Evolution of the Jurassic Comallo volcanic sedimentary complex in the western North Patagonian Massif, Rio Negro province, Argentina

Here we define the Comallo volcanic sedimentary complex of the western North Patagonian Massif. It is composed by two members that include 9 lithofacies and 7 facies associations. The lower member is represented by massive and stratified lapilli tuffs, lithic and tuffaceous breccias, coherent andesite-dacite lava-flows, andesitic breccias, and massive crystalline limestones. The upper member consists of red conglomerates, parallel and cross-stratification sandstones, and mudstones. The lower member is interpreted as having been deposited by dilute and dense pyroclastic currents, block and ash deposits and lava-flow, and hypersaline lacustrine environments. The upper member is considered to have been deposited in fluviatile environments. A new U-Pb age of 192.0 ± 3.0 Ma (Sinemurian) was obtained for this complex. It is coeval with several volcanic sedimentary units that crop out along the western North Patagonian Massif, the Neuquén basin and the Extraandean Chubut in pull-apart or rift depocentres. The movements of the NW-SE strike-slip faults that border the northern and southern limits of the Comallo depocentre made possible the development of a pull-apart basin as well as the volcanism and the fluvial system. This and other coeval units were deposited in similar environmental conditions along the North Patagonian Massif, the Neuquén basin and the extraandean Chubut, indicating a similar tectonomagmatic setting, in a tensional rear-arc during Lower Jurassic times.

本研究界定了北巴塔哥尼亚地块（North Patagonian Massif）西部的科马约（Comallo）火山沉积复合体。该复合体由两个岩性段组成，包含9种岩相（lithofacies）与7种相组合（facies associations）。下段以块状与层状火山砾凝灰岩、岩屑角砾岩、凝灰质角砾岩、致密安山岩-英安岩熔岩流、安山质角砾岩及块状结晶灰岩为代表。上段则由红色砾岩、平行层理与交错层理砂岩以及泥岩构成。研究认为下段形成于稀散火山碎屑流、密集火山碎屑流、块体-火山灰沉积、熔岩流作用以及高盐度湖相环境；上段被认为形成于河流相环境。本次为该复合体测得新的铀-铅（U-Pb）年龄为192.0 ± 3.0 Ma，对应早侏罗世锡内穆阶（Sinemurian）。该复合体与出露于北巴塔哥尼亚地块西部、内乌肯盆地（Neuquén basin）及安第斯外丘布特地区（Extraandean Chubut）拉分或裂谷沉积中心内的多个火山沉积单元时代相当。环绕科马约沉积中心南北边界的北西-南东向走滑断层（strike-slip faults）活动，促成了拉分盆地（pull-apart basin）的形成，同时也为火山活动与河流系统的发育提供了有利条件。该复合体与其他同期单元均形成于早侏罗世（Lower Jurassic）时期北巴塔哥尼亚地块、内乌肯盆地及安第斯外丘布特地区的相似沉积环境中，指示当时区域整体处于张性弧后（tensional rear-arc）环境，具备一致的构造岩浆背景（tectonomagmatic setting）。