扬子地块北缘毕机沟岩体下部带堆晶岩结构定量化分析:对层状岩体冷却早期粒间熔体排出机制的约束

Quantitative textural analysis of cumulates from the lower zone of the Bijigou intrusion in the northern margin of the Yangtze Block:Constraints on the expulsion mechanism of interstitial liquid at the early stage of solidification of layered intrusions

层状岩体冷却中—晚期晶粥中粒间熔体的排出被认为与铁钛氧化物的大量结晶关系密切,但在岩体冷却早期铁钛氧化物尚未大量结晶时,粒间熔体能否被有效排出、以及如何排出还需要深入探究。扬子地块北缘毕机沟岩体是我国目前发现的规模最大、分异程度较高且赋存有钒钛磁铁矿矿床的层状岩体。岩体下部带、中部带和上部带分别由斜长橄榄岩、辉长岩和闪长岩组成。本研究以下部斜长橄榄岩相带为对象,通过对斜长石和单斜辉石进行晶体粒度分布(CSDs)和空间展布形式(SDPs)分析,探讨层状岩体冷却早期粒间熔体的排出方式及其控制因素。模拟计算结果表明,样品中封闭粒间熔体比例(FTL)为9%~13%,暗示岩体冷却早期粒间熔体被有效排出。单斜辉石和斜长石的CSD曲线斜率主要为3.36~2.45和2.99~1.87,截距分别为0.45~3.35和1.06~2.83,均具有斜率一致而截距变化较大的特征,与机械压实的CSD曲线一致。单斜辉石和斜长石在SDP图解上均显示负相关关系,也与机械压实趋势一致。因此,毕机沟岩体冷却早期粒间熔体的排出机制为机械压实。在此基础上,我们将毕机沟与全球其他典型层状岩体从岩体规模、粒间熔体和堆晶矿物密度差、晶体堆积速率与压实速率以及晶粥层厚度等方面进行系统对比后发现,在层状岩体冷却早期没有岩浆补给和对流的情况下,当岩体规模相近时,压实受控于堆晶矿物与粒间熔体密度差,密度差越大压实越容易发生;而当岩体规模不同时,规模越大压实越容易发生。

The expulsion of interstitial liquid from crystal mush is closely related to the abundant crystallization of Fe-Ti oxides during the mid-late stage of the solidification of layered intrusions.However,the mechanism behind the expulsion of interstitial liquid at the early stage,prior to the abundant crystallization of Fe-Ti oxides,remains enigmatic.The highly differentiated Bijigou intrusion,located in the northern margin of the Yangtze Block,is the largest layered intrusion with Fe-Ti-(V)mineralization in China at present.It consists of three main zones:the lower zone(LZ),middle zone(MZ),and upper zone(UZ),composed mainly of plagioclase peridotite,gabbro,and diorite,respectively.In this study,we analyzed the crystal size distributions(CSDs)and spatial distribution patterns(SDPs)for plagioclase and clinopyroxene of samples from the plagioclase peridotite unit in the LZ.The objective was to examine the expulsion mechanism of interstitial liquid from the crystal mush during the early stage of solidification of layered intrusions.Our modeling results showed that the fraction of trapped liquid(FTL in these samples ranged from 9%to 13%,suggesting that the interstitial liquid was effectively expelled from the crystal mush.The consistent slopes of the CSD curves for clinopyroxene and plagioclase of samples from the plagioclase peridotite unit mainly vary from 3.36 to 2.45 and from 2.99 to 1.87,respectively.The intercepts varied from 0.45 to 3.35 for clinopyroxene and from 1.06 to 2.83 for plagioclase.These values are similar to those observed in mechanical compaction.In addition,the SDP R value(ratio of observed and predicted nearest neighbor distance of the same mineral)and the mineral mode,except for plagioclase/clinopyroxene,are negatively correlated,consistent with the trend of mechanical compaction.Therefore,the expulsion of interstitial liquid during the early stage of solidification in the Bijigou intrusion is controlled by mechanical compaction.On the other hand,we compared the Bijigou intrusion with the Skaergaard,Sept Iles,and Kiglapait layered intrusions,in terms of scale,fraction of trapped liquid,density contrast of interstitial liquid and cumulus crystal,accumulation rate and compaction rate relationship,and thickness of mush layer.Our analysis suggests that compaction plays a critical role in expelling interstitial liquid at the early stage of solidification in layered intrusions,particularly in the absence of magma replenishment and convection.However,the factors controlling the degree of compaction are distinct under different conditions.Furthermore,our comparison indicates that the density contrast of interstitial liquid and cumulus crystals primarily controls the onset of compaction when intrusion sizes are similar,with a higher degree of compaction observed for greater density contrasts.In addition,compaction is more likely to operate in large intrusions than small ones at the early stage of solidification.