Petrology and geochemistry of a diamondiferous lherzolite from the Premier diamond mine, South Africa

This paper reports on the petrology and geochemistry of a diamondiferous peridotite xenolith from the Premier diamond mine in South Africa.

The xenolith is altered with pervasive serpentinisation of olivine and orthopyroxene. Garnets are in an advanced state of kelyphitisation but partly fresh. Electron microprobe analyses of the garnets are consistent with a lherzolitic paragenesis (8.5 wt.% Cr₂O₃ and 6.6 wt.% CaO). The garnets show limited variation in trace element composition, with generally low concentrations of most trace elements, e.g. Y (<11 ppm), Zr (<18 ppm) and Sr (<0.5 ppm). Garnet rare earth element concentrations, when normalised against the C1 chondrite of McDonough and Sun (Chem. Geol. 120 (1995) 223), are characterised by a rare earth element pattern similar to garnet from fertile lherzolite.

All diamonds recovered are colourless. Most crystals are sharp-edged octahedra, some with minor development of the dodecahedral form. A number of crystals are twinned octahedral macles, while aggregates of two or more octahedra are also common. Mineral inclusions are rare. Where present they are predominantly small black rosettes believed to consist of sulfide. In one instance a polymineralic (presumably lherzolitic) assemblage of reddish garnet, green clinopyroxene and a colourless mineral is recognised.

Infrared analysis of the xenolith diamonds show nitrogen contents generally lower than 500 ppm and variable nitrogen aggregation state, from 20% to 80% of the ‘B’ form. When plotted on a nitrogen aggregation diagram a well defined trend of increasing nitrogen aggregation state with increasing nitrogen content is observed. Carbon isotopic compositions range from −3.6 ‰ to −1.3 ‰. These are broadly correlated with diamond nitrogen content as determined by infrared spectroscopy, with the most negative C-isotopic compositions correlating with the lowest nitrogen contents.

Xenolith mantle equilibration temperatures, calculated from nitrogen aggregation systematics as well as the Ni in garnet thermometer are on the order of 1100 to 1200 °C.

It is concluded that the xenolith is a fertile lherzolite, and that the lherzolitic character may have resulted from the total metasomatic overprinting of pre-existing harzburgite. Metasomatism occurred prior to, or accompanied, diamond growth.     

Re-evaluation of the Cablac Limestone at its type area,East Timor: Revision of the Miocene stratigraphy of Timor

The Cablac Limestone, widely recorded in Timor, has its type area on Cablac Mountain where it was regarded as a Lower Miocene shallow-marine carbonate-platform succession. The Bahaman-like facies placed in the Cablac Limestone are now known to belong to the Upper Triassic–Lower Jurassic rather than the Lower Miocene. On the northern slopes of Cablac Mountain, a crush breccia, formerly regarded as the basal conglomerate of the formation, is now considered to have developed along a high-angle fault separating Banda Terrane units of Asian affinity from an overthrust limestone stack containing units belonging to the Gondwana and Australian-Margin Megasequences. The Cablac breccia includes rock fragments that were probably derived locally from these tectonostratigraphic units after terrane emplacement and overthrusting. Clasts include peloid and oolitic limestones of the Upper Triassic–Lower Jurassic derived from the Gondwana Megasequence, deep-water carbonate pelagites of the Cretaceous and Paleogene derived from the Australian-Margin Megasequence, Upper Oligocene–Lower Miocene (Te Letter Stage) shallow-water limestone derived from the Banda Terrane, and a younger Neogene calcarenite containing clasts of mixed tectonostratigraphic affinity. There is no evidence for significant sedimentary or tectonic transport of clasts that form the breccia. The clast types and the present understanding of the geological history of Timor suggest that the crush breccia formed late in the Plio-Pleistocene uplift history of Timor. It is not the basal conglomerate of the Cablac Limestone. However, the clasts of an Upper Oligocene–Lower Miocene limestone found in the breccia suggest that a shallow-marine limestone unit of this age either outcrops in the region and has not been detected in the field, or has been eroded completely during late Neogene uplift. The clasts are similar in age and lithology to an Upper Oligocene–Lower Miocene formation that unconformably overlies a metamorphic complex in the Booi region of West Timor, similar to the Lolotoi Metamorphic Complex (Banda Terrane) that is juxtaposed against the crush breccia of Cablac Mountain. The Cablac Limestone at its type area includes a mixed assemblage of carbonate rock units ranging in age from Triassic to Plio-Pleistocene and representing diverse facies. As a formation, the name “Cablac Limestone” should be discarded for a Cenozoic unit. The Upper Oligocene–Lower Miocene shallow-water limestone unit that is typified by outcrops in the Booi region of West Timor, and that has contributed to clasts in the Cablac breccia, is informally named the Booi limestone. It is considered part of the allochthonous Banda Terrane of Asian affinity and represents the only shallow-marine Lower Miocene unit known from Timor. The only other Miocene sedimentary unit known from Timor includes carbonate pelagites – designated the Kolbano beds – probably deposited on an Australian continental terrace at water depths between 1000 and 3000 m. On the northeastern edge of Cablac Mountain, oolitic limestone and associated units of the Gondwana Megasequence, the Kolbano beds of the Australian-Margin Megasequence, and the Booi limestone and associated metasediments of the Banda Terrane were juxtaposed by a Plio-Pleistocene high-angle fault along which the Cablac crush breccia formed.     

