江西东乡—相山中生代火山岩中富铝矿物的发现和成因意义

THE DISCOVERY AND GENETIC SIGNIFICANCE OF AL-RICH MINERALS IN MESOZOIC VOLCANIC ROCKS OF DONGXIANG—XIANGSHAN,JIANGXI PROVINCE

本文讨论在江西东乡上侏罗统虎岩组凝灰岩和相山鹅湖岭组碎斑熔岩中新发现的石榴石、红柱石等富铝硅酸盐矿物。石榴石呈自形的菱形十二面体,常包裹含钛磁铁矿、针状磷灰石等矿物包裹体。电子探针分析表明,东乡产出的石榴石为铁铝榴石,相山为钙质铁铝榴石。根据石榴石—熔体成分计算石榴石形成时温度为1060℃,压力为6×10~8Pa。二长温度计计算长石形成温度为785℃。从而证实石榴石为岩浆成因,厘定了本区火山岩是S型。

Al--rich minerals of magmatic origin, mainly garnet and andalusite, have beenfound in Mesozoic volcanic rocks of the Dongxiang--Xiangshan area of JiangxiProvince. The garnet usually contains some inclusions of titanium-bearing magne-tite and needle apatite. According to the composition obtained from probe analy-sis, the garnet is mainly almandine. Calculation of Fe and Mg partition betweengarnet and melt suggests that the garnet formed under conditions of T=1060℃and P=6 ×10~8Pa, so is a mineral that might crystallize earlier from magmas atdepth. The discovery of Al--rich minerals in volcanic rocks of this area providesan important basis for assuming the volcanic rocks to be of S--type origin.