Geochemistry of eclogites of the Tso Morari complex,Ladakh, NW Himalayas:Insights into trace element behavior during subduction and exhumation

Whole rock major and trace element compositions of seven eclogites from the Tso Morari ultra-high pressure(UHP) complex, Ladakh were determined with the aim of constraining the protolith origins of the subducted crust. The eclogites have major element compositions corresponding to sub-alkaline basalts. Trace element characteristics of the samples show enrichment in LILE's over HFSEs(Rb, Th, K except Ba) with LREE enrichments((La/Lu)n = 1.28-5.96). Absence of Eu anomaly on the Primitive Mantle normalized diagram suggests the absence of plagioclase fractionation. Positive correlation between Mg# with Ni and Cr suggests olivine fractionation of mantle melts. Narrow range of(La/Yb)n(2.1-9.4) and Ce/Yb(6.2-16.2) along with Ti/Y(435-735) ratios calculated for the Tso Morari samples is consistent with generation of melts by partial melting of a garnet free mantle source within the spinel peridotite field. Ternary diagrams(viz. Ti-Zr-Y and Nb-Zr-Y) using immobile and incompatible elements show that the samples range from depleted to enriched and span from within plate basalts(WPB)to enriched MORB(E-MORB) indicating that the eclogite protoliths originated from basaltic magmas.Primitive Mantle normalized multi element plots showing significant Th and LREE enrichment marked by negative Nb anomalies are characteristic of continental flood basalts. Positive Pb, negative Nb, high Th/Ta, a narrow range of Nb/La and the observed wide variation for Ti/Y indicate that the Tso Morari samples have undergone some level of crustal contamination. Observed geochemical characteristics of the Tso Morari samples indicate tholeiitic compositions originated from enriched MORB(E-MORB) type magmas which underwent a limited magmatic evolution through the process of fractional crystallization and probably more by crustal contamination. Observed geochemical similarities(viz. Zr, Nb, La/Yb, La/Gd,La/Nb, Th/Ta ratios and REE) between Tso Morari eclogites and the Group I Panjal Traps make the trap basalt the most likely protoliths for the Tso Morari eclogites.     

Eclogitic metasediments from the Tso Morari area (Ladakh, Himalaya): evidence for continental subduction during India-Asia convergence

Metasediments in the Tso Morari area (Ladakh, Himalaya) provide new insights into the Higher Himalayan metamorphism in the northwestern part of the Himalayan belt. Whole-rock analyses and petrologic observations show that the metasediments correspond to Fe-rich metapelites, Mg-rich metapelites, intermediate metapelites and metagreywackes of the Indian continental margin. Jadeite + chloritoid + paragonite + garnet in the Fe-rich metapelites indicate pressures of 20 ±2 kbar at temperatures of 550 ±50 °C according to major element partitioning thermobarometry, stability fields of minerals and Thermocalc P-T estimates. These results are consistent with P-T estimates on other metasediments and with the occurrence of eclogites. Subsequent retrogression at the eclogite-blueschist facies transition (from 18 to 13 kbar and 540 ±50 °C) was followed by an increase in temperature to 630 ±30 °C at amphibolite facies conditions. The metamorphic evolution is related to subduction of the Indian continental margin beneath the southern Asian margin at the onset of the Indian-Eurasian collision. Received: 17 April 1996 / Accepted: 19 February 1997     

The role of amphiboles in the metamorphic evolution of the UHP rocks: a case study from the Tso Morari Complex, northwest Himalayas