Re-evaluation of the Cablac Limestone at its type area, East Timor: Revision of the Miocene stratigraphy of Timor

The Cablac Limestone, widely recorded in Timor, has its type area on Cablac Mountain where it was regarded as a Lower Miocene shallow-marine carbonate-platform succession. The Bahaman-like facies placed in the Cablac Limestone are now known to belong to the Upper Triassic–Lower Jurassic rather than the Lower Miocene. On the northern slopes of Cablac Mountain, a crush breccia, formerly regarded as the basal conglomerate of the formation, is now considered to have developed along a high-angle fault separating Banda Terrane units of Asian affinity from an overthrust limestone stack containing units belonging to the Gondwana and Australian-Margin Megasequences. The Cablac breccia includes rock fragments that were probably derived locally from these tectonostratigraphic units after terrane emplacement and overthrusting. Clasts include peloid and oolitic limestones of the Upper Triassic–Lower Jurassic derived from the Gondwana Megasequence, deep-water carbonate pelagites of the Cretaceous and Paleogene derived from the Australian-Margin Megasequence, Upper Oligocene–Lower Miocene (Te Letter Stage) shallow-water limestone derived from the Banda Terrane, and a younger Neogene calcarenite containing clasts of mixed tectonostratigraphic affinity. There is no evidence for significant sedimentary or tectonic transport of clasts that form the breccia. The clast types and the present understanding of the geological history of Timor suggest that the crush breccia formed late in the Plio-Pleistocene uplift history of Timor. It is not the basal conglomerate of the Cablac Limestone. However, the clasts of an Upper Oligocene–Lower Miocene limestone found in the breccia suggest that a shallow-marine limestone unit of this age either outcrops in the region and has not been detected in the field, or has been eroded completely during late Neogene uplift. The clasts are similar in age and lithology to an Upper Oligocene–Lower Miocene formation that unconformably overlies a metamorphic complex in the Booi region of West Timor, similar to the Lolotoi Metamorphic Complex (Banda Terrane) that is juxtaposed against the crush breccia of Cablac Mountain. The Cablac Limestone at its type area includes a mixed assemblage of carbonate rock units ranging in age from Triassic to Plio-Pleistocene and representing diverse facies. As a formation, the name “Cablac Limestone” should be discarded for a Cenozoic unit. The Upper Oligocene–Lower Miocene shallow-water limestone unit that is typified by outcrops in the Booi region of West Timor, and that has contributed to clasts in the Cablac breccia, is informally named the Booi limestone. It is considered part of the allochthonous Banda Terrane of Asian affinity and represents the only shallow-marine Lower Miocene unit known from Timor. The only other Miocene sedimentary unit known from Timor includes carbonate pelagites – designated the Kolbano beds – probably deposited on an Australian continental terrace at water depths between 1000 and 3000 m. On the northeastern edge of Cablac Mountain, oolitic limestone and associated units of the Gondwana Megasequence, the Kolbano beds of the Australian-Margin Megasequence, and the Booi limestone and associated metasediments of the Banda Terrane were juxtaposed by a Plio-Pleistocene high-angle fault along which the Cablac crush breccia formed.     

