ALBERT

Description: 〈span〉Crystal aggregates in igneous rocks have been variously ascribed to growth processes (e.g., twinning, heterogeneous nucleation, epitaxial growth, dendritic growth), or dynamical processes (e.g., synneusis, accumulation during settling). We tested these hypotheses by quantifying the relative orientation of adjacent crystals using electron backscatter diffraction. Both olivine aggregates from Kīlauea volcano (Hawaiʻi, USA) and chromite aggregates from the Bushveld Complex (South Africa) show diverse attachment geometries inconsistent with growth processes. Near-random attachments in chromite aggregates are consistent with accumulation by settling of individual crystals. Attachment geometries and prominent geochemical differences across grain boundaries in olivine aggregates are indicative of synneusis.〈/span〉

Description: 〈span〉The Santa Fe structure in northern New Mexico is one of the few confirmed impact craters in the western USA. The history of the impact structure is obscure as it is tectonized and eroded to the extent that an intact crater is not preserved, and what remains is located in a complex geological setting. Shatter cones and shocked quartz were previously cited to confirm an impact origin; however, estimates for both impact age (350−1200 Ma) and crater diameter (6−13 km) remain poorly constrained. To further evaluate the extent of shock deformation, we investigated ∼6600 detrital zircon grains for shock features, using material collected from 15 drainages and other sites within an ∼5 km radius of known shatter cone outcrops. Six detrital shocked zircon grains were found at three locations, including two near shatter cones and one near brecciated granitoid. Follow-up studies of bedrock at two sites proximal to detrital shocked zircon occurrences led to the discovery of shocked zircon in situ in a shatter cone-bearing sample of biotite schist; shocked grains were not found in brecciated granitoid at the second site. Electron backscatter diffraction confirms the presence of {112} shock-twin lamellae in five shocked zircon grains, and secondary ion mass spectrometry U-Pb data for three detrital shocked grains yielded 〈sup〉207〈/sup〉Pb/〈sup〉206〈/sup〉Pb crystallization ages from 1715 ± 22 to 1472 ± 35 Ma. Laser ablation−inductively coupled plasma−mass spectrometry U-Pb ages for detrital zircon grains at five of the investigated sites provide the first broad constraints on the local distribution of Paleo- to Mesoproterozoic bedrock in the area. The presence of shock-twinned zircon indicates that some exposed rocks at the Santa Fe structure may record impact pressures up to ∼20 GPa based on empirical studies, which is higher than previous reports of ∼10 GPa based on planar deformation features in shocked quartz. The 1472 ± 35 Ma date from a shock-twinned zircon yields the first direct radiometric maximum age constraint on the Santa Fe impact event, and expands the possible time period for impact to the Mesoproterozoic. Identification of shocked zircon in modern sediment led to the first discovery of shocked zircon in bedrock at this site, which is notable, as shocked zircon is otherwise not abundant in the studied rock samples. This study thus illustrates that detrital zircon surveys are an efficient way to search for diagnostic evidence of shock deformation at putative impact structures where shocked minerals may be present, but are not abundant in exposed bedrock.〈/span〉

Description: The origin and causes of mineralogical diversity of A-type granites are debated. The series of A-type granite plutons, with distinct mineralogical differences, emplaced along an Upper Paleozoic crustal-scale shear zone in the Cobequid Highlands, Nova Scotia, provide an opportunity to examine the origin of different A-type plutons in a similar tectonic setting. Based on the ferromagnesian minerals present, the plutons are classified into sodic granites with sodic amphibole, calcic granites with calcic amphibole, and biotite granites. Sodic and calcic granites occur exclusively in complex intrusions with subequal amounts of gabbro in the eastern shear zone, whereas plutons in the western shear zone, with lesser gabbro, are solely biotite granites. Trace elements and radiogenic isotopes show that the three granite types have different sources. Intensive parameters including temperature, pressure, and water-in-melt contents were estimated from mineralogical and geochemical data. Modeling of these geochemical data suggests that the biotite and calcic granites were derived by 20%–40% partial melting of intracrustal feldspathic rocks, whereas the sodic granites are extreme fractionates (90%) of coeval mafic magma. We propose that supply of Upper Paleozoic mafic magma, probably related to regional extension and decompression melting beneath the Magdalen Basin, created a deep crustal hot zone in the eastern Cobequid Highlands, and extreme fractionation of underplated mafic sills produced the sodic granites. Heat transfer from crystallizing mafic magma induced partial melting of the surrounding crust, creating batches of biotite and calcic granitic melts in different depths. Fractionated and crustally derived melts segregated along crustal-scale faults, constructing the complex plutons in the east. Melting of the crust was further facilitated by the release of water from the crustal rocks upon heating. In the eastern shear zone, water was released predominantly by magmatic rocks and in lesser amounts compared to the west, where Neoproterozoic sedimentary and volcaniclastic rocks are more abundant. The volatile-rich granitic melts in the western part of the shear zone were crystallized rapidly, stabilizing only biotite. This study demonstrates that the mineralogical variations in A-type granites arise from rather similar magma compositions, but they are important petrogenetic indicators of varying sources, specific magmatic processes, and emplacement conditions.

