ALBERT

Description: The Late Jurassic English Peak plutonic complex was emplaced in an upper crustal retro-arc setting in the central Klamath Mountains province, northern California. Emplacement of the main, central pluton was preceded by intrusion of two satellite bodies: the Uncles Creek pluton crystallized from H 2 O-rich quartz dioritic magma with hornblende as the liquidus mafic phase; in contrast, the Heiney Bar pluton is a c. 2·5 km diameter body zoned from gabbro to granodiorite. Al-in-hornblende barometry from these two plutons indicates a stage of magma storage at c. 600–500 MPa. The central English Peak pluton is a c. 15 km diameter body composed of early and late stages. Early stage rocks range from gabbro to tonalite, with variable proportions of augite, orthopyroxene, hornblende and biotite. The early stage lacks discernible zoning and rock types vary at the outcrop scale. This diversity is reflected in bulk-rock compositions, which do not form a compositional array. The late-stage intrusion consists of three concentric units that are zoned from outer, more mafic rocks (quartz diorite, tonalite, quartz monzodiorite) to inner, compositionally evolved rocks (granodiorite and granite). Late-stage samples plot in smooth, typically linear arrays for most major and trace elements. Al-in-hornblende pressures indicate that late-stage hornblende cores grew in a reservoir at c. 400 MPa and that rims grew at the level of final emplacement (c. 250 MPa). The mid-crustal reservoir was the site of late-stage magma evolution, including episodic magma mixing. Oxygen and Sr isotopes indicate initial evolution of English Peak pluton magmas in a deep crustal region of mixing, assimilation, storage, and homogenization (MASH zone), where they were contaminated by metasedimentary rocks. Thus, the English Peak pluton represents a crustal-scale system, with mantle-derived magmas that differentiated near the Moho, storage and crystallization of satellite-pluton magmas in the middle crust (c. 600–500 MPa), development of a large, episodically recharged, magma chamber in the upper middle crust (c. 400 MPa) and final emplacement in the upper crust.

Description: Records of micrometeorite collisions at down to submicron scales were discovered on dust grains recovered from near-Earth asteroid 25143 (Itokawa). Because the grains were sampled from very near the surface of the asteroid, by the Hayabusa spacecraft, their surfaces reflect the low-gravity space environment influencing the physical nature of the asteroid exterior. The space environment was examined by description of grain surfaces and asteroidal scenes were reconstructed. Chemical and O isotope compositions of five lithic grains, with diameters near 50 μm, indicate that the uppermost layer of the rubble-pile-textured Itokawa is largely composed of equilibrated LL-ordinary-chondrite-like material with superimposed effects of collisions. The surfaces of the grains are dominated by fractures, and the fracture planes contain not only sub-μm-sized craters but also a large number of sub-μm- to several-μm-sized adhered particles, some of the latter composed of glass. The size distribution and chemical compositions of the adhered particles, together with the occurrences of the sub-μm-sized craters, suggest formation by hypervelocity collisions of micrometeorites at down to nm scales, a process expected in the physically hostile environment at an asteroid’s surface. We describe impact-related phenomena, ranging in scale from 10-9 to 104 meters, demonstrating the central role played by impact processes in the long-term evolution of planetary bodies. Impact appears to be an important process shaping the exteriors of not only large planetary bodies, such as the moon, but also low-gravity bodies such as asteroids.

Description: 〈span〉〈div〉Abstract〈/div〉The Paleoproterozoic Usagaran Orogenic Belt in East-Africa preserves relics of Paleoproterozoic volcanic-arc magmas, and subducted and displaced oceanic-crust. This article describes the geochemical characteristics of displaced pillow basalts from the Usagaran Belt (Konse Group).Our data indicate that the Konse pillow basalts have tholeiitic composition and are overprinted by greenschist facies metamorphic conditions but their primary geochemical signatures are preserved by REE and fluid immobile elements. The (La/Sm)〈sub〉N〈/sub〉 ratios (0.62 - 1.09) and REE patterns point to Normal-MORBs and Transitional-MORBs mantle source.The analysis of high-valency elements and trace element patterns points to a mixed signature of MORB tholeiites and island-arc tholeiites with elevation of Ba, Th, U, Eu and Sr. This composition is similar to that of Phanerozoic back-arc suprasubduction-zone ophiolites. Therefore, Usagaran Belt pillow basalts make analogue of the Tethyan-type suprasubduction-zone ophiolite evolution and emplacement in the Precambrian.The emplacement of the Konse pillow basalts is older than the neighbouring 1920 - 1870 Ma volcanic-arc magmas and is probably coeval with the formation of the 2000 Ma Yalumba eclogites with MORB affinity. Therefore, the Konse Konse pillow basalts might have been displaced from its suprasubduction-zone tectonic setting of igneous construction within the Yalumba ocean basin around 2000 Ma.〈/span〉

Description: We want to know when plate tectonics began and will consider any important Earth feature that shows significant temporal evolution. Kimberlites, the primary source of diamonds, are rare igneous features. We analyze their distribution throughout Earth history; most are young (~95% are younger than 0.75 Ga), but rare examples are found as far back as the Archean (older than 2.5 Ga). Although there are differing explanations for this age asymmetry (lack of preservation, lack of exposure, fewer mantle plumes, or lack of old thick lithosphere in the Archean and Proterozoic), we suggest that kimberlite eruptions are a consequence of modern-style plate tectonics, in particular subduction of hydrated oceanic crust and sediments deep into the mantle. This recycling since the onset of modern-style plate tectonics ca. 1 Ga has massively increased mantle CO 2 and H 2 O contents, leading to the rapid and explosive ascent of diamond-bearing kimberlite magmas. The age distribution of kimberlites, combined with other large-scale tectonic indicators that are prevalent only in the past ~1 Ga (blueschists, glaucophane-bearing eclogites; coesite- or diamond-bearing ultrahigh-pressure metamorphic rocks; lawsonite-bearing metamorphic rocks; and jadeitites), indicates that plate tectonics, as observed today, has only operated for 〈25% of Earth history.