Petrology of the Vandfaldsdalen Macrodike, Skaergaard Region, East Greenland

The Vandfaldsdalen macrodike is a layered and differentiatedgabbroic dike approximately 3?5 km long and from 200 to 500m wide. It appears to cut the eastern margin of the Skaergaardintrusion and may have served as a feeder for the Basistoppensill. The macrodike can be divided into three series of rocks:a marginal series of differentiated gabbros adjacent to thewalls of the dike; a central series of differentiated and subhorizontallylayered gabbros and ferrodiorites in the interior of the dike;and an upper felsic series of granophyric rocks with abundantquartzo-feldspathic xenoliths. The mineral and bulk-rock compositionsthrough both the marginal series and central series show progressiveiron enrichment. The most Ca-rich plagioclase (An₆₉) and mostmagnesian pyroxene (Wo₄₂ En₄₆ Fs₁₂) occur in olivine-bearingrocks of the marginal series about 5 m from the contact withwall rocks. The most Na-rich plagioclase (An₃₉) and Fe-richpyroxene (Wo₃₈ En₂₄ Fs₃₈) are in olivine-free ferrodiorite ofthe central series, about 20 m below the contact with the felsicseries. Evidence from field observations, bulk-rock chemical compositions,and Sr and Nd isotopic data indicate the felsic series formedas a mixture of the initial macrodike magma and granitic countryrock. ⁸⁷Sr/⁸⁶Sr ratios of specimens from the felsic series rangebetween 0?7129 and 0?7294. ¹⁴³Nd/¹⁴⁴Nd ratios vary between 0?51208and 0?51118. Both ratios vary serially with the SiO₂ contentsof the specimens. We suggest that the felsic series evolvedas a separate body of low density liquid which floated on thedenser gabbroic magma of the central series. Heat from crystallizationof the gabbroic magma must have diffused into the felsic layer,enabling extensive assimilation of the granitic xenoliths, butour data indicate there was very little exchange of chemicalcomponents between the two liquids.     

Gendered access to customary land in East Timor

Land tenure rights reflect the deeper structures of society, particularly gender distinctions in relation to land. Considering the structural differences between patrilineal and matrilineal customary tenure systems in East Timor are understudied, this paper explores men and women’s experiences in accessing land under such arrangements. The comparative analysis of two patrilineal with one matrilineal land tenure systems in Ainaro and Manufahi districts suggests a significant degree of flexibility within both systems with respect to the norms of gendered inheritance. Therefore, the binary constructs of ‘patrilineal’ and ‘matrilineal’ societies are limiting. Both men and women in these communities may acquire land rights under different circumstances, mainly through negotiations with their parents or hamlet chief. Daughters in the patrilineal communities could inherit family land upon their parents’ death and sons in the matrilineal community could gain land by cultivating and maintaining unclaimed customary land. Empirical evidence show that inheritance principally determines usufruct rights to land, but marriage exchange practices complicates a deeper understanding of traditional East Timorese land rights.

Petrology, geochemistry and paleogeographic reconstruction of the East Sulawesi Ophiolite, Indonesia

The East Sulawesi Ophiolite (ESO) is tectonically dismembered and widely distributed in Central and East Sulawesi. It comprises, from base to top, residual mantle peridotite and mafic–ultramafic cumulate through layered to isotropic gabbro, to sheeted dolerites and basaltic volcanic rocks. Residual peridotite is dominantly spinel lherzolite intercalated with harzburgite and dunite. Ultramafic rocks from different locations display significant differences in rock composition and mineral. However, the clinopyroxene of peridotite displays REE pattern similarities with those of mid-ocean ridge (MOR) origin, rather than those of suprasubduction zone (SSZ) origin. The gabbroic unit consists of massive gabbro, layered gabbro, mafic and ultramafic cumulate and anorthosite. The observed crystallization sequence of gabbroic unit, which is olivine→(spinel)→plagioclase→clinopyroxene→(orthopyroxene)→(hornblende), and the mineral chemistry data indicate that the ESO gabbro has similarities with MOR setting.Major and trace element geochemistry of basalt and dolerite suggests MOR, oceanic plateau and minor SSZ origins. A possible oceanic plateau origin is supported by the following: (i) the 15-km thickness is comparable with the thickness of oceanic plateau rather than normal oceanic lithosphere; (ii) there are no or only minor olivine phenocrysts in the basalt; and (iii) predominance of aphyric texture in the basalts. The REE pattern of ESO basalt exhibits N-MORB-like signatures. However, a negative Nb anomaly in the trace element spider diagram may be attributed to mantle heterogeneity of an OPB source.The geochemical variations and disparities for both peridotite and basalt and the noncogenetic relationship between crust and mantle sections in several locations suggest that the ESO may have been formed at one tectonic setting and was later overprinted by magmatism in different environments through its birth to emplacement. A possible Cretaceous origin of an oceanic plateau component of the ESO is indicated on the basis of calculated paleopositions using plate trajectory analyses together with previously published paleolatitude data. The ESO can be traced back to the proximity of the presently active region of the SW Pacific Superplume.     

