中国大陆科学钻探主孔100～2000米超高压变质岩岩相学特征与变质变形史

中国大陆科学钻探工程的实施，为超高压变质带的研究打开了新的生面，加深了对苏鲁超高压变质带的认识。从100～2000米获得的岩心的岩石学观察，得知主要岩石类型有：(1)榴辉岩及石榴辉石岩；(2)榴辉岩质片麻岩；(3)石榴橄榄岩；(4)黑云(角闪)二长片麻岩和(5)碎裂岩等。榴辉岩可分幔源和壳源两类，壳源榴辉岩在钻孔中分布较广，上部最为集中；幔源榴辉岩，包括石榴辉石岩，在空间上与超镁铁岩有密切共生关系。榴辉岩质的片麻岩是一种中酸性的超高压岩石，与壳源的榴辉岩共生，显微镜下可以追索出它们之间的结构演化关系。石榴橄榄岩以石榴单辉橄榄岩为主，是俯冲带上部地幔楔加入于俯冲板片变质而成。石榴橄榄岩中的石榴子石富镁，单斜辉石为绿辉石并常含钛斜硅镁石，说明其经过超高压变质的过程。从变质岩石的组合，面理和线理产状的差异，地震反射面和构造角砾岩带的发育，发现以1600米为界，可大致分为2个岩片。上部岩片中多金红石榴辉岩而且出现频率很高，下部岩片中多为多硅白云母榴辉岩出现频率较低。由于隆升进入中下地壳，超高压变质岩普遍发生退变质。榴辉岩的早期退变质成为具后成合晶结构的石榴角闪岩，榴辉岩质片麻岩退变质形成绿帘黑云(角闪)斜长片麻岩，变质条件为角闪岩相，它可以部分熔融或受到钾交代作用而转变为黑云角闪二长片麻岩。后期的伸展造成了局部碎裂和角砾岩带，新生矿物为绿泥石，方解石和赤铁矿、绿帘石等，属绿帘角闪岩相和绿片岩相。主孔100～2000米的超高压变质岩石组合的确定，进一步说明了巨量的地壳物质可以深俯冲进入地幔并迅速折返；超高压变质岩石记录了陆壳俯冲折返和壳幔相互作用的过程，它们是板块下覆构造和地幔动力学的信息载体。     

洋壳俯冲过程中的地幔楔上升对流：来自芝麻坊石榴子石二辉橄榄岩早期变质的证据

苏鲁超高压变质地体中发现了大量包裹在超高压（UHP）变质片麻岩和混合岩中的造山带石榴橄榄岩。根据它们的野外产出特征和全岩地球化学成分,其中一部分石榴橄榄岩的原岩来自于亏损地幔,后来被卷入俯冲陆壳并经受过俯冲陆壳产生的熔/流体的交代。但是,对这些岩石早期的亏损过程尚缺乏清晰的认识。本文报道了东海芝麻坊石榴子石二辉橄榄岩早期变质演化的新证据。根据详细的变质反应结构观察和矿物成分研究,芝麻坊石榴子石二辉橄榄岩在经历高压低温俯冲带型超高压变质之前经历了至少两期变质演化。其原岩矿物组合由石榴子石变斑晶的高Ca-Cr核部及其中包裹的高Mg单斜辉石、高Al-Cr斜方辉石和高Mg-Ni橄榄石所记录;指示芝麻坊石榴子石二辉橄榄岩的原岩为高温-高压的富集石榴子石二辉橄榄岩。第二期矿物组合为包裹在低Cr变斑晶石榴子石幔部和细粒新生石榴子石核部的大量富Al铬铁矿和高Mg低Ni橄榄石以及少量高Mg斜方辉石。该期组合未发现单斜辉石,表明岩石随后被转变为高温低压的难熔尖晶石方辉橄榄岩或尖晶石纯橄岩。芝麻坊石榴子石二辉橄榄岩的早期变质演化记录了它们被卷入大陆板片俯冲带之前的地幔楔上升对流过程。笔者认为芝麻坊石榴子石二辉橄榄岩的原岩来源于早期俯冲大洋板片之上的深部高温富集地幔楔,洋壳俯冲过程中的地幔楔对流导致其上升到弧后或岛弧之下的地幔楔浅部,减压部分熔融使原本富集的石榴子石二辉橄榄岩转化为难熔的尖晶石方辉橄榄岩或尖晶石纯橄岩。     

