History of the Formation of the Tsentral'noe Titanium–Zirconium Sand Deposit in the European Part of Russia

Based on the study of the Tsentral'noe deposit, specific features of the formation of mineral assemblages of complex titanium–zirconium placers are considered. The placers formed during the multiple redeposition of clastogenic minerals from source rocks and younger sedimentary rocks (intermediate collectors of titanium–zirconium minerals). The location of erosion and sedimentation zones significantly varied in the Phanerozoic in the adjacent region, resulting in the development of intricate relationships between different-aged terrigenous rocks (possible intermediate collectors) that provided the formation of new mineral assemblages of clastogenic ore minerals. In addition, erosional processes during the continental evolution of the study region could promote the exposure of more ancient rock complexes, the local washout of crystalline basement rocks, and the delivery of ore minerals from the latter rocks to the coastal zone of sedimentary basins. The aim of this communication is to attract the attention of researchers to the issue of the formation of mineral assemblages of complex placers of heavy minerals with similar hydraulic grain dimension and migration capacity for concentration in a rather narrow grain size range. Such mineral assemblages only slightly inherit the primary compositional features of provenances and primarily reflect changes in the sedimentation environment.     

Application of arsenopyrite geothermometry and sphalerite geobarometry to the Taebaek Pb-Zn(-Ag) deposit at Yeonhwa I mine,Republic of Korea

The Taebaek Pb-Zn(-Ag) deposit of the Yeonhwa I mine, Republic of Korea, occurs in a broadly folded and reverse-faulted terrain of Paleozoic sedimentary rocks: the Taebaeksan basin. The orebodies consist of several thin tabular orebodies of hydrothermal replacement type where they are hosted by carbonate rocks. The Pb-Zn(-Ag) mineralization can be divided into four distinct stages based upon the mode of occurrence of ore minerals, ore textural relationships and their composition. Based on temperatures inferred from arsenopyrite compositions by means of electron microprobe and fluid inclusions, the estimated temperatures for the stages I, II, III and IV reach 330 to 350 °C, 270 to 340 °C, 230 to 250 °C, and <220 °C, respectively. The sulphur activity (atm) of ore formation at the Taebaek deposit was estimated for each stage as 10^–11 to 10^–11.5, 10^–9.5 to 10^–13, 10^–13.5 to 10^–15, and <10^–15, respectively. Even though application of sphalerite geobarometry is problematic because of the absence of good mineral assemblages, sphalerite coexisting with pyrite but not with pyrrhotite was used to estimate the minimum mineralization pressure (about 1 kbar).     

Carbon and Oxygen Isotopic Compositions of Carbonates from Precambrian Apatite-Bearing Carbonate Rocks of the Aldan Shield

Geostructural setting, as well as mineral and isotopic compositions, of separate apatite deposits and occurrences in the Aldan Shield composed of Precambrian metasedimentary apatite–carbonate rocks are considered. In terms of carbon and oxygen isotopic compositions, they differ from other carbonate rocks of the Aldan Shield, including carbonatites and Ca–Mg metasomatites, and resemble Phanerozoic and Precambrian metasedimentary carbonates. They formed in oxidizing conditions. The contribution of evaporite processes at different stages of their formation is supported by the carbonate enrichment in ¹³C and the presence of sulfates. It was established that apatite–carbonate rocks represent the product of complex alternation of sedimentary processes at different values of salinity in the basins occasionally characterized by the decomposition of older sediments under subaerial conditions.     

