Magmatism on Iceland is dominated by the eruption of tholeiitic lavas along two NE-SW trending rift zones that represent the loci of new crustal formation. Small volumes of transitional to alkaline magmas erupted along the ~120 km long Snæfellsnes Peninsula in western Iceland. The Snæfellsnes Peninsula is broken into three distinct volcanic systems; Ljósufjöll, Lýsuskard, and Snæfellsjökull. This study examines the geochemical compositions and depths of crystallization of alkaline basaltic glass to see if these features vary spatially along the peninsula. Glass MgO values range from 3.4 to 9.0 wt %, TiO2 ranges from 0.9 – 4.8 wt. % increasing westward along the peninsula. Estimations of crystallization pressures, based on the pressure dependence of ol±plag±cpx, average 4.6 ± 2.8 kbar, translating to depth a depth of 16.1 ± 10 km generally increasing with distance from the main rift zones. In some cases, cox-melt barometry was also used as a check on the glass cotectic pressures. Crustal thickness beneath Snæfellsnes has been estimated to be ~25 km, suggesting mid to lower crustal magma storage depths. Calculated depths in Snæfellsjökull and Lýsuskard are consistent and deep (22 ± 2.4 km) whereas the Ljósufjöll system is more varied. The Ljósufjöll system displays two groupings of depths, split lengthwise. The northern group exhibits depth between 15-26 km, like the rest of the peninsula, while the southern group has no discernable pattern. Previous data on post-glacial lavas reveals broad spatial trends in REEs across the peninsula. Basaltic glasses exhibit a wide range of La/Sm (1 – 5.6) suggesting a variation in melting percentage over a small area. Samples west of 22.33⁰W have higher Dy/Yb (>2.1) indicating a deeper melting depth due to residual garnet as well as a higher La/Sm (>10) indicative of a smaller degree of melting and/or an enriched source. These data have shown that melt storage occurs at near Moho depths along the peninsula but is highly variable in the eastern Ljósufjöll.

The Aegean region contains numerous metamorphic core complexes that reflect post-collision extensional tectonics. The largest of these is the Menderes Massif of western Turkey, which covers an area of ~40,000 km². The Selimiye Shear Zone bounds its southern border and is a key location for studying the metamorphic history of the massif. Models of the tectonic evolution of the region requires an understanding of the peak pressure-temperature (P-T) conditions recorded by rocks in the massif, and the time (t) at which they achieved those conditions. However, limited P-T-t data exists in the Southern Menderes Massif. Here, P-T-t data was obtained from garnet-bearing rocks collected perpendicular to strike along seven transects spaced about 35 km across the Selimiye Shear Zone. Garnets in nine samples from four transects were analyzed using high-resolution back-scattered electron (BSE) imagery, X-ray element (Fe, Mg, Mn, Ca and Y) maps, and quantitative compositional analyses. Both zircon and monazite grains were dated in rock thin section using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and Secondary Ion Mass Spectrometry (SIMS) to ascertain the timing of events recorded by the rocks. Some garnet X-ray element maps show zoning consistent with multiple stages of growth, diffusion, and retrogression. Distinct zones in each sample are visible on high contrast BSE images and can be related to Y, Fe, and Mn contents. The conventional garnet + biotite geothermometer and garnet + plagioclase + muscovite + biotite geobarometer were used to estimate peak metamorphic P-T conditions. These range from 556±10°C to 671±27°C, and 15.3±0.2 kbar to 22.4±0.5 kbar. The temperatures are similar to previous estimates, but the pressures appear about ~10 kbar higher the previous estimates. U-Pb zircon ages range from 2022±30 Ma (13.4% disc.) to 254±5 Ma (13.4% disc.). Based on Th/U contents, the oldest ages are likely related to inherited grains from magmatic sources. The youngest zircon age is the first reported Triassic grain from the Southern Menderes Massif and may relate to the closure event of the Paleo-Tethyan Ocean. A younger history is recorded by U-Pb and Th-Pb monazite ages, which range Cretaceous to Miocene. Monazite geochronology is hindered by the contamination of high amounts of common Pb, but U-Pb and Th-Pb age calculations show Miocene to Jurassic ages. The textures of monazite (i.e. drusy, filling cracks of garnet and in reaction with allanite) imply that ages could record crystallization and/or fluid dissolution/reprecipitation mediated events in the Oligocene. Data reported here support the observation that polymetamorphic events are recorded in the Southern Menderes Massif rocks in close proximity to the Selimiye Shear Zone. Two options are possible environments of continental exhumation of rocks along the zone: (1) a polymetamorphic history that records relict high-pressures from previous metamorphic events or (2) a single-stage exhumation of high-pressure rocks. / text

