PGE reefs as an in situ crystallization phenomenon: the Nadezhda gabbronorite body, Lukkulaisvaara layered intrusion, Fennoscandian Shield, Russia

Summary The 100 m thick and 700 m long Nadezhda sill-like gabbronorite body, emplaced in the mafic-ultramafic Lukkulaisvaara layered intrusion, is associated with Cu-Ni-PGE mineralization that is mostly confined to mottled anorthosites developed from host gabbronorites and norites along the margin of the entire body. PGE and Au show distinct positive correlation with Cu, Ni and S suggesting that the noble metals are mainly controlled by sulphides. A mineralized layer about 1 m thick contains on average 0.74 wt.% S, 1.77 ppm Pt and 7.96 ppm Pd. Marginal location of PGE-rich sulphides and their absence in the Nadezhda body suggest that the mineralizing process must have been gravity-independent and not related to the bulk saturation of the magma in sulphides. We propose that the Nadezhda Cu-Ni-PGE mineralization formed as a result of local sulphide saturation that was achieved in situ at the margins as a result of the flux of ore-bearing fluids from the interior of the body towards its margins. The sulphides became PGE-rich due to interaction with a large volume of ore-bearing fluids that were introduced into the Nadezhda body during a juvenile stage of its development as a conduit through which a large volume of magma has passed on its way to the main chamber. Authors’ addresses: R. M. Latypov and S. Yu. Chistyakova, Kola Science Centre, Geological Institute, Fersman Str. 16, 184200 Apatity, Russia; Present address: Department of Geosciences, University of Oulu, P. O. Box 3000, Oulu, FIN-90014, Finland; T. T. Alapieti, Department of Geosciences, University of Oulu, P. O. Box 3000, Oulu, FIN-90014, Finland     

Infiltration metasomatism in layered intrusions revisited: a reinterpretation of compositional reversals at the base of cyclic units

Summary Irvine (1980) proposed an elegant mechanism termed “infiltration metasomatism” to explain upward displacement of discontinuities in mineral compositional trends relative to contacts between cyclic units in the Muskox intrusion. It was concluded that the offsets of Mg/(Mg + Fe) discontinuities in olivine and chromite are a secondary postmagmatic feature that resulted from reaction between the cumulus minerals and intercumulus liquid that was frontally displaced upwards from the underlying crystal pile as a result of compaction. We reinterpret this feature in the Muskox and other layered intrusions as basal reversals that arise from a temperature gradient-driven flux of low melting point components from the hot magma parental to cyclic units towards a relatively cold cumulate floor. In this interpretation basal reversals are a primary magmatic feature that does not involve intercumulus liquid migration. Authors’ addresses: R. M. Latypov, S. Yu. Chistyakova, Kola Science Centre, Geological Institute, Fersman Str. 16, 184200, Apatity, Russia; Present address: Department of Geosciences, University of Oulu, Oulu, P.O. Box 3000, FIN-90014, Finland; T. T. Alapieti, Department of Geosciences, University of Oulu, Oulu, P.O. Box 3000, FIN-90014, Finland     

PGE mineralization in the late Archaean iron-rich mafic-ultramafic Hanumalapur Complex, Karnataka, India

Summary An unusually thick sulfur-poor mineralized zone enriched in platinum-group elements (PGE) is described in the Hanumalapur Complex, Shimoga District, Karnataka State, India. This promising occurrence was discovered in the early 1990s and the best samples at the time of writing have yielded Pt+Pd concentrations in excess of six ppm. The western part of the area concerned belongs to the late Archaean Dharwar Super Group (3000–2500 Ma), while the eastern part is occupied predominantly by a granite-gneiss terrain ∼3000 Ma in age. Ten mafic-ultramafic complexes which host interesting vanadium-bearing titanomagnetite occurrences are encountered in the western part, one of which is the Hanumalapur Complex. The PGE mineralized zone in this complex may be divided into four mineralogically distinctive types, which are, in descending order of PGE content: 1) a silicate-hosted Pd type, 2) a silicate-hosted Pt type, 3) a base-metal sulfide-hosted Pd type, and 4) an oxide-hosted PGE type. The genesis of the mineralization is somewhat unclear at this point of investigation, especially because of complete re-crystallization, but the evidence gathered so far suggests something different than a traditional orthomagmatic model requiring magma mixing processes and resulting in sulfide immiscibility. This is backed-up by the general lack of base metal sulfides in favor of chromite, although pure chlorite-amphibole and chlorite-albite-epidote-amphibole rocks may contain significant PGE concentrations regardless of the amount of chromite. The PGM textures show little evidence of hydrothermal alteration and remobilization, but the PGE mineralogy itself displays some characteristics of fluid action, as it seems that there are some OH-bearing Pt and Pd minerals present. The first author was Deceased Author’s address: R. J. Kaukonen, Department of Geosciences, University of Oulu, Oulu, P.O. Box 3000, FIN-90014 Finland     

The nature and origin of Fe-Ti-P-rich rocks in the Qareaghaj mafic-ultramafic intrusion, NW Iran

