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1.
在北大巴山高桥地区广泛出露一套镁铁质岩,岩石类型为辉绿岩、辉长辉绿岩、辉绿玢岩和辉石闪长岩等。主要组成矿物为单斜辉石、斜长石、角闪石和铁钛氧化物。本文利用电子探针对单斜辉石进行了详细研究,表明该区所有单斜辉石均属于普通辉石,从辉绿玢岩和辉长辉绿岩到辉石闪长岩,单斜辉石具有从富镁向富铁演化的趋势。单斜辉石内部四面体位置的Al含量为0.02～0.20,AlⅣ/AlTot=85%～100%,指示其形成于不饱和的碱性玄武质岩浆中,与单斜辉石碱性判别图解和岩石地化特征反映的碱性岩系列相一致。单斜辉石的平衡温度为1132～1164℃,压力为0.71×102～9.30×102MPa,显示明显的深源特征。Al对Si的替代所导致的电荷不平衡主要由Fe3+来补偿,Ti大多形成钛辉石,AlⅥ进入八面体形成契尔马克分子Ca（Mg,AlⅥ）,Ca2+和Na+的类质同像置换使得辉石具有较高Ca含量和较低的Na2O含量。综合岩体地质产状以及单斜辉石对构造环境的指示意义,认为高桥岩体形成于与俯冲相关的弧后盆地环境,为弧后拉张作用的产物。  相似文献   

2.
在拉萨地块中部米巴勒地区产出的中新世钠质方沸石(霞石)响岩中的单斜辉石发育大量环带结构,环带结构包括正环带、反环带和韵律环带。其中正环带结构从核部到边部Mg#、MgO、Cr2O3呈逐渐降低趋势,而FeO、Na2O、Al2O3、TiO2呈逐渐升高的趋势,它反映了岩浆正常结晶的历史。反环带结构中Mg#、MgO、Cr2O3从核部到边部呈升高的趋势,FeO、Na2O、Al2O3的含量总体上呈降低趋势,本文认为响岩中单斜辉石的这种环带结构是受到了和它共存的钾质-超钾质火山岩岩浆混合或混染所致,两种岩石应是各自独立的母岩浆的结晶产物。具韵律环带结构的单斜辉石从核部到边部出现成分规律性波动,Na2O、Al2O3、TiO2、Cr2O3含量的变化在正反环带的范围内,Mg#值的变化范围较小,在正环带斑晶和基质的范围内,它是岩浆多次混合作用的结果。该地区钠质过碱性岩的出现可能指示青藏高原在这套火山岩喷发时(13~12Ma),构造体制发生了显著的改变,进入到一个典型的陆内伸展阶段。  相似文献   

3.
西秦岭礼县地区新生代钾霞橄黄长岩系的单斜辉石   总被引：5，自引：1，他引：5  
对西秦岭礼县地区钾霞橄橄黄长质火山岩系中单斜辉石进行了矿物化学研究，斑晶相单斜辉石类型复杂，成分变化范围大，具有多源属性，基质相单斜辉石可以分为高Ti铝透辉石和含Ti透辉石两种基本类型，它们是同源岩浆演化结晶的产物。结晶顺序前者晚于后者，岩系中透辉石Ca(Mg,Fe)Si2O6结晶过程中广泛存在于CaTiAl2O6分子替代，晚期熔体富Ti,Al贫Si,Mg，熔体有向富Ti,Al,Fe,Na贫Mg,Si演化的趋势，在透辉石成分上表现为CaTiAl2O6和NaFe^3 Si2O6端员分子对Ca(Mg,Fe)Si2O6的替代，本区基质相透辉石与世界上典型地区的钾霞橄黄长岩系的透辉石具有不同程度的可比性。  相似文献   

