Stratigraphic records of the dynamic uplift of the Emeishan large igneous province

Abstract

Fluid dynamical and numerical modelling predicts a large-scale regional domal uplift prior to basalt eruptions in large igneous provinces, which can be readily measured when a plume head rises below a shallow marine sedimentary basin. Research on the sedimentology, biostratigraphy, and isotopic chronology of the Emeishan large igneous province demonstrates that the sedimentary environment in the Maokou stage is not uniform carbonate platform facies, but rather sedimentary facies with a north–south linear alignment and west–east different distribution controlled by the syn-depositional normal faulting of the Changhai and Xiaojiang faults, which are the result of underwater dynamic uplift induced by deep mantle activity. The dynamic uplift started in the Maokou stage. Thus, thinning of the Maokou limestone was the product of the difference in the initial depositional thickness caused by the underwater uplift and post-depositional surface uplift and erosion, but post-depositional uplift was much less than kilometre scale. Sedimentary facies differentiation and tectonic–sedimentary evolution in the Maokou stage provide a constraint for the time of the initial eruption and eruption environment before and during the Emeishan basalt eruption. Small-scale magmatic activity might have already begun in the middle of the Maokou stage, whereas submarine and terrestrial sedimentary environments coexisted before and during Emeishan basalt eruption.     

The contribution of the Emeishan large igneous province to the strontium isotope evolution of the Capitanian seawater

ABSTRACT

The ⁸⁷Sr/⁸⁶Sr minimum of the Capitanian seawater is one of the most significant features in the Phanerozoic seawater ⁸⁷Sr/⁸⁶Sr history. In order to assess possible contribution of the Emeishan large igneous provinces (LIPs) to strontium isotope evolution of the Capitanian seawater, ⁸⁷Sr/⁸⁶Sr ratios were measured from the Capitanian limestones which are locally interlayered with the Emeishan basalts. The limestones underlying the Emeishan basalts have high ⁸⁷Sr/⁸⁶Sr ratios (0.7070–0.7074). However, extremely low ⁸⁷Sr/⁸⁶Sr ratios (0.7068–0.7070) were identified in the late Capitanian Jinogondolella prexuanhanensis–J. xuanhanensis zones, which correspond to the eruption time of the Emeishan LIP. The temporal coincidence of these two phenomena supports the idea of a potential linkage between Capitanian ⁸⁷Sr/⁸⁶Sr minimum and eruption of this igneous province. The strong submarine hydrothermal activity and erosion of the Emeishan LIP could have released large amounts of non-radiogenic Sr to the oceans and play an important role in strontium isotope evolution of the seawater.     

丽江玄武岩水含量：对峨眉山大火成岩省源区水含量分布特征的启示

近年来对太古宙科马提岩和显生宙大火成岩省中苦橄岩的水含量、地幔潜热、源区成分等研究表明,这些短时间内喷出巨量岩浆的地表过程都与水化的地幔柱有关。峨眉山大火成岩省位于扬子板块西部,是我国被公认的大火成岩省之一。前人从地球化学的角度将其分为西、中、东三区;并通过对西区丽江、永胜、宾川、大理苦橄岩和中区二滩玄武岩的水含量分析,发现形成峨眉山大火成岩省的地幔柱可能自喷发初期就已普遍存在强烈的水化,且该特征持续至喷发中晚期。然而前人的研究着重于苦橄岩,对作为大火成岩省主体部分的玄武岩研究甚少。本文以位于西区的仕满、大具剖面中的高Ti/Y玄武岩为研究对象,采用单斜辉石斑晶反演原始熔体水含量的方法,得到仕满、大具玄武岩原始熔体的水含量下限分别为1.15%和0.83%,该水含量略低于丽江苦橄岩水含量。而计算出的源区最低水含量分别为1380×10^-6和1245×10^-6,与二滩玄武岩相当。结合前人报道的数据,本次工作的结果证明了峨眉山大火成岩省的地幔柱水化现象普遍且长期存在,地幔柱内部的热化学组成是不均一的,且其热化学结构是随着时间而发生变化的。本次工作还暗示...     

