Summary The Pre-Salt province of the Campos and Santos Basins (SE Brazil) hosts one of the most significant offshore hydrocarbon discoveries of the past 20 years. Several studies have focused on the regional tectonic setting and the depositional and diagenetic characteristics of these Barremian to Aptian lacustrine carbonates. Only recently, a comprehensive tectono-stratigraphic evolution for the deposition of the presalt succession and a model for the time and spatial links between hydrothermal diagenesis and structural evolution was established. Three tectono-diagenetic events were identified through U-Pb geochronology, petrographic and geochemical investigations: Barremian to Aptian, Albo-Cenomanian and Campanian to Maastrichtian. They reveal a diagenetic evolution characterised by an initially strong hydrothermal overprint that progressively attained geothermal equilibrium under deep-burial conditions during the late Cretaceous. These diagenetic episodes are not necessarily coeval with magmatic events but indeed coeval with basin-scale tectonic events linked to changes in the associated tectonic regime, thus characterising events of structural diagenesis. Nevertheless, paragenetic phases related to each of these events are, in most cases, a consequence of the host rock composition.
Rare earth elements (REE) are essential raw materials used in modern technology. Current production of REE is dominated by hard-rock mining, particularly in China, which typically requires high energy input. In order to expand the resource base of the REE, it is important to determine what alternative sources exist. REE placers have been known for many years, and require less energy than mining of hard rock, but the REE ore minerals are typically derived from eroded granitic rocks and are commonly radioactive. Other types of REE placers, such as those derived from volcanic activity, are rare. The Aksu Diamas heavy mineral placer in Turkey has been assessed for potential REE extraction as a by-product of magnetite production, but its genesis was not previously well understood. REE at Aksu Diamas are hosted in an array of mineral phases, including apatite, chevkinite group minerals (CGM), monazite, allanite and britholite, which are concentrated in lenses and channels in unconsolidated Quaternary sands. Fingerprinting of pyroxene, CGM, magnetite and zircon have identified the source of the placer as the nearby Gölcük alkaline volcanic complex, which has a history of eruption throughout the Plio-Quaternary. Heavy minerals were eroded from tephra and reworked into basinal sediments. This type of deposit may represent a potential resource of REE in other areas of alkaline volcanism.
Magmatism between 1.3 and 0.9 Ga at the southwestern margin of Fennoscandia, comprising mainly granitic batholiths and subordinate bimodal volcanic rocks, provides a nearly continuous magmatic record of the Fennoscandian tectonic evolution. Here, we present new and published zircon Hf, K-feldspar Pb and whole-rock Sr isotopic data from the granitic rocks. The εHf isotopic evolution since 1300 Ma starts out as relatively juvenile, with a flat superchondritic trend at 1300–1130 Ma followed by a steeper trend towards lower, but still superchondritic values at 1070–1010 Ma. During the 1000–920 Ma period, the trend flattens out at near-chondritic values. The variations between flat and steep εHf trends correspond to previously documented extensional and compressional periods, respectively. Although the change to a steeper εHf trend at ca. 1100 Ma may indicate the emergence of a new isotopic reservoir (i.e. a colliding continent), there is no corresponding change in the K-feldspar Pb or whole-rock Sr isotopic composition. We argue that the trends are better explained by varying proportions of isotopically evolved crust and juvenile mantle in the magma source regions, similar to Nd and Hf isotopic pull-downs and pull-ups observed in many accretionary orogenic systems. We therefore conclude that continuous accretionary processes without involvement of exotic sources is the best explanation for the isotopic evolution before and during the Sveconorwegian orogeny, and that the orogeny involved generation of significant volumes of new crust to the SW margin of the Fennoscandia.
Application of geochemical proxies to vein minerals - particularly calcite - can fingerprint the source of fluids controlling various important geological processes from seismicity to geothermal systems. Determining fluid source, e.g. meteoric, marine, magmatic or metamorphic waters, can be challenging when using only trace elements and stable isotopes as different fluids can have overlapping geochemical characteristics, such as δ18O. In this contribution we show that by combining the recently developed LA-ICP-MS U-Pb calcite geochronometer with stable isotopes (including clumped isotope palaeothermometry) and trace element analysis, the fluid source of veins can be more readily determined. Calcite veins hosted in the Devonian Montrose Volcanic Formation at Lunan Bay in the Midland Valley Terrane of Central Scotland were used as a case study. δD values of fluid inclusions in the calcite, and parent fluid δ18O values reconstructed from clumped isotope palaeothermometry, gave values which could represent a range of fluid sources: metamorphic or magmatic fluids, or surface waters which had undergone much fluid-rock interaction. Trace elements showed no distinctive patterns and shed no further light on fluid source. LA-ICP-MS U-Pb dating determined the vein calcite precipitation age – 318±30 Ma – which rule out metamorphic or magmatic fluid sources as no metamorphic or magmatic activity was occurring in the area at this time. The vein fluid source was therefore a surface water (meteoric based on paleogeographic reconstruction) which had undergone significant water-rock interaction. This study highlights the importance of combining the recently developed LA-ICP-MS U-Pb calcite geochronometer with stable isotopes and trace elements to help determine fluid sources of veins, and indeed any geological feature where calcite precipitated from a fluid that may have resided in the crust for a period of time (e.g. fault precipitates or cements).
The characterization of the geochemical reservoirs of the Earth's continental crust, including the determination of representative upper and lower crustal compositions, underpins our understanding of crustal evolution. The classic I- and S-type granite classification has often been invoked to distinguish between melts derived from igneous protoliths and those derived from the melting of a sedimentary source. Recent geochemical studies suggest that most granites, even those cited as typical examples of ‘S-type’, show evidence for a mixture of mantle and upper crustal sources, thereby implying that granite formation is evidence for overall crustal growth. We have examined the source of leucogranite bodies in one of the world's youngest collisional orogens using novel zircon techniques that can resolve the presence of even minor mantle contributions. 232 zircons from 12 granites from the Bhutan Himalaya were analysed by in-situ techniques for O, Hf and U–Pb isotopic signatures. In combination with data from the granite host rocks, our data show that the Himalayan leucogranites were derived solely from metamorphosed crustal sediments, and do not record any mantle contribution. This finding is consistent with the time-lag between crustal thickening and widespread crustal melting, and the heat-producing capacities of the pelitic source rocks. We conclude that Himalayan leucogranites provide a more suitable type locality for ‘S-type’ granites than the Lachlan area in South-East Australia where the term was first defined. The Himalayan leucogranites therefore provide evidence that syn-orogenic melting during collisional events does not necessarily result in crustal growth. Importantly, crustal growth models should not always assume that crustal growth is achieved during collisional orogenesis.
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