华北燕山带:构造、埃达克质岩浆活动与地壳演化(英文)

THE YANSHAN BELT OF NORTH CHINA: TECTONICS, ADAKITIC MAGMATISM, AND CRUSTAL EVOLUTION

埃达克质火成岩在中国东部,包括燕山带是很常见的,一般认为它们是下地壳不均匀的镁铁质岩石及/或富集的上地幔岩石在高压(≥1.5 GPa)下部分熔融的结果。在燕山带内埃达克岩浆的形成有一个很长的时间(约190～80 Ma),然而岩浆活动的峰期却与约170～130 Ma间有基底岩石卷入的陆壳收缩期相一致。尽管埃达克质岩浆活动的历史很长,但那种把岩浆活动与岩石圈的拆沉效应相联系的模式似乎是不适当的。在该带内,埃达克质与非埃达克质岩浆活动有一部分是同时的,而且在地理分布上也是相间的,这说明了在下地壳和上地幔岩石的部分熔融中成分是相当不均匀的。侏罗纪及白垩纪熔融作用的热源应当是与古太平洋板块俯冲相关的中生代板底垫托的玄武岩浆。除了局部例外,在燕山带,埃达克质岩浆活动的终结和碱性岩浆活动的开始约在130～120 Ma,在此时期收缩作用使东亚大达200万km^2以上的地区发生了NW—SE向的区域性伸展作用。强烈的地壳伸展仅局限于华北克拉通北缘分布的少数几个变质核杂岩中。陆壳的伸展减薄合理地解释了130～120 Ma间发生高压埃达克质熔融条件的终结,尽管还有局部年轻的埃达克火山活动(约120～80Ma)可以在伸展规模有限而厚的地壳依然存在的地区继续出现。燕山区早白垩世的碱性侵入体中的锆石不存在前寒武纪?

Adakitic igneous rocks are common in eastern China, including the Yanshan belt. They are widely interpreted as the consequence of high pressure (≥1. 5 GPa) partial melting of heterogeneous mafic lower crustal rocks and/or enriched upper mantle. Adakitic magmas were generated within the Yanshan belt over an extended period (ca. 190-80 Ma), although the peak of such magmatism coincided with basement-involved crustal contraction from ca 170-130 Ma. Given the long history of adakit-ic magmatism, scenarios relating such magmatism to the effects of lithospheric delamination seem to be inadequate. Adakitic and non-adakitic magmatism in the belt were in part contemporaneous and inter- spersed geographically. This argues for considerable compositional heterogeneity in lower crustal and upper mantle sources for partial melting. The most likely heat source for Jurassic and Cretaceous melting is judged to be underplated Mesozoic basaltic magmas related to subduction of a paleo-Pacific oceanic plate. With localized exceptions, adakitic magmatism in the Yanshan belt ended and alkalic magmatism began at about the time (ca. 130-120 Ma) when contraction was superceded by regional NW to SE extension in eastern Asia over more than two million square kilometers. Profound crustal extension was limited to a small number of metamorphic core complexes scattered across the northern margin of the North China craton. Extensional thinning of the continental crust can reasonably explain the general termination of high pressure adakitic melt conditions between 130 and 120 Ma, although localized younger adakitic volcanism (ca. 120-80 Ma) may have continued in areas where extension was limited and thick crust survived. Early Cretaceous alkalic plutons in the Yanshan belt show no zircon evidence for crustal assimilation of Precambrian rocks (unlike both Late Jurassic adakitic and non-adakitic plutons), an observation that is compatible with rapid ascent of enriched, mantle-generated melts through a thinned o-verlying crust.