中天山白石泉镁铁-超镁铁质岩体岩石学与矿物学研究

白石泉地区镁铁一超镁铁质岩体处于塔里木板块前缘活动带与中天山地块接合部位，是中天山地块华力西中期岩浆活动的产物。主要岩石类型有辉石橄榄岩（斜方辉石橄榄岩、斜长二辉橄榄岩）、橄榄辉石岩、橄长岩、辉长岩及角闪辉长岩等，主要造岩矿物为橄榄石、斜方辉石、单斜辉石、角闪石、斜长石及黑云母。橄榄石均为贵橄榄石，其Fo值（78-85）位于含铜镍硫化物矿橄榄石的Fo值范围之内；辉石主要有顽火辉石、古铜辉石、紫苏辉石、透辉石等；斜长石的环带构造较为发育；角闪石的FeO含量随着岩浆的演化逐渐增加。它们与造山带环境中的东疆型镁铁一超镁铁杂岩中的造岩矿物具有相同的特征。这些特征表明了白石泉地区的镁铁一超镁铁质岩体的原始岩浆为高镁的拉斑玄武质岩浆。     

甘肃北山红柳沟基性-超基性岩体岩石成因及成矿条件

甘肃北山红柳沟基性-超基性岩体位于塔里木板块北缘北山裂谷带, 岩体侵位于敦煌岩群,主要岩石类型有辉长岩、橄榄辉长岩、橄榄苏长辉长岩、橄榄角闪苏长岩、橄榄辉长苏长岩、二辉橄榄岩、橄榄辉石岩和辉石岩等。橄榄石Fo介于66.97%~82.92%之间,属贵橄榄石,斜方辉石En成分范围为68.49~77.65,属古铜辉石;单斜辉石En成分范围为45.85~48.81,主要为斜顽辉石和透辉石;斜长石An为58.70~72.69,以拉长石为主;角闪石以普通角闪石为主。岩体母岩浆Mg^#值为0.59~0.62,属于高镁拉斑玄武质岩浆,岩浆演化过程中主要发生了橄榄石、斜方辉石、单斜辉石和斜长石的分离结晶作用,主要分离结晶矿物受单斜辉石和斜方辉石的控制,岩浆上升侵位过程中遭受到下地壳物质混染。从构造环境、母岩浆、岩体类型、岩浆分异程度、同化混染等方面综合分析认为红柳沟岩体具有形成铜镍硫化物矿床的较大潜力。     

扬子地块北缘新元古代望江山层状岩体矿物成分和铂族元素特征:对岩浆演化过程和构造环境的制约

望江山层状岩体位于扬子地块北缘新元古代汉南杂岩带中,岩体从底部到顶部由超镁铁质岩过渡为中性岩:底部主要由辉石岩和橄长岩组成;中部为辉长苏长岩和辉长岩;上部为辉长岩和闪长岩。研究以中部岩相带橄榄辉长苏长岩、辉长苏长岩和辉长岩为对象,通过主要矿物的主微量元素和全岩主微量元素的分析,查明望江山岩体来源于尖晶石二辉橄榄岩组成的大陆下岩石圈地幔,并且地幔源区受到了来自俯冲板片流体的交代,岩体中部带的母岩浆为拉斑玄武质岩浆。钛铁矿—磁铁矿矿物对成分计算表明,母岩浆在形成时具有较高氧逸度。通过单斜辉石压力计得到岩体的侵位深度约为12.9~18 km。对岩体母岩浆橄榄石分离结晶过程的模拟计算表明,中部带橄榄石为母岩浆经过~28%分离结晶的产物。此外,铂族元素（PGE）组成暗示岩体并未经历过大规模的硫化物熔离,可能与缺乏地壳物质混染有关。岩体中单斜辉石与岛弧环境堆晶岩中单斜辉石成分相似,不同于裂谷环境中堆晶单斜辉石的成分;同时,全岩Th/Yb和Nb/Yb比值也与岛弧玄武岩比值相似,因此矿物和全岩成分均说明望江山层状岩体应形成于岛弧环境。研究认为扬子北缘在新元古代长期的俯冲过程中,大洋板片断离导致软流圈上涌,提供热源使交代大陆下岩石圈地幔部分熔融形成具有岛弧特征的镁铁质岩浆,在局部伸展环境中上升侵位形成汉南杂岩带中镁铁—超镁铁质层状岩体。      