A Linear Programming Approach to Determine the Normative Composition of Sedimentary Rocks

The analysis of modal and normative composition of sedimentary rocks is widely used for studying their sources and tectonic settings. The normative calculation of the mineral composition of rocks in this study is formulated as a linear programming problem and is solved by means of the simplex method. This enables both simultaneous and successive subtraction of a set of basic minerals from a rock sample represented by its chemical composition {SiO₂...LOI}. Such an approach provides a more exact calculation of the contribution of basic minerals in the rock. This mathematical approach is used to study two representative sets of sandstones and fine-grained rocks from a Meso- to Neoproterozoic marginal basin of southeastern Siberia (Uchur–Maya region, Yakutia) and a Pennsylvanian-Lower Permian uplifted continental block in Colorado, USA. The calculated normative mineral compositions of the Siberian sandstones are consistent with the observed modal compositions. These sandstones vary from K- Feldspar rich arkoses at the base of the sequence (the Uchur Group, lower Riphean) to quartz arenites or lithic sandstones and wacke in transgressive successions of the middle-upper Riphean. Arkoses and quartz arenites are dominant in Meso- to Neoproterozoic Siberia. These samples represent craton interior uplifted basement and quartzose, recycled orogen provenance of a stable craton in Rodinia. There are higher but consistent discrepancies between the calculated and observed compositions for the Pennsylvanian to Lower-Permian arkoses and quartz arenites (Sangre de Cristo, Belden, and Maroon Formations). The differences between the predicted and observed mineralogy may be due to uncertainties in the modes in the matrix and cement of the sandstones. This normative program should supplement modal calculations and provide better genetic constructions, especially in case of matrix-rich sandstones.     

The Stream Sediment Geochemistry of the Walawe Ganga Basin of Sri Lanka - Implications for Gondwana Mineralization

A regional geochemical and mineralogical study aimed at investigating the mineralization in the western-part of the Walawe Ganga (river) Basin in Sri Lanka is represented in this paper. The river basin is the 3^rd largest in the country and has within it a boundary zone between two geologically different crustal blocks, which are marked by granulitic grade rocks and amphibolite grade rocks. Size fractions of stream sediments (< 63 μm; 63–125 μm; 125–177 μm and 177–250 μm) developed on the granulite-grade metamorphic terrain have been analysed at their source for their mineralogical and selected element compositions. Thirty-eight (38) sediment samples and 15 representative probable parent rock samples were chemically analysed giving special emphasis to the High Field Strength trace Elements (HFSE) including the Rare Earth Elements (REE). The granulite grade rocks in the study area is geochemically similar to that of post Archean upper crust. However the stream sediments developed from the high-grade rocks during the intense weathering, are markedly enriched with HFSE and REE. The enrichment of HFSE and LREE is accounted for by the presence of HFSE- and REE- rich accessory mineral phases such as zircon, monazite, apatite, garnet and rutile in the sediments. In some samples, the content of heavy minerals contributes as much as 50 wt. %. These minerals act as a source of elements in the sediments. However, extreme hydraulic sorting of HFSE- and REE-bearing minerals during the sediment deposition cannot be expected within a short distance from near the sources except from a mineralized occurrence. Therefore, the higher enrichment of these elements presumably indicates occurrences of scattered mineral sources such as highly differentiated granites and associated pegmatites within the Walawe Ganga drainage basin. These granitic pegmatites are probably intruded during or soon after the main granulite-facies metamorphic event and similar events are seen in other terrains of East-Gondwana.     

Plagioclase and other mineral equilibria in a contact metamorphic aureole

Plagioclase, microcline, amphibole, clinozoisite, clinopyroxene and biotite from alternating pelitic and calcareous hornfelses of the Wyman Formation, Blanco Mountain Quadrangle, California, were analyzed using an electron microprobe. The metamorphic aureole formed at temperatures of 300–600° C, total pressure 2–3 Kb, and low but variable partial pressure of CO₂. The minerals show some compositional changes with metamorphic grade as well as differences from one assemblage to another. The plagioclases developed in the aureole do not form a continuous series. Rather, coexisting grains of plagioclase in individual rock layers form at certain distinct compositions: An 1–3, 15–17, 28–32, 38–45, 51–55, 59–65, 75 and 80. There is no evidence of disequilibrium in the rocks, although diffusion was limited; the volume for chemical equilibrium for most samples was less than 1 mm. Inspection of the changes in mineral assemblages with increasing degree of metamorphism and with changes in fluid composition suggests a number of reactions between the phases. Neither these reactions nor the compositions of coexisting minerals provide an obvious explanation for the observed gaps in the plagioclase series. Therefore it is postulated that the compositional clustering is structurally controlled.     

