摘要
辽西四合屯地区早白垩世义县组玄武岩包含大量具环带结构单斜辉石。本文对这些单斜辉石进行了系统的矿物学、主微量元素和原位Sr同位素分析,并探讨了义县组玄武岩的岩浆成因和演化历史。四合屯义县组玄武岩中的单斜辉石主要发育正环带结构,少量为反环带结构。反环带单斜辉石核部主要为透辉石,并具有低的Mg^(#)(65.5~79.7)和Cr_(2)O_(3)(<0.48%),以及高的Na_(2)O(0.34%~1.33%)和Al_(2)O_(3)(1.02%~3.20%)。它们具有轻稀土元素富集的配分型式,具有明显的Eu和Sr负异常,说明其与斜长石平衡过。在Mg^(#)-Na_(2)O/Al_(2)O_(3)相关图中,反环带单斜辉石核部总体落入华北古老下地壳麻粒岩中单斜辉石的成分范围,因此其可能是来自下地壳麻粒岩的捕虏晶。反环带单斜辉石幔部和正环带单斜辉石核部主要为普通辉石,它们具有高度一致且变化范围相对窄的化学组成,包括高的Mg^(#)(84.7~89.2)和Cr_(2)O_(3)(0.13%~1.41%),以及低的Na_(2)O(0.17%~0.48%)和Al_(2)O_(3)(1.44%~2.57%)。它们的轻重稀土元素强分馏,与玄武岩全岩组成吻合。利用单斜辉石和基性岩浆之间Fe-Mg及微量元素的分配系数,我们计算了与反环带单斜辉石幔部和正环带单斜辉石核部平衡的岩浆组成,结果表明它们与剔除橄榄石捕虏晶的全岩组成达到平衡,因此这部分单斜辉石是岩浆成因的斑晶。它们相比全岩具有变化范围较宽和整体略高的原位^(87)Sr/^(86)Sr比值(0.7058~0.7085)。岩浆成因单斜辉石结晶温压计算结果(1171~1221℃、0.54~1.08GPa)表明义县组玄武岩的岩浆演化主要发生在下地壳浅部(18~36km)。四合屯义县组玄武岩具有壳幔双重属性,捕虏晶矿物的同化混染和下地壳榴辉岩的拆沉作用并不能合理解释其大陆下地壳属性。本次研究中反环带单斜辉石核部捕虏晶具有较高的HREE含量,它们可能作为源区残留相记录了华北东部古老下地壳在中生�
Mineralogical observations and in-situ elemental and Sr isotopic data for zoned clinopyroxenes from Early Cretaceous basalts of the Yixian Formation in Sihetun,western Liaoning Province,shed light on the basaltic magmatic origin and its evolution history.Two populations of clinopyroxene zoning have been recognized in the basalts including the normally-zoned and subordinate reversely-zoned ones.The cores of the reversely-zoned clinopyroxenes are mainly diopsides,with low Mg^(#)values(65.5~79.7)and Cr_(2)O_(3) contents(<0.48%),and high Na_(2)O(0.34%~1.33%)and Al_(2)O_(3)(1.02%~3.20%)contents.Normalized REE patterns of the cores are LREE-enriched and prominently show negative anomalies of Eu and Sr,indicating the clinopyroxenes were once equilibrated with plagioclase.In the plots of Mg^(#)versus Na_(2)O and Al_(2)O_(3),the cores of the reversely-zoned clinopyroxenes generally fall in the composition range of clinopyroxene in the lower crustal granulite from the North China Craton,indicating that the cores are xenocrysts disaggregated from lower crustal granulites.The mantle parts of the reversely-zoned clinopyroxenes and the cores of the normally-zoned clinopyroxenes are mainly augites,which share similar and uniform geochemical compositions,including high Mg^(#)(84.7~89.2)and Cr_(2)O_(3)(0.13%~1.41%),and low Na_(2)O(0.17%~0.48%)and Al_(2)O_(3)(1.44%~2.57%).In addition,their LREE and HREE are highly fractionated,consistent with the REE patterns of the basalts.Using the partition coefficients of Fe-Mg and trace elements between clinopyroxene and basaltic magma,we calculated the melts equilibrated with the mantle parts of the reversely-zoned clinopyroxenes and the cores of the normally-zoned clinopyroxenes.The results reveal that they are magmatic phenocrysts in equilibrium with the basalts excluding olivine xenocrysts.Compared with the basalts,magmatic clinopyroxenes show various and slightly higher ^(87)Sr/^(86)Sr ratios(0.7058~0.7085).Thermobarometer estimations(1171~1221℃,0.54~1.08GPa)suggest that evolution of t
作者
许瑶
张宏福
XU Yao;ZHANG HongFu(State Key Laboratory of Continental Dynamics,Department of Geology,Northwest University,Xi an 710069,China;School of Earth Sciences,Zhejiang University,Hangzhou 310058,China)
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2022年第12期3799-3818,共20页
Acta Petrologica Sinica
基金
国家自然科学基金项目(41688103)资助.