新疆东昆仑达拉库岸镁铁-超镁铁岩的矿物化学及其属性

达拉库岸镁铁-超镁铁岩位于东昆仑造山带南带之喀拉米兰晚古生代沟弧系,由二辉橄榄岩、单辉橄榄岩、橄榄辉石岩、单辉岩、含长辉石岩和辉长岩组成。单辉橄榄岩和橄榄辉石岩中橄榄石均为贵橄榄石(Fo=84.55~89.08),其MnO含量为0.13%~0.29%,NiO含量为0.09%~0.28%;单斜辉石为透辉石和普通辉石,其MgO含量为15.12%~16.98%,Fe O含量为3.84%~5.34%,CaO含量为21.10%~22.95%;与蛇绿岩套中的同类岩石的橄榄石和单斜辉石成分存在较大差异,与夏日哈木和金川镁铁-超镁铁岩中同类岩石的矿物成分类似,表明达拉库岸岩体不是蛇绿岩套的组成部分,而是陆壳中的侵入体。单斜辉石成分表明其母岩浆为拉斑玄武质岩浆,可能形成于与俯冲有关的大陆边缘裂谷环境。达拉库岸岩体具有形成岩浆型铜镍硫化物矿床的条件。     

云南金宝山镁铁-超镁铁岩的地球化学特征及成因

金宝山超镁铁侵入岩赋存有超大型硫化物铂族元素矿床。单辉橄榄岩是岩区员主要的含矿岩石类型。根据单辉橄榄岩的矿物学、岩石学、岩石化学及微量元素地球化学特征，探讨岩浆的结晶分异演化过程，提出岩体的形成与岩浆结品分异有关，母岩浆成分为拉斑质玄武质岩浆。岩体高镁、高稀土总量，以及橄榄石与辉石的结构特征显示岩体是橄揽石、铬尖晶石等矿物与残留岩浆混合形成。     

五台地区雁门关早元古代镁铁-超镁铁岩体的岩石地球化学特征、成因及其大地构造意义

本文对五台地区侵入于原北金岗库组的雁门关镁铁-超镁铁岩体进行了详细的岩石学、矿物学和岩石地球化学分析研究.雁门关岩体在矿物成份上,特别是单斜辉石、尖晶石和角闪石,具有阿拉斯加型侵入岩体和俯冲带或弧相关的岩浆特征,其元素地球化学特征也可与阿拉斯加型侵入杂岩体类比,具有明显的Nd (Ta),Zr (Hf),Y和Ti负异常.从构造环境判别图来看,雁门关岩体主要为与俯冲相关的成因,具大陆边缘弧的性质.因此,雁门关岩体是侵入在大陆边缘弧环境的岩浆经历了橄榄石和单斜辉石的分离结晶作用的产物.它与恒山-五台-阜平地区广泛分布的同时代花岗岩和火山岩一起,构成了华北克拉通中部造山带内一期重要的古元古代大陆边缘岛弧岩浆活动.     

