distinction of lavas or domes from shallow intrusions is critical for the ordering of volcanic, intrusive, sedimentary and hydrothermal events. Both lavas and shallow intrusions typically have porphyritic textures and may have breccia along their top contacts. Detailed study of these breccias and their contact relationships is a reliable means of distinguishing lavas or domes from shallow intrusions. In this contribution, we describe rhyolites in an Early Paleozoic, submarine, volcano-sedimentary succession in northwestern Argentina, known as the Chiquero Formation, formed in a rapidly subsiding backarc setting. This succession is mainly composed of turbidite interbedded with porphyritic rhyolite-dacite units. Many of the rhyolites have breccias along their upper contacts. Rhyolite clasts in the breccias match the underlying rhyolites in terms of phenocryst populations and texture. The main breccia facies along upper contacts are monomictic rhyolite breccia, mudstone-matrix rhyolite breccia and siliceous mudstone-matrix rhyolite breccia. The monomictic rhyolite breccia is interpreted to be in situ hyaloclastite formed by quench fragmentation of the tops of rhyolite lavas extruded onto the sea floor. The mudstone-matrix rhyolite breccia and the siliceous mudstone-matrix rhyolite breccia are also interpreted as hyaloclastite but the spaces between the rhyolite clasts have been infilled by the locally available sediment and hydrothermal silica, respectively. The mudstone-matrix rhyolite breccia and the siliceous mudstone-matrix rhyolite breccia superficially resemble peperite in being composed of igneous and sedimentary components. However, the infilling sediment is laminated and the laminae are conformable with bedding in the overlying turbidite sequence, implying that the sediment passively infilled pore spaces in pre-existing rhyolite breccia. Hence, the studied rhyolites of the Chiquero Formation were submarine lavas emplaced on the sea floor. Those lavas were probably emplaced during the Middle Ordovician based on correlation with a dated rhyolite interpreted to be syn-sedimentary. However, at some localities, the turbidite sequence contains Tremadocian/Arenigian (Early Ordovician) fossils and a discrepancy exists between the isotopic age of the rhyolites and the age of the host sedimentary sequence based on fossils. It is likely that the Chiquero Formation spans a wider age range than previously thought, from Early Ordovician to Late Ordovician, and that at least four episodes of rhyolitic-dacitic magmatism contributed to this backarc succession.

中文翻译：

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在早古生代海底弧后序列中区分熔岩和侵入体，普纳，阿根廷

摘要 在古火山-沉积序列中，由于暴露不完全、变形、变质和热液蚀变，相结构的重建可能很困难。熔岩或圆顶与浅层侵入体的区别对于火山、侵入、沉积和热液事件的排序至关重要。熔岩和浅侵入体通常都具有斑状结构，并且沿着它们的顶部接触可能有角砾岩。对这些角砾岩及其接触关系的详细研究是区分熔岩或圆顶与浅层侵入体的可靠方法。在这篇文章中，我们描述了阿根廷西北部早古生代海底火山沉积序列中的流纹岩，称为 Chiquero 组，形成于快速下沉的弧后环境。该层序主要由混有斑状流纹岩-英安岩单元的浊积岩组成。许多流纹岩沿其上部接触处具有角砾岩。角砾岩中的流纹岩碎屑在斑晶群和质地方面与下面的流纹岩相匹配。沿上部接触面的角砾岩相主要为单晶流纹岩角砾岩、泥岩-基质流纹岩角砾岩和硅质泥岩-基质流纹岩角砾岩。单晶流纹岩角砾岩被解释为由挤压到海底的流纹岩熔岩顶部淬火破碎形成的原位透明碎屑岩。泥岩-基质流纹岩角砾岩和硅质泥岩-基质流纹岩角砾岩也被解释为透明碎屑岩，但流纹岩碎屑之间的空间分别被当地可用的沉积物和热液二氧化硅填充。泥岩-基体流纹岩角砾岩和硅质泥岩-基体流纹岩角砾岩在由火成岩和沉积成分组成的表面上类似于透皮岩。然而，填充沉积物是层状的，层状结构与上覆浊积岩层序中的层理一致，这意味着沉积物被动地填充了先前存在的流纹岩角砾岩中的孔隙空间。因此，研究的 Chiquero 组流纹岩是位于海底的海底熔岩。这些熔岩很可能是在中奥陶世期间形成的，这是基于与被解释为同沉积的过时流纹岩的相关性。然而，在一些地方，浊积岩层序包含 Tremadocian/Arenigian（早奥陶世）化石，流纹岩的同位素年龄与基于化石的宿主沉积层序的年龄存在差异。从早奥陶世到晚奥陶世，奇克罗组跨越的年龄范围可能比以前认为的更广泛，并且至少有四次流纹质-英安质岩浆作用促成了这种弧后演替。