大别山北坡尖晶石橄榄岩中的高温石榴辉石岩包体：兼论上地幔部分熔融

出露于饶拔寨一带的含尖晶石橄榄岩体是大别山区最大的一个上地幔残片,固态侵位于北大别深成片麻岩中,呈无根侵入体的形式产出。岩体内的石榴辉石岩和榴辉岩包体,具高温成因的特点。属于上地幔部分熔融的产物,在Pl-CIPW值对Al_2O_3图解上落于拉斑玄武岩区;榴辉岩、石榴辉石岩和含尖晶石橄榄岩的REE配分形式相似,均属平坦型,只是尖晶石橄榄岩的∑REE较低。在Ni-Co-Sc三组分图解上,石榴辉石岩投绘于深位玄武岩熔体的趋势线附近,而尖晶石橄榄岩的投点则接近超镁铁质残余的趋势,清楚揭示出地幔部分熔融的迹象。尖晶石的Cr~#=12～21,而共生橄榄石Fo=92～93,说明部分熔融程度不高,估计约15％。尖晶石贫TiO_2暗示熔融过程氧逸度低。饶拔赛含尖晶石橄榄岩的出现表明;伴随超高压岩石单元的折返和隆升,会有大陆地幔残片被携带上来。软流圈的上涌或板片的断离使侵出的岩石圈板片得以维持较高的温度,这也就是石榴辉石岩早期深位退变质为麻粒岩相的原因,是后续的进一步抬升,才出现以韭角闪石 斜长石组合为特征的高角闪岩相的退变质作用。     

大陆碰撞造山带的两类橄榄岩——以柴北缘超高压变质带为例

论述了大陆俯冲碰撞带中地幔橄榄岩的基本特征和成岩类型,并重点讨论柴北缘超高压变质带中不同性质的橄榄岩及其成因。根据岩石学特征,我们确定柴北缘超高压带中发育有两种类型的橄榄岩:(1)石榴橄榄岩,岩石类型包括石榴二辉橄榄岩、石榴方辉橄榄岩、纯橄岩和石榴辉石岩,是大陆型俯冲带的标志性岩石。金刚石包裹体、石榴石和橄榄石的出溶结构、温压计算等均反映其来源深度大于200km。地球化学特征表明该橄榄岩的原岩是岛弧环境下高镁岩浆在地幔环境下堆晶的产物。(2)大洋蛇绿岩型地幔橄榄岩,与变质的堆晶杂岩(包括石榴辉石岩、蓝晶石榴辉岩)和具有大洋玄武岩特征的榴辉岩构成典型的蛇绿岩剖面,代表大洋岩石圈残片。这两类橄榄岩的确定对了解柴北缘超高压变质带的性质和构造演化过程有重要意义。     

北苏鲁荣成地区超高压变质带大理岩中石榴辉石岩的退变质过程

北苏鲁荣成地区超高压变质带大理岩中的退变质石榴辉石岩包裹体具有“核-幔-边”结构,核部主要由石榴子石与单斜辉石组成,幔部含有由细小角闪石与绿帘石组成的后成合晶以及颗粒较大的角闪石与单斜辉石,通过详细的岩相学分析以及矿物成分分析,认为这些后成合晶是由石榴子石、单斜辉石以及来自围岩的流体共同反应而产生,大颗粒的角闪石主要是由辉石转变而来的,在幔部这个转变并不彻底,仍有一些残余辉石颗粒。边部主要由角闪石和绿帘石组成。该石榴辉石岩曾经历榴辉岩相超高压变质阶段。没有柯石英超高压代表性矿物（采样处的其他类型岩石都含有柯石英）的原因是石榴辉石岩的原岩为超基性岩。     

