西藏仲巴微地体中新世马莜木富钾埃达克质岩及其内暗色微粒包体的成因

Petrogenesis of the Miocene Mayum K-rich adakitic rocks and their dark microgranular enclaves from the Zhongba micro-terrane,Tibet

西藏南部的中新世含暗色微粒包体的富钾埃达克质岩被认为是之前的大陆地壳在石榴子石稳定压力下发生部分熔融的熔体与同时期的幔源超钾质火山岩发生岩浆混合或混杂的产物。为检验这一观点,本文对雅鲁藏布江缝合带西段仲巴微地体内的中新世马莜木富钾埃达克质岩及其内的暗色微粒包体开展了岩石学、锆石U-Pb定年、矿物电子探针分析以及全岩主微量元素和Sr-Nd同位素测试等方面的工作。结果显示暗色微粒包体和富钾埃达克质岩具有相似的结晶年龄(~17.5 Ma)、矿物组合(石英+长石+角闪石+黑云母)和矿物成分,但暗色微粒包体比富钾埃达克质岩含更多的角闪石和黑云母,且具有比富钾埃达克质岩低的K_(2)O和SiO_(2)以及高的MgO、TiO_(2)和P_(2)O_(5)含量。除此之外,富钾埃达克质岩和暗色微粒包体还表现出相似的全岩Sr-Nd同位素组成:富钾埃达克质岩的(87Sr/86Sr)i和εNd(t)值分别为0.70933~0.70950和-8.17~-6.72;暗色微粒包体的(87Sr/86Sr)i和εNd(t)值分别为0.70933~0.70941和-7.55~-6.57。这些特征表明,暗色微粒包体不是寄主富钾埃达克质岩与同时期具有更高K_(2)O含量的超钾质火山岩发生混合或混杂的产物,而可能是与寄主富钾埃达克质岩相关的细粒堆晶岩或同源包体。结合区域地质资料,本文认为马莜木富钾埃达克质岩是早期地壳内的富钾组分在石榴子石稳定压力下发生部分熔融的结果,并未与同时期的超钾质火山岩发生过岩浆混合或混杂。

The Miocene K-rich adakitic rocks(KARs)containing dark microgranular enclaves(DMEs)in southern Tibet have been thought to be products of magma mixing/mingling between melts derived from partial melting of pre-existing continental crust in the stability field of garnet and the coeval mantle-derived ultrapotassic volcanic rocks.To test this hypothesis,we conduct an integrated study of petrology,zircon U-Pb dating,mineral geochemistry,and whole-rock elemental and Sr-Nd isotopic geochemistry on the Miocene Mayum KARs and their DMEs from the Zhongba micro-terrane in the western segment of the Yarlung Zangbo suture zone.The results show that the KARs and their DMEs have similar emplacement ages(~17.5 Ma),mineral assemblages(quartz+feldspar+amphibole+biotite)and mineral compositions,but the DMEs contain more amphibole and biotite,and have lower K_(2)O and SiO_(2),and higher MgO,TiO_(2)and P_(2)O_(5)contents than the KARs.In addition,the KARs and their DMEs have similar whole-rock Sr-Nd isotopic compositions:the KARs have(~(87)Sr/~(86)Sr)_i of 0.70933~0.70950 andε_(Nd)(t)of-8.17~-6.72;the DME_S have(~(87)Sr/~(86)Sr)_i of 0.70933~0.70941 andε_(Nd)(t)of-7.55~-6.57.These features indicate that the DMEs are most likely fine-grained cumulates or autholiths related to the host KARs,rather than mixtures formed by magma mixing/mingling of the host KARs and the coeval ultrapotassic volcanic rocks with higher K_(2)O contents.Combined with regional geological data,we conclude that the Mayum KARs resulted from partial melting of earlier crustal components that are rich in K_(2)O in the stability field of garnet,and did not mix or mingle with the coeval ultrapotassic volcanic rocks.