攀西白马岩体的矿物结晶顺序与钒钛磁铁矿成因

距今约260 Ma的白马岩体位于上扬子板块西缘的攀西裂谷中,是一个大型的含钒钛磁铁矿镁铁质-超镁铁质杂岩体,是峨眉山大火成岩省的重要组成部分。含矿岩体主要由磁铁橄长岩和橄榄辉长岩组成,主要工业矿体赋存在下部的橄长岩岩相带中。显微镜下显示橄榄石和角闪石均存在2种不同的结构状态,岩浆具有多次脉动的侵位特点。矿物结构特点及磁铁矿、钛铁矿、橄榄石、角闪石及斜长石等矿物电子探针成分测定显示,矿物的结晶顺序大致为斜长石+橄榄石+辉石→角闪石+磁铁矿+钛铁矿→角闪石。根据角闪石和斜长石成分计算角闪石最低结晶温度为1090℃,斜长石的最高结晶温度是1120℃,推测磁铁矿的结晶温度介于1090~1120℃之间。橄榄石的Fo值由下部的磁铁橄长岩向上部的橄榄辉长岩呈逐渐降低的变化趋势,表明随着岩浆的结晶分异进程,系统的氧逸度是逐步变化的,暗示整个结晶分异过程系统处于封闭状态。磁铁矿中w(V2O3)变化于0.72%~1.37%之间,可近似看成是岩浆演化过程氧逸度较低的量化标志(FMQ+0.5),这种低氧逸度条件下硅酸盐矿物的结晶,会导致粒间熔体氧逸度逐步升高且成分向着富Fe的方向演化。岩浆的这种成分演化特点,是晚期形成不混溶熔浆及富Fe-Ti矿浆的主要原因。     

弧地壳深部岩浆结晶分异过程:来自中祁连西段哈马尔达坂杂岩体的年代学、地球化学和热力学模拟研究SCIEI北大核心CSCD

俯冲带弧岩浆分异是大陆地壳形成的主要过程,对研究大陆地壳的演化与生长机制具有重要意义。本文通过对中祁连弧地体西段盐池湾地区的哈马尔达坂杂岩体进行野外地质、地球化学、年代学和热力学模拟研究,来讨论弧岩浆分异过程。哈马尔达坂杂岩体主要存在两类岩石类型:镁铁质-超镁铁质堆晶岩(包括辉石角闪石岩、角闪辉长岩和角闪石岩)和闪长岩类(包括闪长岩、石英闪长岩)。锆石U-Pb定年结果显示辉石角闪石岩的形成时代为473±1Ma,与相邻高钾岛弧玄武岩的形成时代一致。哈马尔达坂杂岩体与高钾岛弧玄武岩具有相似的全岩Sr-Nd-Hf同位素组成,(^(87)Sr/^(86)Sr)_(i)比值在0.7035~0.7053之间,ε_(Nd)(t)=+3.9~+5.1,ε_(Hf)(t)=+10.8~+13.1,表明它们来自同一岩浆源区。结合野外地质关系与岩石地球化学特征,推测该侵入杂岩体的原始熔体为相邻的高钾玄武质弧岩浆。岩石学和地球化学特征表明:闪长岩类是来源于受沉积物熔体交代的俯冲带地幔楔部分熔融形成的原始玄武质岩浆在中下地壳发生(以角闪石为主)结晶分异的产物;镁铁质-超镁铁质岩石具有堆晶结构,地球化学特征呈现富铁贫硅的特征,具有较高的MgO含量(6.19%~14.29%,Mg^(#)=52.3~74.6),代表原始玄武质岩浆分离结晶形成的堆晶体。热力学模拟计算与实验岩石学资料对比,进一步说明该杂岩体中的闪长岩是玄武质岩浆在中压(约0.7GPa)和氧逸度为NNO+1的条件下、发生50%~67%分离结晶的产物。闪长岩类与镁铁质-超镁铁质堆晶岩具有相似的矿物结晶顺序与全岩同位素组成,指示它们具有发生分离结晶作用的互补成分特征,分别代表了玄武质岩浆分离结晶后的衍生熔体和堆晶岩。因此,本文研究表明角闪石分异是弧岩浆分异的主要机制,它导致富SiO_(2)熔体的形成,推动岩浆成分向富硅的方向演变,从而产生新生的安山质地壳。     

东天山四顶黑山层状岩体地质特征及成矿潜力分析

东天山黄山岩带因发育众多镁铁质-超镁铁质岩体和岩浆铜镍硫化物矿床而备受地质学者关注。四顶黑山岩体位于该岩带最东端,具有层状岩体特征;岩石类型有单辉橄榄岩、橄榄辉石岩、橄榄苏长辉长岩、橄榄辉长岩、辉石角闪岩、辉长岩、角闪辉长岩、闪长岩,主要造岩矿物为贵橄榄石、古铜辉石、透辉石、普通角闪石和斜长石,岩体分异较好,蚀变较发育。通过矿物显微结构特征观察以及计算得出:四顶黑山岩体中橄榄石最先结晶,其开始结晶温度大约在1419℃左右;古铜辉石和透辉石在橄榄石之后开始结晶,结晶温度分别在1100℃左右和900～1100℃之间,两矿物相在岩石中可以共存。岩体中橄榄岩相、苏长岩相发育,以及贵橄榄石+古铜辉石的矿物组合特征,表明岩体具有形成铜镍(铂)矿床的有利条件。     

