Depositional Setting and Enrichment Mechanism of Organic Matter of Lower Cretaceous Shale in Ri-Qing-Wei Basin in the Central Sulu Orogenic Belt

A suite of source rock consists of mudstone and shale, with great thickness and continuous deposition was found in the well LK-1 in Lingshan island in Ri-Qing-Wei basin. In order to evaluate the hydrocarbon generation prospects of these source rock and find the mechanism of organic matter enrichment, shale samples were selected from the core for TOC (total organic carbon) and element geochemistry analysis. The results show that organic matter abundance of the source rocks are generally high with average TOC content of 1.26 wt%, suggesting they are good source rocks. The geochemical features show that the sedimentary environment is mostly anoxic brackish water to salt water environment with arid to semiarid climate condition. The enrichment mechanism of organic matter varied with the evolution of the basin, which was divided into three stages according to the sedimentary characteristics. In the initial-middle period of rifting evolution (stage 1 and early stage 2), paleoproductivity is the major factor of OM-enrichment reflecting by high positive correlation between the TOC contents and paleoproductivity proxies. While with the evolution of the rift basin, redox condition and terrigenous clastic input became more and more important until they became the major factor of OM enrichment in the middle stage of rift evolution (stage 2). In the later stage of rift evolution (latest stage 2 and stage 3), besides terrigenous clastic input, the effect of paleoclimate on OM-enrichment increased gradually from a minor factor to a major factor.

Mineralogical and geochemical characterisation of the Kipawa syenite complex, Quebec: implications for rare-earth element deposits

The Kipawa rare-earth element (REE) deposit is located in the Parautochton zone of the Grenville Province 55 km south of the boundary with the Superior Province. The deposit is part of the Kipawa syenite complex of peralkaline syenites, gneisses, and amphibolites that are intercalated with calc-silicate rocks and marbles overlain by a peralkaline gneissic granite. The REE deposit is principally composed of eudialyte, mosandrite and britholite, and less abundant minerals such as xenotime, monazite or euxenite. The Kipawa Complex outcrops as a series of thin, folded sheet imbricates located between regional metasediments, suggesting a regional tectonic control. Several hypotheses for the origin of the complex have been suggested: crustal contamination of mantle-derived magmas, crustal melting, fluid alteration, metamorphism, and hydrothermal activity. Our objective is to characterize the mineralogical, geochemical, and isotopic composition of the Kipawa complex in order to improve our understanding of the formation and the post-formation processes, and the age of the complex. The complex has been deformed and metamorphosed with evidence of melting-recrystallization textures among REE and Zr rich magmatic and post magmatic minerals. Major and trace element geochemistry obtained by ICP-MS suggest that syenites, granites and monzonite of the complex have within-plate A2 type anorogenic signatures, and our analyses indicate a strong crustal signature based on TIMS whole rock Nd isotopes. We have analyzed zircon grains by SEM, EPMA, ICP-MS and MC-ICP-MS coupled with laser ablation (Lu-Hf). Initial isotopic results also support a strong crustal signature. Taken together, these results suggest that alkaline magmas of the Kipawa complex/deposit could have formed by partial melting of the mantle followed by strong crustal contamination or by melting of metasomatized continental crust. These processes and origins strongly differ compare to most alkaline complexes in the world. Additional TIMS and LA-MC-ICP-MS analyses are planned to investigate whether all lithologies share the same strong crustal signature.

Geochemical Compositions and Detrital Minerals of Stream Sediments around the Zijinshan Copper-Gold Orefield and Their Implications

Regional geochemical anomalies in stream sediments often have close spatial relationships with metallogenic provinces or ore districts, but the relationships between them have not been examined in depth. In this study, stream sediments were collected around the Zijinshan Copper-Gold Orefield, Fujian Province, China. Element geochemistry, U–Pb geochronology and Hf isotope compositions of detrital zircons, and electron microprobe and LA-ICP-MS analyses of iron oxides were conducted. The aims of this study were to investigate the relationship between the provenance of the stream sediments and ore-bearing magmatic rocks in the Zijinshan Copper-Gold Orefield, and to explore the enrichment mechanism of the ore-forming elements in stream sediments. The results show that the ore-forming elements and their associated elements are most significantly enriched in stream sediments near the orefield. U–Pb ages and Hf isotopic compositions of detrital zircons in the sediments closest to the orefield carry information on the ore- bearing magmatic rocks in the orefield. However, as the stream sediments are relatively far from the orefield, the degree of enrichment of ore-forming elements and the detrital zircon U–Pb age signals of the ore-bearing magmatic rocks in the orefield rapidly weaken. This weakening of the geochemical signals may have been affected by many factors, such as lithological resistance to weathering, vegetation coverage, micro-topographic conditions, etc. In-situ elements analysis of iron oxides and elemental correlation analysis of stream sediments indicate iron oxides and clay minerals are the main carrier minerals for the migration of ore-forming elements.

