燕辽地区燕山期火成岩与造山模型

YANSHANIAN IGNEOUS ROCKS AND OROGENY MODEL IN YANSHAN-LIAONING AREA

通过与安第斯、青藏北缘、大陆裂谷带火成岩的比较，阐述了燕辽地区燕山期火成岩具活动大陆边缘靠内陆一侧的构造属性。提出了三种可能的母岩浆（玄武质、粗面质与花岗质）以及它们的混合作用是制约以壳幔混合型为主、组成谱系宽的火成岩的主要机制。基于实验岩石学成果，论述了无负Ｅｕ异常的中酸性火成岩类（正长岩、二长岩、石英闪长岩类）形成于加厚陆壳底部（或山根带）。主要基于岩石学成果，讨论了燕山期本区陆壳厚约６０～７０ｋｍ，岩石圈厚约１００～１５０ｋｍ。通过与印支期岩石圈（厚约１５０～２００ｋｍ）的对比，提出了造山岩石圈的拆沉－去根作用，使岩石圈减薄了约５０ｋｍ。由此，提出了一个大洋俯冲与岩石圈拆沉相结合的造山模型，称为华北式（或燕辽式）造山带模型。这一模型不但可以满意地解释为什么弧火成岩属性的岩浆活动可深入远离海沟达一千多公里的内陆地区以及挤压与拉伸交替的反转构造的发育，而且还可以比较满意地解释为什么火成岩组成极性极不明显，伴随岩浆活动的陆壳不断抬升等，并指出燕山期地幔岩石圈减薄，山根仍存在，所以造山后Ａ型花岗岩（指碱性正长岩类）仍保持无负Ｅｕ异常，而本区新生代处于大陆裂谷发育环境，地幔岩石圈与陆壳均减薄。

ompared with Andes,the north margin of Qinghai-Tibet and other continental rifting zones,igneous rocks of the Yanshan are believed to belong to the inland side of active continental marginal arc. Three possible parental magmas (basaltic, trachytic and granitic) are inferred in this paper.A magma mixture is regarded as major machanism for the formation of the igneous rocks signified by wider compositional spectrum and crust-mantle mixing sources.Based on the data from experimental petrology, the acid-intermediate igneous rocks (synitic,monzonitic, QY-dioritic) without negative Eu-anomaly seem to have formed at the base of thickened crust or in the mountain root zone. Both the crust and the lithosphere of the working area in the Yanshanian stage are estimated to be about 60-70 km and 100-150 km in ness, respectively. In comparison with the Indosinian lithosphere of about 150-200 km thickness, the Yanshanian lithosphere is thought to have thinned down to about 50 km due to the delamination - derooting of the orogenic lithosphere. Therefore, an orogenic model,termed the North China (or Yanshan-Liaoning) type, has been put forward, describing orogenic distinctions of oceanic subduction in combination with the lithosphere delamination.This model appears to be satisfactory in explaination of these phenomena,such as arc-type igneous activities in inland side far from the trench for a considerable distance more than 1000 km, the development of inversion tectonics signified by alternating tension and compression, the absence of igneous compositional polarity, and the regional uplift accompanied with magmatism. It is shown that no negative Eu-anomaly in the alkaline syenite of post-orogenic A-type granite remains owing to mantle lithosphere thinning and the mountain root existing in the Yanshanian period. It is considered that both the mantle lithosphere and the crust in the Cenozoic continental rifting became thinned.