Cumulus Textures Research Articles

Genesis of the Late Cretaceous mafic–intermediate intrusions in the giant Gejiu Sn–Cu ore field and its metallogenic significance

The giant Gejiu ore field, located in the southeastern Yunnan Province, China, developed a large-scale Sn–Cu mineralization related to the Cretaceous felsic–mafic magmatism. However, the genesis of Cretaceous mafic–intermediate intrusions remains controversial, which limits our understanding of the Cu source, Sn–Cu symbiotic mechanism, and geodynamic setting of the Gejiu region. To elucidate these issues, we conducted geochronological and geochemical evaluations on the Gejiu mafic–intermediate intrusions, including gabbro, syenite, and monzonite. The mafic–intermediate intrusions formed in 82.1–83.8 Ma, which are contemporaneous with ore-forming granites and ore bodies within the error range. Combined with the cumulus texture in gabbro, linear or parallel elemental plots, and similar and enriched Sr–Nd–Hf isotopes ((87Sr/86Sr)I = 0.708870–0.709811; εNd(t) = − 5.24–−7.70; εHf(t) = − 4.8–2.5), the gradually increased Sr isotopes from gabbro through syenite to monzonite indicated that the Gejiu mafic–intermediate intrusions were formed by fractional crystallization and varying degrees of crustal assimilation of partial melts of an enriched lithospheric mantle.Based on the above research, we compiled geochronological and geochemical data of magmatic–metallogenic events in the region. The Sr–Nd isotopic and elemental ratios (i.e., Zr/Hf, Nb/Ta, Rb/Sr and Ba/Rb) of the Gejiu granites are located between the mafic–intermediate intrusions and spatially temporally connected Laojunshan granites, which indicate that the magma mixing between the enriched mantle–derived Cu-rich magma and metasedimentary rock–derived Sn-rich granitic magma generated Sn–Cu-rich granitic magma. This magma then evolved into the giant Gejiu Sn–Cu deposit via crystal fractionation and fluid exsolution. Combined with the regional geological data and spatiotemporal framework of Sn–polymetallic deposits, the magmatic–metallogenic model of the giant Gejiu Sn–Cu ore field further demonstrates that the southwestern slab rollback of the Neo-Tethyan oceanic plate during the Late Cretaceous controlled its formation.

The Neoarchean mafic-ultramafic São José do Jacuípe Suite in the Itabuna-Salvador-Curaçá orogen, Brazil: New U-Pb ages and geochemical data

The São José do Jacuípe Suite (SSJJ) comprises an association of mafic and ultramafic rocks that crops out in the northern portion of the Itabuna Salvador-Curaçá Orogen (ISCO), São Francisco Craton. They are represented by small NW-SE elongated bodies comprising metamorphosed anorthosites, leucogabbros, gabbronorites, gabbros, ferrogabbros, and subordinate pyroxenites and serpentinites. The rocks are part of an orogenic belt that comprises orthogranulites and orthogneisses of the Caraíba complex, and metasedimentary rocks of the Tanque Novo-Ipirá complex, which appear to be amalgamated into a folded mélange. Petrographical analysis of the SSJJ rocks revealed relict cumulus textures in the pyroxenites. Moreover, inclusion relationships allowed the identification of two metamorphic phases; the first phase likely occurred at amphibolite facies conditions and was overprinted by granulite facies metamorphism that represents the peak metamorphic conditions recorded by the SSJJ rocks. Analytical results for major and trace elements indicate cogeneticity between the mafic rocks, and arc-related mafic cumulate field compositions. In the geotectonic environment diagrams the rocks plot in the MORB and IAT fields, indicating an origin involving melting of different sources or magma mixing. The characteristic signature of the studied rocks is represented by negative HFSE anomalies (Nb, Ti, Zr and Hf), depletion in HREE and enrichment in LILE (Ba, Rb, and Cs), indicating their derivation from metasomatic mantle in a suprasubduction setting. These data corroborate previous isotope data that point to mildly enriched mantle sources. Incompatible trace elements signatures are similar to ophiolites generated in supra-subduction zone (SSZ) environments worldwide. New U/Pb LA-ICP-MS ages indicate igneous crystallization at 2549 ± 23 Ma and 2563 ± 13 Ma for SSJJ gabbronorites, thus indicating that they postdate the other members of the magmatic arc that formed the ISCO by 20–30 Ma.

Parent magma of the Cretaceous Northern Kyushu batholith and heat source of large-scale igneous activity: Petrological constraints on magmatism of the Northeastern Asian continental margin in the Cretaceous period, example for the Kita-Taku mafic complex in northern Kyushu, SW Japan