Amphiboles represent a crucial phase of the ultra-high-pressure (UHP) metamorphic rocks as their solid solution behavior reflects both bulk compositional and P–T changes. Three different types of amphibole have been reported from the UHP metamafic rocks of the Tso Morari Crystalline Complex, NW Himalayas: Na-rich (glaucophane); Na–Ca-rich (barroisite, taramite, winchite) and Ca-rich (tremolite, magnesio-hornblende, pargasite). The Na-amphibole is presented as a core of the zoned amphibole with Na–Ca-rich rim; Na–Ca-amphibole is presented as inclusion in garnets as well as in matrix, and Ca-amphibole is generally found in the matrix. The Na–Ca-amphibole is observed at two different stages of metamorphism. The first is pre-UHP, and the second is post-garnet–omphacite assemblage though with a significant difference in composition. The pressure–temperature estimations of the formation of these two sets of Na–Ca-amphiboles corroborate their textural associations. Ca-rich amphiboles are generally present in the matrix either as symplectite with plagioclase or as a pseudomorph after garnet along with other secondary minerals like chlorite and biotite. Two different types of zoning have been observed in the amphibole grains: (1) core is Na-rich followed by Na–Ca rim and (2) core of Na–Ca-amphibole is followed by Ca-rich rim. The pre-UHP (or the prograde P–T path) and post-UHP stages (or the retrograde P–T path) of Tso Morari eclogites are defined by characteristic amphibole compositions, viz. Na/Na–Ca-amphibole, Na–Ca-amphibole and Ca-amphibole and thus indicate their utility in inferring crustal evolution of this UHP terrain.     

Monazite begets monazite: evidence for dissolution of detrital monazite and reprecipitation of syntectonic monazite during low-grade regional metamorphism

Back-scattered electron (BSE) imaging and X-ray element mapping of monazite in low-grade metasedimentary rocks from the Paleoproterozoic Stirling Range Formation, southwestern Australia, reveal the presence of distinct, high-Th cores surrounded by low-Th, inclusion-rich rims. Previous geochronology has shown that the monazite cores are older than 1.9 Ga and overlap with the ages of detrital zircon grains (∼3.5–2.0 Ga), consistent with a detrital origin. Many cores have scalloped and embayed surfaces indicating partial dissolution of former detrital grains. Textural evidence links the growth of the monazite rims (∼1.2 Ga) to deformation and regional metamorphism during the Mesoproterozoic Albany-Fraser orogeny. These results indicate that high-Th detrital monazite is unstable under low-grade metamorphic conditions (<400°C) and was partially or completely dissolved. Dissolution was followed by near-instantaneous reprecipitation and the formation of low-Th monazite and ThSiO₄. This reaction is likely to operate in other low-grade metasedimentary rocks, resulting in the progressive replacement of detrital monazite by metamorphic monazite during regional prograde metamorphism.     

Distribution of trace elements during breakdown of mantle garnet: an example from Zabargad

Ion probe investigations on mineral phases forming the Al-Di pyroxenites from the Zabargad peridotite body indicate that porphyroclastic pyroxenes in composite mafic layers record an unusual HREE, Zr, Sc enrichment not registered by pyroxenes in spinel websterites. Orthopyroxene in the opx+sp clusters forming the inner, cpx-free zone of layered pyroxenites shows strongly fractionated REE patterns (HREE_N/LREE_N>1000; Yb>100xch) and very high Zr, Sc and Y abundances (up to 30,672 and 60ppm, respectively). In the outer, cpx-rich zone porphyroclastic clinopyroxene is strongly HREE enriched (HREE_N/LREE_N29; Yb 269xch) and displays very high Sc and Zr abundances (up to 819 and 164 ppm, respectively). It is suggested that the unusual trace element abundances are inherited from a precursor garnet. Composite pyroxenite layers are interpreted as former garnet clinopyroxenites characterized by gnt/cpx modal zoning. The sp+opx(cpx-free) assemblage in the inner part is a product of the break-down reaction of garnet upon decompression, with Ca of the original garnet completely entering the enstatite solid solution. The temperature at which the breakdown reaction occurred is estimated to be higher than 1000°C (P in the range 20–30 kbar). In the outer part, decompression caused the garnet to form a sp+opx assemblage; however, the grossularite component participated in the formation of new clinopyroxene which reacted with the clinopyroxene present in the original mode before the decompression reaction, thus forming a cpx₂+sp+opx assemblage. As a result of garnet breakdown, pyroxenes have peculiar HFSE anomalies. Progressive upwelling during the Red Sea rifting produced incomplete reaction under pl-facies conditions. The geochemical signatures of precursor garnet in pyroxenes were partially crased during the recrystallization from granular spincl-bearing to granoblastic plagioclase-bearing assemblages, being preserved only in a few porphyroclast relies. The finding of pyroxenes with trace element characteristics of precursor garnet has important geodynamic and geochemical implications. Al-Di pyroxenite layers had a long history within the mantle, before the continental lithosphere rifting and thinning took place in the region. It is suggested that Al-Di pyroxenites were formed by deep-seated tholeiitic magmatism unrelated to the Red Sea evolution, thus representing the earliest event in the Zabargad upper mantle. Garnet breakdown significantly preceded the metasomatism induced by hydrous fluids (crystallization of Ti-rich pargasite) and the later intrusion of hydrous (Cr-Di) pyroxenite dykes. During the stages of mantle evolution, the HFSE anomalies in pyroxenes varied significantly. We note that the study of HFSE anomalies in mineral phases reveals complex geochemical histories which are not recorded by the whole-rock system.     