Summary Fe-Ti-P-rich rocks (FTP) are unusual with respect to their mineralogy and bulk composition. Varieties of these rocks are mostly related to Proterozoic massif-type anorthosites and to a lesser extent to the upper parts of mafic-ultramafic intracratonic layered complexes and other igneous rock suites. We present results on the geology, mineralogy and geochemistry of a new occurrence of FTP, associated with mafic rocks in the northwestern part of Iran. The Qareaghaj mafic-ultramafic intrusion (QMUI) is a small igneous body situated between Palaeozoic sedimentary rocks and a Precambrian low grade metamorphic complex. The QMUI is composed mainly of non-mineralized mafic and apatite- and Fe-Ti oxide-rich ultramafic rocks. The mafic rocks, mainly coarse-grained gabbro, microgabbro and amphibolite, have a simple mineral assemblage (plagioclase + clinopyroxene + ilmenite) and based on field observations, mineralogy and chemical composition are comagmatic. The ultramafic rocks with high proportion of olivine (∼40–66 vol.%), apatite (∼0.1–16 vol.%), ilmenite (∼11–19 vol.%) and magnetite (∼2–13 vol.%), have unusual bulk compositions (e.g., SiO₂ ∼ 21–30 wt.%, total iron expressed as Fe₂O₃ ^tot ∼ 26–42 wt.%, TiO₂ ∼ 5–11 wt.%, MgO ∼ 9–20 wt.%, P₂O₅ up to 5.1 wt.%, Cr ∼ 40–160 ppm, Ni ∼ 7–73 ppm). The FTP forms numerous sill-like layers, ranging in thickness from ∼5 cm to few meters. These rocks, totally enclosed in mafic rocks with sharp and concordant contacts, show a magmatic lamination and follow the general NW–SE trend of QMUI. The apatite-rich ultramafic rocks makes up 90–95% of the total ultramafic outcrops and contain Mg-poor olivine (Mg# ∼ 40–58) and low-Mg spinel (Mg# ∼ 30–44) in contrast to apatite-poor ones (∼60–63 and ∼43–46, respectively). Field relationships, mineral compositions and geochemical data suggested that the FTP are not related to the mafic host rocks. On the contrary, they intruded latter into the gabbros during plastic, high temperature deformation in local shear zones. Fractional crystallization of P-rich ferrobasaltic parental magma at depth, probably in an open magmatic system, not far from the QMUI magma chamber, is considered as responsible for the formation of the evolved FTP in QMUI.     

Mineral chemistry of submarine lavas from Hilo Ridge, Hawaii: implications for magmatic processes within Hawaiian rift zones

The crustal history of volcanic rocks can be inferred from the mineralogy and compositions of their phenocrysts which record episodes of magma mixing as well as the pressures and temperatures when magmas cooled. Submarine lavas erupted on the Hilo Ridge, a rift zone directly east of Mauna Kea volcano, contain olivine, plagioclase, augite ±orthopyroxene phenocrysts. The compositions of these phenocryst phases provide constraints on the magmatic processes beneath Hawaiian rift zones. In these samples, olivine phenocrysts are normally zoned with homogeneous cores ranging from ∼ Fo₈₁ to Fo₉₁. In contrast, plagioclase, augite and orthopyroxene phenocrysts display more than one episode of reverse zoning. Within each sample, plagioclase, augite and orthopyroxene phenocrysts have similar zoning profiles. However, there are significant differences between samples. In three samples these phases exhibit large compositional contrasts, e.g., Mg# [100 × Mg/(Mg+Fe⁺²)] of augite varies from 71 in cores to 82 in rims. Some submarine lavas from the Puna Ridge (Kilauea volcano) contain phenocrysts with similar reverse zonation. The compositional variations of these phenocrysts can be explained by mixing of a multiphase (plagioclase, augite and orthopyroxene) saturated, evolved magma with more mafic magma saturated only with olivine. The differences in the compositional ranges of plagioclase, augite and orthopyroxene crystals between samples indicate that these samples were derived from isolated magma chambers which had undergone distinct fractionation and mixing histories. The samples containing plagioclase and pyroxene with small compositional variations reflect magmas that were buffered near the olivine + melt ⇒Low-Ca pyroxene + augite + plagioclase reaction point by frequent intrusions of mafic olivine-bearing magmas. Samples containing plagioclase and pyroxene phenocrysts with large compositional ranges reflect magmas that evolved beyond this reaction point when there was no replenishment with olivine-saturated magma. Two of these samples contain augite cores with Mg# of ∼71, corresponding to Mg# of 36–40 in equilibrium melts, and augite in another sample has Mg# of 63–65 which is in equilibrium with a very evolved melt with a Mg# of ∼30. Such highly evolved magmas also exist beneath the Puna Ridge of Kilauea volcano. They are rarely erupted during the shield building stage, but may commonly form in ephemeral magma pockets in the rift zones. The compositions of clinopyroxene phenocryst rims and associated glass rinds indicate that most of the samples were last equilibrated at 2–3 kbar and 1130–1160 °C. However, in one sample, augite and glass rind compositions reflect crystallization at higher pressures (4–5 kbar). This sample provides evidence for magma mixing at relatively high pressures and perhaps transport of magma from the summit conduits to the rift zone along the oceanic crust-mantle boundary. Received: 8 July 1998 / Accepted: 2 January 1999     