The Japanese island in the Northeastern Asian continental margin is located in a subduction zone, and granitoid covers about 30% of the island arc. The Cretaceous Northern Kyushu batholith (CNKB) is classified as 19 bodies based on previous studies and consists of a granitic body and a small amount of associated mafic bodies. Of these, the Kita-Taku mafic complex body (KMC) mainly consists of gabbroic rocks and can be classified into cumulus textures (Cumulus group: Types 1–4) and those without (Non-cumulus group: Types 5–7), which can provide information on the heat source of the large-scale igneous activity in northern Kyushu from the Northeastern Asian continental margin at Cretaceous period.Melt compositions inferred from the high Mg# pyroxene compositions of the olivine-bearing pyroxene hornblendite (Type 1) member of KMC were estimated using melt-pyroxene partition coefficients. The clinopyroxene-bearing equilibrium melts are similar to the Sanukitic high-Mg andesite (HMA) in multi-element and rare-earth element patterns. Besides, the trace element and rare earth element patterns of the Cretaceous high-Mg diorites (HMDs) in northern Kyushu have similar patterns to the clinopyroxene-bearing equilibrium melts of the KMC and Sanukitic HMA magma. This implies that Sanukitic HMA magmas were substantially activated in northern Kyushu during the Cretaceous period.Analysis of the parent magma (heat source) and the small-scale plutonic bodies that consist of CNKB indicates that the essential granitic magma is formed by partial melting of the lower crust and mixing with mantle-derived Sanukitic HMA magma. The Sanukitic HMA magma could be the parent magma for the heat source and crustal growth. The lithofacies of CNKB can be reconstructed from the examination of small-scale plutonic bodies due to the mixing and assimilation of various end-member magmas and crustal materials, and their associated differentiation and accumulation processes. As the Northeastern Asian continental margin during the Cretaceous is assumed to be situated within the extensional setting due to slab rollback, the results of this study provide new insights into the complicated batholith formation processes and heat source.

Gold occurrences in copper-magnetite-apatite deposit at Seruwila, Sri Lanka

This study presents petrology and evidence for possible gold occurrences in Seruwila copper–iron oxide–apatite (IOA) deposit, hosted in an ultramafic intrusion which is located at the boundary between the Highland and Vijayan complexes, within the intermediate-granitic basement in north-eastern Sri Lanka. The study is complemented with petrological observations and XRD and SEM analysis, respectively, to investigate the petrology/subsurface geology of the deposit and identify possible gold occurrences in the deposit. The ore-bearing rocks are mainly composed of magnetite and apatite in various proportions, hosted in an ultramafic intrusion with cumulate features within the granitic-intermediate basement. The secondary veins contain magnetite, chalcopyrite, pyrrhotite, and pyrite together with apatite and scapolite, tremolite, diopside, and minor actinolite and calcite, serpentinite, anhydrite, or gypsum. The clinopyroxene euhedral crystals show cumulus textures including grain triple junctions and large dihedral angles (∼120°), showing magmatic origin. Texturally two types of amphiboles are identified as coarse-grained (0.5–1 mm), pale green euhedral amphibole that is free of inclusions, and fine-grained (<0.1 mm) and brownish, occurring as anhedral inclusions in clinopyroxene. The deposit contains varying amounts of sulfides in which pyrite is the potential gold carrier in magmatic–hydrothermal processes. By the results of XRD analysis, it is evident that the presence of Au (111), Au (200), Au (220) and Au (311), although with low count values (50–500), probably due to the low concentration of gold. Therefore, particularly in the samples with veins or veinlets, gold was inferred to be present in pyrite/chalcopyrite as invisible structurally-bonded gold and/or gold nanoparticles. Hence, the results of this study, although at non-ore grade, veinlets of gold-bearing pyrite/chalcopyrite may serve as a promising target for gold occurrence, being a potential site of gold-mineralization in the context of the East-Gondwana.

U-Pb zircon age, geochemistry and petrogenesis of Mesoarchean anorthositic rocks from the Holenarsipur Greenstone Belt, Western Dharwar Craton: Implications for accretionary tectonics in southern India

The Doddakadanur Anorthosite Complex is a well-preserved sequence of anorthosites, gabbroic anorthosites, and anorthositic gabbros intrusive into amphibolites of the Mesoarchean Holenarsipur Greenstone Belt in the Western Dharwar Craton of southern India. Although the whole sequence is metamorphosed at upper amphibolite to lower granulite facies conditions, igneous cumulus texture is preserved in many of these rocks. The peak P-T conditions corresponding to metamorphic resetting, synchronous with 2.5 Ga regional metamorphism, is estimated to be ~650–700 °C and 7–11 kbar. The major and trace elemental systematics suggest a moderately fractionated parental magma (La/Yb)n = 3.3) with an overall tholeiitic affinity. The trace elemental systematics of these rocks indicates enrichment of the magma source regions by subduction-derived fluids. Uranium-Pb zircon ages yield an emplacement age of 3242 ± 17 Ma and also help to constrain two metamorphic events at 3159 ± 25 Ma and 2538 ± 11 Ma. Considering the intrusive contact relationship between the anorthositic rocks of the Doddakadanur Anorthositic Complex and ultramafics of the Holenarsipur Greenstone Belt, the new U-Pb ages suggest that the mafic-ultramafic rocks of the Holenarsipur Greenstone Belt are at least 3.24 Ga old. Integrating the new age data with available ages for the different magmatic rocks of the region, a spatial and temporal link between anorthosite emplacement and crustal accretion processes in the Western Dharwar Craton can be established. On the basis of whole-rock geochemistry and mineral chemistry, we propose a convergent margin tectonic setting for the rocks of the Doddakadanur Anorthosite Complex.

