Gabbro Series Research Articles

Dendrochemical Analysis as a Method to Identify Mineralized Zones: Insights from the Marathon Cu-PGE Deposit

The Marathon Cu-platinum-group element (PGE) deposit is located within the Eastern Gabbro Series of the Proterozoic Coldwell Alkalic Complex, 10 km north of Marathon, Ontario. The Cu-PGE mineralization occurs within the Two Duck Lake Gabbro, an olivine gabbro intrusion. This geochemical orientation study tested the trace element analysis of tree cores as a method of identifying mineralized versus background areas in the Marathon deposit, focusing on copper (Cu), sulfur (S), palladium (Pd), and silver (Ag). Tree core samples (n=25) were collected with an increment borer from black spruce (Picea mariana) trees and dated by counting growth rings. Fifteen cores, 12 near/overtop the deposit and three distal control samples, were analysed with Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) over a 10-year interval (1992-2002). Data normalization to 88Sr like in other dendrochemical studies corrected for tree health and growth effects. 32S, 63Cu, and 105Pd were found to have the highest concentrations at 94.198, 2.224, and 0.055 ppm, respectively, in control sample 23KPA2-3004. This tree is located beside the Pic River and receives large amounts of run-off from the entire Eastern Gabbro Series, possibly enhancing the chemical composition of the tree core. Excluding this anomalous sample, significant concentrations of Cu and Ag were found moving SSW along the deposit, aligning with the direction of movement of the Pleistocene Laurentide Ice Sheet. While this study did not clearly identify mineralized zones in the Marathon deposit, it has identified the possibility of mineralized run-off in the Pic River affecting nearshore tree chemical composition. The influence of local terrain and glacial till must be also considered when using this method. Future research will continue to study the factors influencing the uptake of elements in trees and refine methods for detecting mineralization through tree core analysis.

Wet magmatic processes during the accretion of the deep crust of the Oman Ophiolite paleoridge: Phase diagrams and petrological records

The Oman Ophiolite is regarded as an analogue to modern fast-spreading ocean ridge systems in an environment of subduction zone initiation. In contrast to recent mid-ocean ridge basalts from the East Pacific Rise, parental melts at the Oman paleoridge are assumed to be hydrous in nature. In order to constrain the role of water during magmatic accretion processes in the deep crust at the Oman paleoridge, we evaluated several experimental studies in hydrous tholeiitic systems performed at shallow pressures. We concluded that the wehrlitic phase assemblage (olivine coexisting with clinopyroxene but without plagioclase) is the most significant feature indicative of high prevailing water activities. The stability of the wehrlitic assemblage decreases with decreasing pressure (not stable in the upper plutonic crust) and depends on the chemical system (only stable in primitive MORB systems).We applied these results to plutonic rocks from cores drilled as part of the Oman Drilling Project (OmanDP). A key observation is the presence of coherent wehrlitic layers within the layered gabbro series, which are frequent in the lowermost gabbros (20%), relative sparse in the mid-crust (6%), and absent from the top of the plutonic crust at the dike/gabbro transition. Based on the combined phase relations for hydrous MORB-type systems at shallow pressures, we interpret this as a direct consequence of the presence of a significant water activity during the accretion of the plutonic crust of the Oman paleoridge, and not as a local phenomenon related to variations in temperature or bulk chemistry. These findings have implications for the mechanism of accretion of the lower crust at the Oman paleoridge, supporting a model that significant parts of the plutonic crust were produced by in-situ crystallization of primitive melt sills.

Magmatic Processes at the Oman Ophiolite Paleoridge – Perspectives on the Role of Water

AbstractThe Oman ophiolite is regarded as best proxy for accreted oceanic crust from typical fast‐spreading ridge systems on land. However, the Oman ophiolite is influenced by initial subduction zone initiation, and the nature of the details of the subduction zone setting is still under controversial debate. While a first magmatic phase shows features of magmatic accretion very similar to those known from the East Pacific Rise, except that the primary melts were slightly water‐enriched, a second type of magmatism is characterized by an apparent subduction‐zone related imprint, producing rocks like FAB basalts and boninites in the upper crust, as well as cross‐cutting gabbronorites and wehrlites in the deeper crust. In this paper, we apply diverse experimental studies in wet tholeiitic and peridotitic systems performed at lower pressures (100 to 500 MPa) in the experimental lab of the University Hannover, in order to constrain the details of the magmatic processes proceeded at the Oman ophiolite paleoridge during the Cretaceous, with special focus on the influence of water on the phase stabilities and phase relations. The experiments were performed in vertically oriented internally heated pressure vessels (IHPV) (see Berndt et al., 2002; Fig. 1). This facility uses as pressure medium mixtures of Ar and H2 in order to adjust the required fH2 in the vessel, enabling us to control the redox conditions. The fH2 prevailing in the IHPV at high P and T was measured with a Shaw‐membrane made of platinum. The overall variation in fO2 in all experimental series was in the range between ∼FMQ‐1 and ∼FMQ+3.2, thus covering the range of oxygen fugacities prevailing in natural MORB magmas (Bezos and Humler, 2005).For understanding the magmatic processes during the Oman ophiolite paleoridge accretion, transects through the lower (GT1) and middle (GT2) crust have been drilled in the frame of ICDP (International Continental Scientific Drilling Program). Drill sites have been selected in the Wadi Tayin massif, which is known that the influence of magmatic phase 2 characterized by subduction‐related primary melts is minimal. Details and progress obtained in the Oman Drilling Project (OmanDP) can be found here: (https://www.omandrilling.ac.uk/). Regarding the first magmatic phase of the processes at the Oman ophiolite paleoridge, a characteristic observation made during the description of the drilled cores GT1 (lower crust) and GT2 (mid‐curst) was that quite often layers in the layered gabbro series occur which show the presence of clinopyroxene joining olivine instead of plagioclase (under near liquidus conditions). In terms of lithologies this could be interpreted as presence of wehrlitic crystal mushes as early cumulates instead of troctolitic, which are the typical ones for primary magmatism at typical fast‐spreading ridges. This situation could be experimentally simulated by adding a moderate to high water activity to primitive MORB at pressures ≥ 200 MPa, resulting in a shift of the clinopyroxene‐in curve to higher temperatures above the plagioclase‐in curve (Feig et al. 2006; see Fig. 2).Regarding the second, late‐stage magmatic phase, the formation of typical Oman high‐Ca‐boninites could be experimentally simulated by water‐saturated partial melting of Oman harzburgite at 200 MPa and relatively low temperatures between 1100 and 1200°C. Depleted gabbronorites crosscutting layered gabbros of phase 1 magmatism can be regarded as cumulates formed in these boninitic melts. Late wehrlites crosscutting layered gabbro could be produced by accumulation of olivine and clinopyroxene at temperatures between 1040 and 1080°C in a hydrous gabbroic system at pressures > 100 MPa with bulk water content of 2–3 wt%.