Atmospheric pollution indicated by trace elements in snow from the northern slope of Cho Oyu range,Himalayas

Samples collected from a 0.87 m snow pit at a high altitude site in the Cho Oyu range, Himalayas were measured for V, Cr, Mn, Co, Ni, Cu, Zn, As, Rb, Sr, Cd, Sn, Sb, Ba, Tl, Pb, Bi, Th, and U using inductively coupled plasma mass spectrometry. In addition, major ions, oxygen stable isotopes, and microparticles were also measured to assist the interpretation of seasonal variation of trace elements. The trace elements show a distinct seasonality, i.e., higher concentrations during the non-monsoon season than those during the monsoon season. Significant correlation is observed between Ba and the other trace elements. Crustal enrichment factor (EF_c) analysis indicates that V, Mn, Co, Ni, Rb, Sr, and Th originate mainly from crustal dust, while anthropogenic inputs make an important contribution to the other trace elements (i.e., Cu, Zn, As, Cd, Sn, Sb, Ti, Pb, Bi, and U). Evidence from air mass back trajectories suggests that atmospheric trace element pollution reaching the studied area is transported dominantly by Indian summer monsoon during the monsoon season, while it is transported mainly by the westerlies during the non-monsoon season.     

Chemical mobilizations in laterites: evidence from trace elements and U-U-Th disequilibria

Geochemical and mineralogical investigations, including measurements of major and trace elements, Sr isotope ratios, and ²³⁸U-²³⁴U-²³⁰Th activity ratios, were made on an old African laterite to reconstruct its formation steps and assess recent chemical mobilization. The present data support a scenario of discontinuous formation for the laterite, with different bedrock weathering conditions during the formation of each unit, rather than a scenario of continuous formation. Absolute accumulation of Fe, U, and lanthanides in the uppermost ferruginous unit suggests an autochthonous origin of this iron cap by leaching of an older overlying profile. Present chemical distributions of lanthanides, as well as of Rb, K, Ba, and Sr, within the profile cannot be linked to the mineralogical distribution of both relictual primary and authigenic secondary phases. Complementary lanthanide patterns indicate that these elements were primarily accumulated in the uppermost ferruginous unit before further remobilization and accumulation in the underlying horizons. These redistribution processes may be related to the chemical instability of the ferruginous cap. The ²³⁸U-²³⁴U-²³⁰Th disequilibria indicate that recent U mobilization occurs in the whole profile and that, as for lanthanides, there is a vertical redistribution of U from the uppermost ferruginous unit to the underlying horizons. Moreover, these data show that both U losses and gains exist at each level of the profile. A simple modeling of this double U mobilization process is proposed to interpret the ²³⁸U-²³⁴U-²³⁰Th data. Differences in the mobilization and fractionation intensities of the U input and removal processes can account for the two evolution trends, which distinguish the ferruginous unit from the underlying ones. Furthermore, on the basis of this modeling, the profile appears to be in a transient state because of recent changes in the U mobilization conditions, which could correspond to major Pleistocene climatic variations.     

Evidence of Late Quaternary seismicity from Yunam Tso,Lahaul and Spiti,NW Himalaya,India

A relict fluvio-lacustrine sediment of an 8 m thick section exposed at Kilang Sarai along Yunam river, near Baralacha La shows presence of cycloids or pseudonodules, ball and pillow structures, flame-like and pocket structures, sand dyke injections, bed dislocation/faulting and flow folds. Within this section four deformed levels of soft sediment structures have been identified which were dated ca. 25 ka BP at level 1 (～0.4 m from the modern river level (mrl), 20.1 ka BP at level 2 (～1.8 m mrl), 17.7 ka BP at level 3 (～2.56 m mrl) and 12.2 ka BP at level 4 (～4.25 m mrl)). Detailed study of these soft sediment structures allow us to demonstrate that deformation level 3 is not related to seismic trigger, but remaining three deformation levels (1, 2 and 4) are ascribed to seismic origin. From compilation of earlier palaeoseismological studies using soft sediment deformational structures (SSDS) in the palaeolake deposits in the adjoining area, suggest that the deformational events identified in the present study are regional in nature and thus tectonic process plays an important role in the evolution of landform in the Spiti region.     

