中亚造山带北山南部下泥盆统火山—沉积地层的岩相、时代及古地理意义

中亚造山带北山南部的火山—沉积地层对探讨古地理演化有重要意义。本文通过北山南部双鹰山地块5条剖面实测,将该区火山—沉积地层分为4种相组合19种岩相,并通过LA-ICP-MS锆石U-Pb测年获得酸性火山岩夹层年龄(418.9±1.2Ma、417.8±1.2Ma、417.5±1.4Ma),证明各剖面均形成于早泥盆世洛赫科夫期(Lochkovian)。双鹰山地块南缘的下泥盆统由共生火山岩相(coherent facies)和原生火山岩相组合(primary volcanic facies)构成,是熔岩溢流、爆发坠落、火山碎屑流和岩浆侵出的产物,代表陆上隆升的普林尼型(Plinian)古火山作用。双鹰山地块中部墩墩山盆地充填序列自下而上由原生火山岩相组合、含火山碎屑沉积岩相组合(volcanogenic sedimentary facies)和正常沉积的河流相组合构成,代表火山活动逐步减弱和水体深度逐步加深的沉积过程。墩墩山盆地火山—沉积层序指示伸展构造背景,与同造山期磨拉石建造有显著区别。

Lithofacies and chronology of volcano- sedimentary sequence in the southern Beishan Region, Central Asian Orogenic Belt and its paleogeographical implication

Volcano- sedimentary sequences play an important role in paleogeographical reconstruction of the southern Beishan Region, Central Asian Orogenic Belt. These volcano- sedimentary sequences can be subdivided into 19 lithofacies of four associations based on dated sections in the Shuangyingshan Block (SYSB), southern Beishan Region. Three LA- ICP- MS zircon U- Pb ages were reported from the felsic volcanic intercalations (418. 9±1. 2 Ma, 417. 8±1. 2 Ma, and 417. 5±1. 4 Ma) indicating an age range of the Lochkovian, Early Devonian for these sections. Coherent and primary volcanic facies distributed on the southern margin of SYSB were interpreted as sediments in lava flows, pyroclastic fall, subaerial pyroclastic flows, and resurgent doming. These facies indicate a subaerial Plinian type volcano. Sequences measured in the Dundunshan Basin, northern SYSB are composed of, in an ascending order, primary volcanic, volcanogenic sedimentary, and fluvial facies, which supports a deepening- upward trend with waning volcanism. These sequences suggest that the Dundunshan basin was developed in tectonic extension rather than syn- collision compression.