Relationships among the Geordie Lake Cu-Pd deposit, alkaline basalt, and syenites in the Coldwell Complex, Midcontinent Rift, Canada

ABSTRACT The Geordie Lake Cu-Pd deposit is associated with troctolite at the base of the Geordie Lake intrusion, located near the center of the Coldwell Complex (1106.5 + 1.2 Ma). It is the only platinum group element deposit in the Midcontinent Rift associated with alkaline rocks. This study focuses on the long-standing questions regarding genetic relationships among the Geordie Lake gabbros, the Wolfcamp basalt, and the various syenites that make up the east-central portion of the Coldwell Complex. Primitive mantle-normalized trace-element patterns for the Geordie Lake intrusion are nearly flat from Th to Ce and show negative Sr, Eu, and Zr anomalies. Characteristic ratios for the Geordie Lake gabbro and troctolite include Th/Nb (0.12), La/Nb (1.1), La/Lu (150), La/Sm (6.9), Zr/Sm (18), and Gd/Yb (2.8). Trace-element patterns that are useful for determining petrogenesis for gabbros are similar to the Wolfcamp basalt and augite syenite with some key exceptions, notably the middle rare earth element and Zr abundances. Affects due to metasomatism or crustal contamination in Wolfcamp basalt and Geordie Lake gabbros and syenites are negligible. Results of Rayleigh fractionation modeling show (1) the Geordie Lake intrusion and Wolfcamp basalt are very similar but not directly related by crystallization, (2) the gabbros and basalt are not related to the syenites, (3) the lower augite syenite can be related to the upper augite syenite and amphibole quartz syenite by fractionation of a hypothetical crystal cumulate composed of orthoclase (78%), clinopyroxene (15%), olivine (1%), and titanomagnetite (6%). We conclude that the Geordie Lake intrusion, Wolfcamp basalt, and saturated syenites in the Coldwell were derived by separate partial melting events in a common mantle source. The origin of the sulfide mineralization is enigmatic because it exhibits characteristics of both magmatic and hydrothermal processes. The sulfide assemblage changes from disseminated bornite and chalcopyrite in the basal zone to pyrrhotite plus chalcopyrite in the upper zones. Sulfides occur as coarse blebs interstitial to fresh or partly altered silicates, or as very fine grains intergrown with clusters of biotite and actinolite. Primitive mantle-normalized platinum group element patterns exhibit a W-shape for Pd-Pt-Rh-Ir-Ni, indicating a relative depletion of Pt and Ir. The Cu/Pd ratios in the mineralized zones are within the range of mantle values (1000–10,000), Pd/Pt is 14–19, Pd/Rh is 91 + 37, and Pd/Ir >16,000. The Pd/Pt, Pd/Rh, and Pd/Ir are considerably higher than in the Wolfcamp basalt (<1, 17, and 75, respectively). If the sulfides are magmatic in origin, then either the Geordie Lake magma was, unlike the Wolfcamp basalt magma, depleted in Pt, Rh, and Ir, or these elements were selectively removed from the sulfide assemblage. Alternatively, Pd was enriched by late-stage hydrothermal processes. Additional work is recommended to constrain petrogenesis of the sulfides by detailed base-metal and TABS (Te, As, Bi, Sb, and Sn) element analysis.

Various sources of rare earth element enrichment at Manamas volcanic rock, Timor Island

Manamas volcanic rock formed due to crustal thinning in fore arc setting. This research aims to provide information and the enrichment process of rare earth elements in Manamas Formation on the Timor Island and their tectonic implication. Manamas volcanic rock exposed in Bihati River, Baun, Timor consists of two different types of basalts, namely alkaline basalt and sub alkaline basalt. Analysis using ICP-MS method shows enrichment in large ion lithophile element and high field strength element. Subalkaline basalt has N-MORB patterns and alkaline basalt have OIB patterns. The Nb element is relatively impoverished that indicates influence of subduction activities. Thorium and uranium elements also show significant enrichment, due to sedimentary rocks contamination or continental crust or directly from the asthenosphere due to magma upwelling. The two distinctive patterns interpreted due to slab tear phenomenon beneath Timor Island during Australia oceanic plate subduction and recycled oceanic crust beneath Banda Arc.