Low-Sulfide Platinum Group Element Ores of the Norilsk-Talnakh Camp

Abstract Low-sulfide platinum group element (PGE) mineralization of the Norilsk-type intrusions is located within the Upper Gabbroic Series, which comprises rocks heterogeneous in texture and composition. The highest grade of 10 to 50 g/t PGEs is confined primarily to chromitiferous taxitic gabbrodolerite, which forms irregular lens- and vein-like bodies that interfinger with contact gabbrodolerite, intrusion breccia, leucogabbro, and gabbrodolerite variably enriched in olivine, from olivine free up to picritic compositions. The abundant amygdules and pegmatoidal textures in Upper Gabbroic Series taxitic rocks, as well as the high enrichment of halogen in minerals (e.g., ≤4.6 wt % Cl in apatite), indicate a higher volatile content of the local magma compared to the magma that precipitated the Main Series. The observed diversity in spinel compositions, which evolve from chromite to Cr magnetite as well as toward hercynite, titanomagnetite, and ulvöspinel, is also indicative of crystallization from a fluid-saturated mush that subsequently reacted, to varying degrees, with contaminated trapped melt and immiscible fluid. The high PGE/S ratio is a primary feature of this mineralization style, albeit the ratio partly increased during sulfide replacement and resorption. The PGE tenor of bulk sulfides calculated as ΣPGE (g/t) in 100% sulfides exceeds 160 and may reach up to 1,400 to 2,500 in low-S ores (0.2–3 wt % S), whereas the value does not exceed 42 in the Talnakh disseminated ore and ranges from 35 to 120 in the Norilsk disseminated ore (1–10 wt % S). Several PGE peaks in the vertical sections correlate well with Cu, Ni, S, and Cr peaks, as well as with observed elevated proportion of amygdules. Low-sulfide ores are composed of two primary sulfide assemblages of pyrrhotite + pentlandite + chalcopyrite and pentlandite + pyrrhotite. The primary sulfides are depleted in the heavier 34S isotope relative to sulfides of the corresponded main orebodies (e.g., mean δ34S = 8.9‰ versus δ34S = 12.3‰, respectively, in the Kharaelakh intrusion). A secondary pyrite + millerite + chalcopyrite assemblage has isotope composition enriched in 34S by 2 to 6‰ δ34S with respect to primary sulfides. The directly measured PGE content in sulfides (e.g., 11–2,274 g/t Pd in pentlandite and 0.10–33.3 g/t Rh in pyrrhotite) is within the range of the typical Norilsk-type magmatic sulfide compositions. The textural setting and diversity of platinum group minerals (PGMs) favor the hypothesis of fluid-controlled crystallization. However, the distinct PGM assemblages in Norilsk 1 and Talnakh-Kharaelakh low-sulfide ores are comparable with those of the corresponding presumably magmatic disseminated and massive orebodies. The most remarkable characteristic is the widespread Pt-Fe alloys in Norilsk 1 and their absence in Talnakh-Kharaelakh, which is interpreted to reflect better preservation of the high-temperature PGMs in Norilsk 1 in contrast to their substantial replacement in more oxidized fluid-enriched environments in Talnakh-Kharaelakh.