Behaviour of zircon in high-grade metamorphic rocks: evidence from Hf isotopes,trace elements and textural studies

Hf isotopic data of minerals in a mafic pyroxene granulite from the southern Bohemian Massif, together with their major and trace element composition and petrological observations were used to decipher the metamorphic history and behaviour of zircon in the granulite. The Hf isotopic composition in the minerals was used to estimate whether the decompression reaction, namely the consumption of garnet and rutile, could have provided Zr for the formation of newly grown metamorphic zircon. The age of the decompression reaction indicated by the evolution of Hf isotopes in garnet and orthopyroxene is between 333 and 331 Ma, i.e. ca. 7 Ma younger than the available U–Pb zircon ages from the Moldanubian granulites and than the newly obtained 343 ± 2 Ma laser ablation ICP-MS U–Pb age of zircons. The combination of bulk and in-situ Hf isotopic data, major and trace element composition and petrological modeling of P–T evolution revealed that the formation of zircons can not be related to the decompression phase of the evolution of the mafic granulites. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Cretaceous isochron ages from K–Ar and Ar / Ar dating of eclogitic rocks in the Tso Morari Complex, western Himalaya, India

The Tso Morari Complex, which is thought to be originally the margin of the Indian continent, is composed of pelitic gneisses and schists including mafic rock lenses (eclogites and basic schists). Eclogites studied here have the mineral assemblage Grt + Omp + Ca-Amp + Zo + Phn + Pg + Qtz + Rt. They also have coesite pseudomorph in garnet and quartz rods in omphacite, suggesting a record of ultrahigh-pressure metamorphism. They occur only in the cores of meter-scale mafic rock lenses intercalated with the pelitic schists. Small mafic lenses and the rim parts of large lenses have been strongly deformed to form the foliation parallel to that of the pelitic schists and show the mineral assemblages of upper greenschist to amphibolite facies metamorphism. The garnet–omphacite thermometry and the univariant reaction relations for jadeite formation give 13–21 kbar at 600 °C and 16–18 kbar at 750 °C for the eclogite formation using the jadeite content of clinopyroxene (X_Jd = 0.48).

Phengites in pelitic schists show variable Si / Al and Na / K ratios among grains as well as within single grains, and give K–Ar ages of 50–87 Ma. The pelitic schist with paragonite and phengite yielded K–Ar ages of 83.5 Ma (K = 4.9 wt.%) for paragonite–phengite mixture and 85.3 Ma (K = 7.8 wt.%) for phengite and an isochron age of 91 ± 13 Ma from the two dataset. The eclogite gives a plateau age of 132 Ma in Ar/Ar step-heating analyses using single phengite grain and an inverse isochron age of 130 ± 39 Ma with an initial ⁴⁰Ar / ³⁶Ar ratio of 434 ± 90 in Ar/Ar spot analyses of phengites and paragonites. The Cretaceous isochron ages are interpreted to represent the timing of early stage of exhumation of the eclogitic rocks assuming revised high closure temperature (500 °C) for phengite K–Ar system. The phengites in pelitic schists have experienced retrograde reaction which modified their chemistry during intense deformation associated with the exhumation of these rocks with the release of significant radiogenic ⁴⁰Ar from the crystals. The argon release took place in the schists that experienced the retrogression to upper greenschist facies metamorphisms from the eclogite facies conditions.     

