U-Pb zircon age of a sheeted dike complex in ophiolites in the structure of the Revdinsky massif, Ural Рlatinum Belt

Research subject. A tectonic block of ophiolites, corresponding to the root zone of a sheeted dike complex, in the eastern part of the Revdinsky massif of the Platinum-bearing belt of the Urals (UPB).Materials and methods. Zircons for dating were collected in the first-generation dolerite dikes of the sheeted dike complex under study, which had been previously considered as Ordovician. U-Pb dating of zircons was performed by LA-ICP-MS on a NexION 300S quadrupole mass spectrometer with a laser ablation attachment NWR 213.Results. Zircons with an age of 425.6 ± 2.9 Ma are characterized by primary magmatic zoning and apparently correspond to the time of intrusion of the sheeted dike complex. In zircons with an age of 404.0 ± 2.9 Ma, inclusions of metamorphogenic minerals (amphibole, chamosite, quartz, clinozoisite) were discovered. This age reflects rock recrystallization or metamorphism, for example, during the intrusion of later vein rocks of the diorite-tonalite series or dolerite dikes of the second generation. Three points of determination showed a younger age of 362 ± 5.6 Ma, probably reflecting the time of metamorphism at the beginning of collisional processes. Conclusions. For the first time, the Silurian U-Pb age of zircons (LA-ICP-MS) was obtained for the ophiolite block (root zone of the sheeted dike complex) in the structure of the Revdinsky massif UPB. The obtained age of intrusion of the sheeted dike complex (425.6 ± 2.9 Ma) coincides with the age of zircons (428.5 ± 3.7 Ma) from gabbro screens in sheeted dikes of the East Ural zone determined earlier (Smirnov, Ivanov, 2010) and corresponds to the time of spreading over the subduction zone in the Middle Urals.

Implications for sedimentary transport processes in southwestern Africa: a combined zircon morphology and age study including extensive geochronology databases

Extensive morphological and age studies on more than 4600 detrital zircon grains recovered from modern sands of Namibia reveal complex mechanisms of sediment transport. These data are further supplemented by a zircon age database containing more than 100,000 single grain analyses from the entire southern Africa and allow for hypothesising of a large Southern Namibian Sediment Vortex located between the Damara Orogen and the Orange River in southern Namibia. The results of this study also allow assuming a modified model of the Orange River sand highway, whose origin is likely located further south than previously expected. Moreover, studied samples from other parts of Namibia give first insights into sediment movements towards the interior of the continent and highlight the potential impact of very little spatial variations of erosion rates. Finally, this study points out the huge potential of detrital zircon morphology and large geo-databases as an easy-to-use additional tool for provenance analysis.

Detrital zircon ages and the origins of the Nashoba terrane and Merrimack belt in southeastern New England, USA

The fault-bounded Nashoba–Putnam terrane, a metamorphosed early Paleozoic, Ganderian arc/back-arc complex in SE New England, lies between rocks of Avalonian affinity to the southeast and middle Paleozoic sedimentary rocks, interpreted as cover on Ganderian basement, in the Merrimack belt to the northwest. U–Pb detrital zircon laser ablation inductively coupled plasma mass spectrometry analysis were conduced on six samples from the Nashoba terrane in Massachusetts and seven samples associated with the Merrimack belt in Massachusetts and SE New Hampshire to investigate their depositional ages and provenance. Samples from the Nashoba terrane yielded major age populations between ~560 and ~540 Ma, consistent with input from local sources formed during the Ediacaran–Cambrian Penobscot orogenic cycle and its basement rocks. Youngest detrital zircons in the terrane, however, are as young as the Early to Middle Ordovician. Six formations from the Merrimack belt were deposited between ~435 and 420 Ma based on youngest zircon age populations and crosscutting plutons, and yielded large ~470–443 Ma age populations. Three of these formations show only Gondwanan provenance. Three others have a mixed Gondwanan-Laurentian signal, which is known to be typical for younger and/or more westerly sedimentary rocks and may indicate that they are the youngest deposits in the Merrimack belt (late Silurian to early Devonian) and/or have been deposited in the equivalent of the more westerly Central Maine basin. Detrital zircon age populations from the Tower Hill Formation, along the faulted contact between the Merrimack belt and Nashoba terrane, are different from either of these tectonic domains and may indicate that the boundary is complex.