The Aegean region contains numerous metamorphic core complexes that reflect post-collision extensional tectonics. The largest of these is the Menderes Massif of western Turkey, which covers an area of ~40,000 km². The Selimiye Shear Zone bounds its southern border and is a key location for studying the metamorphic history of the massif. Models of the tectonic evolution of the region requires an understanding of the peak pressure-temperature (P-T) conditions recorded by rocks in the massif, and the time (t) at which they achieved those conditions. However, limited P-T-t data exists in the Southern Menderes Massif. Here, P-T-t data was obtained from garnet-bearing rocks collected perpendicular to strike along seven transects spaced about 35 km across the Selimiye Shear Zone. Garnets in nine samples from four transects were analyzed using high-resolution back-scattered electron (BSE) imagery, X-ray element (Fe, Mg, Mn, Ca and Y) maps, and quantitative compositional analyses. Both zircon and monazite grains were dated in rock thin section using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and Secondary Ion Mass Spectrometry (SIMS) to ascertain the timing of events recorded by the rocks. Some garnet X-ray element maps show zoning consistent with multiple stages of growth, diffusion, and retrogression. Distinct zones in each sample are visible on high contrast BSE images and can be related to Y, Fe, and Mn contents. The conventional garnet + biotite geothermometer and garnet + plagioclase + muscovite + biotite geobarometer were used to estimate peak metamorphic P-T conditions. These range from 556±10°C to 671±27°C, and 15.3±0.2 kbar to 22.4±0.5 kbar. The temperatures are similar to previous estimates, but the pressures appear about ~10 kbar higher the previous estimates. U-Pb zircon ages range from 2022±30 Ma (13.4% disc.) to 254±5 Ma (13.4% disc.). Based on Th/U contents, the oldest ages are likely related to inherited grains from magmatic sources. The youngest zircon age is the first reported Triassic grain from the Southern Menderes Massif and may relate to the closure event of the Paleo-Tethyan Ocean. A younger history is recorded by U-Pb and Th-Pb monazite ages, which range Cretaceous to Miocene. Monazite geochronology is hindered by the contamination of high amounts of common Pb, but U-Pb and Th-Pb age calculations show Miocene to Jurassic ages. The textures of monazite (i.e. drusy, filling cracks of garnet and in reaction with allanite) imply that ages could record crystallization and/or fluid dissolution/reprecipitation mediated events in the Oligocene. Data reported here support the observation that polymetamorphic events are recorded in the Southern Menderes Massif rocks in close proximity to the Selimiye Shear Zone. Two options are possible environments of continental exhumation of rocks along the zone: (1) a polymetamorphic history that records relict high-pressures from previous metamorphic events or (2) a single-stage exhumation of high-pressure rocks. / text