The Ryonchon podiform chromitite from the Chongjin ophiolite, northern Korea Peninsula

Ryonchon chromitite is located structurally in the Chongjin area, the northern part of the Korean Peninsula, which crops out at Chongjin terrane, the eastern end of the Jilin-Yanbian-Hambuk Fold Belt. We made on-site geological survey of this deposit and conducted the analyses on the investigated chromitite by means of microscopes, single crystal X-ray diffraction (SXRD) and electron probe micro-analysis (EPMA) to elucidate the phase composition, the chemical composition and the texture and structure of the chromitite. The chromian spinels from the study area are separated into four groups according to chemical composition of spinel as ordinary spinel-group minerals are. The studied chromites have different compositions according to their occurrences, which is not owing to the precipitation of different pararental melts but the subsolidus re-equilibration during the hydrous alternation.The massive/semi-massive unaltered chromite in chromitite from dunite have consistently low Cr (Cr# < 0.6) and low Al (Al2O3 23–26 wt%), whereas Al2O3melt and (FeO/MgO)melt values range 14.58–15.43, 0.40–0.52, respectively, which have been contrasted with that of the typical podiform chromitite. By comparing present analyses with previous works, the parental melt of the Ryonchon chromitite from the Chongjin ophiolite is similar in chemical compostion to Island arc tholeiitic melts rather than boninitic melt, whose tectonic setting corresponds to a forearc environment of a suprasubduction zone. In addition, both cumulus texture and euhedral crystal shapes of the chromite grains in massive/semimassive chromitite from dunite support their magmatic origin. We found that the chromitite from this area have undergone the hydrothermal alteration as appearance of fluidal ferritchromit, Cr# of ferritchromites higher than 0.8, the high MgO content of the ferritchromit, occurrence of typical hydrothermal minerals such as heazlewoodite and the differences in the products formed during several metamorphic processes.

Lithium isotopic composition of Alaskan-type intrusion and its implication

To determine whether fractionation of Li isotopes could occur at high temperatures, we examined major element and Li isotopic compositions of olivine from the Xiadong intrusion, an Alaskan-type complex in the Central Asian Orogenic Belt. Olivine in thirteen dunites, displaying characteristic cumulus textures, yielded large variations in Li concentration (0.10 to 11.18ppm) and isotopic composition (δ7Li=−7.18 to +34.41‰). These variations are too large to be attributed entirely to diffusive processes. The correlations between Li elemental or isotopic composition and differentiation indices such as Fo and MnO contents of olivine, and NiO content of chromite, suggest probable Li isotope fractionation during early stage of differentiation. We speculate that while Li behaves mildly incompatible during differentiation, 7Li is preferentially incorporated into olivine relative to 6Li during early stage crystallization from arc magmas. Relatively high Li concentrations and low δ7Li in arc magmas might be the result of substantial olivine fractionation prior to eruption.

Floresta and Bodocó Mafic–Ultramafic Complexes, western Borborema Province, Brazil: Geochemical and isotope constraints for evolution of a Neoproterozoic arc environment and retro-eclogitic hosted Ti-mineralization

The retrograde high-pressure metamorphosed Floresta and Bodocó mafic–ultramafic complexes (FMUC and BMUC, respectively) occur as clusters of small bodies, lenses and boudins with close association with granitic, gneissic rocks and metasediments in the southern limits of the Alto Pajeú and Piancó–Alto Brígida domains, Borborema Province, northeast Brazil. Ultramafic lithologies including metadunites, olivine cumulate rocks, metapyroxenites, chromitites and massive ilmenomagnetitites showing evidence of cumulus textures and mafic members include garnet amphibolites and coarse metahornblendites. BMUC rocks include Cr–Fe–Ti chromitites in the Fazenda Esperança deposit, and FMUC rocks show important Fe–Ti–V economic mineralization in the mines of the Serrote das Pedras Pretas and Lagoa dos Angicos and the Riacho da Posse deposit in development (54 Mt @13% Ti). The whole-rock and mineral geochemistry of these complexes are akin to arc cumulate rocks, particularly Alaskan-type complexes, which are widely considered to have formed above subduction zones.A coarse metahornblendite from the Serrote das Pedras Pretas mine was dated by U–Pb zircon through laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS). Information obtained from the cores of igneous zircons represent an upper intercept age of crystallization at 1024±18Ma with 4.2 mean square weighted deviation (MSWD), whereas zircon exhibiting coarse rims with low Th/U at 0.008–0.038 have concordia ages of 685±17 and 625±6Ma. According to their zircon textures, these ages may be representative of a high-grade eclogitic or progressive granulitic/retro-eclogitic stage during the Pan-African–Brasiliano event. Additionally, the age of the igneous protolith, 1028±63Ma, is well marked by a Sm–Nd isochron based on 12 samples.Positive ɛNd(t) values between 0.01 and 4.84 suggest a depleted mantle or a source close to the chondritic reservoir for the mafic–ultramafic bodies studied. The TDM model ages between 1.1 and 1.6Ga are consistent with a juvenile Mesoproterozoic source or mixing between Paleoproterozoic and Neoproterozoic sources. The dispersion of initial ratios of 87Sr/86Sr at 0.7016–0.7090 and the high fractionation of light rare earth elements (LREE) relative to heavy rare earth elements (HREE) suggest a depleted mantle source metasomatized by subduction-related melts/fluids.The mafic–ultramafic complexes, volcanic and sedimentary rocks, and some granitic intrusions with post-collisional signature after 975Ma constitute a proto to marginal arc-back-arc basin system that evolved between 1025 and 975Ma. However, FMUC and BMUC which are currently arc cumulate rocks were originally picrite melts in a suprasubduction ophiolite zone.The economic deposits of Fe–Ti associated with retro-eclogitic and ultramafic members of the FMUC and enclosed within the late Ediacaran granite can be linked in part by early accumulation under magmatic conditions, secondary enrichment of rutile up to 12% in the high-pressure (HP) stage, and hydrothermal interactions during granite emplacement.