Layered gabbros and peridotites from Piton des Neiges volcano, La Réunion Island

We conducted a petrological study of the layered gabbros and peridotites outcropping in the cirque of Salazie, combining field investigations, geochemistry, petrography and, mineralogy to constrain the magmatic evolution of Piton des Neiges volcano (La Réunion). Three new hectometer-long outcrops are reported here (Sites 1, 2 and 3) in addition to the gabbroic series (hereafter named Site 4) previously described in the literature. The Sites 1 and 2 are located in the upper parts of the Mât River and show dunite and wehrlite units with no apparent foliation, lineation, or layering. The Sites 3 and 4 are located in the lower parts of the river and are made of olivine-gabbro, gabbro and ferrogabbro units showing marked foliation and lineation, and abundant layering. The mean composition of primary phases varies between Sites 1–2 and Sites 3–4: Fo89.5 to Fo69.8 in olivine, Mg# of 90 to 69 in clinopyroxene, and Mg# of 44 to 0 in Cr-spinel, titanomagnetite and, ilmenite, respectively. Here we show that the layered gabbros and peridotites exposed at the four sites represent multiple levels of magma storage. Based on the results of previous gravimetric studies and a drill hole related to geothermal exploration, we propose that the gabbroic series belongs to a magmatic system predating the building of Piton des Neiges and centered underneath the cirque of Salazie. The high volume of erupted materials in the NE quadrant of the volcanic edifice, and the southward magmatic flow direction in the layered gabbros suggest that the initial eruptive center of Proto-Piton des Neiges was above the cirque of Salazie, ~3 km north of the present-day summit. The landscape of Piton des Neiges may have changed when its NE flank experienced a major lateral collapse, after complete solidification of the gabbroic bodies. The magmatic activity, then, shifted southward after the flank collapse. Finally, we suggest that peridotites outcrops are associated with the present-day Piton des Neiges.

Internal architecture and Fe–Ti–V oxide ore genesis in a Variscan synorogenic layered mafic intrusion, the Beja Layered Gabbroic Sequence (Portugal)

The Beja Layered Gabbroic Sequence (LGS) intruded the SW border of the Ossa Morena Zone at ca. 350Ma during the early stages of the Variscan oblique continental collision in Iberia. It represents an exceptional case-study for the petrogenesis and metallogenesis of the rare class of synorogenic layered intrusions.LGS oxidised parental magmas (+1.7±0.5 ΔFMQ) evolved to relatively reduced ferro-basaltic magmas (±0.5 ΔFMQ) after extensive differentiation (SB I–SB II–ODV I Series: Fo88-54) and gave rise to ODV I Series oxide-rich ferro-gabbros and massive oxide ores, that represent a potential economic resource for Fe, Ti and V. Vanadium enrichment in ODV I melt peaked at the formation of basal ores and was followed by heterogeneous depletion at prevailing high Fe–Ti contents within other oxide-rich domains of the Series. Geological, geophysical and geochemical evidence suggest that density driven processes were fundamental to oxide accumulation and segregation within ODV I basal domains.The SB I–SB II–ODV I suite was followed by monotonous gabbroic Series (ODV II, III, BRG I, II) developed from repeated magma replenishments with similar initial composition (Fo78). The prevailing oxidising conditions (Fo76-66 ΔFMQ+.5 to+1.0) and the buffering effect of recurrent replenishments constrained the Fe–Ti enrichment within a typical calc-alkaline differentiation trend; decreasing recharge/crystallisation rates locally produced oxide accumulations at ≈+1Δ FMQ. The coupled ƒO2 decreasing with Fe-enrichment throughout LGS is consistent with crystallisation under conditions closed to oxygen exchange. It is inferred that the large amount of oxides that precipitated throughout ODV I Series required transient, episodic openings of the system favoured by the tectonically active character of the magma chamber.Modelling of melt compositions supports that LGS parental magmas were high-alumina, primitive basalts (#Mg0.63), which after some differentiation gave rise to SB II Series (#Mg0.59) preserved as a chilled margin. The SB I parental magmas may account for the monotonous Series compositional spectrum, whereas ODV I Series model melts may be derived as residual magmas from SB II Series. ODV I melts show strong Fe–Ti enrichment and Si depletion, consistent with a Fenner trend, wherein the formation of extreme differentiation products was precluded by the arrival of new magma batches related with the monotonous Series.

Mesoproterozoic high Fe–Ti mafic magmatism in western Shandong, North China Craton: Petrogenesis and implications for the final breakup of the Columbia supercontinent

High Fe–Ti basic rocks (FeOt>12wt% and TiO2>2wt%) are rare in the geological record and display a different evolutionary trend (the ‘Fenner’ trend) from the normal ‘Bowen’ trend of SiO2 enrichment. In this paper, we present geochronological, geochemical and Sr–Nd isotopic data for the Fe–Ti-rich gabbros from western Shandong within the Eastern Block of the North China Craton. Zircon U–Pb dating indicates that they were emplaced at ∼1.21Ga. All high Fe–Ti gabbros are characterized by low MgO (<7wt%) and Mg numbers (<50), and high FeOt (>12wt%), TiO2 (>2wt%) and P2O5 (mostly >0.4wt%). Geochemically, they can be divided into two different series (the alkaline series for the Yishui gabbros and the sub-alkaline series for the Feixian gabbros). Both series of Fe–Ti-rich gabbros show similar depletion in Th, U, Nb and Ta, and have positive Eu anomalies. The main difference is that the alkaline series displays slightly more fractionated REE patterns and more depleted Sr–Nd isotopic compositions than the sub-alkaline series; it also lacks positive P anomalies. Elemental geochemistry indicates that both series were likely generated by the partial melting of lherzolitic mantle, with different proportions of garnet in their sources. The mantle sources were re-enriched through varying degrees of hybridization of lower crustal materials prior to partial melting. The results of modeling based on the Sr–Nd isotopic compositions show that their mantle sources could have formed from the hybridization of N-MORB-like depleted mantle through involvement of around 1% lower crustal components for the alkaline series and 2% for the sub-alkaline series: this accounts for their geochemical signatures, especially for the marked depletion in Th and U, and weak negative Nb and Ta anomalies. A combination of Fe-rich mantle source and subsequent fractional crystallization, coupled with low fO2 during magmatic evolution, likely resulted in the Fe–Ti enrichment trend in both series of gabbros. In combination with regional and global studies, their generation could be related to a prolonged superplume that led to the final breakup of the Columbia supercontinent during Mesoproterozoic time.