东帝汶海槽断裂特征与构造演化模式

东帝汶海槽位于澳大利亚板块西北帝汶海与帝汶岛之间,晚中新世澳大利亚板块与东南亚班达岛弧碰撞引发了帝汶岛与帝汶海槽的构造变形,由于弧-陆碰撞过程的复杂性,帝汶海槽的变形时间与机制仍然存在较大争议。为约束帝汶海槽变形时间、探讨其演化过程,依据二维地震数据,对东帝汶海槽的断裂特征进行定量分析,并结合钻井与天然地震事件探讨弧-陆碰撞背景下帝汶海槽的构造演化模式。研究结果表明,晚中新世（约6 Ma）澳大利亚板块与班达岛弧碰撞,引发了帝汶岛的隆升与帝汶海槽的沉降,变形持续至晚上新世（约3 Ma）,形成现今帝汶海槽形态。现今澳大利亚板块与班达岛弧之间的相对运动速率已逐渐减小趋于停止,未来二者可能会以一个整体继续向北运动。     

Deformation and metamorphism of the Aileu Formation,north coast,East Timor and its tectonic significance

The large block of metamorphic rocks along the north coast of East Timor is of special interest as it occurs at the boundary between continental and oceanic crust in an island arc-continent collision zone. A detailed study of the structure and metamorphic history of 400 km² of this formation showed it has a complex history of penetrative deformation but the structure is coherent.Pelites, psammites and limestones interlayered with dolerites and amphibolites have been metamorphosed in a medium pressure environment. They now form a metamorphic province zoned from low greenschist facies in the southwest to upper amphibolite facies in the east. The earliest recognised deformation phase predated the metamorphism and produced a widespread layer—parallel schistosity but no recognisable folds. The second deformation phase post-dated the metamorphic maximum and micropetrological evidence indicates a gradual cooling during this event. This deformation produced tight folds with an axial plane schistosity and transposed the earlier structures. The progressively weaker third and fourth phases developed crenulation cleavages and related folds, under greenschist facies conditions. Open, fifth phase, macroscopic folds were probably synchronous with strike slip faulting parallel to the north coast. Later dip slip faulting juxtaposed the Aileu Formation with Permian and Mesozoic sediments of very low metamorphic grade.Reconnaissance K/Ar radiometric dating using hornblende and biotite showed the prograde metamorphic maximum occurred before 11 Ma ago and implies that the second, and strongest, deformation phase occurred in the late Miocene. This young age establishes the relationship of the deformation events to the collision between Australia and the Inner Banda Arc.The proposed models for the structure of Timor must be modified to fit the deformation history of the Aileu Formation. If Timor is essentially autochthonous, the Aileu Formation was probably deposited in a Palaeozoic graben and the metamorphic maximum may have occurred in the Jurassic. The overthrusting models must be modified in the light of the close correlation in time between penetrative deformation and emplacement of the proposed thrust sheets. The analogy proposed between Timor and ‘normal’ convergent margins is not supported but it may be possible to draw analogies with the Molucca Sea.     

Pacification,resistance, and territoriality: prospects for a space of peace in East Timor

Despite current United Nations-sponsored negotiations on East Timor between Indonesia and Portugal, there is little prospect for success given the overwhelming advantages Indonesia enjoys in the struggle for control over the territory. The territorialization of Indonesian power in East Timor over the last two decades has given Jakarta a level of dominance that has allowed it to avoid serious negotiations aimed at resolving the conflict. By interrogating social power in conjunction with Knight's (1994) three key components of statehood (territory, population, and sovereignty) on a variety of geographical scales, however, it becomes clear that East Timor is a far more contested terrain than it first appears. This paper illustrates that the peace process can succeed in a manner consistent with international law and human rights only by a strengthening of the forces of resistance, which necessarily entails the West's altering its relationship of criminal complicity with Indonesia.     