Spinifex-textured komatiites in the south border of the Carajas ridge, Selva Greenstone belt, Carajás Province, Brazil

Spinifex-textured komatiites in the Selva greenstone belt are the first unequivocal examples of komatiites in the Transition Subdomain of the Carajás Mineral Province. Outcrops of spinifex-textured komatiites, located ∼1.5 km to the south of the Carajás ridge, were discovered during regional exploration for Ni–Cu–(PGE) sulfide deposits by VALE. They are associated with a 3.8 km long unit consisting of variable types of ultramafic rocks (talc schist, serpentinite and spinifex-textured komatiite). This ultramafic unit follows the steep dipping NW–SE trending Selva greenstone belt composed mainly by quartz-chlorite schists (interpreted as metasediments) and chlorite-actinolite schists (interpreted as metabasalts). Greenschist facies metamorphic parageneses characterize all rock types in the Selva greenstone belt.The komatiitic rocks in the Selva belt comprise a sequence of flows consisting of an upper spinifex-textured layer and a lower olivine cumulate layer. Although the spinifex and cumulus textures are well preserved in the field, the primary mineralogy of the komatiites has been completely replaced by greenschist facies metamorphic minerals. Platy olivine spinifex texture, consisting of an array of roughly parallel olivine plates, and random spinifex texture, consisting of randomly oriented olivine plates, are the most common primary volcanic textures in komatiites in the Selva greenstone belt. Platy and random spinifex texture is defined by former plates of olivine replaced by serpentine with minor actinolite, chlorite and magnetite, alternating with former matrix replaced by abundant actinolite and minor chlorite, talc, serpentine, and magnetite. The domains between olivine plates in both platy and random spinifex-textured rocks contain irregular arrays of fine-grained parallel crystals, representing primary fine-grained “quench” clinopyroxene crystals replaced by actinolite.Spinifex-textured komatiites have MgO contents bracket between 22.8 and 26.9 wt.%, and cumulate textured komatiites have MgO contents up to 40.6 wt.%. When plotted vs MgO contents, most major and minor elements fall on well-defined linear trends indicating control by olivine fractionation or accumulation. Komatiites from the Selva and Seringa (located in the Rio Maria Domain) belts are Al-undepleted with Al2O3/TiO2 ratios close to 20. Results for CaO, Na2O, and REE suggest that these elements were mobile and their abundances have been modified during metasomatic alteration. REE contents in some samples are very high (up to 40 times primitive mantle values) and REE patterns vary from flat (La/YbMN ∼ 1) to highly enriched in LREE (La/YbMN up to ∼ 10). The REE mobility may be related to hydrothermal alteration associated to Cu–Au mineralization in the region.The identification of spinifex-textured komatiites close to the Carajás Basin suggests the continuation of 3.0–2.9 Ga greenstone belts of the Rio Maria Domain within the Transition Subdomain, and enlarges the area with potential to host komatiite-associated Ni–Cu–PGE deposits.

Petrography and Geochemistry (Trace, Ree and Pge) of Pedda Cherlo Palle Gabbro-Diorite Pluton, Prakasam Igneous Province, Andhra Pradesh, India

Abstract Prakasam Igneous Province (PIP) is an important geological domain in the Eastern Dharwar Craton (EDC), found in the junction zone between the EDC and Eastern Ghat Mobile Belt (EGMB). The Pedda Cherlo Palle (PCP) gabbros are massive, leucocratic-mesocractic, and show cumulus textures with minerals plagioclase, cpx, and amphiboles. Compositionally, plagioclase is a labradorite-bytownite, cpx is diopside to augite, olivines are hyalosiderites and amphiboles are magnesiohornblendes. PCP gabbros have normal SiO2, high Al2O3, moderate to high TiO2, Na2O and medium Fe2O3, so, classified as subalkaline tholeiitic gabbros. Fractionated rare earth element (REE) patterns, high abundance of large ion lithofile elements (LILE) and transitional metals coupled with light REE (LREE) relative enrichment over heavy REE (HREE) and Nb are characteristics of partial melting of depleted mantle and melts that have undergone fractional crystalisation. These partial melts are enriched in LREE and LILE, due to the addition of slab derived sediment and fluids. PCP gabbros contain low abundance (5.1 to 24.6 ng/g) of platinum group elements (PGE), and show an increase in the order Ir&gt;Os&gt;Pt&gt;Ru»Pd&gt;Rh. We propose that the subduction related intraoceanic island arc might have accreted to the southeastern margin of India to the east of Cuddapah basin in a collisional regime that took place during Ur to Rodinia amalgamations.