Lithostratigraphy and lithogeochemistry of Ediacaran alkaline basaltic rocks of the Musgravetown Group, Bonavista Peninsula, northeastern Newfoundland, Canada: an extensional volcanogenic basin in the type-Avalon terrane

Volcanic rocks of the Ediacaran Musgravetown Group on Bonavista Peninsula, Avalon terrane, Newfoundland, include basal ca. 600 Ma calc-alkaline basalt succeeded by continental tholeiite and alkaline rhyolite of the ca. 592 Ma Plate Cove volcanic belt (Bull Arm Formation), indicating a change from subduction-related to extensionrelated tectonic regimes during that interval. Alkalic basalts on northeastern (Dam Pond area) and southwestern (British Harbour area) Bonavista Peninsula occur below and above, respectively, the ca. 580 Ma glacial Trinity facies. Dam Pond basalt occurs in a structural dome intercalated with and flanked by fine-grained, siliciclastic deposits (Big Head Formation) overlain by Trinity facies. The British Harbour basalt occurs above the Trinity facies, in an upward- coarsening sandstone sequence (Rocky Harbour Formation) overlain by red beds of the Crown Hill Formation (uppermost Musgravetown Group). The Rocky Harbour and Big Head formations are likely stratigraphically interfingered proximal and distal deposits, respectively, derived from erosion of the Bull Arm Formation and older Avalonian assemblages.The Big Head basalts have lower SiO2, Zr, FeOT, P2O5, TiO2 and higher Mg#, Cr, V, Co and Ni contents, and are therefore more primitive than the more FeOT-, TiO2-, and P2O5-rich British Harbour basalts. Large-ionlithophile and rare-earth-element concentrations and ratios indicate that both suites originated from low degree partial melts of deep, weakly garnet-bearing, undepleted asthenospheric peridotite sources, with magma conduits likely focused along regional extensional faults. The protracted and episodic extension-related volcanic activity is consistent with a geodynamic setting that evolved from a mature arc into extensional basins with slowly waning magmatism, possibly involving slab rollback and delamination followed by magmatic underplating. The duration and variation of both volcanism and sedimentation indicate that the Musgravetown Group should be elevated to a Supergroup in order to facilitate future correlation of its constituent parts with other Avalonian basins.

Melt-rock interaction in the lower crust based on silicate melt inclusions in mafic garnet granulite xenoliths, Bakony–Balaton Highland

Major and trace element composition of silicate melt inclusions (SMI) and their rock-forming minerals were studied in mafic garnet granulite xenoliths from the Bakony–Balaton Highland Volcanic Field (Western-Hungary). Primary SMIs occur in clinopyroxene and plagioclase in the plagioclase-rich domains of mafic garnet granulites and in ilmenite in the vicinity of these domains in the wall rock. Based on major and trace elements, we demonstrated that the SMIs have no connection with the xenolith-hosting alkaline basalt as they have rhyodacitic composition with a distinct REE pattern, negative Sr anomaly, and HFSE depletion. The trace element characteristics suggest that the clinopyroxene hosted SMIs are the closest representation of the original melt percolated in the lower crust. In contrast, the plagioclase and ilmenite hosted SMIs are products of interaction between the silicic melt and the wall rock garnet granulite. A further product of this interaction is the clinopyroxene–ilmenite±plagioclase symplectite. Textural observations and mass ­balance calculations reveal that the reaction between titanite and the silicate melt led to the formation of these assemblages. We propose that a tectonic mélange of metapelites and (MOR-related) metabasalts partially melted at 0.3–0.5 GPa to form a dacitic–rhyodacitic melt leaving behind a garnet-free, plagioclase+clinopyroxene+orthopyroxene+ilmenite residuum. The composition of the SMIs (both major and trace elements) is similar to those from the middle Miocene calc-alkaline magmas, widely known from the northern Pannonian Basin (Börzsöny and Visegrád Mts., Cserhát and Mátra volcanic areas and Central Slovakian VF), but the SMIs are probably the result of a later, local process. The study of these SMIs also highlights how crustal contamination changes magma compositions during asthenospheric Miocene ascent.

