新疆博格达东缘色皮口一带辉绿岩地球化学、锆石U-Pb年龄及地质意义

摘要

博格达造山带内出露的晚石炭世辉绿岩，为研究博格达裂谷演化末期地球动力学背景提供了重要信息。色皮口地区辉绿岩主量元素以低TiO2、较高Al2O3、较低MgO、贫P2O5，较低的K2O/Na2O比值（0.12~0.53），ΣREE较高，LREE/HREE为2.45~3.56，铕负异常不明显(δEu=0.82~1.02)等为特征。与原始地幔相比，其不相容元素K，Rb， U，Ba富集，高场强元素Nb，Ta，Zr，Hf无富集，Ti亏损不明显，Nb，Ta，Th表现为明显负异常。U富集可能指示与地壳物质的加入有关，较低的Nb/Zr比值(0.02~0.06)，暗示岩浆源区可能为受地壳混染的亏损地幔。辉绿岩LA-ICP-MS锆石U-Pb定年结果为(300.5±1.7) Ma（MSWD=2.3，Th/U比值为0.36~1.3），为晚石炭世晚期，代表了博格达裂谷闭合后地球动力学环境由挤压变为拉张的转折期。%The Late Carboniferous Diabase in bogda orogenic belt provides important information on the bogda rift tec-tonic evolution.The geochemistry of the major is characterized by low contents of TiO2(0.81%~2.1%),MgO(5.83%~10.48%), K2O and P2O5, high Al2O3 and low K2O/Na2O ration.Meanwhile,it is showing that the diabase is enrichment in light rare earth elements with abundance rare earth elements,LREE/HREE ration contents between 2.45 and 3.56,the dia-base exhibits slightly negative Eu anomalies (δEu=0.82~1.02).Comparing with the primitive mantle,they are enriched in the large-iron-lithophile elements(such as K,Rb,U,Ba ) and depletion of high-field-strength element(such as Nb and Ta) and Th. U enrichment may be indicating contamination by crustal materials,lower Nb/Zr ration (0.02~0.06).The above characteristics indicate that magma source are originated from depleted mantle of contamination by crustal materi-als.Using LA-ICP-MS method of U-Pb age dating,it is the 300.5±1.7Ma(MSWD=2.3,Th/U=0.36~1.33)that represents the crystallized age of the diabase.It represents the important conversion from compression to extension in Geodynamics.