Sulfur and Metal Fertilization of the Lower Continental Crust

Mantle-derived melts and metasomatic fluids are considered to be important in the transport and distribution of trace elements in the subcontinental lithospheric mantle. However, the mechanisms that facilitate sulfur and metal transfer from the upper mantle into the lower continental crust are poorly constrained. This study addresses this knowledge gap by examining a series of sulfide- and hydrous mineral-rich alkaline mafic-ultramafic pipes that intruded the lower continental crust of the Ivrea-Verbano Zone in the Italian Western Alps. The pipes are relatively small (<300 m diameter) and primarily composed of a matrix of subhedral to anhedral amphibole (pargasite), phlogopite and orthopyroxene that enclose sub-centimeter-sized grains of olivine. The 1 to 5 m wide rim portions of the pipes locally contain significant blebby and disseminated Fe-Ni-Cu-PGE sulfide mineralization.Stratigraphic relationships, mineral chemistry, geochemical modeling and phase equilibria suggest that the pipes represent open-ended conduits within a large magmatic plumbing system. The earliest formed pipe rocks were olivine-rich cumulates that reacted with hydrous melts to produce orthopyroxene, amphibole and phlogopite.Sulfides precipitated as immiscible liquid droplets that were retained within a matrix of silicate crystals and scavenged metals from the percolating hydrous melt. New high-precision chemical abrasion TIMS-UPb dating of zircons from one of the pipes indicates that these pipes were emplaced at 249.1+/-0.2 Ma, following partial melting of lithospheric mantle pods that were metasomatized during the Eo-Variscan oceanic to continental subduction (approx. 420-310 Ma). The thermal energy required to generate partial melting of the metasomatized mantle was most likely derived from crustal extension, lithospheric decompression and subsequent asthenospheric rise during the orogenic collapse of the Variscan belt (<300 Ma). Unlike previous models, outcomes from this study suggest a significant temporal gap between the occurrence of mantle metasomatism, subsequent partial melting and emplacement of the pipes.We argue that this multi-stage process is a very effective mechanism to fertilize the commonly dry and refractory lower continental crust in metals and volatiles. During the four-dimensional evolution of the thermo-tectonic architecture of any given terrain, metals and volatiles stored in the lower continental crust may become available as sources for subsequent ore-forming processes, thus enhancing the prospectivity of continental block margins for a wide range of mineral systems

U-Pb (CA-TIMS) zircon geochronology of the late permian of Australia: Constraining the time scale between two major extinctions

The Bowen and Sydney Basins make up a ca. 600 km long, NNW-trending series of basins in eastern Australia, filled with terrestrial sediments of late Paleozoic to early Mesozoic age. The basins are also important sedimentary archives of evolutionary events in eastern Gondwana during times that are characterized by two major extinctions (end Middle Permian and Permian-Triassic extinctions). We present new U-Pb (CA-TIMS) zircon ages for several tuffs interbedded within the sediments, with the purpose of creating a time scale of the Upper Permian as recorded in eastern Australia that allows correlations with different parts of the world. Currently, these correlations mostly depend on carbon-isotope records as well as biostratigraphy and lithostratigraphy that are deemed unsuitable for constructing high-resolution chronostratigraphic framework in absence of robust and precise radio-isotopic ages. The existing SHRIMP U-Pb ages on zircon have been shown to be compromised by inaccuracy and low precision when compared to what is achievable with the CA-TIMS method

Paleomagnetic and age determinations on rocks from Nares Strait

The Nares Strait controversy concerns the debate about whether or not a major sinistral transcurrent fault (the Wegener Fault) separates northern Greenland and Canada. To date no firm evidence has been found for the proposed 200 km sinistral offset, and to the contrary, geological correlations, mainly involving Paleozoic rocks across the Nares Strait, suggest that total left-lateral motion is no more than 70 km. The E-W trending Thule (Greenland) and Devon Island (Canada) dyke swarms lie on opposite sides of Baffin Bay and are offset sinistrally about 200 km, suggesting that if their correlation is established a convincing case for the Wegener Fault can be made. Paleomagnetic, geochemical and petrographic data allow, but do not yet establish, the correlation. Paleomagnetic results for Canadian sites (VGP = 6.9°N, 181.8 °E, A95 = 12.7°, N = 5) and Greenland sites (VGP = 11.5 °N, 178.3 °E, A95 = 13.8°, N = 4) are not significantly different at the 95 % confidence level. These levels are too large to resolve whether or not the Thule and Devon Island swarms have been offset. Geochemical data reveal a distinct and identical pattern in incompatible elements, while petrographically, the dykes are indistinguishable. U-Pb geochronological results for a Canadian dyke (720.2 ±2.0 Ma) and a Thule dyke (720.4 ±2.7 Ma) are identical within error and clearly identify the two sets of dykes as being parts of the same magmatic episode

U-Pb geochronology of the Emeishan large igneous province and the end of the Late Paleozoic Ice Age: connection between magmatism, climate, ocean chemistry, and mass extinction

