湘东北幕阜山含绿帘石花岗闪长岩岩浆的上升速率：岩相学和矿物化学证据

A Magma Ascent Rate of Epidote-Bearing Granodioritic Magma in the Mufushan Complex Batholith of NE Hunan Province:Evidence from Petrography and Mineral Chemistry

出露于扬子地块东南缘的幕阜山复式岩体侵位年龄约为145Ma。该复式岩体的花岗闪长岩及其暗色微粒包体都含有独特的岩浆绿帘石。这些绿帘石呈自形晶,普遍发育熔蚀结构,部分具褐帘石核。微粒包体及花岗闪长岩中绿帘石的Ps值((100Fe3+/(Fe3++Al))均集中在26～28,TiO2含量小于0.2%,为典型的岩浆成因。岩相学的观察显示,主岩与包体中角闪石与绿帘石为同期产物,其后依次结晶的是黑云母、斜长石等矿物。利用角闪石全铝压力计,获得的花岗闪长岩主岩和包体的结晶压力大约为4.05kbar。黑云母矿物化学资料表明,花岗闪长岩和微粒包体的黑云母形成时氧逸度在NNO缓冲附近,结合实验岩石学成果,这一条件下绿帘石与斜长石共存的压力值应不低于8.5kbar。由于岩浆绿帘石的熔解时间与熔蚀宽度具有一定的函数关系,通过统计岩浆绿帘石的最大熔蚀宽度,本文估算幕阜山花岗闪长岩岩浆的上升速率超过170m/year,暗示岩体可能以裂隙的方式上升。根据花岗闪长岩体条带状分布的特征,可反映岩浆上升时该区正处于北东向拉张的构造环境中。

Exposed in the southeastern margin of Yangtze Block,Mufushan complex batholith has an emplacement age of 145 Ma（Zircon U-Pb）.Granodiorite and its mafic microgranular enclaves from the batholith contain characteristic monocrystalline epidotes.Most of these epidotes are idiomorphic,featured frequently with melting corrosion structure and occasionally with allanite-rich cores.The Ps values（（100Fe3＋/（Fe3＋＋Al））of both kinds of epidotes mainly fall between 26 and 28,while TiO2 contents of these epidotes are less than 0.2%.It is suggested that these epidotes are magmatic genesis.Results from petrographic study suggest that the crystallization order in both granodiorite and mafic microgranular enclaves begins with amphibole and epidote,and then comes biotite,plagioclase and other minerals.Mineral assemblage and the compositional analysis of calcic amphibolites from granodiorite and mafic microgranular enclaves show that these calcic amphiboles are suitable for the Anderson and Smith＇s Al-in-hornblende barometer,and the barometer gives a crystallization pressure of about 4.05 kbar for the host granodiorite.Mineral Chemistry shows that biotites in the host granodiorite and mafic microgranular enclaves are formed around the NNO buffer,meaning the coexisting pressure of epidote and plagioclase is higher than 8.5 kbar.Since the melting time of magmatic epidote is related to its dissolution width,we calculated the dissolution width of these epidotes,and based on maximum dissolution width,the minimum ascent rate of the Mufushan granodiorite is 170m/year,implying a quick ascent along fissures.Taken the stripped distribution of the granodiorite into consideration,a northeast tensional tectonic environment is suggested during the emplacement of the magma.