Petrogenesis and tectonic significance of the plagiogranites in the Zhaheba ophiolite, Eastern Junggar orogen, Xinjiang, China

Plagiogranites (albitite and albite granite dikes/lenses) occur in the western section of the Zhaheba ophiolite, middle part of the Eastern Junggar orogen. The Zhaheba albitites (498.0±5.8Ma) and albite granites (494.6±6.9Ma) were roughly coeval and with distinct petrographic textures, i.e., cumulus texture for the albitites and granitic texture for albite granites. The Zhaheba plagiogranites are Na-enriched and LILE-depleted, resembling typical plagiogranites. Geochemical characteristics and cumulus texture of the albitites indicate that they may have been formed by early stage accumulation of albite, whereas the albite granites may have been the products of the residual magma consolidation. The εNd(t) values of the albitites and albite granites vary in the ranges of 7.2–7.7 and 6.4–7.3, respectively, whereas the εHf(t) values vary in the ranges of 11.5–17.9 and 9.8–13.9, respectively. Isotopic compositions and Zr/Hf ratios of the Zhaheba plagiogranites are similar to those of typical MORB, implying a genetic relationship with the oceanic crust. The low TiO2, Nb, Ta content, LREE enrichment and elevated (87Sr/86Sr)i values of the Zhaheba plagiogranites indicate that the rocks were likely derived from the anatexis of amphibolites, which were related to hydrothermal alteration of gabbros in intra-oceanic backarc basin. U–Pb geochronology of the Zhaheba plagiogranites indicates that it is more reasonable to connect the Zhaheba–Armantai and Karamay ophiolites to the Zaysan collision zone.

Petrology and Sm–Nd dating of the Genina Gharbia Alaskan-type complex (Egypt): Insights into deep levels of Neoproterozoic island arcs

Abstract The deep levels of Neoproterozoic island arcs are poorly known due to limited accessibility. The Genina Gharbia Alaskan-type complex (south Eastern Desert, Egypt) is the remains of a magma chamber that crystallized at the base (crust–mantle boundary) of a mature Neoproterozoic island arc. The rock assemblage comprises hornblende-bearing harzburgite, lherzolite, pyroxenite, norite and gabbro. All lithologies show cumulus texture with evidence of extensive cumulus mineral–melt interactions. Clinopyroxenes from all lithologies have similar rare earth element (REE) patterns with slight medium-rare earth element (MREE) enrichment. Hornblendes are slightly enriched in MREE and light rare earth elements (LREE). Island arc signatures are indicated by high contents of large ion-lithophile elements and low concentration of high field-strength elements. Positive initial e Nd (+ 5.7 to + 7.0) and Nd model ages (963 ± 81 Ma) are consistent with the Genina Gharbia magma being extracted from a depleted mantle source. Modeling of estimated parental magma indicates 10% partial melting of a 90% depleted mantle source with a 10% (MORB + sediments)-derived fluid, commencing in the spinel stability field (

Petrology and geochemistry of the ultramafic-mafic Mawpyut complex, Meghalaya: a Sylhet trap differentiation centre in northeastern India

The present article describes, for the first time, petrological and geochemical details of the Mawpyut differentiated complex which is related to the Sylhet trap located at Jaintia Hills district, Meghalaya, northeastern India. The Mawpyut complex occurs as an arcuate body that intrudes into the surrounding Shillong Group rocks. The complex in general contains ‘ultramafic’ and ‘mafic’ rocks, as well as minor syenitic veins that postdate the main units. The lithotypes correspond to cumulate and noncumulate units. The cumulate unit is represented by olivine clinopyroxenite, clinopyroxenite, plagioclase-bearing ultramafic, olivine gabbronorite, mela-gabbronorite, melagabbro, orthopyroxene gabbro, and gabbro, all with a pronounced cumulus texture. The noncumulate unit is marked by gabbro, monzonite, monzodiorite, and quartzsyenite. The use of several major and trace element variation diagrams suggests that magmatic differentiation led to the formation of cumulate and noncumulate units. In chondrite-normalized REE diagrams the cumulate rocks show flat LREE and MREE patterns and a moderate positive Eu anomaly (in plagioclase-bearing ultramafics) due to plagioclase cumulation. The rocks of the noncumulate unit show a strongly fractionated REE pattern and no Eu anomaly. The noncumulate mafic rocks are geochemically comparable to high-phosphorous/high-titanium basalts (HPT) indicative of low pressure fractional crystallization. In a primitive mantle-normalized multielement diagram some of the cumulate rocks show pronounced negative anomalies for K and P, indicating anorogenic mafic magmatism in a within-plate setting. The rocks of the noncumulate unit show a slight negative anomaly for Yb and a Nb–Ta trough, indicating a subduction-related signature that perhaps is inherited from subducted sedimentary rocks incorporated during crustal contamination of the derived magma (left after crystal cumulation) with country rocks. Various trace element ratios for the cumulate mafic rocks indicate parent EMI/EMII/HIMU sources with a very limited crustal signature. The noncumulate mafic rocks (corresponding to the derived evolved magma) indicate EMI/EMII/HIMU sources with a pronounced crustal contamination. The Sr–Nd isotopic compositions of the Mawpyut samples typically plot in the continental flood basalt field, with an affinity to the EMII source. The isotopic compositions of the noncumulate rocks also clearly indicate crustal contamination. We suggest that partial melting (involving garnet in the residue) of the enriched mantle source EMI/EMII/HIMU could have derived the parental melt; this melt, in turn, underwent assimilation and fractional crystallization to produce the variety of cumulate-noncumulate lithologies of the Mawpyut complex. Copyright © 2013 John Wiley & Sons, Ltd.