4.
坡北镁铁-超镁铁质岩体位于塔里木板块东北缘的北山地区,主要由坡一、坡三、坡七和坡十等十几个侵入体组成,已发现坡一和坡十岩体赋存大型Cu-Ni硫化物矿床,目前对坡北含矿岩体成岩成矿构造背景存在不同认识。本文对坡一岩体各类岩石的矿物、主量和微量元素组成进行了系统分析,坡北岩体橄榄石Fo值范围为83.17～90.2,斜方辉石主要为古铜辉石,单斜辉石主要为普通辉石和透辉石;铬尖晶石Cr#和Mg#值变化范围分别为0.45～0.69和0.11～0.41,具有由富铝、富镁向富铬、富铁方向演化的趋势。微量元素具有明显的Nb、Ta和Ti负异常,Pb正异常。岩体主量元素变化特征主要受橄榄石、辉石和斜长石分离结晶/堆晶作用控制。模拟计算表明,岩体母岩浆MgO含量为16.09%,FeO含量为10.38%,岩浆结晶温度为1 331～1 411℃,为高温苦橄质岩浆。坡北岩体是早期受俯冲流体交代的岩石圈地幔在塔里木地幔柱作用下发生较高程度部分熔融的产物。  相似文献   

5.
冈底斯岩基南缘自西向东，从楠木林到米林广泛出露一系列含暗色细粒包体的含紫苏辉石侵入岩。这一系列含紫苏辉石的侵入岩具有斜长石以及紫苏辉石的巨晶（>5mm），呈现出堆晶结构。锆石U-Pb定年表明，这一系列含紫苏辉石侵入岩的结晶年龄为97~77Ma，并不随侵位位置具有显著的经度上的变化。含紫苏辉石的基性岩具有高的Al2O3（17.3%~18.2%）含量，较高的MgO（3.9%~4.1%）含量，FeOT含量在8.7%~9.0%之间；低的Cr（<14.8×10-6）和Ni（<15.0×10-6）含量，基本不具有Eu的异常，富集大离子亲石元素（LILE）和LREE，亏损高场强元素（HFSE）。含紫苏辉石的中-酸性岩具有高的Al2O3（14.9%~18.8%）含量，高的Mg#值（>39.7）；变化较大的Cr（5.7×10-6~260×10-6）和Ni（10.2×10-6~78.2×10-6）含量，具有微弱-强烈的Eu的负异常，富集LILE和LREE，亏损HFSE。暗色细粒包体与含紫苏辉石基性岩相比具有相似的SiO2含量，FeOT（8.1%~9.0%）含量，稍高的MgO（4.7%~5.4%）含量，Al2O3（18.1%~19.4%）含量以及Mg#值（51.0~52.6）；具有与含紫苏辉石基性岩相似的微量元素分布和稀土元素配分模式。这一系列含紫苏辉石的侵入岩具有较低的初始Sr同位素比值（87Sr/86Sr（t）=0.7037~0.7044），较高并变化较大的εNd（t）值（+3.7~+9.4）和εHf（t）值（+9.9~+14.6）。这些特征共同说明，经流体+熔体交代的地幔楔中软流圈部分在俯冲流体存在的情况下发生部分熔融形成母岩浆，其母岩浆随后与俯冲板片熔体发生混合。在岩浆演化过程中经历了单斜辉石、斜方辉石以及斜长石的分离结晶并最终形成了冈底斯岩基南缘出露的含紫苏辉石侵入岩。暗色细粒包体可以代表母岩浆的早期堆晶，是岩浆淬火作用的产物。母岩浆中大量流体的存在，使其结晶顺序为单斜辉石-斜长石（紫苏辉石），随后的堆晶作用使得这一系列侵入岩得以赋存紫苏辉石。  相似文献   

6.
王建  李建平 《矿物学报》2003,23(2):115-123
对西秦岭礼县新生代钾霞橄黄长岩系中的基质相含钛透辉石进行了矿物化学研究，根据透辉石中Ti和Al的含量划分出低Ti—透辉石和高Ti—铝透辉石两种基本类型，它们作为同源岩浆演化结晶的产物，结晶顺序前者先于后者。火山岩系的透辉石[Ca(Mg，Fe)Si2O6]结晶过程中广泛存在着CaTiAl2O6(钛辉石)分子替代，晚期熔体富Ti、Al贫Si、Mg。百草山岩筒是演化岩浆结晶的产物，熔体向富Ti、Al、Fe^3 、Na，贫Mg、Si趋势演化；在透辉石成分上表现为CaTiAl2O6和NaFe^3 Si2O6(锥辉石)端元分子对Ca(Mg，Fe)Si2O6的替代。本地区基质相透辉石与世界上典型地区的钾霞橄黄长岩系的透辉石具有不同程度的可比性，反映了这种特殊的岩浆熔体成分在一定程度上控制着透辉石的结晶过程和阳离子在矿物晶格中的占位。  相似文献   