峨眉山大火成岩省基性墙群几何学研究及对地幔柱中心的指示意义

本文以1:20万和1:5万区调资料为基础,结合野外观察研究了峨眉山大火成岩省主要分布区内的基性(辉绿岩)岩墙群的几何学特征。结果表明,岩墙群的几何学特征表现为中心放射状,由6条巨型岩墙群组成,辐射角度近200°。其中心收敛于永仁一带,与地层学指示的最大隆起位置吻合,指示了二叠纪峨眉山玄武岩事件的地幔柱中心位置。岩墙群分布呈中东部较多而西部少的特点,这可能与不同的剥蚀程度有关。     

峨眉山大火成岩省内带黑谷田含钒钛磁铁矿层状岩体成因

黑谷田岩体产于峨眉山大火成岩省内带,是一个小型含钒钛磁铁矿辉长岩体。与区内其它典型大型基性-超基性层状岩体具有多个旋回岩相的特征不同,黑谷田层状岩体分为下部、上部两个岩相带:下部岩相带从底到顶依次为橄榄辉石岩、磁铁辉长岩、含磷灰石辉长岩和中粒辉长岩,上部岩相带为细粒辉长岩,二者呈突变接触关系。黑谷田岩体的锆石SHRIMP U-Pb年龄为263±5Ma,表明其是~260Ma峨眉山地幔柱岩浆主活动期的产物。岩石的矿物组合(主要为单斜辉石、斜长石,磁铁矿,少量橄榄石等)、元素地球化学(富Fe2O3、TiO2、P2O5,高Sm/Yb及低La/Sm)及低的初始87Sr/86Sr值和亏损的εNd(t)值特征一致指示黑谷田岩体与峨眉山高Ti玄武岩具有密切的内在成因联系。岩体的岩相学及地球化学特征暗示下部岩相带是富FeTi岩浆侵入发生橄榄石、单斜辉石、磁铁矿、斜长石、磷灰石等矿物分离结晶、堆积固结的产物,而上部岩相带是另一期岩浆上侵较为快速冷却固结的结果,矿物堆晶作用不显著,但是二者起源于相同的母岩浆。下部岩相带比上部岩相带具有相对低的初始87Sr/86Sr值(分别为0.7041~0.7051和0.7050~0.7056)和略高的εNd(t)值(分别为2.1~4.4和0.6~1.3),表明后者比前者经历了稍微强烈的地壳物质同化混染。下部岩相带仅有橄榄辉石岩及辉长岩而缺少正长岩和花岗岩、以及较厚的氧化物矿体赋存在岩体底部下凹部位说明黑谷田钒钛磁铁矿形成于岩浆通道系统中,磁铁矿在流动过程中由于重力作用堆积成矿。黑谷田含钒钛磁铁矿岩体的发现表明小型层状岩体也具有重要的Fe-Ti氧化物成矿潜力,在勘探找矿中不容忽视。     

关于峨眉山大火成岩省一些重要问题的讨论

峨眉山大火成岩省(ELIP)近几年来取得了许多新的进展,但在一些重要的问题上仍存在着争论,笔者列举了ELIP火山喷发的时间、分布范围、高钛和低钛玄武岩、层状岩体与玄武岩以及成矿作用的关系、大火成岩省与生物绝灭的关系以及与地幔柱的关系等方面存在的问题,以及由此引发的相关争论.这些问题的解决对于大火成岩省研究的深入将会起到重要作用.     

峨眉山大火成岩省和西伯利亚大火成岩省地球化学特征的比较及其成因启示

峨眉山大火成岩省和西伯利亚大火成岩省是发生于二叠 -三叠纪之交的重要岩浆事件。它们在主要元素、微量元素和Sr、Nd、Pb同位素特征上具有相似姓 ,但是峨眉山大火成岩省的不相容元素比值和同位素比值的变化范围相对要小一些。相对而言 ,峨眉山玄武岩具有高的Fe8和Sm/Yb值 ,暗示了其熔融深度较西伯利亚大火成岩省深 ,而熔融程度较低 ,两者的源区均为石榴石二辉橄榄岩。根据Nd同位素特征估算峨眉山和西伯利亚地幔柱的 Nd≈ 2 ,接近于原始地幔特征。综合其他地球化学特征 ,认为两个大火成岩省可能起源于同一个来自于核 -幔边界的超级地幔柱     

峨眉山大火成岩省东区普安玄武岩系年代学、地球化学及成因研究

广泛分布于中国西南川、滇、黔三省的峨眉山玄武岩是我国最早被国际认可的大火成岩省,受到了国内外学者的广泛关注.前人对大火成岩省西区玄武岩已达成多项共识,而对东区玄武岩的岩石组合、火山活动时限、岩石成因等方面还存在诸多争议.本文以峨眉山大火成岩省东区贵州普安玄武岩系为研究对象,通过解析典型剖面,明确该区玄武岩系岩石类型从底...     