Petrology of the Basistoppen Sill, East Greenland: A Calculated Magma Differentiation Trend

The 660 m thick Basistoppen sill is an Eocene, tholeiitic, layeredintrusion emplaced in the upper part of the Skaergaard complexshortly after solidification of the Skaergaard magma. Despiteits small size, the Basistoppen sill has one of the most extensivedifferentiation sequences known. The ranges of the solid solutionsin olivine, plagioclase, and pyroxene from the Basistoppen arecomparable to those in the Skaergaard and Bushveld intrusions.The rocks of the sill are orthocumulates composed of approximately35% trapped liquid and 65% cumulus minerals and can be dividedinto zones based on changes in the cumulus mineral assemblage.From the base upward those zones are: a Gabbro Picrite Zonecontaining cumulus olivine, Fe-Cr spinel, and minor biotite;a Bronzite Gabbro Zone containing cumulus orthopyroxene, Ca-richclinopyroxene, plagioclase, and minor Fe-Cr spinel; a PigeoniteGabbro Zone containing cumulus plagioclase, Ca-rich clinopyroxene,pigeonite, magnetite, and minor ilmenite; and a Fayalite DioriteZone containing cumulus plagioclase, Ca-rich clinopyroxene,magnetite, ilmenite, apatite, and olivine. The Basistoppen isoverlain by a zoned granophyre sill that was most likely derivedin part from the Basistoppen magma and in part from melted Precambriangneiss. The excellent exposure, uncomplicated structure, goodchilled margin, and lack of strong modal layering facilitatethe calculation of a differentiation trend for the Basistoppensill. During crystallization the Basistoppen magma became progressivelyricher in Fe, P, Na, K, Zn, Rb, Zr, La, Sm, and Th, became progressivelypoorer in Mg, Ca, Al, Cr, and Ni, and remained relatively unchangedin Si, Sc, and Sr through at least the first 90% of crystallization.     

Petrology of spinel lherzolite xenoliths from Youkou volcano,Adamawa Massif,Cameroon Volcanic Line: mineralogical and geochemical fingerprints of sub-rift mantle processes

The basaltic maar of Youkou, situated in the Adamawa Volcanic Massif in the eastern branch of the continental segment of the Cameroon Volcanic Line, contains mantle-derived xenoliths of various types in pyroclastites. Spinel-bearing lherzolite xenoliths from the Youkou volcano generally exhibit protogranular textures with olivine (Fo_89.4?90.5), enstatite (En_89???91Fs_8.7?9.8Wo_0.82?1.13), clinopyroxene, spinel (Cr#_Sp?=?9.4–13.8), and in some cases amphibole (Mg_#?=?88.5–89.1). Mineral equilibration temperatures in the lherzolite xenoliths have been estimated from three–two pyroxene thermometers and range between 835 and 937 °C at pressures of 10–18 kbar, consistent with shallow mantle depths of around 32–58 km. Trends displayed by bulk-rock MgO correlate with Al₂O₃, indicating that the xenoliths are refractory mantle residues after partial melting. The degree of partial melting estimated from spinel compositions is less than 10%: evidences for much higher degrees of depletion are preserved in one sample, but overprinted by refertilization in others. Trace element compositions of the xenoliths are enriched in highly incompatible elements (LREE, Sr, Ba, and U), indicating that the spinel lherzolites underwent later cryptic metasomatic enrichment induced by plume-related hydrous silicate melts. The extreme fertility (Al₂O₃?=?6.07–6.56 wt% in clinopyroxene) and the low CaO/Al₂O₃ ratios in the spinel lherzolites suggest that they could not be a simple residue of partial melting of primitive mantle and must have experienced refertilization processes driven by the infiltration of carbonatite or carbonated silicate melts.     