Two mantle sources, two plumbing systems: tholeiitic and alkaline magmatism of the Maymecha River basin, Siberian flood volcanic province

Rocks of two distinctly different magma series are found in a ∼4000-m-thick sequence of lavas and tuffs in the Maymecha River basin which is part of the Siberian flood-volcanic province. The tholeiites are typical low-Ti continental flood basalts with remarkably restricted, petrologically evolved compositions. They have basaltic MgO contents, moderate concentrations of incompatible trace elements, moderate fractionation of incompatible from compatible elements, distinct negative Ta(Nb) anomalies, and _Nd values of 0 to +2. The primary magmas were derived from a relatively shallow mantle source, and evolved in large crustal magma chambers where they acquired their relatively uniform compositions and became contaminated with continental crust. An alkaline series, in contrast, contains a wide range of rock types, from meymechite and picrite to trachytes, with a wide range of compositions (MgO from 0.7 to 38 wt%, SiO₂ from 40 to 69 wt%, Ce from 14 to 320 ppm), high concentrations of incompatible elements and extreme fractionation of incompatible from compatible elements (Al₂O₃/TiO₂∼1; Sm/Yb up to 11). These rocks lack Ta(Nb) anomalies and have a broad range of _Nd values, from −2 to +5. The parental magmas are believed to have formed by low-degree melting at extreme mantle depths (>200 km). They bypassed the large crustal magma chambers and ascended rapidly to the surface, a consequence, perhaps, of high volatile contents in the primary magmas. The tholeiitic series dominates the lower part of the sequence and the alkaline series the upper part; at the interface, the two types are interlayered. The succession thus provides evidence of a radical change in the site of mantle melting, and the simultaneous operation of two very different crustal plumbing systems, during the evolution of this flood-volcanic province. Received: 6 January 1998 / Accepted: 29 June 1998     

The paleoproterozoic vurechuaivench layered Pt-bearing pluton,Kola Peninsula: New results of the U-Pb (ID-TIMS,SHRIMP) dating of baddeleytte and zircon

The Vurechuaivench layered PGE-bearing pluton (VP) is located in the central part of the Kola Peninsula, at the southeastern contact of the Monchegorsk layered complex with the Paleoproterozoic Imandra-Varzuga rift structure. The VP is composed of gabbronorites with a layered horizon of intercalated gabbronorites and anorthosites, containing sulfide and PGE-bearing mineralization. The U-Pb (ID-TIMS) age of baddeleytte from gabbronorite of the ore zone (sample M-42) was determined on a Finningan MAT-262 (RPQ) seven-channel mass-spectrometer in the Laboratory of Geochronology at the Geological Institute, Kola Scientific Center, Russian Academy of Sciences. Zircons from anorthosites and gabbronorites of the ore zone (samples B-1 and B-2, respectively) were studied on a SHRIMP-II secondary-ionic microprobe in the Center of Isotopic Studies of the Federal State Unitary Enterprise VSEGEI (St. Petersburg). The reliable age of formation of various VP rocks was determined by single grains of accessory baddeleytte and zircon based on additional studies of sample M-42 (2498.2 ± 6.7 Ma) and new studies of samples B-1 and B-2 (2507.9 ± 6.6 and 2504.8.4 Ma). The identical U-Pb ages of anorthosites and gabbronorites from the ore reef indicate that anorthosites are a dependent phase and were formed along with gabbronorites during the intrachamber melt differentiation and crystallization.     

A mafic-ultramafic cumulate sequence derived from boninite-type melts (Niagara Icefalls,northern Victoria Land,Antarctica)

The layered sequence from Niagara Icefalls (northern Victoria Land, Antarctica) is related to the Cambrian-Early Ordovician Ross Orogeny. The sequence consists of dunites, harzburgites, orthopyroxenites, melagabbronorites and gabbronorites of cumulus origin. The Mg# of olivine, spinel, orthopyroxene and clinopyroxene from these rocks yields positive correlations, thus indicating formation from melts that mainly evolved through fractional crystallisation. The following fractionation sequence was identified: olivine (up to 94 mol% forsterite) + Cr-rich spinel → olivine + orthopyroxene ± spinel → orthopyroxene → orthopyroxene + anorthite-rich plagioclase ± clinopyroxene. Clinopyroxenes retain the peculiar trace element signature of boninite melts, such as extremely low concentrations of HREE and HFSE, and LILE enrichment over REE and HFSE. U–Pb isotope data on zircons separated from a gabbronorite have allowed us to constrain the age of emplacement of the Niagara Icefalls sequence at ∼514 Ma. The occurrence of inherited zircons dated at ∼538 Ma indicates that the boninitic melts experienced, at least locally, crustal contamination. The Niagara Icefalls sequence can be related to a regional scale magmatic event that affected the eastern margin of the Gondwana supercontinent in the Middle Cambrian. We propose that the formation of the sequence was associated with the development of an embryonic back-arc basin in an active continental margin. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Geochemistry of the Merensky reef, Rustenburg Section, Bushveld Complex: controls on the silicate framework and distribution of trace elements