The Early−Middle Palaeozoic Oceanic Events Along the Southern European Margin: The Deli Jovan Ophiolite Massif (NE Serbia) and Palaeo-oceanic Zones of the Great Caucasus

The paper deals with the Middle Palaeozoic oceanic events on the northern margin of the Eastern Mediterranean Hercynides. The Balkan-Carpathian Ophiolite Belt (BCO) and palaeo-oceanic zones of the Great Caucasus, all framing the East European Platform from the south, are correlated. The BCO palaeo-oceanic complex was widely thought to be a Late Precambrian-earliest Cambrian oceanic thrust sheet (563±5 Ma), a part of the South European Palaeo-oceanic Suture. The geochronological studies carried out on gabbroic series of the Deli Jovan Massif (BCO, NE Serbia), showed, however, that they are of much younger (Early Devonian) age: (a) Sm-Nd mineral isochron age of 406±24 Ma, \epsilonNdT= 8.32±0.39; ^8^7Sr/^8^6Sr_i_n_i_t= 0.702592±0.000160; and (b) U-Pb SHRIMP zircon age, 405.0±2.6 Ma. All the studied gabbroic rocks represent high-alumina (19-24.5% Al_2O_3) gabbro-troctolites, originating from shallow level ( Keywords: Eastern Mediterranean Hercynides, Palaeozoic oceanic events, northern Balkanides, Great Caucasus, Devonian ophiolite

Age of nephrite-bearing dikes of the Uzunkyr Belt (South Urals): Local U-Pb isotope analysis of zircon and Sr-Nd isotope data of rock-forming minerals

Through local U-Pb isotope analysis of zircon and Sir-Need data on rock-forming minerals, the age of nephrite-bearing monzonite-diorite dikes of the Uzunkyr Belt has been determined. The derived datings coincide with known geological events that took place in the Phanerozoic on the territory of the South Urals. Xenogenic zircons prove the participation of the Upper Ordovician units in the tectonic structure of the studied area. Devonian zircons are associated with assimilation of subvolcanic rocks which are middle and basic in composition and whose formation time correlates with the appearance of the subduction zone with the Magnitogorsk island arc above it. Early Carboniferous datings indicate the relationship between dike formation and formation of the continental arc-shaped structure to which the Syrostan massif (monzodiorite-granite formation) belongs. The age range of the Uzunkyr nephrite-bearing dikes coincides with that of intrusives (350–336 Ma) of the Magnitogorsk Belt, where formation of gabbro series was also changed by formation of subalkali and alkali igneous rocks. According to the analogous data on zircon datings from metamorphic rocks of the Il’menogorskii Complex, the given territory later evolved as a whole.

Geotectonic framework of Permo–Triassic magmatism within the Korean Peninsula

This study presents sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon ages, and whole-rock chemical and isotopic (Sr-Nd) compositions of representative Triassic plutons from South Korea. The plutons from the Gyeonggi massif (Hongseong, Namyang, Yangpyeong and Odesan), the central Okcheon belt (Baeknok and Yongsan), and the Yeongnam massif (Sangju, Gimcheon, Hamyang and Macheon) yield zircon U–Pb ages of ca. 232–226 Ma, 227–226 Ma, and 240–228 Ma, respectively. Among the Triassic plutonic suite in South Korea, those within the Gyeonggi massif are dominated by granite, syenite, monzonite, monzodiorite and gabbro. Plutons within the Okcheon belt are mainly by granite to quartz monzodiorite. The Yeongnam massif mainly incorporates granite to granodiorite and minor monzodiorite intrusions. The geochemical signatures of the Triassic plutons are characterized by Ta–Nb troughs, depletion of P and Ti, and enrichment of LILE. Most plutons except Macheon monzodioritic pluton show high initial 87Sr/ 86Sr ratios (0.708248–0.714678) and strongly negative ε Nd( T) (− 20.3 to − 7.7) values, suggesting contribution from middle to upper crust. In contrast, the Macheon monzodioritic pluton in the Yeongnam massif shows relatively low initial 87Sr/ 86Sr ratios (0.706547-0.706629) and negative ε Nd( T) (− 4.43 to − 3.62) values. The Middle Triassic syenite–monzonite–granite–gabbro series in and around the Gyeonggi massif possess high-K calc-alkaline and shoshonitic affinity suggesting a post-collisional magmatic event following the Permo–Triassic collision between the North and South China blocks. The Triassic plutons in the Yeongnam massif and the Okcheon belt, together with a Permian Yeongdeok pluton in the Gyeongsang basin, show features typical of high- to medium-K calc-alkaline magmatism with LREE and LILE enrichments. This together with a depletion of Y and HREE suggests their formation in a subduction setting. Our results provide robust evidence to consider the Gyeonggi massif as an extension of the Qinling–Dabie–Sulu belt between the North and South China blocks in central China. The Okcheon belt and Yeongnam massif in South Korea, together with the continental margin of South China, are marked by a common Permian to Triassic magmatic episode, probably related to the paleo-Pacific slab subduction.

