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

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。

Interstitial Immiscibility in Xinjie Layered Intrusion in Panxi Region (SW China): Constraints from Zoning and Compositions of Plagioclase

The Xinjie layered intrusion in the Panxi region (SW China) hosts Fe-Ti oxide mineralization. The intrusion can be divided, from the bottom upward, into Unit Ⅰ, Ⅱ and Ⅲ in terms of mineral assemblages. Units Ⅰ and Ⅱ are mainly composed of wehrlite and clinopyroxenite with small amounts of Fe-Ti oxides, whereas Unit Ⅲ is mainly composed of gabbro with thick Fe-Ti oxide-rich layers. Previous researches proposed that the thick Fe-Ti oxide-rich layers crystallized from immiscible, interstitial Fe-rich liquid, however, the evolution of the immiscible Fe-rich liquid is still unclear. In this study, we analyzed the zoning and compositions of plagioclase from the Fe-Ti oxide gabbro and the overlying leucogabbro and aimed to constrain the evolution of the immiscible Fe-Ti rich liquid. In the leucogabbro, the paired non-reactive microstructures composed of conjugate granophyric intergrowths and ilmenite-rich intergrowths are the direct evidence of liquid immiscibility. The plagioclase grains adjacent to interstitial granophyric intergrowths have FeO and TiO2 content decreasing with the decrease of the An contents from core to rim, whereas the plagioclase grains adjacent to interstitial ilmenite-rich intergrowths have FeO and TiO2 increasing with the decrease of the An contents from core to rim. We suggest that the rim of the plagioclase may record the compositional variations of conjugate immiscible Si-rich and Fe-rich liquid. In Fe-Ti oxide gabbro, primary plagioclase is replaced by newly formed plagioclase from Fe-rich melt and overgrowth in local places. Primary plagioclase has An ranging from 57 to 62, FeO from 0.34% to 0.54% and TiO2 from 0.06% to 0.13%, whereas the plagioclase crystallized from immiscible Fe-rich melt has An, FeO and TiO2 contents higher than those for the primary plagioclase. An values of the plagioclases overgrowth crystallized from highly evolved interstitial liquid are lower than those of the primary ones. We consider that the primary plagioclase formed by normal crystallization, whereas plagioclase overgrowth crystallized from interstitial immiscible Fe-rich melt. After the onset of the immiscibility in the interstitial liquid, immiscible Fe-rich melt may migrate downwards to form the Fe-Ti oxide-rich layers. Plagioclase crystallized from the immiscible Fe-rich melt would have relatively high An content so that high-An area occurred when Fe-rich melt penetrated the primary plagioclase. When the Fe-rich melt evolved to the late stage, being hampered by the narrow growth space, discontinuous plagioclase overgrowth occurred in the restricted places along the rims of the primary and newly-formed plagioclase overgrowth, or penetrated the cracks through the early formed plagioclase.