栖霞组和茅口组等厚图:对峨眉山地幔柱成因模式的指示意义

对峨眉山大火成岩省(ELIP)及周边地区的栖霞组和茅口组地层进行厚度统计,并利用Surfer软件分别绘制等厚图.栖霞组和茅口组等厚图表明,峨眉山地幔柱导致的隆升可能在栖霞期已开始.茅口组顶部普遍存在平行不整合界面,说明广大区域内的茅口组地层均曾抬升为陆并遭受剥蚀,与CampbeU和Griffiths提出的经典地幔柱模型相吻合.永仁-大姚-楚雄-石屏以及宜良-曲靖一带存在二叠系地层缺失区域,可能是地壳隆升幅度最大地区.利用实验模型推导出ELIP的最大隆升幅度为1500m,与前人利用沉积学推算的结果基本一致.茅口组差异剥蚀指示的地幔柱中心地区与放射状基性岩墙群收敛中心吻合.隆起幅度最大的中心区域存在海相玄武岩喷发现象则可能是隆升之后快速沉降造成的,并不能作为否定曾经发生隆起的证据.     

黔西南下三叠统飞仙关组沉积物源分析对峨眉山大火成岩省火山剥蚀序列的重建约束

峨眉山大火成岩省在形成后遭受了强烈的风化剥蚀,与右江盆地构成了独特的源-汇沉积体系,相关的碎屑沉积记录是重建峨眉山大火成岩省晚期火山序列、揭示大火成岩省岩浆演化的关键。本文对黔西南地区右江盆地北缘下三叠统下部飞仙关组开展了详细的沉积物源研究。飞仙关组泥岩由长石、石英和黏矿物等组成,具有低SiO₂、高TiO₂含量和低Al₂O₃/TiO₂值的特征,具Eu的弱负异常,微弱的Nb、Ta负异常。基于Al₂O₃/TiO₂和La/Sm物源图解分析,研究样品的主要物源由峨眉山大火成岩省的高-Ti玄武岩和少量酸性火山岩构成,这与右江盆地晚二叠世的碎屑沉积物源组成一致。碎屑锆石具有～260 Ma的年龄峰值,对应锆石具有较低的Th/Nb和较高的Nb/Hf值,均支持峨眉山大火成岩省作为主要的沉积物源区,而同时存在的>400 Ma的碎屑锆石则对应于峨眉山玄武质岩浆上升时所捕获的华南地壳物质。与上二叠统的碎屑锆石U-Pb年龄组成和锆石...     

峨眉山大火成岩省赋Nb-Ta-Zr矿化正长岩脉的形成时代和锆石Hf同位素组成

攀西地区广泛发育Nb-Ta-Zr矿正长岩脉,它们在空间上与峨眉山大火成岩省的玄武岩、辉长岩及长英质岩体(正长岩和花岗岩)共生,但是否有成因联系并不清楚。本文对攀西地区炉库和白草两个Nb-Ta-Zr矿区中的正长岩体、无矿及含矿正长岩脉进行了锆石LA-ICPMS法U-Pb定年和Lu-Hf同位素研究。研究表明,炉库矿区正长岩体和无矿正长岩脉锆石U-Pb年龄为255.6±2.0Ma和255.6±1.5Ma,含矿正长岩脉为256.7±4.4Ma;白草矿区正长岩体和含矿正长岩脉形成时间为257.9±2.3Ma和257.8±1.3Ma。这些年龄结果均显示攀西地区Nb-Ta-Zr矿化的形成时代与正长岩体一致,说明Nb-Ta-Zr矿化的形成与峨眉山大火成岩省紧密相关,为地幔柱成矿系统的一部分。两个矿区含矿正长岩脉的锆石ε_Hf(t)值分别为+0.1~+9.5(多数集中于+2~+6,炉库矿区)和-0.2~+7.7(白草矿区),与正长岩体相似(炉库矿区为+1.0~+6.6,白草矿区为+2.1~+6.2),说明含矿岩脉与正长岩体有紧密成因联系。结合锆石年龄和Hf同位素研究初步认为,正长岩体和正长岩脉是峨眉山地幔柱岩浆底侵到下地壳底部的基性岩再次部分熔融的结果。     