Petrochemistry of Sandstones from the Siberian Hypostratotype of the Riphean in the Uchur–Maya Region

Based on 148 analyses, we report the petrochemical characteristics of Riphean and Vendian coarse-grained sedimentary rocks of the Uchur–Maya region in eastern Siberia, which is referred to the Proterozoic continental-margin domain. Various methods of petrochemical study applied to Precambrian sandstones are examined. Normative mineral compositions of specific groups of arenites, particularly basal potassic arkoses, are considered. Differences in sandstone compositions at certain stratigraphic levels are shown, and the evolution of chemical composition of rocks is traced through sections of sedimentary cover (Maya Plate) and margin region (Yudoma–Maya Trough). The work presents results of the application of data on mineralogical and chemical compositions of arenites for the restoration of paleodynamic environments in Late Proterozoic sedimentation basins at the southeastern margin of the Siberian Platform.     

沉积盆地物质来源综合研究——以南堡老第三纪亚断陷盆地为例

重矿物分析对于判断沉积盆地的物源方向和物源区的母岩性质是有效的,但具有局限性：①对无重矿物或者仅能提供极少量重矿物的母岩判别力较差;②它不能准确判断物源区沉积物的主要运移通道。因此,进行盆地物源分析还需要重视综合研究,即①通过重矿物的矿物学、重矿物组合研究,判断物源区的母岩类型,推测母岩岩性演化;②通过沉积岩矿物成分与结构研究,判别物源区是否存在重矿物无法辨认的母岩类型和沉积物的搬运距离;③通过沉积体系分析,尤其是砂分散体系的编图,能有效地判断物源区的方位和圈定各物源的具体影响范围;④利用古构造图判别物源区的主要沉积物运移通道及其方位;⑤编制沉积盆地－物源区古环境图,以此来弥补重矿物分析物源所带来的缺陷。     

Preliminary mineralogical and petrological study of the Ortosa Au–Bi–Te ore deposit: a reduced gold skarn in the northern part of the Rio Narcea Gold Belt, Asturias, Spain

The Ortosa deposit (NW Spain) in the northern part of the Rio Narcea Gold Belt (RNGB) is located in the Cantabrian Zone of the Iberian Massif. This zone corresponds to the westernmost exposure of the European Hercynides. The deposit is hosted by marine shales, siltstones, calcareous siltstones and interbedded sandy limestones of the upper part of the Silurian Furada Formation. These rocks are intruded by a main stock and numerous sills and dikes consisting of a reduced, ilmenite-bearing quartz-monzodiorite (Ortosa intrusion). Skarn metasomatism and associated gold mineralization overprinted these sedimentary and igneous rocks, forming endo- and exoskarns.The earliest stage of alteration involved potassium metasomatism from which metasomatic biotite developed in the hornfels around the intrusion. In the endoskarn, the first metasomatic mineral to form is actinolite. Subsequently, quartz, pyroxene (Hd_30–45), and sulfides (mainly arsenopyrite and pyrrhotite) formed, followed by a second generation of amphibole (ferroactinolite and ferrohornblende). The exoskarn is a pyroxene-garnet skarn, which is often banded. The prograde minerals are pyroxene (Hd_10–30) and grossular garnet. The retrograde mineralogy consists of hedenbergite-rich pyroxene (Hd_50–87), amphibole (ferroactinolite–ferrohornblende), and the metallic minerals with minor fluorapatite, K-feldspar, albite, epidote–clinozoisite, vesuvianite and calcite. A final stage of retrograde alteration is characterized by calcite, quartz, and chlorite.Pyrrhotite and arsenopyrite are the more abundant metallic minerals, and löllingite, chalcopyrite, pyrite and sphalerite are present in smaller amounts. The gold occurs as native gold and maldonite, and is accompanied by hedleyite, native bismuth, and bismuthinite. These Au–Bi–Te mineral assemblages occupy cavities and fractures in the arsenopyrite or in the pyrrhotite.Estimated physiochemical conditions of formation based on the composition and stability fields of major calc-silicate and sulfide minerals indicate that the hedenbergite-rich pyroxene and the earliest sulfides (löllingite–pyrrhotite–arsenopyrite) crystallized at temperatures between 470 and 535°C at low log fS₂ between −10 and −6.5 and low log fO₂ of −22. The Ortosa skarns can be included in the reduced gold skarn subtype defined by Meinert (Mineralogical Association of Canada, Quebec city, Que., Canada, 1998, 26,359–414 ).     