This whole rock and silicate mineral study focuses on the genesis of the Merensky reef sequence, as well as the footwall and hanging wall norites at an area of Rustenburg Platinum Mines in a demonstrably normal (undisturbed) environment. Continuous sampling provides major and trace element variations and mineral compositions and allows an evaluation of the post- liquidus processes which affected the sequence. Following the formation of liquidus phases three stages are envisaged to have modified the rocks. These are (a) migration of fluid during early compaction of cumulates, (b) circulation of fluids within the crystal mush, and (c) reaction and solidification of trapped liquid. Liquidus compositions are nowhere preserved in the sequence. A strong link is demonstrated between orthopyroxene compositions (e.g. Mg# and TiO₂) and the incompatible trace element content of the whole rocks. The final amount of trapped liquid is shown to have been variable but never exceeded 10%. Calculated liquidus (pre-equilibration) orthopyroxene compositions show an up- sequence progression of evolving compositions from the footwall norite to the hanging wall norite. Initial Sr isotopic values do not support a simple magma mixing model by which radiogenic Main Zone magma mixes with that of the Critical Zone at the level of the Merensky reef. There is evidence that the hanging wall norite formed from a much more evolved magma. These conclusions have implications for the distribution and origin of the PGE-enriched Merensky reef package. Received: 7 October 1998 / Accepted: 5 March 1999     

Relationship between the Layered Series and the overlying evolved rocks in the Bjerkreim-Sokndal Intrusion, southern Norway

The Bjerkreim-Sokndal layered intrusion (BKSK) consists of a > 7000-m-thick Layered Series comprising anorthosites, leuconorites, troctolites, norites, gabbronorites and jotunites (hypersthene monzodiorites), overlain by an unknown thickness of massive, evolved rocks: mangerites (hypersthene monzonites; MG), quartz mangerites (QMG) and charnockites (CH). The Layered Series is subdivided into six megacyclic units that represent the crystallisation products of successive major influxes of magma. We have studied a ca. 2000-m-thick section that straddles the sequence from the uppermost part of the Layered Series to the QMG in the northern part of the intrusion. Mineral compositions in 37 samples change continuously in the lower part of the sequence up to the middle of the MG-unit (plagioclase An37-18; olivine Fo40-7; Ca-poor pyroxene Mg#57-15; Ca-rich pyroxene Mg#65-21). Above this compositions are essentially constant in the upper part of the MG-unit and in the QMG (An21-13; Fo6-4; Mg#opx17-13; Mg#cpx25-20). The amount of interstitial quartz and the amount of normative orthoclase, however, both increase systematically upwards through the QMG-unit, implying that these rocks are cumulates. There is no evidence of a compositional break in the MG-QMG sequence that could reflect influx of relatively primitive magma.

Two types of QMG/CH are known in the uppermost part of BKSK. Olivine-bearing types are comagmatic with the underlying Layered Series; the studied stratigraphic sequence belongs to this suite. Two-pyroxene QMG and amphibole CH define a separate compositional lineage related to jotunites. An intrusive unit of dominantly two-pyroxene QMG is discordant to the olivine-bearing jotunite-MG-QMG sequence near Rapstad, confirming the presence of two compositionally distinct suites of QMG and related lithologies in the upper part of BKSK.

A xenolith-rich unit near the olivine-bearing MG-QMG boundary represents a major collapse of the roof of the magma chamber during the final stages of crystallisation.     

Architecture and emplacement of the Nebo–Babel gabbronorite-hosted magmatic Ni–Cu–PGE sulphide deposit,West Musgrave,Western Australia

The Nebo–Babel Ni–Cu–platinum-group element (PGE) sulphide deposit in the West Musgrave Block, Western Australia, is the largest nickel sulphide discovery in the last 10 years. The deposit is hosted within a concentrically zoned, olivine-free, tube-like (chonolithic), gabbronorite intrusion associated with the, approximately, 1,078-Ma Giles Complex-layered intrusions in the Warakurna large igneous province. Emplaced into sulphide-free amphibolite facies orthogneiss, the fault-offset Nebo–Babel chonolith extends for 5 km and has a cross-section of 1 × 0.5 km. Igneous mineralogy, fabrics, and textures are well preserved. The lithostratigraphy includes variably textured leucogabbronorites (VLGN) that form an outer shell around mineralised gabbronorite (MGN), with barren gabbronorite (BGN) and oxide–apatite gabbronorite (OAGN) in the middle and lower parts of the chonolith. Mineral and whole-rock geochemistry indicate that the units become progressively evolved in the order: VLGN, MGN, BGN, and OAGN, and that incompatible trace-element concentrations increase downwards within the MGN and BGN. The mineralisation, which is confined to the early, more primitive units (VLGN and MGN), occurs as massive sulphide breccias and stringers and as disseminated gabbronorite-hosted sulphides. The massive sulphides were emplaced late in the intrusive sequence, have different PGE chemistry and Cu tenor to the disseminated sulphides, and have undergone sulphide fractionation. The distribution of disseminated sulphides, which are primary magmatic in origin, is related to chonolith geometry and magma flow regimes, rather than to gravitational settling. Sulfur-bearing country rocks are absent in the Nebo–Babel deposit area, and thus, local crustal S addition was unlikely to have been the major mechanism in achieving sulphide immiscibility. The Nebo–Babel intrusion is part of an originally continuous magma chonolith with multiple and related magma pulses. The parental magma was medium- to low-K tholeiite with 8–9 wt% MgO. The initial magma pulse (VLGN), the most primitive and sulphide saturated, was probably emplaced along a linear weakness in the country rock. After crystallisation of VLGN, marginally more fractionated, sulphide-saturated magma was injected through the thermally insulated core of the conduit, forming the MGN. Successive pulse(s) of more fractionated magma (BGN) were emplaced in the core of the intrusion. After magma flow ceased, closed system crystal fractionation produced consistent mineral and chemical fractionation trends within BGN and OAGN. After crystallisation, the intrusion was overturned and then offset by the Jameson Fault resulting in the apparent ‘reverse’ chemical and mineral trends in Nebo–Babel.     