东昆仑白干湖钨锡矿床成矿岩体岩石学、年代学和地球化学

白干湖钨锡矿田是近年来在我国西部地区新发现的一个具超大型远景规模的矿田,由白干湖、柯可卡尔德、巴什尔希和阿瓦尔四个矿床组成。矿床与加里东期巴什尔希岩浆序列具有密切联系,但对其岩石成因及与成矿关系尚有争议。本文选取白干湖矿床与成矿有关的花岗岩开展了岩石学、年代学和地球化学研究。镜下鉴定其为正长花岗岩。LA-MC-ICP-MS锆石U-Pb年龄为413.6±2.4Ma(MSWD=0.36,n=30)。结合前人资料,认为巴什尔希岩浆序列形成于后碰撞构造环境,岩体年龄集中于中志留世-早泥盆世(ca.433~413Ma),具有S型花岗岩和A型花岗岩伴生的特点。其中白干湖正长花岗岩含有原生锰铝榴石和白云母等过铝质矿物,A/CNK和A/NK分布范围分别为1.07~1.11和1.45~1.49,锆石饱和温度和高场强元素含量相对较低,属于弱过铝质高钾钙碱性S型花岗岩,形成于巴什尔希岩浆序列岩浆活动晚期,与区内钨锡成矿关系密切,矿区内下步找矿应对隐伏或岩枝状产出的正长花岗岩更加重视。而巴什尔希岩浆序列中的二长花岗岩和碱长花岗岩含有角闪石和黑云母等暗色矿物并主要呈他形晶充填于长石和石英晶隙,K2O含量为5.25%~6.29%,属于钾玄岩系列,A/CNK为0.92~1.02,其锆石饱和温度(866~917℃)、全碱含量(8.30%~9.69%)、稀土总量(200×10-6~413×10-6)和Zr、Nb、Ce、Y等高场强元素含量(Zr+Nb+Ce+Y=556×10-6~1006×10-6)都明显偏高,为准铝质或弱过铝质A型花岗岩,在该区及外围可能具有形成与A型花岗岩有关的稀有稀土金属矿床的潜力。     

Pyroxenes from lherzolite inclusions of Itinome-gata,Japan

The lherzolites have recrystallized to plagioclase lherzolites consisting of olvine, pyroxenes, chromian spinel, plagioclase and pargasite at a depth of 20 to 25 km in the uppermost part of the mantle. It is believed that the garnet lherzolites and spinel lherzolites were originally derived from depths of 50–75 km and 30–50 km respectively. The clinopyroxenes contained about 10 mol. % of jadeite and Tschermak's molecules, respectively and the orthopyroxenes also included about 5–10% of Tschermak's component. Transported upward, the garnet was transformed through pyroxene-spinel symplectite to olivine, plagioclase and spinel aggregates, and most of the jadeite amd some Tschermak's components in the pyroxenes formed secondary pyroxenes and pargasite, and finally plagioclase under isochemical conditions.     

Kimberlites and lamproites of the East European Platform: Petrology and geochemistry

Several episodes of kimberlite magmatism occurred in the East European Province (EEP) during a long (about 1.5 Gyr) time period, from the Late Paleoproterozoic (ca. 1.8 Ga) in the Archean Ukrainian and Baltic shields to the Middle Paleozoic (ca. 0.36 Ga) mainly in the Arkhangelsk, Timan, and adjacent regions. Based on the analysis of data on 16 kimberlite occurrences and four lamproite occurrences within the EEP, five time stages can be distinguished; one of them, the Middle Paleozoic stage (Middle Ordovician and Devonian), is the most productive epoch for diamond in the northern hemisphere (EEP, Siberian Craton, and part of the China Craton). The analysis of petrological and geochemical characteristics of kimberlites (lamproites were studied less thoroughly) revealed variations in rock composition and their correlation with a number of factors, including the spatial confinement to the northern or southern Archean blocks of the craton, time of formation of the source of kimberlite melts, contents of volatiles and autoliths, etc. Three petrogeochemical types of kimberlites were distinguished: high-, medium-, and low-Ti (TiO₂ > 3 wt %, 1–3 wt %, and <1 wt %, respectively). There are two time intervals of the formation of kimberlite and lamproite sources in the EEP, corresponding to T_Nd(DM) values of about 2 Ga (up to 2.9 Ga in the Por’ya Guba occurrence) and 1 Ga. The latter interval includes two groups of occurrences with model source ages of about 1 Ga (low-and medium-Ti kimberlites of the Zolotitsa and Verkhotina occurrences) and about 0.8 Ga (high-Ti kimberlites of the Kepino and a number of other occurrences); i.e., there seems to be an evolutionary trend in the composition of kimberlites. Concentric zoning patterns were recognized. The role of the crust in kimberlite sources is discussed; it is assumed that buried remnants of the oceanic lithosphere (megaliths) may underlie whole continents. A unique feature of the composition of low-Ti kimberlites, for instance, kimberlites of the Zolotitsa occurrence (to a smaller extent, medium-Ti kimberlites of the V. Grib pipe) is the distinct depletion of highly charged elements and pronounced negative anomalies of Ti, Zr, Th, U, Nb, and Ta in trace-element distribution patterns, which indicates a contribution of crustal material to the source of these kimberlites. It was shown that autoliths exert a significant influence on the differentiation of kimberlite material, resulting in the enrichment of rocks in the whole spectrum of incompatible elements. It was argued that geochemical criteria can be used together with traditional criteria (including those based on indicator minerals) for the assessment of diamond potential in EEP occurrences. We hope that such a combined approach will yield important outcomes in the future.     