吉林红旗岭1号和7号岩体中含矿超镁铁质岩的矿物化学特征:对岩浆演化过程以及铜镍硫化物矿床形成机制的约束

本文对吉林红旗岭1号和7号岩体中含矿超镁铁质岩的主要造岩矿物进行了详细研究。两岩体的主要造岩矿物为贵橄榄石、古铜辉石、单斜辉石、斜长石、角闪石和金云母。岩浆的暗色矿物结晶顺序为:橄榄石→斜方辉石→单斜辉石→角闪石→黑云母,与镜下实际观察一致,是岩浆在不同深度结晶的产物。原始岩浆来自上地幔,两岩体进入高位岩浆房中的熔体的MgO含量分别13.98%和14.22%、Mg#值分别为72.22和71.05,为含水的高镁的苦橄质玄武岩浆。深部岩浆房深度距地表约26～27km,岩浆房内的结晶温度介于1280～1379℃之间,即结晶于下地壳中。岩浆由深部上升到高位(浅部)岩浆房中的过程是近绝热的,也是快速完成的。岩浆可能经历了两次岩浆房的演化过程,岩浆在上升到高位岩浆房之前,在深部曾经历了较短时间的橄榄石和少量辉石的分离结晶作用;但在高位岩浆房中混染了地壳物质,与此同时,还经历了同源岩浆混合作用以及岩浆过冷却作用,这些都有利于岩浆体系中成矿元素含量增高以及硫达到饱和状态,使金属硫化物熔离并晶出,从而使岩体发生铜镍矿化作用。     

Occurrence of an Alaskan-type complex in the Middle Tianshan Massif,Central Asian Orogenic Belt: inferences from petrological and mineralogical studies

The Xiadong mafic–ultramafic complex lies in the central part of the Middle Tianshan Massif (MTM), along the southern margin of the Central Asian Orogenic Belt (CAOB). This complex is composed of dunite, hornblende (Hbl) clinopyroxenite, hornblendite, and Hbl gabbro. These rocks are characterized by adcumulated textures and variable alteration. Orthopyroxene is an extremely rare mineral in all rock units and plagioclase is absent in dunite and Hbl clinopyroxenite. Hbl, Fe-chromite, and Cr-magnetite are common phases. Olivines have forsterite (Fo) contents ranging from 92.3 to 96.6. Clinopyroxenes are Ca-rich, Ti-poor diopsides, and mostly altered to tremolites or actinolites. Chromites display low TiO₂ and Al₂O₃ contents and high Cr# and Fe²⁺/(Fe²⁺ + Mg) values. Primary and secondary Hbls show wide compositional variations. These petrological and mineralogical features as well as mineral chemistry are comparable to typical Alaskan-type complexes worldwide, which are widely considered to have formed above subduction zones. The chemistry of clinopyroxene and chromite supports an arc plate-tectonic origin for the Xiadong complex. Its confirmation as an Alaskan-type complex implies that the MTM, with Precambrian basement, was probably a continental arc during oceanic plate underflow and further supports the hypothesis of southward subduction of the Palaeozoic Junggar Ocean.     

The Early Permian mafic–ultramafic complexes in the Beishan Terrane,NW China: Alaskan-type intrusives or rift cumulates?

