Composition,Structure, and Thermal Regime of the Lithospheric Mantle in the Area of the Highly Diamondiferous V. Grib Kimberlite Pipe,Arkhangelsk Diamondiferous Province: Data on the Chemical Composition of Garnet and Chrome-Diopside Xenocrysts

Doklady Earth Sciences - The contents of the main elements (including Ni) are presented for 1100 garnet and 831 clinopyroxene grains extracted from kimberlite of a diatrem part of the...     

Mineralogy and equilibrium P-T estimates for peridotite assemblages from the V. Grib kimberlite pipe (Arkhangelsk Kimberlite Province)

This study presents mineralogical and thermobarometric data for equilibrium peridotite assemblages from the V. Grib kimberlite pipe of the Arkhangelsk diamond province. We provided the first constraints on the composition, structure, thermal state, and lower boundary of the lithospheric mantle beneath the V. Grib kimberlite pipe. It was found that phlogopite-free and phlogopite-bearing peridotite xenoliths can be distinguished by their mineral chemistry. The occurrence of phlogopite in peridotites may represent evidence for modal metasomatism responsible for variation in the mineral composition of phlogopite-pyrope and pyrope peridotites. On the basis of P-T estimates, we conclude that modal metasomatism may have affected the entire thickness of the lithospheric mantle beneath the V. Grib kimberlite pipe. Comparison of our results with the available data from the literature shows strong vertical and lateral mantle heterogeneity beneath kimberlite pipes of the Lomonosov deposit and the V. Grib pipe.     

Mineralogy and Petrology of the Aries Diamondiferous Kimberlite Pipe, Central Kimberley Block, Western Australia

Aries is a deeply weathered micaceous kimberlite pipe (820 Ma)consisting of four lobes: South, Central, North, and North Extension.It is the largest ( 18 ha) and most diamondiferous of the fewkimberlites currently known on the Australian continent, andis rich in country-rock (dolerite and quartzite) xenoliths.Three textural varieties of Aries kimberlites can be recognized,together with autoclastic breccias: (1) macrocrystalmedium-grained;(2) aphanitic (5 vol. % olivine macrocrysts); and (3) macrocrystalsegregated. The kimberlites contain two generations of olivinepseudomorphs (30–40 vol. %), and two of phlogopite (upto 60 vol. %), in a groundmass of apatite, calcite, diopside,sphene, spinels, serpentine, talc, and accessory groundmassminerals including aeschynite [(Ce, Ca) (Ti, Nb)₂O₆], barite,ilmenite, monazite, rutile, siderite, and unidentified Nb-Fe-titanates.Phlogopite zoning is complex and differs from lobe to lobe,but general compositions and trends resemble phlogopites fromkimberlites (TiO₂ 0–5–4 wt. %, A1₂O₃ 9–16%);tetraferriphlogopite substitution is indicated by low Al insome grains. Diopside is low in Cr, Al, Na, and Ti, with highmg-number [molecular Mg/(Mg + Fe²⁺) 93]. Apatite contains upto 17–5% SrO, calcite up to 1–7% SrO but littleMgO or FeO, sphene up to 1.5% Nb₂O₅, and ilmenite 2.6% Nb2O5and 16% MnO but no detectable MgO. Extremely complex moqftiological, textural, and compositionalvariations are present in spinels. They can be divided intofive textural-genetic types: cognate Groundmass chromian spinels(Type G); Inclusions of chromian spinels in olivine macrocrysts(Type I), probably representing either early phenocrysts ormantle xenocrysts: Macrocryst chromian spinels (Type M), probablyrepresenting xenocrysts; late-stage groundmass Fenian spinels(Type F), derived from serpentinization of olivine; Alterationferrian spinels (Type A), found as inclusions associated withsiliceous melt inclusions, in Types I and M, and probably representinginteraction of these earlier types with late-stage melts. Someof these, particularly Types M and F, show further texturalsub-types with no obvious genetic significance. The pipe formed from several magma-pulses. All four lobes maycontain at least one pulse in common, but Central and SouthLobes include additional pulse(s) which yielded distinctivephlogopite zoning, whereas North Lobe and North Extension includepulsc(s) which may have originated at higher mantle levels andyielded more evolved phlogopites. Aries most resembles South African Group II kimberlites mineralogically,certain West African micaceous kimberlites geochemically, andGroup I kimberlites isotopically. A distinctive mantle source-regionis implied by high Nb/U, Ce/Sr, Ce/P, Rb/Ba, and especiallyNb/Zr ratios. Similar anomalous geochemical signatures are sharedwith two other contemporaneous (800 Ma) lampro-phyric intrusionsin the east Kimberley (at Maude Creek and Bow Hill), suggestingthat a scattered alkaline province exists in the Kimberley Block,generated from a regionally anomalous mantle source.     