北大别洪庙榴辉岩相岩石Sm-Nd年龄:峰期变质时代

前人工作认为北大别榴辉岩在榴辉岩相变质后,经历了麻粒岩相退变质作用,因此获得的Sm-Nd矿物等时线年龄代表了麻粒岩相变质时代.本文对北大别安徽洪庙百丈岩榴辉岩相岩石(辉石石榴石岩)的研究表明,该岩石经历了三叠纪超高压变质作用,经历了角闪岩相退变质.所研究样品在峰期变质之后是否经历了麻粒岩相退变质尚不能明确界定.结合矿物氧同位素体系平衡判据,辉石石榴石岩在峰期变质时达到了Sm-Nd同位素体系的均一化和平衡,并且在后期角闪岩相退变质中该同位素体系未出现明显的扰动,因此其Sm-Nd矿物等时线年龄[(225±14)Ma和(229±13)Ma]代表了榴辉岩相变质时代.同时,样品中矿物的同位素组成表明,在俯冲板块折返和退变质过程中,岩石受到外来和内部流体的不均匀作用,造成退变的单斜辉石在同位素组成上的不均一.     

北大别木子店石榴辉石岩的麻粒岩相退变质作用

详细的岩相学、矿物化学、岩石化学和变质作用温压条件计算表明, 大别变质地体北部角闪二辉麻粒岩是石榴辉石岩在其抬升过程中经历麻粒岩相退变质作用的产物, 峰期变质作用至少是发生在高压榴辉岩相条件下, 而且地温梯度较低.石榴辉石岩的退变质作用p-t轨迹以早期的近等温降压、中期的近等压升温和晚期的降温降压为特征.这一结果表明北大别变质地体的峰期变质作用并非仅达麻粒岩相.      

超镁铁质捕虏体中共存矿物的Cr~#在岩石成因中的意义

中国东部新生代玄武质岩石中的超镁铁质捕虏体,主要包括五种类型:石榴石二辉橄榄岩(±少量尖晶石)、尖晶石二辉橄榄岩、尖晶石方辉橄榄岩、辉石岩和巨晶矿物。不同类型捕虏体间的共存矿物存在系统的化学成分变化。尤以Al_2O_3、Cr_2O_3变化明显。本文提出捕虏体中共存矿物的Cr~#[100Cr/(Al+Cr)]可作为分类和岩石成因的重要标志。并将五种捕虏体划分为三种地幔成因类型:饱满的或原始的地幔(石榴石二辉橄榄岩和尖晶石二辉橄榄岩),亏损的或残余的地幔(尖晶石方辉橄榄岩),地幔条件下熔浆分离的产物(辉石岩和巨晶矿物)。     

青藏高原拉萨地块松多榴辉岩的岩相学特征和变质演化过程

松多榴辉岩出露于拉萨地块的石英片岩中,主要由较为基性的金红石榴辉岩和较为酸性的石英榴辉岩组成。榴辉岩相矿物组合为石榴子石 绿辉石 绿帘石±多硅白云母±石英±金红石。岩石发生了较强烈的退变质作用,退变质矿物有角闪石、绿帘石、石英、钠长石及绿泥石。石榴子石变斑晶具有生长环带结构,变斑晶和基质石榴子石主要落入C类榴辉岩区,少数石榴子石变斑晶边部和基质石榴子石落入B类榴辉岩区;单斜辉石主要为绿辉石,少数Ⅰ世代和Ⅲ世代为普通辉石;角闪石均为钙质角闪石。根据石榴子石-绿辉石-多硅白云母矿物温压计计算,获得的温压范围为630～777℃和2.58～2.70GPa,峰期变质条件接近于石英-柯石英转变线。榴辉岩的原岩经历了从高绿片岩相、角闪岩相、榴辉岩相、角闪岩相到高绿片岩相的变质过程,这反映了与古特提斯洋闭合有关的俯冲进变质作用和随后的折返退变质作用。     