贵州乐康剖面茅口阶-吴家坪阶碳同位素组成变化及峨眉山大火成岩省的响应

随着精细年代学研究的发展,越来越多的证据表明地幔柱作用下的大火成岩省和地质历史时期的生物绝灭之间有着密切的联系。我国西南地区峨眉山溢流玄武岩省是目前我国惟一被国际认可的大火成岩省,为了更好地证明峨眉山大火成岩省与同期生物、环境变化之间的关系,选择贵州望谟乐康剖面石灰岩开展了碳同位素组成研究。结果表明,在峨眉山地幔柱作用之前的茅口阶碳同位素组成显示出较高的正值(δ13C=5.2‰～4.0‰);茅口阶顶部至领薅组第一段δ13C值主要落在2‰～4‰范围内,其时序上对应于峨眉山玄武岩主喷发之前的区域穹状隆升时期;之后由于峨眉山大火成岩省有关的大规模火山作用导致低值碳源加入,当时缓慢复苏的生物作用造成的碳同位素组成增加远不能抵消火山作用造成的碳同位素组成降低,导致期间碳同位素组成明显降低(变化于0‰～2‰范围),并出现负漂移。该剖面碳同位素组成的变化为峨眉山地幔柱作用对生物和环境效应的研究提供了重要信息。     

Geochronology of layered mafic intrusions from the Pan–Xi area in the Emeishan large igneous province, SW China

The Panzhihua–Xichang (Pan-Xi) area hosts mafic/ultramafic intrusions, which are part of the Permian Emeishan large igneous province. Some of these intrusions host giant Fe–Ti–V deposits and minor Ni–Cu–PGE mineralization. In the present study, zircon U–Pb ages of 259.3±1.3 and 260.7±0.8 Ma have been obtained from the giant Fe–Ti–V ore-bearing Hongge and the unmineralized Binggu intrusions, respectively, by isotope dilution thermal ionization mass spectrometry method. In combination with the ages of other ore-bearing intrusions, this age shows that these mafic/ultramafic intrusions were emplaced at ca. 260 Ma. The Hongge and Binggu intrusions cut the lower part of the rapidly deposited Emeishan flood basalt sequence but no further into the upper volcanic sequence in the Pan-Xi area. Thus, emplacement and mineralization of the mafic/ultramafic intrusions were almost contemporaneous with the eruption of the Emeishan flood basalts during a relatively short time span.     

峨眉山大火成岩省岩石成因与空间差异性研究——基于全区高Ti玄武岩地球化学数据分析与模拟

高Ti玄武岩成因是峨眉山大火成岩省(ELIP)研究的热点问题.由于高Ti玄武岩地球化学特征在空间上存在差异,其岩石成因尚未达成共识.本文系统收集了峨眉山大火成岩省中高Ti玄武岩地球化学数据以及锆石ID-TIMS U-Pb测年结果,并进行统一处理分析与模拟.研究结果显示,峨眉山大火成岩省形成于约259～258 Ma,高T...     

峨眉山大火成岩省西部苦橄岩及其共生玄武岩的地球化学：地幔柱头部熔融的证据

丽江地区的苦橄岩位于峨眉山大火成岩省的西部,其与辉斑玄武岩、无斑玄武岩和玄武质火山碎屑岩共生。苦橄岩中的斑晶主要为富镁橄榄石,其F0含量最高达91．6％,CaO含量最高达0．42％,其内含有少量玻璃包裹体,指示了橄榄石是在熔体中结晶形成的。苦橄岩中的铬尖晶石具有高的Cr#值（73-75）。计算的初始岩浆的MgO含量大约为22wt％,初始熔融的温度为1630-1680℃。研究结果表明,玄武质岩石是苦橄质岩浆通过橄榄石和单斜辉石分离结晶形成的。苦橄岩和玄武岩的Nd-Sr-Pb同位素比值差别不大,只落在一个很小的范围内（如εNd（t）=-1．3 to＋4．0）。高的εNd（t）值以及抗蚀变不相容元素的原始地幔标准化图解与洋岛玄武岩相似,并且其重稀土元素特征指示了源区有石榴子石的残余,而且是低部分熔融的产物。同位素比值与抗蚀变不相容元素比值（如Nb／La）的相关性表明,岩浆形成过程中有少量的大陆地壳物质或者相对低εNd（t）组分的大陆岩石圈地幔的混染。因此,总体上,苦橄岩的地球化学特征的研究结果支持了峨眉山大火成岩省是地幔柱头部熔融的成因模型。     