Cyclicity in the Main and Upper Zones of the Bushveld Complex, South Africa: Crystallization from a Zoned Magma Sheet

The major element composition of plagioclase, pyroxene, olivine,and magnetite, and whole-rock ⁸⁷Sr/⁸⁶Sr data are presented forthe uppermost 2·1 km of the layered mafic rocks (upperMain Zone and Upper Zone) at Bierkraal in the western BushveldComplex. Initial ⁸⁷Sr/⁸⁶Sr ratios are near-constant (0·7073± 0·0001) for 24 samples and imply crystallizationfrom a homogeneous magma sheet without major magma rechargeor assimilation. The 2125 m thick section investigated in drillcore comprises 26 magnetitite and six nelsonite (magnetite–ilmenite–apatite)layers and changes up-section from gabbronorite (An₇₂ plagioclase;Mg# 74 clinopyroxene) to magnetite–ilmenite–apatite–fayaliteferrodiorite (An₄₃; Mg# 5 clinopyroxene; Fo₁ olivine). The overallfractionation trend is, however, interrupted by reversals characterizedby higher An% of plagioclase, higher Mg# of pyroxene and olivine,and higher V₂O₅ of magnetite. In the upper half of the successionthere is also the intermittent presence of cumulus olivine andapatite. These reversals in normal fractionation trends definethe bases of at least nine major cycles. We have calculateda plausible composition for the magma from which this entiresuccession formed. Forward fractional crystallization modelingof this composition predicts an initial increase in total iron,near-constant SiO₂ and an increasing density of the residualmagma before magnetite crystallizes. After magnetite beginsto crystallize the residual magma shows a near-constant totaliron, an increase in SiO₂ and decrease in density. We explainthe observed cyclicity by bottom crystallization. Initiallymagma stratification developed during crystallization of thebasal gabbronorites. Once magnetite began to crystallize, periodicdensity inversion led to mixing with the overlying magma layer,producing mineralogical breaks between fractionation cycles.The magnetitite and nelsonite layers mainly occur within fractionationcycles, not at their bases. In at least two cases, crystallizationof thick magnetitite layers may have lowered the density ofthe basal layer of melt dramatically, and triggered the proposeddensity inversion, resulting in close, but not perfect, coincidenceof mineralogical breaks and packages of magnetitite layers. KEY WORDS: layered intrusion; mineral chemistry; isotopes; magma; convection; differentiation     

Sequence of magma emplacement and sulfide saturation in the Gaojiacun–Lengshuiqing intrusive complex (SW China)

The Lengshuiqing area contains several small intrusions made up of peridotite ± quartz diorite ± granite spatially associated with the Gaojiacun pluton (gabbroids + peridotite + diorite). Ni–Cu sulfide ore occur at Lengshuiqing, hosted in peridotite. SHRIMP U–Pb zircon dating produced the ages of 803 ± 4.2 Ma (peridotite), 807 ± 2.6 Ma (oikocrystic hornblende gabbro), 809 ± 4.3 Ma (hornblende gabbronorites) for the Gaojiacun pluton and 807 ± 3.8 Ma (diorite, intrusion I), 817 ± 6.3 Ma (quartz diorite, intrusion II) and 817 ± 5 Ma (peridotite, intrusion 101) for Lengshuiqing. These ages suggest the emplacement of the Gaojiacun pluton later than the intrusions from Lengshuiqing. The olivine from Lengshuiqing does not contain sulfide inclusions and is relatively Ni-rich (1,150–1,550 ppm Ni), suggesting its crystallisation before the sulfide saturation that generated the Ni–Cu deposits. The olivine of the gabbros in the Gaojiacun pluton is Ni-poor (250–800 ppm), which indicates crystallisation from a severely metal-depleted magma after a sulfide saturation event. The olivine in the peridotites from the Gaojiacun pluton has 800–1,150 ppm Ni and contains sulfide inclusions. Moreover, geological evidence suggests the genesis of the peridotites from Gaojiacun in conduits that were ascending through the gabbroids. A sequence of at least three stages of magma emplacement is proposed: (1) Lengshuiqing; (2) gabbroids from Gaojiacun; (3) peridotites from Gaojiacun. Given the age differences, the intrusions at Lengshuiqing and the Gaojiacun pluton might have been produced by different magmatic events.     