雅鲁藏布江缝合带白朗县白岗尖晶石—石榴子石相二辉橄榄岩的相界反应及其意义

西藏白朗县白岗村蛇绿混杂岩中有一罕见的尖晶石石榴子石二辉橄榄岩小岩块,被松软的蛇纹岩化尖晶石二辉橄榄岩包裹其中。岩块中发育有碎基单斜辉石、斜方辉石中出溶单斜辉石、切过出溶单斜辉石的贯入单斜辉石和外来碎粒单斜辉石及钙质辉石+铬尖晶石→钙铁石榴子石相界反应。同时,在岩块和包壳岩石的橄榄石中出现针状硅镁石出溶物。计算这些矿物的温度压力表明,它们的温度压力都处于>800℃,>1.8GPa以上的地幔石榴子石域超高压环境,而且,经历过一个上升→俯冲→上升的"N"字形历程。     

中国大陆科学钻探CCSD-PP3钻孔地幔岩的尖晶石相部分熔融证据

CCSD PP3超镁铁岩表现为高 Mg 和 Cr,低 Ca 和 Al 特征,是中国东部和超高压变质带上极其罕见的亏损地幔岩,通过地球化学和矿物学和 Re-Os 同位素研究发现岩石来自古元古代岩石圈地幔,并在地幔浅部尖晶石相条件下发生部分熔融,最近一次部分熔融作用发生在中-晚元古代,后经岩石圈厚度加大、拆沉或深俯冲过程,岩石转变为石榴石橄榄岩。     

Les péridotites et pyroxénolites à grenat du Bois des feuilles (Monts du lyonnais) (France)

The author describes a new occurrence of garnet peridotite and garnet pyroxenite interlayered in the biotite-sillimanite-garnet gneisses at the top of the granulitic serie of the Monts du Lyonnais (Massif Central français). Its dimensions are rather significant for a crustal gisement (500×100 m). It is only composed of forsterite, enstatite, chromiferous diopside, pyrope and spinel peridotites with their products of retrograde transformations as kelyphites, amphiboles, chlorites, lizardite, ores, etc. The petrographic studies show the heterogeneity of the massif and the anteriority of the red spinel upon the garnet which always forms a corona around the spinel. The peridotites are intermingled with numerous streched and dislocated layers of garnet websterites with rare centimetric levels. These pyroxenites would be derived of particular magmatic processes (partial anatectic melting followed by cristallisation) developped from an upper mantle level in a primary pyrolitic lherzolitic (s. l) or garnet peridotitic material. The garnet peridotite of “Le Bois des Feuilles” would be, in fact, a “secondary garnet lherzolite” derived: - either from a spinel lherzolite intermingled with garnet websterite layers and their “dunitic” remnants, to form a “pseudo-garnet lherzolite” like this of Beni Bouchera described by Kornprobst; - or from a spinel lherzolite associated with garnet websterites and submitted temporarily, at the time of its diapiric rising movement from the mantle towards the crust to the conditions of the spinel garnet lherzolite facies. The plastic deformations and intense laminations form blastomylonites of mixed rocks recristallised ultimately under granulitic facies conditions. These rocks are, pro parte, not very different from the other crustal garnet peridotites, in spite of the frequency of the spinel inclusions in garnet. In corollary, it seems that numerous crustal garnet peridotites would have the same origin.     