Melts of garnet lherzolite: experiments, models and comparison to melts of pyroxenite and carbonated lherzolite

Phase equilibrium experiments on a compositionally modified olivine leucitite from the Tibetan plateau have been carried out from 2.2 to 2.8 GPa and 1,380–1,480 °C. The experiments-produced liquids multiply saturated with spinel and garnet lherzolite phase assemblages (olivine, orthopyroxene, clinopyroxene and spinel ± garnet) under nominally anhydrous conditions. These SiO₂-undersaturated liquids and published experimental data are utilized to develop a predictive model for garnet lherzolite melting of compositionally variable mantle under anhydrous conditions over the pressure range of 1.9–6 GPa. The model estimates the major element compositions of garnet-saturated melts for a range of mantle lherzolite compositions and predicts the conditions of the spinel to garnet lherzolite phase transition for natural peridotite compositions at above-solidus temperatures and pressures. We compare our predicted garnet lherzolite melts to those of pyroxenite and carbonated lherzolite and develop criteria for distinguishing among melts of these different source types. We also use the model in conjunction with a published predictive model for plagioclase and spinel lherzolite to characterize the differences in major element composition for melts in the plagioclase, spinel and garnet facies and develop tests to distinguish between melts of these three lherzolite facies based on major elements. The model is applied to understand the source materials and conditions of melting for high-K lavas erupted in the Tibetan plateau, basanite–nephelinite lavas erupted early in the evolution of Kilauea volcano, Hawaii, as well as younger tholeiitic to alkali lavas from Kilauea.     

中天山东段天湖花岗岩岩石学、地球化学及其成因

天湖花岗岩体由六个侵入体组成,按岩石谱系划分为三个岩相单元,归并为一个超单元,其为同一次岩浆熔融事件过程中曾经发生过三次岩浆侵入活动而形成的圆形深成岩体。LA-ICP-MS锆石U-Pb定年结果表明,天湖岩体侵位于早三叠世(241~243Ma),为印支早期的产物。来自岩体的锆石原位Hf同位素测定结果表明,ε_(Hf)(t)值变化于-2.6~5.5之间,二阶段Hf模式年龄(t_(DM2))变化于941~1435Ma之间,表明其岩浆源区均为来源于亏损地慢的新生地壳岩石。该岩体总体富硅(SiO_2为69.14%~76.62%),富碱(K_2O+Na_2O为7.39%~8.83%),而Ti,Ca,Fe和Mg含量较低,A/CNK为0.95~1.04,(K_2O+Na_2O)/Al_2O_3为0.69~0.82,属于钙碱性花岗岩。总体上富集大离子亲石元素(LILE)K、Rb和高场强元素(HFSE)Ba、Th,但贫Ta、Nb、Ce、Hf、Zr、Sm、Y及Yb。稀土元素的球粒陨石标准化配分曲线呈右倾型,轻重稀土分馏明显,重稀土分异不显著,而轻稀土分异明显,铕呈现弱负异常(δEu=0.24~0.78)。岩石学及地球化学特征研究表明,天湖岩体形成与幔源岩浆的底侵和内侵使得中-新元古代新生的地壳物质发生部分熔融有关。岩浆侵入活动的动力来源于海西晚期至印支早期古特提斯洋向北俯冲和碰撞及随后的板内伸展作用。     

Hornblende lherzolite and the upper mantle

Hornblende lherzolite nodules from the Kirsh volcano, near Ataq, South Arabian Federation, are likely to have been derived from the upper mantle. The hornblende is a pargasitic variety, rich in sodium and chromium.     