Description: The gemstones jadeite and ruby generally form as a result of the plate tectonic processes subduction and collision. Jade made of jadeite (jadeitite) forms when supercritical fluids released from subducting oceanic crust condense in the overlying mantle wedge, 20–120 km deep in the Earth. Jadeitite deposits thus mark the location of exhumed fossil subduction zones. Ruby, the red gem variety of corundum, forms during amphibolite- and granulite-facies metamorphism or melting of mixed Al-rich and Si-poor protoliths, 10–40 km deep in the crust. Suitable conditions generally exist where passive-margin carbonates and shales are involved in continental collision. Most ruby deposits formed during Ediacaran-Cambrian (ca. 550 Ma) collisions that produced the East African–Antarctic orogen and the supercontinent Gondwana, or during Cenozoic collisions in south Asia. Ruby is thus a robust indicator of continental collision. As a result of these diagnostic properties, we propose the term "plate tectonic gemstones" (PTGs) for jadeitite and ruby. The PTGs are a new type of petrotectonic indicator that are mostly found in Neoproterozoic and younger rocks. The PTGs as petrotectonic indicators that form deep in the Earth have the added advantage that their record is unlikely to be obliterated by erosion, although the possibility of destruction via retrogression needs to be further assessed. Recognition of the PTGs links modern concepts of plate tectonics to economic gemstone deposits and ancient concepts of beauty, and may aid in exploration for new deposits.

Description: We want to know when plate tectonics began and will consider any important Earth feature that shows significant temporal evolution. Kimberlites, the primary source of diamonds, are rare igneous features. We analyze their distribution throughout Earth history; most are young (~95% are younger than 0.75 Ga), but rare examples are found as far back as the Archean (older than 2.5 Ga). Although there are differing explanations for this age asymmetry (lack of preservation, lack of exposure, fewer mantle plumes, or lack of old thick lithosphere in the Archean and Proterozoic), we suggest that kimberlite eruptions are a consequence of modern-style plate tectonics, in particular subduction of hydrated oceanic crust and sediments deep into the mantle. This recycling since the onset of modern-style plate tectonics ca. 1 Ga has massively increased mantle CO 2 and H 2 O contents, leading to the rapid and explosive ascent of diamond-bearing kimberlite magmas. The age distribution of kimberlites, combined with other large-scale tectonic indicators that are prevalent only in the past ~1 Ga (blueschists, glaucophane-bearing eclogites; coesite- or diamond-bearing ultrahigh-pressure metamorphic rocks; lawsonite-bearing metamorphic rocks; and jadeitites), indicates that plate tectonics, as observed today, has only operated for 〈25% of Earth history.

Description: 〈span〉〈div〉Abstract〈/div〉The Paleoproterozoic Usagaran Orogenic Belt in East Africa is an exemplary site of preserved relics of Paleoproterozoic volcanic-arc magmas, and subducted and displaced oceanic crust. This paper describes the geochemical characteristics of displaced pillow basalts from the Usagaran Belt (Konse Group). Our data indicate that the Konse pillow basalts have tholeiitic composition and are overprinted by greenschist-facies metamorphic conditions but their primary geochemical signatures are preserved by REE and fluid-immobile elements. The (La/Sm)〈sub〉N〈/sub〉 ratios (0.62–1.09) and REE patterns point to normal mid-ocean ridge basalt (N-MORB) and transitional MORB (T-MORB) mantle source. The analysis of high-valency elements and trace element patterns points to a mixed signature of MORB tholeiites and island-arc tholeiites with elevation of Ba, Th, U, Eu and Sr. This composition is similar to that of Phanerozoic back-arc suprasubduction-zone ophiolites. Therefore, Usagaran Belt pillow basalts are an analogue of the Tethyan-type suprasubduction-zone ophiolite evolution and emplacement in the Precambrian. The emplacement of the Konse pillow basalts is older than the neighbouring 1920–1870 Ma volcanic-arc magmas and is probably coeval with the formation of the 2000 Ma Yalumba eclogites with MORB affinity. Thus, the Konse pillow basalts might have been displaced from their suprasubduction-zone tectonic setting of igneous construction within the Yalumba ocean basin around 2000 Ma.〈strong〉Thematic collection:〈/strong〉 This article is part of the ‘Tethyan ophiolites and Tethyan seaways collection’ available at: 〈a href="https://www.lyellcollection.org/cc/tethyan-ophiolites-and-tethyan-seaways"〉https://www.lyellcollection.org/cc/tethyan-ophiolites-and-tethyan-seaways〈/a〉〈/span〉