The petrogenesis and geodynamic setting of the Early Permian mafic–ultramafic complexes in the Beishan Terrane, NW China have important bearing on the prospects of Ni–Cu–PGE sulfide and Ti–Fe oxide deposits, as well as in understanding the history of evolution of the southern Central Asian Orogenic Belt (CAOB). Here we present results from a detailed study on a representative suit of a mafic–ultramafic rocks from the Bijiashan complex in the Beishan Terrane. The complex is composed of dunite, troctolite, olivine gabbro and gabbro without clinopyroxenite or hornblendite. In addition to olivine and clinopyroxene, orthopyroxene and plagioclase are also present in all these rock types, in the absence of chromite or primary hornblende. The minerals display marked compositional variations with the Fo content in olivines ranging from 66 to 83 and the clinopyroxenes ranging from diopside to augite. The whole-rock geochemistry shows good correlations among the major elements, and the trace elements are characterized by flat REE patterns with Eu positive anomalies, and HFSE depletion relative to LILE. These features are comparable to other mafic–ultramafic complexes in the Beishan and Eastern Tianshan terranes, but obviously deviate from the typical features of Alaskan-type intrusives. The present study suggests that the Beishan complexes were formed in a post-orogenic extensional environment with potential for Ni–Cu mineralization, rather than in arc-related setting with Alaskan-type PGE mineralization. Our study confirms that the Beishan area does not conform to an arc-related setting, but was located within a rift setting in Permian probably related to Permian mantle plume event, suggesting that the subduction of the Paleo-Asian ocean had culminated by this time, and the southern CAOB witnessed a post-orogenic extensional regime in late Paleozoic.     

Ultramafic and Mafic Inclusions from Adak Island: Crystallization History, and Implications for the Nature of Primary Magmas and Crustal Evolution in the Aleutian Arc

Xenolithic inclusions in calc-alkaline andesite from Mt. Moffettvolcano, Adak Island, Aleutian arc, reveal a nearly continuousrecord of crystallization of basaltic magmas in the crust, andpossibly upper mantle, of the arc. The record is more detailedand continuous than that obtained from study of calc-alkalinevolcanic rocks in the arc. Cumulate xenoliths form a progressiveseries in modal mineralogy from ultramafic, hornblende-bearingolivine clinopyroxenite to both hornblende-bearing and hornblende-freegabbros. The cumulate hornblende gabbro xenoliths are typicalof those found in island arc andesites worldwide. Xenolithicinclusions without cumulate textures, here termed compositexenoliths, are characterized by forsteritic olivine, zoned Cr-diopsideand hornblende, and are interpreted to have resulted from reactionand chilling upon magma mixing at depth. The olivine and clinopyroxene in both cumulate and compositexenoliths show the largest and the most complete variation trendsfor Ni, Cr, and FeO/MgO ratio yet reported in igneous xenolithsfrom island arc volcanic rocks. Variation of Ni in olivine indicatesthat the parent magmas for the xenoliths had minimum MgO contentsof 9 wt. per cent. Variation of Cr in clinopyroxene indicatesthat the magmas were basaltic rather than picritic, probablyin equilibrium with spinel lherzolite at near Moho depths. Successiveinjections of batches of primary melt into a magma chamber fractionatingolivine and clinopyroxene can reproduce observed compatibleelement depletion trends. A steady-state process of cotecticcrystallization in a magma chamber continually replenished withbasaltic magma is a possible mechanism for producing large accumulationsof olivine and clinopyroxene, suggesting that Alaskan-type ultramaficcomplexes are related to hydrous basaltic magmas in island arcs.This steady-state open-system crystallization process can alsoyield the abundant high-alumina basalt type in the Aleutianarc. Continued crystallization of high-alumina basalt in lowercrustal magma chambers, recorded in a mineralogically coherentseries of pyroxenite to hornblende gabbro xenoliths, can yieldbasaltic to andesitic magmas of the calc-alkaline series. No xenoliths with a sedimentary protolith have been found atMt Moffett, evidence that the arc crust is igneous in origin,with the lower crust formed of gabbro crystallized from mantle-derivedmelts. Ultramafic cumulates may reside in both the lower crustor upper mantle beneath the arc. A model is proposed wherebythe cumulate crystallization products of hydrous, mantle beneaththe arc. A model is define the upper mantle and lower crustof the arc over time.The net composition added to the crustof the arc is that of high-alumina basalt.     

