层状岩体晶粥中压实作用效率——来自扬子地块北缘望江山岩体中部带矿物晶体粒度分布(CSDs)和空间展布形式(SDPs)的约束

The efficiency of compaction in the crystal mush of layered intrusions:Insights from crystal size distributions(CSDs)and spatial distribution patterns(SDPs)for minerals of the middle zone of the Wangjiangshan intrusion in the northern margin of the Yang

层状岩体冷凝中—晚期有铁钛氧化物结晶时,晶粥中粒间熔体的排出方式常以压实作用为主,但不同层位压实作用的效率往往不同,目前对控制压实作用效率的主要机制尚不清楚。笔者等以扬子北缘出露规模较大、分异程度较高的新元古代望江山层状岩体作为研究对象,利用斜长石、单斜辉石的晶体粒度分布(CSDs)和空间展布形式(SDPs)分析,探讨了岩体中部带下段(MZa)橄榄辉长苏长岩、辉长苏长岩和中部带上段(MZb)氧化物辉长岩粒间熔体的排出机制及其效率。岩体MZa和MZb中斜长石和单斜辉石的CSD曲线均具有相互平行的特征,SDP图解具有负相关趋势,分别与机械压实的特征和趋势一致,表明各岩相带粒间熔体的排出方式均为机械压实。模拟计算得到橄榄辉长苏长岩、辉长苏长岩和氧化物辉长岩中封闭粒间熔体比例(F)分别为:28%~33%,14%~23%和7%~12%,表明从MZa到MZb机械压实逐渐增强。MZa中橄榄辉长苏长岩斜长石CSD截距最大(2.23~3.78),特征长最小(0.29~0.45)且小颗粒斜长石具有较大长宽比(5∶1~11∶1),表明岩浆冷却速率较快,导致压实效率最低,粒间熔体未被有效排出。MZa中辉长苏长岩斜长石CSD截距减小(1.31~2.60),特征长增大(0.43~0.58)且小颗粒斜长石长宽比减小(<5∶1),说明冷却速率变慢,导致压实效率提高,粒间熔体被有效排出。MZb氧化物辉长岩中斜长石CSD截距进一步减小(0.49~1.60),特征长进一步增大(0.53~0.69)且小颗粒斜长石具有低长宽比(<5∶1),说明冷却速率进一步降低,同时相较于橄榄辉长苏长岩和辉长苏长岩堆晶矿物与粒间熔体密度差增大,压实效率进一步提高,粒间熔体排出程度达到最高。因此,层状岩体晶粥中机械压实效率主要受控于堆晶矿物与粒间熔体密度差以及岩浆冷却速率,当冷却速率较低并有大量铁钛氧化物结晶时机械压实作用的效率最高。

Objectives:Compaction is considered to be a common mechanism for the expulsion of interstitial liquid from the crystal mush when the abundant crystallization of Fe—Ti oxides during the mid-late stage of the solidification of layered intrusions.However,the efficiency of compaction is often different from layer to layer,and the main factors controlling the efficiency of compaction is not clear yet.The Neoproterozoic Wangjiangshan intrusion is a large and well-differentiated layered intrusion in the northern margin of the Yangtze block,and is composed of dunite,pyr oxeniteand troctolite in the lower zone(LZ),gabbro and gabbronorite in the middle zone(MZ)and diorite in theupp er zone(UZ).The~2000-m-thick MZ is the main part of the Wangjiangshan intrusion,and can be further di videdinto MZa and MZb based on the occurrence of Fe—Ti—V mineralization.Mineral composition and texture ofrocks are distinct from the MZa to MZb,indicating that the different rock units in the MZ may have different mechanismsand different degrees of expulsion of interstitial liquid from the crystal mush.Therefore,we collected samples from the MZ of the Wangjiangshan intrusion and analyzed the crystal size distributions(CSDs)and spatial distribution patterns(SDPs)for clinopyroxene and plagioclase,in order to study the expulsion mechanism of interstitialliquid from crystal mush and its efficiency.Methods:Three samples from the olivine gabbronorite unit,four samples from the gabbronorite unit of the MZb and five samplesfrom the oxide gabbro unit of the MZb were selected for the CSD and SDP measurements for plagioclase and clinopyroxene.Results:Plagioclase and clinopyroxene of the olivine gabbronorite unit and the gabbronorite unit of the MZb,and the oxide gabbro unit of the MZb overall show parallel CSD curves and the negative correlation in the SDP plot,suggesting that the interstitial iquid was expelled by the mechanical compaction in these three units.On the other hand,the fraction of trapped lliquid(F)of samples from the olivine gabbronorite unit,the gabbronorite unit and the oxide gabbro unitare from 28%to 33%,14%to 23%,and 7%to 12%,respectively,calculated by the equilibrium distribution method using whole-rock composition,indicating thatdifferent degrees of mechanical compaction for t hese three units.Plagioclaseless than 0.1 mm of the olivine gabbronorite unit has a high aspect ratio from 5:1 to 11:1,and the CSD curves have intercept from 2.23 to 3.78 with the characteristic length from 0.29 to 0.45,suggesting a high rate of magma cooling.This results in a low compaction rate with a maximum of 0.33 m/a and a high rate of crystal accu mulation with a minimum of 0.27 m/a of the crystal mush,leading to an inefficient mechanical com pactionand a low degree of expulsion of interstitial liquid.Plagioclase less than 0.1 mm of the gabbronorite unithas a low aspect ratio(<5:1),and the CSD curves have intercept from 1.31 to 2.60 with the characteristic length from 0.43 to 0.58,suggesting a low rate of magma cooling.This results in a high compaction rate with a maximumof 0.54 m/a and a low rate of crystal accumulation with a minimum of 0.18 m/a of the crystal mush,leading to an efficient mechanical compaction and a slightly higher degree of expulsion of interstitial liquid.Plagiocla seless than 0.1 mm of the oxide gabbro unit has a low aspect ratio(<5:1),and the CSD curves have interceptfrom 0.49 to 1.60 with the characteristic length from 0.53 to 0.69,suggesting the lowest rate of magma cooling.This results in a high compaction rate with a maximum of 0.95 m/a and a low rate of crystal accumulation with a minimum of 0.13 m/a of the crystal mush,leading to a more efficient mechanical compaction and a much higher degree of expulsion of interstitial liquid.Conclusions:The interstitial liquid of the three units in the MZ of the Wangjiangshan intrusion was expelled to varying degrees by the mechanical compaction.Mechanic compaction for samples from the olivine gabbronorite unit of MZa is inefficient,due to the fast magma cooling rate and the small density contrast between the cumulus and the interstitial melt.With the decrease of cooling rate,the compaction efficiency of the gabbronorite unit of MZa and oxide gabbro unit of MZb gradually increases.Simultaneously,abundant crystallization of Fe—Ti oxides may progressively increase the density contrast of the cumulus and the interstitial melt,resulting in a further increase of th e compaction efficiency of the oxide gabbro unit.This study indicates that the efficiency of mechanic compaction of layered intrusions is mainly controlled by the cooling rate of magma and the density contrast of the cumulus and interstitial melt,with a distinct enhancement by the low cooling rate of magma and abundant crystallization of Fe—Tioxides.