Eclogites from the Obnazhennaya Kimberlite Pipe,Yakutia, Russia

Mineralogy and petrography of six eelogite xenoliths from the Obnazhennaya kimberlite pipe ar e described. Based upon modal and mineral compositions, these eclogites can be divided into Group A (five samples) and Group B (one sample), as per Coleman et al. (1965) and Shervais et al. (1988). Group-A eclogites are orthopyroxene-bearing, and their constituent minerals have high Mg# and Cr₂O₃ content. The clinopyroxenes in this type of eelogite have low jadeite component. The geochemical features of Group-A eclogites are similar to garnet pyroxenite, and e believed to be the product of high-pressure fractionates from an alkaline basaltic melt in thear upper mantle. Group-B eelogite (0-82/91) contains higher Al₂O₃ and FeO and lower MgO and Cr₂O₃; its composition is similar to a high-aluminum basalt or gabbro. This eelogite could have crystallized under high pressure in the upper mantle from a basaltic melt, without significant fractionation. Alternatively, it also could be the relict of subducted oceanic crust. However, no evidence exists at present that definitively indicates a crustal origin for this Group-B eelogite xenolith.     

甘肃敦煌党河水库TTG地球化学特征、锆石SHRIMP U-Pb定年及其构造意义

甘肃敦煌党河水库岩体以花岗闪长岩为主,并构成英云闪长岩-花岗闪长岩-奥长花岗岩的TTG组合。锆石SHRIMP U-Pb年为440±12Ma (MSWD=2.5),代表TTG系列岩石的侵位时代。岩石化学成分中SiO₂含量约为57.02%~72.75%,随着SiO₂含量的增加,TiO₂、Al₂O₃、MgO、CaO和FeO等的含量相应下降,这表明该套岩石具岩浆成因,是角闪石和斜长石的分离作用控制下岩浆分异的产物。岩石的稀土元素总量较低,轻重稀土分异明显,(La/Yb) _N＝4.70~58.88,无或略具微弱的Eu异常,近于平行的稀土元素配分曲线表明所有岩石具有同成因的亲缘关系。岩石富集大离子亲石元素Rb、K、Th、Sr等,亏损Nb和Ta等高场强元素以及P和Ti。在Rb/30-Hf-3×Ta和Rb/30-Hf-0.25×Nb图解上,所有岩石的投影点落到岛弧到后碰撞区。但从区域地质背景、TTG的产出环境、形成时代及与敦煌地块南北缘火成岩对比分析,认为该TTG系列岩石并非形成于岛孤构造环境,而是加里东造山作用晚期(约440Ma)红柳河缝合带和(或者)阿尔金北缘红柳沟拉配泉俯冲碰撞带,引发富含角闪石的下地壳源岩部分熔融形成岩浆,上侵过程中同化部分陆壳物质形成具有岛弧性质的岩浆岩。     

Diamondiferous peridotite xenoliths from the Argyle (AK1) lamproite pipe,Western Australia

