攀西地区新街层状岩体粒间不混熔作用:来自斜长石环带结构的记录

发布时间：2019-01-30 20:37

【摘要】：位于攀西地区的新街层状岩体赋含大量钒钛磁铁矿,是峨眉山大火成岩省的一部分。岩体下部带和中部带以单斜辉石岩为主,并伴生浸染状钒钛磁铁矿矿化;上部带以辉长岩为主,赋存厚层的钒钛磁铁矿矿体。之前研究认为厚层的钒钛磁铁矿矿体的形成与粒间不混熔的富Fe熔体有关,但对富Fe熔体的演化过程缺乏细致研究。本文通过对新街岩体上部带的富矿辉长岩层和上覆浅色辉长岩中斜长石环带结构和成分的研究,揭示了富Fe熔体的演化过程。在浅色辉长岩中保存的岩浆不混熔的直接证据表现为矿物粒间共轭的富Si交生体和富钛铁矿交生体代表的非反应结构。本次研究发现,与粒间富Si交生体接触的斜长石边部的FeO和TiO_2含量随斜长石牌号(An)值的降低而降低,而与粒间富钛铁矿交生体接触的斜长石边部的FeO和TiO_2含量随An值的降低而升高,说明斜长石的边部成分变化记录了粒间共轭的富Si和富Fe熔体的成分特征。在富矿辉长岩中,斜长石可分为初生和新生两种,初生斜长石的An值介于57~62,FeO含量为0.34%~0.50%,TiO_2含量为0.06%~0.13%,新生斜长石具有相对较高的An值(61~81)和FeO、TiO_2含量,二者的内部和边部还发育增生斜长石,其An值(~50)相对较低;在初生斜长石边部可见不连续的新生斜长石环带和增生斜长石边,造成其内部成分显著不均一,并发育复杂的环带结构。本文认为,初生斜长石是岩浆正常分离结晶作用的产物。在粒间熔体发生不混熔后,不混熔的富Fe熔体逐渐向岩浆房下方迁移并结晶出了一些相对高An值的新生斜长石,或沿一些初生斜长石边部生长形成不连续的高An环带。当富Fe熔体演化至晚期,由于矿物生长空间受限,仅在初生和新生斜长石局部形成了相对低An值的增生边、或沿颗粒裂隙进入斜长石内部形成增生斜长石核。
[Abstract]:Xinjie stratified rock body located in Panxi area is a part of Emeishan great igneous province with large amount of vanadium and titanomagnetite. The lower and middle zones of the rock body are dominated by monocline pyroxenite and associated with the mineralization of vanadium-titanomagnetite, while the upper zone is dominated by gabbro with thick layers of vanadium titanomagnetite ore bodies. It is considered that the formation of thick layer vanadium titanomagnetite ore bodies is related to the intergranular immiscible rich Fe melts, but the evolution process of Fe rich melts is not studied in detail. In this paper, the structure and composition of plagioclase ring zone in the upper zone of Xinjie rock body are studied, and the evolution process of rich Fe melt is revealed by studying the structure and composition of plagioclase ring zone in the upper belt of Xinjie rock body and overlying light color gabbro. The direct evidence of the immiscible melting of the magma preserved in the light-colored gabbro is the nonreactive structure represented by the conjugated Si and ilmenite intersecting bodies. It was found that the contents of FeO and TiO_2 in the edge of plagioclase exposed to the intergranular rich Si cross decreased with the decrease of plagioclase (An). However, the content of FeO and TiO_2 in the edge of the plagioclase in contact with the intergranular ilmenite is increased with the decrease of the An value, which indicates that the change of the edge component of the plagioclase records the composition characteristics of the intergranular conjugated Si and Fe rich melts. The plagioclase can be divided into two types: primary and new. The An value of the primary plagioclase is between 57 and 62, and the content of Feo is 0.34 and 0.50. The content of TiO2 is 0.06 and 0.13. The new plagioclase has a relatively high An value (61 / 81) and a higher FeO,TiO_2 content, and a relatively low An value (~ 50) of the plagioclase developed in the interior and the edge of the two plagioclase. At the edge of the primary plagioclase, discontinuous new plagioclase and accretive plagioclase were observed, which resulted in the significant heterogeneity of the internal composition and the development of complex annular structure. It is believed that primary plagioclase is the product of normal separation and crystallization of magma. After the intergranular melt is immiscible, the immiscible rich Fe melts gradually migrate under the magma chamber and crystallize some relatively high An values of new plagioclase, or grow along the edge of some primary plagioclase to form a discontinuous high An ring. When the rich Fe melt evolves to the late stage, due to the limitation of mineral growth space, only a relatively low An proliferative edge is formed in the primary and newborn plagioclase, or an accretionary plagioclase nucleus is formed along the grain fissures into the plagioclase.
【作者单位】：中国科学院广州地球化学研究所中国科学院矿物学与成矿学重点实验室;广东省矿物物理与材料研究开发重点实验室;中国科学院大学;
【基金】：国家自然科学基金(41325006、41473037和41502048)联合资助
【分类号】：P575

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