Enrichment of PGE through interaction of evolved boninitic magmas with early formed cumulates in a gabbro–breccia zone of the Mesoarchean Nuasahi massif (eastern India)

The Mesoarchean Nuasahi chromite deposits of the Singhbhum Craton in eastern India consist of a lower chromite-bearing ultramafic unit and an upper magnetite-bearing gabbroic unit. The ultramafic unit is a ∼5 km long and ∼400 m wide linear belt trending NNW-SSE with a general north-easterly dip. The chromitite ore bodies are hosted in the dunite that is flanked by the orthopyroxenite. The rocks of the ultramafic unit including the chromitite crystallized from a primitive boninitic magma, whereas the gabbro unit formed from an evolved boninitic magma. A shear zone (10–75 m wide) is present at the upper contact of the ultramafic unit. This shear zone consists of a breccia comprising millimeter- to meter-sized fragments of chromitite and serpentinized rocks of the ultramafic unit enclosed in a pegmatitic and hybridized gabbroic matrix. The shear zone was formed late synkinematically with respect to the main gabbroic intrusion and intruded by a hydrous mafic magma comagmatic with the evolved boninitic magma that formed the gabbro unit. Both sulfide-free and sulfide-bearing zones with platinum group element (PGE) enrichment are present in the breccia zone. The PGE mineralogy in sulfide-rich assemblages is dominated by minerals containing Pd, Pt, Sb, Bi, Te, S, and/or As. Samples from the gabbro unit and the breccia zone have total PGE concentrations ranging from 3 to 116 ppb and 258 to 24,100 ppb, respectively. The sulfide-rich assemblages of the breccia zone are Pd-rich and have Pd/Ir ratios of 13–1,750 and Pd/Pt ratios of 1–73. The PGE-enriched sulfide-bearing assemblages of the breccia zone are characterized by (1) extensive development of secondary hydrous minerals in the altered parts of fragments and in the matrix of the breccia, (2) coarsening of grain size in the altered parts of the chromitite fragments, and (3) extensive alteration of primary chromite to more Fe-rich chromite with inclusions of chlorite, rutile, ilmenite, magnetite, chalcopyrite, and PGE-bearing chalcogenides. Unaltered parts of the massive chromitite fragments from the breccia zone show PGE ratios (Pd/Ir = 2.5) similar to massive chromitite (Pd/Ir = 0.4–6.6) of the ultramafic unit. The Ir-group PGE (IPGE: Ir, Os, Ru) of the sulfide-rich breccia assemblages were contributed from the ultramafic–chromitite breccia. Samples of the gabbro unit have fractionated primitive mantle-normalized patterns, IPGE depletion (Pd/Ir = 24–1,227) and Ni-depletion due to early removal of olivine and chromite from the primitive boninitic magma that formed the ultramafic unit. Samples of the gabbro and the breccia zone have negative Nb, Th, Zr, and Hf anomalies, indicating derivation from a depleted mantle source. The Cu/Pd ratios of the PGE-mineralized samples of the breccia zone (2.0 × 10³–3.2 × 10³) are lower than mantle (6.2 × 10³) suggesting that the parental boninitic magma (Archean high-Mg lava: Cu/Pd ratio ∼1.3 × 10³; komatiite: Cu/Pd ratio ∼8 × 10³) was sulfur-undersaturated. Samples of the ultramafic unit, gabbro and the mineralized breccia zone, have a narrow range of incompatible trace element ratios indicating a cogenetic relationship. The ultramafic rocks and the gabbros have relatively constant subchondritic Nb/Ta ratios (ultramafic rocks: Nb/Ta = 4.1–8.8; gabbro unit: Nb/Ta = 11.5–13.2), whereas samples of the breccia zone are characterized by highly variable Nb/Ta ratios (Nb/Ta = 2.5–16.6) and show evidence of metasomatism. The enrichment of light rare earth element and mobile incompatible elements in the mineralized samples provides supporting evidence for metasomatism. The interaction of the ultramafic fragments with the evolved fluid-rich mafic magma was key to the formation of the PGE mineralization in the Nuasahi massif.     