Spinel-group minerals in rocks of the Khibiny alkaline pluton,Kola Peninsula

Seven spinel-group minerals in various geological settings have been revealed in the rocks of the Khibiny pluton. Hercynite, gahnite, and vuorelainenite occur only in xenoliths of hornfels after volcanic and sedimentary rocks, whereas spinel and magnesiochromite occur in alkaline ultramafic rocks of dike series. Franklinite has been discovered in a low-temperature hydrothermal vein. Ubiquitous magnetite is abundant in foyaite, foidolites, alkaline ultrabasic rocks, and pegmatite and hydrothermal veins and may even be the main mineral in some foidolite varieties. The spinel-group minerals are characterized by various chemical compositions due to the fractionation of nepheline syenites resulting in formation of the Main ring of foidolites and apatite-nepheline ore. Like most other minerals found throughout the pluton, magnetite is characterized by variation in the chemical composition along the radial line from the contact with country Proterozoic volcanic rocks to the geometric center of the pluton. Toward the center, the total Ti and Mn contents in magnetite increase from 5–15 up to 40 at %.     

Formation and mineral chemistry of a calcic skarn from Al-Madhiq, SW Saudi Arabia

A newly identified skarn occurrence is described from the Neoproterozoic rocks of the SW Arabian shield. It is exposed to the SE, E and NE of the Al-Madhiq town. The skarn attributes correspond to those typical of the calcic skarns that host W-deposits. It is characterized as an exoskarn of the proximal type, related to a granitoid contact close to an impure quartzite bed within the regional metamorphic rocks of mixed sedimentary and volcanic derivation. The skarn is localized along a shear zone parallel to the regional faults and other major shear zones. Samples from the studied area contain characteristic skarn minerals that include both the prograde (brownish red grossular, ferrosalite, aluminian titanite-grothite, albite-oligoclase, scapolite), and retrograde (epidote, quartz, hornblende, calcite) assemblages. The pyroxenes are ferrosalites, Mn-bearing, and more like those from “oxidized” skarns; although garnets indicate it to be a “reduced” type skarn. Epidote mimicks that from typical skarns, as it bears a pistacite content of 15.9–20.7%. Grossular composition reflects a largely reduced genetic environment; as it is in solid solution with 6.5–21.6% andradite, 0–0.15% uvarovite, 0–0.47% pyrope, 4.33–18.75% almandine, and 0.4–8.58% spessartine molecules. Titanite composition varies from aluminian titanite to grothite, that may be analogous to the newly described Al-rich titanite from the low-pressure calc-silicate rocks.     

A calculated petrogenetic grid for ultramafic rocks in the system CaO-FeO-MgO-Al2O3-SiO2-CO2-H2O at low pressures

Calculated phase equilibria among the minerals amphibole, chlorite, clinopyroxene, orthopyroxene, olivine, dolomite, magnesite, serpentine, brucite, calcite, quartz and fluid are presented for the system CaO–FeO–MgO–Al₂O₃–SiO₂–CO₂–H₂O (CaF-MASCH), with chlorite and H₂O–CO₂ fluid in excess and for a temperature range of 440°C–600°C and low pressures. The minerals chosen in CaFMASCH represent the great majority of phases encountered in metamorphosed ultramafic rocks. The changes in mineral compositions in terms of FeMg_-1 and (Mg, Fe)SiAl_-1Al_-1 are related to variations in the intensive parameters. For example, equilibria at high in the presence of chlorite involve minerals which are relatively aluminous compared with those at low . The calculated invariant, univariant and divariant equilibria are compared with naturally-occurring greenschist and amphibolite facies ultramafic mineral assemblages. The correspondence of sequences of mineral assemblages and the compositions of the minerals in the assemblages is very good.     