On the article “Weathered potassic volcanic rocks as protoliths of the hematitic phyllites of the southern Serra do Espinhaço (MinasGerais)”: a discussion

Chaves and Knauer (2020) have presented three new whole-rock chemical analyses of phyllitic hematite, a unique metamorphic rock of the southern Serra do Espinhaço. Based on their three samples and a selection of other three samples from the literature, Chaves and Knauer have proposed that the geochemical uniqueness of the rock – i.e., high contents of K2O, Al2O3 and Fe2O3, and depletion in SiO2 – would represent a weathered, feldspathoid-rich alkaline basalt. This contribution is a discussion of their new data, the trace-element contents of which are at odds with those of a potassic, mantle-derived volcanic protolith for the hematitic phyllite. Its Nb/Th ratios of ~3 and chondrite-normalised La/Yb ratios of ~9–17, for instance, are typical of the continental crust. We also point out aspects that escaped the attention of Chaves and Knauer (2020), one of which is the ubiquitous occurrence of tourmaline in the hematitic phyllite.

Occurrence and significance of pumpellyite blasts from the greenschists of the Mt. Medvednica (Croatia)

<p>The Mt. Medvednica is located north of Zagreb, a capital of Croatia, reaching 1033 m in height. It belongs to a complex geological unit located in the border area between Alps, Tisia (crystalline basement of the Pannonian Basin) and Dinarides, that are separated with large and regionally significant tectonic zones. Such geological position inevitably resulted with preservation of characteristics inherited from those large tectonic units, as well as those related to the local scale geological processes. Despite the significant tectonism, the Cretaceous metamorphism of Mt. Medvednica did not exceed P-T conditions of a low-grade metamorphism, as a typical metamorphic rock present is greenschist originated from the mafic igneous rock protolith.</p><p>The investigated Mt. Medvednica greenschists are characterized with weak schistosity, granoblastic to granolepidoblastic texture and typically comprise chlorite (40 vol.%), albite (35 vol.%), opaques (up to 15 vol.%), epidote (5 vol.%) and quartz (5 vol.%) that do not exceed 0.5 mm in size, with accessory minerals like titanite, apatite, zircon and calcite, together with rare finding of pumpellyite. The pumpellyite was so far just sporadically reported in the greenschists and was not investigated in detail. On the contrary, pumpellyite was almost regularly reported in the basic rocks from Jurassic ophiolite mélange that tectonically overly greenschists. Pumpellyite can be found there as a secondary hydrous silicate occurring in the altered extrusive rocks that undergone low-temperature ocean floor hydrothermal metasomatism addressed to the ophiolite emplacement.</p><p>Since blasts of pumpellyite (ca. 0.2‒0.3 mm in size) that we have found in the greenschists are possible indicators for a polyphase metamorphic evolution, we have conducted microtextural analyses combined with a phase equilibrium modeling approach through the construction of P-T pseudosections. Chemical composition of greenschists suggested an origin from the altered calc-alkaline basalt. Therefore, P-T pseudosections in the range of 100‒1000 MPa and 250‒450 °C were constructed with PERPLEX software in the complex MnNCKFMASHTO chemical system, and contoured by isopleths for the mode and chemical composition of major rock-forming minerals.</p><p>Pumpellyite chemistry is characterized with SiO<sub>2</sub>=36.77‒38.38 wt.%, Al<sub>2</sub>O<sub>3</sub>=18.56‒21.00 wt.%, CaO=20.69‒22.89 wt.% and FeO=14.50‒16.85 wt.% that classify this mineral as a pumpellyite-(Fe<sup>2+</sup>). Metamorphic P-T conditions for pumpellyite-(Fe<sup>2+</sup>) blasts in the assemblage with chlorite and albite were modeled to 500 MPa and 270°C. Those values correspond well with the theoretically expected values, as well as with previously obtained peak P-T values for greenschist metamorphism of Mt. Medvednica obtained on the metapelites and metabasites with aid of a classical geothermobarometry. For comparison, different pumpellyite chemistry and slightly higher P-T values obtained in this research with pressures (up to +300 MPa) and temperatures (approx. +40°C) point to metamorphic mineral different from pumpellyite related to Jurassic ophiolite mélange altered basic rocks. Microtextural relations between major mineral assemblage and assemblage with pumpellyite show that prograde part of Cretaceous metamorphism, as a consequence of closure of the Neo-Tethys oceanic crust, preceded the growth of pumpellyite that may be ascribed to the retrograde part of a clockwise P-T path.</p>