7.
藏南特提斯喜马拉雅东部的错那洞淡色花岗岩产出有白云母、石榴子石、电气石等特征性矿物,显示具有高度分异岩浆岩的特征。从二云母花岗岩到电气石花岗岩到石榴子石花岗岩,碱性长石均为正长石,斜长石从更长石演变为钠长石,白云母Al2O3、Na2O含量逐渐升高,反映花岗岩逐渐向富Al富Na演化的趋势。二云母花岗岩中黑云母XFe值、AlⅥ逐渐升高,表明二云母花岗岩之间存在结晶分异演化。电气石Xvac值、Al、Mg和Fe含量之间的变化,反映电气石花岗岩到花岗伟晶岩岩浆结晶环境中Na含量的增加,表明花岗伟晶岩结晶分异演化程度高于电气石花岗岩。石榴子石高XFe、XMn值的特征,也是高分异花岗岩的标志。上述矿物学研究结果表明,错那洞二云母花岗岩是由岩浆早期结晶矿物与残余的母岩浆组成的糊状物经过长期结晶形成的,而电气石花岗岩和石榴子石花岗岩由从晶粥体中分离出来的衍生熔体形成。  相似文献   

8.
西天山哈拉达拉辉长岩的Fe-Ti富集机制及其构造意义   总被引：4，自引：4，他引：0  
贺鹏丽  黄小龙  李洪颜  李洁  于洋  李武显 《岩石学报》2013,29(10):3457-3472
特克斯哈拉达拉辉长岩体是西南天山出露规模最大的层状基性-超基性侵入杂岩体,其富含Fe-Ti氧化物(含量高达15%)。哈拉达拉辉长岩中的橄榄石具有较低的Fo值(64~75),斜长石主要为中-拉长石,单斜辉石相对低Mg#(0.66~0.87),属于次透辉石或普通辉石,总体上显示出较高的岩浆演化程度。Fe-Ti氧化物以钒钛磁铁矿为主(TiO2=0.8%~20.6%;V2O3=0.10%~0.83%),常与钛铁矿呈矿物对共生或出溶钛铁矿。晚期的云母矿物均围绕他形的Fe-Ti氧化物生长,主要为富MgO的金云母,少量属于黑云母;大部分云母成分富含F、Cl(F+Cl高达3.14%),指示岩浆形成于贫水环境。哈拉达拉辉长岩体的主量元素成分变化很大(Mg#=0.48~0.73),橄榄辉长岩主要受橄榄石和斜长石的结晶分异/堆晶作用影响,而辉长岩的成分变化主要受控于斜长石和Fe-Ti氧化物的堆晶作用。哈拉达拉岩体的Fe-Ti氧化物主要为岩浆正常结晶分异作用的结果,该体系具有较低氧逸度、贫水的特点,其地幔源区的熔融程度较高,可能是塔里木地幔柱在天山造山带这一构造薄弱带的早期岩浆活动。  相似文献   

9.
钱兵  张照伟  李文渊  敬志成  邵继 《岩石学报》2018,34(8):2275-2294
盐场北山岩体位于柴北缘造山带西段,是区内近年来新发现的具有良好铜镍矿化的镁铁-超镁铁质岩体,由橄榄岩相、辉石岩相和辉长岩相岩石组成。锆石U-Pb年代学研究表明,岩体形成时代为258.5±1.6Ma,为晚二叠世岩浆作用的产物。岩石中橄榄石Fo分子为82.8~86.7,均为贵橄榄石,辉石由斜方辉石和单斜辉石组成,斜方辉石种属为古铜辉石,单斜辉石为透辉石、普通辉石和顽透辉石。岩石地球化学特征显示,岩石属于亚碱性岩系列,主要氧化物SiO_2、Al_2O_3、CaO、Ti O_2、Na_2O+K_2O等与MgO呈规律性变化趋势,显示不同岩相岩石为同源岩浆演化的产物;岩石相对富集大离子亲石元素(Rb、Ba、Sr),而强烈亏损高场强元素(Nb、Ta、Zr、Hf、Ti),具有岛弧岩浆岩特征;锆石Hf同位素显示εHf(t)均为正值,变化范围为0.1～6.7之间,具有幔源岩浆成因特征;岩石原生岩浆为MgO含量略大于13.21%的高镁拉斑玄武质岩浆,岩浆源区为受俯冲洋壳析出流体交代地幔楔而成的富集地幔。岩石结晶过程中发生了强烈的结晶分异作用和同化混染作用,结晶分异作用在岩浆演化的初期使岩浆中硫发生初步富集,地壳混染作用是岩浆中硫化物达到饱和并发生熔离的主要因素。  相似文献   