Geochronology and geochemistry of the Nantianwan mafic–ultramafic complex, Emeishan large igneous province: metallogenesis of magmatic Ni–Cu sulphide deposits and geodynamic setting

The Nantianwan mafic–ultramafic complex is situated in the northwest part of the Panxi district, southwest China. It consists predominantly of gabbros, gabbronorites, and lherzolites. LA–ICP–MS U–Pb zircon dating of the gabbronorites yields an age of 259.7 ± 0.6 million years, consistent with the ages of other mafic–ultramafic intrusions in the Emeishan large igneous province (ELIP). Gabbronorites and lherzolites host Cu–Ni sulphide ores. Cumulus texture is pronounced in these rocks, containing magnesium-rich olivine (up to 81.4% forsterite). SiO2 contents of the lherzolites range from 42.93 to 44.18 wt.%, whereas those of the gabbronorites vary between 44.89 and 52.76 wt.%. Analysed samples have low rare earth element (REE) contents (23.22–30.16 ppm for lherzolites and 25.21–61.05 ppm for gabbronorites). Both lherzolites and gabbronorites have similar chondrite-normalized REE patterns, suggesting that they are comagmatic. All samples are slightly enriched in large ion lithophile elements (LILEs, e.g. Rb, Ba, and Sr) relative to high field strength elements (HFSEs, e.g. Nb, Ta, and Ti), very similar to those of ocean island basalts (OIBs). The presence of cumulus textures and geochemical signatures indicates that fractional crystallization played an important role in the petrogenesis of these rocks. Initial (87Sr/86Sr) t (t = 260 Ma) ratios and ϵNd(t) values of the mafic–ultramafic suite vary from 0.70542 to 0.70763, and −0.4 to 1.7, respectively. Compared to the Cu–Ni-bearing Baimazhai and Limahe intrusions in the ELIP, which were considerably contaminated by variable crustal materials, the Nantianwan complex exhibits much lower (87Sr/86Sr) t . Their ϵNd(t) versus (Th/Nb)PM ratios also indicate that the ore-bearing magmas did not undergo significant crustal contamination. In combination with (Tb/Yb)PM versus (Yb/Sm)PM modelling, we infer that the magmas originated from an incompatible elements-enriched spinel-facies lherzolite that itself formed by interaction between the Emeishan plume and the lithospheric mantle. Most plots of NiO versus Fo contents of olivine suggest that sulphides are separated from the parental magma by liquid immiscibility, which is also supported by bulk-rock Cu/Zr ratios of the lherzolites (7.04–102.67) and gabbronorites (0.88–5.56). We suggest that the gabbronorites and lherzolites experienced undersaturation to oversaturation of sulphur; the latter may be due to fractional crystallization in a high-level magma chamber, accounting for the sulphide segregation.

Complete preservation of ophiolite suite from south Andaman, India: A mineral-chemical perspective

Field studies supplemented by petrographic analyses clearly reveal complete preservation of ophiolite suite from Port Blair (11°39′N: 92°45′E) to Chiriyatapu (11°30′24″N: 92°42′30″E) stretch of South Andaman. The ophiolite suite reveals serpentinite at the base which is overlain unconformably by cumulate ultramafic-mafic members with discernible cumulus texture and igneous layering. Basaltic dykes are found to cut across the cumulate ultramafic-mafic members. The succession is capped by well exposed pillow basalts interlayered with arkosic sediments. Olivine from the basal serpentinite unit are highly magnesian (Fo80.1–86.2). All clinopyroxene analyses from cumulate pyroxenite, cumulate gabbro and basaltic dyke are discriminated to be ‘Quad’ and are uniformly restricted to the diopside field. Composition of plagioclase in different lithomembers is systematically varying from calcic to sodic endmembers progressively from cumulate pyroxenite to pillow basalt through cumulate gabbro and basaltic dyke. Plagioclase phenocrysts from basaltic dyke are found to be distinctly zoned (An60.7-An35.3) whereas groundmass plagioclase are relatively sodic (An33-An23.5). Deduced thermobarometric data from different lithomembers clearly correspond to the observed preservation of complete ophiolite suite.

Late Archean Lake Harris Komatiite, Central Gawler Craton, South Australia:Geologic Setting and Geochemistry