Description: The age of the Kalamaili orogenic belt, marking the final amalgamation in East Junggar, North Xinjiang, is significant for the reconstruction of Paleozoic evolution of the southern Central Asian orogenic belt. The Tamugang and Songkarsu Formations of terrestrial molasse in the southeastern part of the Kalamaili belt, shed from the rising Kalamaili orogen, record the orogenic history. The strata consist of proximal conglomerate thinning to distal fine-grained sandstone and mudstone. Poorly sorted conglomerate is composed of dominant pyroclastic rocks with lesser andesitic, granitic, and ophiolitic clasts. Imbricated clasts indicate that the paleocurrents were directed to the present-day southwest to west-southwest. Laser ablation inductively coupled plasma–mass spectrometry (LA-ICP-MS) detrital zircon U-Pb dating of sandstones from both formations confirms that the Yemaquan arc northeast of the Kalamaili orogenic belt was the main source. Two granitic cobbles with zircon sensitive high-resolution ion microprobe (SHRIMP) U-Pb ages of 432.2 ± 7.8 Ma and 428.1 ± 6.8 Ma indicate the presence of Silurian magmatism in the Yemaquan arc. The SHRIMP U-Pb age of volcanic rocks from the Batamayineishan Formation, which overlies the molasse on both sides of the Kalamaili belt, is 349.5 ± 6.0 Ma. The depositional age of molasse is confined to between 343.5 Ma and 345 Ma, based on the 2 range of possible ages for the youngest detrital zircons and the overlying volcanic rocks. Combined with the previously dated plagiogranite and biostratigraphic ages on chert in the Kalamaili ophiolite as the lower age limit, the Kalamaili collision is restricted to 373.8–343.5 Ma, taking into account 2 error, suggesting that the termination of Kalamaili paleo-ocean subduction and the final amalgamation in East Junggar occurred before the Visean.

Description: 〈span〉〈div〉Abstract〈/div〉Crystal aggregates in igneous rocks have been variously ascribed to growth processes (e.g., twinning, heterogeneous nucleation, epitaxial growth, dendritic growth), or dynamical processes (e.g., synneusis, accumulation during settling). We tested these hypotheses by quantifying the relative orientation of adjacent crystals using electron backscatter diffraction. Both olivine aggregates from Kīlauea volcano (Hawaiʻi, USA) and chromite aggregates from the Bushveld Complex (South Africa) show diverse attachment geometries inconsistent with growth processes. Near-random attachments in chromite aggregates are consistent with accumulation by settling of individual crystals. Attachment geometries and prominent geochemical differences across grain boundaries in olivine aggregates are indicative of synneusis.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Santa Fe structure in northern New Mexico is one of the few confirmed impact craters in the western USA. The history of the impact structure is obscure as it is tectonized and eroded to the extent that an intact crater is not preserved, and what remains is located in a complex geological setting. Shatter cones and shocked quartz were previously cited to confirm an impact origin; however, estimates for both impact age (350–1200 Ma) and crater diameter (6–13 km) remain poorly constrained. To further evaluate the extent of shock deformation, we investigated ∼6600 detrital zircon grains for shock features, using material collected from 15 drainages and other sites within an ∼5 km radius of known shatter cone outcrops. Six detrital shocked zircon grains were found at three locations, including two near shatter cones and one near brecciated granitoid. Follow-up studies of bedrock at two sites proximal to detrital shocked zircon occurrences led to the discovery of shocked zircon in situ in a shatter cone-bearing sample of biotite schist; shocked grains were not found in brecciated granitoid at the second site. Electron backscatter diffraction confirms the presence of {112} shock-twin lamellae in five shocked zircon grains, and secondary ion mass spectrometry U-Pb data for three detrital shocked grains yielded 〈sup〉207〈/sup〉Pb/〈sup〉206〈/sup〉Pb crystallization ages from 1715 ± 22 to 1472 ± 35 Ma. Laser ablation–inductively coupled plasma–mass spectrometry U-Pb ages for detrital zircon grains at five of the investigated sites provide the first broad constraints on the local distribution of Paleo- to Mesoproterozoic bedrock in the area. The presence of shock-twinned zircon indicates that some exposed rocks at the Santa Fe structure may record impact pressures up to ∼20 GPa based on empirical studies, which is higher than previous reports of ∼10 GPa based on planar deformation features in shocked quartz. The 1472 ± 35 Ma date from a shock-twinned zircon yields the first direct radiometric maximum age constraint on the Santa Fe impact event, and expands the possible time period for impact to the Mesoproterozoic. Identification of shocked zircon in modern sediment led to the first discovery of shocked zircon in bedrock at this site, which is notable, as shocked zircon is otherwise not abundant in the studied rock samples. This study thus illustrates that detrital zircon surveys are an efficient way to search for diagnostic evidence of shock deformation at putative impact structures where shocked minerals may be present, but are not abundant in exposed bedrock.〈/span〉