摘要
北京裂谷北缘的赤城－古北口深断裂，是硅铝质（石英正长岩、富钾／环斑花岗岩）与铁镁质（斜长岩、辉长岩、辉绿岩）的双峰式深成岩带。Ｕ／Ｐｂ一致性年龄为１６９７±０．９Ｍａ，其岩石组合可与河北大庙斜长（辉长）岩－二长岩杂岩体类比，但铁镁质岩石较少。主体硅铝质岩石的富钾、富铁暗色矿物及钾质Ａ型花岗岩地球化学均与世界各地及密云沙厂环斑花岗岩一致。含铁橄榄石石英正长岩，代表了岩浆分离结晶初期还原条件下，太古代母岩派生的初始岩浆组成。该岩带的确定是将裂谷不同构造部位的四条东西向岩带与裂谷作用联系的关键。伴随裂谷作用拉张断裂向南呈台阶状断陷下沉，由隆起区向断陷区顺序形成了深成至浅成侵入，以至火山喷发的非造山岩浆活动。
Along the Chicheng-Gubeikou deep fault of the north boundary of Mid-Proterozoic(1.85-1.60 Ga)Beijing rift,is a plutonic belt made up of bimodal rock association(quartz syenite/K-rich granite/rapakivi granite and anorthosite/gabbro/diabase).Previously these rocks were assigned to Archean migmatites or Mesozoic intrusions,but the U/Pb concordant age of quartz syenite(1697.3±0.9 Ma) proves that they are Mid-Proterozoic age.The mafic rocks are comparable with the Damiao anorthosite(norite)mangerite complex,but the pro- portion is decreased to only 10%.The K-rich magmatic and Fe(Ti)-riched mafic minerals features and potassic A-type WPG granitic geochemistry of granitic rocks are similar to the rapakivi granite in Shachang and elsewhere.Fayalite-bearing quartz syenite could represent the early fractional stage initial granitic magma derived from Archean protolith in a deeper and reductive environment.This rock belt serves as a key linking the abyssal Damiao complex(1735 Ma)in the north with the hypabyssal Shachang rapakivi granite massif(1665 Ma)and Pinggu -Jixian potassic alkaline basaltic and phonolitic volcanites(625 Ma)in the south making up four par- allel rock belts.They represent different successive levels of emplacement which occurred from north to south in response their tectonic setting along the rift and extensional faulting.
基金
IGCP315项及国家自然科学基金
关键词
环斑花岗岩
岩套
深成岩
裂谷作用
rapakivi suite,Chicheng-Gubeikou plutonic rock belt,Proterozoic rifting