The McIntosh Layered Troctolite-Olivine Gabbro Intrusion, East Kimberley, Western Australia

The well-preserved ?lower Proterozoic McIntosh intrusion consistsof 96 macro-layers with a total stratigraphic thickness of about6 km. The lowermost rocks in this possible cone-shaped intrusionare hidden, and the roof and the upper layers were removed byerosion. The layered sequence is dominated by 40 bimodal cyclicunits of troctolite and olivine gabbro. Minor gabbronorite layersoccur throughout the sequence, and are more abundant and morefractionated higher in the sequence. Six imperfect megacycicunits are developed in the upper 2700 m, each unit consistingof several troctolite-olivine gabbro cyclic units followed bya Fe-Ti oxide-bearing gabbronorite. The overall cumulus crystallizationorder in each megacyclic unit was plagioclase first, closelyfollowed by olivine, then augite, orthopyroxene, and magnetitesuccessively. Cryptic composition data for troctolites and olivine gabbrosshow a slight overall decrease of 10 mol per cent An and Fofrom the base to the top of the layered sequence (approximateranges An_80–70 and Fo_78–68). Several major fluctuationsoccur however, and are generally associated with the oxide gabbronorites,which are significantly more fractionated than the adjacentlayers (plagioclase An_53–60, orthopyroxene Mg_52–69Each fluctuation comprises a marked progressive discontinuity(rapid normal fractionation) followed by a gradual to rapidregressive discontinuity (or reversal) in the overlying troctolitesand olivine gabbros. Apparently, such marked progressive discontinuitieshave not been described in layered intrusions. A chilled margin and the overall composition of the intrusionsuggest an olivine tholeiite parent magma, inferred to havecrystallized at P 6 kb, relatively low _PH2O and high f_O2 (>NNO buffer). The troctolite-olivine gabbro cyclic units areinferred to have formed by fractional crystallization of periodicadditions of new magma. However, the oxide gabbronorites seemtoo fractionated relative to the underlying layers to have formedby conventional crystal fractionation mechanisms, and they couldhave resulted from a ‘liquid fractionation’ processin which fractionated residual magma, instead of rising, periodicallybecame denser and ponded on the temporary floor (a density crossover).Gradual, reversed cryptic trends in the cyclic units above theoxide gabbronorite layers may reflect mixing of this fractionatedmagma with successive magma additions.     

Petrochemistry, geochemistry, and genesis of the Chek-Chikan gabbronorite Massif, southeastern framing of the North Asian craton

The Chek-Chikan Massif is a typical representative of basic magmatism, which is widely spread within the Dzhugdzhur-Stanovoi superterrane. The massif consists of gabbronorites, amphibole gabbros, gabbroanorthosites, and anorthosites. The geochemical similarity of the gabbronorites, amphibole gabbros, and anorthosites suggests their genetic link and allows us to consider them as products of intrachamber differentiation. The main geochemical peculiarity of this rock association is the high degree of the melt fractionation. The rocks of the considered massif are enriched in large ion lithophile elements such as Sr (424–1018 ppm) and Ba (50–754 ppm) and have moderate to low contents of such high-field strength elements as Nb (1–17 ppm), Hf (0.4–1.0 ppm), and Th (0.05–1.14 ppm). According to the model calculations, the initial melt had a basaltic composition and crystallized at a temperature of ∼1180 °C and pressure up to 4 kbar. The U-Pb zircon age of the massif is 203 ± 1 Ma. The geochemical peculiarities of the massif and its confinement to the northern framing of the eastern segment of the Mongol-Okhotsk fold belt make it possible to presume that its formation was related to either the activity of the Siberian plume, to one of the stages of closure of the Mongol-Okhotsk paleoocean in the rear part of subduction zone, or to the slab break off.     

Enrichment of basalt and mixing of dacite in the rootzone of a large rhyolite chamber: inclusions and pumices from the Rattlesnake Tuff, Oregon

 A variety of cognate basalt to basaltic andesite inclusions and dacite pumices occur in the 7-Ma Rattlesnake Tuff of eastern Oregon. The tuff represents ∼280 km³ of high-silica rhyolite magma zoned from highly differentiated rhyolite near the roof to less evolved rhyolite at deeper levels. The mafic inclusions provide a window into the processes acting beneath a large silicic chamber. Quenched basaltic andesite inclusions are substantially enriched in incompatible trace elements compared to regional primitive high-alumina olivine tholeiite (HAOT) lavas, but continuous chemical and mineralogical trends indicate a genetic relationship between them. Basaltic andesite evolved from primitive basalt mainly through protracted crystal fractionation and multiple cycles (≥10) of mafic recharge, which enriched incompatible elements while maintaining a mafic bulk composition. The crystal fractionation history is partially preserved in the mineralogy of crystal-rich inclusions (olivine, plagioclase ± clinopyroxene) and the recharge history is supported by the presence of mafic inclusions containing olivines of Fo₈₀. Small amounts of assimilation (∼2%) of high-silica rhyolite magma improves the calculated fit between observed and modeled enrichments in basaltic andesite and reduces the number of fractionation and recharge cycles needed. The composition of dacite pumices is consistent with mixing of equal proportions of basaltic andesite and least-evolved, high-silica rhyolite. In support of the mixing model, most dacite pumices have a bimodal mineral assemblage with crystals of rhyolitic and basaltic parentage. Equilibrium dacite phenocrysts are rare. Dacites are mainly the product of mingling of basaltic andesite and rhyolite before or during eruption and to a lesser extent of equilibration between the two. The Rattlesnake magma column illustrates the feedback between mafic and silicic magmas that drives differentiation in both. Low-density rhyolite traps basalts and induces extensive fractionation and recharge that causes incompatible element enrichment relative to the primitive input. The basaltic root zone, in turn, thermally maintains the rhyolitic magma chamber and promotes compositional zonation. Received: 1 June 1998 / Accepted: 5 February 1999     