Banded iron formations of the calciphyre-metabasite-gneiss association in the East European Craton

The paper presents characteristics of the least studied iron formations of the East European Craton (Archean banded iron formations of the calciphyre-metabasite-gneiss association), a typical member of granulite complexes of the Ukrainian Shield, Belarussian-Baltic region, and Voronezh crystalline massif. They are mainly composed of diverse metasedimentary rocks: aluminous gneisses; silicate-magnetite, magnetite, and barren quartzites; eulysites; calciphyres; and marbles associated with metavolcanic rocks. Data on chemical compositions of the metasedimentary rocks are summarized for the first time and their possible primary mineral composition has been reconstructed using the MINLITH software. It is shown that they could be formed from a lithogenetic series of sediments linked by gradual transitions and geochemical commonness of sediments: from fine-grained terrigenous insufficiently mature sediments to chemogenic sediments depleted in terrigenous material (ferruginous-siliceous, ferruginous-siliceous-carbonate, siliceous-carbonate, and carbonate sediments). The inferred primary mineral assemblage indicates sedimentation in the central parts of large paleobasins in a reducing environment characterized by deficit of oxygen and excess of carbon dioxide. Lithological specifics of the banded iron formations in different regions presumably reflect different distances of sedimentation zones from submarine hydrothermal discharge sites and sources of terrigenous material. The banded iron formations at the present-day erosion section of basement represent metamorphosed fragments of the lateral-facies zoning of rocks of the Archean sedimentary basins (or a single basin) of the East European Craton. Unlike other Early Precambrian banded iron formations of the East European Craton, rocks of the calciphyre-metabasite-gneiss association are marked by a high Mn content.     

A suggested origin of MARID xenoliths in kimberlites by high pressure crystallization of an ultrapotassic rock such as lamproite

Chemical, mineralogical and isotopic studies have been made on nodules of the MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside) xenolith suite in southern African kimberlites. All are ultramafic and ultrapotassic (MgO= 20–25%, K₂O=4–9%), with bulk compositions reflecting the wide variation in relative proportions of the five minerals amongst the nodules. They are comparable in major element compositions to magnesian lamproites, in particular the ultrabasic olivine-lamproites of Western Australia. In a number of high pressure experimental studies on ultra-potassic rocks, the phases produced between 25–30 kbar from compositions comparable to those of MARID rocks (in the presence of additional water), were predominantly phlogopite and diopside (±K-richterite, ±ilmenite, ±rutile). Furthermore the compositions of experimental minerals produced in the synthetic-biotite-mafurite-H₂O system by Edgar et al. (1976) are similar to those in MARID rocks.It is suggested on the basis of these observations and the textural appearance of MARID rocks that they are magmatic compositional equivalents of MgO-rich lamproites that crystallized at high pressures. While lamproites have higher average concentrations of incompatible elements, (including REE), some MARID rocks have comparable abundances. It is suggested that late stage vapour-rich melts carrying substantial REE and other incompatible elements escaped from crystallizing MARID magmas into surrounding subcontinental lithosphere, thus resulting in lower levels of these elements in most MARID rocks. In contrast faster crystallization of lamproitic rocks under volcanic/ hypabyssal conditions would prevent similar losses.The MARID proto-magmas are thought to be either partial melts of metasomatised phlogopite peridotite, or small volume asthenospheric melts which are modified and further enriched by incorporation of small partial melts of enriched subcontinental lithosphere during magma ascent.     