10.
洪山正长岩杂岩体发育较多的具有核-边(核-幔-边)结构的单斜辉石,通过研究单斜辉石成分的变化,可以获得岩石成因及演化信息。本文在详细野外地质调查的基础上,采用锆石U-Pb年代学、矿物学研究,获得洪山正长岩杂岩体内黑云辉石正长岩锆石U-Pb年龄为(126.9±1.2)Ma,是华北克拉通岩石圈减薄峰期的产物;洪山正长岩杂岩体内辉石正长岩与黑云辉石正长岩中单斜辉石Mg~#值分别在39.4～72.5、55.4～81.7,具有较高FeO、Na_2O、CaO含量,较低Al_2O_3、MgO、TiO_2含量的特征;单斜辉石总体具有透辉石→霓石的演化趋势,并与熔体达到平衡状态,单斜辉石在初始演化时具有Fe~(2+)对Mg~(2+)的取代关系,随着演化的进行,岩浆更加富钠、富铁,反映了岩浆体系具有高温、中等氧逸度和富碱的特点。结合单斜辉石核-边(核-幔-边)具有截然的接触关系和不连续的化学组成,表明洪山正长岩杂岩体在形成后还经历了富钠、富铁流体的改造,致使单斜辉石形成了具有富钠、富铁的边部,流体可能是由西向东(或者由洪山正长岩杂岩体中部向外部)对杂岩体进行改造的。  相似文献   

11.
Suzanne Y. Wass 《Lithos》1979,12(2):115-132
Geochemical and textural data on clinopyroxenes in individual alkali basaltic flows from provinces in eastern Australia and the Massif Central can be used to differentiate four different modes of origin (three at high pressure) for these clinopyroxenes. Many single flows from the two provinces contain clinopyroxenes of three, or even four, of these origins. Rare flows contain core clinopyroxene with overgrowths of clinopyroxenes of two distinct generation. Each of the overgrowth is compositionally analogous to clinopyroxenes occurring in xenoliths or as discrete crystals in the same host lava. Such rimming relationships provide evidence that the host magma has undergone high pressure crystallisation and confirm that some xenoliths and megacrysts are cognate. With decreasing pressure the major changes in clinopyroxene chemistry are an increase in the ratio Allv/Alvi, a linear increase of atomic proportions of Ti and Al with decreasing Si, and an increase in the Ti: (100 Mg/(Mg + Σ Fe)) ratio of the pyroxenes. Al2O2 wt, % is an unreliable potential geobarometric indicator; consideration of tetrahedral and octahedral site occupancies by Al (Allv/Alvl ratio) is necessary. High-pressure fractionation dominated by clinopyroxene is postulated for some basaltic-composition, resulting in SiO2 depletion, alkali enrichment and decrease in the Mg/(Mg + Fe2+) value of the host magma.  相似文献   