The Aegean region contains numerous metamorphic core complexes that reflect post-collision extensional tectonics. The largest of these is the Menderes Massif of western Turkey, which covers an area of ~40,000 km². The Selimiye Shear Zone bounds its southern border and is a key location for studying the metamorphic history of the massif. Models of the tectonic evolution of the region requires an understanding of the peak pressure-temperature (P-T) conditions recorded by rocks in the massif, and the time (t) at which they achieved those conditions. However, limited P-T-t data exists in the Southern Menderes Massif. Here, P-T-t data was obtained from garnet-bearing rocks collected perpendicular to strike along seven transects spaced about 35 km across the Selimiye Shear Zone. Garnets in nine samples from four transects were analyzed using high-resolution back-scattered electron (BSE) imagery, X-ray element (Fe, Mg, Mn, Ca and Y) maps, and quantitative compositional analyses. Both zircon and monazite grains were dated in rock thin section using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and Secondary Ion Mass Spectrometry (SIMS) to ascertain the timing of events recorded by the rocks. Some garnet X-ray element maps show zoning consistent with multiple stages of growth, diffusion, and retrogression. Distinct zones in each sample are visible on high contrast BSE images and can be related to Y, Fe, and Mn contents. The conventional garnet + biotite geothermometer and garnet + plagioclase + muscovite + biotite geobarometer were used to estimate peak metamorphic P-T conditions. These range from 556±10°C to 671±27°C, and 15.3±0.2 kbar to 22.4±0.5 kbar. The temperatures are similar to previous estimates, but the pressures appear about ~10 kbar higher the previous estimates. U-Pb zircon ages range from 2022±30 Ma (13.4% disc.) to 254±5 Ma (13.4% disc.). Based on Th/U contents, the oldest ages are likely related to inherited grains from magmatic sources. The youngest zircon age is the first reported Triassic grain from the Southern Menderes Massif and may relate to the closure event of the Paleo-Tethyan Ocean. A younger history is recorded by U-Pb and Th-Pb monazite ages, which range Cretaceous to Miocene. Monazite geochronology is hindered by the contamination of high amounts of common Pb, but U-Pb and Th-Pb age calculations show Miocene to Jurassic ages. The textures of monazite (i.e. drusy, filling cracks of garnet and in reaction with allanite) imply that ages could record crystallization and/or fluid dissolution/reprecipitation mediated events in the Oligocene. Data reported here support the observation that polymetamorphic events are recorded in the Southern Menderes Massif rocks in close proximity to the Selimiye Shear Zone. Two options are possible environments of continental exhumation of rocks along the zone: (1) a polymetamorphic history that records relict high-pressures from previous metamorphic events or (2) a single-stage exhumation of high-pressure rocks. / text

The Toba caldera located in north Sumatra, Indonesia, is the source of the largest volcanic eruption in the Quaternary (Rose & Chesner 1987). Its enormous volume of 2800 km3 has been a matter of debate for decades and it is still unclear where and how the Toba magma was assembled. This study documents oxygen isotope data for a suite of whole rocks and minerals erupted as part of the Young Toba Tuff (YTT), some 74 ka ago (cf. Chesner et al. 1991). Oxygen isotope data has been obtained in-situ from quartz crystals (SIMS), whole rocks (conventional), as well as quartz, feldspar, amphibole and biotite (laser fluorination). In combination with cathodoluminescence (CL) imaging on the quartzes, the data are used to test the relative roles of shallow magmatic processes such as crystal fractionation, magma-crust interaction and crystal recycling within the Toba magmatic system. In addition, thermobarometric calculations have been performed on plagioclase and amphibole phenocrysts from the YTT to help unravel the magma storage and plumbing system that gave rise to the YTT. The combined evidence will be used to derive a model for shallow magma evolution and storage at Toba some 74 ka ago. The CL images of quartz crystals exhibit defined patterns of magmatic zoning, which broadly coincide with fluctuations in δ18O values in the quartz crystals, allowing correlation of textural and compositional data. Measured δ18Oquartz values range from 6.7 ‰ to 9.4 ‰, independent of position on crystal core or rim. Values for δ18Omagma have been calculated from quartz phenocrysts (assuming Δquartz-magma is 0.7 ‰ at magmatic temperatures). The lowest magma value is 6.0 ‰, apparently reflecting a primitive isotopic signal (Taylor & Sheppard 1986). The maximum calculated magma value is 8.7 ‰, indicating a significant crustal component and thus multiple sources to the Toba magmatic system. Several crystals, however, show internal zoning with gradually lower values towards the rims, pointing to a late-stage low-δ18O input, most probably from the shallow volcanic edifice. The crystals therefore record a complex and heterogeneous origin of the YTT magma, comprising a primitive and an evolved magmatic component topped up with several substantial crustal contributions to finally assemble the massive volume of the 74 ka Toba eruption.