中国东部中生代大规模岩浆活动与长英质大火成岩省问题

笔者认为,中国东部中生代大规模岩浆活动很难用太平洋板块的俯冲来解释,中国东部中生代大规模岩浆活动可能相当于几个不同时期发育的长英质大火成岩省,与中生代东亚超级地幔柱的活动有关.世界上存在两类大火成岩省,一类以镁铁质岩为主(M-LIP);另一类以长英质岩为主(F-LIP).中国也存在上述两类大火成岩省,二叠纪的峨眉山玄武岩属于前者,中国东部中生代大规模的岩浆活动属于后者.二者可能均与地幔柱的活动有关,不同在于镁铁质大火成岩省的地幔柱上升停滞在岩石圈底部,在那里发生部分熔融形成大规模玄武岩喷发;而与长英质大火成岩省有关的地幔柱可抵达下地壳底部直接烘烤和加热下地壳,形成长英质成分的岩浆岩.学术界通常认为中国东部中生代大规模岩浆活动与太平洋板块向西俯冲导致的软流圈地幔上升有关,本文却认为它可能与来自下地幔的地幔柱有关.大火成岩省矿产丰富,与镁铁质大火成岩省有关的矿产有铜、镍、铬、铂、钯等,与长英质大火成岩省有关的矿产有金、铜、钨、锡、钼、铋、锑、铀等.     

峨眉山大火成岩省中高Os苦橄岩的发现及地质意义

本文对峨眉山大火成岩省中苦橄岩及其共生的玄武岩进行了铂族元素(PGE)分析,结果表明苦橄岩比玄武岩的PGE含量要高至少一个数量级,并且具有明显高的Os含量,不仅比熔融程度最高的科马提岩要高,而且比原始地幔还要高,另外,还显示出超球粒陨石的Os/Ir比值(2.84～3.88)。其高的Os/Ir比值可能与岩浆上升过程中混入黑色页岩有关。部分熔融计算表明,含有0.01%硫化物的原始地幔 0.5%的外核在7%的熔融程度下,然后又被约10%的黑色页岩混染可以模拟原始岩浆的PGE含量。其Os含量及其他地球化学特征与其同时代的西伯利亚暗色岩系的相似性可能暗示了这两个大火成岩省来自于同一个起源于核-幔边界的超级地幔柱。另外,还根据苦橄岩和玄武岩PGE的含量估算了该地区PGE的成矿潜力。     

Epithermal mineralization controlled by synextensional magmatism in the Guazapares Mining District of the Sierra Madre Occidental silicic large igneous province,Mexico