12.
Summary In the zoned clinopyroxene phenocrysts from the volcanic rocks of Almopia the Mg/Fe ratio and the contents of Mn, and mostly of Na, increase from the core towards the rim of the crystals, whereas the Fe3+/Fe2+ ratio, and in many cases the contents of Ca, decrease. On the contrary, the content of Si decreases from inside outwards or remains almost constant whereas Al and Ti increase.The high Ca-content of the clinopyroxenes, low Ti, and limited Fe-enrichment, with a trend from Ca43.3: Mg47.0: (Fe + Mn)9.7 to Ca46.6: Mg42.2: (Fe + Mn)11.2, illustrate the shoshonitic to cale-alkaline nature of the parental magma.The composition of the clinopyroxenes indicates that, of all possible controlling factors, the high water-vapour pressure is considered to play an important role for the AlIV fluctuations during the crystallization.
Zonierte klinopyroxene der vulkanite von almopia (Voras-Gebirge), Zentral-Mazedonien, Griechenland
Zusammenfassung Bei den zonierten Klinopyroxeneinsprenglingen der Vulkanite von Almopia nehmen das Mg/Fe-Verhältnis und die Gehalte an Mn und meistens an Na in den einzelnen Zonen vom Kern zum Rand zu, während das Fe3+/Fe2+-Verhältnis, und in vielen Fällen die Gehalte an Ca, abnehmen. Im Gegensatz dazu nimmt der Gehalt an Si von innen nach außen ab oder bleibt fast konstant, während Al und Ti zunehmen.Der hohe Ca-Gehalt und gleichzeitig niedrige Ti-Gehalt der Klinopyroxene, zusammen mit ihrer begrenzten Fe-Anreicherung und einem Trend von Ca43.3: Mg47.0: (Fe + Mn)9.7 bis Ca46.6: Mg42.2: (Fe + Mn)11.2 weisen auf die shoshonitische bis kalk-alkalische Natur des Stammagmas hin. Die Zusammensetzung der Klinopyroxene weist darauf hin, daß der hohe Wasserdampfdruck im Magma auch eine wichtige Rolle für Schwankungen im AlIV-Gehalt während der Kristallisation darstellt.
  相似文献   

13.
A crystal chemical investigation of clinopyroxenes from a suite of nepheline-bearing lavas located in the Nyambeni Range of Kenya has delineated the polyhedral site configurations and related intracrystalline relationships. These are distinct from those determined for the clinopyroxene in an analogous suite of leucite-bearing lavas from the Sabatini volcanoes in the Roman Region of Italy (Dal Negro et al. 1985).The Nyambeni clinopyroxene, varying from salite to hedenbergite, preferentially accepts Na in the M2 site to balance increasing Fe2+ and Si, respectively, whereas the Sabatini clinopyroxene is confined within the salite field and preferentially accepts Aliv to balance the effect of increasing (Fe3++Ti4++Alvi+Cr3+)M1.The Fe2+/Fe3+ and K/Na ratios of the host rocks emerge as significant factors in determining the different polyhedral configurations and evolutions of the clinopyroxene from the two lava suites, respectively. The resulting Mg-Fe2+ order-disorder relationships in M1–M2 are also distinct in the two clinopyroxenes. A high degree of MgFe2+ order in M1–M2 corresponds to the largest configurational, hence energetic, difference between M1 and M2 in the Nyambeni clinopyroxene, whereas the converse applies to the Sabatini clinopyroxene.In view of the significant crystal chemical differences and distinct evolution trends, it is proposed that salites from alkali volcanic rocks may be referred to as Nyambeni-type or Sabatini-type, respectively.  相似文献   

14.
The clinopyroxenes mentioned have been investigated by single crystal X-ray diffraction combined with electron microprobe analysis. The aim of this study was to characterize the crystal-chemical variations of clinopyroxenes in order to delineate the intracrystalline constraints which are characteristic of specific magmatic environments. Clinopyroxenes (cpx) crystallized from peralkaline ultrapotassic melt with kamafugitic and lamproitic affinities are characterized by high Si contents, which are insensitive to variations in silica abundance and silica saturation of the melt. The high Si occupancy in clinopyroxenes from kamafugitic magma is coupled to large M1 (i.e. Mg and Fe2+) and M2 (high Ca occupancy) sites, whereas in clinopyroxenes from magmas with lamproitic affinity, high Si content is combined with large M1 but small M2 sites. Clinopyroxenes from Romantype alkaline potassic and ultrapostassic rocks are characterized by an expanded tetrahedron (high IVA1 content) and small M1 site which is combined with small M2 polyhedron in clinopyroxenes from the potassic rocks and large M2 site in those from the ultrapotassic rocks.  相似文献   