东昆仑中灶火地区超镁铁质辉石岩的成因

最近在青海东昆北中灶火地区发现超镁铁质岩的岩石学、地球化学以及成因矿物学等方面的研究成果.岩石主要由单斜辉石、斜方辉石和角闪石组成, 另有少量斜长石、石英、黑云母和铁质不透明矿物.角闪石和黑云母为后期退变质矿物.斜方辉石成因判别分析结果为岩浆成因, 故该超镁铁质岩为辉石岩而非麻粒岩.该辉石岩化学成分上表现为异常的高MgO、高CaO、低Al₂O₃特征, 微量元素表现为Rb、Th富集而Nb、Ti的亏损, 表明其来源于富集地幔.通过岩相学、稀土元素等特征与前人研究结果对比认为该辉石岩是俯冲洋壳部分熔融产生的富Si熔体与地幔橄榄岩发生交代反应产生辉石岩岩浆, 然后底侵到地壳中部冷却结晶形成的.野外地质特征显示辉石岩的侵位晚于发生糜棱岩化的围岩, 即晚于围岩的形成时代, 即早二叠世, 说明该辉石岩是在中二叠世古特提斯洋向北大规模俯冲及其所导致的弧后伸展的构造背景下形成的.      

东帝汶国金属矿产资源潜力与矿业投资环境

东帝汶国的成矿地质条件较优越,矿产资源较丰富。铬铁矿化在马纳图托省、马努法伊省和包考省有明显的找矿信息,成矿环境受环太平洋成矿带超镁铁质岩带控制。其中位于马纳托图省Hili Manu地区的蛇纹石化超基性岩中呈豆荚状产出铬铁矿,Cr2O3含量在36%～51%之间,显示出较好的资源潜力。锰矿化主要分布于包考省、维克克省和马纳图托省,为海相层控型,其中位于马纳托图省Vemasse地区的锰矿床规模较大,品位较高,具有成为中—大型锰矿床的潜力。铜矿化出露于包考省、维克克省、科瓦利马省、马努法伊省、马纳图托省和劳滕省,矿化作用与蛇绿岩套有关,其中位于维克克省Ossu地区的蛇纹岩体,存在与蛇纹石化基性—超基性岩石组合有关的铜/金/银矿化露头,暗示其下可能隐伏着更大的硫化物矿化体。金矿化既有热液脉型,又有浅成低温热液蚀变浸染型,更有河流砂矿型,分布范围广,品位中等到较富,其中马努法伊省Turiscai地区 Sue河、Laclo河及Cler河沿岸砂砾层中存在找寻砂金矿的前景,河流上游存在原生金矿的可能。综上所述,东帝汶国具有明显的找矿前景、矿产资源开发潜力和较好的矿业投资环境,建议加大工作力度,把握机会,适时介入。     

The transition between spinel lherzolite and garnet lherzolite,and its use as a Geobarometer

The equilibrium between spinel lherzolite and garnet lherzolite has been experimentally determined in the CaO-MgO-Al₂O₃-SiO₂ system between 800° and 1,100° C. In confirmation of earlier work and predictions from thermodynamic data, it was found that theP-T slope of the reaction was close to zero, the equilibrium ranging from 16.1 kb at 800° C to 18.7 kb at 1,100° C (±0.3 kb). The addition of Cr₂O₃ to the system raised the stability field of spinel to higher pressures. It was found that the pressure at which both garnet and spinel could exist with olivine+orthopyroxene+clinopyroxene in the system CMAS ?Cr₂O₃ could best be described by the empirical relationship: $$P = P^{\text{O}} + \alpha X_{{\text{Cr}}}^{s{\text{p}}} $$ whereP ⁰ is the equilibrium pressure for the univariant reaction in the Cr₂O₃-free system,α is a constant apparently independent of temperature with a value of 27.9 kilobars, andX _Cr ^sp is the mole fraction of chromium in spinel. Use was made of the extensive literature on Mg-Fe²⁺ solid solutions to quantitatively derive the effect of Fe²⁺ on the equilibrium. The effect of other components (Fe³⁺, Na) was also considered. The equilibrium can be used as a sensitive geobarometer for rocks containing the five phases ol+opx+cpx+gt+sp, and thus provides the only independent check presently available for the more widely applicable geobarometer which uses the alumina content of orthopyroxene in equilibrium with garnet.