We show here that epithermal mineralization in the Guazapares Mining District is closely related to extensional deformation and magmatism during the mid-Cenozoic ignimbrite flare-up of the Sierra Madre Occidental silicic large igneous province, Mexico. Three Late Oligocene–Early Miocene synextensional formations are identified by detailed volcanic lithofacies mapping in the study area: (1) ca. 27.5 Ma Parajes formation, composed of silicic outflow ignimbrite sheets; (2) ca. 27–24.5 Ma Témoris formation, consisting primarily of locally erupted mafic-intermediate composition lavas and interbedded fluvial and debris flow deposits; (3) ca. 24.5–23 Ma Sierra Guazapares formation, composed of silicic vent to proximal ignimbrites, lavas, subvolcanic intrusions, and volcaniclastic deposits. Epithermal low-to intermediate-sulfidation, gold–silver–lead–zinc vein and breccia mineralization appears to be associated with emplacement of Sierra Guazapares formation rhyolite plugs and is favored where pre-to-synvolcanic extensional structures are in close association with these hypabyssal intrusions.Several resource areas in the Guazapares Mining District are located along the easternmost strands of the Guazapares Fault Zone, a NNW-trending normal fault system that hosts most of the epithermal mineralization in the mining district. This study describes the geology that underlies three of these areas, which are, from north to south: (1) The Monte Cristo resource area, which is underlain primarily by Sierra Guazapares formation rhyolite dome collapse breccia, lapilli-tuffs, and fluvially reworked tuffs that interfinger with lacustrine sedimentary rocks in a synvolcanic half-graben bounded by the Sangre de Cristo Fault. Deposition in the hanging wall of this half-graben was concurrent with the development of a rhyolite lava dome-hypabyssal intrusion complex in the footwall; mineralization is concentrated in the high-silica rhyolite intrusions in the footwall and along the syndepositional fault and adjacent hanging wall graben fill. (2) The San Antonio resource area, underlain by interstratified mafic-intermediate lavas and fluvial sandstone of the Témoris formation, faulted and tilted by two en echelon NW-trending normal faults with opposing dip-directions. Mineralization occurs along subvertical structures in the accommodation zone between these faults. There are no silicic intrusions at the surface within the San Antonio resource area, but they outcrop ∼0.5 km to the east, where they are intruded along the La Palmera Fault, and are located ∼120 m-depth in the subsurface. (3) The La Unión resource area, which is underlain by mineralized andesite lavas and lapilli-tuffs of the Témoris Formation. Adjacent to the La Unión resource area is Cerro Salitrera, one of the largest silicic intrusions in the area. The plug that forms Cerro Salitrera was intruded along the La Palmera Fault, and was not recognized as an intrusion prior to our work.We show here that epithermal mineralization is Late Oligocene to Miocene-age and hosted in extensional structures, younger than Laramide (Cretaceous-Eocene) ages of mineralization inferred from unpublished mining reports for the region. We further infer that mineralization was directly related to the emplacement of silicic intrusions of the Sierra Guazapares formation, when the mid-Cenozoic ignimbrite flare-up of the Sierra Madre Occidental swept westward into the study area about 24.5–23 Ma ago.     

峨眉山大火成岩省Cu-Ni-PGE成矿作用——几个典型矿床岩石地球化学特征的分析

峨眉山大火成岩省岩浆型Cu-Ni-PGE矿化岩体广泛分布,构成峨眉山地幔柱成矿系统中一个非常重要的成矿系列。本文剖析了峨眉山大火成岩省该类矿床的分布及部分典型矿床的地质地球化学特征和矿化特征,揭示了成矿岩体统一的地幔柱成因,阐述了Cu-Ni-PGE成矿作用与峨眉山地幔柱岩浆活动体系的关系,探讨了由于岩浆演化过程及硫化物熔离富集过程的差异所导致的矿化类型变异。指出Cu-Ni-PGE矿床成矿岩体原始岩浆为地幔柱高程度熔融的高镁玄武岩浆,成矿岩体与峨眉山低钛玄武岩同源,矿化岩体主要产于峨眉山地幔柱活动模型的内带低钛玄武岩分布区;金宝山、朱布、力马河、杨柳坪矿床分别代表峨眉山地幔柱Cu-Ni-PGE成矿作用不同成矿机制的端员类型。     

峨眉山大火成岩省太和花岗岩的成因及构造意义

攀西地区的太和花岗质岩体和赋存超大型钒钛磁铁矿矿床的辉长岩体在空间上共生,成因上均与峨眉山地幔柱头的上升密切相关.太和花岗质岩体主要由超碱质花岗岩和石英正长岩及少量正长岩组成;富含高场强元素并具高Ga/Al值(3.74～5.63),显示典型A型花岗岩的特征.花岗岩、正长岩和辉长岩的Nb/Ta和Zr/Hf值与洋岛玄武岩(OIB)的相应比值近似.花岗质岩石具较低的87Sr/86Sr初始值(0.7025～0.7049)和正的εNd(t)值(1.9～3.5),与辉长岩的值相近[(87Sr/86Sr)i =0.7049～0.7052; εNd(t) =2.4～3.3].太和花岗质岩体的εNd(t)为正值,显示地幔柱来源的底侵玄武质岩浆对其形成起主要作用.辉长质和花岗质岩石具相似的钕同位素组成,表明其母岩浆来自于同一源区.我们认为太和花岗质侵入体主要由底侵于下地壳的玄武质岩浆分异出的花岗质熔体侵位及随后经结晶分异而形成.因此,晚古生代时幔源岩浆底侵造成的地壳增生在峨眉山大火成岩省中表现极为显著.