The North Caribou Terrane forms the core of Superior Province and records a protracted history of crustal growth and modification. At the centre of the North Caribou Terrane, lies the North Caribou greenstone belt, which is surrounded by granitoids of diverse compositions and ages. This study reports whole-rock geochemistry, zircon and titanite geochronology, and hornblende geobarometry on these plutonic rocks. Although zircons as old as 3132 ± 7 Ma were found, the main magmatic pulse occurred between 2880 and 2830 Ma, and geobarometry indicates tectonic thickening during this period. This was followed by widespread hydrothermal alteration and limited magmatism from 2760 to 2680 Ma, and shallow, brittle-ductile intrusions at circa 2630 Ma. From 2730 to 2630 Ma, intrusions were emplaced at increasingly shallow crustal levels. All of the rocks, except for the youngest pegmatitic intrusions, show similar patterns in major and trace elements, with a general trend toward more evolved compositions through time. These patterns indicate that the granitoids record mostly reworking of early intrusions, which is also consistent with patterns observed in the geochronology.

The North Caribou Terrane forms the core of Superior Province and records a protracted history of crustal growth and modification. At the centre of the North Caribou Terrane, lies the North Caribou greenstone belt, which is surrounded by granitoids of diverse compositions and ages. This study reports whole-rock geochemistry, zircon and titanite geochronology, and hornblende geobarometry on these plutonic rocks. Although zircons as old as 3132 ± 7 Ma were found, the main magmatic pulse occurred between 2880 and 2830 Ma, and geobarometry indicates tectonic thickening during this period. This was followed by widespread hydrothermal alteration and limited magmatism from 2760 to 2680 Ma, and shallow, brittle-ductile intrusions at circa 2630 Ma. From 2730 to 2630 Ma, intrusions were emplaced at increasingly shallow crustal levels. All of the rocks, except for the youngest pegmatitic intrusions, show similar patterns in major and trace elements, with a general trend toward more evolved compositions through time. These patterns indicate that the granitoids record mostly reworking of early intrusions, which is also consistent with patterns observed in the geochronology.

No description available.

The Pacific Rim Terrane in British Columbia is a group of fault-bound forearc metasedimentary and metaigneous rocks subcreted to Wrangellia, comprising three lithological units: the Leech River Complex (LRC), the Pandora Peak Unit (PPU), and the Pacific Rim Complex. Of these three, the LRC and PPU were subject to an elevated thermal metamorphic event which locally overprinted typical low temperature, medium pressure forearc assemblages with low greenschist through amphibolite facies assemblages. The field study shows that biotite, garnet and staurolite isograds occur concentrically in the LRC, centered on the Leech River fault, which separates the Pacific Rim Terrane from the underlying Metchosin Igneous Complex of the Crescent terrane. Local thermal overprint in the PPU is sub-biotitic and is characterized by local replacement of prehnite-pumpellyite and lawsonite-bearing assemblages with muscovite ± chlorite. Multi-method geothermobarometry shows peak metamorphic temperatures from ~230 °C in the northern PPU to ~600 °C near the Leech River fault at ~4 kbar, and isotherms are continuous across the LRC-PPU boundary. The interfoliated Tripp Creek metabasites and Eocene Walker Creek intrusions do not control the distribution of isotherms, and syn-metamorphic felsic sills rarely have contact aureoles. Intercalated metabasites show two distinct rare earth element (REE) patterns, including NMORB-like light REE depletion among most Tripp Creek metabasites, and light REE enrichment in PPU metabasites. The lack of thermal aureoles associated with metabasites, and interlayered garnetite bands with negative Ce-anomalies attributed to seafloor hydrothermal processes, suggest the Tripp Creek metabasites are not syn-metamorphic sills and formed prior to accretion. The subcretion of then recently formed oceanic crust belonging to the Crescent terrane is identified as the probable cause of anomalously high temperature forearc conditions, as well as possible proximity to an Eocene mid ocean ridge. The high temperature metamorphic rocks in the Pacific Rim Terrane document the conversion of inherited primary pyrite to pyrrhotite in carbonaceous metasediments. S-inclusive pseudosections for LRC protoliths predict a low temperature (<420 °C) narrow pyrite desulfidation window that produces pyrrhotite and releases negligible S to the fluid phase. Conversely, sulfide petrography in the LRC shows pyrite can persist up to ~550 °C as inclusions in andalusite and staurolite porphyroblasts, as well as possibly in the rock matrix. S contents in carbonaceous pelites show a marked reduction at medium grade, associated with a dearth of visible sulfide in LRC phyllites. Sluggish pyrite desulfidation, pyrrhotite desulfidation, and terrane-scale S mobility are interpreted as the driver for mobility of intra-terrane sourced Au, leading to the formation of a hypozonal orogenic Au deposit in the central LRC. / Graduate / 2022-06-11