Experimental evidence for mobility of Zr and other trace elements in soils

A Soxhlet extraction was carried out over a period of 27 d on a column comprising 3 cm of quartz overlain by 4 cm of soil from the B horizon and then 1 cm of soil from the A horizon of a granitic podzol. Major and trace elements were leached from the column and accumulated in a reservoir at the base of the column. Total loss of elements from the soil over the course of the experiment ranged from 0.002 to 1 wt% with major elements and the light and heavy rare earth elements (REE) showing the largest percentage losses. Zirconium (0.002%) and then Al (0.008%) showed the lowest percentage loss. The light REE were leached out of the soil preferentially to the mid REE. All elements showed accumulation, by a factor of 2 to 11, in the quartz layers at the base of the column, particularly in the upper first 1 cm of the quartz. Major elements were leached from the column at a rate of 0.02 to 0.59 μmol h⁻¹ whereas Zr, Nd, Sm, Gd, Dy, Rb, and Sr were leached at the rate of 0.5 to 30 × 10⁻⁶ μmol h⁻¹. Concentrations of other REE in the reservoir increased over the duration of the experiment, but they were poorly correlated with time, so leaching rates were not calculated. Normalization of the major element leaching rates to take into account the constant flushing of water through the column, the average annual rainfall in the Allt a’Mharcaidh catchment in Scotland from where the soil was sampled, and the cross-sectional area of the soil in the column, together with the temperature of the soil in the column (70°C) compared with the average annual temperature of the Allt a’Mharcaidh catchment (5.7°C), gave major element release rates from the soil of 0.002 to 0.97 mEq m⁻² yr⁻¹ (depending on the choice of E_a, the dissolution activation energy), which are generally less than those measured in the field of 0.1 to 40.9 mEq m⁻² yr⁻¹.Calculations showed that despite the redistribution and loss of Zr from the column, assumptions of Zr mobility would have had a negligible effect on calculated element release rates of Na, Ca, Fe, and Mg. However, significant underestimates of the release of K (5%), Ti (57%), Al (5%), and Si (10%) as well as some trace elements (e.g., Nd, 23%; Rb, 54%; Sr, 24%) would have occurred. Concentrations of Ca and Sr leached from the column correlated well (RSQ = 0.93, p < 0.01), supporting the idea of the use of Sr release as a proxy for Ca release in weathering rate calculations. The release rates and percentage loss of REE from the soil varied between elements indicating that REE distribution patterns of rocks and soils may not be preserved in drainage waters.     

塔里木克拉通早前寒武纪基底层序与组合：颗粒锆石U-Pb年龄新证据

采用颗粒锆石U-Pb定年方法,测得塔里木东北缘库鲁克塔格地区不整合于原兴地塔格群(现双横山组)之下的红卫庄花岗片麻岩结晶年龄为1943±6(2σ)Ma,表明库鲁克塔格地区早前寒武纪结晶基底之上的第一套稳定盖层应为中元古界,与华北长城系相当。在早前寒武纪表壳岩系斜长角闪岩中得到锆石生成年龄为2492±19(2σ)Ma,托格灰色片麻岩锆石结晶年龄为2337±6(2σ)Ma,并同时获得其中可能有2660±2(2σ)Ma、2782±4(2σ)Ma捕获晶年龄。上述年龄新资料表明,以库鲁克塔格地区为代表的塔里木克拉通早前寒武纪基底中可能发育有新太古代岩石,但其主要以残留包体形式分布于托格灰色片麻岩中,以托格灰色片麻岩为主体的早前寒武纪块体可能形成于(新太古代)-古元古代。     

长白山火山地幔不均一性——微量元素及同位素地球化学研究

大陆火山的复杂性是一个普遍关注的全球性问题,也是一个长期以来悬而未解决的地质问题.长白山火山位于亚洲板块东部的活动边缘,受到西太平洋俯冲的影响,新生代构造和火山活动活跃,为探讨大陆火山成因及其地幔源区特点提供了极佳的天然实验室.天池火山和龙岗火山是长白山地区两个重要的新生代火山区,其火山岩分布具有空间相邻、火山活动时代相似的特点,本文通过对基性火山岩的岩石学和地球化学对比研究,指出天池火山和龙岗火山具有明显的地球化学不均一性,包括微量元素不均一性和Sr、Nd、Pb同位素不均一性,来自于不同的地幔源区,天池火山和龙岗火山火山岩分布相邻,覆盖范围东西最大不过100多公里,因此这是典型的小尺度地幔地球化学不均一的特点.Sr、Nd同位素特征显示龙岗火山来自于原始地幔,而天池火山来自于原始地幔和富集地幔混合源区.不相容元素Nb、Ta显示,天池火山Ta元素含量很低（主要落在0.17～2.15 ppm的范围）,具有明显的负Ta异常和明显的Nb/Ta分异（Nb/Ta=10～50,个别达到100以上）,可能是板片部分熔融产物与金红石等副矿物反应的结果,暗示了西太平洋板块俯冲机制的影响;而龙岗火山没有明显Nb、Ta亏损或者分异.     