Geochemistry of a long in-situ section of intrusive slow-spread oceanic lithosphere: Results from IODP Site U1309 (Atlantis Massif, 30°N Mid-Atlantic-Ridge)

IODP Site U1309 was drilled at Atlantis Massif, an oceanic core complex, at 30°N on the Mid-Atlantic Ridge (MAR). We present the results of a bulk rock geochemical study (major and trace elements) carried out on 228 samples representative of the different lithologies sampled at this location. Over 96% of Hole U1309D is made up of gabbroic rocks. Diabases and basalts cross-cut the upper part of the section; they have depleted MORB compositions similar to basalts sampled at MAR 30°N. Relics of mantle were recovered at shallow depth. Mantle peridotites show petrographic and geochemical evidence of extensive melt–rock interactions. Gabbroic rocks comprise: olivine-rich troctolites (> 70% modal olivine) and troctolites having high Mg# (82–89), high Ni (up to 2300 ppm) and depleted trace element compositions (Yb 0.06–0.8 ppm); olivine gabbros and gabbros (including gabbronorites) with Mg# of 60–86 and low trace element contents (Yb 0.125–2.5 ppm); and oxide gabbros and leucocratic dykes with low Mg# (< 50), low Ni (∼ 65 ppm) and high trace element contents (Yb up to 26 ppm). Troctolites and gabbros are amongst the most primitive and depleted oceanic gabbroic rocks. The main geochemical characteristics of Site U1309 gabbroic rocks are consistent with a formation as a cumulate sequence after a common parental MORB melt, although (lack of systematic) downhole variations indicate that the gabbroic series were built by multiple magma injections. In detail, textural and geochemical variations in olivine-rich troctolites and gabbronorites suggest chemical interaction (assimilation?) between the parental melt and the intruded lithosphere. Site U1309 gabbroic rocks do not represent the complementary magmatic product of 30°N volcanics, although they sample the same mantle source. The bulk trace element composition of Site U1309 gabbroic rocks is similar to primitive MORB melt compositions; this implies that there was no large scale removal of melts from this gabbro section. The occurrence of such a large magmatic sequence implies that a high magmatic activity is associated with the formation of Atlantis Massif. Our results suggest that almost all melts feeding this magmatic system stays trapped into the intruded lithosphere.

EXPERIMENTAL STUDY ON CRUSTAL WEHRLITES OF THE OMAN OPHIOLITE: EXPERIMENTAL OUTLINE AND MICROANALYTICAL RESULTS

The “ophiolite model”, which is mainly based on studies on the Oman ophiolite, imply that typical lower oceanic crust formed at fast-spreading ridges consists of a relatively simple “pan cake” sequence of (from bottom to top) the Moho-transition zone (MTZ), layered gabbro developing to isotropic gabbros, and sheeted dikes. However, the “real” lower Oman ophiolite crust is much more complicated, characterized by numerous complex intrusions of different rock types. Among these, the most enigmatic rocks are wehrlites, which are consisting mainly of olivine and clinopyroxene and can make up in some areas of the Oman ophiolite more than 20% of the crust (Juteau et al., 1988). The genesis of these rocks is up to now not clear. Current models for their formation are based on an origin from melt-impregnated mantle peridotites or on a magmatic accumulation of melts with very special composition (e.g., very depleted melts related to a subduction zone environment, or picritic melts). Koga et al. (2001) showed that the clinopyroxenes of typical wehrlites were in equilibrium with a “normal” MORB melts. This result, however, conflicts with the typical crystallization order observed in typical MORB systems, where plagioclase crystallizes before clinopyroxene. This implies that the parental melts were wet, since it is well-known from experiments in water-rich systems that the crystallization of plagioclase is suppressed. This is supported by new experimental work of Feig et al. (2005, submitted) who studied the influence of water on the phase equilibria in a gabbroic system. Their results clearly show that at pressures > 0.1 MPa under high water activities the order of crystallization is olivine, clinopyroxene, plagioclase which clearly demonstrates that wet MORB-tpye magmas have the potential to form wehrlite by the accumulation of early crystallized olivine and clinopyroxene. In order to evaluate the genesis of the Oman crustal wehrlites we started a study where we combine experimental work using natural wehrlites as starting material with investigations on natural rocks, especially by using microanalytical tools like SIMS and electron microprobe. For this, we investigated a section through the lower oceanic crust of the Oman ophiolite exposed in the Wadi Haymiliyah (Halyn block). Here, distinct discordant wehrlite bodies ranging in size from decameters to hundreds of meters intrude the layered gabbro series at different crustal levels, starting from the MTZ up to the sheeted dikes. Common feature of all wehrlite bodies is the dominance of olivine and clinopyroxene with high Mg# (MgO/(MgO+FeO)*100, molar) varying between 85 and 95. In addition, each body shows characteristic features, e.g., in terms of amount and presence of poikilitic or interstitial minerals like orthopyroxene, pargasite, and plagioclase or in terms of textural details (e.g., “classical” cumulate structures versus recrystallized textures; amount and size of poikilitc clinopyroxene grains, if present). There is a clear relation between petrographical evolution and structural height of the bodies. The wehrlites deep in the section from the MTZ show only olivine and clinopyroxene, strongly recrystallized and often forming monomineralic patches or layers, while the wehrlites from the top of the section show typical cumulate textures, with the crystallization order olivine, clinopyroxene, orthopyroxene, pargasite, plagioclase. Often, pyroxene and pargasite characteristically form mm-sized poikilitic crystals. In addition to the phase equilibria experiments of Feig et al. (2005, submitted), the presence of mm-sized poikilitic pargasite as a primary phase indicates that the associated melts showed a high water activity. SIMS analyses of clinopyroxenes from different wehrlite bodies reveal REE and trace element patterns suggesting that the clinopyroxenes were crystallized from tholeiitic, MORB-type melts which were highly depleted, not comparable to normal primitive MORB. In a sample from the top of the section the clinopyroxene exhibit trace element patterns with depleted character while the coexisting pargasites show strongly enriched pattern suggesting complex multi-stage differentiation processes or magma mixing. These pargasites show high concentration of chlorine suggesting that at least the fluids involved in the latemagmatic stage of the wehrlite formation may influenced by seawater. With the help of an experimental study under controlled fO2 and aH2O at pressures between 100 and 200 MPa we want to evaluate the conditions of the wehrlitic magmas within the lower crust. Especially, we want to clarify whether the crustal wehrlites from the Oman ophiolite are derived from wet tholeiitic magmas or not. Starting material are mixtures of natural wehrlites and a synthesized glass representing the “lost” equilibrium melt. The composition of this glass was calculated using the major and trace element compositions of the corresponding olivines and clinopyroxenes and the well-known relations on element partitioning between minerals and melt in tholeiitic systems from the literature or from own experimental series. First results are presented.