Mineralogical changes during intense chemical weathering of sedimentary rocks in Bangladesh

Bangladesh is situated in a subtropical to tropical climatic zone. A recently weathered crust has developed on sedimentary bedrock (sandstone, siltstone, shale and claystones) of Tertiary–Quaternary age. Weathered samples were collected from 16 sections totaling 68 samples and were analyzed mineralogically. The main primary minerals identified in the weathered crust of sedimentary rocks are quartz, plagioclase, K-feldspar, biotite, muscovite, sparse carbonate and epidote. The secondary minerals are kaolinite, illite, chlorite, gibbsite and goethite. Weathering initiated along the grain boundaries and cleavage planes of the minerals, forming small cloudy materials which were very difficult to identify. In the advanced stage of weathering, these cloudy materials have turned into secondary minerals. In region 1, high rain fall (7100 mm/yr) and monsoonic climate resulted in a kaolinite–gibbsite–goethite suite through the weathering of feldspars and biotite. The occurrence of gibbsite in the relatively elevated lands of Sylhet and Fe-kaolinite throughout the study areas is indicative of a humid–tropical climate during formation of the weathered crust.     

Chemical weathering in Luzon, Philippines from clay mineralogy and major-element geochemistry of river sediments

Clay mineralogy and major-element geochemistry of 35 surface sediment samples collected in 21 major to moderate rivers of Luzon, Philippines are used to evaluate the present chemical weathering process. The clay mineral assemblage consists mainly of smectite (average 86%) with minor kaolinite (9%) and chlorite (5%) and very scarce illite (1%), and does not show strong island-wide differences. The major element results of both bulk and clay-fraction sediments indicate that the formation of clay minerals is accompanied by leaching of Ca and Na first and of Fe and Mn thereafter during the chemical weathering process. A low-moderate chemical weathering degree of bulk sediments and a moderate-intensive degree of clay-fraction sediments are obtained in Luzon rivers based on proxies of chemical index of alteration (CIA) and smectite crystallinity. It is suggested that the majority of andesitic–basaltic volcanic and sedimentary rocks along with the tectonically active geological setting and sub-tropical East Asian monsoon climate are responsible for the predominance of smectite in the clay mineral assemblage.     

Genetic model of uranium mineralization in the Permo-Triassic sedimentary rocks of the Stara Planina eastern Serbia

One deposit and four occurrences of uranium minerals in the Permo-Triassic sedimentary rocks of the Stara Planina in eastern Serbia were ascertained using data from years of multidisciplinary geological research. The minerals are the fissure-filling type, and were assigned to the exogenic mineralization group, being mostly epigene in nature. Relevant geological information was used to derive a genetic model of uranium mineralization in the Permo-Triassic sedimentary rocks of the Stara Planina. The model explains the formation of Permo-Triassic rocks and three stages of the complex process of U mineralization. The genetic model treats: (1) the primary sources of the uranium; (2) the mobility of uranium; and (3) accessory metals from the parent rocks to their deposition in host rocks and the postmineral change in ore bodies. A geochemical barrier zone was identified in the sedimentary rocks that contained uranium mineral ore. This geochemical barrier area included crescent-shaped, flat-lens, or vein-like ore bodies. The U-containing mineral described is comparable with those from the relatively common fissure-filling uranium minerals found around the world.     

Petrochemistry of Metapelites from the Patom Highland and a Possible Composition of Primary Sediments

Metapelites as products of isochemical regional metamorphism retain a memory on paleoclimate and geochemical evolution of clays. At the base of metasedimentary sequences, they are composed of high-Al varieties containing modal and normative pyrophyllite locally in association with chloritoid. The clayey rocks are relatively Al-depleted in middle part of the section but Al-rich and pyrophyllite-free at the top. The lower high-Al metasedimentary rocks could have been derived from mature kaolinite and kaolinite-bearing hydromuscovite–chlorite clays. The middle low-Al metapelites are considered to be relatively immature montmorillonite-bearing illite clays, and the upper high-Al varieties are interpreted as relatively mature illite–chlorite clays without kaolinite. The evolution of clays imprinted in geochemical signatures of metamorphic rocks was predetermined by the cooling of moderately humid climate in middle periods of sedimentary cycles related to the development of alpine glacial topography on the northern Patom Highland. Periodic variations of chemical and mineral compositions of rocks were used as helpful guides for the determination of boundaries and volumes of primary sedimentary cycles in metasedimentary sequences with obliterated stratigraphy.     