Gabbro Akarem mafic-ultramafic complex, Eastern Desert, Egypt: a Late Precambrian analogue of Alaskan-type complexes

Summary ?Gabbro Akarem is a Late-Precambrian concentrically-zoned mafic-ultramafic intrusion located along a major fracture zone trending NE-SW in the Eastern Desert of Egypt. It intruded low-grade metasedimentary rocks, and has a contact metamorphic aureole a few meters wide. This intrusion comprises a dunite core enveloped by clinopyroxene hornblende-bearing lherzolite, olivine-hornblende clinopyroxenite and plagioclase hornblendite. The contacts between the rock types are gradational. They have cumulate textures and the observed crystallization sequence is: olivine ( + cotectic spinel)-orthopyroxene (Opx)-clinopyroxene (Cpx)-hornblende. Mafic minerals from the core of the intrusion are highly magnesian, a consistent increase in the Mg# of olivine (from 69 to 87), Opx (from 62 to 89), Cpx (from 85 to 96) and hornblends (from 62 to 88) is observed from the mafic to the ultramafic units. Spinel has a wide range of Cr# and Mg# ratios. The various rock units define a fractionation trend. The mafic rocks are slightly LREE-enriched relative to the ultramafic units and chondrites. In many aspects, the Gabbro Akarem intrusion is similar to Alaskan-type complexes. Mineralogical and geochemical data suggest that the different rock units were fractionated from a hydrous picritic magma with no apparent crustal contamination. A petrogenetic model involving a rapid rise of hydrous mantle magma along a major fracture zone is proposed. Extensive fractional crystallization led to magma chamber stratification; internal circulation and strong vertical stretching up the center of the rapidly rising diapir increased the rate of magma ascent towards the core. Due to cooling and high viscosity the marginal mafic magma was partly crystallized while the unsolidified core ultramafic magma continued its ascent. As a result, different mineral phases crystallized at different pressure-temperature paths. Field relations, geophysical, petrological and experimental studies support this model which explains many of the characteristics of the Gabbro Akarem and some other concentrically zoned mafic-ultramafic intrusions. Received April 24, 2001; revised version accepted November 20, 2001     

Geochemistry and Os–Nd–Sr isotopes of the Gaositai Alaskan-type ultramafic complex from the northern North China craton: implications for mantle–crust interaction

We report petrological, chemical and Os–Nd–Sr isotopic data for the Gaositai ultramafic complex from northern North China craton (NCC) to reveal its petrogenesis. The complex shows features of Alaskan-type intrusions, including (1) the concentric zoning from dunite core, to clinopyroxenite and hornblendite in the rim, and the common cumulative textures; (2) the abundance of olivine, clinopyroxene and hornblende, and the scarcity of orthopyroxene and plagioclase, and (3) the systematic decrease in Mg# of ferromagnesian phases from core to rim, accompanied by the Fe-enrichment trend of accessory spinel. The different rock types show highly varied, radiogenic Os isotopic ratios (0.129–5.2), and unradiogenic Nd isotopic composition (ε_Nd(t) = −8 to −15), but are homogeneous in I_Sr ratios (0.7054–0.7066). The (¹⁸⁷Os/¹⁸⁸Os)_i ratios are found to be anti-correlated with ε_Nd(t) values and whole-rock Mg# as well. These data suggest significant crustal contamination during magma evolution. The crustal contaminants are dominantly Archean mafic rocks in the lower crust, and subordinate TTG gneisses at shallower crustal levels. The parental magma was hydrous picritic in composition, derived from an enriched lithospheric mantle source above a subduction zone. The zoned pattern of the complex formed probably through “flow differentiation” of a rapidly rising crystal mush along a fracture zone that was developed as a result of lithospheric extension in a back-arc setting in the northern margin of the NCC at ca. 280 Ma.     