Combined iron and magnesium isotope geochemistry of pyroxenite xenoliths from Hannuoba,North China Craton: implications for mantle metasomatism

We present high-precision iron and magnesium isotopic data for diverse mantle pyroxenite xenoliths collected from Hannuoba, North China Craton and provide the first combined iron and magnesium isotopic study of such rocks. Compositionally, these xenoliths range from Cr-diopside pyroxenites and Al-augite pyroxenites to garnet-bearing pyroxenites and are taken as physical evidence for different episodes of melt injection. Our results show that both Cr-diopside pyroxenites and Al-augite pyroxenites of cumulate origin display narrow ranges in iron and magnesium isotopic compositions (δ⁵⁷Fe = ?0.01 to 0.09 with an average of 0.03 ± 0.08 (2SD, n = 6); δ²⁶Mg = ? 0.28 to ?0.25 with an average of ?0.26 ± 0.03 (2SD, n = 3), respectively). These values are identical to those in the normal upper mantle and show equilibrium inter-mineral iron and magnesium isotope fractionation between coexisting mantle minerals. In contrast, the garnet-bearing pyroxenites, which are products of reactions between peridotites and silicate melts from an ancient subducted oceanic slab, exhibit larger iron isotopic variations, with δ⁵⁷Fe ranging from 0.12 to 0.30. The δ⁵⁷Fe values of minerals in these garnet-bearing pyroxenites also vary widely (?0.25 to 0.08 in olivines, ?0.04 to 0.25 in orthopyroxenes, ?0.07 to 0.31 in clinopyroxenes, 0.07 to 0.48 in spinels and 0.31–0.42 in garnets). In addition, the garnet-bearing pyroxenite shows light δ²⁶Mg (?0.43) relative to the mantle. The δ²⁶Mg of minerals in the garnet-bearing pyroxenite range from ?0.35 for olivine and orthopyroxene, to ?0.34 for clinopyroxene, 0.04 for spinel and ?0.68 for garnet. These measured values stand in marked contrast to calculated equilibrium iron and magnesium isotope fractionation between coexisting mantle minerals at mantle temperatures derived from theory, indicating disequilibrium isotope fractionation. Notably, one phlogopite clinopyroxenite with an apparent later metasomatic overprint has the heaviest δ⁵⁷Fe (as high as 1.00) but the lightest δ²⁶Mg (as low as ?1.50) values of all investigated samples. Overall, there appears to be a negative co-variation between δ⁵⁷Fe and δ²⁶Mg in the Hannuoba garnet-bearing pyroxenite and in the phlogopite clinopyroxenite xenoliths and minerals therein. These features may reflect kinetic isotopic fractionation due to iron and magnesium inter-diffusion during melt–rock interaction. Such processes play an important role in producing inter-mineral iron and magnesium isotopic disequilibrium and local iron and magnesium isotopic heterogeneity in the subcontinental mantle.     

铬尖晶石和石榴石的相变: 大陆科学钻探CCSD-PP3孔超镁铁岩超高压变质作用的证据

PP3超镁铁岩主要岩石类型有纯橄岩和石榴石橄榄岩,两者为渐变,主要矿物为橄榄石、铬尖晶石、石榴石、单斜辉石和斜方辉石.铬尖晶石的Cr#[Cr/(Cr+Mg) ×100]从51~89变化,TiO₂和MnO₂值分别低于0.26%和0.46%.铬尖晶石矿物表现为4期次演化的特点,反映了从岩浆期、榴辉岩相、角闪岩相和绿片岩相演化特征.随着超镁铁岩的演化,铬尖晶石表现为Cr#不断增大,而Mg#[Mg×100/(Mg+Fe2+) ]不断减少、氧逸度不断增加的过程.PP3铬尖晶石反映了地幔来源,为大陆岩石圈超镁铁岩特征,后期随折返而演化.从石榴石与铬尖晶石相互转变过程看出,PP3超镁铁岩经历了深度加大的过程,超镁铁岩曾经到达100km以上的岩石圈地幔深处.在绿片岩相-绿片角闪岩相变质过程中,铬尖晶石中Cr、Mg和Al减少,Fe相对增加,产生富Cr尖晶石变质作用样式.晚期剪切变形等次生变化影响了铬尖晶石矿物成分.      