15.
富钛钙铁榴石是硅不饱和碱性火成岩中的特征矿物,其Ti含量和Ti进入石榴石的方式可直接反映岩浆体系的硅饱和度和氧逸度。河北矾山超镁铁岩-正长岩杂岩体是华北克拉通北缘东西向展布三叠纪碱性岩带的重要岩体之一,其岩石中发育三种类型的富钛钙铁榴石:类型Ⅰ发育于岩体外带的石榴石辉石正长岩中,与辉石、黑云母呈岩浆共生关系,端元组成为Adr_(25-65)Mmt_(15-37)Slo_(8-28)Grs_(10-14),以高TiO_(2)含量(6.08%~18.61%)、较低的SiO_(2)含量(25.46%~33.26%)为特征,为原生岩浆成因;类型Ⅱ见于各类岩石,呈细粒他形充填于其他矿物颗粒之间,端元组成为Adr_(57-69)Mmt_(5-19)Slo_(0-6)Grs_(19-27),以高Al_(2)O_(3)含量(3.95%~5.56%)为特征,为岩浆演化后期熔体富Al_(2)O_(3)时结晶而成;类型Ⅲ亦见于各类岩石,主要呈细粒他形或细碎状发育于其他矿物内,端元组成为Adr_(68-79)Mmt_(8-14)Slo_(0-5)Grs_(6-14),以低的Al_(2)O_(3)含量(1.18%~2.89%)和较高的FeO含量(21.65%~24.62%)为特征,为岩浆期后热液成因。在矾山杂岩体的富钛钙铁榴石中,Ti主要以Si→Ti^(4+)和2Fe^(3+)→Ti^(4+)+Fe^(2+)/Mg替代方式进入到晶体结构中,反映了矾山杂岩体的母岩浆体系为二氧化硅不饱和的碱性岩浆,并具有较高的氧逸度。受石榴石的主量元素组成和相应的晶体结构控制,在微量元素组成上,矾山杂岩体中的富钛钙铁榴石富集轻稀土或中稀土、亏损重稀土,大离子亲石元素(Rb、Ba、Pb和Sr)含量较低或极低,而高场强元素(如Th、U、Nb、Ta、Zr和Hf等)含量普遍较高。本文对石榴石的成因研究结果支持矾山杂岩体不同类型的岩石形成于封闭体系下同一母岩浆系统充分的结晶-分异和堆晶作用;此演化模式可以较好地解释矾山杂岩体的同心环状特征和韵律层状结构,以及磁铁矿和磷矿的成因。  相似文献   

16.
The Kamarbon alkaline gabbroic intrusion crops out in Central Alborz, north Iran, along the northern margin of the Alpine-Himalayan belt. The gabbroic intrusion includes theralites at margins which replace with teschenites toward the center. In teschenitic rocks, the main minerals are diopside, clinopyroxene, and rhönite. In this research, the occurrence of rhönite is reported in Kamarbon teschenitic gabbros, and also its mineralogical properties, paragenesis, and source magma are investigated. Based on whole rock and microprobe analysis data, we try to clarify the formation of Kamarbon gabbro and the crystallization condition of rhönite. In teschenitic gabbros, rhönite reveals the composition (Na, Ca)1.97(Ti, VIAl, Fe+3, Fe+2, Mn, Mg)5.99(Si, IVAl)6.02O20. On the basis of petrographical observations and mineral chemistry, we suggest that the teschenites were formed in distinctive lower pressures and temperatures than theralites, below 1.9 kbar and 1075 °C. Rhönite was crystallized (at the mentioned P-T condition) as a primary phase, in the late stage of crystallization at shallow depth corresponding with 6–10 km, in teschenites. Important factors of the rhönite crystallization in undersaturated magmas can be regardarded as Al and Ti enrichment and Si depletion; the same enrichment and depletion are also observed in the associated clinopyroxenes.  相似文献   