Coarse-grained and porphyritic inclusions in phonolitic lavas, Roccamonfina volcano, Italy

The inclusions in the 446–213-ka phonolite lavas of the Roccamonfina volcano (Italy) can be classified as coarse-grained and porphyritic types. Both are wide-varying in composition. The coarse-grained inclusions belong to a cumulate clinopyroxenite–gabbro–monzonitic clinopyroxenite–monzonitic gabbro series, and contain some interstitial, vesicular glass. The porphyritic inclusions show non-cumulate structures, and a coarse-grained, crystalline, vesicular matrix. They correspond to tephrites, phonolitic tephrites, trachybasalts, and basaltic trachyandesites. The glass in the coarse-grained inclusions is similar in composition to the porphyritic inclusions, but in part it deviates compositionally from the latter. These glasses are referred to, respectively, as trapped interstitial liquid from which the cumulates crystallized, and as injected, ‘foreign’ (non-comagmatic) melt. The phonolites vary in composition from tephritic phonolites to phonolites and nepheline phonolites, and do not show any traces of vesicularity. Despite the large eruptive time span, and the broad differentiation, they constitute a coherent magmatic group. Mineralogical and chemical similarities between both inclusion categories, and compositional similarity of the trapped glass to the porphyritic types, suggest that both groups of inclusions are cogenetic. By contrast, deviations of many elements of this glass and of the inclusions from the ideal mixing line inclusions–host phonolites rule out any genetic correlation among these rocks. Least-squares mass-balance calculations reinforce this lack of correlation. This conclusion is also supported by the fact that the phonolites are the most evolved derivatives of the basic and middle terms of the Roccamonfina high-K series, whereas the inclusions appear to constitute a potassic suite compositionally intermediate between both the Roccamonfina high-K and low-K series. Mineralogical and petrologic features suggest that the parental magma of the porphyritic types underwent prolonged fractionation in one or more crustal reservoirs at depths exceeding the stability conditions of plagioclase. The magma then migrated upwards to shallower reservoirs. Fractionation and crystal accumulation formed one (or more) layered crystal mush(es) of coarse-grained rock types, whereas the residual magma migrated toward the surface being stored in one (or more) shallower reservoir(s). When the residual melts of both cumulate and porphyritic layers, still incompletely consolidated, disrupted into coherent fragments and crystals, they were brought to the surface by pulses of phonolitic magma coming from below, and acting during a long time span. The residual melts of cumulate and porphyritic types vesiculated during ascent to the surface, quenching during eruption of the host phonolitic bodies.

Petrology and geochemistry of the Kruuse Fjord Gabbro Complex, East Greenland

The Kruuse Fjord Gabbro Complex is a composite intrusion of layered gabbro and troctolite with subordinate ultramafic rocks and minor trondhjemitic bodies. It was emplaced into Archaean continental crust of East Greenland during early Tertiary rifting of Greenland from Eurasia. The work to date has identified an outer gabbro series and an inner troctolite series, and these are separated by a narrow zone of trondhjemitic intrusions. In the southeast, the partially crystallized cumulates of the gabbro series were intruded by a lenticular, ultramafic pluton 800 m in thickness. Volumetrically minor, syenite–trachyandesite net-veined dykes and later, diabase dykes cross-cut the plutonic rocks. Structural and topographic features suggest that the layered rocks were affected by synmagmatic subsidence and deformation but not by monoclinal coastal flexure.The gabbro series is composed of a marginal gabbro unit, about 20 m wide, bordering more than a 2 km thickness of layered olivine and magnetite gabbro cumulates. The marginal gabbro is interpreted to be chilled magma. The layered cumulates are the product of repeated injections of magma that fractionated in an open-system magma chamber. Anorthositic and troctolitic layers in the lower part of the sequence may represent inputs of magma and suggest that the order of cumulus mineral crystallization was (1) plagioclase (An39–85), (2) olivine (Fo46–82), (3) augite (Wo28–47En39–58Fs8–18 ) and (4) magnetite. The disappearance of cumulus magnetite and a reversal in mineral compositions at 1.5 km from the base of the succession suggests a major input of magma occurred at this height. In the troctolite series, the composition of cumulus minerals, mineral crystallization sequence and style of emplacement are similar to those in the gabbro series. The ultramafic pluton is composed of coarse-grained wehrlite, olivine melagabbro and troctolite that were formed by at least three injections of magma. The typical mineral crystallization sequence was (1) cumulus chromite and olivine (Fo84–88); (2) poikilitic chrome diopside (Wo29–51En43–63Fs3–13 ); and (3) intercumulus plagioclase (An75–90), phlogopite, apatite and localized disseminated sulphides containing Au and platinum-group elements.Comparison of crystallization sequences and the major and trace element compositions of clinopyroxene suggests that the gabbroic and troctolitic rocks formed from a magma represented by the chilled marginal gabbro, a tholeiitic basalt magma similar to E-MORB, whereas the ultramafic rocks formed from a magma that was relatively enriched in incompatible trace elements and volatiles. The association of these two magma types is an example of bimodal mafic–ultramafic magmatism in a rifting environment.