Thermal and redox equilibrium conditions of the upper-mantle xenoliths from the Quaternary volcanoes of NW Spitsbergen,Svalbard Archipelago

Upper-mantle xenoliths in Cenozoic basalts of northwestern Spitsbergen are rocks of peridotite (spinel lherzolites) and pyroxenite (amphibole-containing garnet and garnet-free clinopyroxenites, garnet clinopyroxenites, and garnet and garnet-free websterites) series. The upper-mantle section in the depth range 50–100 km is composed of spinel peridotites; at depths of 80–100 km pyroxenites (probably, dikes or sills) appear. The equilibrium conditions of parageneses are as follows: in the peridotites—730–1180 °C, 13–27 kbar, and oxygen fugacity of − 1.5 to + 0.3 log. un.; in the pyroxenites—1100–1310 °C, 22–33 kbar. The pyroxenite minerals have been found to contain exsolved structures, such as orthopyroxene lamellae in clinopyroxene and, vice versa, clinopyroxene lamella in orthopyroxene. The formation temperatures of unexsolved phases in orthopyroxene and clinopyroxene are nearly 100–150 °C higher than the temperatures of the lamellae–matrix equilibrium and the equilibrium of minerals in the rock. The normal distribution of cations in the spinel structure and the equilibrium distribution of Fe^2 + between the M1 and M2 sublattices in the orthopyroxenes point to the high rate of xenolith ascent from the rock crystallization zone to the surface. All studied Spitsbergen rock-forming minerals from mantle xenoliths contain volatiles in their structure: OH⁻, crystal hydrate water H₂O_cryst, and molecules with characteristic CH and CO groups. The first two components are predominant, and the total content of water (OH– + H₂O_cryst) increases in the series olivine → garnet → orthopyroxene → clinopyroxene. The presence of these volatiles in the nominally anhydrous minerals (NAM) crystallized at high temperatures and pressures in the peridotites and pyroxenites testifies to the high strength of the volatile–mineral bond. The possibility of preservation of volatiles is confirmed by the results of comprehensive thermal and mass-spectral analyses of olivines and clinopyroxene, whose structures retain these components up to 1300 °C. The composition of hypothetic C–O–H fluid in equilibrium (in the presence of free carbon) with the underlying mantle rocks varies from aqueous (> 80% H₂O) to aqueous–carbonic (~ 60% H₂O). The fluid becomes essentially aqueous when the oxygen activity in the system decreases. However, there is no strict dependence of the redox conditions on the depth of formation of xenoliths.     

Subduction of lithospheric upper mantle recorded by solid phase inclusions and compositional zoning in garnet: Example from the Bohemian Massif

This paper presents monomineral and multiphase inclusions in garnet from eclogites and clinopyroxenites, which form layers and boudins in garnet peridotites from two areas in the Moldanubian zone of the Bohemian Massif. The garnet peridotites occur in felsic granulites and reached UHP conditions prior to their granulite facies overprint. In addition to complex compositional zoning, garnets from hosting eclogites and clinopyroxenites preserve inclusions of hydrous phases and alkali silicate minerals including: amphiboles, chlorites, micas and feldspars. Amphibole, biotite and apatite inclusions in garnet have a high concentration of halogens; CO₂ and sulfur are involved in carbonates and sulfide inclusions, respectively. The inclusion patterns and compositional zoning in garnet in combination with textural relations among minerals, suggest that the ultramafic and mafic bodies are derived from lithospheric mantle above the subduction zone and were transformed into garnet pyroxenites and eclogites in the subduction zone. Based on compositional, mineral and textural relations, all of these rocks along with the surrounding crustal material were overprinted by granulite facies metamorphism during their exhumation.     