Geomorphology and Geochemistry of Back Arc Basins in the Havre Trough, Southwest Pacific

<p>The Havre Trough back arc system located behind the Kermadec Arc, in the southwest Pacific, is a classic example of an intra-oceanic back arc system. Subduction driven magmatism is focused at the arc front, and melting in the back arc is accompanied by back arc rifting. This study examines the deep back arc basins of the southern Havre Trough. Compared to the well-studied Kermadec Arc front volcanoes, the back arc basins remain poorly explored, yet are important features in understanding key structural and geochemical dynamics of the subduction system. The back arc is characterised by areas of deeper basins and constructional cross-arc volcanic edifices, which had previously been attributed to ‘rift regime’ and ‘arc regime’, respectively. In this study, recently acquired multibeam data was used to produce digital terrain maps that show individual basins within the Havre Trough that host a range of different morphological features, such as elongated ridges, nearly-flat basin floors, and small volcanic cones. Lavas dredged from the 10 basins were analysed, eight of which sample the rift regime and two sample the arc regime. The back arc basin lavas are basalts to basaltic-andesites and show fractionation of olivine + pyroxene ± plagioclase mineral assemblages. Olivine phenocrysts were tested for chemical equilibrium and predominantly show that crystallisation occurred in equilibrium with host melts. However, petrographic features such as dissolution and zoning within plagioclase show evidence of multistage magmatic evolution. Whole rock trace element geochemistry reveals trace element characteristics typical of volcanic arc lavas, such as enrichments in large ion lithophile elements (LILE) and Pb relative to high field strength elements (HFSE). From west to east, the back arc basin lavas show a decrease in NbN/YbN, consistent with trench perpendicular flow and progressive melt extraction towards the volcanic front. There is also a broad correlation between NbN/YbN and distance along the strike of the subduction zone. This may suggest a component of trench parallel flow of the mantle wedge, with increasing depletion northwards, although further evidence is needed to rule out pre-existing mantle heterogeneity. Ba/Th values, which trace the addition of slab-derived aqueous fluids, decrease with distance from the arc front. This indicates that the aqueous fluid component becomes less prominent with increasing distance from the arc front. Conversely, the basin lavas exhibit broadly increasing LaN/SmN values with distance from the arc front. As LaN/SmN can be used to trace the deep subduction component, i.e. sediment melt contribution, greater LaN/SmN suggests increasing contribution of a sediment signature away from the arc front. The parameters that measure recycled component flux are comparable between rift and arc regimes, so it is unlikely that increased volatile fluxing leads to the larger concentrations of magmatic activity displayed in arc regimes. Gill volcano (arc regime) has similar to higher NbN/YbN than lavas from adjacent basins, suggesting increased magmatic activity may in part relate to pockets of more fertile mantle. This study shows that back arcs and associated volcanism can be complicated, further research is integral in determining mechanisms for voluminous magmatic activity spread throughout the back arc.</p>

Geomorphology and Geochemistry of Back Arc Basins in the Havre Trough, Southwest Pacific

<p>The Havre Trough back arc system located behind the Kermadec Arc, in the southwest Pacific, is a classic example of an intra-oceanic back arc system. Subduction driven magmatism is focused at the arc front, and melting in the back arc is accompanied by back arc rifting. This study examines the deep back arc basins of the southern Havre Trough. Compared to the well-studied Kermadec Arc front volcanoes, the back arc basins remain poorly explored, yet are important features in understanding key structural and geochemical dynamics of the subduction system. The back arc is characterised by areas of deeper basins and constructional cross-arc volcanic edifices, which had previously been attributed to ‘rift regime’ and ‘arc regime’, respectively. In this study, recently acquired multibeam data was used to produce digital terrain maps that show individual basins within the Havre Trough that host a range of different morphological features, such as elongated ridges, nearly-flat basin floors, and small volcanic cones. Lavas dredged from the 10 basins were analysed, eight of which sample the rift regime and two sample the arc regime. The back arc basin lavas are basalts to basaltic-andesites and show fractionation of olivine + pyroxene ± plagioclase mineral assemblages. Olivine phenocrysts were tested for chemical equilibrium and predominantly show that crystallisation occurred in equilibrium with host melts. However, petrographic features such as dissolution and zoning within plagioclase show evidence of multistage magmatic evolution. Whole rock trace element geochemistry reveals trace element characteristics typical of volcanic arc lavas, such as enrichments in large ion lithophile elements (LILE) and Pb relative to high field strength elements (HFSE). From west to east, the back arc basin lavas show a decrease in NbN/YbN, consistent with trench perpendicular flow and progressive melt extraction towards the volcanic front. There is also a broad correlation between NbN/YbN and distance along the strike of the subduction zone. This may suggest a component of trench parallel flow of the mantle wedge, with increasing depletion northwards, although further evidence is needed to rule out pre-existing mantle heterogeneity. Ba/Th values, which trace the addition of slab-derived aqueous fluids, decrease with distance from the arc front. This indicates that the aqueous fluid component becomes less prominent with increasing distance from the arc front. Conversely, the basin lavas exhibit broadly increasing LaN/SmN values with distance from the arc front. As LaN/SmN can be used to trace the deep subduction component, i.e. sediment melt contribution, greater LaN/SmN suggests increasing contribution of a sediment signature away from the arc front. The parameters that measure recycled component flux are comparable between rift and arc regimes, so it is unlikely that increased volatile fluxing leads to the larger concentrations of magmatic activity displayed in arc regimes. Gill volcano (arc regime) has similar to higher NbN/YbN than lavas from adjacent basins, suggesting increased magmatic activity may in part relate to pockets of more fertile mantle. This study shows that back arcs and associated volcanism can be complicated, further research is integral in determining mechanisms for voluminous magmatic activity spread throughout the back arc.</p>