This paper describes a suite of peridotite xenoliths. some carrying diamonds at high grades, from the richly diamondiferous early Proterozoic (1180 Ma) Argyle (AK1) lamproite pipe, in northwestern Australia. The peridotites are mostly coarse garnet lherzolites but also include garnet harzburgite, chromite — garnet peridotite, a garnet wehrlite, and an altered spinel peridotite with extremely Cr-rich chromite. In all cases the garnet has been replaced by a kelyphite-like, symplectic intergrowth of Alrich pyroxenes, Al-spinel and secondary silicates. The peridotites have refractory compositions characterized by high Mg/(Mg+Fe) and depletion in lithophile elements (Al₂O₃ and CaO < 1%, Na₂O0.03%) and high field strength cations such as Ti, Zr, Y, and Yb. Olivines have high Mg/(Mg+Fe) (Mg ^91–93 ) and, like olivine inclusions in diamonds from the Argyle pipe, contain detectable amounts of Cr₂O₃ (0.03%–0.07%) but have very low CaO contents (typically 0.04%–0.05%). Enstatites (Mg ^92–94 ) have comparatively high Cr₂O₃ (0.2%–0.45%) and Na₂O (up to 0.18%) but very low Al₂O₃ contents (0.5%–0.7%). Diopsides (Mg ^92–94 , Ca/(Ca+Mg+Fe)=0.37–0.43) are Cr-rich (0.7%–1.9% Cr₂O₃) and have low Al₂O₃ (0.7%–2.2%) and Na₂O (0.5%–1.6%) contents. Many have high K₂O contents, typically 0.1%–0.4% but up to 1.3% K₂O in one xenolith. The chromite coexisting with former garnet is Mg-and Cr-rich [Mg/(Mg+Fe²⁺)=0.68–0.72, Cr/(Cr+Al)=0.72–0.79] whereas chromite in the spinel peridotite is even more Cr-rich (65% Cr₂O₃, Cr/(Cr+Al)=0.85, resembling inclusions in diamond. One highly serpentinized former garnet peridotite contains a Cr-rich (up to 13% Cr₂O₃) titanate resembling armalcolite but containing significant K₂O (1%–2.5%), CaO (0.6%–2.2%), ZrO₂ (0.1%–0.8%), SrO (0.1%–0.3%), and BaO (up to 0.58%): this appears to have formed as an overprint of the primary mineralogy. Temperatures and pressures estimated from coexisting pyroxenes and reconstructed garnet compositions indicate that the garnet lherzolites equilibrated at 1140°–1290° C and 5.0–5.9 GPa (160–190 km depth), within the stability field of diamond. Oxygen fugacties within the diamond forming environment are estimated from spinel-bearing assemblages to be reducing, with f _O2 between MW and IW. The presence of significant K in the diopsides from the peridotite xenoliths and in diopsides from heavy mineral concentrate from the Argyle pipe implies metasomatic enrichment of the subcontinental lithosphere within the diamond stability field. The P-T conditions estimated for the Argyle peridotites demonstrate that diamondiferous lamproite magmas incorporate mantle xenoliths from similar depths to kimberlites in cratonic settings, and imply that Proterozoic cratonized orogenic belts can have lithospheric roots of comparable thickness to beneath Archaean cratons. These roots lie at the base of the lithosphere within the stability field of diamond. The xenoliths, the calcic nature of chrome pyropes from heavy mineral concentrate, and the diamond inclusion assemblage indicate that the lighosphere beneath the Western Australian lamproites is mostly depleted lherozolite rather than the harzburgite commonly found beneath Archaean cratons. Nevertheless, the dominance of eclogitic paragenesis inclusions in Argyle diamonds indicates a significant proportion of diamondiferous eclogite is also present. The form, mineral inclusion assemblage, and the C-isotopic composition of diamonds in the peridotite xenoliths suggest that disaggregated diamondiferous peridotites are the source of the planar octahedral diamonds which constitute a minor component of the Argyle production. These diamonds are believed to have formed from mantle carbon in reduced, refractory peridotite (Iherzolite-harzburgite) in contrast to the predominant strongly ¹³C-depleted eclogitic suite diamonds which contain a recycled crustal carbon component. The source region of the lamproites has undergone long-term (2 Ga) enrichment in incompatible elements.     

Sheared peridotite xenolith from the V. Grib kimberlite pipe,Arkhangelsk Diamond Province,Russia: Texture,composition, and origin

The petrography and mineral composition of a mantle-derived garnet peridotite xenolith from the V. Grib kimberlite pipe (Arkhangelsk Diamond Province, Russia) was studied. Based on petrographic characteristics, the peridotite xenolith reflects a sheared peridotite. The sheared peridotite experienced a complex evolution with formation of three main mineral assemblages: (1) a relict harzburgite assemblage consist of olivine and orthopyroxene porphyroclasts and cores of garnet grains (Gar1) with sinusoidal rare earth elements (REE) chondrite C1 normalized patterns; (2) a neoblastic olivine and orthopyroxene assemblage; (3) the last assemblage associated with the formation of clinopyroxene and garnet marginal zones (Gar2). Major and trace element compositions of olivine, orthopyroxene, clinopyroxene and garnet indicate that both the neoblast and clinopyroxene-Gar2 mineral assemblages were in equilibrium with a high Fe-Ti carbonate-silicate metasomatic agent. The nature of the metasomatic agent was estimated based on high field strength elements (HFSE) composition of olivine neoblasts, the garnet-clinopyroxene equilibrium condition and calculated by REE-composition of Gar2 and clinopyroxene. All these evidences indicate that the agent was a high temperature carbonate-silicate melt that is geochemically linked to the formation of the protokimberlite melt.     