Some subcalcic clinopyroxenites from Salt Lake Crater,Oahu, and their petrogenetic significance

Some inclusions from Salt Lake Crater are essentially single-phase subcalcic clinopyroxenites whose original clinopyroxenes, prior to extensive unmixing, were tschermakitic subcalcic varieties with compositions close to Ca₃₄Mg₅₄Fe₁₂. In addition to copious amounts of orthopyroxene, very minor garnet and spinel also were exsolved from the subcalcic clinopyroxenes.The genesis of the garnet pyroxenite suite at Salt Lake Crater has been examined in terms of three models, namely: (i) cumulates from alkali basaltic magmas; (ii) fractional fusion of basanitic garnet clinopyroxenite; and (iii) anatexis of upper mantle lherzolites. Field, mineralogical, chemical and experimental data collectively favour model (iii) and indicate that the nodules are genetically unrelated to their nephelinitic hosts. The Salt Lake garnet pyroxenites can be closely equated with the garnet pyroxenites in magmatictype layers in certain alpine-type ultramafic massifs and they are also similar to many garnet pyroxenite xenoliths in alkaline volcanics from other localities.Liquids produced by anhydrous partial melting of spinel Iherzolite at pressures of approximately 20 kb commonly have picritic chemistries. The crystallization behaviour of picritic liquids at elevated pressures ( 20 kb) indicates that the initial crystallization products may be either essentially single-phase subcalcic clinopyroxenites (with minimal high pressure fractionation) or a range of olivine-aluminous orthopyroxene-aluminous subcalcic clinopyroxene-garnet-(spinel) assemblages with variable 100 Mg/(Mg+Fe) ratios (when fractionation has been operative). All these assemblages may be subsequently modified by subsolidus exsolution and recrystallization.     

Pressure-temperature dependent compositional variation of phlogopitic micas in upper mantle peridotites

Compositional variation of phlogopitic micas in upper mantle peridotites is reviewed. Phlogopitic micas in garnet peridotites are systematically lower in Al (or eastonite component) than those in spinel peridotites. The core of phlogopite megacryst and phenocryst of kimberlite is always lower in Al than the rim. It is apparent that Al/(Al + Si) ratio or eastonite component in phlogopitic micas in ultramafic rocks is controlled by the equilibrium pressure and temperature. In the upper mantle peridotites containing garnet or spinel, the Al/(Al + Si) ratio of phlogopitic mica decreases with increasing pressure at constant temperature. Phlogopitic mica is a potential thermo-barometer in both garnet- and spinel-peridotite facies.     

High-pressure ultramafics in the lower crustal rocks of the Pekul’ney complex, central Chukchi Peninsula. 1. Petrography and mineralogy

Dunites, peridotites, olivine and spinel pyroxenites, and metagabbroids have been described in the tectonic blocks of the Pekul’ney complex of the central Chukchi Peninsula together with garnet-hornblende-clinopyroxene and zoisite (clinozoisite)-garnet-hornblende rocks, which are indicative of high-pressure complexes. However, the interpretations of previous researchers on the composition, structure, setting, and processes of formation of this rock association are highly controversial. The petrographic and mineralogical results reported in this paper indicate that the blocks of the complex host bodies of cumulate ultramafics among metamorphic rocks. These relationships were supported by the finding of xenoliths and xenocrysts of metamorphic rocks in the ultramafics. The metamorphic country rocks are lower crustal amphibolites and schists with peak metamorphic parameters corresponding to the high-pressure portion of the epidoteamphibolite facies (610–680°C and 9–14 kbar). All the varieties of ultramafic rocks studied in the blocks of the complex are assigned to a single cumulate series (from dunite to clinozoisite-garnet hornblendite), and the compositions of their primary minerals show regular correlations similar to crystallization differentiation trends. Specific features of the ultramafics of the Pekul’ney complex are the early crystallization of hornblende (which is present already in peridotites), wide range of garnet crystallization (associating with clinopyroxene, ceylonite, and hornblende), presence of magmatic clinozoisite in the most evolved assemblages (with garnet, hornblende, and clinopyroxene), and absence of evidence for plagioclase crystallization. Clinopyroxene from the most evolved ultramafic rocks contains more than 15 wt % Al₂O₃. The classification of the rocks of the complex provides a basis for the interpretation of geological relations between them and the elucidation of the characteristics of the internal structure of the blocks of the complex and bodies of cumulate ultramafic rocks in them.