The Lake Harris Komatiite in the central Gawler craton of South Australia is the first documented komatiite outside the West Australian craton and the easternmost occurrence of such primitive ultramafic rocks in Australia. A U-Pb zircon age of ca. 2520 Ga for the komatiitic sequence indicates a previously unknown period of mantle-plume activity in the Late Archean. An integrated program of airborne magnetic surveys, gravity surveys, and core drilling was successful in defining the distribution and volcanic architecture of the komatiitic flows and associated greenstones through an extensive thin cover of Cenozoic alluvial sediments. Surface exposure of the komatiitic rocks is restricted to one small outcrop near Lake Harris. The greenstones form a series of subparallel east-northeast–trending sinuous magnetic highs flanked by large ovoid to elongate magnetic highs and lows that correlate with Archean-Proterozoic granitic bodies associated with province-wide shear systems, similar to the Archean greenstone terranes in the Yilgarn craton of Western Australia. The steeply dipping greenstone sequence was metamorphosed to middle amphibolite facies during the ca. 2440 Ma Sleafordian orogeny and sheared during the ca. 1700 Ma Kimban and ca. 1540 Ma Kararan orogenies. The greenstones consist of komatiite cumulates (43–32% MgO, anhydrous), high to low Mg komatiite (32–18% MgO), komatiitic and tholeiitic basalt (<18% MgO), pyroxenite cumulates, felsic volcanic rocks, minor metasedimentary rocks, pyroclastic rocks, and rare banded iron formation. They extend over 300 km in three subparallel belts that appear to be isoclinally folded around east-northeast axes and tectonically dismembered to the south by the Yerda shear zone. Komatiitic rocks have been confirmed by drilling in all three belts, but the absence of outcrop and structural complexities prevent detailed stratigraphic correlations within and between the belts. The komatiitic rocks display a range of quenched and cumulus textures defined by the different habits of olivine and its alteration products. Trace sulfides (pyrrhotite, chalcopyrite, pentlandite, pyrite, marcasite, polydymite violarite, heazelwoodite, millerite) form very small (0.01–0.2 mm) single-phase disseminated grains and coarser disaggregated grains. Their distribution largely reflects metamorphic and serpentinization processes, with high Ni/S ratios and probable sulfur loss from the more magnesian parts of the flows. Rare composite pyrrhotite-pentlandite-chalcopyrite blebs (0.1–0.5 mm) characterize some low Mg flows. Locally, there is supergene pyrite-marcasite and native copper-bornite-chalcocite(?) assemblage infilling of late low-temperature serpentine-chlorite veinlets. Thick ponded lava lake and distal composite sheet flow facies have been identified from different parts of the komatiitic sequences. Systematic whole-rock and mineral chemical trends indicate that despite the effects of recrystallization and reequilibration during amphibolite-facies metamorphism, the original magmatic geochemical profiles are largely preserved. The whole-rock data for the Lake Harris Komatiite does not show any obvious Ni depletion during fractionation but indicate a strong olivine control in dominantly sulfur undersaturated environments. Low sulfur (100–600 ppm S) and high Pd + Pt (5–30 ppb) contents, and Ti/Pd ratios of 2 to 4 × 10 5 for the komatiitic rocks are similar to sulfur-undersaturated Archean komatiites hosting Ni-Cu-PGE deposits, i.e., there is little evidence for sulfur saturation in the sampled komatiites. Identification of a pre-2.5 Ga source of sulfur in the substrate would be a positive indicator of potential sulfur saturation of the lavas elsewhere in the greenstone belt and a possible target for mineralization. The Lake Harris Komatiite has chemical (parent magma composition of 29% MgO, Al 2 O 3 /TiO 2 = 16, depleted light (L)REE and initial Nd isotope ( ϵ Nd = 2.8–3.0 at 2520 Ma) characteristics similar to typical Al-depleted Archean komatiites, and there is no clear evidence of chemical modification by processes associated with contemporary subduction. Coherent patterns of trace elements (Th, Nb, REE, Ti, Y, Zr, and P) and typical initial ϵ Nd signatures indicate a Late Archean komatiitic system involving a depleted mantle source and no obvious crustal contamination. This system probably exploited a lithosphere that was stretched and thinned by extension and/or thermal erosion in an intraplate environment. Dynamic tectonic environments involving mantle-plume activity, long-lived active plate margins, and widespread arc volcanism during the Late Archean are now favored for the evolution of the western Gawler craton.

TEXTURAL AND MINERALOGICAL CONSTRAINTS ON CHROMITITE GENESIS: EVIDENCES FROM THE CUMULATE SEQUENCE OF NURALI MASSIF (SOUTHERN URALS, RUSSIA)

TEXTURAL AND MINERALOGICAL CONSTRAINTS ON CHROMITITE GENESIS: EVIDENCES FROM THE CUMULATE SEQUENCE OF NURALI MASSIF (SOUTHERN URALS, RUSSIA)

Magmatic and Tectonic Structures from the Chimalpahad Layered Complex, Andhra Pradesh, India

The Chimalpahad Layered Complex (CLC) is the largest (>200 sq km) metamorphosed anorthosite complex in the Precambrian shield of Peninsular India. It exhibits spectacular magmatic layering together with superposed tectonic structures. Modal layering, with primary cumulus texture, is the most prominent magmatic structure displayed by the complex. A variety of magmatic structures (rhythmic mineral-graded layering, cyclic and cross layering, and troughs), practically unaffected (or weakly modified) by later tectonic overprinting, form the relict igneous structures in low strain zones within the CLC; they are described and depicted in sufficient detail, and the possible mechanisms for their formation are enumerated. The structural features in the CLC owe their origin to dual play of magmatic and tectonic processes. The original magmatic structures in high strain zones are variably overprinted by: (1) planar foliation (S1) and mineral lineation (L1) of the D 1 deformation, (2) the F1a and F1b folding event of the D 2 ductile deformation and (3) brittle-ductile fractures/faults of the D 3 deformation that are inferred to have developed during exhumation of the CLC by transpressional tectonism. Micro-textural features of the CLC provide useful criteria for identification of superimposition of solid-state recrystallization on magmatic fabrics. Simultaneous compression and strike-slip shear mechanism (transpression and transtension) have been proposed to account for the magmato-tectonic evolution of the CLC.