滇西北红山铜多金属矿床的成因类型:黄铁矿和磁黄铁矿LA-ICPMS微量元素制约

红山大型铜多金属矿床位于云南中甸地区,包括红山、红牛和恩卡3个矿段,矿体主要呈层状-似层状产于石榴石夕卡岩、角岩、大理岩和硅质岩之中,或者呈细脉浸染-网脉状赋存于深部隐伏花岗斑岩体之中。目前学术界关于该矿床的成因类型仍然存在不同认识,本文使用激光剥蚀电感耦合等离子体质谱仪(LAICPMS)对矿区硅质岩、夕卡岩矿石中的黄铁矿和磁黄铁矿进行了微区原位成分的测试,进而根据微量元素特征来约束矿床的成因。分析结果表明,不同产状的黄铁矿具有明显不同的微量元素组成,硅质岩中的黄铁矿相对富集Ti、Mn、Ni、As、Pb、Bi、Te、Ag和Sb等微量元素,Co/Ni比值小于1,表现为典型同生沉积黄铁矿的微量元素特征;而夕卡岩矿石中的黄铁矿则相对富集Co和Cu,亏损As、Se和Sb等低温元素,且Co/Ni比值多数大于1,显示高温岩浆热液黄铁矿的微量元素特征。此外,硅质岩中磁黄铁矿的Co、Ni和Se等微量元素组成与黄铁矿的组成十分类似,表明它们的化学组成主要受到沉积环境的控制。夕卡岩中的磁黄铁矿与黄铁矿相比强烈亏损Co,这可能归因于早期黄铁矿沉淀时带走了大量的Co,从而导致残余热液中Co浓度的大幅降低。通过研究,我们还查明红山矿区黄铁矿中的Co、Ni、As和Se等元素主要以固溶体的形成存在,而Pb、Bi、Ag、Cu和Mn等元素则主要以显微包裹体的形式存在。黄铁矿和磁黄铁矿中Pb和Bi均表现为正相关关系,暗示它们可能以显微包裹体或纳米微粒的形式分布于这两种硫化物中。结合野外地质产状与前人已有研究,我们认为红山矿区至少存在两期成矿作用,其中晚三叠世沉积成岩作用形成的黄铁矿富集了一定的Ag、Bi和Pb等成矿元素,而晚白垩世的岩浆-热液活动则带来了大量的Cu和Mo等金属元素,从而在红山矿区形成了复合型的Cu-Mo-Pb-Zn-Ag多金属矿化体系。     