Blueschist facies conditions in Tethyan passive margin metabasaltic rocks of the easternmost Lower Alpujarride Units (Internal Betic Zone, Spain)

<p>Jurassic shallow-intrusive basic bodies within the Permian-Triassic Tethyan passive margin sedimentary sequences of the Lower Alpujarride units (Internal Betic Zone, Spain) locally show Alpine low-grade metamorphism in the greenschist and blueschist facies. A small sill-like mafic body near Redován town (Callosa Range) partially preserves igneous ophitic/subophitic texture and relics of augite, ferrohornblende-ferroedenite, kaersutite and K-feldspar (orthoclase). The metamorphic overprint corresponds to high-pressure and low-temperature mineral assemblages that comprise magnesioriebeckite, actinolite, albite, stilpnomelane, phengite and chlorite, with rutile, apatite and titanite as accessory minerals. Major and trace element geochemical data reveal igneous protoliths derived from magmas of alkaline basalt composition enriched in incompatible elements and E-MORB geochemical affinity. The intrusion emplacement occurred at shallow crustal levels in an extensional geodynamic setting (within-plate basalts) related to the breakoff of Pangea. Pressure-Temperature (P-T) conditions estimated by means of pseudosection calculations and the intersection of phengite (Si) and chlorite (Mg#) isopleths indicate a cold thermal gradient with calculated peak metamorphic conditions of ca. 8 kbar at 310 ºC. These conditions are consistent with metamorphism during burial down to ca. 24 km depth and a thermal gradient of ca. 13 ºC/km. Although the easternmost Lower Alpujarride units have been traditionally described as reaching only lower-greenschist to greenschist metamorphic peak conditions, the textures, mineral compositions and P-T conditions of the studied metagabbroic body reveal blueschist facies conditions that attest for a regional early stage (Eocene) of subduction of the lower Alpujarride units. This event predates the late Oligocene - early Miocene subduction-related metamorphism of the Intermediate and Upper Alpujarride units.</p>

Petrogenesis and Geodynamic Implications of a Newly Discovered Basanite Dike in Zaolin, Jingdezhen City, South China

A recently discovered basanite dike in the Zaolin area of Jingdezhen, South China, contains mantle xenocrysts such as kink-banded olivines, olivines + orthopyroxenes assemblage, and chromites. In addition, polymorphic carbonates of the MgCO3–FeCO3 series occur as augens, either independently or interspersed with diopside and spinel in the matrix. The rock is characterized by high Cr and Ni contents, high whole-rock Mg# values (0.66–0.72), and high Ca/Al (0.72–1.03) and TFeO/MgO (1.1–1.3) ratios and is alkali-rich with Na2O > K2O. The trace-element partition patterns are similar to those of other basanites in eastern China as well as ocean island basalts. Whole-rock geochemical analyses show depleted Sr and Nd isotopic compositions (86Sr/87Sr=0.70358–0.703853, εNd=2.52–6.73). These data indicate that the rock has experienced negligible crustal contamination, should be derived from asthenospheric mantle, or mixed by the MORB with EMI/EMII mantle and have been carbonated. The calculated T–P conditions of the melt in equilibrium with xeno-olivine are 1160–1320°C at the mantle depth. The high Cr# values of the spinel xenocrysts indicate that the lithospheric mantle under the Jingdezhen area was probably relict Proterozoic mantle. The Ar–Ar plateau age and the isochron and inverse isochron ages for the matrix of the basanite are all 44 Ma. The basanite, as well as other alkaline basalt or lamprophyre dikes in southeastern China, formed in a rifting regime during the Eocene.