Superposition of replacements in the mafic granulites of the Jijal complex of the Kohistan arc, northern Pakistan: dehydration and rehydration within deep arc crust

A deep-level crustal section of the Cretaceous Kohistan arc is exposed in the northern part of the Jijal complex. The occurrence of mafic to ultramafic granulite-facies rocks exhibits the nature and metamorphic evolution of the lower crust. Mafic granulites are divided into two rock types: two-pyroxene granulite (orthopyroxene+clinopyroxene+plagioclase±quartz [1]); and garnet–clinopyroxene granulite (garnet+clinopyroxene+plagioclase+quartz [2]). Two-pyroxene granulite occurs in the northeastern part of the Jijal complex as a relict host rock of garnet–clinopyroxene granulite, where the orthopyroxene-rich host is transected by elongated patches and bands of garnet–clinopyroxene granulite. Garnet–clinopyroxene granulite, together with two-pyroxene granulite, has been partly replaced by amphibolite (hornblende±garnet+plagioclase+quartz [3]). The garnet-bearing assemblage [2] is expressed by a compression–dehydration reaction: hornblende+orthopyroxene+plagioclase=garnet+clinopyroxene+quartz+H₂O↑. Subsequent amphibolitization to form the assemblage [3] is expressed by two hydration reactions: garnet+clinopyroxene+plagioclase+H₂O=hornblende+quartz and plagioclase+hornblende+H₂O=zoisite+chlorite+quartz. The mafic granulites include pod- and lens-shaped bodies of ultramafic granulites which consist of garnet hornblendite (garnet+hornblende+clinopyroxene [4]) associated with garnet clinopyroxenite, garnetite, and hornblendite. Field relation and comparisons in modal–chemical compositions between the mafic and ultramafic granulites indicate that the ultramafic granulites were originally intrusive rocks which dissected the protoliths of the mafic granulites and then have been metamorphosed simultaneously with the formation of garnet–clinopyroxene granulite. The results combined with isotopic ages reported elsewhere give the following tectonic constraints: (1) crustal thickening through the development of the Kohistan arc and the subsequent Kohistan–Asia collision caused the high-pressure granulite-facies metamorphism in the Jijal complex; (2) local amphibolitization of the mafic granulites occurred after the collision.     

STAGES OF KIMBERLITE DEVELOPMENT AND EVOLVING CONDITIONS OF DIAMOND FORMATION

Ultramafic/mafic complexes hosting Fe-Ni-Cu mineralization occur as small, lensoidal bodies within the Svecofennian, molasse-like metasedimentary rocks of the Vammala Nickel Belt (VNB) in southwestern Finland. One of them, the Sääksjärvi metaperidotitemetagabbro complex, has been studied to gain a better understanding of their petrogenesis and timing of emplacement. These ultramafic rocks were emplaced before the regional upper-amphibolite-facies metamorphism of the Svecofennian orogeny. They recrystallized to amphibole-dominated assemblages comprising: (1) in metaperidotiteolivine + magnesian hornblende ± chromite ± enstatite ± augite ± phlogopite; (2) in hornblendite-actinolitic hornblende ± augite ± plagioclase ± Fe-Ti oxides; and (3) in metagabbro-actinolitic hornblende + plagioclase ± Fe-Ti oxides ± biotite. The recrystallization was accompanied by changes that involved the formation of a lattice-preferred orientation in olivine and porphyroclastic, poikiloblastic, and equigranular textures.

Geochemical modeling indicates that the ultramafic rocks were derived from a tholeiitic magma (Mg/Mg + Fe = 0.58 to 0.62; Ni = 90 to 120 ppm; low Ti content) by olivine (Fo_78-84) accumulation and, in the case of the gabbro differentiates, accumulation of olivine with subordinate clinopyroxene and plagioclase. The geochemical character is that of island-arc low-Ti tholeiites and, like other VNB intrusions, involves enrichment of light-ion-lithophile elements and rare-earth elements relative to high-field-strength elements compared with normalized mid-oceanic-ridge basalts; this is particularly evident in the Nd/Nb, Zr/Nb, and Th/ Nb ratios. In the studied cumulate body, the sheared margins and the contact-parallel foliation indicate that the ultramafic bodies underwent plastic deformation and possibly were displaced along the evolving foliation in the more ductile migmatitic country rocks. This is contrary to previous interpretations of the VNB ultramafic bodies, which have been treated essentially as unmodified in situ magmatic intrusions.     