Xenoliths of Eclogites with Diamonds from the Yubileinaya Kimberlite Pipe,Yakutia

The first results of study of minerals and diamonds of diamond-bearing eclogites from kimberlites of the Yubileinaya pipe with a variable percent amount of clinopyroxene and garnet are presented. Samples with a garnet content from 30 to 90% of the xenolith volume are dominant among the round to oval xenoliths with diamonds. Five eclogite samples contain grains of accessory rutile, as well as corundum and kyanite. Some samples host two or more diamond crystals.     

Peculiarities of Diamonds in Eclogitic Xenoliths from the Komsomolskaya Kimberlite Pipe,Yakutia

The first studies of diamonds in eclogitic xenoliths from the Komsomolskaya kimberlite pipe are described. Among round and oval-shaped xenoliths with diamond ingrowths, samples with a garnet content of 40–90% of the xenolith volume dominate. Two eclogite samples contain grains of accessory rutile; a kyanite sample is also revealed. Certain samples contain two or more crystals of diamonds. Diamonds with an octahedral habit and crystals with transitional habits, which belong to an octahedral-rhombic dodecahedral row, dominate in eclogites; there are many variety VIII aggregates. A high concentration of structural nitrogen, commonly in the A form, was registered in most of the crystals. Diamonds with a small content of nitrogen impurities, 40–67% in the B1 form, are present in a number of xenoliths. The calculated temperatures of the formation of eclogitic xenoliths is 1100–1300°C. Diversity in the impurity compositions of diamonds in the same xenolith shows that these diamonds were formed at various times and in different settings. The diamond position in xenoliths, the various level of nitrogen aggregation in the diamonds, and a number of other factors point to the later formation of the diamonds, as compared to minerals of eclogites, from fluid or fluid-melts in the process of metasomatosis.

     

赣南铁山垅钨矿田花岗斑岩地球化学特征、锆石U- Pb年龄及Hf同位素特征

赣南铁山垅钨矿田位于南岭钨锡多金属成矿带东段,已探获黑钨矿资源量超10万吨。铁山垅复式岩体包括主体似斑状黑云母花岗岩和补体细粒二云母花岗岩两部分,花岗斑岩呈脉状分布。矿田内花岗岩具有相似的地球化学特征,都属过铝质高钾钙碱性花岗岩类,表现出高硅、富铝、富碱、高钾、富成矿元素（W、Sn、Cu、Mo）和亏损Ba、Sr、Ti、P、REE、Eu,稀土配分曲线呈典型的“海鸥式”分布和M型四分组效应等特征。利用LA- ICP- MS锆石U- Pb定年方法获得花岗斑岩206Pb/238U 年龄为146. 7±0. 5 Ma（MSDW=0. 5）,成岩时代属晚侏罗世。锆石的n(176Lu)/n(177Hf)=0. 000973～0. 001989,fLu/Hf=-0. 97～-0. 94,εHf(t)=-17. 9～-10. 3,二阶段模式年龄（TDM2）为1. 86～2. 33 Ga,显示原岩为古元古代地壳。综合分析认为,铁山垅矿田岩浆活动可划分为170～155 Ma、155～150 Ma、150～145 Ma三个阶段,钨锡矿成矿主要集中在第二阶段,且第三阶段花岗斑岩与铜多金属矿成矿关系密切,推测铜岭矿区深部具有较大的找矿前景。     