Petrochemical constrains on the origin and tectonic setting of mafic to intermediate dykes from Tikar plain, Central Cameroon Shear Zone

The Tikar plain is located on the Cameroon Central Shear Zone. It is also part of the North Equatorial Pan-African Belt. It is formed of granitoids intruded in places by mafic and intermediate dykes. The mafic dykes are essentially banded gabbros composed of plagioclases, pyroxenes, amphiboles, biotites and opaques. Their textures range from porphyroblastic to porphyritic. The intermediate dykes are monzonites and monzodiorites and are characterized, respectively, by cataclastic and mylonitic textures. The minerals identified are amphiboles, potassium feldspar, pyroxenes, epidotes, sphenes and opaques. Seritization reaction is mostly present on the mafic and intermediate dykes, while chloritization is much more pronounced on the intermediate dykes. The Tikar plain dykes are high-k calc-alkaline to shoshonitic. They are characterized by low to moderate SiO2 content (42.08–61.96 wt%), low to high TiO2 (0.47–2 wt%) and low Ni (1.48–99.18 ppm) contents. The mafic dykes show fractional trends with negative anomalies of Zr, U and P and positive Rb, Ba, Ta, Pb and Sr in multi-element diagrams, while the intermediate dykes present negative anomalies of Nb, Ta, Zr, Sr P and Ti and relative positive anomalies of Rb, Ba and Pb. The rare-earth elements (REE) patterns show positive Eu anomalies for the mafic dykes and negative anomalies for the intermediate dykes. The REE spectrum of all the dykes shows enrichment in LREE with relatively flat HREE, which can indicate arc magmatism. In the Zr–Ti/100–Sr/2 diagram, the mafic dykes plot in the island arc tholeiite and calc-alkaline basalt fields. The Th, Nb and LREE concentrations indicate that the subducted lithosphere with crustal component contributed to generation of the intermediate dykes of the Tikar plain. The geochemical characteristics of the mafic to intermediate dykes suggest their derivation from a various degree of partial melting in the garnet spinel facies, probably between depths of 80 and 100 km. The collision between the Central African Fold Belt and the northern edge of the Congo craton resulting in crustal thickening, sub-crustal lithospheric delamination and upwelling of the asthenosphere may have been the principal process in the generation of the intermediate dykes in the Tikar plain. The magma for the mafic and intermediate dyke would have migrated through the faults network of the Central Cameroon Shear Zone before crystallizing in the granito-gneissic basement rocks.

New data on the genesis and evolution of the primitive magmas of Mount Cameroon: contribution of melt inclusions

Mount Cameroon is a Plio-Quaternary volcanic massif, without a central crater, made up ofmore than 140 pyroclastic cones. It is one of the active volcanoes of the Cameroon Line. Mount Cameroon magmatic inclusions are found in microdroplets trapped in the early minerals (olivines) from the pyroclastic products. The analysis of these magmatic inclusions allowed us to find primitive liquids compared to lavas. Major elements study of the magmatic inclusions, trapped in the most magnesian olivines (Mg#84-86) of Mount Cameroon revealed "primitive" liquids of basanite and alkaline basalt type with variable composition compared to the much more uniform basalts of the magmatic series of Mount Cameroon. The study of these trapped liquids shows that: (i)- the original primitive lavas did not undergo the process of evolution by FC, but rather underwent fundamentally (or exclusively) the process of partial melting; (ii) the emitted lavas, evolved essentially by FC; (iii) the variations in the trace element contents of the primitive liquids directly reflect a variation in the rate of partial melting of a homogeneous mantelic source. The very high La/Yb ratios of the Mount Cameroon inclusions (> 20) characterize a garnet lherzolite source. Spectra of the magmatic inclusions show a negative anomaly or depletion in K, Rb and Ba as those of HIMU. The "primitive" liquids and lavas of Mount Cameroon represent a co-genetic sequence formed by varying degrees of partial melting of a source considered as homogeneous.