Northwest Africa 773: lunar mare breccia with a shallow-formed olivine-cumulate component, inferred very-low-Ti (VLT) heritage, and a KREEP connection

Lunar meteorite Northwest Africa 773 (herein referred to as NWA773) is a breccia composed predominantly of mafic volcanic components, including a prominent igneous clast lithology. The clast lithology is an olivine-gabbro cumulate, which, on the basis of mineral and bulk compositions, is a hypabyssal igneous rock related compositionally to volcanic components in the meteorite. The olivine-gabbro lithology exhibits cumulus textures and, in our largest section of it, includes some 48% olivine (Fo 64 to Fo 70, average Fo 67), 27% pigeonite (En 60Fs 24Wo 16 to En 67Fs 27Wo 6), 11% augite (En 50Fs 17Wo 33 to En 47Fs 13Wo 40), 2% orthopyroxene (En 70Fs 26Wo 4), 11% plagioclase (An 80 to An 94), and trace barian K-feldspar, ilmenite, Cr-spinel, RE-merrillite, troilite, and Fe-Ni metal. The Mg/Fe ratios of the mafic silicates indicate equilibration of Fe and Mg; however, the silicates retain compositional variations in minor and trace elements that are consistent with intercumulus crystallization. Accessory mineralogy reflects crystallization of late-stage residual melt. Both lithologies (breccia and olivine cumulate) of the meteorite have very-low-Ti (VLT) major-element compositions, but with an unusual trace-element signature compared to most lunar VLT volcanic compositions, i.e., relative enrichment in light REE and large-ion-lithophile elements, and greater depletion in Eu than almost all other known lunar volcanic rocks. The calculated composition of the melt that was in equilibrium with pyroxene and plagioclase of the cumulate lithology exhibits a KREEP-like REE pattern, but at lower concentrations. Melt of a composition calculated to have been in equilibrium with the cumulate assemblage, plus excess olivine, yields a major-element composition that is similar to known green volcanic glasses. One volcanic glass type from Apollo 14 in particular, green glass B, type 1, has a very low Ti concentration and REE characteristics, including extremely low Eu concentration, that make it a candidate parent melt for the olivine-gabbro cumulate. We infer an origin for the parent melt of NWA773 volcanic components by assimilation of a trace-element-rich partial or residual melt by a magnesian, VLT magma deep in the lunar crust or in the mantle prior to transportation to the near-surface, accumulation of olivine and pyroxene in a shallow chamber, eruption onto a volcanic surface, and incorporation of components into local, predominantly volcanic regolith, prior to impact mixing of the volcanic terrain and related hypabyssal setting, and ejection from the surface of the Moon. Volcanic components such as these probably occur in the Oceanus Procellarum region near the site of origin of the green volcanic glasses found in the Apollo 14 regolith.

Crystallization of the basaltic shergottites: Insights from crystal size distribution (CSD) analysis of pyroxenes

Abstract— Quantitative petrographic analysis, using the crystal size distribution (CSD) method, provides a novel approach for examining the crystallization histories of basaltic shergottites. Grain number densities at different sizes are plotted against grain size, and the resulting curve relates to the geologic processes involved with the crystallization of the grain population. Most basaltic shergottites are dominated by pigeonite and augite; and because plagioclase is primarily interstitial, and therefore constrained in its growth by the surrounding pyroxenes, we limited our size measurements to the pyroxene phases.The groundmasses of Elephant Moraine (EET) A79001 lithology A and Dar al Gani (DaG) 476 are fine grained with cumulus pyroxene and interstitial plagioclase glass. Their simple linear CSD plots record a single stage of pyroxene crystallization under steady‐state conditions of continuous nucleation and growth. The textures of Queen Alexandra Range (QUE) 94201 and EETA79001 lithology B are quite different from the other shergottites, with intergrown pyroxene and plagioclase. Likewise, their CSD plots are also distinct, with curved trends that suggest a lack of large grains, most likely because of interference between simultaneously growing silicate phases. However, the CSD plot shapes are smooth, also implying a single stage of growth. Shergotty and Zagami, with coarser cumulus textures, display CSD plots that are generally linear over most grain sizes. This implies that conditions of nucleation and growth were dominant during formation of the pyroxene populations. Both plots, however, also display kinks, implying multiple stages of growth.A similar kink is also visible in a CSD plot of only the Mg‐rich cores of Shergotty pyroxenes, which suggests the feature represents changes in conditions during core crystallization, rather than an event coincident with the change in composition to the Fe‐rich rims. The plot may be interpreted as representing two stages of core growth with an intervening short hiatus of nucleation, with continued crystallization associated with ascent of the magma. Eruption onto the surface probably triggered the compositional change to Fe‐rich rims. The CSD analysis of products from a controlled crystallization study agree with experimental and petrologic estimates that cooling rates for Zagami were on the order of a few tenths of a degree per hour. Growth rates derived from these cooling rates suggest crystallization of Shergotty and Zagami pyroxenes occurred over a period of a few weeks to months.