中国东北地区中生代构造体制的转变:来自火山岩时空分布与岩石组合的制约

东北地区中生代经历了蒙古-鄂霍茨克构造体系向太平洋构造体系的转换,形成了不同期次火山活动。本文归纳总结了露头区与覆盖区中生代火山岩的年代学、空间分布、岩石组合以及地球化学特征,揭示了两个构造域的时空分布范围。该区火山岩锆石U-Pb年龄统计结果表明中生代存在五期火山活动:早-中侏罗世(190~160Ma)、晚侏罗世(160~145Ma)、早白垩世早期(145~120Ma)、早白垩世晚期(120~100Ma)、晚白垩世早期(100~90Ma)。早-中侏罗世火山岩分布较少,火山岩仅分布在大兴安岭西部满洲里地区和东部张广才岭以及南侧辽宁北票-朝阳地区,火山岩属于高钾钙碱性系列,为蒙古-鄂霍茨克海闭合和法拉隆板块双俯冲作用的产物。晚侏罗世东北地区火山活动明显增强,主要分布在大兴安岭地区,张广才岭以及小兴安岭也有少量分布。西部大兴安岭地区以粗面安山岩、粗面岩为主,属于同碰撞造山成因,为蒙古-鄂霍茨克海闭合造山环境产物。东部以中酸性、酸性岩为主,为法拉隆板块背离欧亚大陆,岩石圈伸展引起的壳源物质熔融产物。早白垩世早期火山活动最为强烈,火山岩主要分布在大兴安岭地区。岩性以高钾钙碱性系列的粗面玄武安山岩、粗面安山岩、安山岩、粗面岩为主,为蒙古-鄂霍茨克海闭合造山后伸展环境产物。早白垩世晚期火山岩主要分布在松辽盆地内部。火山岩以中酸性岩为主,属于中钾-高钾钙碱性系列,为伊泽奈崎板块俯冲引起的弧后拉张,软流圈上涌导致年轻地壳熔融的产物。晚白垩世早期火山岩仅分布在小兴安岭及吉林、黑龙江省东部地区。火山岩为一套玄武岩、玄武安山岩、安山岩和英安岩组合,属于中钾钙碱性系列,是伊泽奈崎-库拉板块高角度俯冲的大陆边缘岩浆活动产物。东北地区中生代不同期次火山岩记录了蒙古-鄂霍茨克构造域向太平洋构造域转换过程及其时空影响范围。

Mesozoic tectonic regimes transition in the Northeast China: Constriants from temporal-spatial distribution and associations of volcanic rocks

In the Mesozoic, the Northeast (NE) China experienced transformation from Mongol-Okhotsk (MO) to circum-Pacific tectonic regime, forming different temporal volcanic activities. This paper summarizes geochoronology, spatial-temporal variations, association and geochemistry, with the aim to constrain evolution of those two regimes. Together with previous information, U-Pb age data indicate that Mesozoic volcanism in NE China can be subdivided into five episodes: Early-Middle Jurassic (190~160Ma), Late Jurassic (160~145Ma), early Early Cretaceous (145~120Ma), late Early Cretaceous (120~100Ma), and early Late Cretaceous (100~90Ma). The Early-Middle Jurassic volcanic rocks, occurring not only in the Lesser Xing'an-Zhangguangcai Ranges and western Liaoning Province, but also in the Manzhouli region, belong chemically to the high-k calc-alkaline series, implying the double subduction of MO oceanic plate and Farallon plate. Volcanic rocks of Late Jurassic mainly occur in the Great Xing'an Range (GXR), and a little in the Lesser Xing'an Range (LXR) and Zhangguangcai Range. The volcanic rocks in GXR which are trachyandesite and trachyte imply syn-collisional setting, and they were due to collisional orogeny accompanied the closure of MO Bay, and those in LXR and Zhangguangcai Range are intermediate-silicic rocks, which are formed under an extensional environment related to the deviation of Farallon plate. The most widespread volcanism in NE China was during in the early Early Cretaceous. Volcanic rocks largely distributed in GXR and around Songliao Basin are trachy-basaltic andesite, trachyandesite, andesite, trachyte and so on, which belong chemically to high-K calc-alkaline series and post-orogenic tectonic setting. The rocks formed during delamination of a thickened continental crust related to the post-orogenic diffuse extension due to the closure of MO suture bay. In the late Early Cretaceous, volcanism in GXR gradually decreased, whereas those in Songliao Basin increased. The volcanic rocks belong chemically to high-K calc-alkaline series and within plate setting, which formed under an extensional environment related to a backarc setting accompanied a low angle subduction of the Kula-Izanagi plate underneath the Eurasian continent. In the early Late Cretaceous, volcanic rocks were limited to the LXR and eastern Jilin-Heilongjiang provinces, and they are consist of medium-K calc-alkaline rocks such as basalt, basaltic andesite, andesite and dacite, implying active continental margin setting. Volcanism in early Late Cretaceous was affected by right-lateral strike-slip motions in continental margin in process of the oblique subduction of the Kula-Izanagi plate. Taking all this into account, we can conclude that the Mesozoic volcanism in Northeast China was both controlled by the closure of MO Bay and subduction of Paleo-Pacific plate. Mesozoic volcanic rocks were tectonic overprint that MO regime transferred to the circum-Pacific regime.