A layered basic intrusion, deformed and metamorphosed in granulite faciès of the Sri Lanka basement

A layered basic intrusion has been found in the Central Granulite Belt of the Sri Lanka continental basement. It intruded parallel to bedding, before all or early during deformation of neighbouring metasediments. Deformation, affecting metasediments and the intrusion alike, includes flattening to c. 1/20 of the original thickness and NNW-stretching to c. 20 times the original length. The intrusion is now 170–300 m thick. Most of the deformation was acquired under granulite facies metamorphism. The intrusion was then folded, still at high T, by a large F4-synform with an axis parallel to str1 and a steep axial plane. A steep axial plane cleavage and minor folds are related to this big fold. Stretching continued along its axis. Late during formation of this fold a granite intruded, mainly following S4 cleavage planes. The intrusion shows a homogeneous gabbroic series at the bottom, followed upwards by a differentiated and layered series. A thin sequence of ultramafic rocks occurs near the middle. This indicates multiple melt-injection. More homogeneous partly biotite-bearing amphibolites form the top of the succession. Magmatic layering is well preserved, but no magmatic minerals or grain fabrics have escaped deformation or metamorphism. Static annealing under granulite facies conditions outlasted all deformation and was accompanied and followed by the beginning of cooling. Hornblende-Plagioclase coronas formed round garnets at this stage. Geochemical work, carried out by STOSCH (1991) on our samples, confirms the cumulate nature of the rocks.

Platinum group element ,mineralisation in the Munni Munni Complex, western Australia

The late Archaean Munni Munni Complex occupies an elliptical area of 9 by 25 km, the southern half of which is covered unconformably by a 2.7 Ga volcanic sequence. The Complex consists of a lower 1850 m thick Ultramafic Series (UMS) and an upper Gabbroic Series (GS) at least 3600 m thick, and is in the form of an elongate funnel. The UMS is made up of macrorhythmic cycles of dunite, wehrlite and clinopyroxenite, while the GS shows an uninterrupted fractionation trend from pigeonite gabbros through pigeonite-magnetite gabbros to granophyres. The base of the GS is very sharp, and marked by simultaneous appearance of cumulus plagioclase and pigeonite. GS cumulates show a monotonous upward increase in Fe/Mg and an absence of cyclic layering, indicating crystallization in a closed chamber.

Petrology and Tectonic Significance of Gabbros, Tonalites, Shoshonites, and Anorthosites in a Late Paleozoic Arc-Root Complex in the Wrangellia Terrane, Southern Alaska

Plutonic rocks intrusive into the late Paleozoic Tetelna Formation of southern Alaska are the underpinnings of the late Paleozoic Skolai arc of the Wrangellia Terrane. There are four groups of intrusive rocks within the Skolai arc: (1) Gabbro-diorite plutons that contain gabbroic to anorthositic cumulates along with a differentiated series of gabbros and diorites of basaltic to andesitic composition; (2) Silicic intrusions including tonalite, granodiorite, and granite; (3) Monzonitic to syenitic plutonic rocks of the Ahtell complex and related dikes and sills; (4) Fault-bounded bytownite anorthosite of uncertain age and association. These anorthosites may be related to post-Skolai, Nikolai Greenstone magmatism. The silicic rocks yield discordant U-Pb zircon ages of 290-320 Ma (early to late Pennsylvanian). Relative age relations suggest that the oldest intrusive rocks are the gabbro-diorite plutons, the youngest are the monzonitic rocks, and that the silicic rocks span this range. The gabbro-diorite plutons are similar to gabbroic plutonic rocks in modern and other ancient island arc complexes. They record the differentiation of calc-alkaline basalt to andesite by the fractionation of plagioclase, pyroxene, olivine, and Fe-Ti oxides. The silicic rocks do not appear to be related to either the gabbros or the monzonites. They may represent partial melts of Skolai arc crust. The monzonitic rocks of the Ahtell complex have shoshonitic chemistry. Similar shoshonitic rocks are widespread in both the Wrangellia terrane and the neighboring Alexander terrane and intrude the contact between the two. In modern oceanic arcs, shoshonitic rocks are typically associated with tectonic instability occurring during the initial stages of subduction or just prior to or during termination or flip of an established subduction zone. The nature of any tectonic instability which may have led to the cessation of subduction in the Skolai arc is unclear. Possibilities include collision of the arc with a ridge, an oceanic plateau, another arc, or a continental fragment. One possibility is that the shoshonitic magmatism marks the late Paleozoic amalgamation of Wrangellia and the Alexander terrane. The scarcity of arc rocks predating the shoshonites in the Alexander terrane supports this possibility, but structural corroboration is lacking.