徐淮早白垩世埃达克质岩中含橄榄石单斜辉石岩的成因及其岩石学意义

报道了徐淮地区早白垩世埃达克质岩中首次发现的含橄榄石单斜辉石岩捕虏体的岩相学与矿物化学资料, 该类捕虏体显示堆积结构、块状构造, 主要由单斜辉石(~80%)、斜方辉石(~5%)、橄榄石(~5%)和普通角闪石(~10%)组成.橄榄石外侧发育有斜方辉石反应边, 角闪石沿辉石粒间分布, 呈嵌晶结构.矿物化学分析结果表明: 橄榄石的镁橄榄石分子值(Fo)=77.7~79.3, Ni=623×10-6~773×10-6; 斜方辉石的Mg#=75.6~80.2, Cr=161×10-6~684×10-6, Ni=79×10-6~708×10-6; 单斜辉石的Mg#=84.5~86.4, CaO=21.59%~23.13%, Al₂O₃=1.72%~2.44%.上述矿物与中、新生代玄武岩中橄榄石、斜方辉石和单斜辉石斑晶以及堆积成因辉石岩中的斜方辉石和单斜辉石成分类似.此外, 单斜辉石的稀土配分型式以相对富含中稀土元素的上凸型为特征, 稀土元素含量较低(∑REE=10.14×10-6~12.71×10-6), 无明显的铕异常(δEu=0.90~1.16), 类似于新生代玄武岩中单斜辉石斑晶.捕虏体中的普通角闪石的Mg#=74.0~80.4、SiO₂=43.2%~44.5%、Na₂O=2.04%~2.29%, 稀土元素分馏不明显, 显示亏损高场强元素(HFSEs, 如Nb、Ta、Zr、Hf), 富集Sr、Rb、Ba的特征, 与新生代玄武岩中角闪石捕虏晶成分不同.结合其嵌晶结构, 普通角闪石应是寄主岩浆贯入结晶的产物.综合上述特征, 可以看出含橄榄石单斜辉石捕虏体为镁铁质岩浆高压堆晶成因.结合华北克拉通东部早白垩世双峰式火山岩组合的出现, 推断含橄榄石单斜辉石岩捕虏体可能是早白垩世基性岩浆底侵的产物.      

Geochemistry of Proterozoic volcanic and granitic rocks from the Gold Hill-Wheeler Peak area,northern New Mexico

Early Proterozoic supracrustal and plutonic rocks from the Gold Hill-Wheeler Peak area in northern New Mexico define three populations: amphibolite—diorite—tonalite, hornblendite—cumulus amphibolite and felsic volcanics and porphyries. Also present are mid-Proterozoic granites. Amphibolites are similar in Ti, Zr, Cr, Ni and REE contents to young calc-alkaline and arc basalts and diorites and tonalites are similar in composition to young andesites and to high-Al₂O₃ tonalites, respectively. Felsic volcanics resemble young felsic volcanics from mature arc systems in their immobile-element contents. Geochemical model studies suggest that the amphibolites, hornblendites, diorites and tonalites are related by progressive fractional crystallization of a hydrous parent tholeiite magma produced from partial melting of undepleted lherzolite. Amphibolites represent parent tholeiites modified by olivine removal. Hornblendite is an early solid residue comprised chiefly of hornblende, clinopyroxene, and olivine; diorite and cumulus amphibolite represent respectively residual solid (clinopyroxene, plagioclase, hornblende) and liquid, after 50% crystallization. Tonalite represents a residual liquid after 80% crystallization. Felsic volcanic rocks are produced by partial melting of a tonalite or diorite source with granulite-facies mineralogy in the lower crust. Granites have a similar origin to felsic volcanics although requiring an inhomogeneous source with the presence of residual hornblende or garnet.The calc-alkaline igneous rocks in the Gold Hill-Wheeler Peak area suggest the presence of an arc system in northern New Mexico during the Early Proterozoic. The fact that these rocks interfinger with and are overlain by mature clastic sediments favors a model in which a continental arc system is uplited, eroded and buried by cratonic sediments from the north.