赣南铁山垅钨矿田花岗斑岩地球化学特征、锆石U- Pb年龄及Hf同位素特征

赣南铁山垅钨矿田位于南岭钨锡多金属成矿带东段,已探获黑钨矿资源量超10万吨。铁山垅复式岩体包括主体似斑状黑云母花岗岩和补体细粒二云母花岗岩两部分,花岗斑岩呈脉状分布。矿田内花岗岩具有相似的地球化学特征,都属过铝质高钾钙碱性花岗岩类,表现出高硅、富铝、富碱、高钾、富成矿元素（W、Sn、Cu、Mo）和亏损Ba、Sr、Ti、P、REE、Eu,稀土配分曲线呈典型的“海鸥式”分布和M型四分组效应等特征。利用LA- ICP- MS锆石U- Pb定年方法获得花岗斑岩206Pb/238U 年龄为146. 7±0. 5 Ma（MSDW=0. 5）,成岩时代属晚侏罗世。锆石的n(176Lu)/n(177Hf)=0. 000973～0. 001989,fLu/Hf=-0. 97～-0. 94,εHf(t)=-17. 9～-10. 3,二阶段模式年龄（TDM2）为1. 86～2. 33 Ga,显示原岩为古元古代地壳。综合分析认为,铁山垅矿田岩浆活动可划分为170～155 Ma、155～150 Ma、150～145 Ma三个阶段,钨锡矿成矿主要集中在第二阶段,且第三阶段花岗斑岩与铜多金属矿成矿关系密切,推测铜岭矿区深部具有较大的找矿前景。     

Olivine in a Coesite-bearing Eclogite from the Udachnaya Kimberlite Pipe

Doklady Earth Sciences - Olivine (high-Fe forsterite; Fo# 69-76) and associated minerals have been studied in a coesite-bearing diamondiferous eclogite from the Udachnaya-East kimberlite pipe....     

安徽女山麻粒岩包体的地球化学特征:下地壳组成及其构造属性初探

女山麻粒岩包体的Mg#值总体较低（28－51），在化学成分上主要表现为偏中性，没有明显的辉石、石榴子石或Fe-Ti氧化物的堆晶作用，但部分有斜长石的堆晶作用，略具Eu,Sr正异常，部分则有斜长石的分离结晶，略具Eu,Sr负异常。另外，女山麻粒岩包体亏损Rb,Th,Nb,Ta,富集Ba,LREE,Zr,Hf等，具较高的Nb/Ta,Zr/Hf比值，麻粒岩相变质作用过程造成了Rb的强烈亏损，但对Nb,Ta,U,Th的影响较弱。女山麻粒岩包体的形成过程中存在岩浆的结晶分异作用，通过分离结晶混染作用模型（AFC）模拟，表明华北克拉通太古宙基性火山岩和中必一麻粒岩分别为较可能的初始岩浆和混合源，初始岩浆经过10％－40％的AFC可以形成女山麻粒岩包体的微量元素组成。总体上，女山麻粒岩包体的主元素、微量元素特征均相似于华北克拉通太古宙中性麻粒岩、华北克拉通下地壳平均值，而与扬子克拉通下地壳平均值得明显差别。因此女山地区下地壳可能仍属华北克拉通的一部分，因而支持Li(1994)的碰撞拆离模式。     

The Petrology of Grospydite Xenoliths from the Zagadochnaya Kimberlite Pipe in Yakutia

The xenoliths of garnet–clinopyroxene–disthene rocks(grospydites and associated kyanite eclogites) from the Zagadochnayakimberlite pipe in Yakutia have been studied in detail. Contraryto previous data, the presence of a continuous range in thepyrope-grossular series of garnets is shown on the basis ofnumerous X-ray data and 17 chemical analyses of garnets. Thisconclusion is confirmed by the study of separate grains withkyanite intergrowths from the kimberlite heavy fraction, whichare present in the kimberlite as the result of destruction ofgrospydite xenoliths, and possible, of garnet-kyanite rocksalso. A close connection of the calcium content in the garnetwith the sodium content in the coexisting clinopyroxes is alsoshown. An increase in the chemical potential of sodium resultsin the stability of the pryoxene-kyanite assemblage insteadof a garnet of intermediate composition (50 percent of grossular).The interval of the miscibility gap between calcium-rich andcalcium-poor garnets is increased in this way. The data of chemicalanalyses of 14 pyroxenes from the xenoliths indicate that theydiffer in the high aluminium and sodium content from other pyroxenesof eclogite-facies rocks. Chromium-rich bands with a high chromiumcontent in coexisting garnet, pyroxene, and kyantic have beenoccasionally found in the xenoliths. A study has been made ofthe chrome-kyanite with 12.86 per Cr202. The presence of chromium-richminerals in the grospydite xenoliths confrms their connectionwith ultrabasic rocks.