秦岭杂岩清油河斜长角闪岩多期变质的证据——来自锆石微量元素和包裹体的启示

北秦岭清油河地区广泛发育以透镜状、条带状或岩墙状赋存于秦岭杂岩石英二长片麻岩中的斜长角闪岩,矿物组合为角闪石、斜长石以及少量的石榴子石、单斜辉石等。锆石阴极发光图像、不同微区矿物包裹体和稀土元素分析,以及不同性质锆石LA-ICP-MS U-Pb定年和微量元素分析等综合研究结果表明,该斜长角闪岩锆石成因十分复杂,可进一步划分为三种类型。第一类锆石呈半自形-他形晶,阴极发光显示条带状分带,锆石稀土总量较高,球粒陨石标准化稀土配分模式中重稀土富集,Eu负异常明显,记录的206Pb/238U加权平均值为774±13Ma,代表了该岩石原岩形成时代;第二类锆石呈他形晶,为典型的变质锆石,阴极发光图像为扇状或面状分带,锆石稀土总量较低,球粒陨石标准化稀土配分模式中重稀土相对平坦,没有Eu的负异常,表明该类锆石结晶时岩石中含有石榴子石,而没有长石,该类锆石矿物包裹体主要为Grt+Cpx+Q,可能代表榴辉岩相(榴辉岩或者石榴子石辉石岩)变质矿物组合,记录的~(206)Pb/~(238)U加权平均值为493±5Ma;第三类锆石呈他形晶或构成第二类锆石的薄边,亦为典型变质锆石,阴极发光为面状分带,锆石稀土总量较低,球粒陨石标准化稀土配分模式中重稀土平坦,具有Eu的异常,锆石包裹体矿物组合为Grt+Cpx+Pl+Amp+Q,应代表退变质矿物组合,记录的206Pb/238U加权平均值为448±4Ma。该斜长角闪岩的原岩年龄(774±13Ma)、峰期变质年龄(493±5Ma)和后期退变质年龄(448±4Ma)与北秦岭已厘定的高压-超高压岩石的原岩年龄、峰期变质和退变质年龄非常吻合,说明该岩石可能是北秦岭高压-超高压变质岩带的一部分。据此,并结合清油河斜长角闪岩中发现残存金刚石的研究成果,可以推测现今秦岭杂岩中出露的一些低级变质相岩石峰期也可能经历了高压-超高压变质,只是由于后期强烈的退变质作用的改造而难于识别和辨认,北秦岭高压-超高压岩石的分布可能远比目前观察到的丰富。此外,本次研究还显示清油河斜长角闪岩原岩具有大陆拉斑玄武岩特征,是陆壳岩石的一部分。这些资料为进一步论证北秦岭早古生代高压-超高压岩石(包括榴辉岩、长英质片麻岩、石榴子石辉岩、榴闪岩和部分斜长角闪岩等)是陆壳俯冲-深俯冲作用的产物提供了新的证据。     

Variations in trapping temperatures and trace elements in peridotite-suite inclusions from African diamonds: evidence for two inclusion suites,and implications for lithosphere stratigraphy

The proton microprobe has been used to measure the trace element contents of Cr-pyrope, olivine and orthopyroxene inclusion (DI) in>60 diamonds from southern Africa, and in concentrate garnets from the host kimberlites. Olivine inclusions show a negative correlation of Ca with Fo content, but olivines coexisting with very subcalcic garnets are anomously depleted in Ca relative to Fo. The maximum and median values of Ti, Zn, Ga, Zr and Y in DI garnets decrease as Ca decreases relative to Cr, consistent with increasing depletion by removal of silicate melts. However, the most subcalcic garnets are anomously enriched in Sr (and LREE). The contrasting depletion in Zr, Y, Ti etc. and enrichment in Sr is not consistent with a single-stage depletion event. The extreme Ca depletion of the most subcalcic garnets and their coexisting olivines, and the Sr enrichment, are interpreted as the result of metasomatism by a carbonatitic fluid following depletion. Trapping temperatures of garnet DI, estimated by nickel thermometry range from 950 to > 1500°C. Most DI with T>1200°C are lherzolitic, rather than subcalcic. In most pipes studied, the trapping T of DI garnets is higher than the T range of equivalent garnets from concentrates. The high Ts are interpreted as reflecting the formation of diamonds during short-lived thermal pulses, followed by cooling toward a conductive geotherm. The T distribution of calcic and subcalcic garnets in concentrates from kimberlites suggests that lherzolite and harzburgite are intimately intermixed over the depth range 150–180 km beneath the Kalahari craton. The abundance of calcic garnets with T>1200°C suggests the presence of significant amounts of lherzolite at greater depths. This deeper lherzolite may be associated with eclogite, and may be the source region for some high-T lherzolitic DI garnets.     

新疆北天山黄山东含铜镍矿镁铁-超镁铁岩体的岩石成因：主要元素、微量元素和Sr-Nd同位素证据

本文对新疆黄山-镜儿泉铜镍成矿带中黄山东岩体进行了主量元素、微量元素和Sr-Nd同位素的研究,探讨了岩体成因和区域构造演化.黄山东岩体有三次岩浆侵入,第一次侵入形成了橄榄辉长岩、角闪辉长岩和闪长岩,构成岩体的主体,其中橄榄辉长岩位于岩体中部,角闪辉长岩位于岩体上部和下部,闪长岩沿岩体边缘分布;第二次侵入形成辉长苏长岩,...