The Late Paleozoic Ice Age involved several pulses of glaciation from the early Carboniferous through mid-Permian in southern Gondwana. In eastern Australia, biostratigraphy suggested the final pulse to be c. 265 Ma. New U-Pb zircon ages from volcanic layers revise the age of latest glacigenic deposits to c. 255 Ma, making the late stage of the LPIA broadly synchronous with emplacement of the Emeishan large igneous province (ELIP) of China. Recent zircon U-Pb ages indicate rapid emplacement of the intrusive phase of the ELIP, from 260-257 Ma. New zircon U-Pb ages from volcanics overlying the youngest basalts indicate a short-lived effusive component, ending by 258 Ma. Emeishan volcanism has been linked with the end-Mid-Permian mass extinction, though radioisotopic ages from sedimentary units that record the extinction are limited. The ELIP is smaller than other magmatic provinces that have been associated with mass extinctions, though in terms of devolatilization reactions that can affect climate, volume is less significant than eruption rate and composition of the host rock. An important test of the impact of the ELIP on climate is the stable isotopic record of ocean sediments. Initial stable Ca isotope data from marine carbonates indicate the change in ocean chemistry in the Mid- to Late Permian was smaller than that associated with the end-Permian extinction, but suggest ocean anoxia. Efforts are ongoing to establish a robust chronology for the end-Mid-Permian extinction

Sulfur and metal fertilization of the lower continental crust

Mantle-derived melts and metasomatic fluids are considered to be important in the transport and distribution of trace elements in the subcontinental lithospheric mantle. However, the mechanisms that facilitate sulfur and metal transfer from the upper mantle into the lower continental crust are poorly constrained. This study addresses this knowledge gap by examining a series of sulfide- and hydrous mineral-rich alkaline mafic–ultramafic pipes that intruded the lower continental crust of the Ivrea–Verbano Zone in the Italian Western Alps. The pipes are relatively small (<300 m diameter) and primarily composed of a matrix of subhedral to anhedral amphibole (pargasite), phlogopite and orthopyroxene that enclose sub-centimeter-sized grains of olivine. The 1 to 5 m wide rim portions of the pipes locally contain significant blebby and disseminated Fe–Ni–Cu–PGE sulfide mineralization. Stratigraphic relationships, mineral chemistry, geochemical modeling and phase equilibria suggest that the pipes represent open-ended conduits within a large magmatic plumbing system. The earliest formed pipe rocks were olivine-rich cumulates that reacted with hydrous melts to produce orthopyroxene, amphibole and phlogopite. Sulfides precipitated as immiscible liquid droplets that were retained within a matrix of silicate crystals and scavenged metals from the percolating hydrous melt. New high-precision chemical abrasion TIMS U-Pb dating of zircons from one of the pipes indicates that these pipes were emplaced at 249.1 ± 0.2 Ma, following partial melting of lithospheric mantle pods that were metasomatized during the Eo-Variscan oceanic to continental subduction (~420-310 Ma). The thermal energy required to generate partial melting of the metasomatized mantle was most likely derived from crustal extension, lithospheric decompression and subsequent asthenospheric rise during the orogenic collapse of the Variscan belt (<300 Ma). Unlike previous models, outcomes from this study suggest a significant temporal gap between the occurrence of mantle metasomatism, subsequent partial melting and emplacement of the pipes. We argue that this multi-stage process is a very effective mechanism to fertilize the commonly dry and refractory lower continental crust in metals and volatiles. During the four-dimensional evolution of the thermo-tectonic architecture of any given terrain, metals and volatiles stored in the lower continental crust may become available as sources for subsequent ore-forming processes, thus enhancing the prospectivity of continental block margins for a wide range of mineral systems.20 page(s

On the Formation of Magmatic Sulphide Systems in the Lower Crust by Long-Lived Mass Transfer through the Lithosphere: Insights from the Valmaggia Pipe, Ivrea Verbano Zone, Italy

The lower crustal domain of the Ivrea-Verbano Zone (NW Italy) hosts five ~300-m-wide pipe-like ultramafic intrusions that are metasomatized and exhibit Ni-Cu-PGE sulphide mineralization. To better constrain the role of metasomatism in the ore genesis, we studied the best-preserved pipe at Valmaggia which was emplaced 249 Myrs ago. Phlogopite 40Ar/39Ar analyses show that the pipe was infiltrated by metasomatic fluids derived from the subcontinental lithospheric mantle (SCLM) in two pulses at ~208 Ma and ~189 Ma which introduced sulphides into the pipe. Consequently, the pipe repeatedly acted as a preferred path for mass transfer from the SCLM into the lower crust over \u3e60 Myrs (i.e., emplacement to second metasomatic pulse). Uplifted block margins, such as the Ivrea-Verbano Zone, are potentially important exploration targets for magmatic sulphides. We argue that exploration strategies should focus on structures such as pipes that can focus metasomatic agents during ascent through the lithosphere