CAJ | 학술논문

北京西山地区出露的早白垩世阳坊岩体主要由二长花岗岩组成,含少量石英二长岩、白岗岩以及闪长质包体.本文首次报道了阳坊岩体的主量元素和微量元素高精度分析测试结果.黑云母二长花岗岩是典型的高钾钙碱性花岗岩;具有富集Rb、Ba、LREE等强不相容元素,Th、U、Nb、Ta相对LREE亏损,负Eu异常较弱的元素地球化学特征.石英二长岩具有高钾、相对高碱、富镁贫铁、富集Rb、Ba、LREE、Sr等强不相容元素,Sr/Y比值高,Th、U、Nb、Ta相对LREE亏损,Eu异常不明显的特点,具有类似于高Sr低Y型中酸性火成岩(adakite)的元素地球化学特征.白岗岩具有明显亏损Sr、Ba、REE尤其是MREE,具明显负Eu异常的地球化学特征.闪长质包体MgO含量和g#值较高,具有富集K、Rb、Ba、LREE、Sr等强不相容元素,Sr/Y比值高,Th、U、Nb、Ta相对LREE亏损,无Eu异常的特点,与玻基方辉安山岩的地球化学特征相近似,属于典型的钾玄岩系列岩石.阳坊岩体的闪长质包体起源于交代岩石圈地幔的部分熔融,石英二长岩是幔源岩浆与下地壳岩石相互作用的产物,黑云母二长花岗岩形成于下地壳富钾基性岩的部分熔融过程,而白岗岩是上地壳岩石部分熔融的产物;表明燕山西段在早白垩世晚期具有高地温梯度.地质证据和岩石化学、微量元素判别图解均显示阳坊岩体形成于造山带崩塌阶段.
북경서산지구출로적조백성세양방암체주요유이장화강암조성,함소량석영이장암、백강암이급섬장질포체.본문수차보도료양방암체적주량원소화미량원소고정도분석측시결과.흑운모이장화강암시전형적고갑개감성화강암;구유부집Rb、Ba、LREE등강불상용원소,Th、U、Nb、Ta상대LREE우손,부Eu이상교약적원소지구화학특정.석영이장암구유고갑、상대고감、부미빈철、부집Rb、Ba、LREE、Sr등강불상용원소,Sr/Y비치고,Th、U、Nb、Ta상대LREE우손,Eu이상불명현적특점,구유유사우고Sr저Y형중산성화성암(adakite)적원소지구화학특정.백강암구유명현우손Sr、Ba、REE우기시MREE,구명현부Eu이상적지구화학특정.섬장질포체MgO함량화g#치교고,구유부집K、Rb、Ba、LREE、Sr등강불상용원소,Sr/Y비치고,Th、U、Nb、Ta상대LREE우손,무Eu이상적특점,여파기방휘안산암적지구화학특정상근사,속우전형적갑현암계렬암석.양방암체적섬장질포체기원우교대암석권지만적부분용융,석영이장암시만원암장여하지각암석상호작용적산물,흑운모이장화강암형성우하지각부갑기성암적부분용융과정,이백강암시상지각암석부분용융적산물;표명연산서단재조백성세만기구유고지온제도.지질증거화암석화학、미량원소판별도해균현시양방암체형성우조산대붕탑계단.
The Yangfang Pluton exposed in Western Hills of Beijing is dominantly composed of biotite monzogranite, and much less quartz monzonite, alaskite and dioritic enclaves. The high-precise analysis of major and trace elements for these rocks are firstly reported by this study. The biotite monzogranite is a typical high-potassic calc-alkaline granite, and has the characteristics of Rb, Ba, and LREE enrichment, depletion of Th, U, Nb, Ta relative to LREE, and weak negative Eu anomaly. The quartz monzodiorite is high potassic and total alkali, magnesium enrichment and iron poor characteristics, and exhibits enrichment of Rb, Ba, LREE and Sr, as well as high Sr/Y ratio, but depletion of Th, U, Nb, Ta relative to LREE, and no siginificant Eu anomaly. So the quartz monzodiorite is similar to adakite (high Sr low Y type intermediate and acid igneous rocks) in geochemistry. The alaskite has obvious depletion of Sr, Ba, and REE (especially MREE), and negative Eu anomaly. The dioritic enclave is high in magnesium content, Mg# value and Sr/Y ratio, and enriches K, Rb, Ba, LREE, and Sr, but deplets Th, U, Nb, Ta relative to LREE. It does not show any Eu anomaly. The geochemistry of diorite is the same as that of sanukite, and belongs to typical shoshonitic series. The diorite of the Yangfang Pluton was originated by partial melting of metasomatic lithospheric mantle; and the quartz monzonite is the product of mantle-derived magma and lower crust interaction. Meanwhile, the biotite monzogranite and alaskite came from the partial melting of K-rich mafic rocks in the lower crust and upper crustal rocks, respectively. These petrogenetic models imply a very high thermal gradient underneath the western segment of Yanshan orogenic belt during the late stage of Early Creataceous. Geological evidence and wholerock chemistry as well as geochemical discrimination diagrams indicate that the Yangfang Pluton formed during the collapse stage of orogenic evolution.