Duality of magmatism in the plutonic sequence of the Sumail Nappe, Oman

Detailed field mapping of the crustal sequence in several areas of the Sumail Ophiolite Nappe, mainly in the Fizh, the Salahi and the Haylayn blocks, and comparisons with other well-studied areas, have shown that the crustal sequence was constructed by two distinct, although nearly contemporaneous, magmatic series: The first, a gabbroic series, was emplaced during the accretion stage at an oceanic spreading centre which formed the layered gabbro unit, the high-level isotropic gabbro unit and their by-products, the sheeted dyke complex and the lower extrusives. Fine-grained homogeneous, planar-laminated gabbros commonly occur at the top of the layered gabbro sequence. In some specific areas (Wadi Haymiliyah and Wadi Rajmi), these are strongly differentiated Fe-Ti-rich gabbronorites and norites, which coincide with large-scale discontinuities within the plutonic sequence, interpreted as axial discontinuities of the spreading magma chambers. The second series, an ultramafic wehrlitic series, is locally rooted in the upper mantle (i.e., the impregnated dunitic transition zone). This has intruded the whole crustal sequence up to the pillow lavas. Countless discordant and concordant wehrlitic bodies intruded the plutonic sequence, making up 20–40% of it in volume. We describe the various settings and forms of these intrusions, which were largely underestimated in previous studies. We also describe their mineralogy and textures, and show with chemical and mineralogical arguments that these bodies evolved as relatively closed systems, independently from each other. We infer from the study of the contacts that the gabbros were still hot and consolidated just at the moment of the wehrlitic intrusion. At the base of the crust however, indications of immiscibility between gabbros and wehrlites are common, suggesting that both magmatic series were quite contemporaneous. The wehrlites are commonly isotropic and may exhibit an internal layering of the dunite-wehrlite-clinopyroxenite type, sometimes appearing at the upper or lower margins of the intrusions. In this case, the ultramafic layered cumulates show lobate intrusive contacts with the adjacent layered gabbros. These plagioclase-free ultramafic layered cumulates, therefore, are not interbedded within the gabbro cumulates, but actually belong to the wehrlitic intrusive series. Picritic dykes radiating from the uppermost intrusions can be traced up to the upper extrusives, which are for the main part cogenetic with the wehrlites. The time of this second magmatic event coincides with the cessation of the accretion process and with the beginning of the intraoceanic detachment of the ophiolite at the spreading centre. We believe that the compressive regime that initiated the detachment acted as an efficient filter press and helped the extraction and ascent of the wehrlitic magmas towards the uppermost levels of the crust.

The intrusion of the Carrock Fell Gabbro Series (Cumbria) as a sub-horizontal tabular body

SUMMARY The existence of grain size and modally graded layering in the Carrock Fell Gabbro Series is recorded and evidence relating to the original emplacement attitude reviewed. Reinterpretation of the palaeomagnetic data and the structural and geological history are considered, together with isotopic dating, field relationships, gabbroic layering, xenolith distribution and geochemical evidence. All this evidence, in conjunction with the grain size and modally graded layering, is found to be consistent with the intrusion of the Gabbro Series as a sub-horizontal tabular body which has subsequently been rotated into its present-day orientation. This conclusion is contrary to previous interpretations, which have favoured a sub-vertical intrusion.

The geology and geochemistry of the Waterfall Gorge section of the Insizwa Complex with particular reference to the origin of the nickel sulfide deposits

The Jurassic-aged mafic rocks from the Waterfall Gorge section of the Insizwa layered complex, Transkei, range in composition from olivine gabbro and picrite (Basal zone), through a series of gabbros (Central zone), to monzonites (Roof zone). The compositional data for the mineral phases (olivine, plagioclase, bronzite, and augite) show significant features: in the basal gabbro the trend in compositional variation is the reverse of that seen in the Central zone of the intrusion; and olivines from the picrite unit are the most forsteritic in the intrusion, but they are depleted in Ni compared to olivines from the basal gabbro and Central zone. The data for olivines from the picrite unit at Tabankulu exhibit a similar but more significant degree of depletion.Mineral compositional and whole-rock geochemical data for the basal olivine gabbro suggest that olivine control is primarily responsible for the compositional variation in this unit at Insizwa, however, to account for the upward increase in the forsterite content of the cumulus olivine at the base, the intrusion of successively less-fractionated batches of a low MgO magma is suggested.We suggest that the depletion of the olivines in Ni can be attributed to the equilibration of the parent magma with sulfide and we show that these data are consistent with experimental distribution coefficient data for Ni between olivine and sulfide, permitting broad constraints to be placed on the grade, as well as the tonnage and location of the ore. Extrapolation of these conclusions to the Tabankulu intrusion suggest that the highly Ni-depleted olivines of the picrite have equilibrated with a magma from which a particularly high proportion of sulfide has segregated. Geophysical data fail to detect a massive orebody at the base of the intrusion, though several small bodies may exist. It is suggested that saturation may have occurred at depth, and only a small amount of sulfide has been transported to the surface in the picrite and concentrated in depressions at the base of the intrusion.Massive and disseminated magmatic Ni sulfide mineralization is confined to the lower Basal zone and shows a consistent fractionation into pyrrhotite-pentlandite-rich ore at the base and chalcopyrite-rich ore at the top (in both massive sheets and globular ore).The cause of sulfur saturation cannot be attributed to the reduction of the solubility of sulfur in the magma by increasing the SiO 2 concentration (contamination), despite the close association of the ore with granophyric segregations in the basal olivine gabbro. Indeed, the major and trace element data argue strongly against wholesale assimilation of crust at any stage in the production of the basal gabbro.