The vast distribution and long duration of the Late Mesozoic magmatism in the eastern part of South China presents a unique case in the world. This offers a natural laboratory to study the process of magma genesis, the magma emplacement mode, the relationship between magmatism and tectonics, the geodynamic role on the magma emplacement and lithospheric evolution. Since 50's, particularly 90's of the last century, geoscientists have made important efforts in geological cartography and carried out numerous studies with remarkable scientific achievements, building a solid background to understand the tectonic evolution of the South China Block (SCB). However, certain fundamental questions mentioned above remain unsolved and/or are in hot debate. In order to make progress in these scientific issues, we have carried out in a multi-disciplinary study in the Late Mesozoic Qingyang-Jiuhua massif, Hengshan massif and Fujian coastal zone according to their distance with respect to the paleo subduction zone of the Paleo-Pacific plate, the ages of granitic massifs and related tectonics, including field observation on the structure geology, micro-observation on thin section, U-Pb dating on monazite, AMS, paleomagnetism, gravity modeling and P condition concern the granite emplacement. In the view of deformation in these granitic massifs and their country rocks, mode and influence of regional tectonics on the emplacement, though each studied zone reveals its distinguished characteristics, they show some intrinsic and common relationships between them. With our new results and integrating previous data, in this thesis, we discuss the tectonic context of emplacement of these Late Mesozoic magmatic massifs and the geodynamic evolution of the SCB., We propose a 3-step geodynamic model: (1) during 145-130 Ma period, the Paleo-Pacific plate subducted northwestwardly, the West Philippines micro-continent, approaching to SCB, important subduction-related arc volcanism was produced in the coastal areas of Southeast China coast (Zhejiang-Fujian-Guangdong), forming a back-arc extension tectonic system in SCB; (2) during 130-110 Ma period, due to the collision between the West Philippines microcontinent and SCB, the compressional tectonic structures were developed in the Changle-Na'ao coastal zone, producing ductile deformation zones. However, the inland of the eastern part of SCB was under a NW-SE extensional tectonic regime; (3) during 105-90 Ma period, a new subduction zone was developed in the SE flank of the West Philippines micro-continent, the subducting slab reached the Changle-Nan'ao tectonic belt, with the possible break-off of slab, the asthenospheric ascent was responsible for the important emplacement of plutonic massifs and dykes. The tectonics of the eastern part of SCB was characterized by a general extensional system in this period. This tectonic pattern has been significantly disturbed by the Oligocene-Eocene opening of the South China sea,and the Miocene shortening of the SCB margin in Taiwan. Of course, this model should be improved by more geological, geophysical and geochemical investigations.