17.
 The crystal chemical response of basalt clinopyroxene to increasing pressure was investigated by means of crystal-structure simulation (a procedure that enables modeling of the structural parameters of a clinopyroxene of known chemistry without requiring direct X-ray diffraction analysis) using available experimental chemical data. Pressure proved the main physical variable governing clinopyroxene behavior in a magmatic environment. The general internal consistency of the simulation data permitted construction of an empirical geobarometer based on the relationship of cell volume (Vc) vs M1-site volume (VM1). The straightforward geobarometric formulation in the absence of direct X-ray analysis is: P(kbar)=698.443+4.985⋅AlT−26.826⋅Fe2+ M1−3.764⋅Fe3+ +53.989⋅AlM1+3.948⋅Ti+14.651⋅Cr −700.431⋅Ca−666.629⋅Na−682.848⋅MgM2−691.138⋅Fe2+ M2−688.384⋅Mn−6.267⋅(MgM2)2−4.144⋅(Fe2+ M2 where: (Fe2+ M1⋅MgM2)/(Fe2+ M2⋅MgM1)=e**(0.238⋅R3++0.289⋅CNM−2.315), CNM=Ca+Na+Mn, and R3+=AlM1+Fe3++Ti+Cr, with cations in atoms per formula unit. The geobarometer reproduces experimental pressures within ±2 kbar (=1σ; max. dev. ≤5 kbar; N=29) in the range 0–24 kbar and is applicable to near-liquidus C2/c clinopyroxenes crystallized from basaltic melts in the absence of garnet (excepting high-Al2O3 basalts). It is therefore suitable for many natural clinopyroxenes occurring as mega- or phenocrysts or forming well-preserved cumulate pyroxenites. If the above restrictions are not wholly satisfied, the Vc vs VM1 plot can also be used qualitatively to deduce the relative pressure conditions of clinopyroxenes forming from similar batches of magma. The structural simulation of experimental data also provided insight into the influence of minor chemical changes of the parental magma on the crystal chemistry of clinopyroxene at high pressure. Within the considered compositional space at given P-T, a CaO and a SiO2 in the melt have opposite effects on M2- and T-site cation populations. As a result, under similar physical conditions, clinopyroxenes from higher-CaO or more undersaturated basalts have higher VM2, VT and Vc and lower VM1. For basalts with normal contents of Al2O3 (<18 wt %), variations of major elements in the melt do not reduce the accuracy of the geobarometer. Received: 3 April 1994 / Accepted: 23 December 1995  相似文献   

18.
Clinopyroxene megacrysts and mineral aggregates with clinopyroxene occur in the volcanic deposits at Mont Briançon and Marais de Limagne, which are located in the northern part of the Devès volcanic field (Massif Central, France). The clinopyroxenes can be subdivided into five groups based upon their major and trace element chemistry. Types 1a, 1b and 1c have mg# ～0.80 and are relatively Al-rich and low in Na and Fe3+. Subdivision into three groups is based on differing trace element signatures. Type 2 clinopyroxenes have mg# = 0.63–0.65 and higher Na and Fe3+ (Fe3+/ΣFe > 0.4) contents and may contain apatite inclusions. A type 3 megacryst is Fe-rich (mg# = ～0.52) and has the highest Na and Fe3+ contents, as well as containing titanite and apatite inclusions. High Fe3+ contents in all clinopyroxenes investigated emphasises the need to consider Fe3+/Fe2+ when assessing the petrologic origin of such megacrysts. The large range in mg# means that the clinopyroxenes could not all have crystallised from the same melt; in fact comparison with the basanitic host lavas from the two localities reveal that nearly all of the megacrysts are xenocrystic in the strict sense. The clinopyroxenes are mostly genetically related, having crystallised from related melts within the magmatic system that had undergone various degrees of differentiation. Similarities in clinopyroxene chemistry indicate that both volcanic centres are linked to the same magmatic system at depth. Assessing the depth of crystallisation reveals that types 1a and 1b formed in the lithospheric mantle, near the asthenosphere–lithosphere boundary, whereas types 1c, 2 and 3 formed in crustal magma chambers or conduits. Eruption was induced by a pulse of Mg-rich magma from the asthenosphere that entered the existing magmatic system, entraining clinopyroxene as megacrysts at several stages of ascent, before erupting at the surface. The style of eruption at Mont Briançon (cinder cone) and Marais de Limagne (maar) is different and most likely reflects local differences in near-surface hydrology. The essentially identical variety in megacrysts at the two localities suggests that eruption must have been nearly contemporaneous.  相似文献   

