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Sediments deposited into foreland basins can provide valuable insights related to the geological evolution of their hinterlands. Located in the peripheral foreland of the South Sistan Suture Zone (SE Iran), the Karvandar Basin exhibits a several-kilometer-thick shallow-marine to continental clastic sedimentary sequence forming elongated sub-circular synclines. These synclines overlie a mud-dominated formation with exotic volcanic blocks that hosts one of Iran's largest mud volcano, known as Pirgel. In this study, we present a ∼3.5-km-thick magnetostratigraphic section and U-Pb zircon ages of interlayered tuffs that constrain a depositional age of the Karvandar Basin of ∼24–17 Ma. Sandstone and microconglomerate framework analyses and paleocurrent directions suggest a first-cycle active volcanic arc source to the northeast of the basin. We interpret the mud-dominated lithology with volcanic blocks as an olistostrome originating from a similar source as the overlying clastic sequence. The deposition of the olistostrome is dated at ∼24.5 Ma by a U-Pb calcite age from a coral block. The absence of large-scale anticlines and the occurrence of angular unconformities suggest that the sub-circular synclines in the Karvandar Basin formed by gravity-driven downbuilding into the unconsolidated fluid-saturated olistostrome, resembling salt-related minibasins. Integrated results indicate that a late Oligocene to early Miocene Makran volcanic arc represents the source of the clastic sequence. Hence, our results provide new constraints on the initiation of arc volcanism related to the Makran subduction zone, predating earliest reported ages from the Mirabad pluton (19 Ma) to the northeast of the Karvandar Basin by ∼5 Myr

Difficulties in dating Mississippi Valley-type (MVT) mineralization and the often closely-associated regional dolomitization events have led to considerable controversy regarding the typical tectonic contexts in which they form. This paper presents the first radiometric datings for the Gorno district in Lombardy, Italy, a classic example of the Alpine subclass of MVT deposits. U-Pb dating of hydrothermal carbonate minerals pre- and post-dating the main ore-forming event shows that both regional dolomitization and base-metal mineralization must have occurred within <10 Ma of the deposition of the Carnian host rocks. This implies that the ore deposits of the Gorno district, and probably other Alpine-type deposits, formed at shallow burial, prior to the western Tethys rifting phase. Concurrent Triassic magmatism and extensional tectonics likely contributed to the high geothermal heat fluxes necessary to drive the associated large-scale hydrothermal system. These findings contradict the general paradigm for MVT genesis, i.e., a preferred occurrence in deep-burial settings associated with compressional tectonics.

The formation of the Pannonian basin by major lithospheric extension during the early to mid-Miocene was accompanied by intensive volcanic activity. Among it, explosive volcanism fed by silicic (dacite to rhyolite) magmas is considered to have been the largest (accumulated tephra volume exceeded 4000 km3) in Europe in the last 20 Myr. Zircon U-Pb geochronology indicates that this occurred between 18.2 and 14.4 Ma (Bükkalja volcanic field; BVF) and involved at least four major eruption episodes. Distal ash materials related to these volcanic eruptions are recognized around the Pannonian basin and even farther, over thousand kilometre away, based on zircon and glass shard data. Another major silicic volcanic activity developed eastward (Tokaj Mts., TM) from 13.2 to 11.5 Ma. In this case, explosive and effusive volcanic products were equally preserved. Explosive events in the TM are also clearly distinguished by zircon geochronology as well as zircon and glass shard trace element compositions. The diverse zircon and glass geochemical data from these two areas suggest that reservoirs with distinct silicic magmas developed in the shallow continental crust for about 7 Myr. The epsilon-Hf values of zircon crystals are in a similar range from -10 to +2 and show a temporal increase, implying increasing role of mantle-derived magmas. In the BVF, an abrupt change in the epsilon-Hf values occurred after 16.2 Ma, suggesting that by this time the crust, and the lithospheric mantle was considerably thinned, while in the TM, this occurred later and in two smaller steps. MELTS modelling results, zircon and glass trace element data suggest different magma evolution in the BVF and TM as shown particularly by the Eu and Ce anomalies. In the TM, silicic magmas evolved under more reduced and drier magmatic environments. In contrast, wetter, and more oxidised conditions, with the presence of amphibole in the mineral assemblage, dominated in some of the BVF silicic magmas. The silicic magm