Timescales and thermal evolution of large silicic magma reservoirs during an ignimbrite flare-up: perspectives from zircon

Four voluminous ignimbrites (150–500 km3) erupted in rapid succession at 27 Ma in the central San Juan caldera cluster, Colorado. To reconstruct the timescales and thermal evolution of these magma reservoirs, we used zircon ID-TIMS U–Pb geochronology, zircon LA-ICP-MS geochemistry, thermal modeling, and zircon age and crystallization modeling. Zircon geochronology reveals dispersed zircon age spectra in all ignimbrites, with decreasing age dispersion through time that we term a ‘chimney sweeping’ event. Zircon whole-grain age modeling suggests that 2σ zircon age spans represent approximately one-quarter of total zircon crystallization timescales due to the averaging effect of whole-grain, individual zircon ages, resulting in zircon crystallization timescales of 0.8–2.7 m.y. Thermal and zircon crystallization modeling combined with Ti-in-zircon temperatures indicates that magma reservoirs were built over millions of years at relatively low magmatic vertical accretion rates (VARs) of 2–5 × 10–3 m y−1 (2–5 × 10–6 km3 y−1 km−2), and we suggest that such low VARs were characteristic of the assembly of the greater San Juan magmatic body. Though we cannot unequivocally discern between dispersed zircon age spectra caused by inheritance (xenocrystic or antecrystic) versus prolonged crystallization from the same magma reservoir (autocrystic), our findings suggest that long-term magma input at relatively low VARs produced thermally mature upper crustal magma reservoirs resulting in protracted zircon crystallization timescales. Compiling all U–Pb ID-TIMS zircon ages of large ignimbrites, we interpret the longer timescales of subduction-related ignimbrites as a result of longer term, lower flux magmatism, and the shorter timescales of Snake River Plain ignimbrites as a result of shorter term, higher flux magmatism.

Detrital geochronology and lithologic signatures of Weddell Sea Embayment ice streams, Antarctica—Implications for subglacial geology and ice sheet history

Tills from moraines adjacent to major ice streams of the Weddell Sea Embayment contain distinct detrital zircon (n = 5304) and K-bearing mineral age populations (n = 323) that, when combined with pebble composition data, can be used to better understand Antarctica’s subglacial geology and ice sheet history. Till representing the Institute, Foundation, Academy, Recovery and Slessor Ice Streams each have distinct detrital zircon age populations. Detrital Ar-Ar ages are mostly younger than zircon ages, and distinctive populations include 270−300 Ma (Institute), 170−190 Ma (Foundation), and 1200−1400 Ma (Recovery), which are not easily explained by known outcrops. Pebble fractions of the Foundation and Academy tills are most diverse with up to >40% exotic erratics. The southern side of the Recovery Glacier has fossiliferous limestone erratics. Mixing models created using a nonlinear squares curve-fitting approach were developed to evaluate contributors of zircons to Foundation Ice Stream till. These model results and pebble lithology data both indicate that unexposed (subglacial) bedrock is mixed with exposed rocks to produce the observed till. Notably, the model required limited local Patuxent Formation input to the Foundation till’s zircon population. Our data suggest that sandstones underlie the Foundation Ice Stream and Recovery Glacier troughs, which has a bearing on basal ice flow conditions and results in geological controls on ice stream location. This geo- and thermo-chronological characterization of the ice streams will enable ice-rafted debris in Weddell Sea marine sediments to be traced back to its sources and interpreted in terms of ice stream dynamics.