Genesis of the Precambrian copper-rich Caraiba hypersthenite-norite complex,Brazil

Caraiba, the largest Brazilian copper deposit under exploitation, consists mostly of disseminated and remobilised bornite and chalcopyrite hosted in early Proterozoic norite and hypersthenite. The mafic igneous complex comprises multiple intrusions of dykes, veins and breccias of norites and hypersthenites, with minor proportions of amphibolised gabbronorite and peridotite xenoliths transported by the magma from deeper levels in the lithosphere. The country rocks are high-grade gneisses, granulites and metasediments. Compositions of plagioclase(An_60-40) and orthopyroxene(En_70-60) fall in a narrow range similar to the Koperberg Suite from the Okiep copper district, South Africa, and to that in many massif-type anorthosites. Whole-rock major and trace element geochemistry indicate a parental magma enriched in Fe, LREE, P, K, and Cu. Negative Nb anomalies on multi-element plots and fractionated REE patterns, along with sulphide sulphur isotopes in the range ³⁴S = –1.495 to + 0.643, suggest a primary mantle lithosphere source, although a lower crustal source for the gabbronorite and peridotite xenoliths cannot be excluded. Geochronological and field evidence indicate that both norite and hypersthenite are likely to have been emplaced during a major sinistral transcurrent (partly transpressional) shearing event associated with the waning stage of evolution of the early Proterozoic Salvador-Curaçá orogen.     

Geochemistry of mafic rocks of the PGE-bearing Vurechuaivench Massif (Monchegorsk Complex,Kola region)

The platinum-bearing Paleoproterozoic Vurechuaivench Massif in the Monchegorsk Pluton is made up of amphibolized and saussuritized gabbronorites, anorthosites, and norites. The geochemical features of the massif rocks are considered at four detailed areas. It was confirmed that the Vurechuaivench and Nyud-Poaz massifs are geochemically similar. The rare-earth element (REE) distribution in the rocks of the Vurechuaivench Massif is peculiar in the low total REE content (9.4–27.6 ppm), negative REE slope, significant LREE enrichment [La/Yb] _n = 3.7–8.7), and distinctly expressed positive Eu anomaly ([Eu/Eu*] _n = 1.2–2.2). The REE distribution pattern remains unchangeable throughout the entire section, including the rocks of the Pt reef, with a gradual upsection REE increase. It is suggested that the PGE reef of the Vurechuaivench Massif, as the Platinova Reef (Skaergaard massif) and Sonju-Lake Intrusion (Duluth complex), was formed during fractional crystallization in a large magma chamber without new magma influx. It is conceivable that the Vurechuaivench Massif is the allochthonous fragment of a large loppolith-like body, the lower portions of which compose the Nyud-Poaz Massif, while the middle part was almost completely eroded.     

Petrologic and experimental evidence for the movement and heating of the pre-eruptive Minoan rhyodacite (Santorini, Greece)

Hydrothermal experiments combined with petrologic observations form the basis for a new two-stage model for the evolution of the pre-eruption Minoan magma chamber at Santorini, Greece. Ninety-nine percent of the erupted volume is two-pyroxene, rhyodacitic magma that had been stored at a temperature of ∼885 °C, based on magnetite-ilmenite and QUILF geothermometry. The rest of the volume is basaltic to andesitic magma, which occurs as <10 cm scoria clasts and as small inclusions in rhyodacite pumices. Petrologic observations show that these magmas mixed at different scales and at different times (i.e., multiple batches of mafic magma). Hydrothermal experiments were carried out on samples of rhyodacite and a mafic scoria in order to determine magma storage conditions and the mixing history of the two magmas. At 885 °C, the rhyodacite must have been stored at water-saturated pressures of ∼50 MPa, based on its phase assemblage, matrix-glass composition, and crystal content. However, glass inclusions inside rhyodacitic plagioclase phenocrysts contain more than 6 wt% H₂O, indicating they formed at pressures >200 MPa. In addition, the composition of the plagioclase hosts (An_{56 ± 6}) of the inclusions require temperatures of 825 ± 25 °C at pressures >200 MPa. This demonstrates that the Minoan rhyodacitic magma underwent a two-stage evolution, first crystallizing at ∼825^∘C and >200 MPa, and then rinsing to a shallow ∼50 MPa storage region with a concomitant rise in temperature to ∼885 °C. We suggest that the episodic intrusion of mafic magmas provided the necessary heat and perhaps contributed to the ascent of the magma to shallow crustal depths where it reequilibrated before the cataclysmic eruption. Phase equilibria suggest that much of the heating of the rhyodacite occurred in the shallow storage region. Thermal budget calculations suggest that the rhyodacite magma could have been heated by intrusions of basalt rising at reasonable upwelling rates and injected into the storage zone over several hundred years. Preservation of amphibole in the mafic scoria indicate that injection of mafic magma continued up until days before the cataclysmic eruption, perhaps triggering the event. Received: 30 September 1997 / Accepted: 5 October 1998