The Mid-Miocene volcanism in the northern Pannonian basin has a peculiar nature. The first volcanic products (metaluminous andesite to peraluminous dacite-rhyodacite) are unique since the dacites and rhyodacites contain almandine garnet, which rarely occurs in volcanic rocks. The primary almandine crystals have a moderate Ca content (CaO=4.5–8.1 wt%), although low-Ca (CaO<3 wt%) almandine derived from lower crustal metapelites is also found. Trace element variations in garnet are consistent with progressive crystal fractionation at high-pressure (>700 MPa). Zircon U-Pb geochronology results suggest eruption ages between 15.6 and 15.0 Ma. Remarkably, this volcanism has no relationship with coeval subduction, but it occurred in response to continental rifting. The nature of these magmas was further explored with the help of trace element and Hf-isotope composition of zircon. Zircon in the peraluminous rhyodacites has a unique compositional feature, an elevated Al content (Al=10–15 ppm) not found in other silicic volcanic rocks in this region. Notably, zircon in the coeval metaluminous volcanic rocks has also relatively high Al-content (5–10 ppm). Zircon crystals in the peraluminous rocks have relatively high Dy/Yb and low Th/U ratios consistent with early garnet crystallization. Strong depletion of heavy rare earth elements is observed also in the glass shards. The epsilon Hf values of zircon crystals show a variation from -4 to +2, transitional values compared to the Miocene volcanic rocks of the region and suggest mixing of mantle-derived and crustal-derived magmas. This is consistent with petrogenetic modelling calculations based on bulk rock isotope and trace element data, implying interaction between mafic magmas derived from enriched lithospheric mantle and metasedimentary lower crust. Thus, the peraluminous garnet-bearing magmas in the northern Pannonian basin are hybrids, and are not of pure anatectic origin, as being proposed for compositionally similar magma

A silicic ignimbrite flare-up episode occurred in the Pannonian Basin during the Miocene, coeval with the syn-extensional period in the region. It produced important correlation horizons in the regional stratigraphy; however, they lacked precise and accurate geochronology. Here, we used U–Pb (LA-ICP-MS and ID-TIMS) and (U–Th)/He dating of zircons to determine the eruption ages of the youngest stage of this volcanic activity and constrain the longevity of the magma storage in crustal reservoirs. Reliability of the U–Pb data is supported by (U–Th)/He zircon dating and magnetostratigraphic constraints. We distinguish four eruptive phases from 15.9 ± 0.3 to 14.1 ± 0.3 Ma, each of which possibly includes multiple eruptive events. Among these, at least two large volume eruptions (>10 km3) occurred at 14.8 ± 0.3 Ma (Demjén ignimbrite) and 14.1 ± 0.3 Ma (Harsány ignimbrite). The in situ U–Pb zircon dating shows wide age ranges (up to 700 kyr) in most of the crystal-poor pyroclastic units, containing few to no xenocrysts, which implies efficient recycling of antecrysts. We propose that long-lived silicic magma reservoirs, mostly kept as high-crystallinity mushes, have existed in the Pannonian Basin during the 16–14 Ma period. Small but significant differences in zircon, bulk rock and glass shard composition among units suggest the presence of spatially separated reservoirs, sometimes existing contemporaneously. Our results also better constrain the time frame of the main tectonic events that occurred in the Northern Pannonian Basin: We refined the upper temporal boundary (15 Ma) of the youngest counterclockwise block rotation and the beginning of a new deformation phase, which structurally characterized the onset of the youngest volcanic and sedimentary phase.

Many studies now acknowledge the occurrence of systematic discrepancies between U–Pb ages determined in zircons in situ by LA-ICP-MS and the benchmark analytical method ID-TIMS. In this study, we present detailed investigations into the ablation characteristics of zircons that suggest an underlying mechanism responsible for these age biases relative to ID-TIMS. Confocal laser scanning microscopy of laser ablation pits reveals that there are small but significant differences in the amount of material removed by the laser between different zircons. Based on numerous pit depth and LA-ICP-MS 206Pb/238U ratio measurements of a suite of natural zircon reference materials and samples, we demonstrate that a systematic age bias is strongly correlated with the offset in ablation rates between the primary reference material and sample zircons. We offer further insights concerning the effects of thermal annealing on the ablation behaviour of zircons and demonstrate that, although there is a change in laser ablation rates for annealed zircons, the variations between different zircons are not eliminated. Finally, we show that slight variations in laser focus also influence the ablation behaviour of zircons and may further degrade the accuracy of U–Pb age determinations.