19.
An analcimite sill, which intrudes Carboniferous sedimentary rocks northwest of the township of Barraba in northeastern New South Wales, is exceedingly rich in ultramafic and mafic inclusions and also contains a varied megacryst assemblage. The majority of inclusions belong to an ultramafic-mafic granulite suite whose members generally contain a Cr-poor green spinel. Layering is preserved in many inclusions and their textures are appropriate to those arising from recrystallization at subsolidus temperatures. Ultramafic granulites of the Al-spinel suite are mainly pyroxenites, with rarer lherzolites, and mafic granulites usually consist of the assemblage plagioclasea-luminous pyroxenes-spinel. Ca-rich tschermakitic clinopyroxenes and coexisting aluminous Ca-poor orthopyroxenes define a trend of moderate iron enrichment. Spinels also display significant Fe2+ → Mg substitution. Plagioclase in some plagioclase-bearing pyroxenites and mafic granulites contains numerous rod-like inclusions of spinel, compositionally similar to the discrete spinels unassociated with plagioclase. The formation of spinel in plagioclase is believed to have resulted largely from the migration of (Mg, Fe2+) to Al-rich nucleation sites in the feldspar. Other inclusion types include Cr-spinel lherzolites —more Fe-rich than Cr-diopside lherzolite inclusions in alkaline volcanics — and rare wehrlite heteradcumulates, probably cognate with the host analcimite. The megacryst assemblage is dominated by anorthoclase megacrysts, which are accompanied, in order of decreasing abundance, by megacrysts of tschermakitic clinopyroxene, titanbiotite, kaersutite, and aluminous titanomagnetite. The Al-spinel mafic granulites have low Ti, K and P contents and their petrochemical affinities are high-alumina mafic alkaline to transitional. They compare closely in major and minor element chemistry with some ocean ridge basalts. The Al-spinel ultramafic-mafic inclusions suite is interpreted as the remnants of a layered ultramafic-mafic “pluton ” which initially crystallized at pressures in the vicinity of 10 kb and subsequently re-equilibrated at subsolidus temperatures (ca 950° C) and comparable pressures. The parent magma was K-poor, ol-normative subalkaline and its fractionation at moderate pressures, controlled mainly by olivine and subcalcic clinopyroxene, resulted in decreases in the derivative liquids in their saturation levels and ol contents, and increases in Al and Ca. These trends are reflected in the compositions of the mafic granulites. The pressure regime of megacryst formation apparently was greater than 10–12 kb i.e. the megacrysts precipitated before acquisition of xenoliths of the Al-spinel granulite suite by the analcimite host. Anorthoclase fractionation produced only limited compositional changes in the original alkali basaltic melt.  相似文献   

20.
Layers of Ca-rich garnet–clinopyroxene rocks enclosedin a serpentinite body at Hujialin, in the Su–Lu terraneof eastern China, preserve igneous textures, relict spinel ingarnet, and exsolution lamellae of Ca-rich garnet, ilmenite/magnetite,Fe-rich spinel, and also amphibole in clinopyroxene. In termsof their major and trace element compositions, the studied samplesform a trend from arc cumulates towards Fe–Ti gabbros.Reconstructed augite compositions plot on the trend for clinopyroxenein arc cumulates. These data suggest that the rocks crystallizedfrom mantle-derived magmas differentiated to various extentsbeneath an arc. The Ca-rich garnet + diopside assemblage isinferred to have formed by compressing Ca-rich augite, whereasthe relatively Mg-rich cores of garnet porphyroblasts may haveformed at the expense of spinel. The protolith cumulates weresubducted from near the crust–mantle boundary (c. 1 GPa)deep into the upper mantle (4·8 ± 0·6 GPaand 750 ± 50°C). Negatively sloped P–T pathsfor the garnet–clinopyroxene rocks and the corollary ofcorner flow induced subduction of mantle wedge peridotite arenot supported by the available data. Cooling with, or without,decompression of the cumulates after the igneous stage probablyoccurred prior to deep subduction. KEY WORDS: arc cumulates; Ca-rich garnet; garnet–clinopyroxene rocks; Su–Lu terrane; UHP metamorphism  相似文献   

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