Sub-volcanic infiltration and syn-eruptive quenching of liquids in cumulate wall-rocks: the example of the gabbroic nodules of Stromboli (Aeolian Islands, Italy)

Summary ¶Fine- to coarse-grained plutonic nodules within the Petrazza pyroclastics (Paleo-Stromboli I period) consist of gabbroic rocks with variable amounts of interstitial material. They are characterised by cumulate textures and low pressure modal mineralogy formed by plagioclase (An_96–87)+clinopyroxene (Mg-v 82–94)+olivine (Fo_83–74)±amphibole±opaque minerals; the interstitial material consists of newly crystallised microlites (quenching) of plagioclase (An_73–55)+amphibole+clinopyroxene±olivine±biotite±opaques and highly variable amounts of residual glasses that range in composition from shoshonite and high-K basaltic andesite to high-K andesite and latite. The interstitial material has a relatively high but variable degree of vesicularity. The whole rock incompatible element abundances are lower than – but the patterns are typical of – in subduction related magmas and the incompatible trace-elements are well correlated with the amount of the interstitial material. The Sr, Pb and Nd isotopic ratios resemble those of the extrusive rocks of Stromboli older series and the mineral chemistry of the gabbros is similar to that of the HKCA Paleo-Stromboli lavas. Modal mineralogy, mineral chemistry and chemical-isotopic whole rock compositions suggest that the cumulus portions of the gabbroic nodules crystallised from basaltic magmas compositionally compatible with those erupted by Stromboli volcano. The interstitial material does not represent the residual liquid after in situ crystallisation of the gabbros; it is also distinct from the juvenile host andesite magma. Textural evidence, Fe–Mg mineral/liquid partioning and mass balance calculations indicate that the interstitial material (quench crystals and vesicular glass) derived from infiltrated hydrous basaltic liquid undercooling and vesiculation of which occurred during the eruption of the Petrazza pyroclastics.Received April 17, 2002; revised version accepted November 14, 2002 Published online June 2, 2003     

碎屑石榴石地球化学物源分析与解释:粒度的影响

单矿物地球化学分析是沉积物源分析研究的重要手段,运用广泛。沉积物中碎屑矿物的粒度组成,即水动力分选作用,是否影响到基于这种方法的物源解释,目前尚不清楚且受到的关注不多。通过研究柴达木盆地北缘地区第三系沉积物中的680颗碎屑石榴石(0.068~0.557 mm)的主量元素地球化学组成数据,解释了不同粒度的碎屑石榴石物源。结果显示,0.063~ 0.125 mm的碎屑石榴石的地球化学结果中Fe^2+与Mn^2+含量更高,表明其母岩的变质结晶程度较弱,故物源解释结果更可能为中—低级角闪岩相变沉积岩,而其他粒径的碎屑石榴石Ca^2+与Mg^2+含量更高,这说明其物源区的温压条件较前者高,因而物源解释结果为低级变质相,中酸性火成岩,榴辉岩等宽泛结果。这种碎屑石榴石粒度差异导致的地球化学组成的不同,说明碎屑石榴石的颗粒大小在一定程度上会影响物源解释结果。因此,在进行单矿物地球化学物源研究时,选取特定粒径(0.063~ 0.125 mm或0.125~ 0.25 mm)组成的碎屑矿物进行分析,可以消除水动力分选作用的影响,有助于获得更为准确的物源解释结果。